Anatomy & Physiology





Anatomy & Physiology













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Table of Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Unit 1: Levels of Organization


Chapter 1: An Introduction to the Human Body . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1 Overview of Anatomy and Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.2 Structural Organization of the Human Body . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3 Functions of Human Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4 Requirements for Human Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.5 Homeostasis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.6 Anatomical Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.7 Medical Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30


Chapter 2: The Chemical Level of Organization . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.1 Elements and Atoms: The Building Blocks of Matter . . . . . . . . . . . . . . . . . . . 42
2.2 Chemical Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.3 Chemical Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.4 Inorganic Compounds Essential to Human Functioning . . . . . . . . . . . . . . . . . 57
2.5 Organic Compounds Essential to Human Functioning . . . . . . . . . . . . . . . . . . 63


Chapter 3: The Cellular Level of Organization . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.1 The Cell Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.2 The Cytoplasm and Cellular Organelles . . . . . . . . . . . . . . . . . . . . . . . . . 95
3.3 The Nucleus and DNA Replication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
3.4 Protein Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
3.5 Cell Growth and Division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
3.6 Cellular Differentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118


Chapter 4: The Tissue Level of Organization . . . . . . . . . . . . . . . . . . . . . . . . . . 131
4.1 Types of Tissues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.2 Epithelial Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
4.3 Connective Tissue Supports and Protects . . . . . . . . . . . . . . . . . . . . . . . . 146
4.4 Muscle Tissue and Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
4.5 Nervous Tissue Mediates Perception and Response . . . . . . . . . . . . . . . . . . . 156
4.6 Tissue Injury and Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158


Unit 2: Support and Movement
Chapter 5: The Integumentary System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171


5.1 Layers of the Skin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
5.2 Accessory Structures of the Skin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
5.3 Functions of the Integumentary System . . . . . . . . . . . . . . . . . . . . . . . . . . 187
5.4 Diseases, Disorders, and Injuries of the Integumentary System . . . . . . . . . . . . . 191


Chapter 6: Bone Tissue and the Skeletal System . . . . . . . . . . . . . . . . . . . . . . . . 203
6.1 The Functions of the Skeletal System . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
6.2 Bone Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
6.3 Bone Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
6.4 Bone Formation and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
6.5 Fractures: Bone Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
6.6 Exercise, Nutrition, Hormones, and Bone Tissue . . . . . . . . . . . . . . . . . . . . . 227
6.7 Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems . 231


Chapter 7: Axial Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
7.1 Divisions of the Skeletal System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
7.2 The Skull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
7.3 The Vertebral Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
7.4 The Thoracic Cage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
7.5 Embryonic Development of the Axial Skeleton . . . . . . . . . . . . . . . . . . . . . . 274


Chapter 8: The Appendicular Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
8.1 The Pectoral Girdle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
8.2 Bones of the Upper Limb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
8.3 The Pelvic Girdle and Pelvis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
8.4 Bones of the Lower Limb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
8.5 Development of the Appendicular Skeleton . . . . . . . . . . . . . . . . . . . . . . . . 316


Chapter 9: Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
9.1 Classification of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
9.2 Fibrous Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
9.3 Cartilaginous Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338
9.4 Synovial Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340




9.5 Types of Body Movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
9.6 Anatomy of Selected Synovial Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . 354
9.7 Development of Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369


Chapter 10: Muscle Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
10.1 Overview of Muscle Tissues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
10.2 Skeletal Muscle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
10.3 Muscle Fiber Contraction and Relaxation . . . . . . . . . . . . . . . . . . . . . . . . 389
10.4 Nervous System Control of Muscle Tension . . . . . . . . . . . . . . . . . . . . . . . 397
10.5 Types of Muscle Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402
10.6 Exercise and Muscle Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 403
10.7 Cardiac Muscle Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
10.8 Smooth Muscle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
10.9 Development and Regeneration of Muscle Tissue . . . . . . . . . . . . . . . . . . . . 410


Chapter 11: The Muscular System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419
11.1 Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems 420
11.2 Naming Skeletal Muscles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
11.3 Axial Muscles of the Head, Neck, and Back . . . . . . . . . . . . . . . . . . . . . . . 427
11.4 Axial Muscles of the Abdominal Wall and Thorax . . . . . . . . . . . . . . . . . . . . 437
11.5 Muscles of the Pectoral Girdle and Upper Limbs . . . . . . . . . . . . . . . . . . . . 443
11.6 Appendicular Muscles of the Pelvic Girdle and Lower Limbs . . . . . . . . . . . . . . 452


Unit 3: Regulation, Integration, and Control
Chapter 12: The Nervous System and Nervous Tissue . . . . . . . . . . . . . . . . . . . . . 473


12.1 Basic Structure and Function of the Nervous System . . . . . . . . . . . . . . . . . . 474
12.2 Nervous Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481
12.3 The Function of Nervous Tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
12.4 The Action Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
12.5 Communication Between Neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499


Chapter 13: Anatomy of the Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . 517
13.1 The Embryologic Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518
13.2 The Central Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524
13.3 Circulation and the Central Nervous System . . . . . . . . . . . . . . . . . . . . . . 537
13.4 The Peripheral Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544


Chapter 14: The Somatic Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565
14.1 Sensory Perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566
14.2 Central Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586
14.3 Motor Responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 599


Chapter 15: The Autonomic Nervous System . . . . . . . . . . . . . . . . . . . . . . . . . . 617
15.1 Divisions of the Autonomic Nervous System . . . . . . . . . . . . . . . . . . . . . . 618
15.2 Autonomic Reflexes and Homeostasis . . . . . . . . . . . . . . . . . . . . . . . . . 627
15.3 Central Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635
15.4 Drugs that Affect the Autonomic System . . . . . . . . . . . . . . . . . . . . . . . . . 639


Chapter 16: The Neurological Exam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653
16.1 Overview of the Neurological Exam . . . . . . . . . . . . . . . . . . . . . . . . . . . 654
16.2 The Mental Status Exam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 658
16.3 The Cranial Nerve Exam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665
16.4 The Sensory and Motor Exams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673
16.5 The Coordination and Gait Exams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678


Chapter 17: The Endocrine System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 691
17.1 An Overview of the Endocrine System . . . . . . . . . . . . . . . . . . . . . . . . . . 692
17.2 Hormones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695
17.3 The Pituitary Gland and Hypothalamus . . . . . . . . . . . . . . . . . . . . . . . . . 703
17.4 The Thyroid Gland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711
17.5 The Parathyroid Glands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716
17.6 The Adrenal Glands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719
17.7 The Pineal Gland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722
17.8 Gonadal and Placental Hormones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723
17.9 The Endocrine Pancreas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724
17.10 Organs with Secondary Endocrine Functions . . . . . . . . . . . . . . . . . . . . . 729
17.11 Development and Aging of the Endocrine System . . . . . . . . . . . . . . . . . . . 731


Unit 4: Fluids and Transport
Chapter 18: The Cardiovascular System: Blood . . . . . . . . . . . . . . . . . . . . . . . . . 743


18.1 An Overview of Blood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744
18.2 Production of the Formed Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . 748


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18.3 Erythrocytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751
18.4 Leukocytes and Platelets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758
18.5 Hemostasis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763
18.6 Blood Typing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 768


Chapter 19: The Cardiovascular System: The Heart . . . . . . . . . . . . . . . . . . . . . . 783
19.1 Heart Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784
19.2 Cardiac Muscle and Electrical Activity . . . . . . . . . . . . . . . . . . . . . . . . . . 805
19.3 Cardiac Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818
19.4 Cardiac Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822
19.5 Development of the Heart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 832


Chapter 20: The Cardiovascular System: Blood Vessels and Circulation . . . . . . . . . . . 843
20.1 Structure and Function of Blood Vessels . . . . . . . . . . . . . . . . . . . . . . . . 844
20.2 Blood Flow, Blood Pressure, and Resistance . . . . . . . . . . . . . . . . . . . . . . 856
20.3 Capillary Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865
20.4 Homeostatic Regulation of the Vascular System . . . . . . . . . . . . . . . . . . . . 867
20.5 Circulatory Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 877
20.6 Development of Blood Vessels and Fetal Circulation . . . . . . . . . . . . . . . . . . 909


Chapter 21: The Lymphatic and Immune System . . . . . . . . . . . . . . . . . . . . . . . . 925
21.1 Anatomy of the Lymphatic and Immune Systems . . . . . . . . . . . . . . . . . . . . 926
21.2 Barrier Defenses and the Innate Immune Response . . . . . . . . . . . . . . . . . . 939
21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types . . . . . 945
21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies . . . . . . . . . . . 953
21.5 The Immune Response against Pathogens . . . . . . . . . . . . . . . . . . . . . . . 958
21.6 Diseases Associated with Depressed or Overactive Immune Responses . . . . . . . . 962
21.7 Transplantation and Cancer Immunology . . . . . . . . . . . . . . . . . . . . . . . . 966


Unit 5: Energy, Maintenance, and Environmental Exchange
Chapter 22: The Respiratory System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 981


22.1 Organs and Structures of the Respiratory System . . . . . . . . . . . . . . . . . . . . 982
22.2 The Lungs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993
22.3 The Process of Breathing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996
22.4 Gas Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005
22.5 Transport of Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011
22.6 Modifications in Respiratory Functions . . . . . . . . . . . . . . . . . . . . . . . . 1017
22.7 Embryonic Development of the Respiratory System . . . . . . . . . . . . . . . . . . 1018


Chapter 23: The Digestive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031
23.1 Overview of the Digestive System . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032
23.2 Digestive System Processes and Regulation . . . . . . . . . . . . . . . . . . . . . 1037
23.3 The Mouth, Pharynx, and Esophagus . . . . . . . . . . . . . . . . . . . . . . . . . 1041
23.4 The Stomach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051
23.5 The Small and Large Intestines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057
23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder . . . . . . . . 1067
23.7 Chemical Digestion and Absorption: A Closer Look . . . . . . . . . . . . . . . . . . 1071


Chapter 24: Metabolism and Nutrition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1091
24.1 Overview of Metabolic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . 1092
24.2 Carbohydrate Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1097
24.3 Lipid Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1109
24.4 Protein Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115
24.5 Metabolic States of the Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1120
24.6 Energy and Heat Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1124
24.7 Nutrition and Diet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1126


Chapter 25: The Urinary System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1141
25.1 Physical Characteristics of Urine . . . . . . . . . . . . . . . . . . . . . . . . . . . 1142
25.2 Gross Anatomy of Urine Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145
25.3 Gross Anatomy of the Kidney . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1149
25.4 Microscopic Anatomy of the Kidney . . . . . . . . . . . . . . . . . . . . . . . . . . 1154
25.5 Physiology of Urine Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158
25.6 Tubular Reabsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1161
25.7 Regulation of Renal Blood Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1170
25.8 Endocrine Regulation of Kidney Function . . . . . . . . . . . . . . . . . . . . . . . 1171
25.9 Regulation of Fluid Volume and Composition . . . . . . . . . . . . . . . . . . . . . 1173
25.10 The Urinary System and Homeostasis . . . . . . . . . . . . . . . . . . . . . . . . 1176


Chapter 26: Fluid, Electrolyte, and Acid-Base Balance . . . . . . . . . . . . . . . . . . . . 1187
26.1 Body Fluids and Fluid Compartments . . . . . . . . . . . . . . . . . . . . . . . . . 1188




26.2 Water Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1196
26.3 Electrolyte Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1199
26.4 Acid-Base Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1204
26.5 Disorders of Acid-Base Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209


Unit 6: Human Development and the Continuity of Life
Chapter 27: The Reproductive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1217


27.1 Anatomy and Physiology of the Male Reproductive System . . . . . . . . . . . . . . 1218
27.2 Anatomy and Physiology of the Female Reproductive System . . . . . . . . . . . . 1228
27.3 Development of the Male and Female Reproductive Systems . . . . . . . . . . . . 1244


Chapter 28: Development and Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255
28.1 Fertilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256
28.2 Embryonic Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1260
28.3 Fetal Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1271
28.4 Maternal Changes During Pregnancy, Labor, and Birth . . . . . . . . . . . . . . . . 1276
28.5 Adjustments of the Infant at Birth and Postnatal Stages . . . . . . . . . . . . . . . . 1282
28.6 Lactation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285
28.7 Patterns of Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1287


Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1327


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PREFACE
Welcome to Human Anatomy and Physiology, an OpenStax College resource. We created this textbook with several goals
in mind: accessibility, customization, and student engagement—helping students reach high levels of academic scholarship.
Instructors and students alike will find that this textbook offers a thorough introduction to the content in an accessible
format.
About OpenStax College
OpenStax College is a nonprofit organization committed to improving student access to quality learning materials. Our
free textbooks are developed and peer-reviewed by educators to ensure that they are readable, accurate, and organized in
accordance with the scope and sequence requirements of today’s college courses. Unlike traditional textbooks, OpenStax
College resources live online and are owned by the community of educators using them. Through partnerships with
companies and foundations committed to reducing costs for students, we are working to improve access to higher education
for all. OpenStax College is an initiative of Rice University and is made possible through the generous support of several
philanthropic foundations.
About OpenStax College’s Resources
OpenStax College resources provide quality academic instruction. Three key features set our materials apart from others:
1) They can be easily customized by instructors for each class, 2) they are “living” resources that grow online through
contributions from science educators, and 3) they are available for free or for a minimal cost.
Customization
OpenStax College learning resources are conceived and written with flexibility in mind so that they can be customized for
each course. Our textbooks provide a solid foundation on which instructors can build their own texts. Instructors can select
the sections that are most relevant to their curricula and create a textbook that speaks directly to the needs of their students.
Instructors are encouraged to expand on existing examples in the text by adding unique context via geographically localized
applications and topical connections.
Human Anatomy and Physiology can be easily customized using our online platform (https://openstaxcollege.org/textbooks/
anatomy-and-physiology/adapt). The text is arranged in a modular chapter format. Simply select the content most relevant
to your syllabus and create a textbook that addresses the needs of your class. This customization feature will ensure that
your textbook reflects the goals of your course.
Curation
To broaden access and encourage community curation, Human Anatomy and Physiology is “open source” under a Creative
Commons Attribution (CC BY) license. Members of the scientific community are invited to submit examples, emerging
research, and other feedback to enhance and strengthen the material, keeping it current and relevant for today’s students.
Submit your suggestions to info@openstaxcollege.org, and check in on edition status, alternate versions, errata, and news
on the StaxDash at http://openstaxcollege.org.
Cost
Our textbooks are available for free online, and in low-cost print and tablet editions.
About Human Anatomy and Physiology
Human Anatomy and Physiology is designed for the two-semester anatomy and physiology course taken by life science
and allied health students. It supports effective teaching and learning, and prepares students for further learning and future
careers. The text focuses on the most important concepts and aims to minimize distracting students with more minor details.
The development choices for this textbook were made with the guidance of hundreds of faculty who are deeply involved
in teaching this course. These choices led to innovations in art, terminology, career orientation, practical applications, and
multimedia-based learning, all with a goal of increasing relevance to students. We strove to make the discipline meaningful
and memorable to students, so that they can draw from it a working knowledge that will enrich their future studies.
Coverage and Scope
The units of ourHuman Anatomy and Physiology textbook adhere to the scope and sequence followed by most two-semester
courses nationwide.
Unit 1: Levels of Organization
Chapters 1–4 provide students with a basic understanding of human anatomy and physiology, including its language, the
levels of organization, and the basics of chemistry and cell biology. These chapters provide a foundation for the further study


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of the body. They also focus particularly on how the body’s regions, important chemicals, and cells maintain homeostasis.
Chapter 1 An Introduction to the Human Body
Chapter 2 The Chemical Level of Organization
Chapter 3 The Cellular Level of Organization
Chapter 4 The Tissue Level of Organization
Unit 2: Support and Movement
In Chapters 5–11, students explore the skin, the largest organ of the body, and examine the body’s skeletal and muscular
systems, following a traditional sequence of topics. This unit is the first to walk students through specific systems of the
body, and as it does so, it maintains a focus on homeostasis as well as those diseases and conditions that can disrupt it.
Chapter 5 The Integumentary System
Chapter 6 Bone and Skeletal Tissue
Chapter 7 The Axial Skeleton
Chapter 8 The Appendicular Skeleton
Chapter 9 Joints
Chapter 10 Muscle Tissue
Chapter 11 The Muscular System
Unit 3: Regulation, Integration, and Control
Chapters 12–17 help students answer questions about nervous and endocrine system control and regulation. In a break with
the traditional sequence of topics, the special senses are integrated into the chapter on the somatic nervous system. The
chapter on the neurological examination offers students a unique approach to understanding nervous system function using
five simple but powerful diagnostic tests.
Chapter 12 Introduction to the Nervous System
Chapter 13 The Anatomy of the Nervous System
Chapter 14 The Somatic Nervous System
Chapter 15 The Autonomic Nervous System
Chapter 16 The Neurological Exam
Chapter 17 The Endocrine System
Unit 4: Fluids and Transport
In Chapters 18–21, students examine the principal means of transport for materials needed to support the human body,
regulate its internal environment, and provide protection.
Chapter 18 Blood
Chapter 19 The Cardiovascular System: The Heart
Chapter 20 The Cardiovascular System: Blood Vessels and Circulation
Chapter 21 The Lymphatic System and Immunity
Unit 5: Energy, Maintenance, and Environmental Exchange
In Chapters 22–26, students discover the interaction between body systems and the outside environment for the exchange
of materials, the capture of energy, the release of waste, and the overall maintenance of the internal systems that regulate
the exchange. The explanations and illustrations are particularly focused on how structure relates to function.
Chapter 22 The Respiratory System
Chapter 23 The Digestive System
Chapter 24 Nutrition and Metabolism
Chapter 25 The Urinary System
Chapter 26 Fluid, Electrolyte, and Acid–Base Balance
Unit 6: Human Development and the Continuity of Life
The closing chapters examine the male and female reproductive systems, describe the process of human development and
the different stages of pregnancy, and end with a review of the mechanisms of inheritance.
Chapter 27 The Reproductive System
Chapter 28 Development and Genetic Inheritance
Pedagogical Foundation and Features
Human Anatomy and Physiology is designed to promote scientific literacy. Throughout the text, you will find features that
engage the students by taking selected topics a step further.


Homeostatic Imbalances discusses the effects and results of imbalances in the body.
Disorders showcases a disorder that is relevant to the body system at hand. This feature may focus on a specific
disorder, or a set of related disorders.
Diseases showcases a disease that is relevant to the body system at hand.


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Aging explores the effect aging has on a body’s system and specific disorders that manifest over time.
Career Connections presents information on the various careers often pursued by allied health students, such as
medical technician, medical examiner, and neurophysiologist. Students are introduced to the educational requirements
for and day-to-day responsibilities in these careers.
Everyday Connections tie anatomical and physiological concepts to emerging issues and discuss these in terms of
everyday life. Topics include “Anabolic Steroids” and “The Effect of Second-Hand Tobacco Smoke.”
Interactive Links direct students to online exercises, simulations, animations, and videos to add a fuller context
to core content and help improve understanding of the material. Many features include links to the University of
Michigan’s interactive WebScopes, which allow students to zoom in on micrographs in the collection. These resources
were vetted by reviewers and other subject matter experts to ensure that they are effective and accurate. We strongly
urge students to explore these links, whether viewing a video or inputting data into a simulation, to gain the fullest
experience and to learn how to search for information independently.


Dynamic, Learner-Centered Art
Our unique approach to visuals is designed to emphasize only the components most important in any given illustration.
The art style is particularly aimed at focusing student learning through a powerful blend of traditional depictions and
instructional innovations.
Much of the art in this book consists of black line illustrations. The strongest line is used to highlight the most important
structures, and shading is used to show dimension and shape. Color is used sparingly to highlight and clarify the primary
anatomical or functional point of the illustration. This technique is intended to draw students’ attention to the critical
learning point in the illustration, without distraction from excessive gradients, shadows, and highlights. Full color is used
when the structure or process requires it (for example, muscle diagrams and cardiovascular system illustrations).


By highlighting the most important portions of the illustration, the artwork helps students focus on the most important
points, without overwhelming them.


Micrographs
Micrograph magnifications have been calculated based on the objective provided with the image. If a micrograph was
recorded at 40×, and the image was magnified an additional 2×, we calculated the final magnification of the micrograph to
be 80×.
Please note that, when viewing the textbook electronically, the micrograph magnification provided in the text does not take
into account the size and magnification of the screen on your electronic device. There may be some variation.


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These glands secrete oils that lubricate and protect the skin. LM × 400. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)


Learning Resources
The following resources are (or will be) available in addition to main text:


PowerPoint slides: For each chapter, the illustrations are presented, one per slide, with their respective captions.
Pronunciation guide: A subset of the text’s key terms are presented with easy-to-follow phonetic transcriptions. For
example, blastocyst is rendered as “blas'to-sist”


About Our Team
Senior Contributors


J. Gordon Betts Tyler Junior College
Peter Desaix University of North Carolina at Chapel Hill
Eddie Johnson Central Oregon Community College
Jody E. Johnson Arapahoe Community College
Oksana Korol Aims Community College
Dean Kruse Portland Community College
Brandon Poe Springfield Technical Community College
James A. Wise Hampton University
Mark Womble Youngstown State University
Kelly A. Young California State University, Long Beach


Advisor
Robin J. Heyden
Other Contributors


Kim Aaronson Aquarius Institute; Triton College
Lopamudra Agarwal Augusta Technical College
Gary Allen Dalhousie University
Robert Allison McLennan Community College
Heather Armbruster Southern Union State Community College


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Timothy Ballard University of North Carolina Wilmington
Matthew Barlow Eastern New Mexico University
William Blaker Furman University
Julie Bowers East Tennessee State University
Emily Bradshaw Florida Southern College
Nishi Bryska University of North Carolina, Charlotte
Susan Caley Opsal Illinois Valley Community College
Boyd Campbell Southwest College of Naturopathic Medicine and Health Sciences
Ann Caplea Walsh University
Marnie Chapman University of Alaska, Sitka
Barbara Christie-Pope Cornell College
Kenneth Crane Texarkana College
Maurice Culver Florida State College at Jacksonville
Heather Cushman Tacoma Community College
Noelle Cutter Molloy College
Lynnette Danzl-Tauer Rock Valley College
Jane Davis Aurora University
AnnMarie DelliPizzi Dominican College
Susan Dentel Washtenaw Community College
Pamela Dobbins Shelton State Community College
Patty Dolan Pacific Lutheran University
Sondra Dubowsky McLennan Community College
Peter Dukehart Three Rivers Community College
Ellen DuPré Central College
Elizabeth DuPriest Warner Pacific College
Pam Elf University of Minnesota
Sharon Ellerton Queensborough Community College
Carla Endres Utah State University - College of Eastern Utah: San Juan Campus
Myriam Feldman Lake Washington Institute of Technology; Cascadia Community College
Greg Fitch Avila University
Lynn Gargan Tarant County College
Michael Giangrande Oakland Community College
Chaya Gopalan St. Louis College of Pharmacy
Victor Greco Chattahoochee Technical College
Susanna Heinze Skagit Valley College
Ann Henninger Wartburg College
Dale Horeth Tidewater Community College
Michael Hortsch University of Michigan
Rosemary Hubbard Marymount University
Mark Hubley Prince George's Community College
Branko Jablanovic College of Lake County
Norman Johnson University of Massachusetts Amherst
Mark Jonasson North Arkansas College
Jeff Keyte College of Saint Mary


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William Kleinelp Middlesex County College
Leigh Kleinert Grand Rapids Community College
Brenda Leady University of Toledo
John Lepri University of North Carolina, Greensboro
Sarah Leupen University of Maryland, Baltimore County
Lihua Liang Johns Hopkins University
Robert Mallet University of North Texas Health Science Center
Bruce Maring Daytona State College
Elisabeth Martin College of Lake County
Natalie Maxwell Carl Albert State College, Sallisaw
Julie May William Carey University
Debra McLaughlin University of Maryland University College
Nicholas Mitchell St. Bonaventure University
Shobhana Natarajan Brookhaven College
Phillip Nicotera St. Petersburg College
Mary Jane Niles University of San Francisco
Ikemefuna Nwosu Parkland College; Lake Land College
Betsy Ott Tyler Junior College
Ivan Paul John Wood Community College
Aaron Payette College of Southern Nevada
Scott Payne Kentucky Wesleyan College
Cameron Perkins South Georgia College
David Pfeiffer University of Alaska, Anchorage
Thomas Pilat Illinois Central College
Eileen Preston Tarrant County College
Mike Pyle Olivet Nazarene University
Robert Rawding Gannon University
Jason Schreer State University of New York at Potsdam
Laird Sheldahl Mt. Hood Community College
Brian Shmaefsky Lone Star College System
Douglas Sizemore Bevill State Community College
Susan Spencer Mount Hood Community College
Cynthia Standley University of Arizona
Robert Sullivan Marist College
Eric Sun Middle Georgia State College
Tom Swenson Ithaca College
Kathleen Tallman Azusa Pacific University
Rohinton Tarapore University of Pennsylvania
Elizabeth Tattersall Western Nevada College
Mark Thomas University of Northern Colorado
Janis Thompson Lorain County Community College
Rita Thrasher Pensacola State College
David Van Wylen St. Olaf College
Lynn Wandrey Mott Community College


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Margaret Weck St. Louis College of Pharmacy
Kathleen Weiss George Fox University
Neil Westergaard Williston State College
David Wortham West Georgia Technical College
Umesh Yadav University of Texas Medical Branch
Tony Yates Oklahoma Baptist University
Justin York Glendale Community College
Cheri Zao North Idaho College
Elena Zoubina Bridgewater State University; Massasoit Community College
Shobhana Natarajan Alcon Laboratories, Inc.


Special Thanks
OpenStax College wishes to thank the Regents of University of Michigan Medical School for the use of their extensive
micrograph collection. Many of the UM micrographs that appear in Human Anatomy and Physiology are interactive
WebScopes, which students can explore by zooming in and out.
We also wish to thank the Open Learning Initiative at Carnegie Mellon University, with whom we shared and exchanged
resources during the development of Human Anatomy and Physiology.


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1 | AN INTRODUCTION TO
THE HUMAN BODY


Figure 1.1 Blood Pressure A proficiency in anatomy and physiology is fundamental to any career in the health
professions. (credit: Bryan Mason/flickr)


Introduction
Chapter Objectives


After studying this chapter, you will be able to:
• Distinguish between anatomy and physiology, and identify several branches of each
• Describe the structure of the body, from simplest to most complex, in terms of the six levels of organization
• Identify the functional characteristics of human life
• Identify the four requirements for human survival
• Define homeostasis and explain its importance to normal human functioning
• Use appropriate anatomical terminology to identify key body structures, body regions, and directions in the
body


• Compare and contrast at least four medical imagining techniques in terms of their function and use in
medicine


Though you may approach a course in anatomy and physiology strictly as a requirement for your field of study, the
knowledge you gain in this course will serve you well in many aspects of your life. An understanding of anatomy and
physiology is not only fundamental to any career in the health professions, but it can also benefit your own health.


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 9




Familiarity with the human body can help you make healthful choices and prompt you to take appropriate action when signs
of illness arise. Your knowledge in this field will help you understand news about nutrition, medications, medical devices,
and procedures and help you understand genetic or infectious diseases. At some point, everyone will have a problem with
some aspect of his or her body and your knowledge can help you to be a better parent, spouse, partner, friend, colleague, or
caregiver.
This chapter begins with an overview of anatomy and physiology and a preview of the body regions and functions. It then
covers the characteristics of life and how the body works to maintain stable conditions. It introduces a set of standard
terms for body structures and for planes and positions in the body that will serve as a foundation for more comprehensive
information covered later in the text. It ends with examples of medical imaging used to see inside the living body.


1.1 | Overview of Anatomy and Physiology
By the end of this section, you will be able to:
• Compare and contrast anatomy and physiology, including their specializations and methods of study
• Discuss the fundamental relationship between anatomy and physiology


Human anatomy is the scientific study of the body’s structures. Some of these structures are very small and can only
be observed and analyzed with the assistance of a microscope. Other larger structures can readily be seen, manipulated,
measured, and weighed. The word “anatomy” comes from a Greek root that means “to cut apart.” Human anatomy was
first studied by observing the exterior of the body and observing the wounds of soldiers and other injuries. Later, physicians
were allowed to dissect bodies of the dead to augment their knowledge. When a body is dissected, its structures are cut apart
in order to observe their physical attributes and their relationships to one another. Dissection is still used in medical schools,
anatomy courses, and in pathology labs. In order to observe structures in living people, however, a number of imaging
techniques have been developed. These techniques allow clinicians to visualize structures inside the living body such as a
cancerous tumor or a fractured bone.
Like most scientific disciplines, anatomy has areas of specialization. Gross anatomy is the study of the larger structures of
the body, those visible without the aid of magnification (Figure 1.2a). Macro- means “large,” thus, gross anatomy is also
referred to as macroscopic anatomy. In contrast, micro- means “small,” andmicroscopic anatomy is the study of structures
that can be observed only with the use of a microscope or other magnification devices (Figure 1.2b). Microscopic anatomy
includes cytology, the study of cells and histology, the study of tissues. As the technology of microscopes has advanced,
anatomists have been able to observe smaller and smaller structures of the body, from slices of large structures like the heart,
to the three-dimensional structures of large molecules in the body.


Figure 1.2 Gross and Microscopic Anatomy (a) Gross anatomy considers large structures such as the brain. (b)
Microscopic anatomy can deal with the same structures, though at a different scale. This is a micrograph of nerve
cells from the brain. LM × 1600. (credit a: “WriterHound”/Wikimedia Commons; credit b: Micrograph provided by the
Regents of University of Michigan Medical School © 2012)


Anatomists take two general approaches to the study of the body’s structures: regional and systemic. Regional anatomy is
the study of the interrelationships of all of the structures in a specific body region, such as the abdomen. Studying regional
anatomy helps us appreciate the interrelationships of body structures, such as how muscles, nerves, blood vessels, and other
structures work together to serve a particular body region. In contrast, systemic anatomy is the study of the structures that
make up a discrete body system—that is, a group of structures that work together to perform a unique body function. For
example, a systemic anatomical study of the muscular system would consider all of the skeletal muscles of the body.


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Whereas anatomy is about structure, physiology is about function. Human physiology is the scientific study of the chemistry
and physics of the structures of the body and the ways in which they work together to support the functions of life. Much
of the study of physiology centers on the body’s tendency toward homeostasis. Homeostasis is the state of steady internal
conditions maintained by living things. The study of physiology certainly includes observation, both with the naked eye and
with microscopes, as well as manipulations and measurements. However, current advances in physiology usually depend
on carefully designed laboratory experiments that reveal the functions of the many structures and chemical compounds that
make up the human body.
Like anatomists, physiologists typically specialize in a particular branch of physiology. For example, neurophysiology is
the study of the brain, spinal cord, and nerves and how these work together to perform functions as complex and diverse as
vision, movement, and thinking. Physiologists may work from the organ level (exploring, for example, what different parts
of the brain do) to the molecular level (such as exploring how an electrochemical signal travels along nerves).
Form is closely related to function in all living things. For example, the thin flap of your eyelid can snap down to clear away
dust particles and almost instantaneously slide back up to allow you to see again. At the microscopic level, the arrangement
and function of the nerves and muscles that serve the eyelid allow for its quick action and retreat. At a smaller level of
analysis, the function of these nerves and muscles likewise relies on the interactions of specific molecules and ions. Even
the three-dimensional structure of certain molecules is essential to their function.
Your study of anatomy and physiology will make more sense if you continually relate the form of the structures you are
studying to their function. In fact, it can be somewhat frustrating to attempt to study anatomy without an understanding
of the physiology that a body structure supports. Imagine, for example, trying to appreciate the unique arrangement of the
bones of the human hand if you had no conception of the function of the hand. Fortunately, your understanding of how
the human hand manipulates tools—from pens to cell phones—helps you appreciate the unique alignment of the thumb
in opposition to the four fingers, making your hand a structure that allows you to pinch and grasp objects and type text
messages.


1.2 | Structural Organization of the Human Body
By the end of this section, you will be able to:
• Describe the structure of the human body in terms of six levels of organization
• List the eleven organ systems of the human body and identify at least one organ and one major function of each


Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic
architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of
the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules,
organelles, cells, tissues, organs, organ systems, organisms and biosphere (Figure 1.3).


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 11




Figure 1.3 Levels of Structural Organization of the Human Body The organization of the body often is discussed
in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human
organism.


The Levels of Organization
To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles,
atoms and molecules. All matter in the universe is composed of one or more unique pure substances called elements,
familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these
pure substances (elements) is an atom. Atoms are made up of subatomic particles such as the proton, electron and neutron.
Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things.
Molecules are the chemical building blocks of all body structures.
A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small,
independently-living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human
anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells.
A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with
a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life. A


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tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform
a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each
organ performs one or more specific physiological functions. An organ system is a group of organs that work together to
perform major functions or meet physiological needs of the body.
This book covers eleven distinct organ systems in the human body (Figure 1.4 and Figure 1.5). Assigning organs to organ
systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In
fact, most organs contribute to more than one system.


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 13




Figure 1.4 Organ Systems of the Human Body Organs that work together are grouped into organ systems.


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Figure 1.5 Organ Systems of the Human Body (continued) Organs that work together are grouped into organ
systems.


The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that
can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all
cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 15




1.3 | Functions of Human Life
By the end of this section, you will be able to:
• Explain the importance of organization to the function of the human organism
• Distinguish between metabolism, anabolism, and catabolism
• Provide at least two examples of human responsiveness and human movement
• Compare and contrast growth, differentiation, and reproduction


The different organ systems each have different functions and therefore unique roles to perform in physiology. These many
functions can be summarized in terms of a few that we might consider definitive of human life: organization, metabolism,
responsiveness, movement, development, and reproduction.


Organization
A human body consists of trillions of cells organized in a way that maintains distinct internal compartments. These
compartments keep body cells separated from external environmental threats and keep the cells moist and nourished. They
also separate internal body fluids from the countless microorganisms that grow on body surfaces, including the lining of
certain tracts, or passageways. The intestinal tract, for example, is home to even more bacteria cells than the total of all
human cells in the body, yet these bacteria are outside the body and cannot be allowed to circulate freely inside the body.
Cells, for example, have a cell membrane (also referred to as the plasma membrane) that keeps the intracellular
environment—the fluids and organelles—separate from the extracellular environment. Blood vessels keep blood inside
a closed circulatory system, and nerves and muscles are wrapped in connective tissue sheaths that separate them from
surrounding structures. In the chest and abdomen, a variety of internal membranes keep major organs such as the lungs,
heart, and kidneys separate from others.
The body’s largest organ system is the integumentary system, which includes the skin and its associated structures, such
as hair and nails. The surface tissue of skin is a barrier that protects internal structures and fluids from potentially harmful
microorganisms and other toxins.


Metabolism
The first law of thermodynamics holds that energy can neither be created nor destroyed—it can only change form. Your
basic function as an organism is to consume (ingest) energy and molecules in the foods you eat, convert some of it into fuel
for movement, sustain your body functions, and build and maintain your body structures. There are two types of reactions
that accomplish this: anabolism and catabolism.
• Anabolism is the process whereby smaller, simpler molecules are combined into larger, more complex substances.
Your body can assemble, by utilizing energy, the complex chemicals it needs by combining small molecules derived
from the foods you eat


• Catabolism is the process by which larger more complex substances are broken down into smaller simpler molecules.
Catabolism releases energy. The complex molecules found in foods are broken down so the body can use their parts to
assemble the structures and substances needed for life.


Taken together, these two processes are called metabolism. Metabolism is the sum of all anabolic and catabolic reactions
that take place in the body (Figure 1.6). Both anabolism and catabolism occur simultaneously and continuously to keep you
alive.


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Figure 1.6 Metabolism Anabolic reactions are building reactions, and they consume energy. Catabolic reactions
break materials down and release energy. Metabolism includes both anabolic and catabolic reactions.


Every cell in your body makes use of a chemical compound, adenosine triphosphate (ATP), to store and release energy.
The cell stores energy in the synthesis (anabolism) of ATP, then moves the ATP molecules to the location where energy is
needed to fuel cellular activities. Then the ATP is broken down (catabolism) and a controlled amount of energy is released,
which is used by the cell to perform a particular job.


View this animation (http://openstaxcollege.org/l/metabolic) to learn more about metabolic processes. What kind of
catabolism occurs in the heart?


Responsiveness
Responsiveness is the ability of an organism to adjust to changes in its internal and external environments. An example
of responsiveness to external stimuli could include moving toward sources of food and water and away from perceived
dangers. Changes in an organism’s internal environment, such as increased body temperature, can cause the responses of
sweating and the dilation of blood vessels in the skin in order to decrease body temperature, as shown by the runners in
Figure 1.7.


Movement
Human movement includes not only actions at the joints of the body, but also the motion of individual organs and even
individual cells. As you read these words, red and white blood cells are moving throughout your body, muscle cells are
contracting and relaxing to maintain your posture and to focus your vision, and glands are secreting chemicals to regulate
body functions. Your body is coordinating the action of entire muscle groups to enable you to move air into and out of your
lungs, to push blood throughout your body, and to propel the food you have eaten through your digestive tract. Consciously,
of course, you contract your skeletal muscles to move the bones of your skeleton to get from one place to another (as the
runners are doing in Figure 1.7), and to carry out all of the activities of your daily life.


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Figure 1.7 Marathon Runners Runners demonstrate two characteristics of living humans—responsiveness and
movement. Anatomic structures and physiological processes allow runners to coordinate the action of muscle groups
and sweat in response to rising internal body temperature. (credit: Phil Roeder/flickr)


Development, growth and reproduction
Development is all of the changes the body goes through in life. Development includes the processes of differentiation,
growth, and renewal.
Growth is the increase in body size. Humans, like all multicellular organisms, grow by increasing the number of existing
cells, increasing the amount of non-cellular material around cells (such as mineral deposits in bone), and, within very narrow
limits, increasing the size of existing cells.
Reproduction is the formation of a new organism from parent organisms. In humans, reproduction is carried out by the
male and female reproductive systems. Because death will come to all complex organisms, without reproduction, the line
of organisms would end.


1.4 | Requirements for Human Life
By the end of this section, you will be able to:
• Discuss the role of oxygen and nutrients in maintaining human survival
• Explain why extreme heat and extreme cold threaten human survival
• Explain how the pressure exerted by gases and fluids influences human survival


Humans have been adapting to life on Earth for at least the past 200,000 years. Earth and its atmosphere have provided
us with air to breathe, water to drink, and food to eat, but these are not the only requirements for survival. Although you
may rarely think about it, you also cannot live outside of a certain range of temperature and pressure that the surface of our
planet and its atmosphere provides. The next sections explore these four requirements of life.


Oxygen
Atmospheric air is only about 20 percent oxygen, but that oxygen is a key component of the chemical reactions that keep
the body alive, including the reactions that produce ATP. Brain cells are especially sensitive to lack of oxygen because of
their requirement for a high-and-steady production of ATP. Brain damage is likely within five minutes without oxygen, and
death is likely within ten minutes.


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Nutrients
A nutrient is a substance in foods and beverages that is essential to human survival. The three basic classes of nutrients are
water, the energy-yielding and body-building nutrients, and the micronutrients (vitamins and minerals).
The most critical nutrient is water. Depending on the environmental temperature and our state of health, we may be able to
survive for only a few days without water. The body’s functional chemicals are dissolved and transported in water, and the
chemical reactions of life take place in water. Moreover, water is the largest component of cells, blood, and the fluid between
cells, and water makes up about 70 percent of an adult’s body mass. Water also helps regulate our internal temperature and
cushions, protects, and lubricates joints and many other body structures.
The energy-yielding nutrients are primarily carbohydrates and lipids, while proteins mainly supply the amino acids that are
the building blocks of the body itself. You ingest these in plant and animal foods and beverages, and the digestive system
breaks them down into molecules small enough to be absorbed. The breakdown products of carbohydrates and lipids can
then be used in the metabolic processes that convert them to ATP. Although you might feel as if you are starving after
missing a single meal, you can survive without consuming the energy-yielding nutrients for at least several weeks.
Water and the energy-yielding nutrients are also referred to as macronutrients because the body needs them in large
amounts. In contrast, micronutrients are vitamins and minerals. These elements and compounds participate in many
essential chemical reactions and processes, such nerve impulses, and some, such as calcium, also contribute to the body’s
structure. Your body can store some of the micronutrients in its tissues, and draw on those reserves if you fail to consume
them in your diet for a few days or weeks. Some others micronutrients, such as vitamin C and most of the B vitamins, are
water-soluble and cannot be stored, so you need to consume them every day or two.


Narrow Range of Temperature
You have probably seen news stories about athletes who died of heat stroke, or hikers who died of exposure to cold. Such
deaths occur because the chemical reactions upon which the body depends can only take place within a narrow range of
body temperature, from just below to just above 37°C (98.6°F). When body temperature rises well above or drops well
below normal, certain proteins (enzymes) that facilitate chemical reactions lose their normal structure and their ability to
function and the chemical reactions of metabolism cannot proceed.
That said, the body can respond effectively to short-term exposure to heat (Figure 1.8) or cold. One of the body’s responses
to heat is, of course, sweating. As sweat evaporates from skin, it removes some thermal energy from the body, cooling it.
Adequate water (from the extracellular fluid in the body) is necessary to produce sweat, so adequate fluid intake is essential
to balance that loss during the sweat response. Not surprisingly, the sweat response is much less effective in a humid
environment because the air is already saturated with water. Thus, the sweat on the skin’s surface is not able to evaporate,
and internal body temperature can get dangerously high.


Figure 1.8 Extreme Heat Humans adapt to some degree to repeated exposure to high temperatures. (credit: McKay
Savage/flickr)


The body can also respond effectively to short-term exposure to cold. One response to cold is shivering, which is random
muscle movement that generates heat. Another response is increased breakdown of stored energy to generate heat. When
that energy reserve is depleted, however, and the core temperature begins to drop significantly, red blood cells will lose
their ability to give up oxygen, denying the brain of this critical component of ATP production. This lack of oxygen can


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cause confusion, lethargy, and eventually loss of consciousness and death. The body responds to cold by reducing blood
circulation to the extremities, the hands and feet, in order to prevent blood from cooling there and so that the body’s core can
stay warm. Even when core body temperature remains stable, however, tissues exposed to severe cold, especially the fingers
and toes, can develop frostbite when blood flow to the extremities has been much reduced. This form of tissue damage can
be permanent and lead to gangrene, requiring amputation of the affected region.


Controlled Hypothermia
As you have learned, the body continuously engages in coordinated physiological processes to maintain a stable
temperature. In some cases, however, overriding this system can be useful, or even life-saving. Hypothermia is the
clinical term for an abnormally low body temperature (hypo- = “below” or “under”). Controlled hypothermia is
clinically induced hypothermia performed in order to reduce the metabolic rate of an organ or of a person’s entire body.
Controlled hypothermia often is used, for example, during open-heart surgery because it decreases the metabolic needs
of the brain, heart, and other organs, reducing the risk of damage to them. When controlled hypothermia is used
clinically, the patient is given medication to prevent shivering. The body is then cooled to 25–32°C (79–89°F). The
heart is stopped and an external heart-lung pump maintains circulation to the patient’s body. The heart is cooled further
and is maintained at a temperature below 15°C (60°F) for the duration of the surgery. This very cold temperature helps
the heart muscle to tolerate its lack of blood supply during the surgery.
Some emergency department physicians use controlled hypothermia to reduce damage to the heart in patients who
have suffered a cardiac arrest. In the emergency department, the physician induces coma and lowers the patient’s
body temperature to approximately 91 degrees. This condition, which is maintained for 24 hours, slows the patient’s
metabolic rate. Because the patient’s organs require less blood to function, the heart’s workload is reduced.


Narrow Range of Atmospheric Pressure
Pressure is a force exerted by a substance that is in contact with another substance. Atmospheric pressure is pressure
exerted by the mixture of gases (primarily nitrogen and oxygen) in the Earth’s atmosphere. Although you may not perceive
it, atmospheric pressure is constantly pressing down on your body. This pressure keeps gases within your body, such as the
gaseous nitrogen in body fluids, dissolved. If you were suddenly ejected from a space ship above Earth’s atmosphere, you
would go from a situation of normal pressure to one of very low pressure. The pressure of the nitrogen gas in your blood
would be much higher than the pressure of nitrogen in the space surrounding your body. As a result, the nitrogen gas in your
blood would expand, forming bubbles that could block blood vessels and even cause cells to break apart.
Atmospheric pressure does more than just keep blood gases dissolved. Your ability to breathe—that is, to take in oxygen
and release carbon dioxide—also depends upon a precise atmospheric pressure. Altitude sickness occurs in part because
the atmosphere at high altitudes exerts less pressure, reducing the exchange of these gases, and causing shortness of breath,
confusion, headache, lethargy, and nausea. Mountain climbers carry oxygen to reduce the effects of both low oxygen levels
and low barometric pressure at higher altitudes (Figure 1.9).


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Figure 1.9 Harsh Conditions Climbers on Mount Everest must accommodate extreme cold, low oxygen levels, and
low barometric pressure in an environment hostile to human life. (credit: Melanie Ko/flickr)


Decompression Sickness
Decompression sickness (DCS) is a condition in which gases dissolved in the blood or in other body tissues are no
longer dissolved following a reduction in pressure on the body. This condition affects underwater divers who surface
from a deep dive too quickly, and it can affect pilots flying at high altitudes in planes with unpressurized cabins. Divers
often call this condition “the bends,” a reference to joint pain that is a symptom of DCS.
In all cases, DCS is brought about by a reduction in barometric pressure. At high altitude, barometric pressure is much
less than on Earth’s surface because pressure is produced by the weight of the column of air above the body pressing
down on the body. The very great pressures on divers in deep water are likewise from the weight of a column of water
pressing down on the body. For divers, DCS occurs at normal barometric pressure (at sea level), but it is brought on by
the relatively rapid decrease of pressure as divers rise from the high pressure conditions of deep water to the now low,
by comparison, pressure at sea level. Not surprisingly, diving in deep mountain lakes, where barometric pressure at the
surface of the lake is less than that at sea level is more likely to result in DCS than diving in water at sea level.
In DCS, gases dissolved in the blood (primarily nitrogen) come rapidly out of solution, forming bubbles in the blood
and in other body tissues. This occurs because when pressure of a gas over a liquid is decreased, the amount of gas
that can remain dissolved in the liquid also is decreased. It is air pressure that keeps your normal blood gases dissolved
in the blood. When pressure is reduced, less gas remains dissolved. You have seen this in effect when you open a
carbonated drink. Removing the seal of the bottle reduces the pressure of the gas over the liquid. This in turn causes
bubbles as dissolved gases (in this case, carbon dioxide) come out of solution in the liquid.
The most common symptoms of DCS are pain in the joints, with headache and disturbances of vision occurring in
10 percent to 15 percent of cases. Left untreated, very severe DCS can result in death. Immediate treatment is with
pure oxygen. The affected person is then moved into a hyperbaric chamber. A hyperbaric chamber is a reinforced,
closed chamber that is pressurized to greater than atmospheric pressure. It treats DCS by repressurizing the body so
that pressure can then be removed much more gradually. Because the hyperbaric chamber introduces oxygen to the
body at high pressure, it increases the concentration of oxygen in the blood. This has the effect of replacing some of
the nitrogen in the blood with oxygen, which is easier to tolerate out of solution.


The dynamic pressure of body fluids is also important to human survival. For example, blood pressure, which is the pressure
exerted by blood as it flows within blood vessels, must be great enough to enable blood to reach all body tissues, and yet
low enough to ensure that the delicate blood vessels can withstand the friction and force of the pulsating flow of pressurized
blood.


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1.5 | Homeostasis
By the end of this section, you will be able to:
• Discuss the role of homeostasis in healthy functioning
• Contrast negative and positive feedback, giving one physiologic example of each mechanism


Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to
blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A set point is the
physiological value around which the normal range fluctuates. A normal range is the restricted set of values that is
optimally healthful and stable. For example, the set point for normal human body temperature is approximately 37°C
(98.6°F) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range
a few degrees above and below that point. Control centers in the brain and other parts of the body monitor and react to
deviations from homeostasis using negative feedback. Negative feedback is a mechanism that reverses a deviation from
the set point. Therefore, negative feedback maintains body parameters within their normal range. The maintenance of
homeostasis by negative feedback goes on throughout the body at all times, and an understanding of negative feedback is
thus fundamental to an understanding of human physiology.


Negative Feedback
A negative feedback system has three basic components (Figure 1.10a). A sensor, also referred to a receptor, is a
component of a feedback system that monitors a physiological value. This value is reported to the control center. The
control center is the component in a feedback system that compares the value to the normal range. If the value deviates too
much from the set point, then the control center activates an effector. An effector is the component in a feedback system
that causes a change to reverse the situation and return the value to the normal range.


Figure 1.10 Negative Feedback Loop In a negative feedback loop, a stimulus—a deviation from a set point—is
resisted through a physiological process that returns the body to homeostasis. (a) A negative feedback loop has four
basic parts. (b) Body temperature is regulated by negative feedback.


In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is,
beyond homeostasis). This stimulus is “heard” by a specific sensor. For example, in the control of blood glucose, specific
endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. These pancreatic beta cells respond
to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. The insulin signals skeletal
muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. As
glucose concentration in the bloodstream drops, the decrease in concentration—the actual negative feedback—is detected
by pancreatic alpha cells, and insulin release stops. This prevents blood sugar levels from continuing to drop below the
normal range.
Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain
(Figure 1.10b). When the brain’s temperature regulation center receives data from the sensors indicating that the body’s
temperature exceeds its normal range, it stimulates a cluster of brain cells referred to as the “heat-loss center.” This
stimulation has three major effects:


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• Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the surface of the skin
allowing the heat to radiate into the environment.


• As blood flow to the skin increases, sweat glands are activated to increase their output. As the sweat evaporates from
the skin surface into the surrounding air, it takes heat with it.


• The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal
passageways. This further increases heat loss from the lungs.


In contrast, activation of the brain’s heat-gain center by exposure to cold reduces blood flow to the skin, and blood returning
from the limbs is diverted into a network of deep veins. This arrangement traps heat closer to the body core and restricts heat
loss. If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and
producing shivering. The muscle contractions of shivering release heat while using up ATP. The brain triggers the thyroid
gland in the endocrine system to release thyroid hormone, which increases metabolic activity and heat production in cells
throughout the body. The brain also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes
the breakdown of glycogen into glucose, which can be used as an energy source. The breakdown of glycogen into glucose
also results in increased metabolism and heat production.


Water concentration in the body is critical for proper functioning. A person’s body retains very tight control on water
levels without conscious control by the person. Watch this video (http://openstaxcollege.org/l/H2Ocon) to learn more
about water concentration in the body. Which organ has primary control over the amount of water in the body?


Positive Feedback
Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the
normal range results in more change, and the system moves farther away from the normal range. Positive feedback in the
body is normal only when there is a definite end point. Childbirth and the body’s response to blood loss are two examples
of positive feedback loops that are normal but are activated only when needed.
Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired.
Enormous changes in the mother’s body are required to expel the baby at the end of pregnancy. And the events of childbirth,
once begun, must progress rapidly to a conclusion or the life of the mother and the baby are at risk. The extreme muscular
work of labor and delivery are the result of a positive feedback system (Figure 1.11).


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Figure 1.11 Positive Feedback Loop Normal childbirth is driven by a positive feedback loop. A positive feedback
loop results in a change in the body’s status, rather than a return to homeostasis.


The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix
contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). These nerve cells send messages
to the brain, which in turn causes the pituitary gland at the base of the brain to release the hormone oxytocin into the
bloodstream. Oxytocin causes stronger contractions of the smooth muscles in of the uterus (the effectors), pushing the baby
further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and
increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts,
stopping the release of oxytocin.
A second example of positive feedback centers on reversing extreme damage to the body. Following a penetrating wound,
the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced
perfusion (penetration of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut
down and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood
vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting
substances. This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area
based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.


1.6 | Anatomical Terminology
By the end of this section, you will be able to:
• Demonstrate the anatomical position
• Describe the human body using directional and regional terms
• Identify three planes most commonly used in the study of anatomy
• Distinguish between the posterior (dorsal) and the anterior (ventral) body cavities, identifying their subdivisions and
representative organs found in each


• Describe serous membrane and explain its function


Anatomists and health care providers use terminology that can be bewildering to the uninitiated. However, the purpose of
this language is not to confuse, but rather to increase precision and reduce medical errors. For example, is a scar “above the
wrist” located on the forearm two or three inches away from the hand? Or is it at the base of the hand? Is it on the palm-
side or back-side? By using precise anatomical terminology, we eliminate ambiguity. Anatomical terms derive from ancient
Greek and Latin words. Because these languages are no longer used in everyday conversation, the meaning of their words
does not change.


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Anatomical terms are made up of roots, prefixes, and suffixes. The root of a term often refers to an organ, tissue, or
condition, whereas the prefix or suffix often describes the root. For example, in the disorder hypertension, the prefix “hyper-
” means “high” or “over,” and the root word “tension” refers to pressure, so the word “hypertension” refers to abnormally
high blood pressure.


Anatomical Position
To further increase precision, anatomists standardize the way in which they view the body. Just as maps are normally
oriented with north at the top, the standard body “map,” or anatomical position, is that of the body standing upright, with
the feet at shoulder width and parallel, toes forward. The upper limbs are held out to each side, and the palms of the hands
face forward as illustrated in Figure 1.12. Using this standard position reduces confusion. It does not matter how the body
being described is oriented, the terms are used as if it is in anatomical position. For example, a scar in the “anterior (front)
carpal (wrist) region” would be present on the palm side of the wrist. The term “anterior” would be used even if the hand
were palm down on a table.


Figure 1.12 Regions of the Human Body The human body is shown in anatomical position in an (a) anterior view
and a (b) posterior view. The regions of the body are labeled in boldface.


A body that is lying down is described as either prone or supine. Prone describes a face-down orientation, and supine
describes a face up orientation. These terms are sometimes used in describing the position of the body during specific
physical examinations or surgical procedures.


Regional Terms
The human body’s numerous regions have specific terms to help increase precision (see Figure 1.12). Notice that the term
“brachium” or “arm” is reserved for the “upper arm” and “antebrachium” or “forearm” is used rather than “lower arm.”
Similarly, “femur” or “thigh” is correct, and “leg” or “crus” is reserved for the portion of the lower limb between the knee
and the ankle. You will be able to describe the body’s regions using the terms from the figure.


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Directional Terms
Certain directional anatomical terms appear throughout this and any other anatomy textbook (Figure 1.13). These terms
are essential for describing the relative locations of different body structures. For instance, an anatomist might describe one
band of tissue as “inferior to” another or a physician might describe a tumor as “superficial to” a deeper body structure.
Commit these terms to memory to avoid confusion when you are studying or describing the locations of particular body
parts.
• Anterior (or ventral) Describes the front or direction toward the front of the body. The toes are anterior to the foot.
• Posterior (or dorsal) Describes the back or direction toward the back of the body. The popliteus is posterior to the
patella.


• Superior (or cranial) describes a position above or higher than another part of the body proper. The orbits are superior
to the oris.


• Inferior (or caudal) describes a position below or lower than another part of the body proper; near or toward the tail
(in humans, the coccyx, or lowest part of the spinal column). The pelvis is inferior to the abdomen.


• Lateral describes the side or direction toward the side of the body. The thumb (pollex) is lateral to the digits.
• Medial describes the middle or direction toward the middle of the body. The hallux is the medial toe.
• Proximal describes a position in a limb that is nearer to the point of attachment or the trunk of the body. The brachium
is proximal to the antebrachium.


• Distal describes a position in a limb that is farther from the point of attachment or the trunk of the body. The crus is
distal to the femur.


• Superficial describes a position closer to the surface of the body. The skin is superficial to the bones.
• Deep describes a position farther from the surface of the body. The brain is deep to the skull.


Figure 1.13 Directional Terms Applied to the Human Body Paired directional terms are shown as applied to the
human body.


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Body Planes
A section is a two-dimensional surface of a three-dimensional structure that has been cut. Modern medical imaging
devices enable clinicians to obtain “virtual sections” of living bodies. We call these scans. Body sections and scans can be
correctly interpreted, however, only if the viewer understands the plane along which the section was made. A plane is an
imaginary two-dimensional surface that passes through the body. There are three planes commonly referred to in anatomy
and medicine, as illustrated in Figure 1.14.
• The sagittal plane is the plane that divides the body or an organ vertically into right and left sides. If this vertical
plane runs directly down the middle of the body, it is called the midsagittal or median plane. If it divides the body into
unequal right and left sides, it is called a parasagittal plane or less commonly a longitudinal section.


• The frontal plane is the plane that divides the body or an organ into an anterior (front) portion and a posterior (rear)
portion. The frontal plane is often referred to as a coronal plane. (“Corona” is Latin for “crown.”)


• The transverse plane is the plane that divides the body or organ horizontally into upper and lower portions. Transverse
planes produce images referred to as cross sections.


Figure 1.14 Planes of the Body The three planes most commonly used in anatomical and medical imaging are the
sagittal, frontal (or coronal), and transverse plane.


Body Cavities and Serous Membranes
The body maintains its internal organization by means of membranes, sheaths, and other structures that separate
compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments (Figure
1.15). These cavities contain and protect delicate internal organs, and the ventral cavity allows for significant changes in
the size and shape of the organs as they perform their functions. The lungs, heart, stomach, and intestines, for example, can
expand and contract without distorting other tissues or disrupting the activity of nearby organs.


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Figure 1.15 Dorsal and Ventral Body Cavities The ventral cavity includes the thoracic and abdominopelvic cavities
and their subdivisions. The dorsal cavity includes the cranial and spinal cavities.


Subdivisions of the Posterior (Dorsal) and Anterior (Ventral) Cavities
The posterior (dorsal) and anterior (ventral) cavities are each subdivided into smaller cavities. In the posterior (dorsal)
cavity, the cranial cavity houses the brain, and the spinal cavity (or vertebral cavity) encloses the spinal cord. Just as the
brain and spinal cord make up a continuous, uninterrupted structure, the cranial and spinal cavities that house them are also
continuous. The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal
fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the posterior (dorsal) cavity.
The anterior (ventral) cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity (see Figure
1.15). The thoracic cavity is the more superior subdivision of the anterior cavity, and it is enclosed by the rib cage. The
thoracic cavity contains the lungs and the heart, which is located in the mediastinum. The diaphragm forms the floor of the
thoracic cavity and separates it from the more inferior abdominopelvic cavity. The abdominopelvic cavity is the largest
cavity in the body. Although no membrane physically divides the abdominopelvic cavity, it can be useful to distinguish
between the abdominal cavity, the division that houses the digestive organs, and the pelvic cavity, the division that houses
the organs of reproduction.
Abdominal Regions and Quadrants
To promote clear communication, for instance about the location of a patient’s abdominal pain or a suspicious mass, health
care providers typically divide up the cavity into either nine regions or four quadrants (Figure 1.16).


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Figure 1.16 Regions and Quadrants of the Peritoneal Cavity There are (a) nine abdominal regions and (b) four
abdominal quadrants in the peritoneal cavity.


The more detailed regional approach subdivides the cavity with one horizontal line immediately inferior to the ribs and
one immediately superior to the pelvis, and two vertical lines drawn as if dropped from the midpoint of each clavicle
(collarbone). There are nine resulting regions. The simpler quadrants approach, which is more commonly used in medicine,
subdivides the cavity with one horizontal and one vertical line that intersect at the patient’s umbilicus (navel).
Membranes of the Anterior (Ventral) Body Cavity
A serous membrane (also referred to a serosa) is one of the thin membranes that cover the walls and organs in the thoracic
and abdominopelvic cavities. The parietal layers of the membranes line the walls of the body cavity (pariet- refers to a
cavity wall). The visceral layer of the membrane covers the organs (the viscera). Between the parietal and visceral layers is
a very thin, fluid-filled serous space, or cavity (Figure 1.17).


Figure 1.17 Serous Membrane Serous membrane lines the pericardial cavity and reflects back to cover the
heart—much the same way that an underinflated balloon would form two layers surrounding a fist.


There are three serous cavities and their associated membranes. The pleura is the serous membrane that surrounds the lungs
in the pleural cavity; the pericardium is the serous membrane that surrounds the heart in the pericardial cavity; and the
peritoneum is the serous membrane that surrounds several organs in the abdominopelvic cavity. The serous fluid produced
by the serous membranes reduces friction between the walls of the cavities and the internal organs when they move, such
as when the lungs inflate or the heart beats. Both the parietal and visceral serosa secrete the thin, slippery serous fluid
that prevents friction when an organ slides past the walls of a cavity. In the pleural cavities, pleural fluid prevents friction
between the lungs and the walls of the cavity. In the pericardial sac, pericardial fluid prevents friction between the heart and
the walls of the pericardial sac. And in the peritoneal cavity, peritoneal fluid prevents friction between abdominal and pelvic
organs and the wall of the cavity. The serous membranes therefore provide additional protection to the viscera they enclose
by reducing friction that could lead to inflammation of the organs.


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 29




1.7 | Medical Imaging
By the end of this section, you will be able to:
• Discuss the uses and drawbacks of X-ray imaging
• Identify four modern medical imaging techniques and how they are used


For thousands of years, fear of the dead and legal sanctions limited the ability of anatomists and physicians to study the
internal structures of the human body. An inability to control bleeding, infection, and pain made surgeries infrequent, and
those that were performed—such as wound suturing, amputations, tooth and tumor removals, skull drilling, and cesarean
births—did not greatly advance knowledge about internal anatomy. Theories about the function of the body and about
disease were therefore largely based on external observations and imagination. During the fourteenth and fifteenth centuries,
however, the detailed anatomical drawings of Italian artist and anatomist Leonardo da Vinci and Flemish anatomist Andreas
Vesalius were published, and interest in human anatomy began to increase. Medical schools began to teach anatomy using
human dissection; although some resorted to grave robbing to obtain corpses. Laws were eventually passed that enabled
students to dissect the corpses of criminals and those who donated their bodies for research. Still, it was not until the late
nineteenth century that medical researchers discovered non-surgical methods to look inside the living body.


X-Rays
German physicist Wilhelm Röntgen (1845–1923) was experimenting with electrical current when he discovered that a
mysterious and invisible “ray” would pass through his flesh but leave an outline of his bones on a screen coated with a metal
compound. In 1895, Röntgen made the first durable record of the internal parts of a living human: an “X-ray” image (as it
came to be called) of his wife’s hand. Scientists around the world quickly began their own experiments with X-rays, and by
1900, X-rays were widely used to detect a variety of injuries and diseases. In 1901, Röntgen was awarded the first Nobel
Prize for physics for his work in this field.
The X-ray is a form of high energy electromagnetic radiation with a short wavelength capable of penetrating solids and
ionizing gases. As they are used in medicine, X-rays are emitted from an X-ray machine and directed toward a specially
treated metallic plate placed behind the patient’s body. The beam of radiation results in darkening of the X-ray plate. X-rays
are slightly impeded by soft tissues, which show up as gray on the X-ray plate, whereas hard tissues, such as bone, largely
block the rays, producing a light-toned “shadow.” Thus, X-rays are best used to visualize hard body structures such as teeth
and bones (Figure 1.18). Like many forms of high energy radiation, however, X-rays are capable of damaging cells and
initiating changes that can lead to cancer. This danger of excessive exposure to X-rays was not fully appreciated for many
years after their widespread use.


Figure 1.18 X-Ray of a Hand High energy electromagnetic radiation allows the internal structures of the body, such
as bones, to be seen in X-rays like these. (credit: Trace Meek/flickr)


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Refinements and enhancements of X-ray techniques have continued throughout the twentieth and twenty-first centuries.
Although often supplanted by more sophisticated imaging techniques, the X-ray remains a “workhorse” in medical imaging,
especially for viewing fractures and for dentistry. The disadvantage of irradiation to the patient and the operator is now
attenuated by proper shielding and by limiting exposure.


Modern Medical Imaging
X-rays can depict a two-dimensional image of a body region, and only from a single angle. In contrast, more recent medical
imaging technologies produce data that is integrated and analyzed by computers to produce three-dimensional images or
images that reveal aspects of body functioning.
Computed Tomography
Tomography refers to imaging by sections. Computed tomography (CT) is a noninvasive imaging technique that uses
computers to analyze several cross-sectional X-rays in order to reveal minute details about structures in the body (Figure
1.19a). The technique was invented in the 1970s and is based on the principle that, as X-rays pass through the body, they
are absorbed or reflected at different levels. In the technique, a patient lies on a motorized platform while a computerized
axial tomography (CAT) scanner rotates 360 degrees around the patient, taking X-ray images. A computer combines these
images into a two-dimensional view of the scanned area, or “slice.”


Figure 1.19 Medical Imaging Techniques (a) The results of a CT scan of the head are shown as successive
transverse sections. (b) An MRI machine generates a magnetic field around a patient. (c) PET scans use
radiopharmaceuticals to create images of active blood flow and physiologic activity of the organ or organs being
targeted. (d) Ultrasound technology is used to monitor pregnancies because it is the least invasive of imaging
techniques and uses no electromagnetic radiation. (credit a: Akira Ohgaki/flickr; credit b: “Digital Cate”/flickr; credit c:
“Raziel”/Wikimedia Commons; credit d: “Isis”/Wikimedia Commons)


Since 1970, the development of more powerful computers and more sophisticated software has made CT scanning routine
for many types of diagnostic evaluations. It is especially useful for soft tissue scanning, such as of the brain and the thoracic
and abdominal viscera. Its level of detail is so precise that it can allow physicians to measure the size of a mass down to a
millimeter. The main disadvantage of CT scanning is that it exposes patients to a dose of radiation many times higher than
that of X-rays. In fact, children who undergo CT scans are at increased risk of developing cancer, as are adults who have
multiple CT scans.


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 31




A CT or CAT scan relies on a circling scanner that revolves around the patient’s body. Watch this video
(http://openstaxcollege.org/l/CATscan) to learn more about CT and CAT scans. What type of radiation does a CT
scanner use?


Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique based on a phenomenon of nuclear
physics discovered in the 1930s, in which matter exposed to magnetic fields and radio waves was found to emit radio
signals. In 1970, a physician and researcher named Raymond Damadian noticed that malignant (cancerous) tissue gave
off different signals than normal body tissue. He applied for a patent for the first MRI scanning device, which was in use
clinically by the early 1980s. The early MRI scanners were crude, but advances in digital computing and electronics led
to their advancement over any other technique for precise imaging, especially to discover tumors. MRI also has the major
advantage of not exposing patients to radiation.
Drawbacks of MRI scans include their much higher cost, and patient discomfort with the procedure. The MRI scanner
subjects the patient to such powerful electromagnets that the scan room must be shielded. The patient must be enclosed in
a metal tube-like device for the duration of the scan (see Figure 1.19b), sometimes as long as thirty minutes, which can
be uncomfortable and impractical for ill patients. The device is also so noisy that, even with earplugs, patients can become
anxious or even fearful. These problems have been overcome somewhat with the development of “open” MRI scanning,
which does not require the patient to be entirely enclosed in the metal tube. Patients with iron-containing metallic implants
(internal sutures, some prosthetic devices, and so on) cannot undergo MRI scanning because it can dislodge these implants.
Functional MRIs (fMRIs), which detect the concentration of blood flow in certain parts of the body, are increasingly being
used to study the activity in parts of the brain during various body activities. This has helped scientists learn more about the
locations of different brain functions and more about brain abnormalities and diseases.


A patient undergoing an MRI is surrounded by a tube-shaped scanner. Watch this video (http://openstaxcollege.org/
l/MRI) to learn more about MRIs. What is the function of magnets in an MRI?


Positron Emission Tomography
Positron emission tomography (PET) is a medical imaging technique involving the use of so-called radiopharmaceuticals,
substances that emit radiation that is short-lived and therefore relatively safe to administer to the body. Although the
first PET scanner was introduced in 1961, it took 15 more years before radiopharmaceuticals were combined with the
technique and revolutionized its potential. The main advantage is that PET (see Figure 1.19c) can illustrate physiologic
activity—including nutrient metabolism and blood flow—of the organ or organs being targeted, whereas CT and MRI scans


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can only show static images. PET is widely used to diagnose a multitude of conditions, such as heart disease, the spread of
cancer, certain forms of infection, brain abnormalities, bone disease, and thyroid disease.


PET relies on radioactive substances administered several minutes before the scan. Watch this video
(http://openstaxcollege.org/l/PET) to learn more about PET. How is PET used in chemotherapy?


Ultrasonography
Ultrasonography is an imaging technique that uses the transmission of high-frequency sound waves into the body to
generate an echo signal that is converted by a computer into a real-time image of anatomy and physiology (see Figure
1.19d). Ultrasonography is the least invasive of all imaging techniques, and it is therefore used more freely in sensitive
situations such as pregnancy. The technology was first developed in the 1940s and 1950s. Ultrasonography is used to study
heart function, blood flow in the neck or extremities, certain conditions such as gallbladder disease, and fetal growth and
development. The main disadvantages of ultrasonography are that the image quality is heavily operator-dependent and that
it is unable to penetrate bone and gas.


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 33




abdominopelvic cavity
anabolism
anatomical position
anatomy
anterior
anterior cavity


catabolism
caudal


cell


computed tomography (CT)


control center
cranial
cranial cavity
deep
development
differentiation
distal
dorsal
dorsal cavity
effector
frontal plane
gross anatomy


growth
homeostasis
inferior


lateral
magnetic resonance imaging (MRI)


medial


KEY TERMS
division of the anterior (ventral) cavity that houses the abdominal and pelvic viscera


assembly of more complex molecules from simpler molecules
standard reference position used for describing locations and directions on the human body


science that studies the form and composition of the body’s structures
describes the front or direction toward the front of the body; also referred to as ventral


larger body cavity located anterior to the posterior (dorsal) body cavity; includes the serous membrane-
lined pleural cavities for the lungs, pericardial cavity for the heart, and peritoneal cavity for the abdominal and
pelvic organs; also referred to as ventral cavity


breaking down of more complex molecules into simpler molecules
describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the


coccyx, or lowest part of the spinal column); also referred to as inferior
smallest independently functioning unit of all organisms; in animals, a cell contains cytoplasm, composed of fluid
and organelles


medical imaging technique in which a computer-enhanced cross-sectional X-ray image
is obtained


compares values to their normal range; deviations cause the activation of an effector
describes a position above or higher than another part of the body proper; also referred to as superior


division of the posterior (dorsal) cavity that houses the brain
describes a position farther from the surface of the body


changes an organism goes through during its life
process by which unspecialized cells become specialized in structure and function


describes a position farther from the point of attachment or the trunk of the body
describes the back or direction toward the back of the body; also referred to as posterior


posterior body cavity that houses the brain and spinal cord; also referred to the posterior body cavity
organ that can cause a change in a value


two-dimensional, vertical plane that divides the body or organ into anterior and posterior portions
study of the larger structures of the body, typically with the unaided eye; also referred to macroscopic


anatomy
process of increasing in size


steady state of body systems that living organisms maintain
describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the


coccyx, or lowest part of the spinal column); also referred to as caudal
describes the side or direction toward the side of the body


medical imaging technique in which a device generates a magnetic field to
obtain detailed sectional images of the internal structures of the body
describes the middle or direction toward the middle of the body


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metabolism
microscopic anatomy
negative feedback


normal range
nutrient
organ
organ system
organism


pericardium
peritoneum
physiology
plane
pleura
positive feedback
positron emission tomography (PET)


posterior
posterior cavity
pressure
prone
proximal
regional anatomy
renewal
reproduction
responsiveness
sagittal plane
section
sensor
serosa
serous membrane
set point


spinal cavity
superficial


sum of all of the body’s chemical reactions
study of very small structures of the body using magnification


homeostatic mechanism that tends to stabilize an upset in the body’s physiological condition by
preventing an excessive response to a stimulus, typically as the stimulus is removed


range of values around the set point that do not cause a reaction by the control center
chemical obtained from foods and beverages that is critical to human survival


functionally distinct structure composed of two or more types of tissues
group of organs that work together to carry out a particular function


living being that has a cellular structure and that can independently perform all physiologic functions
necessary for life


sac that encloses the heart
serous membrane that lines the abdominopelvic cavity and covers the organs found there
science that studies the chemistry, biochemistry, and physics of the body’s functions


imaginary two-dimensional surface that passes through the body
serous membrane that lines the pleural cavity and covers the lungs


mechanism that intensifies a change in the body’s physiological condition in response to a stimulus
medical imaging technique in which radiopharmaceuticals are traced to


reveal metabolic and physiological functions in tissues
describes the back or direction toward the back of the body; also referred to as dorsal


posterior body cavity that houses the brain and spinal cord; also referred to as dorsal cavity
force exerted by a substance in contact with another substance


face down
describes a position nearer to the point of attachment or the trunk of the body


study of the structures that contribute to specific body regions
process by which worn-out cells are replaced


process by which new organisms are generated
ability of an organisms or a system to adjust to changes in conditions


two-dimensional, vertical plane that divides the body or organ into right and left sides
in anatomy, a single flat surface of a three-dimensional structure that has been cut through
(also, receptor) reports a monitored physiological value to the control center
membrane that covers organs and reduces friction; also referred to as serous membrane


membrane that covers organs and reduces friction; also referred to as serosa
ideal value for a physiological parameter; the level or small range within which a physiological parameter


such as blood pressure is stable and optimally healthful, that is, within its parameters of homeostasis
division of the dorsal cavity that houses the spinal cord; also referred to as vertebral cavity


describes a position nearer to the surface of the body


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 35




superior
supine
systemic anatomy
thoracic cavity
tissue
transverse plane
ultrasonography


ventral
ventral cavity


X-ray


describes a position above or higher than another part of the body proper; also referred to as cranial
face up


study of the structures that contribute to specific body systems
division of the anterior (ventral) cavity that houses the heart, lungs, esophagus, and trachea


group of similar or closely related cells that act together to perform a specific function
two-dimensional, horizontal plane that divides the body or organ into superior and inferior portions
application of ultrasonic waves to visualize subcutaneous body structures such as tendons and


organs
describes the front or direction toward the front of the body; also referred to as anterior


larger body cavity located anterior to the posterior (dorsal) body cavity; includes the serous membrane-
lined pleural cavities for the lungs, pericardial cavity for the heart, and peritoneal cavity for the abdominal and
pelvic organs; also referred to as anterior body cavity
form of high energy electromagnetic radiation with a short wavelength capable of penetrating solids and ionizing
gases; used in medicine as a diagnostic aid to visualize body structures such as bones


CHAPTER REVIEW
1.1 Overview of Anatomy and Physiology
Human anatomy is the scientific study of the body’s structures. In the past, anatomy has primarily been studied via
observing injuries, and later by the dissection of anatomical structures of cadavers, but in the past century, computer-assisted
imaging techniques have allowed clinicians to look inside the living body. Human physiology is the scientific study of the
chemistry and physics of the structures of the body. Physiology explains how the structures of the body work together to
maintain life. It is difficult to study structure (anatomy) without knowledge of function (physiology). The two disciplines
are typically studied together because form and function are closely related in all living things.


1.2 Structural Organization of the Human Body
Life processes of the human body are maintained at several levels of structural organization. These include the chemical,
cellular, tissue, organ, organ system, and the organism level. Higher levels of organization are built from lower levels.
Therefore, molecules combine to form cells, cells combine to form tissues, tissues combine to form organs, organs combine
to form organ systems, and organ systems combine to form organisms.


1.3 Functions of Human Life
Most processes that occur in the human body are not consciously controlled. They occur continuously to build, maintain,
and sustain life. These processes include: organization, in terms of the maintenance of essential body boundaries;
metabolism, including energy transfer via anabolic and catabolic reactions; responsiveness; movement; and growth,
differentiation, reproduction, and renewal.


1.4 Requirements for Human Life
Humans cannot survive for more than a few minutes without oxygen, for more than several days without water, and for
more than several weeks without carbohydrates, lipids, proteins, vitamins, and minerals. Although the body can respond to
high temperatures by sweating and to low temperatures by shivering and increased fuel consumption, long-term exposure
to extreme heat and cold is not compatible with survival. The body requires a precise atmospheric pressure to maintain its
gases in solution and to facilitate respiration—the intake of oxygen and the release of carbon dioxide. Humans also require
blood pressure high enough to ensure that blood reaches all body tissues but low enough to avoid damage to blood vessels.


1.5 Homeostasis
Homeostasis is the activity of cells throughout the body to maintain the physiological state within a narrow range that is
compatible with life. Homeostasis is regulated by negative feedback loops and, much less frequently, by positive feedback
loops. Both have the same components of a stimulus, sensor, control center, and effector; however, negative feedback loops


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work to prevent an excessive response to the stimulus, whereas positive feedback loops intensify the response until an end
point is reached.


1.6 Anatomical Terminology
Ancient Greek and Latin words are used to build anatomical terms. A standard reference position for mapping the body’s
structures is the normal anatomical position. Regions of the body are identified using terms such as “occipital” that are more
precise than common words and phrases such as “the back of the head.” Directional terms such as anterior and posterior
are essential for accurately describing the relative locations of body structures. Images of the body’s interior commonly
align along one of three planes: the sagittal, frontal, or transverse. The body’s organs are organized in one of two main
cavities—dorsal (also referred to posterior) and ventral (also referred to anterior)—which are further sub-divided according
to the structures present in each area. The serous membranes have two layers—parietal and visceral—surrounding a fluid
filled space. Serous membranes cover the lungs (pleural serosa), heart (pericardial serosa), and some abdominopelvic organs
(peritoneal serosa).


1.7 Medical Imaging
Detailed anatomical drawings of the human body first became available in the fifteenth and sixteenth centuries; however,
it was not until the end of the nineteenth century, and the discovery of X-rays, that anatomists and physicians discovered
non-surgical methods to look inside a living body. Since then, many other techniques, including CT scans, MRI scans, PET
scans, and ultrasonography, have been developed, providing more accurate and detailed views of the form and function of
the human body.


INTERACTIVE LINK QUESTIONS
1. View this animation (http://openstaxcollege.org/l/
metabolic) to learn more about metabolic processes. What
kind of catabolism occurs in the heart?
2. Water concentration in the body is critical for proper
functioning. A person’s body retains very tight control on
water levels without conscious control by the person.
Watch this video (http://openstaxcollege.org/l/H2Ocon)
to learn more about water concentration in the body. Which
organ has primary control over the amount of water in the
body?
3. A CT or CAT scan relies on a circling scanner that
revolves around the patient’s body. Watch this video


(http://openstaxcollege.org/l/CATscan) to learn more
about CT and CAT scans. What type of radiation does a CT
scanner use?
4. A patient undergoing an MRI is surrounded by a tube-
shaped scanner. Watch this video
(http://openstaxcollege.org/l/MRI) to learn more about
MRIs. What is the function of magnets in an MRI?
5. PET relies on radioactive substances administered
several minutes before the scan. Watch this video
(http://openstaxcollege.org/l/PET) to learn more about
PET. How is PET used in chemotherapy?


REVIEW QUESTIONS
6. Which of the following specialties might focus on
studying all of the structures of the ankle and foot?


a. microscopic anatomy
b. muscle anatomy
c. regional anatomy
d. systemic anatomy


7. A scientist wants to study how the body uses foods and
fluids during a marathon run. This scientist is most likely
a(n) ________.


a. exercise physiologist
b. microscopic anatomist
c. regional physiologist
d. systemic anatomist


8. The smallest independently functioning unit of an
organism is a(n) ________.


a. cell
b. molecule
c. organ
d. tissue


9. A collection of similar tissues that performs a specific
function is an ________.


a. organ
b. organelle
c. organism
d. organ system


10. The body system responsible for structural support and
movement is the ________.


a. cardiovascular system
b. endocrine system
c. muscular system
d. skeletal system


11. Metabolism can be defined as the ________.
a. adjustment by an organism to external or internal
changes


b. process whereby all unspecialized cells become
specialized to perform distinct functions


c. process whereby new cells are formed to replace
worn-out cells


d. sum of all chemical reactions in an organism


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 37




12. Adenosine triphosphate (ATP) is an important
molecule because it ________.


a. is the result of catabolism
b. release energy in uncontrolled bursts
c. stores energy for use by body cells
d. All of the above


13. Cancer cells can be characterized as “generic” cells that
perform no specialized body function. Thus cancer cells
lack ________.


a. differentiation
b. reproduction
c. responsiveness
d. both reproduction and responsiveness


14. Humans have the most urgent need for a continuous
supply of ________.


a. food
b. nitrogen
c. oxygen
d. water


15. Which of the following statements about nutrients is
true?


a. All classes of nutrients are essential to human
survival.


b. Because the body cannot store any
micronutrients, they need to be consumed nearly
every day.


c. Carbohydrates, lipids, and proteins are
micronutrients.


d. Macronutrients are vitamins and minerals.
16. C.J. is stuck in her car during a bitterly cold blizzard.
Her body responds to the cold by ________.


a. increasing the blood to her hands and feet
b. becoming lethargic to conserve heat
c. breaking down stored energy
d. significantly increasing blood oxygen levels


17. After you eat lunch, nerve cells in your stomach
respond to the distension (the stimulus) resulting from the
food. They relay this information to ________.


a. a control center
b. a set point
c. effectors
d. sensors


18. Stimulation of the heat-loss center causes ________.


a. blood vessels in the skin to constrict
b. breathing to become slow and shallow
c. sweat glands to increase their output
d. All of the above


19. Which of the following is an example of a normal
physiologic process that uses a positive feedback loop?


a. blood pressure regulation
b. childbirth
c. regulation of fluid balance
d. temperature regulation


20. What is the position of the body when it is in the
“normal anatomical position?”


a. The person is prone with upper limbs, including
palms, touching sides and lower limbs touching
at sides.


b. The person is standing facing the observer, with
upper limbs extended out at a ninety-degree angle
from the torso and lower limbs in a wide stance
with feet pointing laterally


c. The person is supine with upper limbs, including
palms, touching sides and lower limbs touching
at sides.


d. None of the above
21. To make a banana split, you halve a banana into two
long, thin, right and left sides along the ________.


a. coronal plane
b. longitudinal plane
c. midsagittal plane
d. transverse plane


22. The lumbar region is ________.
a. inferior to the gluteal region
b. inferior to the umbilical region
c. superior to the cervical region
d. superior to the popliteal region


23. The heart is within the ________.
a. cranial cavity
b. mediastinum
c. posterior (dorsal) cavity
d. All of the above


24. In 1901, Wilhelm Röntgen was the first person to win
the Nobel Prize for physics. For what discovery did he win?


a. nuclear physics
b. radiopharmaceuticals
c. the link between radiation and cancer
d. X-rays


25. Which of the following imaging techniques would be
best to use to study the uptake of nutrients by rapidly
multiplying cancer cells?


a. CT
b. MRI
c. PET
d. ultrasonography


26. Which of the following imaging studies can be used
most safely during pregnancy?


a. CT scans
b. PET scans
c. ultrasounds
d. X-rays


27. What are two major disadvantages of MRI scans?


a. release of radiation and poor quality images
b. high cost and the need for shielding from the
magnetic signals


c. can only view metabolically active tissues and
inadequate availability of equipment


d. release of radiation and the need for a patient to
be confined to metal tube for up to 30 minutes


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CRITICAL THINKING QUESTIONS
28. Name at least three reasons to study anatomy and
physiology.
29. For whom would an appreciation of the structural
characteristics of the human heart come more easily: an
alien who lands on Earth, abducts a human, and dissects his
heart, or an anatomy and physiology student performing a
dissection of the heart on her very first day of class? Why?
30. Name the six levels of organization of the human body.
31. The female ovaries and the male testes are a part of
which body system? Can these organs be members of more
than one organ system? Why or why not?
32. Explain why the smell of smoke when you are sitting
at a campfire does not trigger alarm, but the smell of smoke
in your residence hall does.
33. Identify three different ways that growth can occur in
the human body.
34. When you open a bottle of sparkling water, the carbon
dioxide gas in the bottle form bubbles. If the bottle is left
open, the water will eventually “go flat.” Explain these
phenomena in terms of atmospheric pressure.


35. On his midsummer trek through the desert, Josh ran out
of water. Why is this particularly dangerous?
36. Identify the four components of a negative feedback
loop and explain what would happen if secretion of a body
chemical controlled by a negative feedback system became
too great.
37. What regulatory processes would your body use if
you were trapped by a blizzard in an unheated, uninsulated
cabin in the woods?
38. In which direction would an MRI scanner move to
produce sequential images of the body in the frontal plane,
and in which direction would an MRI scanner move to
produce sequential images of the body in the sagittal plane?
39. If a bullet were to penetrate a lung, which three anterior
thoracic body cavities would it enter, and which layer of the
serous membrane would it encounter first?
40.Which medical imaging technique is most dangerous to
use repeatedly, and why?
41. Explain why ultrasound imaging is the technique of
choice for studying fetal growth and development.


CHAPTER 1 | AN INTRODUCTION TO THE HUMAN BODY 39




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2 | THE CHEMICAL LEVEL
OF ORGANIZATION


Figure 2.1 Human DNA Human DNA is described as a double helix that resembles a molecular spiral staircase. In
humans the DNA is organized into 46 chromosomes.


Introduction
Chapter Objectives


After studying this chapter, you will be able to:
• Describe the fundamental composition of matter
• Identify the three subatomic particles
• Identify the four most abundant elements in the body
• Explain the relationship between an atom’s number of electrons and its relative stability
• Distinguish between ionic bonds, covalent bonds, and hydrogen bonds
• Explain how energy is invested, stored, and released via chemical reactions, particularly those reactions that
are critical to life


• Explain the importance of the inorganic compounds that contribute to life, such as water, salts, acids, and
bases


CHAPTER 2 | THE CHEMICAL LEVEL OF ORGANIZATION 41




• Compare and contrast the four important classes of organic (carbon-based) compounds—proteins,
carbohydrates, lipids and nucleic acids—according to their composition and functional importance to human
life


The smallest, most fundamental material components of the human body are basic chemical elements. In fact, chemicals
called nucleotide bases are the foundation of the genetic code with the instructions on how to build and maintain the human
body from conception through old age. There are about three billion of these base pairs in human DNA.
Human chemistry includes organic molecules (carbon-based) and biochemicals (those produced by the body). Human
chemistry also includes elements. In fact, life cannot exist without many of the elements that are part of the earth. All of
the elements that contribute to chemical reactions, to the transformation of energy, and to electrical activity and muscle
contraction—elements that include phosphorus, carbon, sodium, and calcium, to name a few—originated in stars.
These elements, in turn, can form both the inorganic and organic chemical compounds important to life, including, for
example, water, glucose, and proteins. This chapter begins by examining elements and how the structures of atoms, the
basic units of matter, determine the characteristics of elements by the number of protons, neutrons, and electrons in the
atoms. The chapter then builds the framework of life from there.


2.1 | Elements and Atoms: The Building Blocks of Matter
By the end of this section, you will be able to:
• Discuss the relationships between matter, mass, elements, compounds, atoms, and subatomic particles
• Distinguish between atomic number and mass number
• Identify the key distinction between isotopes of the same element
• Explain how electrons occupy electron shells and their contribution to an atom’s relative stability


The substance of the universe—from a grain of sand to a star—is called matter. Scientists define matter as anything that
occupies space and has mass. An object’s mass and its weight are related concepts, but not quite the same. An object’s mass
is the amount of matter contained in the object, and the object’s mass is the same whether that object is on Earth or in the
zero-gravity environment of outer space. An object’s weight, on the other hand, is its mass as affected by the pull of gravity.
Where gravity strongly pulls on an object’s mass its weight is greater than it is where gravity is less strong. An object of a
certain mass weighs less on the moon, for example, than it does on Earth because the gravity of the moon is less than that
of Earth. In other words, weight is variable, and is influenced by gravity. A piece of cheese that weighs a pound on Earth
weighs only a few ounces on the moon.


Elements and Compounds
All matter in the natural world is composed of one or more of the 92 fundamental substances called elements. An element
is a pure substance that is distinguished from all other matter by the fact that it cannot be created or broken down by
ordinary chemical means. While your body can assemble many of the chemical compounds needed for life from their
constituent elements, it cannot make elements. They must come from the environment. A familiar example of an element
that you must take in is calcium (Ca++). Calcium is essential to the human body; it is absorbed and used for a number of
processes, including strengthening bones. When you consume dairy products your digestive system breaks down the food
into components small enough to cross into the bloodstream. Among these is calcium, which, because it is an element,
cannot be broken down further. The elemental calcium in cheese, therefore, is the same as the calcium that forms your
bones. Some other elements you might be familiar with are oxygen, sodium, and iron. The elements in the human body are
shown in Figure 2.2, beginning with the most abundant: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). Each
element’s name can be replaced by a one- or two-letter symbol; you will become familiar with some of these during this
course. All the elements in your body are derived from the foods you eat and the air you breathe.


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Figure 2.2 Elements of the Human Body The main elements that compose the human body are shown from most
abundant to least abundant.


In nature, elements rarely occur alone. Instead, they combine to form compounds. A compound is a substance composed
of two or more elements joined by chemical bonds. For example, the compound glucose is an important body fuel. It is
always composed of the same three elements: carbon, hydrogen, and oxygen. Moreover, the elements that make up any
given compound always occur in the same relative amounts. In glucose, there are always six carbon and six oxygen units
for every twelve hydrogen units. But what, exactly, are these “units” of elements?


Atoms and Subatomic Particles
An atom is the smallest quantity of an element that retains the unique properties of that element. In other words, an atom
of hydrogen is a unit of hydrogen—the smallest amount of hydrogen that can exist. As you might guess, atoms are almost
unfathomably small. The period at the end of this sentence is millions of atoms wide.
Atomic Structure and Energy
Atoms are made up of even smaller subatomic particles, three types of which are important: the proton, neutron, and
electron. The number of positively-charged protons and non-charged (“neutral”) neutrons, gives mass to the atom, and the
number of each in the nucleus of the atom determine the element. The number of negatively-charged electrons that “spin”
around the nucleus at close to the speed of light equals the number of protons. An electron has about 1/2000th the mass of
a proton or neutron.
Figure 2.3 shows two models that can help you imagine the structure of an atom—in this case, helium (He). In the planetary
model, helium’s two electrons are shown circling the nucleus in a fixed orbit depicted as a ring. Although this model
is helpful in visualizing atomic structure, in reality, electrons do not travel in fixed orbits, but whiz around the nucleus
erratically in a so-called electron cloud.


CHAPTER 2 | THE CHEMICAL LEVEL OF ORGANIZATION 43




Figure 2.3 Two Models of Atomic Structure (a) In the planetary model, the electrons of helium are shown in
fixed orbits, depicted as rings, at a precise distance from the nucleus, somewhat like planets orbiting the sun. (b) In
the electron cloud model, the electrons of carbon are shown in the variety of locations they would have at different
distances from the nucleus over time.


An atom’s protons and electrons carry electrical charges. Protons, with their positive charge, are designated p+. Electrons,
which have a negative charge, are designated e–. An atom’s neutrons have no charge: they are electrically neutral. Just
as a magnet sticks to a steel refrigerator because their opposite charges attract, the positively charged protons attract the
negatively charged electrons. This mutual attraction gives the atom some structural stability. The attraction by the positively
charged nucleus helps keep electrons from straying far. The number of protons and electrons within a neutral atom are equal,
thus, the atom’s overall charge is balanced.
Atomic Number and Mass Number
An atom of carbon is unique to carbon, but a proton of carbon is not. One proton is the same as another, whether it is found
in an atom of carbon, sodium (Na), or iron (Fe). The same is true for neutrons and electrons. So, what gives an element its
distinctive properties—what makes carbon so different from sodium or iron? The answer is the unique quantity of protons
each contains. Carbon by definition is an element whose atoms contain six protons. No other element has exactly six protons
in its atoms. Moreover, all atoms of carbon, whether found in your liver or in a lump of coal, contain six protons. Thus, the
atomic number, which is the number of protons in the nucleus of the atom, identifies the element. Because an atom usually
has the same number of electrons as protons, the atomic number identifies the usual number of electrons as well.
In their most common form, many elements also contain the same number of neutrons as protons. The most common form
of carbon, for example, has six neutrons as well as six protons, for a total of 12 subatomic particles in its nucleus. An
element’s mass number is the sum of the number of protons and neutrons in its nucleus. So the most common form of
carbon’s mass number is 12. (Electrons have so little mass that they do not appreciably contribute to the mass of an atom.)
Carbon is a relatively light element. Uranium (U), in contrast, has a mass number of 238 and is referred to as a heavy metal.
Its atomic number is 92 (it has 92 protons) but it contains 146 neutrons; it has the most mass of all the naturally occurring
elements.
The periodic table of the elements, shown in Figure 2.4, is a chart identifying the 92 elements found in nature, as well
as several larger, unstable elements discovered experimentally. The elements are arranged in order of their atomic number,
with hydrogen and helium at the top of the table, and the more massive elements below. The periodic table is a useful device
because for each element, it identifies the chemical symbol, the atomic number, and the mass number, while organizing


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elements according to their propensity to react with other elements. The number of protons and electrons in an element are
equal. The number of protons and neutrons may be equal for some elements, but are not equal for all.


Figure 2.4 The Periodic Table of the Elements (credit: R.A. Dragoset, A. Musgrove, C.W. Clark, W.C. Martin)


Visit this website (http://openstaxcollege.org/l/ptable) to view the periodic table. In the periodic table of the
elements, elements in a single column have the same number of electrons that can participate in a chemical reaction.
These electrons are known as “valence electrons.” For example, the elements in the first column all have a single
valence electron, an electron that can be “donated” in a chemical reaction with another atom. What is the meaning of a
mass number shown in parentheses?


Isotopes
Although each element has a unique number of protons, it can exist as different isotopes. An isotope is one of the different
forms of an element, distinguished from one another by different numbers of neutrons. The standard isotope of carbon
is 12C, commonly called carbon twelve. 12C has six protons and six neutrons, for a mass number of twelve. All of the
isotopes of carbon have the same number of protons; therefore, 13C has seven neutrons, and 14C has eight neutrons. The


CHAPTER 2 | THE CHEMICAL LEVEL OF ORGANIZATION 45




different isotopes of an element can also be indicated with the mass number hyphenated (for example, C-12 instead of 12C).
Hydrogen has three common isotopes, shown in Figure 2.5.


Figure 2.5 Isotopes of Hydrogen Protium, designated 1H, has one proton and no neutrons. It is by far the
most abundant isotope of hydrogen in nature. Deuterium, designated 2H, has one proton and one neutron. Tritium,
designated 3H, has two neutrons.


An isotope that contains more than the usual number of neutrons is referred to as a heavy isotope. An example is 14C.
Heavy isotopes tend to be unstable, and unstable isotopes are radioactive. A radioactive isotope is an isotope whose
nucleus readily decays, giving off subatomic particles and electromagnetic energy. Different radioactive isotopes (also
called radioisotopes) differ in their half-life, the time it takes for half of any size sample of an isotope to decay. For example,
the half-life of tritium—a radioisotope of hydrogen—is about 12 years, indicating it takes 12 years for half of the tritium
nuclei in a sample to decay. Excessive exposure to radioactive isotopes can damage human cells and even cause cancer and
birth defects, but when exposure is controlled, some radioactive isotopes can be useful in medicine. For more information,
see the Career Connections.


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Interventional Radiologist
The controlled use of radioisotopes has advanced medical diagnosis and treatment of disease. Interventional
radiologists are physicians who treat disease by using minimally invasive techniques involving radiation. Many
conditions that could once only be treated with a lengthy and traumatic operation can now be treated non-surgically,
reducing the cost, pain, length of hospital stay, and recovery time for patients. For example, in the past, the only options
for a patient with one or more tumors in the liver were surgery and chemotherapy (the administration of drugs to treat
cancer). Some liver tumors, however, are difficult to access surgically, and others could require the surgeon to remove
too much of the liver. Moreover, chemotherapy is highly toxic to the liver, and certain tumors do not respond well to it
anyway. In some such cases, an interventional radiologist can treat the tumors by disrupting their blood supply, which
they need if they are to continue to grow. In this procedure, called radioembolization, the radiologist accesses the liver
with a fine needle, threaded through one of the patient’s blood vessels. The radiologist then inserts tiny radioactive
“seeds” into the blood vessels that supply the tumors. In the days and weeks following the procedure, the radiation
emitted from the seeds destroys the vessels and directly kills the tumor cells in the vicinity of the treatment.
Radioisotopes emit subatomic particles that can be detected and tracked by imaging technologies. One of the most
advanced uses of radioisotopes in medicine is the positron emission tomography (PET) scanner, which detects the
activity in the body of a very small injection of radioactive glucose, the simple sugar that cells use for energy. The
PET camera reveals to the medical team which of the patient’s tissues are taking up the most glucose. Thus, the most
metabolically active tissues show up as bright “hot spots” on the images (Figure 2.6). PET can reveal some cancerous
masses because cancer cells consume glucose at a high rate to fuel their rapid reproduction.


Figure 2.6 PET Scan PET highlights areas in the body where there is relatively high glucose use, which is
characteristic of cancerous tissue. This PET scan shows sites of the spread of a large primary tumor to other
sites.


The Behavior of Electrons
In the human body, atoms do not exist as independent entities. Rather, they are constantly reacting with other atoms to
form and to break down more complex substances. To fully understand anatomy and physiology you must grasp how atoms
participate in such reactions. The key is understanding the behavior of electrons.


CHAPTER 2 | THE CHEMICAL LEVEL OF ORGANIZATION 47




Although electrons do not follow rigid orbits a set distance away from the atom’s nucleus, they do tend to stay within certain
regions of space called electron shells. An electron shell is a layer of electrons that encircle the nucleus at a distinct energy
level.
The atoms of the elements found in the human body have from one to five electron shells, and all electron shells hold eight
electrons except the first shell, which can only hold two. This configuration of electron shells is the same for all atoms.
The precise number of shells depends on the number of electrons in the atom. Hydrogen and helium have just one and two
electrons, respectively. If you take a look at the periodic table of the elements, you will notice that hydrogen and helium
are placed alone on either sides of the top row; they are the only elements that have just one electron shell (Figure 2.7). A
second shell is necessary to hold the electrons in all elements larger than hydrogen and helium.
Lithium (Li), whose atomic number is 3, has three electrons. Two of these fill the first electron shell, and the third spills over
into a second shell. The second electron shell can accommodate as many as eight electrons. Carbon, with its six electrons,
entirely fills its first shell, and half-fills its second. With ten electrons, neon (Ne) entirely fills its two electron shells. Again,
a look at the periodic table reveals that all of the elements in the second row, from lithium to neon, have just two electron
shells. Atoms with more than ten electrons require more than two shells. These elements occupy the third and subsequent
rows of the periodic table.


Figure 2.7 Electron Shells Electrons orbit the atomic nucleus at distinct levels of energy called electron shells.
(a) With one electron, hydrogen only half-fills its electron shell. Helium also has a single shell, but its two electrons
completely fill it. (b) The electrons of carbon completely fill its first electron shell, but only half-fills its second. (c) Neon,
an element that does not occur in the body, has 10 electrons, filling both of its electron shells.


The factor that most strongly governs the tendency of an atom to participate in chemical reactions is the number of electrons
in its valence shell. A valence shell is an atom’s outermost electron shell. If the valence shell is full, the atom is stable;
meaning its electrons are unlikely to be pulled away from the nucleus by the electrical charge of other atoms. If the valence
shell is not full, the atom is reactive; meaning it will tend to react with other atoms in ways that make the valence shell full.
Consider hydrogen, with its one electron only half-filling its valence shell. This single electron is likely to be drawn into
relationships with the atoms of other elements, so that hydrogen’s single valence shell can be stabilized.
All atoms (except hydrogen and helium with their single electron shells) are most stable when there are exactly eight
electrons in their valence shell. This principle is referred to as the octet rule, and it states that an atom will give up, gain,
or share electrons with another atom so that it ends up with eight electrons in its own valence shell. For example, oxygen,
with six electrons in its valence shell, is likely to react with other atoms in a way that results in the addition of two electrons
to oxygen’s valence shell, bringing the number to eight. When two hydrogen atoms each share their single electron with
oxygen, covalent bonds are formed, resulting in a molecule of water, H2O.


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In nature, atoms of one element tend to join with atoms of other elements in characteristic ways. For example, carbon
commonly fills its valence shell by linking up with four atoms of hydrogen. In so doing, the two elements form the simplest
of organic molecules, methane, which also is one of the most abundant and stable carbon-containing compounds on Earth.
As stated above, another example is water; oxygen needs two electrons to fill its valence shell. It commonly interacts
with two atoms of hydrogen, forming H2O. Incidentally, the name “hydrogen” reflects its contribution to water (hydro- =
“water”; -gen = “maker”). Thus, hydrogen is the “water maker.”


2.2 | Chemical Bonds
By the end of this section, you will be able to:
• Explain the relationship between molecules and compounds
• Distinguish between ions, cations, and anions
• Identify the key difference between ionic and covalent bonds
• Distinguish between nonpolar and polar covalent bonds
• Explain how water molecules link via hydrogen bonds


Atoms separated by a great distance cannot link; rather, they must come close enough for the electrons in their valence shells
to interact. But do atoms ever actually touch one another? Most physicists would say no, because the negatively charged
electrons in their valence shells repel one another. No force within the human body—or anywhere in the natural world—is
strong enough to overcome this electrical repulsion. So when you read about atoms linking together or colliding, bear in
mind that the atoms are not merging in a physical sense.
Instead, atoms link by forming a chemical bond. A bond is a weak or strong electrical attraction that holds atoms in the same
vicinity. The new grouping is typically more stable—less likely to react again—than its component atoms were when they
were separate. A more or less stable grouping of two or more atoms held together by chemical bonds is called a molecule.
The bonded atoms may be of the same element, as in the case of H2, which is called molecular hydrogen or hydrogen gas.
When a molecule is made up of two or more atoms of different elements, it is called a chemical compound. Thus, a unit of
water, or H2O, is a compound, as is a single molecule of the gas methane, or CH4.
Three types of chemical bonds are important in human physiology, because they hold together substances that are used by
the body for critical aspects of homeostasis, signaling, and energy production, to name just a few important processes. These
are ionic bonds, covalent bonds, and hydrogen bonds.


Ions and Ionic Bonds
Recall that an atom typically has the same number of positively charged protons and negatively charged electrons. As long
as this situation remains, the atom is electrically neutral. But when an atom participates in a chemical reaction that results
in the donation or acceptance of one or more electrons, the atom will then become positively or negatively charged. This
happens frequently for most atoms in order to have a full valence shell, as described previously. This can happen either
by gaining electrons to fill a shell that is more than half-full, or by giving away electrons to empty a shell than is less
than half-full, thereby leaving the next smaller electron shell as the new, full, valence shell. An atom that has an electrical
charge—whether positive or negative—is an ion.


Visit this website (http://openstaxcollege.org/l/electenergy) to learn about electrical energy and the attraction/
repulsion of charges. What happens to the charged electroscope when a conductor is moved between its plastic sheets,
and why?


CHAPTER 2 | THE CHEMICAL LEVEL OF ORGANIZATION 49




Potassium (K), for instance, is an important element in all body cells. Its atomic number is 19. It has just one electron in
its valence shell. This characteristic makes potassium highly likely to participate in chemical reactions in which it donates
one electron. (It is easier for potassium to donate one electron than to gain seven electrons.) The loss will cause the positive
charge of potassium’s protons to be more influential than the negative charge of potassium’s electrons. In other words, the
resulting potassium ion will be slightly positive. A potassium ion is written K+, indicating that it has lost a single electron.
A positively charged ion is known as a cation.
Now consider fluorine (F), a component of bones and teeth. Its atomic number is nine, and it has seven electrons in its
valence shell. Thus, it is highly likely to bond with other atoms in such a way that fluorine accepts one electron (it is easier
for fluorine to gain one electron than to donate seven electrons). When it does, its electrons will outnumber its protons by
one, and it will have an overall negative charge. The ionized form of fluorine is called fluoride, and is written as F–. A
negatively charged ion is known as an anion.
Atoms that have more than one electron to donate or accept will end up with stronger positive or negative charges. A cation
that has donated two electrons has a net charge of +2. Using magnesium (Mg) as an example, this can be written Mg++ or
Mg2+. An anion that has accepted two electrons has a net charge of –2. The ionic form of selenium (Se), for example, is
typically written Se2–.
The opposite charges of cations and anions exert a moderately strong mutual attraction that keeps the atoms in close
proximity forming an ionic bond. An ionic bond is an ongoing, close association between ions of opposite charge. The table
salt you sprinkle on your food owes its existence to ionic bonding. As shown in Figure 2.8, sodium commonly donates an
electron to chlorine, becoming the cation Na+. When chlorine accepts the electron, it becomes the chloride anion, Cl–. With
their opposing charges, these two ions strongly attract each other.


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Figure 2.8 Ionic Bonding (a) Sodium readily donates the solitary electron in its valence shell to chlorine, which needs
only one electron to have a full valence shell. (b) The opposite electrical charges of the resulting sodium cation and
chloride anion result in the formation of a bond of attraction called an ionic bond. (c) The attraction of many sodium
and chloride ions results in the formation of large groupings called crystals.


Water is an essential component of life because it is able to break the ionic bonds in salts to free the ions. In fact, in
biological fluids, most individual atoms exist as ions. These dissolved ions produce electrical charges within the body. The
behavior of these ions produces the tracings of heart and brain function observed as waves on an electrocardiogram (EKG
or ECG) or an electroencephalogram (EEG). The electrical activity that derives from the interactions of the charged ions is
why they are also called electrolytes.


Covalent Bonds
Unlike ionic bonds formed by the attraction between a cation’s positive charge and an anion’s negative charge, molecules
formed by a covalent bond share electrons in a mutually stabilizing relationship. Like next-door neighbors whose kids hang
out first at one home and then at the other, the atoms do not lose or gain electrons permanently. Instead, the electrons move
back and forth between the elements. Because of the close sharing of pairs of electrons (one electron from each of two
atoms), covalent bonds are stronger than ionic bonds.


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Nonpolar Covalent Bonds
Figure 2.9 shows several common types of covalent bonds. Notice that the two covalently bonded atoms typically share
just one or two electron pairs, though larger sharings are possible. The important concept to take from this is that in covalent
bonds, electrons in the outermost valence shell are shared to fill the valence shells of both atoms, ultimately stabilizing both
of the atoms involved. In a single covalent bond, a single electron is shared between two atoms, while in a double covalent
bond, two pairs of electrons are shared between two atoms. There even are triple covalent bonds, where three atoms are
shared.


Figure 2.9 Covalent Bonding


You can see that the covalent bonds shown in Figure 2.9 are balanced. The sharing of the negative electrons is relatively
equal, as is the electrical pull of the positive protons in the nucleus of the atoms involved. This is why covalently bonded
molecules that are electrically balanced in this way are described as nonpolar; that is, no region of the molecule is either
more positive or more negative than any other.
Polar Covalent Bonds
Groups of legislators with completely opposite views on a particular issue are often described as “polarized” by news
writers. In chemistry, a polar molecule is a molecule that contains regions that have opposite electrical charges. Polar
molecules occur when atoms share electrons unequally, in polar covalent bonds.
The most familiar example of a polar molecule is water (Figure 2.10). The molecule has three parts: one atom of oxygen,
the nucleus of which contains eight protons, and two hydrogen atoms, whose nuclei each contain only one proton. Because
every proton exerts an identical positive charge, a nucleus that contains eight protons exerts a charge eight times greater
than a nucleus that contains one proton. This means that the negatively charged electrons present in the water molecule are
more strongly attracted to the oxygen nucleus than to the hydrogen nuclei. Each hydrogen atom’s single negative electron
therefore migrates toward the oxygen atom, making the oxygen end of their bond slightly more negative than the hydrogen
end of their bond.


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Figure 2.10 Polar Covalent Bonds in a Water Molecule


What is true for the bonds is true for the water molecule as a whole; that is, the oxygen region has a slightly negative
charge and the regions of the hydrogen atoms have a slightly positive charge. These charges are often referred to as “partial
charges” because the strength of the charge is less than one full electron, as would occur in an ionic bond. As shown in
Figure 2.10, regions of weak polarity are indicated with the Greek letter delta (∂) and a plus (+) or minus (–) sign.
Even though a single water molecule is unimaginably tiny, it has mass, and the opposing electrical charges on the molecule
pull that mass in such a way that it creates a shape somewhat like a triangular tent (see Figure 2.10b). This dipole, with the
positive charges at one end formed by the hydrogen atoms at the “bottom” of the tent and the negative charge at the opposite
end (the oxygen atom at the “top” of the tent) makes the charged regions highly likely to interact with charged regions
of other polar molecules. For human physiology, the resulting bond is one of the most important formed by water—the
hydrogen bond.


Hydrogen Bonds
A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for
example, the oxygen in the water molecule) is attracted to another electronegative atom from another molecule. In other
words, hydrogen bonds always include hydrogen that is already part of a polar molecule.
The most common example of hydrogen bonding in the natural world occurs between molecules of water. It happens before
your eyes whenever two raindrops merge into a larger bead, or a creek spills into a river. Hydrogen bonding occurs because
the weakly negative oxygen atom in one water molecule is attracted to the weakly positive hydrogen atoms of two other
water molecules (Figure 2.11).


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Figure 2.11 Hydrogen Bonds between Water Molecules Notice that the bonds occur between the weakly positive
charge on the hydrogen atoms and the weakly negative charge on the oxygen atoms. Hydrogen bonds are relatively
weak, and therefore are indicated with a dotted (rather than a solid) line.


Water molecules also strongly attract other types of charged molecules as well as ions. This explains why “table salt,” for
example, actually is a molecule called a “salt” in chemistry, which consists of equal numbers of positively-charged sodium
(Na+) and negatively-charged chloride (Cl–), dissolves so readily in water, in this case forming dipole-ion bonds between
the water and the electrically-charged ions (electrolytes). Water molecules also repel molecules with nonpolar covalent
bonds, like fats, lipids, and oils. You can demonstrate this with a simple kitchen experiment: pour a teaspoon of vegetable
oil, a compound formed by nonpolar covalent bonds, into a glass of water. Instead of instantly dissolving in the water, the
oil forms a distinct bead because the polar water molecules repel the nonpolar oil.


2.3 | Chemical Reactions
By the end of this section, you will be able to:
• Distinguish between kinetic and potential energy, and between exergonic and endergonic chemical reactions
• Identify four forms of energy important in human functioning
• Describe the three basic types of chemical reactions
• Identify several factors influencing the rate of chemical reactions


One characteristic of a living organism is metabolism, which is the sum total of all of the chemical reactions that go
on to maintain that organism’s health and life. The bonding processes you have learned thus far are anabolic chemical
reactions; that is, they form larger molecules from smaller molecules or atoms. But recall that metabolism can proceed in
another direction: in catabolic chemical reactions, bonds between components of larger molecules break, releasing smaller
molecules or atoms. Both types of reaction involve exchanges not only of matter, but of energy.


The Role of Energy in Chemical Reactions
Chemical reactions require a sufficient amount of energy to cause the matter to collide with enough precision and force
that old chemical bonds can be broken and new ones formed. In general, kinetic energy is the form of energy powering
any type of matter in motion. Imagine you are building a brick wall. The energy it takes to lift and place one brick atop
another is kinetic energy—the energy matter possesses because of its motion. Once the wall is in place, it stores potential
energy. Potential energy is the energy of position, or the energy matter possesses because of the positioning or structure of
its components. If the brick wall collapses, the stored potential energy is released as kinetic energy as the bricks fall.
In the human body, potential energy is stored in the bonds between atoms and molecules. Chemical energy is the form of
potential energy in which energy is stored in chemical bonds. When those bonds are formed, chemical energy is invested,
and when they break, chemical energy is released. Notice that chemical energy, like all energy, is neither created nor
destroyed; rather, it is converted from one form to another. When you eat an energy bar before heading out the door for a
hike, the honey, nuts, and other foods the bar contains are broken down and rearranged by your body into molecules that
your muscle cells convert to kinetic energy.
Chemical reactions that release more energy than they absorb are characterized as exergonic. The catabolism of the foods
in your energy bar is an example. Some of the chemical energy stored in the bar is absorbed into molecules your body uses
for fuel, but some of it is released—for example, as heat. In contrast, chemical reactions that absorb more energy than they
release are endergonic. These reactions require energy input, and the resulting molecule stores not only the chemical energy


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in the original components, but also the energy that fueled the reaction. Because energy is neither created nor destroyed,
where does the energy needed for endergonic reactions come from? In many cases, it comes from exergonic reactions.


Forms of Energy Important in Human Functioning
You have already learned that chemical energy is absorbed, stored, and released by chemical bonds. In addition to chemical
energy, mechanical, radiant, and electrical energy are important in human functioning.
• Mechanical energy, which is stored in physical systems such as machines, engines, or the human body, directly powers
the movement of matter. When you lift a brick into place on a wall, your muscles provide the mechanical energy that
moves the brick.


• Radiant energy is energy emitted and transmitted as waves rather than matter. These waves vary in length from long
radio waves and microwaves to short gamma waves emitted from decaying atomic nuclei. The full spectrum of radiant
energy is referred to as the electromagnetic spectrum. The body uses the ultraviolet energy of sunlight to convert a
compound in skin cells to vitamin D, which is essential to human functioning. The human eye evolved to see the
wavelengths that comprise the colors of the rainbow, from red to violet, so that range in the spectrum is called “visible
light.”


• Electrical energy, supplied by electrolytes in cells and body fluids, contributes to the voltage changes that help transmit
impulses in nerve and muscle cells.


Characteristics of Chemical Reactions
All chemical reactions begin with a reactant, the general term for the one or more substances that enter into the reaction.
Sodium and chloride ions, for example, are the reactants in the production of table salt. The one or more substances
produced by a chemical reaction are called the product.
In chemical reactions, the components of the reactants—the elements involved and the number of atoms of each—are all
present in the product(s). Similarly, there is nothing present in the products that are not present in the reactants. This is
because chemical reactions are governed by the law of conservation of mass, which states that matter cannot be created or
destroyed in a chemical reaction.
Just as you can express mathematical calculations in equations such as 2 + 7 = 9, you can use chemical equations
to show how reactants become products. As in math, chemical equations proceed from left to right, but instead of
an equal sign, they employ an arrow or arrows indicating the direction in which the chemical reaction proceeds. For
example, the chemical reaction in which one atom of nitrogen and three atoms of hydrogen produce ammonia would
be written as N + 3H → NH3 . Correspondingly, the breakdown of ammonia into its components would be written as
NH3 → N + 3H.


Notice that, in the first example, a nitrogen (N) atom and three hydrogen (H) atoms bond to form a compound. This anabolic
reaction requires energy, which is then stored within the compound’s bonds. Such reactions are referred to as synthesis
reactions. A synthesis reaction is a chemical reaction that results in the synthesis (joining) of components that were
formerly separate (Figure 2.12a). Again, nitrogen and hydrogen are reactants in a synthesis reaction that yields ammonia
as the product. The general equation for a synthesis reaction is A + B → AB.


Figure 2.12 The Three Fundamental Chemical Reactions The atoms and molecules involved in the three
fundamental chemical reactions can be imagined as words.


In the second example, ammonia is catabolized into its smaller components, and the potential energy that had been stored
in its bonds is released. Such reactions are referred to as decomposition reactions. A decomposition reaction is a chemical


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reaction that breaks down or “de-composes” something larger into its constituent parts (see Figure 2.12b). The general
equation for a decomposition reaction is: AB → A + B .
An exchange reaction is a chemical reaction in which both synthesis and decomposition occur, chemical bonds are both
formed and broken, and chemical energy is absorbed, stored, and released (see Figure 2.12c). The simplest form of an
exchange reaction might be: A + BC → AB + C . Notice that, to produce these products, B and C had to break apart in a
decomposition reaction, whereas A and B had to bond in a synthesis reaction. A more complex exchange reaction might be:
AB + CD → AC + BD . Another example might be: AB + CD → AD + BC .
In theory, any chemical reaction can proceed in either direction under the right conditions. Reactants may synthesize into a
product that is later decomposed. Reversibility is also a quality of exchange reactions. For instance, A + BC → AB + C
could then reverse to AB + C → A + BC . This reversibility of a chemical reaction is indicated with a double arrow:
A + BC ⇄ AB + C . Still, in the human body, many chemical reactions do proceed in a predictable direction, either one
way or the other. You can think of this more predictable path as the path of least resistance because, typically, the alternate
direction requires more energy.


Factors Influencing the Rate of Chemical Reactions
If you pour vinegar into baking soda, the reaction is instantaneous; the concoction will bubble and fizz. But many chemical
reactions take time. A variety of factors influence the rate of chemical reactions. This section, however, will consider only
the most important in human functioning.
Properties of the Reactants
If chemical reactions are to occur quickly, the atoms in the reactants have to have easy access to one another. Thus, the
greater the surface area of the reactants, the more readily they will interact. When you pop a cube of cheese into your mouth,
you chew it before you swallow it. Among other things, chewing increases the surface area of the food so that digestive
chemicals can more easily get at it. As a general rule, gases tend to react faster than liquids or solids, again because it takes
energy to separate particles of a substance, and gases by definition already have space between their particles. Similarly, the
larger the molecule, the greater the number of total bonds, so reactions involving smaller molecules, with fewer total bonds,
would be expected to proceed faster.
In addition, recall that some elements are more reactive than others. Reactions that involve highly reactive elements like
hydrogen proceed more quickly than reactions that involve less reactive elements. Reactions involving stable elements like
helium are not likely to happen at all.
Temperature
Nearly all chemical reactions occur at a faster rate at higher temperatures. Recall that kinetic energy is the energy of matter
in motion. The kinetic energy of subatomic particles increases in response to increases in thermal energy. The higher the
temperature, the faster the particles move, and the more likely they are to come in contact and react.
Concentration and Pressure
If just a few people are dancing at a club, they are unlikely to step on each other’s toes. But as more and more people
get up to dance—especially if the music is fast—collisions are likely to occur. It is the same with chemical reactions: the
more particles present within a given space, the more likely those particles are to bump into one another. This means that
chemists can speed up chemical reactions not only by increasing the concentration of particles—the number of particles
in the space—but also by decreasing the volume of the space, which would correspondingly increase the pressure. If there
were 100 dancers in that club, and the manager abruptly moved the party to a room half the size, the concentration of the
dancers would double in the new space, and the likelihood of collisions would increase accordingly.
Enzymes and Other Catalysts
For two chemicals in nature to react with each other they first have to come into contact, and this occurs through random
collisions. Because heat helps increase the kinetic energy of atoms, ions, and molecules, it promotes their collision. But in
the body, extremely high heat—such as a very high fever—can damage body cells and be life-threatening. On the other
hand, normal body temperature is not high enough to promote the chemical reactions that sustain life. That is where catalysts
come in.
In chemistry, a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any change.
You can think of a catalyst as a chemical change agent. They help increase the rate and force at which atoms, ions, and
molecules collide, thereby increasing the probability that their valence shell electrons will interact.
The most important catalysts in the human body are enzymes. An enzyme is a catalyst composed of protein or ribonucleic
acid (RNA), both of which will be discussed later in this chapter. Like all catalysts, enzymes work by lowering the level of
energy that needs to be invested in a chemical reaction. A chemical reaction’s activation energy is the “threshold” level of
energy needed to break the bonds in the reactants. Once those bonds are broken, new arrangements can form. Without an
enzyme to act as a catalyst, a much larger investment of energy is needed to ignite a chemical reaction (Figure 2.13).


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Figure 2.13 Enzymes Enzymes decrease the activation energy required for a given chemical reaction to occur. (a)
Without an enzyme, the energy input needed for a reaction to begin is high. (b) With the help of an enzyme, less
energy is needed for a reaction to begin.


Enzymes are critical to the body’s healthy functioning. They assist, for example, with the breakdown of food and its
conversion to energy. In fact, most of the chemical reactions in the body are facilitated by enzymes.


2.4 | Inorganic Compounds Essential to Human
Functioning
By the end of this section, you will be able to:
• Compare and contrast inorganic and organic compounds
• Identify the properties of water that make it essential to life
• Explain the role of salts in body functioning
• Distinguish between acids and bases, and explain their role in pH
• Discuss the role of buffers in helping the body maintain pH homeostasis


The concepts you have learned so far in this chapter govern all forms of matter, and would work as a foundation for geology
as well as biology. This section of the chapter narrows the focus to the chemistry of human life; that is, the compounds
important for the body’s structure and function. In general, these compounds are either inorganic or organic.
• An inorganic compound is a substance that does not contain both carbon and hydrogen. A great many inorganic
compounds do contain hydrogen atoms, such as water (H2O) and the hydrochloric acid (HCl) produced by your
stomach. In contrast, only a handful of inorganic compounds contain carbon atoms. Carbon dioxide (CO2) is one of
the few examples.


• An organic compound, then, is a substance that contains both carbon and hydrogen. Organic compounds are
synthesized via covalent bonds within living organisms, including the human body. Recall that carbon and hydrogen
are the second and third most abundant elements in your body. You will soon discover how these two elements
combine in the foods you eat, in the compounds that make up your body structure, and in the chemicals that fuel your
functioning.


The following section examines the three groups of inorganic compounds essential to life: water, salts, acids, and bases.
Organic compounds are covered later in the chapter.


Water
As much as 70 percent of an adult’s body weight is water. This water is contained both within the cells and between the
cells that make up tissues and organs. Its several roles make water indispensable to human functioning.
Water as a Lubricant and Cushion
Water is a major component of many of the body’s lubricating fluids. Just as oil lubricates the hinge on a door, water
in synovial fluid lubricates the actions of body joints, and water in pleural fluid helps the lungs expand and recoil with


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breathing. Watery fluids help keep food flowing through the digestive tract, and ensure that the movement of adjacent
abdominal organs is friction free.
Water also protects cells and organs from physical trauma, cushioning the brain within the skull, for example, and protecting
the delicate nerve tissue of the eyes. Water cushions a developing fetus in the mother’s womb as well.
Water as a Heat Sink
A heat sink is a substance or object that absorbs and dissipates heat but does not experience a corresponding increase in
temperature. In the body, water absorbs the heat generated by chemical reactions without greatly increasing in temperature.
Moreover, when the environmental temperature soars, the water stored in the body helps keep the body cool. This cooling
effect happens as warm blood from the body’s core flows to the blood vessels just under the skin and is transferred to the
environment. At the same time, sweat glands release warm water in sweat. As the water evaporates into the air, it carries
away heat, and then the cooler blood from the periphery circulates back to the body core.
Water as a Component of Liquid Mixtures
A mixture is a combination of two or more substances, each of which maintains its own chemical identity. In other
words, the constituent substances are not chemically bonded into a new, larger chemical compound. The concept is easy
to imagine if you think of powdery substances such as flour and sugar; when you stir them together in a bowl, they
obviously do not bond to form a new compound. The room air you breathe is a gaseous mixture, containing three discrete
elements—nitrogen, oxygen, and argon—and one compound, carbon dioxide. There are three types of liquid mixtures, all
of which contain water as a key component. These are solutions, colloids, and suspensions.
For cells in the body to survive, they must be kept moist in a water-based liquid called a solution. In chemistry, a liquid
solution consists of a solvent that dissolves a substance called a solute. An important characteristic of solutions is that they
are homogeneous; that is, the solute molecules are distributed evenly throughout the solution. If you were to stir a teaspoon
of sugar into a glass of water, the sugar would dissolve into sugar molecules separated by water molecules. The ratio of
sugar to water in the left side of the glass would be the same as the ratio of sugar to water in the right side of the glass.
If you were to add more sugar, the ratio of sugar to water would change, but the distribution—provided you had stirred
well—would still be even.
Water is considered the “universal solvent” and it is believed that life cannot exist without water because of this. Water is
certainly the most abundant solvent in the body; essentially all of the body’s chemical reactions occur among compounds
dissolved in water. Because water molecules are polar, with regions of positive and negative electrical charge, water
readily dissolves ionic compounds and polar covalent compounds. Such compounds are referred to as hydrophilic, or
“water-loving.” As mentioned above, sugar dissolves well in water. This is because sugar molecules contain regions of
hydrogen-oxygen polar bonds, making it hydrophilic. Nonpolar molecules, which do not readily dissolve in water, are called
hydrophobic, or “water-fearing.”
Concentrations of Solutes
Various mixtures of solutes and water are described in chemistry. The concentration of a given solute is the number of
particles of that solute in a given space (oxygen makes up about 21 percent of atmospheric air). In the bloodstream of
humans, glucose concentration is usually measured in milligram (mg) per deciliter (dL), and in a healthy adult averages
about 100 mg/dL. Another method of measuring the concentration of a solute is by its molarilty—which is moles (M) of the
molecules per liter (L). The mole of an element is its atomic weight, while a mole of a compound is the sum of the atomic
weights of its components, called the molecular weight. An often-used example is calculating a mole of glucose, with the
chemical formula C6H12O6. Using the periodic table, the atomic weight of carbon (C) is 12.011 grams (g), and there are six
carbons in glucose, for a total atomic weight of 72.066 g. Doing the same calculations for hydrogen (H) and oxygen (O),
the molecular weight equals 180.156g (the “gram molecular weight” of glucose). When water is added to make one liter of
solution, you have one mole (1M) of glucose. This is particularly useful in chemistry because of the relationship of moles
to “Avogadro’s number.” A mole of any solution has the same number of particles in it: 6.02 × 1023. Many substances in
the bloodstream and other tissue of the body are measured in thousandths of a mole, or millimoles (mM).
A colloid is a mixture that is somewhat like a heavy solution. The solute particles consist of tiny clumps of molecules large
enough to make the liquid mixture opaque (because the particles are large enough to scatter light). Familiar examples of
colloids are milk and cream. In the thyroid glands, the thyroid hormone is stored as a thick protein mixture also called a
colloid.
A suspension is a liquid mixture in which a heavier substance is suspended temporarily in a liquid, but over time, settles
out. This separation of particles from a suspension is called sedimentation. An example of sedimentation occurs in the blood
test that establishes sedimentation rate, or sed rate. The test measures how quickly red blood cells in a test tube settle out
of the watery portion of blood (known as plasma) over a set period of time. Rapid sedimentation of blood cells does not
normally happen in the healthy body, but aspects of certain diseases can cause blood cells to clump together, and these
heavy clumps of blood cells settle to the bottom of the test tube more quickly than do normal blood cells.
The Role of Water in Chemical Reactions
Two types of chemical reactions involve the creation or the consumption of water: dehydration synthesis and hydrolysis.


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• In dehydration synthesis, one reactant gives up an atom of hydrogen and another reactant gives up a hydroxyl group
(OH) in the synthesis of a new product. In the formation of their covalent bond, a molecule of water is released as a
byproduct (Figure 2.14). This is also sometimes referred to as a condensation reaction.


• In hydrolysis, a molecule of water disrupts a compound, breaking its bonds. The water is itself split into H and OH.
One portion of the severed compound then bonds with the hydrogen atom, and the other portion bonds with the
hydroxyl group.


These reactions are reversible, and play an important role in the chemistry of organic compounds (which will be discussed
shortly).


Figure 2.14 Dehydration Synthesis and Hydrolysis Monomers, the basic units for building larger molecules, form
polymers (two or more chemically-bonded monomers). (a) In dehydration synthesis, two monomers are covalently
bonded in a reaction in which one gives up a hydroxyl group and the other a hydrogen atom. A molecule of water is
released as a byproduct during dehydration reactions. (b) In hydrolysis, the covalent bond between two monomers is
split by the addition of a hydrogen atom to one and a hydroxyl group to the other, which requires the contribution of
one molecule of water.


Salts
Recall that salts are formed when ions form ionic bonds. In these reactions, one atom gives up one or more electrons, and
thus becomes positively charged, whereas the other accepts one or more electrons and becomes negatively charged. You
can now define a salt as a substance that, when dissolved in water, dissociates into ions other than H+ or OH–. This fact is
important in distinguishing salts from acids and bases, discussed next.
A typical salt, NaCl, dissociates completely in water (Figure 2.15). The positive and negative regions on the water molecule
(the hydrogen and oxygen ends respectively) attract the negative chloride and positive sodium ions, pulling them away
from each other. Again, whereas nonpolar and polar covalently bonded compounds break apart into molecules in solution,
salts dissociate into ions. These ions are electrolytes; they are capable of conducting an electrical current in solution. This
property is critical to the function of ions in transmitting nerve impulses and prompting muscle contraction.


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Figure 2.15 Dissociation of Sodium Chloride in Water Notice that the crystals of sodium chloride dissociate not
into molecules of NaCl, but into Na+ cations and Cl– anions, each completely surrounded by water molecules.


Many other salts are important in the body. For example, bile salts produced by the liver help break apart dietary fats, and
calcium phosphate salts form the mineral portion of teeth and bones.


Acids and Bases
Acids and bases, like salts, dissociate in water into electrolytes. Acids and bases can very much change the properties of the
solutions in which they are dissolved.
Acids
An acid is a substance that releases hydrogen ions (H+) in solution (Figure 2.16a). Because an atom of hydrogen has just
one proton and one electron, a positively charged hydrogen ion is simply a proton. This solitary proton is highly likely to
participate in chemical reactions. Strong acids are compounds that release all of their H+ in solution; that is, they ionize
completely. Hydrochloric acid (HCl), which is released from cells in the lining of the stomach, is a strong acid because it
releases all of its H+ in the stomach’s watery environment. This strong acid aids in digestion and kills ingested microbes.
Weak acids do not ionize completely; that is, some of their hydrogen ions remain bonded within a compound in solution.
An example of a weak acid is vinegar, or acetic acid; it is called acetate after it gives up a proton.


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Figure 2.16 Acids and Bases (a) In aqueous solution, an acid dissociates into hydrogen ions (H+) and anions. Nearly
every molecule of a strong acid dissociates, producing a high concentration of H+. (b) In aqueous solution, a base
dissociates into hydroxyl ions (OH–) and cations. Nearly every molecule of a strong base dissociates, producing a high
concentration of OH–.


Bases
A base is a substance that releases hydroxyl ions (OH–) in solution, or one that accepts H+ already present in solution (see
Figure 2.16b). The hydroxyl ions or other base combine with H+ present to form a water molecule, thereby removing H+
and reducing the solution’s acidity. Strong bases release most or all of their hydroxyl ions; weak bases release only some
hydroxyl ions or absorb only a few H+. Food mixed with hydrochloric acid from the stomach would burn the small intestine,
the next portion of the digestive tract after the stomach, if it were not for the release of bicarbonate (HCO3–), a weak base
that attracts H+. Bicarbonate accepts some of the H+ protons, thereby reducing the acidity of the solution.
The Concept of pH
The relative acidity or alkalinity of a solution can be indicated by its pH. A solution’s pH is the negative, base-10 logarithm
of the hydrogen ion (H+) concentration of the solution. As an example, a pH 4 solution has an H+ concentration that is ten
times greater than that of a pH 5 solution. That is, a solution with a pH of 4 is ten times more acidic than a solution with a
pH of 5. The concept of pH will begin to make more sense when you study the pH scale, like that shown in Figure 2.17.
The scale consists of a series of increments ranging from 0 to 14. A solution with a pH of 7 is considered neutral—neither
acidic nor basic. Pure water has a pH of 7. The lower the number below 7, the more acidic the solution, or the greater
the concentration of H+. The concentration of hydrogen ions at each pH value is 10 times different than the next pH. For
instance, a pH value of 4 corresponds to a proton concentration of 10–4M, or 0.0001M, while a pH value of 5 corresponds
to a proton concentration of 10–5 M, or 0.00001M. The higher the number above 7, the more basic (alkaline) the solution,
or the lower the concentration of H+. Human urine, for example, is ten times more acidic than pure water, and HCl is
10,000,000 times more acidic than water.


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Figure 2.17 The pH Scale


Buffers
The pH of human blood normally ranges from 7.35 to 7.45, although it is typically identified as pH 7.4. At this slightly basic
pH, blood can reduce the acidity resulting from the carbon dioxide (CO2) constantly being released into the bloodstream
by the trillions of cells in the body. Homeostatic mechanisms (along with exhaling CO2 while breathing) normally keep the
pH of blood within this narrow range. This is critical, because fluctuations—either too acidic or too alkaline—can lead to
life-threatening disorders.
All cells of the body depend on homeostatic regulation of acid–base balance at a pH of approximately 7.4. The body
therefore has several mechanisms for this regulation, involving breathing, the excretion of chemicals in urine, and the
internal release of chemicals collectively called buffers into body fluids. A buffer is a solution of a weak acid and its
conjugate base. A buffer can neutralize small amounts of acids or bases in body fluids. For example, if there is even a slight
decrease below 7.35 in the pH of a bodily fluid, the buffer in the fluid—in this case, acting as a weak base—will bind the
excess hydrogen ions. In contrast, if pH rises above 7.45, the buffer will act as a weak acid and contribute hydrogen ions.


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Acids and Bases
Excessive acidity of the blood and other body fluids is known as acidosis. Common causes of acidosis are situations
and disorders that reduce the effectiveness of breathing, especially the person’s ability to exhale fully, which causes a
buildup of CO2 (and H+) in the bloodstream. Acidosis can also be caused by metabolic problems that reduce the level
or function of buffers that act as bases, or that promote the production of acids. For instance, with severe diarrhea,
too much bicarbonate can be lost from the body, allowing acids to build up in body fluids. In people with poorly
managed diabetes (ineffective regulation of blood sugar), acids called ketones are produced as a form of body fuel.
These can build up in the blood, causing a serious condition called diabetic ketoacidosis. Kidney failure, liver failure,
heart failure, cancer, and other disorders also can prompt metabolic acidosis.
In contrast, alkalosis is a condition in which the blood and other body fluids are too alkaline (basic). As with acidosis,
respiratory disorders are a major cause; however, in respiratory alkalosis, carbon dioxide levels fall too low. Lung
disease, aspirin overdose, shock, and ordinary anxiety can cause respiratory alkalosis, which reduces the normal
concentration of H+.
Metabolic alkalosis often results from prolonged, severe vomiting, which causes a loss of hydrogen and chloride ions
(as components of HCl). Medications also can prompt alkalosis. These include diuretics that cause the body to lose
potassium ions, as well as antacids when taken in excessive amounts, for instance by someone with persistent heartburn
or an ulcer.


2.5 | Organic Compounds Essential to Human
Functioning
By the end of this section, you will be able to:
• Identify four types of organic molecules essential to human functioning
• Explain the chemistry behind carbon’s affinity for covalently bonding in organic compounds
• Provide examples of three types of carbohydrates, and identify the primary functions of carbohydrates in the body
• Discuss four types of lipids important in human functioning
• Describe the structure of proteins, and discuss their importance to human functioning
• Identify the building blocks of nucleic acids, and the roles of DNA, RNA, and ATP in human functioning


Organic compounds typically consist of groups of carbon atoms covalently bonded to hydrogen, usually oxygen, and
often other elements as well. Created by living things, they are found throughout the world, in soils and seas, commercial
products, and every cell of the human body. The four types most important to human structure and function are
carbohydrates, lipids, proteins, and nucleotides. Before exploring these compounds, you need to first understand the
chemistry of carbon.


The Chemistry of Carbon
What makes organic compounds ubiquitous is the chemistry of their carbon core. Recall that carbon atoms have four
electrons in their valence shell, and that the octet rule dictates that atoms tend to react in such a way as to complete
their valence shell with eight electrons. Carbon atoms do not complete their valence shells by donating or accepting four
electrons. Instead, they readily share electrons via covalent bonds.
Commonly, carbon atoms share with other carbon atoms, often forming a long carbon chain referred to as a carbon skeleton.
When they do share, however, they do not share all their electrons exclusively with each other. Rather, carbon atoms tend
to share electrons with a variety of other elements, one of which is always hydrogen. Carbon and hydrogen groupings are
called hydrocarbons. If you study the figures of organic compounds in the remainder of this chapter, you will see several
with chains of hydrocarbons in one region of the compound.
Many combinations are possible to fill carbon’s four “vacancies.” Carbon may share electrons with oxygen or nitrogen or
other atoms in a particular region of an organic compound. Moreover, the atoms to which carbon atoms bond may also be
part of a functional group. A functional group is a group of atoms linked by strong covalent bonds and tending to function


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in chemical reactions as a single unit. You can think of functional groups as tightly knit “cliques” whose members are
unlikely to be parted. Five functional groups are important in human physiology; these are the hydroxyl, carboxyl, amino,
methyl and phosphate groups (Table 2.1).


Functional Groups Important in Human Physiology
Functional
group


Structural
formula Importance


Hydroxyl —O—H
Hydroxyl groups are polar. They are components of all four types of organic
compounds discussed in this chapter. They are involved in dehydration
synthesis and hydrolysis reactions.


Carboxyl O—C—OH Carboxyl groups are found within fatty acids, amino acids, and many otheracids.
Amino —N—H2 Amino groups are found within amino acids, the building blocks of proteins.
Methyl —C—H3 Methyl groups are found within amino acids.
Phosphate —P—O42– Phosphate groups are found within phospholipids and nucleotides.
Table 2.1


Carbon’s affinity for covalent bonding means that many distinct and relatively stable organic molecules nevertheless readily
form larger, more complex molecules. Any large molecule is referred to as macromolecule (macro- = “large”), and the
organic compounds in this section all fit this description. However, some macromolecules are made up of several “copies”
of single units called monomer (mono- = “one”; -mer = “part”). Like beads in a long necklace, these monomers link by
covalent bonds to form long polymers (poly- = “many”). There are many examples of monomers and polymers among the
organic compounds.
Monomers form polymers by engaging in dehydration synthesis (see Figure 2.14). As was noted earlier, this reaction results
in the release of a molecule of water. Each monomer contributes: One gives up a hydrogen atom and the other gives up a
hydroxyl group. Polymers are split into monomers by hydrolysis (-lysis = “rupture”). The bonds between their monomers
are broken, via the donation of a molecule of water, which contributes a hydrogen atom to one monomer and a hydroxyl
group to the other.


Carbohydrates
The term carbohydrate means “hydrated carbon.” Recall that the root hydro- indicates water. A carbohydrate is a molecule
composed of carbon, hydrogen, and oxygen; in most carbohydrates, hydrogen and oxygen are found in the same two-to-one
relative proportions they have in water. In fact, the chemical formula for a “generic” molecule of carbohydrate is (CH2O)n.
Carbohydrates are referred to as saccharides, a word meaning “sugars.” Three forms are important in the body.
Monosaccharides are the monomers of carbohydrates. Disaccharides (di- = “two”) are made up of two monomers.
Polysaccharides are the polymers, and can consist of hundreds to thousands of monomers.
Monosaccharides
Amonosaccharide is a monomer of carbohydrates. Five monosaccharides are important in the body. Three of these are the
hexose sugars, so called because they each contain six atoms of carbon. These are glucose, fructose, and galactose, shown
in Figure 2.18a. The remaining monosaccharides are the two pentose sugars, each of which contains five atoms of carbon.
They are ribose and deoxyribose, shown in Figure 2.18b.


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Figure 2.18 Five Important Monosaccharides


Disaccharides
A disaccharide is a pair of monosaccharides. Disaccharides are formed via dehydration synthesis, and the bond linking
them is referred to as a glycosidic bond (glyco- = “sugar”). Three disaccharides (shown in Figure 2.19) are important to
humans. These are sucrose, commonly referred to as table sugar; lactose, or milk sugar; and maltose, or malt sugar. As you
can tell from their common names, you consume these in your diet; however, your body cannot use them directly. Instead,
in the digestive tract, they are split into their component monosaccharides via hydrolysis.


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Figure 2.19 Three Important Disaccharides All three important disaccharides form by dehydration synthesis.


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Watch this video (http://openstaxcollege.org/l/disaccharide) to observe the formation of a disaccharide. What
happens when water encounters a glycosidic bond?


Polysaccharides
Polysaccharides can contain a few to a thousand or more monosaccharides. Three are important to the body (Figure 2.20):
• Starches are polymers of glucose. They occur in long chains called amylose or branched chains called amylopectin,
both of which are stored in plant-based foods and are relatively easy to digest.


• Glycogen is also a polymer of glucose, but it is stored in the tissues of animals, especially in the muscles and liver. It is
not considered a dietary carbohydrate because very little glycogen remains in animal tissues after slaughter; however,
the human body stores excess glucose as glycogen, again, in the muscles and liver.


• Cellulose, a polysaccharide that is the primary component of the cell wall of green plants, is the component of plant
food referred to as “fiber”. In humans, cellulose/fiber is not digestible; however, dietary fiber has many health benefits.
It helps you feel full so you eat less, it promotes a healthy digestive tract, and a diet high in fiber is thought to reduce
the risk of heart disease and possibly some forms of cancer.


Figure 2.20 Three Important Polysaccharides Three important polysaccharides are starches, glycogen, and fiber.


Functions of Carbohydrates
The body obtains carbohydrates from plant-based foods. Grains, fruits, and legumes and other vegetables provide most of
the carbohydrate in the human diet, although lactose is found in dairy products.
Although most body cells can break down other organic compounds for fuel, all body cells can use glucose. Moreover,
nerve cells (neurons) in the brain, spinal cord, and through the peripheral nervous system, as well as red blood cells, can use
only glucose for fuel. In the breakdown of glucose for energy, molecules of adenosine triphosphate, better known as ATP,
are produced. Adenosine triphosphate (ATP) is composed of a ribose sugar, an adenine base, and three phosphate groups.
ATP releases free energy when its phosphate bonds are broken, and thus supplies ready energy to the cell. More ATP is
produced in the presence of oxygen (O2) than in pathways that do not use oxygen. The overall reaction for the conversion
of the energy in glucose to energy stored in ATP can be written:


C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP


In addition to being a critical fuel source, carbohydrates are present in very small amounts in cells’ structure. For instance,
some carbohydrate molecules bind with proteins to produce glycoproteins, and others combine with lipids to produce
glycolipids, both of which are found in the membrane that encloses the contents of body cells.


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Lipids
A lipid is one of a highly diverse group of compounds made up mostly of hydrocarbons. The few oxygen atoms they contain
are often at the periphery of the molecule. Their nonpolar hydrocarbons make all lipids hydrophobic. In water, lipids do not
form a true solution, but they may form an emulsion, which is the term for a mixture of solutions that do not mix well.
Triglycerides
A triglyceride is one of the most common dietary lipid groups, and the type found most abundantly in body tissues. This
compound, which is commonly referred to as a fat, is formed from the synthesis of two types of molecules (Figure 2.21):
• A glycerol backbone at the core of triglycerides, consists of three carbon atoms.
• Three fatty acids, long chains of hydrocarbons with a carboxyl group and a methyl group at opposite ends, extend from
each of the carbons of the glycerol.


Figure 2.21 Triglycerides Triglycerides are composed of glycerol attached to three fatty acids via dehydration
synthesis. Notice that glycerol gives up a hydrogen atom, and the carboxyl groups on the fatty acids each give up a
hydroxyl group.


Triglycerides form via dehydration synthesis. Glycerol gives up hydrogen atoms from its hydroxyl groups at each bond,
and the carboxyl group on each fatty acid chain gives up a hydroxyl group. A total of three water molecules are thereby
released.
Fatty acid chains that have no double carbon bonds anywhere along their length and therefore contain the maximum number
of hydrogen atoms are called saturated fatty acids. These straight, rigid chains pack tightly together and are solid or semi-
solid at room temperature (Figure 2.22a). Butter and lard are examples, as is the fat found on a steak or in your own body.
In contrast, fatty acids with one double carbon bond are kinked at that bond (Figure 2.22b). These monounsaturated fatty
acids are therefore unable to pack together tightly, and are liquid at room temperature. Polyunsaturated fatty acids contain
two or more double carbon bonds, and are also liquid at room temperature. Plant oils such as olive oil typically contain both
mono- and polyunsaturated fatty acids.


Figure 2.22 Fatty Acid Shapes The level of saturation of a fatty acid affects its shape. (a) Saturated fatty acid chains
are straight. (b) Unsaturated fatty acid chains are kinked.


Whereas a diet high in saturated fatty acids increases the risk of heart disease, a diet high in unsaturated fatty acids is thought
to reduce the risk. This is especially true for the omega-3 unsaturated fatty acids found in cold-water fish such as salmon.
These fatty acids have their first double carbon bond at the third hydrocarbon from the methyl group (referred to as the
omega end of the molecule).


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Finally, trans fatty acids found in some processed foods, including some stick and tub margarines, are thought to be even
more harmful to the heart and blood vessels than saturated fatty acids. Trans fats are created from unsaturated fatty acids
(such as corn oil) when chemically treated to produce partially hydrogenated fats.
As a group, triglycerides are a major fuel source for the body. When you are resting or asleep, a majority of the energy
used to keep you alive is derived from triglycerides stored in your fat (adipose) tissues. Triglycerides also fuel long, slow
physical activity such as gardening or hiking, and contribute a modest percentage of energy for vigorous physical activity.
Dietary fat also assists the absorption and transport of the nonpolar fat-soluble vitamins A, D, E, and K. Additionally, stored
body fat protects and cushions the body’s bones and internal organs, and acts as insulation to retain body heat.
Fatty acids are also components of glycolipids, which are sugar-fat compounds found in the cell membrane. Lipoproteins
are compounds in which the hydrophobic triglycerides are packaged in protein envelopes for transport in body fluids.
Phospholipids
As its name suggests, a phospholipid is a bond between the glycerol component of a lipid and a phosphorous molecule.
In fact, phospholipids are similar in structure to triglycerides. However, instead of having three fatty acids, a phospholipid
is generated from a diglyceride, a glycerol with just two fatty acid chains (Figure 2.23). The third binding site on the
glycerol is taken up by the phosphate group, which in turn is attached to a polar “head” region of the molecule. Recall that
triglycerides are nonpolar and hydrophobic. This still holds for the fatty acid portion of a phospholipid compound. However,
the phosphate-containing group at the head of the compound is polar and thereby hydrophilic. In other words, one end of
the molecule can interact with oil, and the other end with water. This makes phospholipids ideal emulsifiers, compounds
that help disperse fats in aqueous liquids, and enables them to interact with both the watery interior of cells and the watery
solution outside of cells as components of the cell membrane.


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Figure 2.23 Other Important Lipids (a) Phospholipids are composed of two fatty acids, glycerol, and a phosphate
group. (b) Sterols are ring-shaped lipids. Shown here is cholesterol. (c) Prostaglandins are derived from unsaturated
fatty acids. Prostaglandin E2 (PGE2) includes hydroxyl and carboxyl groups.


Steroids
A steroid compound (referred to as a sterol) has as its foundation a set of four hydrocarbon rings bonded to a variety of
other atoms and molecules (see Figure 2.23b). Although both plants and animals synthesize sterols, the type that makes the
most important contribution to human structure and function is cholesterol, which is synthesized by the liver in humans and
animals and is also present in most animal-based foods. Like other lipids, cholesterol’s hydrocarbons make it hydrophobic;
however, it has a polar hydroxyl head that is hydrophilic. Cholesterol is an important component of bile acids, compounds
that help emulsify dietary fats. In fact, the word root chole- refers to bile. Cholesterol is also a building block of many
hormones, signaling molecules that the body releases to regulate processes at distant sites. Finally, like phospholipids,
cholesterol molecules are found in the cell membrane, where their hydrophobic and hydrophilic regions help regulate the
flow of substances into and out of the cell.
Prostaglandins
Like a hormone, a prostaglandin is one of a group of signaling molecules, but prostaglandins are derived from unsaturated
fatty acids (see Figure 2.23c). One reason that the omega-3 fatty acids found in fish are beneficial is that they stimulate
the production of certain prostaglandins that help regulate aspects of blood pressure and inflammation, and thereby reduce
the risk for heart disease. Prostaglandins also sensitize nerves to pain. One class of pain-relieving medications called
nonsteroidal anti-inflammatory drugs (NSAIDs) works by reducing the effects of prostaglandins.


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Proteins
You might associate proteins with muscle tissue, but in fact, proteins are critical components of all tissues and organs.
A protein is an organic molecule composed of amino acids linked by peptide bonds. Proteins include the keratin in the
epidermis of skin that protects underlying tissues, the collagen found in the dermis of skin, in bones, and in the meninges
that cover the brain and spinal cord. Proteins are also components of many of the body’s functional chemicals, including
digestive enzymes in the digestive tract, antibodies, the neurotransmitters that neurons use to communicate with other cells,
and the peptide-based hormones that regulate certain body functions (for instance, growth hormone). While carbohydrates
and lipids are composed of hydrocarbons and oxygen, all proteins also contain nitrogen (N), and many contain sulfur (S),
in addition to carbon, hydrogen, and oxygen.
Microstructure of Proteins
Proteins are polymers made up of nitrogen-containing monomers called amino acids. An amino acid is a molecule
composed of an amino group and a carboxyl group, together with a variable side chain. Just 20 different amino acids
contribute to nearly all of the thousands of different proteins important in human structure and function. Body proteins
contain a unique combination of a few dozen to a few hundred of these 20 amino acid monomers. All 20 of these amino
acids share a similar structure (Figure 2.24). All consist of a central carbon atom to which the following are bonded:
• a hydrogen atom
• an alkaline (basic) amino group NH2 (see Table 2.1)
• an acidic carboxyl group COOH (see Table 2.1)
• a variable group


Figure 2.24 Structure of an Amino Acid


Notice that all amino acids contain both an acid (the carboxyl group) and a base (the amino group) (amine = “nitrogen-
containing”). For this reason, they make excellent buffers, helping the body regulate acid–base balance. What distinguishes
the 20 amino acids from one another is their variable group, which is referred to as a side chain or an R-group. This group
can vary in size and can be polar or nonpolar, giving each amino acid its unique characteristics. For example, the side
chains of two amino acids—cysteine and methionine—contain sulfur. Sulfur does not readily participate in hydrogen bonds,
whereas all other amino acids do. This variation influences the way that proteins containing cysteine and methionine are
assembled.
Amino acids join via dehydration synthesis to form protein polymers (Figure 2.25). The unique bond holding amino acids
together is called a peptide bond. A peptide bond is a covalent bond between two amino acids that forms by dehydration
synthesis. A peptide, in fact, is a very short chain of amino acids. Strands containing fewer than about 100 amino acids are
generally referred to as polypeptides rather than proteins.


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Figure 2.25 Peptide Bond Different amino acids join together to form peptides, polypeptides, or proteins via
dehydration synthesis. The bonds between the amino acids are peptide bonds.


The body is able to synthesize most of the amino acids from components of other molecules; however, nine cannot be
synthesized and have to be consumed in the diet. These are known as the essential amino acids.
Free amino acids available for protein construction are said to reside in the amino acid pool within cells. Structures within
cells use these amino acids when assembling proteins. If a particular essential amino acid is not available in sufficient
quantities in the amino acid pool, however, synthesis of proteins containing it can slow or even cease.
Shape of Proteins
Just as a fork cannot be used to eat soup and a spoon cannot be used to spear meat, a protein’s shape is essential to its
function. A protein’s shape is determined, most fundamentally, by the sequence of amino acids of which it is made (Figure
2.26a). The sequence is called the primary structure of the protein.


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Figure 2.26 The Shape of Proteins (a) The primary structure is the sequence of amino acids that make up the
polypeptide chain. (b) The secondary structure, which can take the form of an alpha-helix or a beta-pleated sheet,
is maintained by hydrogen bonds between amino acids in different regions of the original polypeptide strand. (c) The
tertiary structure occurs as a result of further folding and bonding of the secondary structure. (d) The quaternary
structure occurs as a result of interactions between two or more tertiary subunits. The example shown here is
hemoglobin, a protein in red blood cells which transports oxygen to body tissues.


Although some polypeptides exist as linear chains, most are twisted or folded into more complex secondary structures that
form when bonding occurs between amino acids with different properties at different regions of the polypeptide. The most
common secondary structure is a spiral called an alpha-helix. If you were to take a length of string and simply twist it into
a spiral, it would not hold the shape. Similarly, a strand of amino acids could not maintain a stable spiral shape without
the help of hydrogen bonds, which create bridges between different regions of the same strand (see Figure 2.26b). Less
commonly, a polypeptide chain can form a beta-pleated sheet, in which hydrogen bonds form bridges between different
regions of a single polypeptide that has folded back upon itself, or between two or more adjacent polypeptide chains.
The secondary structure of proteins further folds into a compact three-dimensional shape, referred to as the protein’s tertiary
structure (see Figure 2.26c). In this configuration, amino acids that had been very distant in the primary chain can be
brought quite close via hydrogen bonds or, in proteins containing cysteine, via disulfide bonds. A disulfide bond is a
covalent bond between sulfur atoms in a polypeptide. Often, two or more separate polypeptides bond to form an even
larger protein with a quaternary structure (see Figure 2.26d). The polypeptide subunits forming a quaternary structure
can be identical or different. For instance, hemoglobin, the protein found in red blood cells is composed of four tertiary
polypeptides, two of which are called alpha chains and two of which are called beta chains.
When they are exposed to extreme heat, acids, bases, and certain other substances, proteins will denature. Denaturation is
a change in the structure of a molecule through physical or chemical means. Denatured proteins lose their functional shape
and are no longer able to carry out their jobs. An everyday example of protein denaturation is the curdling of milk when
acidic lemon juice is added.
The contribution of the shape of a protein to its function can hardly be exaggerated. For example, the long, slender shape of
protein strands that make up muscle tissue is essential to their ability to contract (shorten) and relax (lengthen). As another
example, bones contain long threads of a protein called collagen that acts as scaffolding upon which bone minerals are
deposited. These elongated proteins, called fibrous proteins, are strong and durable and typically hydrophobic.
In contrast, globular proteins are globes or spheres that tend to be highly reactive and are hydrophilic. The hemoglobin
proteins packed into red blood cells are an example (see Figure 2.26d); however, globular proteins are abundant throughout


CHAPTER 2 | THE CHEMICAL LEVEL OF ORGANIZATION 73




the body, playing critical roles in most body functions. Enzymes, introduced earlier as protein catalysts, are examples of
this. The next section takes a closer look at the action of enzymes.
Proteins Function as Enzymes
If you were trying to type a paper, and every time you hit a key on your laptop there was a delay of six or seven minutes
before you got a response, you would probably get a new laptop. In a similar way, without enzymes to catalyze chemical
reactions, the human body would be nonfunctional. It functions only because enzymes function.
Enzymatic reactions—chemical reactions catalyzed by enzymes—begin when substrates bind to the enzyme. A substrate
is a reactant in an enzymatic reaction. This occurs on regions of the enzyme known as active sites (Figure 2.27). Any given
enzyme catalyzes just one type of chemical reaction. This characteristic, called specificity, is due to the fact that a substrate
with a particular shape and electrical charge can bind only to an active site corresponding to that substrate.


Figure 2.27 Steps in an Enzymatic Reaction (a) Substrates approach active sites on enzyme. (b) Substrates bind to
active sites, producing an enzyme–substrate complex. (c) Changes internal to the enzyme–substrate complex facilitate
interaction of the substrates. (d) Products are released and the enzyme returns to its original form, ready to facilitate
another enzymatic reaction.


Binding of a substrate produces an enzyme–substrate complex. It is likely that enzymes speed up chemical reactions in part
because the enzyme–substrate complex undergoes a set of temporary and reversible changes that cause the substrates to be
oriented toward each other in an optimal position to facilitate their interaction. This promotes increased reaction speed. The
enzyme then releases the product(s), and resumes its original shape. The enzyme is then free to engage in the process again,
and will do so as long as substrate remains.
Other Functions of Proteins
Advertisements for protein bars, powders, and shakes all say that protein is important in building, repairing, and maintaining
muscle tissue, but the truth is that proteins contribute to all body tissues, from the skin to the brain cells. Also, certain
proteins act as hormones, chemical messengers that help regulate body functions, For example, growth hormone is
important for skeletal growth, among other roles.
As was noted earlier, the basic and acidic components enable proteins to function as buffers in maintaining acid–base
balance, but they also help regulate fluid–electrolyte balance. Proteins attract fluid, and a healthy concentration of proteins
in the blood, the cells, and the spaces between cells helps ensure a balance of fluids in these various “compartments.”
Moreover, proteins in the cell membrane help to transport electrolytes in and out of the cell, keeping these ions in a healthy
balance. Like lipids, proteins can bind with carbohydrates. They can thereby produce glycoproteins or proteoglycans, both
of which have many functions in the body.
The body can use proteins for energy when carbohydrate and fat intake is inadequate, and stores of glycogen and adipose
tissue become depleted. However, since there is no storage site for protein except functional tissues, using protein for energy
causes tissue breakdown, and results in body wasting.


Nucleotides
The fourth type of organic compound important to human structure and function are the nucleotides (Figure 2.28). A
nucleotide is one of a class of organic compounds composed of three subunits:
• one or more phosphate groups
• a pentose sugar: either deoxyribose or ribose
• a nitrogen-containing base: adenine, cytosine, guanine, thymine, or uracil
Nucleotides can be assembled into nucleic acids (DNA or RNA) or the energy compound adenosine triphosphate.


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Figure 2.28 Nucleotides (a) The building blocks of all nucleotides are one or more phosphate groups, a pentose
sugar, and a nitrogen-containing base. (b) The nitrogen-containing bases of nucleotides. (c) The two pentose sugars
of DNA and RNA.


Nucleic Acids
The nucleic acids differ in their type of pentose sugar. Deoxyribonucleic acid (DNA) is nucleotide that stores genetic
information. DNA contains deoxyribose (so-called because it has one less atom of oxygen than ribose) plus one phosphate
group and one nitrogen-containing base. The “choices” of base for DNA are adenine, cytosine, guanine, and thymine.
Ribonucleic acid (RNA) is a ribose-containing nucleotide that helps manifest the genetic code as protein. RNA contains
ribose, one phosphate group, and one nitrogen-containing base, but the “choices” of base for RNA are adenine, cytosine,
guanine, and uracil.
The nitrogen-containing bases adenine and guanine are classified as purines. A purine is a nitrogen-containing molecule
with a double ring structure, which accommodates several nitrogen atoms. The bases cytosine, thymine (found in DNA
only) and uracil (found in RNA only) are pyramidines. A pyramidine is a nitrogen-containing base with a single ring
structure
Bonds formed by dehydration synthesis between the pentose sugar of one nucleic acid monomer and the phosphate group of
another form a “backbone,” from which the components’ nitrogen-containing bases protrude. In DNA, two such backbones
attach at their protruding bases via hydrogen bonds. These twist to form a shape known as a double helix (Figure 2.29).
The sequence of nitrogen-containing bases within a strand of DNA form the genes that act as a molecular code instructing
cells in the assembly of amino acids into proteins. Humans have almost 22,000 genes in their DNA, locked up in the 46
chromosomes inside the nucleus of each cell (except red blood cells which lose their nuclei during development). These
genes carry the genetic code to build one’s body, and are unique for each individual except identical twins.


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Figure 2.29 DNA In the DNA double helix, two strands attach via hydrogen bonds between the bases of the
component nucleotides.


In contrast, RNA consists of a single strand of sugar-phosphate backbone studded with bases. Messenger RNA (mRNA) is
created during protein synthesis to carry the genetic instructions from the DNA to the cell’s protein manufacturing plants in
the cytoplasm, the ribosomes.
Adenosine Triphosphate
The nucleotide adenosine triphosphate (ATP), is composed of a ribose sugar, an adenine base, and three phosphate groups
(Figure 2.30). ATP is classified as a high energy compound because the two covalent bonds linking its three phosphates
store a significant amount of potential energy. In the body, the energy released from these high energy bonds helps fuel the
body’s activities, from muscle contraction to the transport of substances in and out of cells to anabolic chemical reactions.


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Figure 2.30 Structure of Adenosine Triphosphate (ATP)


When a phosphate group is cleaved from ATP, the products are adenosine diphosphate (ADP) and inorganic phosphate (Pi).
This hydrolysis reaction can be written:


ATP + H2O → ADP + Pi + energy


Removal of a second phosphate leaves adenosine monophosphate (AMP) and two phosphate groups. Again, these reactions
also liberate the energy that had been stored in the phosphate-phosphate bonds. They are reversible, too, as when ADP
undergoes phosphorylation. Phosphorylation is the addition of a phosphate group to an organic compound, in this case,
resulting in ATP. In such cases, the same level of energy that had been released during hydrolysis must be reinvested to
power dehydration synthesis.
Cells can also transfer a phosphate group from ATP to another organic compound. For example, when glucose first enters
a cell, a phosphate group is transferred from ATP, forming glucose phosphate (C6H12O6—P) and ADP. Once glucose is
phosphorylated in this way, it can be stored as glycogen or metabolized for immediate energy.


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acid


activation energy


adenosine triphosphate (ATP)
amino acid


anion
atom
atomic number
base


bond
buffer
carbohydrate


catalyst
cation
chemical energy


colloid
compound
concentration
covalent bond
decomposition reaction


denaturation
deoxyribonucleic acid (DNA)
disaccharide
disulfide bond


electron
electron shell
element
enzyme
exchange reaction


KEY TERMS
compound that releases hydrogen ions (H+) in solution


amount of energy greater than the energy contained in the reactants, which must be overcome for a
reaction to proceed


nucleotide containing ribose and an adenine base that is essential in energy transfer
building block of proteins; characterized by an amino and carboxyl functional groups and a variable side-


chain
atom with a negative charge
smallest unit of an element that retains the unique properties of that element


number of protons in the nucleus of an atom


compound that accepts hydrogen ions (H+) in solution
electrical force linking atoms
solution containing a weak acid or a weak base that opposes wide fluctuations in the pH of body fluids


class of organic compounds built from sugars, molecules containing carbon, hydrogen, and oxygen in a
1-2-1 ratio


substance that increases the rate of a chemical reaction without itself being changed in the process
atom with a positive charge


form of energy that is absorbed as chemical bonds form, stored as they are maintained, and released
as they are broken
liquid mixture in which the solute particles consist of clumps of molecules large enough to scatter light


substance composed of two or more different elements joined by chemical bonds
number of particles within a given space
chemical bond in which two atoms share electrons, thereby completing their valence shells


type of catabolic reaction in which one or more bonds within a larger molecule are broken,
resulting in the release of smaller molecules or atoms


change in the structure of a molecule through physical or chemical means
deoxyribose-containing nucleotide that stores genetic information


pair of carbohydrate monomers bonded by dehydration synthesis via a glycosidic bond
covalent bond formed within a polypeptide between sulfide groups of sulfur-containing amino acids,


for example, cysteine
subatomic particle having a negative charge and nearly no mass; found orbiting the atom’s nucleus


area of space a given distance from an atom’s nucleus in which electrons are grouped
substance that cannot be created or broken down by ordinary chemical means
protein or RNA that catalyzes chemical reactions


type of chemical reaction in which bonds are both formed and broken, resulting in the transfer of
components


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functional group


hydrogen bond


inorganic compound
ion
ionic bond
isotope
kinetic energy
lipid


macromolecule
mass number
matter
molecule
monosaccharide
neutron
nucleotide
organic compound
peptide bond
periodic table of the elements


pH


phospholipid
phosphorylation
polar molecule


polysaccharide


potential energy
product
prostaglandin
protein
proton
purine
pyrimidine


group of atoms linked by strong covalent bonds that tends to behave as a distinct unit in chemical
reactions with other atoms


dipole-dipole bond in which a hydrogen atom covalently bonded to an electronegative atom is weakly
attracted to a second electronegative atom


substance that does not contain both carbon and hydrogen
atom with an overall positive or negative charge


attraction between an anion and a cation
one of the variations of an element in which the number of neutrons differ from each other


energy that matter possesses because of its motion
class of nonpolar organic compounds built from hydrocarbons and distinguished by the fact that they are not
soluble in water


large molecule formed by covalent bonding
sum of the number of protons and neutrons in the nucleus of an atom


physical substance; that which occupies space and has mass
two or more atoms covalently bonded together


monomer of carbohydrate; also known as a simple sugar
heavy subatomic particle having no electrical charge and found in the atom’s nucleus
class of organic compounds composed of one or more phosphate groups, a pentose sugar, and a base


substance that contains both carbon and hydrogen
covalent bond formed by dehydration synthesis between two amino acids


arrangement of the elements in a table according to their atomic number; elements
having similar properties because of their electron arrangements compose columns in the table, while elements
having the same number of valence shells compose rows in the table


negative logarithm of the hydrogen ion (H+) concentration of a solution
a lipid compound in which a phosphate group is combined with a diglyceride
addition of one or more phosphate groups to an organic compound
molecule with regions that have opposite charges resulting from uneven numbers of electrons in the


nuclei of the atoms participating in the covalent bond
compound consisting of more than two carbohydrate monomers bonded by dehydration synthesis via


glycosidic bonds
stored energy matter possesses because of the positioning or structure of its components


one or more substances produced by a chemical reaction
lipid compound derived from fatty acid chains and important in regulating several body processes


class of organic compounds that are composed of many amino acids linked together by peptide bonds
heavy subatomic particle having a positive charge and found in the atom’s nucleus
nitrogen-containing base with a double ring structure; adenine and guanine


nitrogen-containing base with a single ring structure; cytosine, thiamine, and uracil


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radioactive isotope


reactant
ribonucleic acid (RNA)
solution
steroid


substrate
suspension
synthesis reaction


triglyceride
valence shell


unstable, heavy isotope that gives off subatomic particles, or electromagnetic energy, as it decays;
also called radioisotopes


one or more substances that enter into the reaction
ribose-containing nucleotide that helps manifest the genetic code as protein


homogeneous liquid mixture in which a solute is dissolved into molecules within a solvent
(also, sterol) lipid compound composed of four hydrocarbon rings bonded to a variety of other atoms and


molecules
reactant in an enzymatic reaction
liquid mixture in which particles distributed in the liquid settle out over time


type of anabolic reaction in which two or more atoms or molecules bond, resulting in the
formation of a larger molecule


lipid compound composed of a glycerol molecule bonded with three fatty acid chains
outermost electron shell of an atom


CHAPTER REVIEW
2.1 Elements and Atoms: The Building Blocks of Matter
The human body is composed of elements, the most abundant of which are oxygen (O), carbon (C), hydrogen (H) and
nitrogen (N). You obtain these elements from the foods you eat and the air you breathe. The smallest unit of an element that
retains all of the properties of that element is an atom. But, atoms themselves contain many subatomic particles, the three
most important of which are protons, neutrons, and electrons. These particles do not vary in quality from one element to
another; rather, what gives an element its distinctive identification is the quantity of its protons, called its atomic number.
Protons and neutrons contribute nearly all of an atom’s mass; the number of protons and neutrons is an element’s mass
number. Heavier and lighter versions of the same element can occur in nature because these versions have different numbers
of neutrons. Different versions of an element are called isotopes.
The tendency of an atom to be stable or to react readily with other atoms is largely due to the behavior of the electrons
within the atom’s outermost electron shell, called its valence shell. Helium, as well as larger atoms with eight electrons in
their valence shell, is unlikely to participate in chemical reactions because they are stable. All other atoms tend to accept,
donate, or share electrons in a process that brings the electrons in their valence shell to eight (or in the case of hydrogen, to
two).


2.2 Chemical Bonds
Each moment of life, atoms of oxygen, carbon, hydrogen, and the other elements of the human body are making and
breaking chemical bonds. Ions are charged atoms that form when an atom donates or accepts one or more negatively charged
electrons. Cations (ions with a positive charge) are attracted to anions (ions with a negative charge). This attraction is called
an ionic bond. In covalent bonds, the participating atoms do not lose or gain electrons, but rather share them. Molecules
with nonpolar covalent bonds are electrically balanced, and have a linear three-dimensional shape. Molecules with polar
covalent bonds have “poles”—regions of weakly positive and negative charge—and have a triangular three-dimensional
shape. An atom of oxygen and two atoms of hydrogen form water molecules by means of polar covalent bonds. Hydrogen
bonds link hydrogen atoms already participating in polar covalent bonds to anions or electronegative regions of other polar
molecules. Hydrogen bonds link water molecules, resulting in the properties of water that are important to living things.


2.3 Chemical Reactions
Chemical reactions, in which chemical bonds are broken and formed, require an initial investment of energy. Kinetic energy,
the energy of matter in motion, fuels the collisions of atoms, ions, and molecules that are necessary if their old bonds are to
break and new ones to form. All molecules store potential energy, which is released when their bonds are broken.
Four forms of energy essential to human functioning are: chemical energy, which is stored and released as chemical bonds
are formed and broken; mechanical energy, which directly powers physical activity; radiant energy, emitted as waves such
as in sunlight; and electrical energy, the power of moving electrons.


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Chemical reactions begin with reactants and end with products. Synthesis reactions bond reactants together, a process that
requires energy, whereas decomposition reactions break the bonds within a reactant and thereby release energy. In exchange
reactions, bonds are both broken and formed, and energy is exchanged.
The rate at which chemical reactions occur is influenced by several properties of the reactants: temperature, concentration
and pressure, and the presence or absence of a catalyst. An enzyme is a catalytic protein that speeds up chemical reactions
in the human body.


2.4 Inorganic Compounds Essential to Human Functioning
Inorganic compounds essential to human functioning include water, salts, acids, and bases. These compounds are inorganic;
that is, they do not contain both hydrogen and carbon. Water is a lubricant and cushion, a heat sink, a component of liquid
mixtures, a byproduct of dehydration synthesis reactions, and a reactant in hydrolysis reactions. Salts are compounds that,
when dissolved in water, dissociate into ions other than H+ or OH–. In contrast, acids release H+ in solution, making it more
acidic. Bases accept H+, thereby making the solution more alkaline (caustic).
The pH of any solution is its relative concentration of H+. A solution with pH 7 is neutral. Solutions with pH below 7 are
acids, and solutions with pH above 7 are bases. A change in a single digit on the pH scale (e.g., from 7 to 8) represents
a ten-fold increase or decrease in the concentration of H+. In a healthy adult, the pH of blood ranges from 7.35 to 7.45.
Homeostatic control mechanisms important for keeping blood in a healthy pH range include chemicals called buffers, weak
acids and weak bases released when the pH of blood or other body fluids fluctuates in either direction outside of this normal
range.


2.5 Organic Compounds Essential to Human Functioning
Organic compounds essential to human functioning include carbohydrates, lipids, proteins, and nucleotides. These
compounds are said to be organic because they contain both carbon and hydrogen. Carbon atoms in organic compounds
readily share electrons with hydrogen and other atoms, usually oxygen, and sometimes nitrogen. Carbon atoms also may
bond with one or more functional groups such as carboxyls, hydroxyls, aminos, or phosphates. Monomers are single units
of organic compounds. They bond by dehydration synthesis to form polymers, which can in turn be broken by hydrolysis.
Carbohydrate compounds provide essential body fuel. Their structural forms include monosaccharides such as glucose,
disaccharides such as lactose, and polysaccharides, including starches (polymers of glucose), glycogen (the storage form of
glucose), and fiber. All body cells can use glucose for fuel. It is converted via an oxidation-reduction reaction to ATP.
Lipids are hydrophobic compounds that provide body fuel and are important components of many biological compounds.
Triglycerides are the most abundant lipid in the body, and are composed of a glycerol backbone attached to three fatty acid
chains. Phospholipids are compounds composed of a diglyceride with a phosphate group attached at the molecule’s head.
The result is a molecule with polar and nonpolar regions. Steroids are lipids formed of four hydrocarbon rings. The most
important is cholesterol. Prostaglandins are signaling molecules derived from unsaturated fatty acids.
Proteins are critical components of all body tissues. They are made up of monomers called amino acids, which contain
nitrogen, joined by peptide bonds. Protein shape is critical to its function. Most body proteins are globular. An example is
enzymes, which catalyze chemical reactions.
Nucleotides are compounds with three building blocks: one or more phosphate groups, a pentose sugar, and a nitrogen-
containing base. DNA and RNA are nucleic acids that function in protein synthesis. ATP is the body’s fundamental molecule
of energy transfer. Removal or addition of phosphates releases or invests energy.


INTERACTIVE LINK QUESTIONS
1. Visit this website (http://openstaxcollege.org/l/ptable)
to view the periodic table. In the periodic table of the
elements, elements in a single column have the same
number of electrons that can participate in a chemical
reaction. These electrons are known as “valence electrons.”
For example, the elements in the first column all have a
single valence electron—an electron that can be “donated”
in a chemical reaction with another atom. What is the
meaning of a mass number shown in parentheses?


2. Visit this website (http://openstaxcollege.org/l/
electenergy) to learn about electrical energy and the
attraction/repulsion of charges. What happens to the
charged electroscope when a conductor is moved between
its plastic sheets, and why?
3. Watch this video (http://openstaxcollege.org/l/
disaccharide) to observe the formation of a disaccharide.
What happens when water encounters a glycosidic bond?


REVIEW QUESTIONS


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4. Together, just four elements make up more than 95
percent of the body’s mass. These include ________.


a. calcium, magnesium, iron, and carbon
b. oxygen, calcium, iron, and nitrogen
c. sodium, chlorine, carbon, and hydrogen
d. oxygen, carbon, hydrogen, and nitrogen


5. The smallest unit of an element that still retains the
distinctive behavior of that element is an ________.


a. electron
b. atom
c. elemental particle
d. isotope


6. The characteristic that gives an element its distinctive
properties is its number of ________.


a. protons
b. neutrons
c. electrons
d. atoms


7. On the periodic table of the elements, mercury (Hg) has
an atomic number of 80 and a mass number of 200.59.
It has seven stable isotopes. The most abundant of these
probably have ________.


a. about 80 neutrons each
b. fewer than 80 neutrons each
c. more than 80 neutrons each
d. more electrons than neutrons


8. Nitrogen has an atomic number of seven. How many
electron shells does it likely have?


a. one
b. two
c. three
d. four


9. Which of the following is a molecule, but not a
compound?


a. H2O
b. 2H
c. H2
d. H+


10. A molecule of ammonia contains one atom of nitrogen
and three atoms of hydrogen. These are linked with
________.


a. ionic bonds
b. nonpolar covalent bonds
c. polar covalent bonds
d. hydrogen bonds


11. When an atom donates an electron to another atom, it
becomes


a. an ion
b. an anion
c. nonpolar
d. all of the above


12. A substance formed of crystals of equal numbers of
cations and anions held together by ionic bonds is called
a(n) ________.


a. noble gas
b. salt


c. electrolyte
d. dipole


13. Which of the following statements about chemical
bonds is true?


a. Covalent bonds are stronger than ionic bonds.
b. Hydrogen bonds occur between two atoms of
hydrogen.


c. Bonding readily occurs between nonpolar and
polar molecules.


d. A molecule of water is unlikely to bond with an
ion.


14. The energy stored in a foot of snow on a steep roof is
________.


a. potential energy
b. kinetic energy
c. radiant energy
d. activation energy


15. The bonding of calcium, phosphorus, and other
elements produces mineral crystals that are found in bone.
This is an example of a(n) ________ reaction.


a. catabolic
b. synthesis
c. decomposition
d. exchange


16. AB → A + B is a general notation for a(n) ________
reaction.


a. anabolic
b. endergonic
c. decomposition
d. exchange


17. ________ reactions release energy.
a. Catabolic
b. Exergonic
c. Decomposition
d. Catabolic, exergonic, and decomposition


18. Which of the following combinations of atoms is most
likely to result in a chemical reaction?


a. hydrogen and hydrogen
b. hydrogen and helium
c. helium and helium
d. neon and helium


19. Chewing a bite of bread mixes it with saliva and
facilitates its chemical breakdown. This is most likely due
to the fact that ________.


a. the inside of the mouth maintains a very high
temperature


b. chewing stores potential energy
c. chewing facilitates synthesis reactions
d. saliva contains enzymes


20. CH4 is methane. This compound is ________.


a. inorganic
b. organic
c. reactive
d. a crystal


21. Which of the following is most likely to be found
evenly distributed in water in a homogeneous solution?


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a. sodium ions and chloride ions
b. NaCl molecules
c. salt crystals
d. red blood cells


22. Jenny mixes up a batch of pancake batter, then stirs in
some chocolate chips. As she is waiting for the first few
pancakes to cook, she notices the chocolate chips sinking to
the bottom of the clear glass mixing bowl. The chocolate-
chip batter is an example of a ________.


a. solvent
b. solute
c. solution
d. suspension


23. A substance dissociates into K+ and Cl– in solution.
The substance is a(n) ________.


a. acid
b. base
c. salt
d. buffer


24. Ty is three years old and as a result of a “stomach bug”
has been vomiting for about 24 hours. His blood pH is 7.48.
What does this mean?


a. Ty’s blood is slightly acidic.
b. Ty’s blood is slightly alkaline.
c. Ty’s blood is highly acidic.
d. Ty’s blood is within the normal range


25. C6H12O6 is the chemical formula for a ________.


a. polymer of carbohydrate
b. pentose monosaccharide
c. hexose monosaccharide
d. all of the above


26. What organic compound do brain cells primarily rely
on for fuel?


a. glucose
b. glycogen
c. galactose
d. glycerol


27.Which of the following is a functional group that is part
of a building block of proteins?


a. phosphate
b. adenine
c. amino
d. ribose


28. A pentose sugar is a part of the monomer used to build
which type of macromolecule?


a. polysaccharides
b. nucleic acids
c. phosphorylated glucose
d. glycogen


29. A phospholipid ________.
a. has both polar and nonpolar regions
b. is made up of a triglyceride bonded to a
phosphate group


c. is a building block of ATP
d. can donate both cations and anions in solution


30. In DNA, nucleotide bonding forms a compound with a
characteristic shape known as a(n) ________.


a. beta chain
b. pleated sheet
c. alpha helix
d. double helix


31. Uracil ________.
a. contains nitrogen
b. is a pyrimidine
c. is found in RNA
d. all of the above


32. The ability of an enzyme’s active sites to bind only
substrates of compatible shape and charge is known as
________.


a. selectivity
b. specificity
c. subjectivity
d. specialty


CRITICAL THINKING QUESTIONS
33. The most abundant elements in the foods and beverages
you consume are oxygen, carbon, hydrogen, and nitrogen.
Why might having these elements in consumables be
useful?
34. Oxygen, whose atomic number is eight, has three stable
isotopes: 16O, 17O, and 18O. Explain what this means in
terms of the number of protons and neutrons.
35.Magnesium is an important element in the human body,
especially in bones. Magnesium’s atomic number is 12. Is
it stable or reactive? Why? If it were to react with another
atom, would it be more likely to accept or to donate one or
more electrons?
36. Explain why CH4 is one of the most common
molecules found in nature. Are the bonds between the
atoms ionic or covalent?
37. In a hurry one day, you merely rinse your lunch dishes
with water. As you are drying your salad bowl, you notice


that it still has an oily film. Why was the water alone not
effective in cleaning the bowl?
38. Could two atoms of oxygen engage in ionic bonding?
Why or why not?
39. AB + CD → AD + BE Is this a legitimate example
of an exchange reaction? Why or why not?
40. When you do a load of laundry, why do you not just
drop a bar of soap into the washing machine? In other
words, why is laundry detergent sold as a liquid or powder?
41. The pH of lemon juice is 2, and the pH of orange juice
is 4. Which of these is more acidic, and by how much?
What does this mean?
42. During a party, Eli loses a bet and is forced to drink
a bottle of lemon juice. Not long thereafter, he begins
complaining of having difficulty breathing, and his friends


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take him to the local emergency room. There, he is given an
intravenous solution of bicarbonate. Why?
43. If the disaccharide maltose is formed from two glucose
monosaccharides, which are hexose sugars, how many


atoms of carbon, hydrogen, and oxygen does maltose
contain and why?
44. Once dietary fats are digested and absorbed, why can
they not be released directly into the bloodstream?


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3 | THE CELLULAR LEVEL
OF ORGANIZATION


Figure 3.1 Fluorescence-stained Cell Undergoing Mitosis A lung cell from a newt, commonly studied for its
similarity to human lung cells, is stained with fluorescent dyes. The green stain reveals mitotic spindles, red is the
cell membrane and part of the cytoplasm, and the structures that appear light blue are chromosomes. This cell is in
anaphase of mitosis. (credit: “Mortadelo2005”/Wikimedia Commons)


Introduction
Chapter Objectives


After studying this chapter, you will be able to:
• Describe the structure and function of the cell membrane, including its regulation of materials into and out
of the cell


• Describe the functions of the various cytoplasmic organelles
• Explain the structure and contents of the nucleus, as well as the process of DNA replication
• Explain the process by which a cell builds proteins using the DNA code
• List the stages of the cell cycle in order, including the steps of cell division in somatic cells
• Discuss how a cell differentiates and becomes more specialized
• List the morphological and physiological characteristics of some representative cell types in the human body


You developed from a single fertilized egg cell into the complex organism containing trillions of cells that you see when
you look in a mirror. During this developmental process, early, undifferentiated cells differentiate and become specialized
in their structure and function. These different cell types form specialized tissues that work in concert to perform all of the
functions necessary for the living organism. Cellular and developmental biologists study how the continued division of a
single cell leads to such complexity and differentiation.


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Consider the difference between a structural cell in the skin and a nerve cell. A structural skin cell may be shaped like
a flat plate (squamous) and live only for a short time before it is shed and replaced. Packed tightly into rows and sheets,
the squamous skin cells provide a protective barrier for the cells and tissues that lie beneath. A nerve cell, on the other
hand, may be shaped something like a star, sending out long processes up to a meter in length and may live for the entire
lifetime of the organism. With their long winding appendages, nerve cells can communicate with one another and with other
types of body cells and send rapid signals that inform the organism about its environment and allow it to interact with that
environment. These differences illustrate one very important theme that is consistent at all organizational levels of biology:
the form of a structure is optimally suited to perform particular functions assigned to that structure. Keep this theme in mind
as you tour the inside of a cell and are introduced to the various types of cells in the body.
A primary responsibility of each cell is to contribute to homeostasis. Homeostasis is a term used in biology that refers to
a dynamic state of balance within parameters that are compatible with life. For example, living cells require a water-based
environment to survive in, and there are various physical (anatomical) and physiological mechanisms that keep all of the
trillions of living cells in the human body moist. This is one aspect of homeostasis. When a particular parameter, such as
blood pressure or blood oxygen content, moves far enough out of homeostasis (generally becoming too high or too low),
illness or disease—and sometimes death—inevitably results.
The concept of a cell started with microscopic observations of dead cork tissue by scientist Robert Hooke in 1665. Without
realizing their function or importance, Hook coined the term “cell” based on the resemblance of the small subdivisions
in the cork to the rooms that monks inhabited, called cells. About ten years later, Antonie van Leeuwenhoek became the
first person to observe living and moving cells under a microscope. In the century that followed, the theory that cells
represented the basic unit of life would develop. These tiny fluid-filled sacs house components responsible for the thousands
of biochemical reactions necessary for an organism to grow and survive. In this chapter, you will learn about the major
components and functions of a prototypical, generalized cell and discover some of the different types of cells in the human
body.


3.1 | The Cell Membrane
By the end of this section, you will be able to:
• Describe the molecular components that make up the cell membrane
• Explain the major features and properties of the cell membrane
• Differentiate between materials that can and cannot diffuse through the lipid bilayer
• Compare and contrast different types of passive transport with active transport, providing examples of each


Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane.
As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma
membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective
barrier around the cell and regulates which materials can pass in or out.


Structure and Composition of the Cell Membrane
The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”).
Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded
within the membrane that have a variety of functions.
A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty
acids that make up the lipid tails (Figure 3.2). The phosphate group is negatively charged, making the head polar and
hydrophilic—or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The
phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid
tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.” A hydrophobic molecule
(or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain
unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are
thus amphipathic molecules. An amphipathicmolecule is one that contains both a hydrophilic and a hydrophobic region. In
fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve
in water while the hydrophobic portion can trap grease in micelles that then can be washed away.


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Figure 3.2 Phospholipid Structure A phospholipid molecule consists of a polar phosphate “head,” which is
hydrophilic and a non-polar lipid “tail,” which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic
tails.


The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the
other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior
of the cell and one layer exposed to the exterior (Figure 3.3). Because the phosphate groups are polar and hydrophilic,
they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. The phosphate
groups are also attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment outside the enclosure
of the cell membrane. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels.
Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and
extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that
are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid; the lipids and
proteins in the cell membrane are not rigidly locked in place.


Figure 3.3 Phospolipid Bilayer The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged
tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads
contact the fluid inside and outside of the cell.


Membrane Proteins
The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins. Two different
types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral protein
(Figure 3.4). As its name suggests, an integral protein is a protein that is embedded in the membrane. A channel protein


CHAPTER 3 | THE CELLULAR LEVEL OF ORGANIZATION 87




is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of
the cell.


Figure 3.4 Cell Membrane The cell membrane of the cell is a phospholipid bilayer containing many different
molecular components, including proteins and cholesterol, some with carbohydrate groups attached.


Another important group of integral proteins are cell recognition proteins, which serve to mark a cell’s identity so that it
can be recognized by other cells. A receptor is a type of recognition protein that can selectively bind a specific molecule
outside the cell, and this binding induces a chemical reaction within the cell. A ligand is the specific molecule that binds
to and activates a receptor. Some integral proteins serve dual roles as both a receptor and an ion channel. One example of a
receptor-ligand interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine
molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to
flow into the cell.
Some integral membrane proteins are glycoproteins. A glycoprotein is a protein that has carbohydrate molecules attached,
which extend into the extracellular matrix. The attached carbohydrate tags on glycoproteins aid in cell recognition. The
carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx.
The glycocalyx is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to
the cell membrane. The glycocalyx can have various roles. For example, it may have molecules that allow the cell to bind
to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients. The glycocalyces
found in a person’s body are products of that person’s genetic makeup. They give each of the individual’s trillions of cells
the “identity” of belonging in the person’s body. This identity is the primary way that a person’s immune defense cells
“know” not to attack the person’s own body cells, but it also is the reason organs donated by another person might be
rejected.
Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the
internal or external surface of an integral protein. These proteins typically perform a specific function for the cell. Some
peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes
that can pass through the cells and into the bloodstream.


Transport across the Cell Membrane
One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These
substances include ions such as Ca++, Na+, K+, and Cl–; nutrients including sugars, fatty acids, and amino acids; and waste
products, particularly carbon dioxide (CO2), which must leave the cell.
The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and
the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane
that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the
cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the
membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However,
water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because
they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do
so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is
the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport
is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).


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Passive Transport
In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration
gradients and diffusion. A concentration gradient is the difference in concentration of a substance across a space.
Molecules (or ions) will spread/diffuse from where they are more concentrated to where they are less concentrated until they
are equally distributed in that space. (When molecules move in this way, they are said to move down their concentration
gradient.) Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.
A couple of common examples will help to illustrate this concept. Imagine being inside a closed bathroom. If a bottle of
perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of
the bathroom, and this diffusion would go on until no more concentration gradient remains. Another example is a spoonful
of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains.
In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each
other and spreading out faster than at cooler temperatures. Having an internal body temperature around 98.6° F thus also
aids in diffusion of particles within the body.


Visit this link (http://openstaxcollege.org/l/diffusion) to see diffusion and how it is propelled by the kinetic energy
of molecules in solution. How does temperature affect diffusion rate, and why?


Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the cell
membranes, any substance that can move down its concentration gradient across the membrane will do so. Consider
substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2. O2
generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because
it is more concentrated inside of them. Neither of these examples requires any energy on the part of the cell, and therefore
they use passive transport to move across the membrane.
Before moving on, you need to review the gases that can diffuse across a cell membrane. Because cells rapidly use up
oxygen during metabolism, there is typically a lower concentration of O2 inside the cell than outside. As a result, oxygen
will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within
the cell. On the other hand, because cells produce CO2 as a byproduct of metabolism, CO2 concentrations rise within
the cytoplasm; therefore, CO2 will move from the cell through the lipid bilayer and into the interstitial fluid, where its
concentration is lower. This mechanism of molecules spreading from where they are more concentrated to where they are
less concentration is a form of passive transport called simple diffusion (Figure 3.5).


Figure 3.5 Simple Diffusion across the Cell (Plasma) Membrane The structure of the lipid bilayer allows only
small, non-polar substances such as oxygen and carbon dioxide to pass through the cell membrane, down their
concentration gradient, by simple diffusion.


CHAPTER 3 | THE CELLULAR LEVEL OF ORGANIZATION 89




Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but
because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to
protein channels and specialized transport mechanisms in the membrane. Facilitated diffusion is the diffusion process used
for those substances that cannot cross the lipid bilayer due to their size and/or polarity (Figure 3.6). A common example
of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be
more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar.
To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to
facilitate its inward diffusion.


Figure 3.6 Facilitated Diffusion (a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes
place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than
carrier proteins, and usually mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are
more selective, often only allowing one particular type of molecule to cross.


As an example, even though sodium ions (Na+) are highly concentrated outside of cells, these electrolytes are polarized
and cannot pass through the nonpolar lipid bilayer of the membrane. Their diffusion is facilitated by membrane proteins
that form sodium channels (or “pores”), so that Na+ ions can move down their concentration gradient from outside the cells
to inside the cells. There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino
acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy
expenditure by the cell.
Water also can move freely across the cell membrane of all cells, either through protein channels or by slipping between the
lipid tails of the membrane itself. Osmosis is the diffusion of water through a semipermeable membrane (Figure 3.7).


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Figure 3.7 Osmosis Osmosis is the diffusion of water through a semipermeable membrane down its concentration
gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by
diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on
the left, the solution on the right side of the membrane is hypertonic.


The movement of water molecules is not itself regulated by cells, so it is important that cells are exposed to an environment
in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes
inside the cells (in the cytoplasm). Two solutions that have the same concentration of solutes are said to be isotonic (equal
tension). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same
outside and inside the cells, and the cells maintain their normal shape (and function).
Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. A solution that has a higher
concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic
solution (Figure 3.8). Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution
that has a lower concentration of solutes than another solution is said to be hypotonic, and water molecules tend to diffuse
out of a hypotonic solution. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually
bursting. A critical aspect of homeostasis in living things is to create an internal environment in which all of the body’s cells
are in an isotonic solution. Various organ systems, particularly the kidneys, work to maintain this homeostasis.


Figure 3.8 Concentration of Solutions A hypertonic solution has a solute concentration higher than another
solution. An isotonic solution has a solute concentration equal to another solution. A hypotonic solution has a solute
concentration lower than another solution.


Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion
of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that
pushes the fluid—and the solutes within it—from a higher pressure area to a lower pressure area. Filtration is an extremely
important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the
endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. Filtration pressure in the
kidneys provides the mechanism to remove wastes from the bloodstream.
Active Transport
For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive
transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a
membrane, often with the help of protein carriers, and usually against its concentration gradient.
One of the most common types of active transport involves proteins that serve as pumps. The word “pump” probably
conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, energy from ATP is
required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against
their concentration gradients (from an area of low concentration to an area of high concentration).


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The sodium-potassium pump, which is also called Na+/K+ ATPase, transports sodium out of a cell while moving
potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These
pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions
to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge
across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the
cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient
is maintained because each Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP
molecule that is used (Figure 3.9). This process is so important for nerve cells that it accounts for the majority of their ATP
usage.


Figure 3.9 Sodium-Potassium Pump The sodium-potassium pump is found in many cell (plasma) membranes.
Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration
gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into
the cell.


Active transport pumps can also work together with other active or passive transport systems to move substances across
the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell.
Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of
the sodium ions will drive them to diffuse into the cell. In this way, the action of an active transport pump (the sodium-
potassium pump) powers the passive transport of sodium ions by creating a concentration gradient. When active transport
powers the transport of another substance in this way, it is called secondary active transport.
Symporters are secondary active transporters that move two substances in the same direction. For example, the sodium-
glucose symporter uses sodium ions to “pull” glucose molecules into the cell. Because cells store glucose for energy,
glucose is typically at a higher concentration inside of the cell than outside. However, due to the action of the sodium-
potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions
through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its
concentration gradient.
Conversely, antiporters are secondary active transport systems that transport substances in opposite directions. For example,
the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H+) out
of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell's interior.
Other forms of active transport do not involve membrane carriers. Endocytosis (bringing “into the cell”) is the process of
a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane
(Figure 3.10). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle.
A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often
brings materials into the cell that must to be broken down or digested. Phagocytosis (“cell eating”) is the endocytosis
of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to
patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. In contrast
to phagocytosis, pinocytosis (“cell drinking”) brings fluid containing dissolved substances into a cell through membrane
vesicles.


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Figure 3.10 Three Forms of Endocytosis Endocytosis is a form of active transport in which a cell envelopes
extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a
large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis
is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.


Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective
in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis.
Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are
specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the
receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron,
a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called
transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the
cell endocytoses the receptor-ligand complexes.
In contrast with endocytosis, exocytosis (taking “out of the cell”) is the process of a cell exporting material using vesicular
transport (Figure 3.11). Many cells manufacture substances that must be secreted, like a factory manufacturing a product for
export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane
fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes
part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis
(Figure 3.12). Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells
produce and secrete large amounts of histamine, a chemical important for immune responses.


Figure 3.11 Exocytosis Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into
a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released
into the extracellular space.


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Figure 3.12 Pancreatic Cells' Enzyme Products The pancreatic acinar cells produce and secrete many enzymes
that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will
be exported from the cells via exocytosis. LM × 2900. (Micrograph provided by the Regents of University of Michigan
Medical School © 2012)


View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/EMsmallCharts/
3%20Image%20Scope%20finals/226%20-%20Pancreas_001.svs/view.apml (http://openstaxcollege.org/l/pcells)
to explore the tissue sample in greater detail.


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Cell: Cystic Fibrosis
Cystic fibrosis (CF) affects approximately 30,000 people in the United States, with about 1,000 new cases reported
each year. The genetic disease is most well known for its damage to the lungs, causing breathing difficulties and
chronic lung infections, but it also affects the liver, pancreas, and intestines. Only about 50 years ago, the prognosis
for children born with CF was very grim—a life expectancy rarely over 10 years. Today, with advances in medical
treatment, many CF patients live into their 30s.
The symptoms of CF result from a malfunctioning membrane ion channel called the cystic fibrosis transmembrane
conductance regulator, or CFTR. In healthy people, the CFTR protein is an integral membrane protein that transports
Cl– ions out of the cell. In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a
defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell.
The CFTR requires ATP in order to function, making its Cl– transport a form of active transport. This characteristic
puzzled researchers for a long time because the Cl– ions are actually flowing down their concentration gradient when
transported out of cells. Active transport generally pumps ions against their concentration gradient, but the CFTR
presents an exception to this rule.
In normal lung tissue, the movement of Cl– out of the cell maintains a Cl–-rich, negatively charged environment
immediately outside of the cell. This is particularly important in the epithelial lining of the respiratory system.
Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. A cilium (plural = cilia)
is one of the hair-like appendages found on certain cells. Cilia on the epithelial cells move the mucus and its trapped
particles up the airways away from the lungs and toward the outside. In order to be effectively moved upward, the
mucus cannot be too viscous; rather it must have a thin, watery consistency. The transport of Cl– and the maintenance
of an electronegative environment outside of the cell attract positive ions such as Na+ to the extracellular space.
The accumulation of both Cl– and Na+ ions in the extracellular space creates solute-rich mucus, which has a low
concentration of water molecules. As a result, through osmosis, water moves from cells and extracellular matrix into
the mucus, “thinning” it out. This is how, in a normal respiratory system, the mucus is kept sufficiently watered-down
to be propelled out of the respiratory system.
If the CFTR channel is absent, Cl– ions are not transported out of the cell in adequate numbers, thus preventing
them from drawing positive ions. The absence of ions in the secreted mucus results in the lack of a normal water
concentration gradient. Thus, there is no osmotic pressure pulling water into the mucus. The resulting mucus is thick
and sticky, and the ciliated epithelia cannot effectively remove it from the respiratory system. Passageways in the lungs
become blocked with mucus, along with the debris it carries. Bacterial infections occur more easily because bacterial
cells are not effectively carried away from the lungs.


3.2 | The Cytoplasm and Cellular Organelles
By the end of this section, you will be able to:
• Describe the structure and function of the cellular organelles associated with the endomembrane system, including
the endoplasmic reticulum, Golgi apparatus, and lysosomes


• Describe the structure and function of mitochondria and peroxisomes
• Explain the three components of the cytoskeleton, including their composition and functions


Now that you have learned that the cell membrane surrounds all cells, you can dive inside of a prototypical human cell
to learn about its internal components and their functions. All living cells in multicellular organisms contain an internal
cytoplasmic compartment, and a nucleus within the cytoplasm. Cytosol, the jelly-like substance within the cell, provides
the fluid medium necessary for biochemical reactions. Eukaryotic cells, including all animal cells, also contain various
cellular organelles. An organelle (“little organ”) is one of several different types of membrane-enclosed bodies in the
cell, each performing a unique function. Just as the various bodily organs work together in harmony to perform all of a
human’s functions, the many different cellular organelles work together to keep the cell healthy and performing all of its
important functions. The organelles and cytosol, taken together, compose the cell’s cytoplasm. The nucleus is a cell’s
central organelle, which contains the cell’s DNA (Figure 3.13).


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Figure 3.13 Prototypical Human Cell While this image is not indicative of any one particular human cell, it is a
prototypical example of a cell containing the primary organelles and internal structures.


Organelles of the Endomembrane System
A set of three major organelles together form a system within the cell called the endomembrane system. These organelles
work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular
products. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a system of channels that is continuous with the nuclear membrane (or “envelope”)
covering the nucleus and composed of the same lipid bilayer material. The ER can be thought of as a series of winding
thoroughfares similar to the waterway canals in Venice. The ER provides passages throughout much of the cell that function
in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface
area that supports its many functions (Figure 3.14).


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Figure 3.14 Endoplasmic Reticulum (ER) (a) The ER is a winding network of thin membranous sacs found in
close association with the cell nucleus. The smooth and rough endoplasmic reticula are very different in appearance
and function (source: mouse tissue). (b) Rough ER is studded with numerous ribosomes, which are sites of protein
synthesis (source: mouse tissue). EM × 110,000. (c) Smooth ER synthesizes phospholipids, steroid hormones,
regulates the concentration of cellular Ca++, metabolizes some carbohydrates, and breaks down certain toxins (source:
mouse tissue). EM × 110,510. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)


Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very
different functions and can be found in very different amounts depending on the type of cell. Rough ER (RER) is so-
called because its membrane is dotted with embedded granules—organelles called ribosomes, giving the RER a bumpy
appearance. A ribosome is an organelle that serves as the site of protein synthesis. It is composed of two ribosomal RNA
subunits that wrap around mRNA to start the process of translation, followed by protein synthesis. Smooth ER (SER) lacks
these ribosomes.
One of the main functions of the smooth ER is in the synthesis of lipids. The smooth ER synthesizes phospholipids, the
main component of biological membranes, as well as steroid hormones. For this reason, cells that produce large quantities
of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER. In addition
to lipid synthesis, the smooth ER also sequesters (i.e., stores) and regulates the concentration of cellular Ca++, a function
extremely important in cells of the nervous system where Ca++ is the trigger for neurotransmitter release. The smooth ER
additionally metabolizes some carbohydrates and performs a detoxification role, breaking down certain toxins.
In contrast with the smooth ER, the primary job of the rough ER is the synthesis and modification of proteins destined for
the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER (giving it the
studded appearance of rough ER). Typically, a protein is synthesized within the ribosome and released inside the channel of
the rough ER, where sugars can be added to it (by a process called glycosylation) before it is transported within a vesicle to
the next stage in the packaging and shipping process: the Golgi apparatus.
The Golgi Apparatus
The Golgi apparatus is responsible for sorting, modifying, and shipping off the products that come from the rough ER,
much like a post-office. The Golgi apparatus looks like stacked flattened discs, almost like stacks of oddly shaped pancakes.
Like the ER, these discs are membranous. The Golgi apparatus has two distinct sides, each with a different role. One side
of the apparatus receives products in vesicles. These products are sorted through the apparatus, and then they are released


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from the opposite side after being repackaged into new vesicles. If the product is to be exported from the cell, the vesicle
migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted (Figure 3.15).


Figure 3.15 Golgi Apparatus (a) The Golgi apparatus manipulates products from the rough ER, and also produces
new organelles called lysosomes. Proteins and other products of the ER are sent to the Golgi apparatus, which
organizes, modifies, packages, and tags them. Some of these products are transported to other areas of the cell and
some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new lysosomes (or packaged
and sent for fusion with existing lysosomes). (b) An electron micrograph of the Golgi apparatus.


Lysosomes
Some of the protein products packaged by the Golgi include digestive enzymes that are meant to remain inside the cell for
use in breaking down certain materials. The enzyme-containing vesicles released by the Golgi may form new lysosomes,
or fuse with existing, lysosomes. A lysosome is an organelle that contains enzymes that break down and digest unneeded
cellular components, such as a damaged organelle. (A lysosome is similar to a wrecking crew that takes down old and
unsound buildings in a neighborhood.) Autophagy (“self-eating”) is the process of a cell digesting its own structures.
Lysosomes are also important for breaking down foreign material. For example, when certain immune defense cells (white
blood cells) phagocytize bacteria, the bacterial cell is transported into a lysosome and digested by the enzymes inside. As
one might imagine, such phagocytic defense cells contain large numbers of lysosomes.
Under certain circumstances, lysosomes perform a more grand and dire function. In the case of damaged or unhealthy cells,
lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell, killing the cell.
This “self-destruct” mechanism is called autolysis, and makes the process of cell death controlled (a mechanism called
“apoptosis”).


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Watch this video (http://openstaxcollege.org/l/endomembrane1) to learn about the endomembrane system, which
includes the rough and smooth ER and the Golgi body as well as lysosomes and vesicles. What is the primary role of
the endomembrane system?


Organelles for Energy Production and Detoxification
In addition to the jobs performed by the endomembrane system, the cell has many other important functions. Just as you
must consume nutrients to provide yourself with energy, so must each of your cells take in nutrients, some of which convert
to chemical energy that can be used to power biochemical reactions. Another important function of the cell is detoxification.
Humans take in all sorts of toxins from the environment and also produce harmful chemicals as byproducts of cellular
processes. Cells called hepatocytes in the liver detoxify many of these toxins.
Mitochondria
A mitochondrion (plural = mitochondria) is a membranous, bean-shaped organelle that is the “energy transformer” of
the cell. Mitochondria consist of an outer lipid bilayer membrane as well as an additional inner lipid bilayer membrane
(Figure 3.16). The inner membrane is highly folded into winding structures with a great deal of surface area, called cristae.
It is along this inner membrane that a series of proteins, enzymes, and other molecules perform the biochemical reactions
of cellular respiration. These reactions convert energy stored in nutrient molecules (such as glucose) into adenosine
triphosphate (ATP), which provides usable cellular energy to the cell. Cells use ATP constantly, and so the mitochondria
are constantly at work. Oxygen molecules are required during cellular respiration, which is why you must constantly
breathe it in. One of the organ systems in the body that uses huge amounts of ATP is the muscular system because ATP
is required to sustain muscle contraction. As a result, muscle cells are packed full of mitochondria. Nerve cells also need
large quantities of ATP to run their sodium-potassium pumps. Therefore, an individual neuron will be loaded with over
a thousand mitochondria. On the other hand, a bone cell, which is not nearly as metabolically-active, might only have a
couple hundred mitochondria.


Figure 3.16 Mitochondrion The mitochondria are the energy-conversion factories of the cell. (a) A mitochondrion
is composed of two separate lipid bilayer membranes. Along the inner membrane are various molecules that work
together to produce ATP, the cell’s major energy currency. (b) An electron micrograph of mitochondria. EM × 236,000.
(Micrograph provided by the Regents of University of Michigan Medical School © 2012)


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Peroxisomes
Like lysosomes, a peroxisome is a membrane-bound cellular organelle that contains mostly enzymes (Figure 3.17).
Peroxisomes perform a couple of different functions, including lipid metabolism and chemical detoxification. In contrast to
the digestive enzymes found in lysosomes, the enzymes within peroxisomes serve to transfer hydrogen atoms from various
molecules to oxygen, producing hydrogen peroxide (H2O2). In this way, peroxisomes neutralize poisons such as alcohol. In
order to appreciate the importance of peroxisomes, it is necessary to understand the concept of reactive oxygen species.


Figure 3.17 Peroxisome Peroxisomes are membrane-bound organelles that contain an abundance of enzymes for
detoxifying harmful substances and lipid metabolism.


Reactive oxygen species (ROS) such as peroxides and free radicals are the highly reactive products of many normal cellular
processes, including the mitochondrial reactions that produce ATP and oxygen metabolism. Examples of ROS include the
hydroxyl radical OH, H2O2, and superoxide (O2− ). Some ROS are important for certain cellular functions, such as cell
signaling processes and immune responses against foreign substances. Free radicals are reactive because they contain free
unpaired electrons; they can easily oxidize other molecules throughout the cell, causing cellular damage and even cell death.
Free radicals are thought to play a role in many destructive processes in the body, from cancer to coronary artery disease.
Peroxisomes, on the other hand, oversee reactions that neutralize free radicals. Peroxisomes produce large amounts of the
toxic H2O2 in the process, but peroxisomes contain enzymes that convert H2O2 into water and oxygen. These byproducts
are safely released into the cytoplasm. Like miniature sewage treatment plants, peroxisomes neutralize harmful toxins so
that they do not wreak havoc in the cells. The liver is the organ primarily responsible for detoxifying the blood before it
travels throughout the body, and liver cells contain an exceptionally high number of peroxisomes.
Defense mechanisms such as detoxification within the peroxisome and certain cellular antioxidants serve to neutralize
many of these molecules. Some vitamins and other substances, found primarily in fruits and vegetables, have antioxidant
properties. Antioxidants work by being oxidized themselves, halting the destructive reaction cascades initiated by the free
radicals. Sometimes though, ROS accumulate beyond the capacity of such defenses.
Oxidative stress is the term used to describe damage to cellular components caused by ROS. Due to their characteristic
unpaired electrons, ROS can set off chain reactions where they remove electrons from other molecules, which then become
oxidized and reactive, and do the same to other molecules, causing a chain reaction. ROS can cause permanent damage to
cellular lipids, proteins, carbohydrates, and nucleic acids. Damaged DNA can lead to genetic mutations and even cancer. A
mutation is a change in the nucleotide sequence in a gene within a cell’s DNA, potentially altering the protein coded by that
gene. Other diseases believed to be triggered or exacerbated by ROS include Alzheimer’s disease, cardiovascular diseases,
diabetes, Parkinson’s disease, arthritis, Huntington’s disease, and schizophrenia, among many others. It is noteworthy that
these diseases are largely age-related. Many scientists believe that oxidative stress is a major contributor to the aging
process.


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Cell: The Free Radical Theory
The free radical theory on aging was originally proposed in the 1950s, and still remains under debate. Generally
speaking, the free radical theory of aging suggests that accumulated cellular damage from oxidative stress contributes
to the physiological and anatomical effects of aging. There are two significantly different versions of this theory: one
states that the aging process itself is a result of oxidative damage, and the other states that oxidative damage causes age-
related disease and disorders. The latter version of the theory is more widely accepted than the former. However, many
lines of evidence suggest that oxidative damage does contribute to the aging process. Research has shown that reducing
oxidative damage can result in a longer lifespan in certain organisms such as yeast, worms, and fruit flies. Conversely,
increasing oxidative damage can shorten the lifespan of mice and worms. Interestingly, a manipulation called calorie-
restriction (moderately restricting the caloric intake) has been shown to increase life span in some laboratory animals.
It is believed that this increase is at least in part due to a reduction of oxidative stress. However, a long-term study
of primates with calorie-restriction showed no increase in their lifespan. A great deal of additional research will be
required to better understand the link between reactive oxygen species and aging.


The Cytoskeleton
Much like the bony skeleton structurally supports the human body, the cytoskeleton helps the cells to maintain their
structural integrity. The cytoskeleton is a group of fibrous proteins that provide structural support for cells, but this is only
one of the functions of the cytoskeleton. Cytoskeletal components are also critical for cell motility, cell reproduction, and
transportation of substances within the cell.
The cytoskeleton forms a complex thread-like network throughout the cell consisting of three different kinds of protein-
based filaments: microfilaments, intermediate filaments, and microtubules (Figure 3.18). The thickest of the three is the
microtubule, a structural filament composed of subunits of a protein called tubulin. Microtubules maintain cell shape and
structure, help resist compression of the cell, and play a role in positioning the organelles within the cell. Microtubules
also make up two types of cellular appendages important for motion: cilia and flagella. Cilia are found on many cells of
the body, including the epithelial cells that line the airways of the respiratory system. Cilia move rhythmically; they beat
constantly, moving waste materials such as dust, mucus, and bacteria upward through the airways, away from the lungs and
toward the mouth. Beating cilia on cells in the female fallopian tubes move egg cells from the ovary towards the uterus. A
flagellum (plural = flagella) is an appendage larger than a cilium and specialized for cell locomotion. The only flagellated
cell in humans is the sperm cell that must propel itself towards female egg cells.


Figure 3.18 The Three Components of the Cytoskeleton The cytoskeleton consists of (a) microtubules, (b)
microfilaments, and (c) intermediate filaments. The cytoskeleton plays an important role in maintaining cell shape and
structure, promoting cellular movement, and aiding cell division.


A very important function of microtubules is to set the paths (somewhat like railroad tracks) along which the genetic
material can be pulled (a process requiring ATP) during cell division, so that each new daughter cell receives the appropriate
set of chromosomes. Two short, identical microtubule structures called centrioles are found near the nucleus of cells. A


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centriole can serve as the cellular origin point for microtubules extending outward as cilia or flagella or can assist with the
separation of DNA during cell division. Microtubules grow out from the centrioles by adding more tubulin subunits, like
adding additional links to a chain.
In contrast with microtubules, the microfilament is a thinner type of cytoskeletal filament (see Figure 3.18b). Actin, a
protein that forms chains, is the primary component of these microfilaments. Actin fibers, twisted chains of actin filaments,
constitute a large component of muscle tissue and, along with the protein myosin, are responsible for muscle contraction.
Like microtubules, actin filaments are long chains of single subunits (called actin subunits). In muscle cells, these long actin
strands, called thin filaments, are “pulled” by thick filaments of the myosin protein to contract the cell.
Actin also has an important role during cell division. When a cell is about to split in half during cell division, actin filaments
work with myosin to create a cleavage furrow that eventually splits the cell down the middle, forming two new cells from
the original cell.
The final cytoskeletal filament is the intermediate filament. As its name would suggest, an intermediate filament is a
filament intermediate in thickness between the microtubules and microfilaments (see Figure 3.18c). Intermediate filaments
are made up of long fibrous subunits of a protein called keratin that are wound together like the threads that compose a rope.
Intermediate filaments, in concert with the microtubules, are important for maintaining cell shape and structure. Unlike the
microtubules, which resist compression, intermediate filaments resist tension—the forces that pull apart cells. There are
many cases in which cells are prone to tension, such as when epithelial cells of the skin are compressed, tugging them in
different directions. Intermediate filaments help anchor organelles together within a cell and also link cells to other cells by
forming special cell-to-cell junctions.


3.3 | The Nucleus and DNA Replication
By the end of this section, you will be able to:
• Describe the structure and features of the nuclear membrane
• List the contents of the nucleus
• Explain the organization of the DNA molecule within the nucleus
• Describe the process of DNA replication


The nucleus is the largest and most prominent of a cell’s organelles (Figure 3.19). The nucleus is generally considered
the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some
cells in the body, such as muscle cells, contain more than one nucleus (Figure 3.20), which is known as multinucleated.
Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature,
making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body (Figure 3.21). Without
nuclei, the life span of RBCs is short, and so the body must produce new ones constantly.


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Figure 3.19 The Nucleus The nucleus is the control center of the cell. The nucleus of living cells contains the genetic
material that determines the entire structure and function of that cell.


Figure 3.20 Multinucleate Muscle Cell Unlike cardiac muscle cells and smooth muscle cells, which have a single
nucleus, a skeletal muscle cell contains many nuclei, and is referred to as “multinucleated.” These muscle cells are
long and fibrous (often referred to as muscle fibers). During development, many smaller cells fuse to form a mature
muscle fiber. The nuclei of the fused cells are conserved in the mature cell, thus imparting a multinucleate characteristic
to mature muscle cells. LM × 104.3. (Micrograph provided by the Regents of University of Michigan Medical School ©
2012)


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View the University of Michigan WebScope at http://141.214.65.171/Histology/Basic%20Tissues/Muscle/
058thin_HISTO_83X.svs/view.apml (http://openstaxcollege.org/l/mnucleate) to explore the tissue sample in
greater detail.


Figure 3.21 Red Blood Cell Extruding Its Nucleus Mature red blood cells lack a nucleus. As they mature,
erythroblasts extrude their nucleus, making room for more hemoglobin. The two panels here show an erythroblast
before and after ejecting its nucleus, respectively. (credit: modification of micrograph provided by the Regents of
University of Michigan Medical School © 2012)


View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/EMsmallCharts/
3%20Image%20Scope%20finals/139%20-%20Erythroblast_001.svs/view.apml (http://openstaxcollege.org/l/
RBC) to explore the tissue sample in greater detail.


Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This
information is stored within DNA. The nucleus sends “commands” to the cell via molecular messengers that translate the
information from DNA. Each cell in your body (with the exception of germ cells) contains the complete set of your DNA.
When a cell divides, the DNA must be duplicated so that the each new cell receives a full complement of DNA. The
following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication.


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Organization of the Nucleus and Its DNA
Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope. This
membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two
bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the
nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into
and out of the nucleus.
Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There
also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The
nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes.
Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.
The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of
DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins (Figure 3.22). Along the
chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone
complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is
the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into
chromosomes, so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA
and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46
chromosomes.


Figure 3.22 DNA Macrostructure Strands of DNA are wrapped around supporting histones. These proteins are
increasingly bundled and condensed into chromatin, which is packed tightly into chromosomes when the cell is ready
to divide.


DNA Replication
In order for an organism to grow, develop, and maintain its health, cells must reproduce themselves by dividing to produce
two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are
produced in an adult human every day. Only very few cell types in the body do not divide, including nerve cells, skeletal
muscle fibers, and cardiac muscle cells. The division time of different cell types varies. Epithelial cells of the skin and
gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off of the surface
by friction.
A DNA molecule is made of two strands that “complement” each other in the sense that the molecules that compose the
strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder.
Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups (Figure 3.23). The two sides of the
ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding
bases, each bonded pair forming one “rung,” or cross member. The four DNA bases are adenine (A), thymine (T), cytosine
(C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine


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always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA
molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could
infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one strand has
a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA.


Figure 3.23 Molecular Structure of DNA The DNA double helix is composed of two complementary strands. The
strands are bonded together via their nitrogenous base pairs using hydrogen bonds.


DNA replication is the copying of DNA that occurs before cell division can take place. After a great deal of debate and
experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew
Meselson and Franklin Stahl. This method is illustrated in Figure 3.24 and described below.


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Figure 3.24 DNA Replication DNA replication faithfully duplicates the entire genome of the cell. During DNA
replication, a number of different enzymes work together to pull apart the two strands so each strand can be used as
a template to synthesize new complementary strands. The two new daughter DNA molecules each contain one pre-
existing strand and one newly synthesized strand. Thus, DNA replication is said to be “semiconservative.”


Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including
helicase, untwist and separate the two strands of DNA.
Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerase
brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase
is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand
continues to be built until it has fully complemented the template strand.
Stage 3: Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA
replication is stopped and the two new identical DNA molecules are complete.
Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term
for this mode of replication is “semiconservative,” because half of the original DNA molecule is conserved in each new
DNA molecule. This process continues until the cell’s entire genome, the entire complement of an organism’s DNA, is
replicated. As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body
contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental
addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are
mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan
the newly synthesized molecule for mistakes and corrects them. Once the process of DNA replication is complete, the cell
is ready to divide. You will explore the process of cell division later in the chapter.


Watch this video (http://openstaxcollege.org/l/DNArep) to learn about DNA replication. DNA replication proceeds
simultaneously at several sites on the same molecule. What separates the base pair at the start of DNA replication?


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3.4 | Protein Synthesis
By the end of this section, you will be able to:
• Explain how the genetic code stored within DNA determines the protein that will form
• Describe the process of transcription
• Describe the process of translation
• Discuss the function of ribosomes


It was mentioned earlier that DNA provides a “blueprint” for the cell structure and physiology. This refers to the fact
that DNA contains the information necessary for the cell to build one very important type of molecule: the protein. Most
structural components of the cell are made up, at least in part, by proteins and virtually all the functions that a cell carries
out are completed with the help of proteins. One of the most important classes of proteins is enzymes, which help speed
up necessary biochemical reactions that take place inside the cell. Some of these critical biochemical reactions include
building larger molecules from smaller components (such as occurs during DNA replication or synthesis of microtubules)
and breaking down larger molecules into smaller components (such as when harvesting chemical energy from nutrient
molecules). Whatever the cellular process may be, it is almost sure to involve proteins. Just as the cell’s genome describes
its full complement of DNA, a cell’s proteome is its full complement of proteins. Protein synthesis begins with genes. A
gene is a functional segment of DNA that provides the genetic information necessary to build a protein. Each particular gene
provides the code necessary to construct a particular protein.Gene expression, which transforms the information coded in a
gene to a final gene product, ultimately dictates the structure and function of a cell by determining which proteins are made.
The interpretation of genes works in the following way. Recall that proteins are polymers, or chains, of many amino acid
building blocks. The sequence of bases in a gene (that is, its sequence of A, T, C, G nucleotides) translates to an amino acid
sequence. A triplet is a section of three DNA bases in a row that codes for a specific amino acid. Similar to the way in
which the three-letter code d-o-g signals the image of a dog, the three-letter DNA base code signals the use of a particular
amino acid. For example, the DNA triplet CAC (cytosine, adenine, and cytosine) specifies the amino acid valine. Therefore,
a gene, which is composed of multiple triplets in a unique sequence, provides the code to build an entire protein, with
multiple amino acids in the proper sequence (Figure 3.25). The mechanism by which cells turn the DNA code into a protein
product is a two-step process, with an RNA molecule as the intermediate.


Figure 3.25 The Genetic Code DNA holds all of the genetic information necessary to build a cell’s proteins. The
nucleotide sequence of a gene is ultimately translated into an amino acid sequence of the gene’s corresponding
protein.


From DNA to RNA: Transcription
DNA is housed within the nucleus, and protein synthesis takes place in the cytoplasm, thus there must be some sort of
intermediate messenger that leaves the nucleus and manages protein synthesis. This intermediate messenger is messenger
RNA (mRNA), a single-stranded nucleic acid that carries a copy of the genetic code for a single gene out of the nucleus
and into the cytoplasm where it is used to produce proteins.
There are several different types of RNA, each having different functions in the cell. The structure of RNA is similar to
DNA with a few small exceptions. For one thing, unlike DNA, most types of RNA, including mRNA, are single-stranded
and contain no complementary strand. Second, the ribose sugar in RNA contains an additional oxygen atom compared with


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DNA. Finally, instead of the base thymine, RNA contains the base uracil. This means that adenine will always pair up with
uracil during the protein synthesis process.
Gene expression begins with the process called transcription, which is the synthesis of a strand of mRNA that is
complementary to the gene of interest. This process is called transcription because the mRNA is like a transcript, or copy, of
the gene’s DNA code. Transcription begins in a fashion somewhat like DNA replication, in that a region of DNA unwinds
and the two strands separate, however, only that small portion of the DNA will be split apart. The triplets within the gene on
this section of the DNA molecule are used as the template to transcribe the complementary strand of RNA (Figure 3.26). A
codon is a three-base sequence of mRNA, so-called because they directly encode amino acids. Like DNA replication, there
are three stages to transcription: initiation, elongation, and termination.


Figure 3.26 Transcription: from DNA to mRNA In the first of the two stages of making protein from DNA, a gene on
the DNA molecule is transcribed into a complementary mRNA molecule.


Stage 1: Initiation. A region at the beginning of the gene called a promoter—a particular sequence of nucleotides—triggers
the start of transcription.
Stage 2: Elongation. Transcription starts when RNA polymerase unwinds the DNA segment. One strand, referred to as the
coding strand, becomes the template with the genes to be coded. The polymerase then aligns the correct nucleic acid (A,
C, G, or U) with its complementary base on the coding strand of DNA. RNA polymerase is an enzyme that adds new
nucleotides to a growing strand of RNA. This process builds a strand of mRNA.
Stage 3: Termination.When the polymerase has reached the end of the gene, one of three specific triplets (UAA, UAG, or
UGA) codes a “stop” signal, which triggers the enzymes to terminate transcription and release the mRNA transcript.
Before the mRNA molecule leaves the nucleus and proceeds to protein synthesis, it is modified in a number of ways. For
this reason, it is often called a pre-mRNA at this stage. For example, your DNA, and thus complementary mRNA, contains
long regions called non-coding regions that do not code for amino acids. Their function is still a mystery, but the process
called splicing removes these non-coding regions from the pre-mRNA transcript (Figure 3.27). A spliceosome—a structure
composed of various proteins and other molecules—attaches to the mRNA and “splices” or cuts out the non-coding regions.
The removed segment of the transcript is called an intron. The remaining exons are pasted together. An exon is a segment
of RNA that remains after splicing. Interestingly, some introns that are removed from mRNA are not always non-coding.
When different coding regions of mRNA are spliced out, different variations of the protein will eventually result, with
differences in structure and function. This process results in a much larger variety of possible proteins and protein functions.
When the mRNA transcript is ready, it travels out of the nucleus and into the cytoplasm.


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Figure 3.27 Splicing DNA In the nucleus, a structure called a spliceosome cuts out introns (noncoding regions)
within a pre-mRNA transcript and reconnects the exons.


From RNA to Protein: Translation
Like translating a book from one language into another, the codons on a strand of mRNA must be translated into the
amino acid alphabet of proteins. Translation is the process of synthesizing a chain of amino acids called a polypeptide.
Translation requires two major aids: first, a “translator,” the molecule that will conduct the translation, and second, a
substrate on which the mRNA strand is translated into a new protein, like the translator’s “desk.” Both of these requirements
are fulfilled by other types of RNA. The substrate on which translation takes place is the ribosome.
Remember that many of a cell’s ribosomes are found associated with the rough ER, and carry out the synthesis of proteins
destined for the Golgi apparatus. Ribosomal RNA (rRNA) is a type of RNA that, together with proteins, composes the
structure of the ribosome. Ribosomes exist in the cytoplasm as two distinct components, a small and a large subunit. When
an mRNA molecule is ready to be translated, the two subunits come together and attach to the mRNA. The ribosome
provides a substrate for translation, bringing together and aligning the mRNA molecule with the molecular “translators”
that must decipher its code.
The other major requirement for protein synthesis is the translator molecules that physically “read” the mRNA codons.
Transfer RNA (tRNA) is a type of RNA that ferries the appropriate corresponding amino acids to the ribosome, and
attaches each new amino acid to the last, building the polypeptide chain one-by-one. Thus tRNA transfers specific amino
acids from the cytoplasm to a growing polypeptide. The tRNA molecules must be able to recognize the codons on mRNA
and match them with the correct amino acid. The tRNA is modified for this function. On one end of its structure is a binding
site for a specific amino acid. On the other end is a base sequence that matches the codon specifying its particular amino
acid. This sequence of three bases on the tRNA molecule is called an anticodon. For example, a tRNA responsible for
shuttling the amino acid glycine contains a binding site for glycine on one end. On the other end it contains an anticodon
that complements the glycine codon (GGA is a codon for glycine, and so the tRNAs anticodon would read CCU). Equipped
with its particular cargo and matching anticodon, a tRNA molecule can read its recognized mRNA codon and bring the
corresponding amino acid to the growing chain (Figure 3.28).


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Figure 3.28 Translation from RNA to Protein During translation, the mRNA transcript is “read” by a functional
complex consisting of the ribosome and tRNA molecules. tRNAs bring the appropriate amino acids in sequence to the
growing polypeptide chain by matching their anti-codons with codons on the mRNA strand.


Much like the processes of DNA replication and transcription, translation consists of three main stages: initiation,
elongation, and termination. Initiation takes place with the binding of a ribosome to an mRNA transcript. The elongation
stage involves the recognition of a tRNA anticodon with the next mRNA codon in the sequence. Once the anticodon and
codon sequences are bound (remember, they are complementary base pairs), the tRNA presents its amino acid cargo and the
growing polypeptide strand is attached to this next amino acid. This attachment takes place with the assistance of various
enzymes and requires energy. The tRNA molecule then releases the mRNA strand, the mRNA strand shifts one codon over
in the ribosome, and the next appropriate tRNA arrives with its matching anticodon. This process continues until the final
codon on the mRNA is reached which provides a “stop” message that signals termination of translation and triggers the
release of the complete, newly synthesized protein. Thus, a gene within the DNAmolecule is transcribed into mRNA, which
is then translated into a protein product (Figure 3.29).


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Figure 3.29 From DNA to Protein: Transcription through Translation Transcription within the cell nucleus
produces an mRNA molecule, which is modified and then sent into the cytoplasm for translation. The transcript is
decoded into a protein with the help of a ribosome and tRNA molecules.


Commonly, an mRNA transcription will be translated simultaneously by several adjacent ribosomes. This increases the
efficiency of protein synthesis. A single ribosome might translate an mRNA molecule in approximately one minute; so
multiple ribosomes aboard a single transcript could produce multiple times the number of the same protein in the same
minute. A polyribosome is a string of ribosomes translating a single mRNA strand.


Watch this video (http://openstaxcollege.org/l/ribosome) to learn about ribosomes. The ribosome binds to the mRNA
molecule to start translation of its code into a protein. What happens to the small and large ribosomal subunits at the
end of translation?


3.5 | Cell Growth and Division
By the end of this section, you will be able to:
• Describe the stages of the cell cycle
• Discuss how the cell cycle is regulated
• Describe the implications of losing control over the cell cycle
• Describe the stages of mitosis and cytokinesis, in order


So far in this chapter, you have read numerous times of the importance and prevalence of cell division. While there are a
few cells in the body that do not undergo cell division (such as gametes, red blood cells, most neurons, and some muscle
cells), most somatic cells divide regularly. A somatic cell is a general term for a body cell, and all human cells, except for
the cells that produce eggs and sperm (which are referred to as germ cells), are somatic cells. Somatic cells contain two


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copies of each of their chromosomes (one copy received from each parent). A homologous pair of chromosomes is the two
copies of a single chromosome found in each somatic cell. The human is a diploid organism, having 23 homologous pairs
of chromosomes in each of the somatic cells. The condition of having pairs of chromosomes is known as diploidy.
Cells in the body replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract
must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell
to divide, and how does it prepare for and complete cell division? The cell cycle is the sequence of events in the life of the
cell from the moment it is created at the end of a previous cycle of cell division until it then divides itself, generating two
new cells.


The Cell Cycle
One “turn” or cycle of the cell cycle consists of two general phases: interphase, followed by mitosis and cytokinesis.
Interphase is the period of the cell cycle during which the cell is not dividing. The majority of cells are in interphase
most of the time.Mitosis is the division of genetic material, during which the cell nucleus breaks down and two new, fully
functional, nuclei are formed. Cytokinesis divides the cytoplasm into two distinctive cells.
Interphase
A cell grows and carries out all normal metabolic functions and processes in a period called G1 (Figure 3.30). G1 phase
(gap 1 phase) is the first gap, or growth phase in the cell cycle. For cells that will divide again, G1 is followed by replication
of the DNA, during the S phase. The S phase (synthesis phase) is period during which a cell replicates its DNA.


Figure 3.30 Cell Cycle The two major phases of the cell cycle include mitosis (cell division), and interphase, when
the cell grows and performs all of its normal functions. Interphase is further subdivided into G1, S, and G2 phases.


After the synthesis phase, the cell proceeds through the G2 phase. The G2 phase is a second gap phase, during which the
cell continues to grow and makes the necessary preparations for mitosis. Between G1, S, and G2 phases, cells will vary
the most in their duration of the G1 phase. It is here that a cell might spend a couple of hours, or many days. The S phase
typically lasts between 8-10 hours and the G2 phase approximately 5 hours. In contrast to these phases, the G0 phase is a
resting phase of the cell cycle. Cells that have temporarily stopped dividing and are resting (a common condition) and cells
that have permanently ceased dividing (like nerve cells) are said to be in G0.
The Structure of Chromosomes
Billions of cells in the human body divide every day. During the synthesis phase (S, for DNA synthesis) of interphase,
the amount of DNA within the cell precisely doubles. Therefore, after DNA replication but before cell division, each cell
actually contains two copies of each chromosome. Each copy of the chromosome is referred to as a sister chromatid and is
physically bound to the other copy. The centromere is the structure that attaches one sister chromatid to another. Because
a human cell has 46 chromosomes, during this phase, there are 92 chromatids (46 × 2) in the cell. Make sure not to confuse
the concept of a pair of chromatids (one chromosome and its exact copy attached during mitosis) and a homologous pair of
chromosomes (two paired chromosomes which were inherited separately, one from each parent) (Figure 3.31).


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Figure 3.31 A Homologous Pair of Chromosomes with their Attached Sister Chromatids The red and blue
colors correspond to a homologous pair of chromosomes. Each member of the pair was separately inherited from one
parent. Each chromosome in the homologous pair is also bound to an identical sister chromatid, which is produced by
DNA replication, and results in the familiar “X” shape.


Mitosis and Cytokinesis
The mitotic phase of the cell typically takes between 1 and 2 hours. During this phase, a cell undergoes two major
processes. First, it completes mitosis, during which the contents of the nucleus are equitably pulled apart and distributed
between its two halves. Cytokinesis then occurs, dividing the cytoplasm and cell body into two new cells. Mitosis is divided
into four major stages that take place after interphase (Figure 3.32) and in the following order: prophase, metaphase,
anaphase, and telophase. The process is then followed by cytokinesis.


Figure 3.32 Cell Division: Mitosis Followed by Cytokinesis The stages of cell division oversee the separation of
identical genetic material into two new nuclei, followed by the division of the cytoplasm.


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Prophase is the first phase of mitosis, during which the loosely packed chromatin coils and condenses into visible
chromosomes. During prophase, each chromosome becomes visible with its identical partner attached, forming the familiar
X-shape of sister chromatids. The nucleolus disappears early during this phase, and the nuclear envelope also disintegrates.
A major occurrence during prophase concerns a very important structure that contains the origin site for microtubule
growth. Recall the cellular structures called centrioles that serve as origin points from which microtubules extend. These
tiny structures also play a very important role during mitosis. A centrosome is a pair of centrioles together. The cell contains
two centrosomes side-by-side, which begin to move apart during prophase. As the centrosomes migrate to two different
sides of the cell, microtubules begin to extend from each like long fingers from two hands extending toward each other. The
mitotic spindle is the structure composed of the centrosomes and their emerging microtubules.
Near the end of prophase there is an invasion of the nuclear area by microtubules from the mitotic spindle. The nuclear
membrane has disintegrated, and the microtubules attach themselves to the centromeres that adjoin pairs of sister
chromatids. The kinetochore is a protein structure on the centromere that is the point of attachment between the mitotic
spindle and the sister chromatids. This stage is referred to as late prophase or “prometaphase” to indicate the transition
between prophase and metaphase.
Metaphase is the second stage of mitosis. During this stage, the sister chromatids, with their attached microtubules, line
up along a linear plane in the middle of the cell. A metaphase plate forms between the centrosomes that are now located at
either end of the cell. The metaphase plate is the name for the plane through the center of the spindle on which the sister
chromatids are positioned. The microtubules are now poised to pull apart the sister chromatids and bring one from each pair
to each side of the cell.
Anaphase is the third stage of mitosis. Anaphase takes place over a few minutes, when the pairs of sister chromatids are
separated from one another, forming individual chromosomes once again. These chromosomes are pulled to opposite ends
of the cell by their kinetochores, as the microtubules shorten. Each end of the cell receives one partner from each pair of
sister chromatids, ensuring that the two new daughter cells will contain identical genetic material.
Telophase is the final stage of mitosis. Telophase is characterized by the formation of two new daughter nuclei at either end
of the dividing cell. These newly formed nuclei surround the genetic material, which uncoils such that the chromosomes
return to loosely packed chromatin. Nucleoli also reappear within the new nuclei, and the mitotic spindle breaks apart, each
new cell receiving its own complement of DNA, organelles, membranes, and centrioles. At this point, the cell is already
beginning to split in half as cytokinesis begins.
The cleavage furrow is a contractile band made up of microfilaments that forms around the midline of the cell during
cytokinesis. (Recall that microfilaments consist of actin.) This contractile band squeezes the two cells apart until they finally
separate. Two new cells are now formed. One of these cells (the “stem cell”) enters its own cell cycle; able to grow and
divide again at some future time. The other cell transforms into the functional cell of the tissue, typically replacing an “old”
cell there.
Imagine a cell that completed mitosis but never underwent cytokinesis. In some cases, a cell may divide its genetic material
and grow in size, but fail to undergo cytokinesis. This results in larger cells with more than one nucleus. Usually this is an
unwanted aberration and can be a sign of cancerous cells.


Cell Cycle Control
A very elaborate and precise system of regulation controls direct the way cells proceed from one phase to the next in the cell
cycle and begin mitosis. The control system involves molecules within the cell as well as external triggers. These internal
and external control triggers provide “stop” and “advance” signals for the cell. Precise regulation of the cell cycle is critical
for maintaining the health of an organism, and loss of cell cycle control can lead to cancer.
Mechanisms of Cell Cycle Control
As the cell proceeds through its cycle, each phase involves certain processes that must be completed before the cell should
advance to the next phase. A checkpoint is a point in the cell cycle at which the cycle can be signaled to move forward or
stopped. At each of these checkpoints, different varieties of molecules provide the stop or go signals, depending on certain
conditions within the cell. A cyclin is one of the primary classes of cell cycle control molecules (Figure 3.33). A cyclin-
dependent kinase (CDK) is one of a group of molecules that work together with cyclins to determine progression past cell
checkpoints. By interacting with many additional molecules, these triggers push the cell cycle forward unless prevented
from doing so by “stop” signals, if for some reason the cell is not ready. At the G1 checkpoint, the cell must be ready for
DNA synthesis to occur. At the G2 checkpoint the cell must be fully prepared for mitosis. Even during mitosis, a crucial stop
and go checkpoint in metaphase ensures that the cell is fully prepared to complete cell division. The metaphase checkpoint
ensures that all sister chromatids are properly attached to their respective microtubules and lined up at the metaphase plate
before the signal is given to separate them during anaphase.


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Figure 3.33 Control of the Cell Cycle Cells proceed through the cell cycle under the control of a variety of molecules,
such as cyclins and cyclin-dependent kinases. These control molecules determine whether or not the cell is prepared
to move into the following stage.


The Cell Cycle Out of Control: Implications
Most people understand that cancer or tumors are caused by abnormal cells that multiply continuously. If the abnormal cells
continue to divide unstopped, they can damage the tissues around them, spread to other parts of the body, and eventually
result in death. In healthy cells, the tight regulation mechanisms of the cell cycle prevent this from happening, while failures
of cell cycle control can cause unwanted and excessive cell division. Failures of control may be caused by inherited genetic
abnormalities that compromise the function of certain “stop” and “go” signals. Environmental insult that damages DNA can
also cause dysfunction in those signals. Often, a combination of both genetic predisposition and environmental factors lead
to cancer.
The process of a cell escaping its normal control system and becoming cancerous may actually happen throughout the
body quite frequently. Fortunately, certain cells of the immune system are capable of recognizing cells that have become
cancerous and destroying them. However, in certain cases the cancerous cells remain undetected and continue to proliferate.
If the resulting tumor does not pose a threat to surrounding tissues, it is said to be benign and can usually be easily removed.
If capable of damage, the tumor is considered malignant and the patient is diagnosed with cancer.


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Cancer Arises from Homeostatic Imbalances
Cancer is an extremely complex condition, capable of arising from a wide variety of genetic and environmental
causes. Typically, mutations or aberrations in a cell’s DNA that compromise normal cell cycle control systems lead to
cancerous tumors. Cell cycle control is an example of a homeostatic mechanism that maintains proper cell function and
health. While progressing through the phases of the cell cycle, a large variety of intracellular molecules provide stop
and go signals to regulate movement forward to the next phase. These signals are maintained in an intricate balance so
that the cell only proceeds to the next phase when it is ready. This homeostatic control of the cell cycle can be thought
of like a car’s cruise control. Cruise control will continually apply just the right amount of acceleration to maintain
a desired speed, unless the driver hits the brakes, in which case the car will slow down. Similarly, the cell includes
molecular messengers, such as cyclins, that push the cell forward in its cycle.
In addition to cyclins, a class of proteins that are encoded by genes called proto-oncogenes provide important signals
that regulate the cell cycle and move it forward. Examples of proto-oncogene products include cell-surface receptors
for growth factors, or cell-signaling molecules, two classes of molecules that can promote DNA replication and cell
division. In contrast, a second class of genes known as tumor suppressor genes sends stop signals during a cell cycle.
For example, certain protein products of tumor suppressor genes signal potential problems with the DNA and thus stop
the cell from dividing, while other proteins signal the cell to die if it is damaged beyond repair. Some tumor suppressor
proteins also signal a sufficient surrounding cellular density, which indicates that the cell need not presently divide. The
latter function is uniquely important in preventing tumor growth: normal cells exhibit a phenomenon called “contact
inhibition;” thus, extensive cellular contact with neighboring cells causes a signal that stops further cell division.
These two contrasting classes of genes, proto-oncogenes and tumor suppressor genes, are like the accelerator and
brake pedal of the cell’s own “cruise control system,” respectively. Under normal conditions, these stop and go signals
are maintained in a homeostatic balance. Generally speaking, there are two ways that the cell’s cruise control can
lose control: a malfunctioning (overactive) accelerator, or a malfunctioning (underactive) brake. When compromised
through a mutation, or otherwise altered, proto-oncogenes can be converted to oncogenes, which produce oncoproteins
that push a cell forward in its cycle and stimulate cell division even when it is undesirable to do so. For example, a cell
that should be programmed to self-destruct (a process called apoptosis) due to extensive DNA damage might instead
be triggered to proliferate by an oncoprotein. On the other hand, a dysfunctional tumor suppressor gene may fail to
provide the cell with a necessary stop signal, also resulting in unwanted cell division and proliferation.
A delicate homeostatic balance between the many proto-oncogenes and tumor suppressor genes delicately controls the
cell cycle and ensures that only healthy cells replicate. Therefore, a disruption of this homeostatic balance can cause
aberrant cell division and cancerous growths.


Visit this link (http://openstaxcollege.org/l/mitosis) to learn about mitosis. Mitosis results in two identical diploid
cells. What structures forms during prophase?


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3.6 | Cellular Differentiation
By the end of this section, you will be able to:
• Discuss how the generalized cells of a developing embryo or the stem cells of an adult organism become
differentiated into specialized cells


• Distinguish between the categories of stem cells


How does a complex organism such as a human develop from a single cell—a fertilized egg—into the vast array of
cell types such as nerve cells, muscle cells, and epithelial cells that characterize the adult? Throughout development
and adulthood, the process of cellular differentiation leads cells to assume their final morphology and physiology.
Differentiation is the process by which unspecialized cells become specialized to carry out distinct functions.


Stem Cells
A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate
into specialized cells. Stem cells are divided into several categories according to their potential to differentiate.
The first embryonic cells that arise from the division of the zygote are the ultimate stem cells; these stems cells are described
as totipotent because they have the potential to differentiate into any of the cells needed to enable an organism to grow and
develop.
The embryonic cells that develop from totipotent stem cells and are precursors to the fundamental tissue layers of the
embryo are classified as pluripotent. A pluripotent stem cell is one that has the potential to differentiate into any type of
human tissue but cannot support the full development of an organism. These cells then become slightly more specialized,
and are referred to as multipotent cells.
A multipotent stem cell has the potential to differentiate into different types of cells within a given cell lineage or small
number of lineages, such as a red blood cell or white blood cell.
Finally, multipotent cells can become further specialized oligopotent cells. An oligopotent stem cell is limited to becoming
one of a few different cell types. In contrast, a unipotent cell is fully specialized and can only reproduce to generate more
of its own specific cell type.
Stem cells are unique in that they can also continually divide and regenerate new stem cells instead of further specializing.
There are different stem cells present at different stages of a human’s life. They include the embryonic stem cells of the
embryo, fetal stem cells of the fetus, and adult stem cells in the adult. One type of adult stem cell is the epithelial stem
cell, which gives rise to the keratinocytes in the multiple layers of epithelial cells in the epidermis of skin. Adult bone
marrow has three distinct types of stem cells: hematopoietic stem cells, which give rise to red blood cells, white blood cells,
and platelets (Figure 3.34); endothelial stem cells, which give rise to the endothelial cell types that line blood and lymph
vessels; and mesenchymal stem cells, which give rise to the different types of muscle cells.


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Figure 3.34 Hematopoiesis The process of hematopoiesis involves the differentiation of multipotent cells into blood
and immune cells. The multipotent hematopoietic stem cells give rise to many different cell types, including the cells of
the immune system and red blood cells.


Differentiation
When a cell differentiates (becomes more specialized), it may undertake major changes in its size, shape, metabolic activity,
and overall function. Because all cells in the body, beginning with the fertilized egg, contain the same DNA, how do the
different cell types come to be so different? The answer is analogous to a movie script. The different actors in a movie all
read from the same script, however, they are each only reading their own part of the script. Similarly, all cells contain the
same full complement of DNA, but each type of cell only “reads” the portions of DNA that are relevant to its own function.
In biology, this is referred to as the unique genetic expression of each cell.
In order for a cell to differentiate into its specialized form and function, it need only manipulate those genes (and thus those
proteins) that will be expressed, and not those that will remain silent. The primary mechanism by which genes are turned
“on” or “off” is through transcription factors. A transcription factor is one of a class of proteins that bind to specific genes
on the DNA molecule and either promote or inhibit their transcription (Figure 3.35).


Figure 3.35 Transcription Factors Regulate Gene Expression While each body cell contains the organism’s entire
genome, different cells regulate gene expression with the use of various transcription factors. Transcription factors are
proteins that affect the binding of RNA polymerase to a particular gene on the DNA molecule.


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Stem Cell Research
Stem cell research aims to find ways to use stem cells to regenerate and repair cellular damage. Over time, most adult
cells undergo the wear and tear of aging and lose their ability to divide and repair themselves. Stem cells do not display
a particular morphology or function. Adult stem cells, which exist as a small subset of cells in most tissues, keep
dividing and can differentiate into a number of specialized cells generally formed by that tissue. These cells enable the
body to renew and repair body tissues.
The mechanisms that induce a non-differentiated cell to become a specialized cell are poorly understood. In a
laboratory setting, it is possible to induce stem cells to differentiate into specialized cells by changing the physical and
chemical conditions of growth. Several sources of stem cells are used experimentally and are classified according to
their origin and potential for differentiation. Human embryonic stem cells (hESCs) are extracted from embryos and
are pluripotent. The adult stem cells that are present in many organs and differentiated tissues, such as bone marrow
and skin, are multipotent, being limited in differentiation to the types of cells found in those tissues. The stem cells
isolated from umbilical cord blood are also multipotent, as are cells from deciduous teeth (baby teeth). Researchers
have recently developed induced pluripotent stem cells (iPSCs) from mouse and human adult stem cells. These cells
are genetically reprogrammed multipotent adult cells that function like embryonic stem cells; they are capable of
generating cells characteristic of all three germ layers.
Because of their capacity to divide and differentiate into specialized cells, stem cells offer a potential treatment for
diseases such as diabetes and heart disease (Figure 3.36). Cell-based therapy refers to treatment in which stem cells
induced to differentiate in a growth dish are injected into a patient to repair damaged or destroyed cells or tissues.
Many obstacles must be overcome for the application of cell-based therapy. Although embryonic stem cells have a
nearly unlimited range of differentiation potential, they are seen as foreign by the patient’s immune system and may
trigger rejection. Also, the destruction of embryos to isolate embryonic stem cells raises considerable ethical and legal
questions.


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Figure 3.36 Stem CellsThe capacity of stem cells to differentiate into specialized cells make them potentially
valuable in therapeutic applications designed to replace damaged cells of different body tissues.


In contrast, adult stem cells isolated from a patient are not seen as foreign by the body, but they have a limited range of
differentiation. Some individuals bank the cord blood or deciduous teeth of their child, storing away those sources of
stem cells for future use, should their child need it. Induced pluripotent stem cells are considered a promising advance
in the field because using them avoids the legal, ethical, and immunological pitfalls of embryonic stem cells.


CHAPTER 3 | THE CELLULAR LEVEL OF ORGANIZATION 121




active transport
amphipathic


anaphase


anticodon


autolysis
autophagy
cell cycle
cell membrane


centriole


centromere
centrosome
channel protein


checkpoint


chromatin
chromosome
cilia


cleavage furrow
codon
concentration gradient
cyclin
cyclin-dependent kinase (CDK)


cytokinesis
cytoplasm


cytoskeleton


cytosol
diffusion


KEY TERMS
form of transport across the cell membrane that requires input of cellular energy


describes a molecule that exhibits a difference in polarity between its two ends, resulting in a difference in
water solubility


third stage of mitosis (and meiosis), during which sister chromatids separate into two new nuclear regions of
a dividing cell


consecutive sequence of three nucleotides on a tRNA molecule that is complementary to a specific codon on
an mRNA molecule


breakdown of cells by their own enzymatic action
lysosomal breakdown of a cell’s own components
life cycle of a single cell, from its birth until its division into two new daughter cells


membrane surrounding all animal cells, composed of a lipid bilayer interspersed with various
molecules; also known as plasma membrane


small, self-replicating organelle that provides the origin for microtubule growth and moves DNA during cell
division


region of attachment for two sister chromatids
cellular structure that organizes microtubules during cell division
membrane-spanning protein that has an inner pore which allows the passage of one or more


substances
progress point in the cell cycle during which certain conditions must be met in order for the cell to proceed


to a subsequence phase
substance consisting of DNA and associated proteins
condensed version of chromatin


small appendage on certain cells formed by microtubules and modified for movement of materials across the
cellular surface


contractile ring that forms around a cell during cytokinesis that pinches the cell into two halves
consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid


difference in the concentration of a substance between two regions
one of a group of proteins that function in the progression of the cell cycle


one of a group of enzymes associated with cyclins that help them perform their
functions


final stage in cell division, where the cytoplasm divides to form two separate daughter cells
internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid


called cytosol, within which are all the other organelles and cellular solute and suspended materials
“skeleton” of a cell; formed by rod-like proteins that support the cell’s shape and provide, among other


functions, locomotive abilities
clear, semi-fluid medium of the cytoplasm, made up mostly of water
movement of a substance from an area of higher concentration to one of lower concentration


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diploid


DNA polymerase


DNA replication
electrical gradient
endocytosis
endoplasmic reticulum (ER)


exocytosis
exon
extracellular fluid (ECF)


facilitated diffusion
flagellum
G0 phase


G1 phase
G2 phase
gene
gene expression
genome
glycocalyx
glycoprotein
Golgi apparatus


helicase
histone
homologous
hydrophilic
hydrophobic
hypertonic
hypotonic
integral protein
intermediate filament


interphase
interstitial fluid (IF)


condition marked by the presence of a double complement of genetic material (two sets of chromosomes, one
set inherited from each of two parents)


enzyme that functions in adding new nucleotides to a growing strand of DNA during DNA
replication


process of duplicating a molecule of DNA
difference in the electrical charge (potential) between two regions


import of material into the cell by formation of a membrane-bound vesicle
cellular organelle that consists of interconnected membrane-bound tubules, which may


or may not be associated with ribosomes (rough type or smooth type, respectively)
export of a substance out of a cell by formation of a membrane-bound vesicle


one of the coding regions of an mRNA molecule that remain after splicing
fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluid found in other


reservoirs in the body
diffusion of a substance with the aid of a membrane protein


appendage on certain cells formed by microtubules and modified for movement
phase of the cell cycle, usually entered from the G1 phase; characterized by long or permanent periods where


the cell does not move forward into the DNA synthesis phase
first phase of the cell cycle, after a new cell is born
third phase of the cell cycle, after the DNA synthesis phase


functional length of DNA that provides the genetic information necessary to build a protein
active interpretation of the information coded in a gene to produce a functional gene product


entire complement of an organism’s DNA; found within virtually every cell
coating of sugar molecules that surrounds the cell membrane
protein that has one or more carbohydrates attached
cellular organelle formed by a series of flattened, membrane-bound sacs that functions in protein


modification, tagging, packaging, and transport
enzyme that functions to separate the two DNA strands of a double helix during DNA replication
family of proteins that associate with DNA in the nucleus to form chromatin


describes two copies of the same chromosome (not identical), one inherited from each parent
describes a substance or structure attracted to water
describes a substance or structure repelled by water


describes a solution concentration that is higher than a reference concentration
describes a solution concentration that is lower than a reference concentration


membrane-associated protein that spans the entire width of the lipid bilayer
type of cytoskeletal filament made of keratin, characterized by an intermediate thickness, and


playing a role in resisting cellular tension
entire life cycle of a cell, excluding mitosis


fluid in the small spaces between cells not contained within blood vessels


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intracellular fluid (ICF)
intron
isotonic
kinetochore
ligand
lysosome
messenger RNA (mRNA)


metaphase


metaphase plate
microfilament


microtubule


mitochondrion


mitosis


mitotic phase
mitotic spindle


multipotent


mutation
nuclear envelope
nuclear pore
nucleolus
nucleosome
nucleus
oligopotent


organelle


osmosis
passive transport
peripheral protein


peroxisome


fluid in the cytosol of cells
non-coding regions of a pre-mRNA transcript that may be removed during splicing
describes a solution concentration that is the same as a reference concentration
region of a centromere where microtubules attach to a pair of sister chromatids


molecule that binds with specificity to a specific receptor molecule
membrane-bound cellular organelle originating from the Golgi apparatus and containing digestive enzymes


nucleotide molecule that serves as an intermediate in the genetic code between DNA and
protein


second stage of mitosis (and meiosis), characterized by the linear alignment of sister chromatids in the
center of the cell


linear alignment of sister chromatids in the center of the cell, which takes place during metaphase
the thinnest of the cytoskeletal filaments; composed of actin subunits that function in muscle


contraction and cellular structural support
the thickest of the cytoskeletal filaments, composed of tubulin subunits that function in cellular movement


and structural support
one of the cellular organelles bound by a double lipid bilayer that function primarily in the production


of cellular energy (ATP)
division of genetic material, during which the cell nucleus breaks down and two new, fully functional, nuclei


are formed
phase of the cell cycle in which a cell undergoes mitosis
network of microtubules, originating from centrioles, that arranges and pulls apart chromosomes


during mitosis
describes the condition of being able to differentiate into different types of cells within a given cell lineage


or small number of lineages, such as a red blood cell or white blood cell
change in the nucleotide sequence in a gene within a cell’s DNA


membrane that surrounds the nucleus; consisting of a double lipid-bilayer
one of the small, protein-lined openings found scattered throughout the nuclear envelope


small region of the nucleus that functions in ribosome synthesis
unit of chromatin consisting of a DNA strand wrapped around histone proteins


cell’s central organelle; contains the cell’s DNA
describes the condition of being more specialized than multipotency; the condition of being able to


differentiate into one of a few possible cell types
any of several different types of membrane-enclosed specialized structures in the cell that perform specific


functions for the cell
diffusion of molecules down their concentration across a selectively permeable membrane


form of transport across the cell membrane that does not require input of cellular energy
membrane-associated protein that does not span the width of the lipid bilayer, but is attached


peripherally to integral proteins, membrane lipids, or other components of the membrane
membrane-bound organelle that contains enzymes primarily responsible for detoxifying harmful


substances


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phagocytosis
pinocytosis
pluripotent
polypeptide
polyribosome
promoter
prophase


proteome
reactive oxygen species (ROS)


receptor
receptor-mediated endocytosis
ribosomal RNA (rRNA)
ribosome
RNA polymerase


S phase
selective permeability
sister chromatid
sodium-potassium pump


somatic cell
spliceosome
splicing
stem cell


telophase


totipotent
transcription
transcription factor
transfer RNA (tRNA)


translation
triplet


unipotent


endocytosis of large particles
endocytosis of fluid
describes the condition of being able to differentiate into a large variety of cell types
chain of amino acids linked by peptide bonds
simultaneous translation of a single mRNA transcript by multiple ribosomes


region of DNA that signals transcription to begin at that site within the gene
first stage of mitosis (and meiosis), characterized by breakdown of the nuclear envelope and condensing of


the chromatin to form chromosomes
full complement of proteins produced by a cell (determined by the cell’s specific gene expression)


a group of extremely reactive peroxides and oxygen-containing radicals that may
contribute to cellular damage


protein molecule that contains a binding site for another specific molecule (called a ligand)
endocytosis of ligands attached to membrane-bound receptors


RNA that makes up the subunits of a ribosome
cellular organelle that functions in protein synthesis


enzyme that unwinds DNA and then adds new nucleotides to a growing strand of RNA for the
transcription phase of protein synthesis


stage of the cell cycle during which DNA replication occurs
feature of any barrier that allows certain substances to cross but excludes others


one of a pair of identical chromosomes, formed during DNA replication


(also, Na+/K+ ATP-ase) membrane-embedded protein pump that uses ATP to move Na+
out of a cell and K+ into the cell


all cells of the body excluding gamete cells
complex of enzymes that serves to splice out the introns of a pre-mRNA transcript


the process of modifying a pre-mRNA transcript by removing certain, typically non-coding, regions
cell that is oligo-, multi-, or pleuripotent that has the ability to produce additional stem cells rather than


becoming further specialized
final stage of mitosis (and meiosis), preceding cytokinesis, characterized by the formation of two new


daughter nuclei
embryonic cells that have the ability to differentiate into any type of cell and organ in the body
process of producing an mRNA molecule that is complementary to a particular gene of DNA


one of the proteins that regulate the transcription of genes
molecules of RNA that serve to bring amino acids to a growing polypeptide strand and properly


place them into the sequence
process of producing a protein from the nucleotide sequence code of an mRNA transcript


consecutive sequence of three nucleotides on a DNA molecule that, when transcribed into an mRNA codon,
corresponds to a particular amino acid


describes the condition of being committed to a single specialized cell type


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vesicle membrane-bound structure that contains materials within or outside of the cell


CHAPTER REVIEW
3.1 The Cell Membrane
The cell membrane provides a barrier around the cell, separating its internal components from the extracellular environment.
It is composed of a phospholipid bilayer, with hydrophobic internal lipid “tails” and hydrophilic external phosphate “heads.”
Various membrane proteins are scattered throughout the bilayer, both inserted within it and attached to it peripherally. The
cell membrane is selectively permeable, allowing only a limited number of materials to diffuse through its lipid bilayer.
All materials that cross the membrane do so using passive (non energy-requiring) or active (energy-requiring) transport
processes. During passive transport, materials move by simple diffusion or by facilitated diffusion through the membrane,
down their concentration gradient. Water passes through the membrane in a diffusion process called osmosis. During active
transport, energy is expended to assist material movement across the membrane in a direction against their concentration
gradient. Active transport may take place with the help of protein pumps or through the use of vesicles.


3.2 The Cytoplasm and Cellular Organelles
The internal environmental of a living cell is made up of a fluid, jelly-like substance called cytosol, which consists mainly
of water, but also contains various dissolved nutrients and other molecules. The cell contains an array of cellular organelles,
each one performing a unique function and helping to maintain the health and activity of the cell. The cytosol and organelles
together compose the cell’s cytoplasm. Most organelles are surrounded by a lipid membrane similar to the cell membrane
of the cell. The endoplasmic reticulum (ER), Golgi apparatus, and lysosomes share a functional connectivity and are
collectively referred to as the endomembrane system. There are two types of ER: smooth and rough. While the smooth ER
performs many functions, including lipid synthesis and ion storage, the rough ER is mainly responsible for protein synthesis
using its associated ribosomes. The rough ER sends newly made proteins to the Golgi apparatus where they are modified
and packaged for delivery to various locations within or outside of the cell. Some of these protein products are enzymes
destined to break down unwanted material and are packaged as lysosomes for use inside the cell.
Cells also contain mitochondria and peroxisomes, which are the organelles responsible for producing the cell’s energy
supply and detoxifying certain chemicals, respectively. Biochemical reactions within mitochondria transform energy-
carrying molecules into the usable form of cellular energy known as ATP. Peroxisomes contain enzymes that transform
harmful substances such as free radicals into oxygen and water. Cells also contain a miniaturized “skeleton” of protein
filaments that extend throughout its interior. Three different kinds of filaments compose this cytoskeleton (in order of
increasing thickness): microfilaments, intermediate filaments, and microtubules. Each cytoskeletal component performs
unique functions as well as provides a supportive framework for the cell.


3.3 The Nucleus and DNA Replication
The nucleus is the command center of the cell, containing the genetic instructions for all of the materials a cell will make
(and thus all of its functions it can perform). The nucleus is encased within a membrane of two interconnected lipid bilayers,
side-by-side. This nuclear envelope is studded with protein-lined pores that allow materials to be trafficked into and out of
the nucleus. The nucleus contains one or more nucleoli, which serve as sites for ribosome synthesis. The nucleus houses
the genetic material of the cell: DNA. DNA is normally found as a loosely contained structure called chromatin within the
nucleus, where it is wound up and associated with a variety of histone proteins. When a cell is about to divide, the chromatin
coils tightly and condenses to form chromosomes.
There is a pool of cells constantly dividing within your body. The result is billions of new cells being created each day.
Before any cell is ready to divide, it must replicate its DNA so that each new daughter cell will receive an exact copy of the
organism’s genome. A variety of enzymes are enlisted during DNA replication. These enzymes unwind the DNA molecule,
separate the two strands, and assist with the building of complementary strands along each parent strand. The original
DNA strands serve as templates from which the nucleotide sequence of the new strands are determined and synthesized.
When replication is completed, two identical DNA molecules exist. Each one contains one original strand and one newly
synthesized complementary strand.


3.4 Protein Synthesis
DNA stores the information necessary for instructing the cell to perform all of its functions. Cells use the genetic code
stored within DNA to build proteins, which ultimately determine the structure and function of the cell. This genetic code
lies in the particular sequence of nucleotides that make up each gene along the DNA molecule. To “read” this code, the
cell must perform two sequential steps. In the first step, transcription, the DNA code is converted into a RNA code. A
molecule of messenger RNA that is complementary to a specific gene is synthesized in a process similar to DNA replication.


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The molecule of mRNA provides the code to synthesize a protein. In the process of translation, the mRNA attaches to
a ribosome. Next, tRNA molecules shuttle the appropriate amino acids to the ribosome, one-by-one, coded by sequential
triplet codons on the mRNA, until the protein is fully synthesized. When completed, the mRNA detaches from the ribosome,
and the protein is released. Typically, multiple ribosomes attach to a single mRNA molecule at once such that multiple
proteins can be manufactured from the mRNA concurrently.


3.5 Cell Growth and Division
The life of cell consists of stages that make up the cell cycle. After a cell is born, it passes through an interphase before it
is ready to replicate itself and produce daughter cells. This interphase includes two gap phases (G1 and G2), as well as an
S phase, during which its DNA is replicated in preparation for cell division. The cell cycle is under precise regulation by
chemical messengers both inside and outside the cell that provide “stop” and “go” signals for movement from one phase to
the next. Failures of these signals can result in cells that continue to divide uncontrollably, which can lead to cancer.
Once a cell has completed interphase and is ready for cell division, it proceeds through four separate stages of mitosis
(prophase, metaphase, anaphase, and telophase). Telophase is followed by the division of the cytoplasm (cytokinesis), which
generates two daughter cells. This process takes place in all normally dividing cells of the body except for the germ cells
that produce eggs and sperm.


3.6 Cellular Differentiation
One of the major areas of research in biology is that of how cells specialize to assume their unique structures and functions,
since all cells essentially originate from a single fertilized egg. Cell differentiation is the process of cells becoming
specialized as they body develops. A stem cell is an unspecialized cell that can divide without limit as needed and can,
under specific conditions, differentiate into specialized cells. Stem cells are divided into several categories according to
their potential to differentiate. While all somatic cells contain the exact same genome, different cell types only express some
of those genes at any given time. These differences in gene expression ultimately dictate a cell’s unique morphological and
physiological characteristics. The primary mechanism that determines which genes will be expressed and which ones will
not is through the use of different transcription factor proteins, which bind to DNA and promote or hinder the transcription
of different genes. Through the action of these transcription factors, cells specialize into one of hundreds of different cell
types in the human body.


INTERACTIVE LINK QUESTIONS
1. Visit this link (http://openstaxcollege.org/l/diffusion)
to see diffusion and how it is propelled by the kinetic
energy of molecules in solution. How does temperature
affect diffusion rate, and why?
2. Watch this video (http://openstaxcollege.org/l/
endomembrane1) to learn about the endomembrane
system, which includes the rough and smooth ER and the
Golgi body as well as lysosomes and vesicles. What is the
primary role of the endomembrane system?
3. Watch this video (http://openstaxcollege.org/l/
DNArep) to learn about DNA replication. DNA replication


proceeds simultaneously at several sites on the same
molecule. What separates the base pair at the start of DNA
replication?
4. Watch this video (http://openstaxcollege.org/l/
ribosome) to learn about ribosomes. The ribosome binds
to the mRNA molecule to start translation of its code into
a protein. What happens to the small and large ribosomal
subunits at the end of translation?
5. Visit this link (http://openstaxcollege.org/l/mitosis) to
learn about mitosis. Mitosis results in two identical diploid
cells. What structures form during prophase?


REVIEW QUESTIONS
6. Because they are embedded within the membrane, ion
channels are examples of ________.


a. receptor proteins
b. integral proteins
c. peripheral proteins
d. glycoproteins


7. The diffusion of substances within a solution tends to
move those substances ________ their ________ gradient.


a. up; electrical
b. up; electrochemical
c. down; pressure
d. down; concentration


8. Ion pumps and phagocytosis are both examples of
________.


a. endocytosis
b. passive transport
c. active transport
d. facilitated diffusion


9. Choose the answer that best completes the following
analogy: Diffusion is to ________ as endocytosis is to
________.


a. filtration; phagocytosis
b. osmosis; pinocytosis
c. solutes; fluid
d. gradient; chemical energy


CHAPTER 3 | THE CELLULAR LEVEL OF ORGANIZATION 127




10. Choose the term that best completes the following
analogy: Cytoplasm is to cytosol as a swimming pool
containing chlorine and flotation toys is to ________.


a. the walls of the pool
b. the chlorine
c. the flotation toys
d. the water


11. The rough ER has its name due to what associated
structures?


a. Golgi apparatus
b. ribosomes
c. lysosomes
d. proteins


12. Which of the following is a function of the rough ER?


a. production of proteins
b. detoxification of certain substances
c. synthesis of steroid hormones
d. regulation of intracellular calcium concentration


13.Which of the following is a feature common to all three
components of the cytoskeleton?


a. They all serve to scaffold the organelles within
the cell.


b. They are all characterized by roughly the same
diameter.


c. They are all polymers of protein subunits.
d. They all help the cell resist compression and
tension.


14. Which of the following organelles produces large
quantities of ATP when both glucose and oxygen are
available to the cell?


a. mitochondria
b. peroxisomes
c. lysosomes
d. ER


15. The nucleus and mitochondria share which of the
following features?


a. protein-lined membrane pores
b. a double cell membrane
c. the synthesis of ribosomes
d. the production of cellular energy


16. Which of the following structures could be found
within the nucleolus?


a. chromatin
b. histones
c. ribosomes
d. nucleosomes


17. Which of the following sequences on a DNA molecule
would be complementary to GCTTATAT?


a. TAGGCGCG
b. ATCCGCGC
c. CGAATATA
d. TGCCTCTC


18. Place the following structures in order from least to
most complex organization: chromatin, nucleosome, DNA,
chromosome


a. DNA, nucleosome, chromatin, chromosome
b. nucleosome, DNA, chromosome, chromatin


c. DNA, chromatin, nucleosome, chromosome
d. nucleosome, chromatin, DNA, chromosome


19.Which of the following is part of the elongation step of
DNA synthesis?


a. pulling apart the two DNA strands
b. attaching complementary nucleotides to the
template strand


c. untwisting the DNA helix
d. none of the above


20. Which of the following is not a difference between
DNA and RNA?


a. DNA contains thymine whereas RNA contains
uracil


b. DNA contains deoxyribose and RNA contains
ribose


c. DNA contains alternating sugar-phosphate
molecules whereas RNA does not contain sugars


d. RNA is single stranded and DNA is double
stranded


21. Transcription and translation take place in the
________ and ________, respectively.


a. nucleus; cytoplasm
b. nucleolus; nucleus
c. nucleolus; cytoplasm
d. cytoplasm; nucleus


22. How many “letters” of an RNA molecule, in sequence,
does it take to provide the code for a single amino acid?


a. 1
b. 2
c. 3
d. 4


23. Which of the following is not made out of RNA?


a. the carriers that shuffle amino acids to a growing
polypeptide strand


b. the ribosome
c. the messenger molecule that provides the code
for protein synthesis


d. the intron
24. Which of the following phases is characterized by
preparation for DNA synthesis?


a. G0
b. G1
c. G2
d. S


25. A mutation in the gene for a cyclin protein might result
in which of the following?


a. a cell with additional genetic material than
normal


b. cancer
c. a cell with less genetic material than normal
d. any of the above


26.What is a primary function of tumor suppressor genes?


a. stop all cells from dividing
b. stop certain cells from dividing
c. help oncogenes produce oncoproteins


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d. allow the cell to skip certain phases of the cell
cycle


27. Arrange the following terms in order of increasing
specialization: oligopotency, pleuripotency, unipotency,
multipotency.


a. multipotency, pleuripotency, oligopotency,
unipotency


b. pleuripotency, oligopotency, multipotency
unipotency


c. oligopotency, pleuripotency, unipotency,
multipotency


d. pleuripotency, multipotency, oligopotency,
unipotency


28. Which type of stem cell gives rise to red and white
blood cells?


a. endothelial
b. epithelial
c. hematopoietic
d. mesenchymal


29. What multipotent stem cells from children sometimes
banked by parents?


a. fetal stem cells
b. embryonic stem cells
c. cells from the umbilical cord and from baby teeth
d. hematopoietic stem cells from red and white
blood cells


CRITICAL THINKING QUESTIONS
30. What materials can easily diffuse through the lipid
bilayer, and why?
31. Why is receptor-mediated endocytosis said to be more
selective than phagocytosis or pinocytosis?
32. What do osmosis, diffusion, filtration, and the
movement of ions away from like charge all have in
common? In what way do they differ?
33. Explain why the structure of the ER, mitochondria, and
Golgi apparatus assist their respective functions.
34. Compare and contrast lysosomes with peroxisomes:
name at least two similarities and one difference.
35. Explain in your own words why DNA replication is
said to be “semiconservative”?
36. Why is it important that DNA replication take place
before cell division? What would happen if cell division of
a body cell took place without DNA replication, or when
DNA replication was incomplete?


37. Briefly explain the similarities between transcription
and DNA replication.
38. Contrast transcription and translation. Name at least
three differences between the two processes.
39.What would happen if anaphase proceeded even though
the sister chromatids were not properly attached to their
respective microtubules and lined up at the metaphase
plate?
40. What are cyclins and cyclin-dependent kinases, and
how do they interact?
41. Explain how a transcription factor ultimately
determines whether or not a protein will be present in a
given cell?
42. Discuss two reasons why the therapeutic use of
embryonic stem cells can present a problem.


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4 | THE TISSUE LEVEL OF
ORGANIZATION


Figure 4.1 Micrograph of Cervical Tissue This figure is a view of the regular architecture of normal tissue contrasted
with the irregular arrangement of cancerous cells. (credit: “Haymanj”/Wikimedia Commons)


Introduction
Chapter Objectives


After studying this chapter, you will be able to:
• Identify the main tissue types and discuss their roles in the human body
• Identify the four types of tissue membranes and the characteristics of each that make them functional
• Explain the functions of various epithelial tissues and how their forms enable their functions
• Explain the functions of various connective tissues and how their forms enable their functions
• Describe the characteristics of muscle tissue and how these enable function
• Discuss the characteristics of nervous tissue and how these enable information processing and control of
muscular and glandular activities


The body contains at least 200 distinct cell types. These cells contain essentially the same internal structures yet they vary
enormously in shape and function. The different types of cells are not randomly distributed throughout the body; rather they
occur in organized layers, a level of organization referred to as tissue. The micrograph that opens this chapter shows the


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high degree of organization among different types of cells in the tissue of the cervix. You can also see how that organization
breaks down when cancer takes over the regular mitotic functioning of a cell.
The variety in shape reflects the many different roles that cells fulfill in your body. The human body starts as a single cell at
fertilization. As this fertilized egg divides, it gives rise to trillions of cells, each built from the same blueprint, but organizing
into tissues and becoming irreversibly committed to a developmental pathway.


4.1 | Types of Tissues
By the end of this section, you will be able to:
• Identify the four main tissue types
• Discuss the functions of each tissue type
• Relate the structure of each tissue type to their function
• Discuss the embryonic origin of tissue
• Identify the three major germ layers
• Identify the main types of tissue membranes


The term tissue is used to describe a group of cells found together in the body. The cells within a tissue share a common
embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged
in an orderly pattern that achieves the tissue’s functions. From the evolutionary perspective, tissues appear in more complex
organisms. For example, multicellular protists, ancient eukaryotes, do not have cells organized into tissues.
Although there are many types of cells in the human body, they are organized into four broad categories of tissues: epithelial,
connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall
health and maintenance of the body. A disruption of the structure is a sign of injury or disease. Such changes can be detected
through histology, the microscopic study of tissue appearance, organization, and function.


The Four Types of Tissues
Epithelial tissue, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, lines
internal cavities and passageways, and forms certain glands. Connective tissue, as its name implies, binds the cells and
organs of the body together and functions in the protection, support, and integration of all parts of the body. Muscle
tissue is excitable, responding to stimulation and contracting to provide movement, and occurs as three major types:
skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. Nervous tissue is also excitable, allowing the
propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the
body (Figure 4.2).
The next level of organization is the organ, where several types of tissues come together to form a working unit. Just as
knowing the structure and function of cells helps you in your study of tissues, knowledge of tissues will help you understand
how organs function. The epithelial and connective tissues are discussed in detail in this chapter. Muscle and nervous tissues
will be discussed only briefly in this chapter.


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Figure 4.2 Four Types of Tissue: Body The four types of tissues are exemplified in nervous tissue, stratified
squamous epithelial tissue, cardiac muscle tissue, and connective tissue in small intestine. Clockwise from nervous
tissue, LM × 872, LM × 282, LM × 460, LM × 800. (Micrographs provided by the Regents of University of Michigan
Medical School © 2012)


Embryonic Origin of Tissues
The zygote, or fertilized egg, is a single cell formed by the fusion of an egg and sperm. After fertilization the zygote gives
rise to rapid mitotic cycles, generating many cells to form the embryo. The first embryonic cells generated have the ability
to differentiate into any type of cell in the body and, as such, are called totipotent, meaning each has the capacity to divide,
differentiate, and develop into a new organism. As cell proliferation progresses, three major cell lineages are established
within the embryo. Each of these lineages of embryonic cells forms the distinct germ layers from which all the tissues
and organs of the human body eventually form. Each germ layer is identified by its relative position: ectoderm (ecto-
= “outer”), mesoderm (meso- = “middle”), and endoderm (endo- = “inner”). Figure 4.3 shows the types of tissues and
organs associated with the each of the three germ layers. Note that epithelial tissue originates in all three layers, whereas
nervous tissue derives primarily from the ectoderm and muscle tissue from mesoderm.


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Figure 4.3 Embryonic Origin of Tissues and Major Organs


View this slideshow (http://openstaxcollege.org/l/stemcells) to learn more about stem cells. How do somatic stem
cells differ from embryonic stem cells?


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Tissue Membranes
A tissue membrane is a thin layer or sheet of cells that covers the outside of the body (for example, skin), the organs (for
example, pericardium), internal passageways that lead to the exterior of the body (for example, abdominal mesenteries), and
the lining of the moveable joint cavities. There are two basic types of tissue membranes: connective tissue and epithelial
membranes (Figure 4.4).


Figure 4.4 Tissue Membranes The two broad categories of tissue membranes in the body are (1) connective tissue
membranes, which include synovial membranes, and (2) epithelial membranes, which include mucous membranes,
serous membranes, and the cutaneous membrane, in other words, the skin.


Connective Tissue Membranes
The connective tissue membrane is formed solely from connective tissue. These membranes encapsulate organs, such as
the kidneys, and line our movable joints. A synovial membrane is a type of connective tissue membrane that lines the
cavity of a freely movable joint. For example, synovial membranes surround the joints of the shoulder, elbow, and knee.
Fibroblasts in the inner layer of the synovial membrane release hyaluronan into the joint cavity. The hyaluronan effectively
traps available water to form the synovial fluid, a natural lubricant that enables the bones of a joint to move freely against
one another without much friction. This synovial fluid readily exchanges water and nutrients with blood, as do all body
fluids.


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Epithelial Membranes
The epithelial membrane is composed of epithelium attached to a layer of connective tissue, for example, your skin. The
mucous membrane is also a composite of connective and epithelial tissues. Sometimes called mucosae, these epithelial
membranes line the body cavities and hollow passageways that open to the external environment, and include the digestive,
respiratory, excretory, and reproductive tracts. Mucous, produced by the epithelial exocrine glands, covers the epithelial
layer. The underlying connective tissue, called the lamina propria (literally “own layer”), help support the fragile epithelial
layer.
A serous membrane is an epithelial membrane composed of mesodermally derived epithelium called the mesothelium that
is supported by connective tissue. These membranes line the coelomic cavities of the body, that is, those cavities that do
not open to the outside, and they cover the organs located within those cavities. They are essentially membranous bags,
with mesothelium lining the inside and connective tissue on the outside. Serous fluid secreted by the cells of the thin
squamous mesothelium lubricates the membrane and reduces abrasion and friction between organs. Serous membranes are
identified according locations. Three serous membranes line the thoracic cavity; the two pleura that cover the lungs and the
pericardium that covers the heart. A fourth, the peritoneum, is the serous membrane in the abdominal cavity that covers
abdominal organs and forms double sheets of mesenteries that suspend many of the digestive organs.
The skin is an epithelial membrane also called the cutaneous membrane. It is a stratified squamous epithelial membrane
resting on top of connective tissue. The apical surface of this membrane is exposed to the external environment and is
covered with dead, keratinized cells that help protect the body from desiccation and pathogens.


4.2 | Epithelial Tissue
By the end of this section, you will be able to:
• Explain the structure and function of epithelial tissue
• Distinguish between tight junctions, anchoring junctions, and gap junctions
• Distinguish between simple epithelia and stratified epithelia, as well as between squamous, cuboidal, and columnar
epithelia


• Describe the structure and function of endocrine and exocrine glands and their respective secretions


Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world
and lining the outside of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of
the body exposed to the outside. Other areas include the airways, the digestive tract, as well as the urinary and reproductive
systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the
body, which includes, blood vessels and serous membranes, are lined by endothelium (plural = endothelia), which is a type
of epithelium.
Epithelial cells derive from all three major embryonic layers. The epithelia lining the skin, parts of the mouth and nose, and
the anus develop from the ectoderm. Cells lining the airways and most of the digestive system originate in the endoderm.
The epithelium that lines vessels in the lymphatic and cardiovascular system derives from the mesoderm and is called an
endothelium.
All epithelia share some important structural and functional features. This tissue is highly cellular, with little or no
extracellular material present between cells. Adjoining cells form a specialized intercellular connection between their cell
membranes called a cell junction. The epithelial cells exhibit polarity with differences in structure and function between the
exposed or apical facing surface of the cell and the basal surface close to the underlying body structures. The basal lamina,
a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying
connective tissue. The basal lamina attaches to a reticular lamina, which is secreted by the underlying connective tissue,
forming a basement membrane that helps hold it all together.
Epithelial tissues are nearly completely avascular. For instance, no blood vessels cross the basement membrane to enter the
tissue, and nutrients must come by diffusion or absorption from underlying tissues or the surface. Many epithelial tissues are
capable of rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a characteristic of surface
epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells.


Generalized Functions of Epithelial Tissue
Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear. The cells
of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a
physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features
that allow the selective transport of molecules and ions across their cell membranes.


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Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical
surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract
secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells.


The Epithelial Cell
Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between
their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions.
Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present,
are on the apical surface.
Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and
move fluids as well as trapped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the
cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and
pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous
with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases,
the transported materials are usually swallowed, and end up in the acidic environment of your stomach.


Cell to Cell Junctions
Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections
allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions (Figure 4.5).


CHAPTER 4 | THE TISSUE LEVEL OF ORGANIZATION 137




Figure 4.5 Types of Cell Junctions The three basic types of cell-to-cell junctions are tight junctions, gap junctions,
and anchoring junctions.


At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments. An
anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are
common on the lateral and basal surfaces of cells where they provide strong and flexible connections. There are three types
of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of
cells. The patches are structural proteins on the inner surface of the cell’s membrane. The adhesion molecule, cadherin, is
embedded in these patches and projects through the cell membrane to link with the cadherin molecules of adjacent cells.
These connections are especially important in holding cells together. Hemidesmosomes, which look like half a desmosome,
link cells to the extracellular matrix, for example, the basal lamina. While similar in appearance to desmosomes, they
include the adhesion proteins called integrins rather than cadherins. Adherens junctions use either cadherins or integrins
depending on whether they are linking to other cells or matrix. The junctions are characterized by the presence of the
contractile protein actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or
form a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue.
In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the
membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent
cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups
of cells.


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Classification of Epithelial Tissues
Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed (Figure 4.6). Cell
shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is
wide). Similarly, the number of cell layers in the tissue can be one—where every cell rests on the basal lamina—which is
a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal
lamina. Pseudostratified (pseudo- = “false”) describes tissue with a single layer of irregularly shaped cells that give the
appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of
the cells can vary.


Figure 4.6 Cells of Epithelial Tissue Simple epithelial tissue is organized as a single layer of cells and stratified
epithelial tissue is formed by several layers of cells.


Simple Epithelium
The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple
squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical,
mirroring the form of the cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular
system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the
cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments
of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The mesothelium is
a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal
organs. Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells
that secrete a fluid that lubricates the mesothelium.
In simple cuboidal epithelium, the nucleus of the box-like cells appears round and is generally located near the center of
the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed in
the lining of the kidney tubules and in the ducts of glands.
In simple columnar epithelium, the nucleus of the tall column-like cells tends to be elongated and located in the basal
end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple
columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract.
Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These
epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the
cilia helps remove particulate matter.


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Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified but instead consists of a single
layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells
appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but
in fact all the cells are in contact with the basal lamina, although some do not reach the apical surface. Pseudostratified
columnar epithelium is found in the respiratory tract, where some of these cells have cilia.
Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types
of cells interspersed among the epithelial cells. For example, a goblet cell is a mucous-secreting unicellular “gland”
interspersed between the columnar epithelial cells of mucous membranes (Figure 4.7).


(a)


(b)


Figure 4.7 Goblet Cell (a) In the lining of the small intestine, columnar epithelium cells are interspersed with goblet
cells. (b) The arrows in this micrograph point to the mucous-secreting goblet cells. LM × 1600. (Micrograph provided
by the Regents of University of Michigan Medical School © 2012)


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View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/Digestive%20System/
Intestines/169_HISTO_40X.svs/view.apml (http://openstaxcollege.org/l/goblet) to explore the tissue sample in
greater detail.


Stratified Epithelium
A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical
wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space.
Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical cells are
squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells
filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of
the mouth cavity is an example of an unkeratinized, stratified squamous epithelium. Stratified cuboidal epithelium and
stratified columnar epithelium can also be found in certain glands and ducts, but are uncommon in the human body.
Another kind of stratified epithelium is transitional epithelium, so-called because of the gradual changes in the shapes of
the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary
bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella
shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition
from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out
and less stratified when the bladder is full and distended. Figure 4.8 summarizes the different categories of epithelial cell
tissue cells.


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Figure 4.8 Summary of Epithelial Tissue Cells


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Watch this video (http://openstaxcollege.org/l/etissues) to find out more about the anatomy of epithelial tissues.
Where in the body would one find non-keratinizing stratified squamous epithelium?


Glandular Epithelium
A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands
consist of groups of epithelial cells. A gland can be classified as an endocrine gland, a ductless gland that releases
secretions directly into surrounding tissues and fluids (endo- = “inside”), or an exocrine gland whose secretions leave
through a duct that opens directly, or indirectly, to the external environment (exo- = “outside”).
Endocrine Glands
The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the
bloodstream, and delivered to targets, in other words, cells that have receptors to bind the hormones. The endocrine system
is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of
endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads.
Exocrine Glands
Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast
milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the
lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.
Glandular Structure
Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as
goblet cells, found in the mucous membranes of the small and large intestine.
The multicellular exocrine glands known as serous glands develop from simple epithelium to form a secretory surface that
secretes directly into an inner cavity. These glands line the internal cavities of the abdomen and chest and release their
secretions directly into the cavities. Other multicellular exocrine glands release their contents through a tubular duct. The
duct is single in a simple gland but in compound glands is divided into one or more branches (Figure 4.9). In tubular glands,
the ducts can be straight or coiled, whereas tubes that form pockets are alveolar (acinar), such as the exocrine portion of the
pancreas. Combinations of tubes and pockets are known as tubuloalveolar (tubuloacinar) compound glands. In a branched
gland, a duct is connected to more than one secretory group of cells.


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Figure 4.9 Types of Exocrine Glands Exocrine glands are classified by their structure.


Methods and Types of Secretion
Exocrine glands can be classified by their mode of secretion and the nature of the substances released, as well as by
the structure of the glands and shape of ducts (Figure 4.10). Merocrine secretion is the most common type of exocrine
secretion. The secretions are enclosed in vesicles that move to the apical surface of the cell where the contents are released
by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen
protection is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example.


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Figure 4.10 Modes of Glandular Secretion (a) In merocrine secretion, the cell remains intact. (b) In apocrine
secretion, the apical portion of the cell is released, as well. (c) In holocrine secretion, the cell is destroyed as it releases
its product and the cell itself becomes part of the secretion.


Apocrine secretion accumulates near the apical portion of the cell. That portion of the cell and its secretory contents pinch
off from the cell and are released. The sweat glands of the armpit are classified as apocrine glands. Both merocrine and
apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus and
golgi regions remain intact after secretion.
In contrast, the process of holocrine secretion involves the rupture and destruction of the entire gland cell. The cell
accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the
surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the skin and hair are
holocrine glands/cells (Figure 4.11).


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Figure 4.11 Sebaceous Glands These glands secrete oils that lubricate and protect the skin. They are holocrine
glands and they are destroyed after releasing their contents. New glandular cells form to replace the cells that are lost.
LM × 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)


Glands are also named after the products they produce. The serous gland produces watery, blood-plasma-like secretions
rich in enzymes such as alpha amylase, whereas the mucous gland releases watery to viscous products rich in the
glycoprotein mucin. Both serous and mucous glands are common in the salivary glands of the mouth. Mixed exocrine glands
contain both serous and mucous glands and release both types of secretions.


4.3 | Connective Tissue Supports and Protects
By the end of this section, you will be able to:
• Identify and distinguish between the types of connective tissue: proper, supportive, and fluid
• Explain the functions of connective tissues


As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike
epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective
tissue cells are dispersed in a matrix. The matrix usually includes a large amount of extracellular material produced by
the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue.
The major component of the matrix is a ground substance often crisscrossed by protein fibers. This ground substance is
usually a fluid, but it can also be mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet
they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and
protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground
substance in bones supporting the body to the inclusion of specialized cells; for example, a phagocytic cell that engulfs
pathogens and also rids tissue of cellular debris.


Functions of Connective Tissues
Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from
the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that
supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules
and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the
body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured
by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and
contribute to the thermal insulation of the body.


Embryonic Connective Tissue
All connective tissues derive from the mesodermal layer of the embryo (see Figure 4.3). The first connective tissue to
develop in the embryo is mesenchyme, the stem cell line from which all connective tissues are later derived. Clusters


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of mesenchymal cells are scattered throughout adult tissue and supply the cells needed for replacement and repair after
a connective tissue injury. A second type of embryonic connective tissue forms in the umbilical cord, called mucous
connective tissue or Wharton’s jelly. This tissue is no longer present after birth, leaving only scattered mesenchymal cells
throughout the body.


Classification of Connective Tissues
The three broad categories of connective tissue are classified according to the characteristics of their ground substance
and the types of fibers found within the matrix (Table 4.1). Connective tissue proper includes loose connective tissue
and dense connective tissue. Both tissues have a variety of cell types and protein fibers suspended in a viscous ground
substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection.
In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective
tissue—bone and cartilage—provide structure and strength to the body and protect soft tissues. A few distinct cell types and
densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of
the deposited calcium salts. In fluid connective tissue, in other words, lymph and blood, various specialized cells circulate
in a watery fluid containing salts, nutrients, and dissolved proteins.


Connective Tissue Examples
Connective tissue proper Supportive connective tissue Fluid connective tissue


Loose connective tissue
Areolar
Adipose
Reticular


Cartilage
Hyaline
Fibrocartilage
Elastic


Blood


Dense connective tissue
Regular elastic
Irregular elastic


Bones
Compact bone
Cancellous bone


Lymph


Table 4.1


Connective Tissue Proper
Fibroblasts are present in all connective tissue proper (Figure 4.12). Fibrocytes, adipocytes, and mesenchymal cells are
fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue
in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in
connective tissue proper but are actually part of the immune system protecting the body.


Figure 4.12 Connective Tissue Proper Fibroblasts produce this fibrous tissue. Connective tissue proper includes
the fixed cells fibrocytes, adipocytes, and mesenchymal cells. LM × 400. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)


Cell Types
The most abundant cell in connective tissue proper is the fibroblast. Polysaccharides and proteins secreted by fibroblasts
combine with extra-cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the


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extra-cellular matrix. As you might expect, a fibrocyte, a less active form of fibroblast, is the second most common cell
type in connective tissue proper.
Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes:
white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white
fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large
amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and
vary among individuals in the population.
Themesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cells
needed for repair and healing of damaged tissue.
The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue
matrix from the blood vessels. The macrophage cells are an essential component of the immune system, which is the
body’s defense against potential pathogens and degraded host cells. When stimulated, macrophages release cytokines, small
proteins that act as chemical messengers. Cytokines recruit other cells of the immune system to infected sites and stimulate
their activities. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and
cellular debris. In contrast, fixed macrophages are permanent residents of their tissues.
The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical
signals histamine and heparin. When irritated or damaged, mast cells release histamine, an inflammatory mediator, which
causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness you
recognize as an allergic response. Like blood cells, mast cells are derived from hematopoietic stem cells and are part of the
immune system.
Connective Tissue Fibers and Ground Substance
Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is
made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have
great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These
fibers hold connective tissues together, even during the movement of the body.
Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property
of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic
tissues found in skin and the elastic ligaments of the vertebral column.
Reticular fiber is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and
are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft
organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells,
blood vessels, and nerves of the organ).
All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of
polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and
polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix
you now know as ground substance.
Loose Connective Tissue
Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows
water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues.
Adipose tissue consists mostly of fat storage cells, with little extracellular matrix (Figure 4.13). A large number of
capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear
yellow and owes its color to carotene and related pigments from plant food. White fat contributes mostly to lipid storage
and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting
the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term “baby
fat.” In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body.
The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat.
Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing
adenosine triphosphate (ATP), a key molecule used in metabolism.


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Figure 4.13 Adipose Tissue This is a loose connective tissue that consists of fat cells with little extracellular matrix. It
stores fat for energy and provides insulation. LM × 800. (Micrograph provided by the Regents of University of Michigan
Medical School © 2012)


Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed
in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports
organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of
epithelial membranes, which are described further in a later section.
Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver
(Figure 4.14). Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its
name from the Latin reticulus, which means “little net.”


Figure 4.14 Reticular Tissue This is a loose connective tissue made up of a network of reticular fibers that provides
a supportive framework for soft organs. LM × 1600. (Micrograph provided by the Regents of University of Michigan
Medical School © 2012)


Dense Connective Tissue
Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays
greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense
regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the
direction of the fiber orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments not
all fibers are parallel. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the
ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the
vertebral column are elastic.
In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength
in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other
tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each
layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular
connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to
regain original shape after stretching (Figure 4.15).


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Figure 4.15 Dense Connective Tissue (a) Dense regular connective tissue consists of collagenous fibers packed
into parallel bundles. (b) Dense irregular connective tissue consists of collagenous fibers interwoven into a mesh-like
network. From top, LM × 1000, LM × 200. (Micrographs provided by the Regents of University of Michigan Medical
School © 2012)


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Connective Tissue: Tendinitis
Your opponent stands ready as you prepare to hit the serve, but you are confident that you will smash the ball past your
opponent. As you toss the ball high in the air, a burning pain shoots across your wrist and you drop the tennis racket.
That dull ache in the wrist that you ignored through the summer is now an unbearable pain. The game is over for now.
After examining your swollen wrist, the doctor in the emergency room announces that you have developed wrist
tendinitis. She recommends icing the tender area, taking non-steroidal anti-inflammatory medication to ease the pain
and to reduce swelling, and complete rest for a few weeks. She interrupts your protests that you cannot stop playing.
She issues a stern warning about the risk of aggravating the condition and the possibility of surgery. She consoles you
by mentioning that well known tennis players such as Venus and Serena Williams and Rafael Nadal have also suffered
from tendinitis related injuries.
What is tendinitis and how did it happen? Tendinitis is the inflammation of a tendon, the thick band of fibrous
connective tissue that attaches a muscle to a bone. The condition causes pain and tenderness in the area around a
joint. On rare occasions, a sudden serious injury will cause tendinitis. Most often, the condition results from repetitive
motions over time that strain the tendons needed to perform the tasks.
Persons whose jobs and hobbies involve performing the same movements over and over again are often at the greatest
risk of tendinitis. You hear of tennis and golfer’s elbow, jumper's knee, and swimmer’s shoulder. In all cases, overuse
of the joint causes a microtrauma that initiates the inflammatory response. Tendinitis is routinely diagnosed through a
clinical examination. In case of severe pain, X-rays can be examined to rule out the possibility of a bone injury. Severe
cases of tendinitis can even tear loose a tendon. Surgical repair of a tendon is painful. Connective tissue in the tendon
does not have abundant blood supply and heals slowly.
While older adults are at risk for tendinitis because the elasticity of tendon tissue decreases with age, active people
of all ages can develop tendinitis. Young athletes, dancers, and computer operators; anyone who performs the same
movements constantly is at risk for tendinitis. Although repetitive motions are unavoidable in many activities and
may lead to tendinitis, precautions can be taken that can lessen the probability of developing tendinitis. For active
individuals, stretches before exercising and cross training or changing exercises are recommended. For the passionate
athlete, it may be time to take some lessons to improve technique. All of the preventive measures aim to increase the
strength of the tendon and decrease the stress put on it. With proper rest and managed care, you will be back on the
court to hit that slice-spin serve over the net.


Watch this animation (http://openstaxcollege.org/l/tendonitis) to learn more about tendonitis, a painful condition
caused by swollen or injured tendons.


Supportive Connective Tissues
Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect
internal organs.
Cartilage
The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground
substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells,
and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the


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perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the
matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.
The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage (Figure 4.16). Hyaline
cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large
amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth.
Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints.
It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone
allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed
through its matrix. The knee and jaw joints and the the intervertebral discs are examples of fibrocartilage. Elastic cartilage
contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently
at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.


Figure 4.16 Types of Cartilage Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm
matrix of chondroitin sulfates. (a) Hyaline cartilage provides support with some flexibility. The example is from dog
tissue. (b) Fibrocartilage provides some compressibility and can absorb pressure. (c) Elastic cartilage provides firm but
elastic support. From top, LM × 300, LM × 1200, LM × 1016. (Micrographs provided by the Regents of University of
Michigan Medical School © 2012)


Bone
Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone’s rigid
extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing
hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the
unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones


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would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology
of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal.
Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time.
Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone
proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones. Compact bone
is solid and has greater structural strength.


Fluid Connective Tissue
Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements
circulating in blood are all derived from hematopoietic stem cells located in bone marrow (Figure 4.17). Erythrocytes, red
blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against
potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood
cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and
wastes are dissolved in the liquid matrix and transported through the body.
Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger
molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels,
delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic
capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.


Figure 4.17 Blood: A Fluid Connective Tissue Blood is a fluid connective tissue containing erythrocytes and various
types of leukocytes that circulate in a liquid extracellular matrix. LM × 1600. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)


View the University of Michigan Webscope at http://virtualslides.med.umich.edu/Histology/
Cardiovascular%20System/081-3_HISTO_40X.svs/view.apml (http://openstaxcollege.org/l/cardiovascular) to
explore the tissue sample in greater detail.


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Visit this link (http://openstaxcollege.org/l/10quiz) to test your connective tissue knowledge with this 10-question
quiz. Can you name the 10 tissue types shown in the histology slides?


4.4 | Muscle Tissue and Motion
By the end of this section, you will be able to:
• Identify the three types of muscle tissue
• Compare and contrast the functions of each muscle tissue type
• Explain how muscle tissue can enable motion


Muscle tissue is characterized by properties that allow movement. Muscle cells are excitable; they respond to a stimulus.
They are contractile, meaning they can shorten and generate a pulling force. When attached between two movable objects,
in other words, bones, contractions of the muscles cause the bones to move. Some muscle movement is voluntary, which
means it is under conscious control. For example, a person decides to open a book and read a chapter on anatomy. Other
movements are involuntary, meaning they are not under conscious control, such as the contraction of your pupil in bright
light. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth (Table
4.2).


Comparison of Structure and Properties of Muscle Tissue Types
Tissue Histology Function Location


Skeletal
Long cylindrical fiber,
striated, many
peripherally located
nuclei


Voluntary movement, produces heat, protects
organs


Attached to bones and
around entrance points
to body (e.g., mouth,
anus)


Cardiac
Short, branched,
striated, single central
nucleus


Contracts to pump blood Heart


Smooth
Short, spindle-shaped,
no evident striation,
single nucleus in each
fiber


Involuntary movement, moves food,
involuntary control of respiration, moves
secretions, regulates flow of blood in arteries
by contraction


Walls of major organs
and passageways


Table 4.2


Skeletal muscle is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other
voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Skeletal muscles
generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary
contraction of skeletal muscles in response to perceived lower than normal body temperature. The muscle cell, ormyocyte,
develops from myoblasts derived from the mesoderm. Myocytes and their numbers remain relatively constant throughout
life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle
cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the


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contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective
tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fiber.
Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear
striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells typically with a single centrally
located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without
any external stimulation. Cardiomyocyte attach to one another with specialized cell junctions called intercalated discs.
Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle
fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The
cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent
cells together across the dynamic pressures changes of the cardiac cycle.
Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile
component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped
with a single nucleus and no visible striations (Figure 4.18).


Figure 4.18 Muscle Tissue (a) Skeletal muscle cells have prominent striation and nuclei on their periphery. (b)
Smooth muscle cells have a single nucleus and no visible striations. (c) Cardiac muscle cells appear striated and have
a single nucleus. From top, LM × 1600, LM × 1600, LM × 1600. (Micrographs provided by the Regents of University of
Michigan Medical School © 2012)


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Watch this video (http://openstaxcollege.org/l/musctissue) to learn more about muscle tissue. In looking through a
microscope how could you distinguish skeletal muscle tissue from smooth muscle?


4.5 | Nervous Tissue Mediates Perception and Response
By the end of this section, you will be able to:
• Identify the classes of cells that make up nervous tissue
• Discuss how nervous tissue mediates perception and response


Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide
the body with information. Two main classes of cells make up nervous tissue: the neuron and neuroglia (Figure 4.19).
Neurons propagate information via electrochemical impulses, called action potentials, which are biochemically linked to
the release of chemical signals. Neuroglia play an essential role in supporting neurons and modulating their information
propagation.


Figure 4.19 The Neuron The cell body of a neuron, also called the soma, contains the nucleus and mitochondria.
The dendrites transfer the nerve impulse to the soma. The axon carries the action potential away to another excitable
cell. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)


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Follow this link (http://openstaxcollege.org/l/nobel) to learn more about nervous tissue. What are the main parts of a
nerve cell?


Neurons display distinctive morphology, well suited to their role as conducting cells, with three main parts. The cell body
includes most of the cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell body and appear as thin
extensions. A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as
myelin, which is formed by accessory cells. The synapse is the gap between nerve cells, or between a nerve cell and
its target, for example, a muscle or a gland, across which the impulse is transmitted by chemical compounds known as
neurotransmitters. Neurons categorized as multipolar neurons have several dendrites and a single prominent axon. Bipolar
neurons possess a single dendrite and axon with the cell body, while unipolar neurons have only a single process extending
out from the cell body, which divides into a functional dendrite and into a functional axon. When a neuron is sufficiently
stimulated, it generates an action potential that propagates down the axon towards the synapse. If enough neurotransmitters
are released at the synapse to stimulate the next neuron or target, a response is generated.
The second class of neural cells comprises the neuroglia or glial cells, which have been characterized as having a simple
support role. The word “glia” comes from the Greek word for glue. Recent research is shedding light on the more complex
role of neuroglia in the function of the brain and nervous system. Astrocyte cells, named for their distinctive star shape,
are abundant in the central nervous system. The astrocytes have many functions, including regulation of ion concentration
in the intercellular space, uptake and/or breakdown of some neurotransmitters, and formation of the blood-brain barrier,
the membrane that separates the circulatory system from the brain. Microglia protect the nervous system against infection
but are not nervous tissue because they are related to macrophages. Oligodendrocyte cells produce myelin in the central
nervous system (brain and spinal cord) while the Schwann cell produces myelin in the peripheral nervous system (Figure
4.20).


Figure 4.20 Nervous Tissue Nervous tissue is made up of neurons and neuroglia. The cells of nervous tissue are
specialized to transmit and receive impulses. LM × 872. (Micrograph provided by the Regents of University of Michigan
Medical School © 2012)


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4.6 | Tissue Injury and Aging
By the end of this section, you will be able to:
• Identify the cardinal signs of inflammation
• List the body’s response to tissue injury
• Explain the process of tissue repair
• Discuss the progressive impact of aging on tissue
• Describe cancerous mutations’ effect on tissue


Tissues of all types are vulnerable to injury and, inevitably, aging. In the former case, understanding how tissues respond
to damage can guide strategies to aid repair. In the latter case, understanding the impact of aging can help in the search for
ways to diminish its effects.


Tissue Injury and Repair
Inflammation is the standard, initial response of the body to injury. Whether biological, chemical, physical, or radiation
burns, all injuries lead to the same sequence of physiological events. Inflammation limits the extent of injury, partially or
fully eliminates the cause of injury, and initiates repair and regeneration of damaged tissue. Necrosis, or accidental cell
death, causes inflammation.Apoptosis is programmed cell death, a normal step-by-step process that destroys cells no longer
needed by the body. By mechanisms still under investigation, apoptosis does not initiate the inflammatory response. Acute
inflammation resolves over time by the healing of tissue. If inflammation persists, it becomes chronic and leads to diseased
conditions. Arthritis and tuberculosis are examples of chronic inflammation. The suffix “-itis” denotes inflammation of
a specific organ or type, for example, peritonitis is the inflammation of the peritoneum, and meningitis refers to the
inflammation of the meninges, the tough membranes that surround the central nervous system
The four cardinal signs of inflammation—redness, swelling, pain, and local heat—were first recorded in antiquity. Cornelius
Celsus is credited with documenting these signs during the days of the Roman Empire, as early as the first century AD. A
fifth sign, loss of function, may also accompany inflammation.
Upon tissue injury, damaged cells release inflammatory chemical signals that evoke local vasodilation, the widening of the
blood vessels. Increased blood flow results in apparent redness and heat. In response to injury, mast cells present in tissue
degranulate, releasing the potent vasodilator histamine. Increased blood flow and inflammatory mediators recruit white
blood cells to the site of inflammation. The endothelium lining the local blood vessel becomes “leaky” under the influence
of histamine and other inflammatory mediators allowing neutrophils, macrophages, and fluid to move from the blood into
the interstitial tissue spaces. The excess liquid in tissue causes swelling, more properly called edema. The swollen tissues
squeezing pain receptors cause the sensation of pain. Prostaglandins released from injured cells also activate pain neurons.
Non-steroidal anti-inflammatory drugs (NSAIDs) reduce pain because they inhibit the synthesis of prostaglandins. High
levels of NSAIDs reduce inflammation. Antihistamines decrease allergies by blocking histamine receptors and as a result
the histamine response.
After containment of an injury, the tissue repair phase starts with removal of toxins and waste products. Clotting
(coagulation) reduces blood loss from damaged blood vessels and forms a network of fibrin proteins that trap blood cells
and bind the edges of the wound together. A scab forms when the clot dries, reducing the risk of infection. Sometimes
a mixture of dead leukocytes and fluid called pus accumulates in the wound. As healing progresses, fibroblasts from the
surrounding connective tissues replace the collagen and extracellular material lost by the injury. Angiogenesis, the growth
of new blood vessels, results in vascularization of the new tissue known as granulation tissue. The clot retracts pulling the
edges of the wound together, and it slowly dissolves as the tissue is repaired. When a large amount of granulation tissue
forms and capillaries disappear, a pale scar is often visible in the healed area. A primary union describes the healing of a
wound where the edges are close together. When there is a gaping wound, it takes longer to refill the area with cells and
collagen. The process called secondary union occurs as the edges of the wound are pulled together by what is called wound
contraction. When a wound is more than one quarter of an inch deep, sutures (stitches) are recommended to promote a
primary union and avoid the formation of a disfiguring scar. Regeneration is the addition of new cells of the same type as
the ones that were injured (Figure 4.21).


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Figure 4.21 Tissue Healing During wound repair, collagen fibers are laid down randomly by fibroblasts that move into
repair the area.


Watch this video (http://openstaxcollege.org/l/healinghand) to see a hand heal. Over what period of time do you
think these images were taken?


Tissue and Aging
According to poet Ralph Waldo Emerson, “The surest poison is time.” In fact, biology confirms that many functions of
the body decline with age. All the cells, tissues, and organs are affected by senescence, with noticeable variability between
individuals owing to different genetic makeup and lifestyles. The outward signs of aging are easily recognizable. The skin
and other tissues become thinner and drier, reducing their elasticity, contributing to wrinkles and high blood pressure.
Hair turns gray because follicles produce less melanin, the brown pigment of hair and the iris of the eye. The face looks
flabby because elastic and collagen fibers decrease in connective tissue and muscle tone is lost. Glasses and hearing aids
may become parts of life as the senses slowly deteriorate, all due to reduced elasticity. Overall height decreases as the
bones lose calcium and other minerals. With age, fluid decreases in the fibrous cartilage disks intercalated between the
vertebrae in the spine. Joints lose cartilage and stiffen. Many tissues, including those in muscles, lose mass through a
process called atrophy. Lumps and rigidity become more widespread. As a consequence, the passageways, blood vessels,
and airways become more rigid. The brain and spinal cord lose mass. Nerves do not transmit impulses with the same speed
and frequency as in the past. Some loss of thought clarity and memory can accompany aging. More severe problems are not
necessarily associated with the aging process and may be symptoms of underlying illness.
As exterior signs of aging increase, so do the interior signs, which are not as noticeable. The incidence of heart diseases,
respiratory syndromes, and type 2 diabetes increases with age, though these are not necessarily age-dependent effects.
Wound healing is slower in the elderly, accompanied by a higher frequency of infection as the capacity of the immune
system to fend off pathogen declines.
Aging is also apparent at the cellular level because all cells experience changes with aging. Telomeres, regions of the
chromosomes necessary for cell division, shorten each time cells divide. As they do, cells are less able to divide and
regenerate. Because of alterations in cell membranes, transport of oxygen and nutrients into the cell and removal of carbon
dioxide and waste products from the cell are not as efficient in the elderly. Cells may begin to function abnormally, which
may lead to diseases associated with aging, including arthritis, memory issues, and some cancers.


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The progressive impact of aging on the body varies considerably among individuals, but Studies indicate, however, that
exercise and healthy lifestyle choices can slow down the deterioration of the body that comes with old age.


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Tissues and Cancer
Cancer is a generic term for many diseases in which cells escape regulatory signals. Uncontrolled growth, invasion into
adjacent tissues, and colonization of other organs, if not treated early enough, are its hallmarks. Health suffers when
tumors “rob” blood supply from the “normal” organs.
A mutation is defined as a permanent change in the DNA of a cell. Epigenetic modifications, changes that do not affect
the code of the DNA but alter how the DNA is decoded, are also known to generate abnormal cells. Alterations in the
genetic material may be caused by environmental agents, infectious agents, or errors in the replication of DNA that
accumulate with age. Many mutations do not cause any noticeable change in the functions of a cell. However, if the
modification affects key proteins that have an impact on the cell’s ability to proliferate in an orderly fashion, the cell
starts to divide abnormally. As changes in cells accumulate, they lose their ability to form regular tissues. A tumor,
a mass of cells displaying abnormal architecture, forms in the tissue. Many tumors are benign, meaning they do not
metastasize nor cause disease. A tumor becomes malignant, or cancerous, when it breaches the confines of its tissue,
promotes angiogenesis, attracts the growth of capillaries, and metastasizes to other organs (Figure 4.22). The specific
names of cancers reflect the tissue of origin. Cancers derived from epithelial cells are referred to as carcinomas. Cancer
in myeloid tissue or blood cells form myelomas. Leukemias are cancers of white blood cells, whereas sarcomas derive
from connective tissue. Cells in tumors differ both in structure and function. Some cells, called cancer stem cells,
appear to be a subtype of cell responsible for uncontrolled growth. Recent research shows that contrary to what was
previously assumed, tumors are not disorganized masses of cells, but have their own structures.


CHAPTER 4 | THE TISSUE LEVEL OF ORGANIZATION 161




Figure 4.22 Development of Cancer Note the change in cell size, nucleus size, and organization in the tissue.


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Watch this video (http://openstaxcollege.org/l/tumor) to learn more about tumors. What is a tumor?


Cancer treatments vary depending on the disease’s type and stage. Traditional approaches, including surgery, radiation,
chemotherapy, and hormonal therapy, aim to remove or kill rapidly dividing cancer cells, but these strategies have their
limitations. Depending on a tumor’s location, for example, cancer surgeons may be unable to remove it. Radiation and
chemotherapy are difficult, and it is often impossible to target only the cancer cells. The treatments inevitably destroy
healthy tissue as well. To address this, researchers are working on pharmaceuticals that can target specific proteins
implicated in cancer-associated molecular pathways.


CHAPTER 4 | THE TISSUE LEVEL OF ORGANIZATION 163




adipocytes
adipose tissue
anchoring junction
apical
apocrine secretion
apoptosis
areolar tissue


astrocyte


atrophy
basal lamina
basement membrane


cardiac muscle


cell junction
chondrocytes
clotting
collagen fiber
connective tissue
connective tissue membrane
connective tissue proper
cutaneous membrane


dense connective tissue


ectoderm
elastic cartilage
elastic fiber


endocrine gland


endoderm


endothelium


KEY TERMS
lipid storage cells
specialized areolar tissue rich in stored fat


mechanically attaches adjacent cells to each other or to the basement membrane
that part of a cell or tissue which, in general, faces an open space


release of a substance along with the apical portion of the cell
programmed cell death
(also, loose connective tissue) a type of connective tissue proper that shows little specialization with


cells dispersed in the matrix
star-shaped cell in the central nervous system that regulates ions and uptake and/or breakdown of some


neurotransmitters and contributes to the formation of the blood-brain barrier
loss of mass and function


thin extracellular layer that lies underneath epithelial cells and separates them from other tissues
in epithelial tissue, a thin layer of fibrous material that anchors the epithelial tissue to the


underlying connective tissue; made up of the basal lamina and reticular lamina
heart muscle, under involuntary control, composed of striated cells that attach to form fibers, each cell


contains a single nucleus, contracts autonomously
point of cell-to-cell contact that connects one cell to another in a tissue
cells of the cartilage


also called coagulation; complex process by which blood components form a plug to stop bleeding
flexible fibrous proteins that give connective tissue tensile strength


type of tissue that serves to hold in place, connect, and integrate the body’s organs and systems
connective tissue that encapsulates organs and lines movable joints


connective tissue containing a viscous matrix, fibers, and cells.
skin; epithelial tissue made up of a stratified squamous epithelial cells that cover the outside of


the body
connective tissue proper that contains many fibers that provide both elasticity and


protection
outermost embryonic germ layer from which the epidermis and the nervous tissue derive


type of cartilage, with elastin as the major protein, characterized by rigid support as well as elasticity
fibrous protein within connective tissue that contains a high percentage of the protein elastin that allows


the fibers to stretch and return to original size
groups of cells that release chemical signals into the intercellular fluid to be picked up and


transported to their target organs by blood
innermost embryonic germ layer from which most of the digestive system and lower respiratory system


derive
tissue that lines vessels of the lymphatic and cardiovascular system, made up of a simple squamous


epithelium


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epithelial membrane
epithelial tissue


exocrine gland


fibroblast
fibrocartilage


fibrocyte
fluid connective tissue


gap junction
goblet cell
ground substance
histamine


histology
holocrine secretion
hyaline cartilage


inflammation
lacunae
lamina propria
loose connective tissue


matrix
merocrine secretion
mesenchymal cell
mesenchyme
mesoderm
mesothelium


mucous connective tissue
mucous gland


mucous membrane


muscle tissue


epithelium attached to a layer of connective tissue
type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also


functions in absorption, transport, and secretion
group of epithelial cells that secrete substances through ducts that open to the skin or to internal body


surfaces that lead to the exterior of the body
most abundant cell type in connective tissue, secretes protein fibers and matrix into the extracellular space
tough form of cartilage, made of thick bundles of collagen fibers embedded in chondroitin sulfate


ground substance
less active form of fibroblast


specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved
proteins


allows cytoplasmic communications to occur between cells
unicellular gland found in columnar epithelium that secretes mucous


fluid or semi-fluid portion of the matrix
chemical compound released by mast cells in response to injury that causes vasodilation and endothelium


permeability
microscopic study of tissue architecture, organization, and function


release of a substance caused by the rupture of a gland cell, which becomes part of the secretion
most common type of cartilage, smooth and made of short collagen fibers embedded in a chondroitin


sulfate ground substance
response of tissue to injury


(singular = lacuna) small spaces in bone or cartilage tissue that cells occupy
areolar connective tissue underlying a mucous membrane


(also, areolar tissue) type of connective tissue proper that shows little specialization with
cells dispersed in the matrix
extracellular material which is produced by the cells embedded in it, containing ground substance and fibers


release of a substance from a gland via exocytosis
adult stem cell from which most connective tissue cells are derived


embryonic tissue from which connective tissue cells derive
middle embryonic germ layer from which connective tissue, muscle tissue, and some epithelial tissue derive
simple squamous epithelial tissue which covers the major body cavities and is the epithelial portion of


serous membranes
specialized loose connective tissue present in the umbilical cord


group of cells that secrete mucous, a thick, slippery substance that keeps tissues moist and acts as a
lubricant


tissue membrane that is covered by protective mucous and lines tissue exposed to the outside
environment


type of tissue that is capable of contracting and generating tension in response to stimulation; produces
movement.


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myelin
myocyte
necrosis
nervous tissue
neuroglia
neuron
oligodendrocyte
parenchyma


primary union


pseudostratified columnar epithelium


reticular fiber


reticular lamina


reticular tissue


Schwann cell
secondary union
serous gland
serous membrane
simple columnar epithelium


simple cuboidal epithelium


simple squamous epithelium


skeletal muscle


smooth muscle


stratified columnar epithelium


stratified cuboidal epithelium
stratified squamous epithelium


striation
supportive connective tissue


layer of lipid inside some neuroglial cells that wraps around the axons of some neurons
muscle cells
accidental death of cells and tissues


type of tissue that is capable of sending and receiving impulses through electrochemical signals.
supportive neural cells


excitable neural cell that transfer nerve impulses
neuroglial cell that produces myelin in the brain


functional cells of a gland or organ, in contrast with the supportive or connective tissue of a gland or
organ


edges of a wound are close enough together to promote healing without the use of stitches to hold them
close


tissue that consists of a single layer of irregularly shaped and sized cells
that give the appearance of multiple layers; found in ducts of certain glands and the upper respiratory tract


fine fibrous protein, made of collagen subunits, which cross-link to form supporting “nets” within
connective tissue


matrix containing collagen and elastin secreted by connective tissue; a component of the basement
membrane


type of loose connective tissue that provides a supportive framework to soft organs, such as lymphatic
tissue, spleen, and the liver


neuroglial cell that produces myelin in the peripheral nervous system
wound healing facilitated by wound contraction


group of cells within the serous membrane that secrete a lubricating substance onto the surface
type of tissue membrane that lines body cavities and lubricates them with serous fluid


tissue that consists of a single layer of column-like cells; promotes secretion and
absorption in tissues and organs


tissue that consists of a single layer of cube-shaped cells; promotes secretion and
absorption in ducts and tubules


tissue that consists of a single layer of flat scale-like cells; promotes diffusion and
filtration across surface


usually attached to bone, under voluntary control, each cell is a fiber that is multinucleated and
striated


under involuntary control, moves internal organs, cells contain a single nucleus, are spindle-shaped,
and do not appear striated; each cell is a fiber


tissue that consists of two or more layers of column-like cells, contains glands and is
found in some ducts


tissue that consists of two or more layers of cube-shaped cells, found in some ducts
tissue that consists of multiple layers of cells with the most apical being flat scale-


like cells; protects surfaces from abrasion
alignment of parallel actin and myosin filaments which form a banded pattern


type of connective tissue that provides strength to the body and protects soft tissue


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synovial membrane


tight junction
tissue
tissue membrane
totipotent
transitional epithelium


vasodilation
wound contraction


connective tissue membrane that lines the cavities of freely movable joints, producing synovial
fluid for lubrication


forms an impermeable barrier between cells
group of cells that are similar in form and perform related functions


thin layer or sheet of cells that covers the outside of the body, organs, and internal cavities
embryonic cells that have the ability to differentiate into any type of cell and organ in the body


form of stratified epithelium found in the urinary tract, characterized by an apical layer of
cells that change shape in response to the presence of urine


widening of blood vessels
process whereby the borders of a wound are physically drawn together


CHAPTER REVIEW
4.1 Types of Tissues
The human body contains more than 200 types of cells that can all be classified into four types of tissues: epithelial,
connective, muscle, and nervous. Epithelial tissues act as coverings controlling the movement of materials across the
surface. Connective tissue integrates the various parts of the body and provides support and protection to organs. Muscle
tissue allows the body to move. Nervous tissues propagate information.
The study of the shape and arrangement of cells in tissue is called histology. All cells and tissues in the body derive from
three germ layers in the embryo: the ectoderm, mesoderm, and endoderm.
Different types of tissues form membranes that enclose organs, provide a friction-free interaction between organs, and keep
organs together. Synovial membranes are connective tissue membranes that protect and line the joints. Epithelial membranes
are formed from epithelial tissue attached to a layer of connective tissue. There are three types of epithelial membranes:
mucous, which contain glands; serous, which secrete fluid; and cutaneous which makes up the skin.


4.2 Epithelial Tissue
In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basal lamina that separates
the epithelium from underlying tissue. The main functions of epithelia are protection from the environment, coverage,
secretion and excretion, absorption, and filtration. Cells are bound together by tight junctions that form an impermeable
barrier. They can also be connected by gap junctions, which allow free exchange of soluble molecules between cells, and
anchoring junctions, which attach cell to cell or cell to matrix. The different types of epithelial tissues are characterized by
their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers form simple epithelia,
whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues.
Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products
through ducts. Endocrine glands secrete hormones directly into the interstitial fluid and blood stream. Glands are classified
both according to the type of secretion and by their structure. Merocrine glands secrete products as they are synthesized.
Apocrine glands release secretions by pinching off the apical portion of the cell, whereas holocrine gland cells store their
secretions until they rupture and release their contents. In this case, the cell becomes part of the secretion.


4.3 Connective Tissue Supports and Protects
Connective tissue is a heterogeneous tissue with many cell shapes and tissue architecture. Structurally, all connective tissues
contain cells that are embedded in an extracellular matrix stabilized by proteins. The chemical nature and physical layout
of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective
tissue fulfills in the body. Connective tissues separate and cushion organs, protecting them from shifting or traumatic injury.
Connect tissues provide support and assist movement, store and transport energy molecules, protect against infections, and
contribute to temperature homeostasis.
Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and
differentiate from mesenchyme and hematopoietic tissue in the bone marrow. Fibroblasts are the most abundant and secrete
many protein fibers, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the
blood cells, chondrocytes form cartilage, and osteocytes form bone. The extracellular matrix contains fluid, proteins,
polysaccharide derivatives, and, in the case of bone, mineral crystals. Protein fibers fall into three major groups: collagen


CHAPTER 4 | THE TISSUE LEVEL OF ORGANIZATION 167




fibers that are thick, strong, flexible, and resist stretch; reticular fibers that are thin and form a supportive mesh; and elastin
fibers that are thin and elastic.
The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective
tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place
and, in the case of adipose tissue, isolate and store energy reserves. The matrix is the most abundant feature for loose tissue
although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibers and may
be regular, with fibers oriented in parallel as in ligaments and tendons, or irregular, with fibers oriented in several directions.
Organ capsules (collagenous type) and walls of arteries (elastic type) contain dense irregular connective tissue. Cartilage
and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible. Hyaline cartilage is smooth and
clear, covers joints, and is found in the growing portion of bones. Fibrocartilage is tough because of extra collagen fibers
and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because
of its high content of elastic fibers. The matrix contains very few blood vessels. Bones are made of a rigid, mineralized
matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized. Cancellous
bone is spongy and less solid than compact bone. Fluid tissue, for example blood and lymph, is characterized by a liquid
matrix and no supporting fibers.


4.4 Muscle Tissue and Motion
The three types of muscle cells are skeletal, cardiac, and smooth. Their morphologies match their specific functions in the
body. Skeletal muscle is voluntary and responds to conscious stimuli. The cells are striated and multinucleated appearing
as long, unbranched cylinders. Cardiac muscle is involuntary and found only in the heart. Each cell is striated with a single
nucleus and they attach to one another to form long fibers. Cells are attached to one another at intercalated disks. The cells
are interconnected physically and electrochemically to act as a syncytium. Cardiac muscle cells contract autonomously and
involuntarily. Smooth muscle is involuntary. Each cell is a spindle-shaped fiber and contains a single nucleus. No striations
are evident because the actin and myosin filaments do not align in the cytoplasm.


4.5 Nervous Tissue Mediates Perception and Response
The most prominent cell of the nervous tissue, the neuron, is characterized mainly by its ability to receive stimuli and
respond by generating an electrical signal, known as an action potential, which can travel rapidly over great distances in
the body. A typical neuron displays a distinctive morphology: a large cell body branches out into short extensions called
dendrites, which receive chemical signals from other neurons, and a long tail called an axon, which relays signals away
from the cell to other neurons, muscles, or glands. Many axons are wrapped by a myelin sheath, a lipid derivative that acts
as an insulator and speeds up the transmission of the action potential. Other cells in the nervous tissue, the neuroglia, include
the astrocytes, microglia, oligodendrocytes, and Schwann cells.


4.6 Tissue Injury and Aging
Inflammation is the classic response of the body to injury and follows a common sequence of events. The area is red,
feels warm to the touch, swells, and is painful. Injured cells, mast cells, and resident macrophages release chemical signals
that cause vasodilation and fluid leakage in the surrounding tissue. The repair phase includes blood clotting, followed
by regeneration of tissue as fibroblasts deposit collagen. Some tissues regenerate more readily than others. Epithelial and
connective tissues replace damaged or dead cells from a supply of adult stem cells. Muscle and nervous tissues undergo
either slow regeneration or do not repair at all.
Age affects all the tissues and organs of the body. Damaged cells do not regenerate as rapidly as in younger people.
Perception of sensation and effectiveness of response are lost in the nervous system. Muscles atrophy, and bones lose mass
and become brittle. Collagen decreases in some connective tissue, and joints stiffen.


INTERACTIVE LINK QUESTIONS
1. View this slideshow (http://openstaxcollege.org/l/
stemcells) to learn more about stem cells. How do somatic
stem cells differ from embryonic stem cells?
2. Watch this video (http://openstaxcollege.org/l/
etissues) to find out more about the anatomy of epithelial
tissues. Where in the body would one find non-keratinizing
stratified squamous epithelium?
3. Visit this link (http://openstaxcollege.org/l/10quiz) to
test your connective tissue knowledge with this 10-question


quiz. Can you name the 10 tissue types shown in the
histology slides?
4. Watch this video (http://openstaxcollege.org/l/
musctissue) to learn more about muscle tissue. In looking
through a microscope how could you distinguish skeletal
muscle tissue from smooth muscle?
5. Follow this link (http://openstaxcollege.org/l/nobel) to
learn more about nervous tissue. What are the main parts of
a nerve cell?


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6. Watch this video (http://openstaxcollege.org/l/
healinghand) to see a hand heal. Over what period of time
do you think these images were taken?


7. Watch this video (http://openstaxcollege.org/l/tumor)
to learn more about tumors. What is a tumor?


REVIEW QUESTIONS
8.Which of the following is not a type of tissue?


a. muscle
b. nervous
c. embryonic
d. epithelial


9. The process by which a less specialized cell matures into
a more specialized cell is called ________.


a. differentiation
b. maturation
c. modification
d. specialization


10. Differentiated cells in a developing embryo derive from
________.


a. endothelium, mesothelium, and epithelium
b. ectoderm, mesoderm, and endoderm
c. connective tissue, epithelial tissue, and muscle
tissue


d. epidermis, mesoderm, and endothelium
11.Which of the following lines the body cavities exposed
to the external environment?


a. mesothelium
b. lamina propria
c. mesenteries
d. mucosa


12. In observing epithelial cells under a microscope, the
cells are arranged in a single layer and look tall and narrow,
and the nucleus is located close to the basal side of the cell.
The specimen is what type of epithelial tissue?


a. columnar
b. stratified
c. squamous
d. transitional


13.Which of the following is the epithelial tissue that lines
the interior of blood vessels?


a. columnar
b. pseudostratified
c. simple squamous
d. transitional


14. Which type of epithelial tissue specializes in moving
particles across its surface and is found in airways and
lining of the oviduct?


a. transitional
b. stratified columnar
c. pseudostratified ciliated columnar
d. stratified squamous


15. The ________ exocrine gland stores its secretion until
the glandular cell ruptures, whereas the ________ gland
releases its apical region and reforms.


a. holocrine; apocrine
b. eccrine; endocrine
c. apocrine; holocrine
d. eccrine; apocrine


16. Connective tissue is made of which three essential
components?


a. cells, ground substance, and carbohydrate fibers
b. cells, ground substance, and protein fibers
c. collagen, ground substance, and protein fibers
d. matrix, ground substance, and fluid


17. Under the microscope, a tissue specimen shows cells
located in spaces scattered in a transparent background.
This is probably ________.


a. loose connective tissue
b. a tendon
c. bone
d. hyaline cartilage


18. Which connective tissue specializes in storage of fat?


a. tendon
b. adipose tissue
c. reticular tissue
d. dense connective tissue


19. Ligaments connect bones together and withstand a lot
of stress. What type of connective tissue should you expect
ligaments to contain?


a. areolar tissue
b. adipose tissue
c. dense regular connective tissue
d. dense irregular connective tissue


20. In adults, new connective tissue cells originate from the
________.


a. mesoderm
b. mesenchyme
c. ectoderm
d. endoderm


21. In bone, the main cells are ________.
a. fibroblasts
b. chondrocytes
c. lymphocytes
d. osteocytes


22. Striations, cylindrical cells, and multiple nuclei are
observed in ________.


a. skeletal muscle only
b. cardiac muscle only
c. smooth muscle only
d. skeletal and cardiac muscles


23. The cells of muscles, myocytes, develop from
________.


a. myoblasts
b. endoderm
c. fibrocytes
d. chondrocytes


24. Skeletal muscle is composed of very hard working
cells. Which organelles do you expect to find in abundance
in skeletal muscle cell?


a. nuclei


CHAPTER 4 | THE TISSUE LEVEL OF ORGANIZATION 169




b. striations
c. golgi bodies
d. mitochondria


25. The cells responsible for the transmission of the nerve
impulse are ________.


a. neurons
b. oligodendrocytes
c. astrocytes
d. microglia


26. The nerve impulse travels down a(n) ________, away
from the cell body.


a. dendrite
b. axon
c. microglia
d. collagen fiber


27. Which of the following central nervous system cells
regulate ions, regulate the uptake and/or breakdown of
some neurotransmitters, and contribute to the formation of
the blood-brain barrier?


a. microglia
b. neuroglia
c. oligodendrocytes
d. astrocytes


28. Which of the following processes is not a cardinal sign
of inflammation?


a. redness
b. heat
c. fever
d. swelling


29. When a mast cell reacts to an irritation, which of the
following chemicals does it release?


a. collagen
b. histamine
c. hyaluronic acid
d. meylin


30. Atrophy refers to ________.
a. loss of elasticity
b. loss of mass
c. loss of rigidity
d. loss of permeability


31. Individuals can slow the rate of aging by modifying all
of these lifestyle aspects except for ________.


a. diet
b. exercise
c. genetic factors
d. stress


CRITICAL THINKING QUESTIONS
32. Identify the four types of tissue in the body, and
describe the major functions of each tissue.
33. The zygote is described as totipotent because it
ultimately gives rise to all the cells in your body including
the highly specialized cells of your nervous system.
Describe this transition, discussing the steps and processes
that lead to these specialized cells.
34.What is the function of synovial membranes?
35. The structure of a tissue usually is optimized for its
function. Describe how the structure of individual cells and
tissue arrangement of the intestine lining matches its main
function, to absorb nutrients.
36. One of the main functions of connective tissue is to
integrate organs and organ systems in the body. Discuss
how blood fulfills this role.
37. Why does an injury to cartilage, especially hyaline
cartilage, heal much more slowly than a bone fracture?
38. You are watching cells in a dish spontaneously
contract. They are all contracting at different rates; some
fast, some slow. After a while, several cells link up and they
begin contracting in synchrony. Discuss what is going on
and what type of cells you are looking at.


39.Why does skeletal muscle look striated?
40. Which morphological adaptations of neurons make
them suitable for the transmission of nerve impulse?
41.What are the functions of astrocytes?
42. Why is it important to watch for increased redness,
swelling and pain after a cut or abrasion has been cleaned
and bandaged?
43. Aspirin is a non-steroidal anti-inflammatory drug
(NSAID) that inhibits the formation of blood clots and
is taken regularly by individuals with a heart condition.
Steroids such as cortisol are used to control some
autoimmune diseases and severe arthritis by down-
regulating the inflammatory response. After reading the
role of inflammation in the body’s response to infection,
can you predict an undesirable consequence of taking anti-
inflammatory drugs on a regular basis?
44. As an individual ages, a constellation of symptoms
begins the decline to the point where an individual’s
functioning is compromised. Identify and discuss two
factors that have a role in factors leading to the
compromised situation.
45. Discuss changes that occur in cells as a person ages.


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5 | THE INTEGUMENTARY
SYSTEM


Figure 5.1 Your skin is a vital part of your life and appearance (a–d). Some people choose to embellish it with tattoos
(a), makeup (b), and even piercings (c). (credit a: Steve Teo; credit b: “spaceodissey”/flickr; credit c: Mark/flickr; credit
d: Lisa Schaffer)


Introduction
Chapter Objectives


After studying the chapter, you will be able to:
• Describe the integumentary system and the role it plays in homeostasis
• Describe the layers of the skin and the functions of each layer
• Describe the accessory structures of the skin and the functions of each
• Describe the changes that occur in the integumentary system during the aging process
• Discuss several common diseases, disorders, and injuries that affect the integumentary system
• Explain treatments for some common diseases, disorders, and injuries of the integumentary system


CHAPTER 5 | THE INTEGUMENTARY SYSTEM 171




What do you think when you look at your skin in the mirror? Do you think about covering it with makeup, adding a tattoo,
or maybe a body piercing? Or do you think about the fact that the skin belongs to one of the body’s most essential and
dynamic systems: the integumentary system? The integumentary system refers to the skin and its accessory structures, and
it is responsible for much more than simply lending to your outward appearance. In the adult human body, the skin makes up
about 16 percent of body weight and covers an area of 1.5 to 2 m2. In fact, the skin and accessory structures are the largest
organ system in the human body. As such, the skin protects your inner organs and it is in need of daily care and protection
to maintain its health. This chapter will introduce the structure and functions of the integumentary system, as well as some
of the diseases, disorders, and injuries that can affect this system.


5.1 | Layers of the Skin
By the end of this section, you will be able to:
• Identify the components of the integumentary system
• Describe the layers of the skin and the functions of each layer
• Identify and describe the hypodermis and deep fascia
• Describe the role of keratinocytes and their life cycle
• Describe the role of melanocytes in skin pigmentation


Although you may not typically think of the skin as an organ, it is in fact made of tissues that work together as a single
structure to perform unique and critical functions. The skin and its accessory structures make up the integumentary system,
which provides the body with overall protection. The skin is made of multiple layers of cells and tissues, which are held to
underlying structures by connective tissue (Figure 5.2). The deeper layer of skin is well vascularized (has numerous blood
vessels). It also has numerous sensory, and autonomic and sympathetic nerve fibers ensuring communication to and from
the brain.


Figure 5.2 Layers of Skin The skin is composed of two main layers: the epidermis, made of closely packed epithelial
cells, and the dermis, made of dense, irregular connective tissue that houses blood vessels, hair follicles, sweat glands,
and other structures. Beneath the dermis lies the hypodermis, which is composed mainly of loose connective and fatty
tissues.


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The skin consists of two main layers and a closely associated layer. View this animation (http://openstaxcollege.org/
l/layers) to learn more about layers of the skin. What are the basic functions of each of these layers?


The Epidermis
The epidermis is composed of keratinized, stratified squamous epithelium. It is made of four or five layers of epithelial
cells, depending on its location in the body. It does not have any blood vessels within it (i.e., it is avascular). Skin that
has four layers of cells is referred to as “thin skin.” From deep to superficial, these layers are the stratum basale, stratum
spinosum, stratum granulosum, and stratum corneum. Most of the skin can be classified as thin skin. “Thick skin” is found
only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, located between the
stratum corneum and the stratum granulosum (Figure 5.3).


Figure 5.3 Thin Skin versus Thick Skin These slides show cross-sections of the epidermis and dermis of (a) thin
and (b) thick skin. Note the significant difference in the thickness of the epithelial layer of the thick skin. From top, LM
× 40, LM × 40. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)


The cells in all of the layers except the stratum basale are called keratinocytes. A keratinocyte is a cell that manufactures
and stores the protein keratin. Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and


CHAPTER 5 | THE INTEGUMENTARY SYSTEM 173




water-resistant properties. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by
cells from the deeper layers (Figure 5.4).


Figure 5.4 Epidermis The epidermis is epithelium composed of multiple layers of cells. The basal layer consists of
cuboidal cells, whereas the outer layers are squamous, keratinized cells, so the whole epithelium is often described
as being keratinized stratified squamous epithelium. LM × 40. (Micrograph provided by the Regents of University of
Michigan Medical School © 2012)


View the University of Michigan WebScope at http://virtualslides.med_umich.edu/Histology/Basic%20Tissues/
Epithelium%20and%20CT/106_HISTO_40X.svs/view.apml? (http://openstaxcollege.org/l/Epidermis) to
explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what
do you notice about the cells?


Stratum Basale
The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the
basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining
collagen fibers, referred to as the basement membrane. A finger-like projection, or fold, known as the dermal papilla
(plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the
connection between the epidermis and dermis; the greater the folding, the stronger the connections made (Figure 5.5).


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Figure 5.5 Layers of the Epidermis The epidermis of thick skin has five layers: stratum basale, stratum spinosum,
stratum granulosum, stratum lucidum, and stratum corneum.


The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that
is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells,
which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed
superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum
basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that
the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a
melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the
living cells of the epidermis from ultraviolet (UV) radiation damage.
In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer
(papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are
unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging
processes.
Stratum Spinosum
As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells
via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells. It
is interesting to note that the “spiny” nature of this layer is an artifact of the staining process. Unstained epidermis samples
do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes,
formed as a result of cell division in the stratum basale (Figure 5.6). Interspersed among the keratinocytes of this layer is a
type of dendritic cell called the Langerhans cell, which functions as a macrophage by engulfing bacteria, foreign particles,
and damaged cells that occur in this layer.


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Figure 5.6 Cells of the Epidermis The cells in the different layers of the epidermis originate from basal cells located
in the stratum basale, yet the cells of each layer are distinctively different. EM × 2700. (Micrograph provided by the
Regents of University of Michigan Medical School © 2012)


View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/EMsmallCharts/
3%20Image%20Scope%20finals/065%20-%20Epidermis_001.svs/view.apml (http://openstaxcollege.org/l/
basal) to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of
skin, what do you notice about the cells?


The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps
prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum
basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum.
Stratum Granulosum
The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the
stratum spinosum. The cells (three to five layers deep) become flatter, their cell membranes thicken, and they generate large
amounts of the proteins keratin, which is fibrous, and keratohyalin, which accumulates as lamellar granules within the
cells (see Figure 5.5). These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give
the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin,


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keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of
hair and nails.
Stratum Lucidum
The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum
and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The
keratinocytes that compose the stratum lucidum are dead and flattened (see Figure 5.5). These cells are densely packed
with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid)
appearance and provides a barrier to water.
Stratum Corneum
The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment
(see Figure 5.5). The increased keratinization (also called cornification) of the cells in this layer gives it its name. There are
usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the
dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate, underlying
layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (or stratum
lucidum in the case of the palms and soles of feet). The entire layer is replaced during a period of about 4 weeks. Cosmetic
procedures, such as microdermabrasion, help remove some of the dry, upper layer and aim to keep the skin looking “fresh”
and healthy.


Dermis
The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis
(epi- = “upon” or “over”) and hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other
structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an
interconnected mesh of elastin and collagenous fibers, produced by fibroblasts (Figure 5.7).


Figure 5.7 Layers of the Dermis This stained slide shows the two components of the dermis—the papillary layer
and the reticular layer. Both are made of connective tissue with fibers of collagen extending from one to the other,
making the border between the two somewhat indistinct. The dermal papillae extending into the epidermis belong to
the papillary layer, whereas the dense collagen fiber bundles below belong to the reticular layer. LM × 10. (credit:
modification of work by “kilbad”/Wikimedia Commons)


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Papillary Layer
The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer
form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like
dermal papillae (see Figure 5.7). Within the papillary layer are fibroblasts, a small number of fat cells (adipocytes), and
an abundance of small blood vessels. In addition, the papillary layer contains phagocytes, defensive cells that help fight
bacteria or other infections that have breached the skin. This layer also contains lymphatic capillaries, nerve fibers, and
touch receptors called the Meissner corpuscles.
Reticular Layer
Underlying the papillary layer is the much thicker reticular layer, composed of dense, irregular connective tissue. This
layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated (net-
like) due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen
fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the
hypodermis. In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help
restore skin turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively.


Hypodermis
The hypodermis (also called the subcutaneous layer or superficial fascia) is a layer directly below the dermis and serves
to connect the skin to the underlying fascia (fibrous tissue) of the bones and muscles. It is not strictly a part of the skin,
although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-
vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides
insulation and cushioning for the integument.


Lipid Storage
The hypodermis is home to most of the fat that concerns people when they are trying to keep their weight under control.
Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes. This stored fat can serve as
an energy reserve, insulate the body to prevent heat loss, and act as a cushion to protect underlying structures from
trauma.
Where the fat is deposited and accumulates within the hypodermis depends on hormones (testosterone, estrogen,
insulin, glucagon, leptin, and others), as well as genetic factors. Fat distribution changes as our bodies mature and age.
Men tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do women (breasts, hips,
thighs, and buttocks). The body mass index (BMI) is often used as a measure of fat, although this measure is, in fact,
derived from a mathematical formula that compares body weight (mass) to height. Therefore, its accuracy as a health
indicator can be called into question in individuals who are extremely physically fit.
In many animals, there is a pattern of storing excess calories as fat to be used in times when food is not readily
available. In much of the developed world, insufficient exercise coupled with the ready availability and consumption
of high-calorie foods have resulted in unwanted accumulations of adipose tissue in many people. Although periodic
accumulation of excess fat may have provided an evolutionary advantage to our ancestors, who experienced
unpredictable bouts of famine, it is now becoming chronic and considered a major health threat. Recent studies indicate
that a distressing percentage of our population is overweight and/or clinically obese. Not only is this a problem for the
individuals affected, but it also has a severe impact on our healthcare system. Changes in lifestyle, specifically in diet
and exercise, are the best ways to control body fat accumulation, especially when it reaches levels that increase the risk
of heart disease and diabetes.


Pigmentation
The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin
is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The
melanin is transferred into the keratinocytes via a cellular vesicle called a melanosome (Figure 5.8).


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Figure 5.8 Skin Pigmentation The relative coloration of the skin depends of the amount of melanin produced by
melanocytes in the stratum basale and taken up by keratinocytes.


Melanin occurs in two primary forms. Eumelanin exists as black and brown, whereas pheomelanin provides a red color.
Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning
salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete
chemicals that stimulate melanocytes. The accumulation of melanin in keratinocytes results in the darkening of the skin, or
a tan. This increased melanin accumulation protects the DNA of epidermal cells from UV ray damage and the breakdown of
folic acid, a nutrient necessary for our health and well-being. In contrast, too much melanin can interfere with the production
of vitamin D, an important nutrient involved in calcium absorption. Thus, the amount of melanin present in our skin is
dependent on a balance between available sunlight and folic acid destruction, and protection from UV radiation and vitamin
D production.
It requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend
to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns, but are more protected than are
pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes;
this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent.
Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and
in severe cases, can cause sufficient DNA damage to result in skin cancer. When there is an irregular accumulation of
melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should
be monitored for changes that might indicate the presence of cancer (Figure 5.9).


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Figure 5.9 Moles Moles range from benign accumulations of melanocytes to melanomas. These structures populate
the landscape of our skin. (credit: the National Cancer Institute)


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Integumentary System
The first thing a clinician sees is the skin, and so the examination of the skin should be part of any thorough physical
examination. Most skin disorders are relatively benign, but a few, including melanomas, can be fatal if untreated.
A couple of the more noticeable disorders, albinism and vitiligo, affect the appearance of the skin and its accessory
organs. Although neither is fatal, it would be hard to claim that they are benign, at least to the individuals so afflicted.
Albinism is a genetic disorder that affects (completely or partially) the coloring of skin, hair, and eyes. The defect
is primarily due to the inability of melanocytes to produce melanin. Individuals with albinism tend to appear white
or very pale due to the lack of melanin in their skin and hair. Recall that melanin helps protect the skin from the
harmful effects of UV radiation. Individuals with albinism tend to need more protection from UV radiation, as they
are more prone to sunburns and skin cancer. They also tend to be more sensitive to light and have vision problems due
to the lack of pigmentation on the retinal wall. Treatment of this disorder usually involves addressing the symptoms,
such as limiting UV light exposure to the skin and eyes. In vitiligo, the melanocytes in certain areas lose their ability
to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches (Figure 5.10).
Neither albinism nor vitiligo directly affects the lifespan of an individual.


Figure 5.10 Vitiligo Individuals with vitiligo experience depigmentation that results in lighter colored patches of
skin. The condition is especially noticeable on darker skin. (credit: Klaus D. Peter)


Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems.
Liver disease or liver cancer can cause the accumulation of bile and the yellow pigment bilirubin, leading to the skin
appearing yellow or jaundiced (jaune is the French word for “yellow”). Tumors of the pituitary gland can result in
the secretion of large amounts of melanocyte-stimulating hormone (MSH), which results in a darkening of the skin.
Similarly, Addison’s disease can stimulate the release of excess amounts of adrenocorticotropic hormone (ACTH),
which can give the skin a deep bronze color. A sudden drop in oxygenation can affect skin color, causing the skin
to initially turn ashen (white). With a prolonged reduction in oxygen levels, dark red deoxyhemoglobin becomes
dominant in the blood, making the skin appear blue, a condition referred to as cyanosis (kyanos is the Greek word for
“blue”). This happens when the oxygen supply is restricted, as when someone is experiencing difficulty in breathing
because of asthma or a heart attack. However, in these cases the effect on skin color has nothing do with the skin’s
pigmentation.


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This ABC video follows the story of a pair of fraternal African-American twins, one of whom is albino. Watch this
video (http://openstaxcollege.org/l/albino) to learn about the challenges these children and their family face. Which
ethnicities do you think are exempt from the possibility of albinism?


5.2 | Accessory Structures of the Skin
By the end of this section, you will be able to:
• Identify the accessory structures of the skin
• Describe the structure and function of hair and nails
• Describe the structure and function of sweat glands and sebaceous glands


Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. These structures embryologically
originate from the epidermis and can extend down through the dermis into the hypodermis.


Hair
Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair
originate in an epidermal penetration of the dermis called the hair follicle. The hair shaft is the part of the hair not anchored
to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies
below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb, and
includes a layer of mitotically active basal cells called the hair matrix. The hair bulb surrounds the hair papilla, which is
made of connective tissue and contains blood capillaries and nerve endings from the dermis (Figure 5.11).


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Figure 5.11 Hair Hair follicles originate in the epidermis and have many different parts.


Just as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the
surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft as the hair grows. The
medulla forms the central core of the hair, which is surrounded by the cortex, a layer of compressed, keratinized cells that
is covered by an outer layer of very hard, keratinized cells known as the cuticle. These layers are depicted in a longitudinal
cross-section of the hair follicle (Figure 5.12), although not all hair has a medullary layer. Hair texture (straight, curly)
is determined by the shape and structure of the cortex, and to the extent that it is present, the medulla. The shape and
structure of these layers are, in turn, determined by the shape of the hair follicle. Hair growth begins with the production of
keratinocytes by the basal cells of the hair bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed through
the follicle toward the surface. Keratinization is completed as the cells are pushed to the skin surface to form the shaft of
hair that is externally visible. The external hair is completely dead and composed entirely of keratin. For this reason, our
hair does not have sensation. Furthermore, you can cut your hair or shave without damaging the hair structure because the
cut is superficial. Most chemical hair removers also act superficially; however, electrolysis and yanking both attempt to
destroy the hair bulb so hair cannot grow.


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Figure 5.12 Hair Follicle The slide shows a cross-section of a hair follicle. Basal cells of the hair matrix in the center
differentiate into cells of the inner root sheath. Basal cells at the base of the hair root form the outer root sheath. LM ×
4. (credit: modification of work by “kilbad”/Wikimedia Commons)


The wall of the hair follicle is made of three concentric layers of cells. The cells of the internal root sheath surround the
root of the growing hair and extend just up to the hair shaft. They are derived from the basal cells of the hair matrix. The
external root sheath, which is an extension of the epidermis, encloses the hair root. It is made of basal cells at the base of
the hair root and tends to be more keratinous in the upper regions. The glassy membrane is a thick, clear connective tissue
sheath covering the hair root, connecting it to the tissue of the dermis.


The hair follicle is made of multiple layers of cells that form from basal cells in the hair matrix and the hair root. Cells
of the hair matrix divide and differentiate to form the layers of the hair. Watch this video (http://openstaxcollege.org/
l/follicle) to learn more about hair follicles.


Hair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example,
hair on the head protects the skull from the sun. The hair in the nose and ears, and around the eyes (eyelashes) defends the
body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat
and other particles from dripping into and bothering the eyes. Hair also has a sensory function due to sensory innervation
by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other
disturbances in the environment, much more so than the skin surface. This feature is also useful for the detection of the
presence of insects or other potentially damaging substances on the skin surface. Each hair root is connected to a smooth
muscle called the arrector pili that contracts in response to nerve signals from the sympathetic nervous system, making
the external hair shaft “stand up.” The primary purpose for this is to trap a layer of air to add insulation. This is visible in
humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is
much more obvious in organisms with a heavier coat than most humans, such as dogs and cats.
Hair Growth
Hair grows and is eventually shed and replaced by new hair. This occurs in three phases. The first is the anagen phase,
during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. The length of this phase is
measured in years, typically from 2 to 7 years. The catagen phase lasts only 2 to 3 weeks, and marks a transition from the


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hair follicle’s active growth. Finally, during the telogen phase, the hair follicle is at rest and no new growth occurs. At the
end of this phase, which lasts about 2 to 4 months, another anagen phase begins. The basal cells in the hair matrix then
produce a new hair follicle, which pushes the old hair out as the growth cycle repeats itself. Hair typically grows at the rate
of 0.3 mm per day during the anagen phase. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is
more hair shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from the
aging process, or the influence of hormones.
Hair Color
Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different
hair color results from differences in the type of melanin, which is genetically determined. As a person ages, the melanin
production decreases, and hair tends to lose its color and becomes gray and/or white.


Nails
The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. The nail body is
formed on the nail bed, and protects the tips of our fingers and toes as they are the farthest extremities and the parts of
the body that experience the maximum mechanical stress (Figure 5.13). In addition, the nail body forms a back-support for
picking up small objects with the fingers. The nail body is composed of densely packed dead keratinocytes. The epidermis
in this part of the body has evolved a specialized structure upon which nails can form. The nail body forms at the nail root,
which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously. The lateral nail
fold overlaps the nail on the sides, helping to anchor the nail body. The nail fold that meets the proximal end of the nail body
forms the nail cuticle, also called the eponychium. The nail bed is rich in blood vessels, making it appear pink, except at
the base, where a thick layer of epithelium over the nail matrix forms a crescent-shaped region called the lunula (the “little
moon”). The area beneath the free edge of the nail, furthest from the cuticle, is called the hyponychium. It consists of a
thickened layer of stratum corneum.


Figure 5.13 Nails The nail is an accessory structure of the integumentary system.


Nails are accessory structures of the integumentary system. Visit this link (http://openstaxcollege.org/l/nails) to learn
more about the origin and growth of fingernails.


Sweat Glands
When the body becomes warm, sudoriferous glands produce sweat to cool the body. Sweat glands develop from epidermal
projections into the dermis and are classified as merocrine glands; that is, the secretions are excreted by exocytosis through a
duct without affecting the cells of the gland. There are two types of sweat glands, each secreting slightly different products.


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An eccrine sweat gland is type of gland that produces a hypotonic sweat for thermoregulation. These glands are found all
over the skin’s surface, but are especially abundant on the palms of the hand, the soles of the feet, and the forehead (Figure
5.14). They are coiled glands lying deep in the dermis, with the duct rising up to a pore on the skin surface, where the
sweat is released. This type of sweat, released by exocytosis, is hypotonic and composed mostly of water, with some salt,
antibodies, traces of metabolic waste, and dermicidin, an antimicrobial peptide. Eccrine glands are a primary component of
thermoregulation in humans and thus help to maintain homeostasis.


Figure 5.14 Eccrine Gland Eccrine glands are coiled glands in the dermis that release sweat that is mostly water.


An apocrine sweat gland is usually associated with hair follicles in densely hairy areas, such as armpits and genital regions.
Apocrine sweat glands are larger than eccrine sweat glands and lie deeper in the dermis, sometimes even reaching the
hypodermis, with the duct normally emptying into the hair follicle. In addition to water and salts, apocrine sweat includes
organic compounds that make the sweat thicker and subject to bacterial decomposition and subsequent smell. The release
of this sweat is under both nervous and hormonal control, and plays a role in the poorly understood human pheromone
response. Most commercial antiperspirants use an aluminum-based compound as their primary active ingredient to stop
sweat. When the antiperspirant enters the sweat gland duct, the aluminum-based compounds precipitate due to a change in
pH and form a physical block in the duct, which prevents sweat from coming out of the pore.


Sweating regulates body temperature. The composition of the sweat determines whether body odor is a byproduct of
sweating. Visit this link (http://openstaxcollege.org/l/sweating) to learn more about sweating and body odor.


Sebaceous Glands
A sebaceous gland is a type of oil gland that is found all over the body and helps to lubricate and waterproof the skin and
hair. Most sebaceous glands are associated with hair follicles. They generate and excrete sebum, a mixture of lipids, onto


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the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells of the stratum corneum, keeping
it pliable. The fatty acids of sebum also have antibacterial properties, and prevent water loss from the skin in low-humidity
environments. The secretion of sebum is stimulated by hormones, many of which do not become active until puberty. Thus,
sebaceous glands are relatively inactive during childhood.


5.3 | Functions of the Integumentary System
By the end of this section, you will be able to:
• Describe the different functions of the skin and the structures that enable them
• Explain how the skin helps maintain body temperature


The skin and accessory structures perform a variety of essential functions, such as protecting the body from invasion by
microorganisms, chemicals, and other environmental factors; preventing dehydration; acting as a sensory organ; modulating
body temperature and electrolyte balance; and synthesizing vitamin D. The underlying hypodermis has important roles in
storing fats, forming a “cushion” over underlying structures, and providing insulation from cold temperatures.


Protection
The skin protects the rest of the body from the basic elements of nature such as wind, water, and UV sunlight. It acts as a
protective barrier against water loss, due to the presence of layers of keratin and glycolipids in the stratum corneum. It also
is the first line of defense against abrasive activity due to contact with grit, microbes, or harmful chemicals. Sweat excreted
from sweat glands deters microbes from over-colonizing the skin surface by generating dermicidin, which has antibiotic
properties.


Tattoos and Piercings
The word “armor” evokes several images. You might think of a Roman centurion or a medieval knight in a suit of
armor. The skin, in its own way, functions as a form of armor—body armor. It provides a barrier between your vital,
life-sustaining organs and the influence of outside elements that could potentially damage them.
For any form of armor, a breach in the protective barrier poses a danger. The skin can be breached when a child skins a
knee or an adult has blood drawn—one is accidental and the other medically necessary. However, you also breach this
barrier when you choose to “accessorize” your skin with a tattoo or body piercing. Because the needles involved in
producing body art and piercings must penetrate the skin, there are dangers associated with the practice. These include
allergic reactions; skin infections; blood-borne diseases, such as tetanus, hepatitis C, and hepatitis D; and the growth of
scar tissue. Despite the risk, the practice of piercing the skin for decorative purposes has become increasingly popular.
According to the American Academy of Dermatology, 24 percent of people from ages 18 to 50 have a tattoo.


Tattooing has a long history, dating back thousands of years ago. The dyes used in tattooing typically derive from
metals. A person with tattoos should be cautious when having a magnetic resonance imaging (MRI) scan because an
MRI machine uses powerful magnets to create images of the soft tissues of the body, which could react with the metals
contained in the tattoo dyes. Watch this video (http://openstaxcollege.org/l/tattoo) to learn more about tattooing.


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Sensory Function
The fact that you can feel an ant crawling on your skin, allowing you to flick it off before it bites, is because the skin, and
especially the hairs projecting from hair follicles in the skin, can sense changes in the environment. The hair root plexus
surrounding the base of the hair follicle senses a disturbance, and then transmits the information to the central nervous
system (brain and spinal cord), which can then respond by activating the skeletal muscles of your eyes to see the ant and the
skeletal muscles of the body to act against the ant.
The skin acts as a sense organ because the epidermis, dermis, and the hypodermis contain specialized sensory nerve
structures that detect touch, surface temperature, and pain. These receptors are more concentrated on the tips of the fingers,
which are most sensitive to touch, especially the Meissner corpuscle (tactile corpuscle) (Figure 5.15), which responds to
light touch, and the Pacinian corpuscle (lamellated corpuscle), which responds to vibration. Merkel cells, seen scattered in
the stratum basale, are also touch receptors. In addition to these specialized receptors, there are sensory nerves connected to
each hair follicle, pain and temperature receptors scattered throughout the skin, and motor nerves innervate the arrector pili
muscles and glands. This rich innervation helps us sense our environment and react accordingly.


Figure 5.15 Light Micrograph of a Meissner Corpuscle In this micrograph of a skin cross-section, you can see a
Meissner corpuscle (arrow), a type of touch receptor located in a dermal papilla adjacent to the basement membrane
and stratum basale of the overlying epidermis. LM × 100. (credit: “Wbensmith”/Wikimedia Commons)


Thermoregulation
The integumentary system helps regulate body temperature through its tight association with the sympathetic nervous
system, the division of the nervous system involved in our fight-or-flight responses. The sympathetic nervous system is
continuously monitoring body temperature and initiating appropriate motor responses. Recall that sweat glands, accessory
structures to the skin, secrete water, salt, and other substances to cool the body when it becomes warm. Even when the body
does not appear to be noticeably sweating, approximately 500 mL of sweat (insensible perspiration) are secreted a day. If
the body becomes excessively warm due to high temperatures, vigorous activity (Figure 5.16ac), or a combination of the
two, sweat glands will be stimulated by the sympathetic nervous system to produce large amounts of sweat, as much as 0.7
to 1.5 L per hour for an active person. When the sweat evaporates from the skin surface, the body is cooled as body heat is
dissipated.
In addition to sweating, arterioles in the dermis dilate so that excess heat carried by the blood can dissipate through the skin
and into the surrounding environment (Figure 5.16b). This accounts for the skin redness that many people experience when
exercising.


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Figure 5.16 Thermoregulation During strenuous physical activities, such as skiing (a) or running (c), the dermal
blood vessels dilate and sweat secretion increases (b). These mechanisms prevent the body from overheating. In
contrast, the dermal blood vessels constrict to minimize heat loss in response to low temperatures (b). (credit a:
“Trysil”/flickr; credit c: Ralph Daily)


When body temperatures drop, the arterioles constrict to minimize heat loss, particularly in the ends of the digits and tip of
the nose. This reduced circulation can result in the skin taking on a whitish hue. Although the temperature of the skin drops
as a result, passive heat loss is prevented, and internal organs and structures remain warm. If the temperature of the skin
drops too much (such as environmental temperatures below freezing), the conservation of body core heat can result in the
skin actually freezing, a condition called frostbite.


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Integumentary System
All systems in the body accumulate subtle and some not-so-subtle changes as a person ages. Among these changes
are reductions in cell division, metabolic activity, blood circulation, hormonal levels, and muscle strength (Figure
5.17). In the skin, these changes are reflected in decreased mitosis in the stratum basale, leading to a thinner epidermis.
The dermis, which is responsible for the elasticity and resilience of the skin, exhibits a reduced ability to regenerate,
which leads to slower wound healing. The hypodermis, with its fat stores, loses structure due to the reduction and
redistribution of fat, which in turn contributes to the thinning and sagging of skin.


Figure 5.17 Aging Generally, skin, especially on the face and hands, starts to display the first noticeable signs of
aging, as it loses its elasticity over time. (credit: Janet Ramsden)


The accessory structures also have lowered activity, generating thinner hair and nails, and reduced amounts of sebum
and sweat. A reduced sweating ability can cause some elderly to be intolerant to extreme heat. Other cells in the skin,
such as melanocytes and dendritic cells, also become less active, leading to a paler skin tone and lowered immunity.
Wrinkling of the skin occurs due to breakdown of its structure, which results from decreased collagen and elastin
production in the dermis, weakening of muscles lying under the skin, and the inability of the skin to retain adequate
moisture.
Many anti-aging products can be found in stores today. In general, these products try to rehydrate the skin and thereby
fill out the wrinkles, and some stimulate skin growth using hormones and growth factors. Additionally, invasive
techniques include collagen injections to plump the tissue and injections of BOTOX® (the name brand of the botulinum
neurotoxin) that paralyze the muscles that crease the skin and cause wrinkling.


Vitamin D Synthesis
The epidermal layer of human skin synthesizes vitamin D when exposed to UV radiation. In the presence of sunlight, a
form of vitamin D3 called cholecalciferol is synthesized from a derivative of the steroid cholesterol in the skin. The liver
converts cholecalciferol to calcidiol, which is then converted to calcitriol (the active chemical form of the vitamin) in the
kidneys. Vitamin D is essential for normal absorption of calcium and phosphorous, which are required for healthy bones.
The absence of sun exposure can lead to a lack of vitamin D in the body, leading to a condition called rickets, a painful
condition in children where the bones are misshapen due to a lack of calcium, causing bowleggedness. Elderly individuals
who suffer from vitamin D deficiency can develop a condition called osteomalacia, a softening of the bones. In present day
society, vitamin D is added as a supplement to many foods, including milk and orange juice, compensating for the need for
sun exposure.
In addition to its essential role in bone health, vitamin D is essential for general immunity against bacterial, viral, and fungal
infections. Recent studies are also finding a link between insufficient vitamin D and cancer.


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5.4 | Diseases, Disorders, and Injuries of the
Integumentary System
By the end of this section, you will be able to:
• Describe several different diseases and disorders of the skin
• Describe the effect of injury to the skin and the process of healing


The integumentary system is susceptible to a variety of diseases, disorders, and injuries. These range from annoying but
relatively benign bacterial or fungal infections that are categorized as disorders, to skin cancer and severe burns, which can
be fatal. In this section, you will learn several of the most common skin conditions.


Diseases
One of the most talked about diseases is skin cancer. Cancer is a broad term that describes diseases caused by abnormal
cells in the body dividing uncontrollably. Most cancers are identified by the organ or tissue in which the cancer originates.
One common form of cancer is skin cancer. The Skin Cancer Foundation reports that one in five Americans will experience
some type of skin cancer in their lifetime. The degradation of the ozone layer in the atmosphere and the resulting increase
in exposure to UV radiation has contributed to its rise. Overexposure to UV radiation damages DNA, which can lead to the
formation of cancerous lesions. Although melanin offers some protection against DNA damage from the sun, often it is not
enough. The fact that cancers can also occur on areas of the body that are normally not exposed to UV radiation suggests
that there are additional factors that can lead to cancerous lesions.
In general, cancers result from an accumulation of DNA mutations. These mutations can result in cell populations that
do not die when they should and uncontrolled cell proliferation that leads to tumors. Although many tumors are benign
(harmless), some produce cells that can mobilize and establish tumors in other organs of the body; this process is referred
to as metastasis. Cancers are characterized by their ability to metastasize.
Basal Cell Carcinoma
Basal cell carcinoma is a form of cancer that affects the mitotically active stem cells in the stratum basale of the epidermis.
It is the most common of all cancers that occur in the United States and is frequently found on the head, neck, arms, and
back, which are areas that are most susceptible to long-term sun exposure. Although UV rays are the main culprit, exposure
to other agents, such as radiation and arsenic, can also lead to this type of cancer. Wounds on the skin due to open sores,
tattoos, burns, etc. may be predisposing factors as well. Basal cell carcinomas start in the stratum basale and usually spread
along this boundary. At some point, they begin to grow toward the surface and become an uneven patch, bump, growth,
or scar on the skin surface (Figure 5.18). Like most cancers, basal cell carcinomas respond best to treatment when caught
early. Treatment options include surgery, freezing (cryosurgery), and topical ointments (Mayo Clinic 2012).


Figure 5.18 Basal Cell Carcinoma Basal cell carcinoma can take several different forms. Similar to other forms of
skin cancer, it is readily cured if caught early and treated. (credit: John Hendrix, MD)


Squamous Cell Carcinoma
Squamous cell carcinoma is a cancer that affects the keratinocytes of the stratum spinosum and presents as lesions
commonly found on the scalp, ears, and hands (Figure 5.19). It is the second most common skin cancer. The American
Cancer Society reports that two of 10 skin cancers are squamous cell carcinomas, and it is more aggressive than basal
cell carcinoma. If not removed, these carcinomas can metastasize. Surgery and radiation are used to cure squamous cell
carcinoma.


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Figure 5.19 Squamous Cell Carcinoma Squamous cell carcinoma presents here as a lesion on an individual’s nose.
(credit: the National Cancer Institute)


Melanoma
A melanoma is a cancer characterized by the uncontrolled growth of melanocytes, the pigment-producing cells in the
epidermis. Typically, a melanoma develops from a mole. It is the most fatal of all skin cancers, as it is highly metastatic
and can be difficult to detect before it has spread to other organs. Melanomas usually appear as asymmetrical brown and
black patches with uneven borders and a raised surface (Figure 5.20). Treatment typically involves surgical excision and
immunotherapy.


Figure 5.20 Melanoma Melanomas typically present as large brown or black patches with uneven borders and a
raised surface. (credit: the National Cancer Institute)


Doctors often give their patients the following ABCDE mnemonic to help with the diagnosis of early-stage melanoma. If
you observe a mole on your body displaying these signs, consult a doctor.
• Asymmetry – the two sides are not symmetrical
• Borders – the edges are irregular in shape
• Color – the color is varied shades of brown or black
• Diameter – it is larger than 6 mm (0.24 in)
• Evolving – its shape has changed
Some specialists cite the following additional signs for the most serious form, nodular melanoma:
• Elevated – it is raised on the skin surface
• Firm – it feels hard to the touch
• Growing – it is getting larger


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Skin Disorders
Two common skin disorders are eczema and acne. Eczema is an inflammatory condition and occurs in individuals of all
ages. Acne involves the clogging of pores, which can lead to infection and inflammation, and is often seen in adolescents.
Other disorders, not discussed here, include seborrheic dermatitis (on the scalp), psoriasis, cold sores, impetigo, scabies,
hives, and warts.
Eczema
Eczema is an allergic reaction that manifests as dry, itchy patches of skin that resemble rashes (Figure 5.21). It may be
accompanied by swelling of the skin, flaking, and in severe cases, bleeding. Many who suffer from eczema have antibodies
against dust mites in their blood, but the link between eczema and allergy to dust mites has not been proven. Symptoms are
usually managed with moisturizers, corticosteroid creams, and immunosuppressants.


Figure 5.21 Eczema Eczema is a common skin disorder that presents as a red, flaky rash. (credit:
“Jambula”/Wikimedia Commons)


Acne
Acne is a skin disturbance that typically occurs on areas of the skin that are rich in sebaceous glands (face and back).
It is most common along with the onset of puberty due to associated hormonal changes, but can also occur in infants
and continue into adulthood. Hormones, such as androgens, stimulate the release of sebum. An overproduction and
accumulation of sebum along with keratin can block hair follicles. This plug is initially white. The sebum, when oxidized by
exposure to air, turns black. Acne results from infection by acne-causing bacteria (Propionibacterium and Staphylococcus),
which can lead to redness and potential scarring due to the natural wound healing process (Figure 5.22).


Figure 5.22 Acne Acne is a result of over-productive sebaceous glands, which leads to formation of blackheads and
inflammation of the skin.


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Dermatologist
Have you ever had a skin rash that did not respond to over-the-counter creams, or a mole that you were concerned
about? Dermatologists help patients with these types of problems and more, on a daily basis. Dermatologists are
medical doctors who specialize in diagnosing and treating skin disorders. Like all medical doctors, dermatologists
earn a medical degree and then complete several years of residency training. In addition, dermatologists may then
participate in a dermatology fellowship or complete additional, specialized training in a dermatology practice. If
practicing in the United States, dermatologists must pass the United States Medical Licensing Exam (USMLE),
become licensed in their state of practice, and be certified by the American Board of Dermatology.
Most dermatologists work in a medical office or private-practice setting. They diagnose skin conditions and rashes,
prescribe oral and topical medications to treat skin conditions, and may perform simple procedures, such as mole or
wart removal. In addition, they may refer patients to an oncologist if skin cancer that has metastasized is suspected.
Recently, cosmetic procedures have also become a prominent part of dermatology. Botox injections, laser treatments,
and collagen and dermal filler injections are popular among patients, hoping to reduce the appearance of skin aging.
Dermatology is a competitive specialty in medicine. Limited openings in dermatology residency programs mean that
many medical students compete for a few select spots. Dermatology is an appealing specialty to many prospective
doctors, because unlike emergency room physicians or surgeons, dermatologists generally do not have to work
excessive hours or be “on-call” weekends and holidays. Moreover, the popularity of cosmetic dermatology has made
it a growing field with many lucrative opportunities. It is not unusual for dermatology clinics to market themselves
exclusively as cosmetic dermatology centers, and for dermatologists to specialize exclusively in these procedures.
Consider visiting a dermatologist to talk about why he or she entered the field and what the field of dermatology is
like. Visit this site (http://www.Diplomaguide.com) for additional information.


Injuries
Because the skin is the part of our bodies that meets the world most directly, it is especially vulnerable to injury. Injuries
include burns and wounds, as well as scars and calluses. They can be caused by sharp objects, heat, or excessive pressure
or friction to the skin.
Skin injuries set off a healing process that occurs in several overlapping stages. The first step to repairing damaged skin
is the formation of a blood clot that helps stop the flow of blood and scabs over with time. Many different types of cells
are involved in wound repair, especially if the surface area that needs repair is extensive. Before the basal stem cells of the
stratum basale can recreate the epidermis, fibroblasts mobilize and divide rapidly to repair the damaged tissue by collagen
deposition, forming granulation tissue. Blood capillaries follow the fibroblasts and help increase blood circulation and
oxygen supply to the area. Immune cells, such as macrophages, roam the area and engulf any foreign matter to reduce the
chance of infection.
Burns
A burn results when the skin is damaged by intense heat, radiation, electricity, or chemicals. The damage results in the death
of skin cells, which can lead to a massive loss of fluid. Dehydration, electrolyte imbalance, and renal and circulatory failure
follow, which can be fatal. Burn patients are treated with intravenous fluids to offset dehydration, as well as intravenous
nutrients that enable the body to repair tissues and replace lost proteins. Another serious threat to the lives of burn patients
is infection. Burned skin is extremely susceptible to bacteria and other pathogens, due to the loss of protection by intact
layers of skin.
Burns are sometimes measured in terms of the size of the total surface area affected. This is referred to as the “rule of nines,”
which associates specific anatomical areas with a percentage that is a factor of nine (Figure 5.23). Burns are also classified
by the degree of their severity. A first-degree burn is a superficial burn that affects only the epidermis. Although the skin
may be painful and swollen, these burns typically heal on their own within a few days. Mild sunburn fits into the category
of a first-degree burn. A second-degree burn goes deeper and affects both the epidermis and a portion of the dermis. These
burns result in swelling and a painful blistering of the skin. It is important to keep the burn site clean and sterile to prevent
infection. If this is done, the burn will heal within several weeks. A third-degree burn fully extends into the epidermis and
dermis, destroying the tissue and affecting the nerve endings and sensory function. These are serious burns that may appear
white, red, or black; they require medical attention and will heal slowly without it. A fourth-degree burn is even more
severe, affecting the underlying muscle and bone. Oddly, third and fourth-degree burns are usually not as painful because
the nerve endings themselves are damaged. Full-thickness burns cannot be repaired by the body, because the local tissues
used for repair are damaged and require excision (debridement), or amputation in severe cases, followed by grafting of the
skin from an unaffected part of the body, or from skin grown in tissue culture for grafting purposes.


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Figure 5.23 Calculating the Size of a Burn The size of a burn will guide decisions made about the need for
specialized treatment. Specific parts of the body are associated with a percentage of body area.


Skin grafts are required when the damage from trauma or infection cannot be closed with sutures or staples. Watch this
video (http://openstaxcollege.org/l/skingraft) to learn more about skin grafting procedures.


Scars and Keloids
Most cuts or wounds, with the exception of ones that only scratch the surface (the epidermis), lead to scar formation. A scar
is collagen-rich skin formed after the process of wound healing that differs from normal skin. Scarring occurs in cases in
which there is repair of skin damage, but the skin fails to regenerate the original skin structure. Fibroblasts generate scar
tissue in the form of collagen, and the bulk of repair is due to the basket-weave pattern generated by collagen fibers and
does not result in regeneration of the typical cellular structure of skin. Instead, the tissue is fibrous in nature and does not
allow for the regeneration of accessory structures, such as hair follicles, sweat glands, or sebaceous glands.
Sometimes, there is an overproduction of scar tissue, because the process of collagen formation does not stop when the
wound is healed; this results in the formation of a raised or hypertrophic scar called a keloid. In contrast, scars that result
from acne and chickenpox have a sunken appearance and are called atrophic scars.
Scarring of skin after wound healing is a natural process and does not need to be treated further. Application of mineral oil
and lotions may reduce the formation of scar tissue. However, modern cosmetic procedures, such as dermabrasion, laser


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treatments, and filler injections have been invented as remedies for severe scarring. All of these procedures try to reorganize
the structure of the epidermis and underlying collagen tissue to make it look more natural.
Bedsores and Stretch Marks
Skin and its underlying tissue can be affected by excessive pressure. One example of this is called a bedsore. Bedsores,
also called decubitis ulcers, are caused by constant, long-term, unrelieved pressure on certain body parts that are bony,
reducing blood flow to the area and leading to necrosis (tissue death). Bedsores are most common in elderly patients who
have debilitating conditions that cause them to be immobile. Most hospitals and long-term care facilities have the practice of
turning the patients every few hours to prevent the incidence of bedsores. If left untreated by removal of necrotized tissue,
bedsores can be fatal if they become infected.
The skin can also be affected by pressure associated with rapid growth. A stretch mark results when the dermis is stretched
beyond its limits of elasticity, as the skin stretches to accommodate the excess pressure. Stretch marks usually accompany
rapid weight gain during puberty and pregnancy. They initially have a reddish hue, but lighten over time. Other than for
cosmetic reasons, treatment of stretch marks is not required. They occur most commonly over the hips and abdomen.
Calluses
When you wear shoes that do not fit well and are a constant source of abrasion on your toes, you tend to form a callus at the
point of contact. This occurs because the basal stem cells in the stratum basale are triggered to divide more often to increase
the thickness of the skin at the point of abrasion to protect the rest of the body from further damage. This is an example of a
minor or local injury, and the skin manages to react and treat the problem independent of the rest of the body. Calluses can
also form on your fingers if they are subject to constant mechanical stress, such as long periods of writing, playing string
instruments, or video games. A corn is a specialized form of callus. Corns form from abrasions on the skin that result from
an elliptical-type motion.


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acne
albinism
anagen
apocrine sweat gland
arrector pili


basal cell


basal cell carcinoma
bedsore


callus
catagen
corn
cortex


cuticle


dermal papilla


dermis


desmosome
eccrine sweat gland


eczema
elastin fibers
eleiden


epidermis
eponychium
external root sheath
first-degree burn
fourth-degree burn
glassy membrane
hair
hair bulb


KEY TERMS
skin condition due to infected sebaceous glands
genetic disorder that affects the skin, in which there is no melanin production
active phase of the hair growth cycle


type of sweat gland that is associated with hair follicles in the armpits and genital regions
smooth muscle that is activated in response to external stimuli that pull on hair follicles and make the hair


“stand up”
type of stem cell found in the stratum basale and in the hair matrix that continually undergoes cell division,


producing the keratinocytes of the epidermis
cancer that originates from basal cells in the epidermis of the skin


sore on the skin that develops when regions of the body start necrotizing due to constant pressure and lack of
blood supply; also called decubitis ulcers
thickened area of skin that arises due to constant abrasion
transitional phase marking the end of the anagen phase of the hair growth cycle


type of callus that is named for its shape and the elliptical motion of the abrasive force
in hair, the second or middle layer of keratinocytes originating from the hair matrix, as seen in a cross-section of


the hair bulb
in hair, the outermost layer of keratinocytes originating from the hair matrix, as seen in a cross-section of the hair


bulb
(plural = dermal papillae) extension of the papillary layer of the dermis that increases surface contact


between the epidermis and dermis
layer of skin between the epidermis and hypodermis, composed mainly of connective tissue and containing


blood vessels, hair follicles, sweat glands, and other structures
structure that forms an impermeable junction between cells


type of sweat gland that is common throughout the skin surface; it produces a hypotonic sweat
for thermoregulation
skin condition due to an allergic reaction, which resembles a rash


fibers made of the protein elastin that increase the elasticity of the dermis
clear protein-bound lipid found in the stratum lucidum that is derived from keratohyalin and helps to prevent


water loss
outermost tissue layer of the skin
nail fold that meets the proximal end of the nail body, also called the cuticle


outer layer of the hair follicle that is an extension of the epidermis, which encloses the hair root
superficial burn that injures only the epidermis
burn in which full thickness of the skin and underlying muscle and bone is damaged
layer of connective tissue that surrounds the base of the hair follicle, connecting it to the dermis


keratinous filament growing out of the epidermis
structure at the base of the hair root that surrounds the dermal papilla


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hair follicle
hair matrix
hair papilla
hair root
hair shaft
hypodermis
hyponychium
integumentary system
internal root sheath
keloid
keratin
keratinocyte
keratohyalin
Langerhans cell
lunula
medulla
Meissner corpuscle
melanin
melanocyte
melanoma
melanosome
Merkel cell
metastasis
nail bed
nail body
nail cuticle
nail fold
nail root
Pacinian corpuscle
papillary layer
reticular layer


rickets
scar


cavity or sac from which hair originates
layer of basal cells from which a strand of hair grows
mass of connective tissue, blood capillaries, and nerve endings at the base of the hair follicle


part of hair that is below the epidermis anchored to the follicle
part of hair that is above the epidermis but is not anchored to the follicle
connective tissue connecting the integument to the underlying bone and muscle
thickened layer of stratum corneum that lies below the free edge of the nail


skin and its accessory structures
innermost layer of keratinocytes in the hair follicle that surround the hair root up to the hair shaft


type of scar that has layers raised above the skin surface
type of structural protein that gives skin, hair, and nails its hard, water-resistant properties


cell that produces keratin and is the most predominant type of cell found in the epidermis
granulated protein found in the stratum granulosum
specialized dendritic cell found in the stratum spinosum that functions as a macrophage


basal part of the nail body that consists of a crescent-shaped layer of thick epithelium
in hair, the innermost layer of keratinocytes originating from the hair matrix


(also, tactile corpuscle) receptor in the skin that responds to light touch
pigment that determines the color of hair and skin
cell found in the stratum basale of the epidermis that produces the pigment melanin
type of skin cancer that originates from the melanocytes of the skin
intercellular vesicle that transfers melanin from melanocytes into keratinocytes of the epidermis


receptor cell in the stratum basale of the epidermis that responds to the sense of touch
spread of cancer cells from a source to other parts of the body


layer of epidermis upon which the nail body forms
main keratinous plate that forms the nail
fold of epithelium that extends over the nail bed, also called the eponychium


fold of epithelium at that extend over the sides of the nail body, holding it in place
part of the nail that is lodged deep in the epidermis from which the nail grows


(also, lamellated corpuscle) receptor in the skin that responds to vibration
superficial layer of the dermis, made of loose, areolar connective tissue
deeper layer of the dermis; it has a reticulated appearance due to the presence of abundant collagen and


elastin fibers
disease in children caused by vitamin D deficiency, which leads to the weakening of bones


collagen-rich skin formed after the process of wound healing that is different from normal skin


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sebaceous gland


sebum
second-degree burn
squamous cell carcinoma
stratum basale
stratum corneum
stratum granulosum
stratum lucidum


stratum spinosum


stretch mark


sudoriferous gland
telogen


third-degree burn
vitamin D
vitiligo


type of oil gland found in the dermis all over the body and helps to lubricate and waterproof the skin
and hair by secreting sebum
oily substance that is composed of a mixture of lipids that lubricates the skin and hair


partial-thickness burn that injures the epidermis and a portion of the dermis
type of skin cancer that originates from the stratum spinosum of the epidermis


deepest layer of the epidermis, made of epidermal stem cells
most superficial layer of the epidermis
layer of the epidermis superficial to the stratum spinosum


layer of the epidermis between the stratum granulosum and stratum corneum, found only in thick
skin covering the palms, soles of the feet, and digits


layer of the epidermis superficial to the stratum basale, characterized by the presence of
desmosomes


mark formed on the skin due to a sudden growth spurt and expansion of the dermis beyond its elastic
limits


sweat gland
resting phase of the hair growth cycle initiated with catagen and terminated by the beginning of a new anagen


phase of hair growth
burn that penetrates and destroys the full thickness of the skin (epidermis and dermis)


compound that aids absorption of calcium and phosphates in the intestine to improve bone health
skin condition in which melanocytes in certain areas lose the ability to produce melanin, possibly due an


autoimmune reaction that leads to loss of color in patches


CHAPTER REVIEW
5.1 Layers of the Skin
The skin is composed of two major layers: a superficial epidermis and a deeper dermis. The epidermis consists of several
layers beginning with the innermost (deepest) stratum basale (germinatum), followed by the stratum spinosum, stratum
granulosum, stratum lucidum (when present), and ending with the outermost layer, the stratum corneum. The topmost layer,
the stratum corneum, consists of dead cells that shed periodically and is progressively replaced by cells formed from the
basal layer. The stratum basale also contains melanocytes, cells that produce melanin, the pigment primarily responsible for
giving skin its color. Melanin is transferred to keratinocytes in the stratum spinosum to protect cells from UV rays.
The dermis connects the epidermis to the hypodermis, and provides strength and elasticity due to the presence of collagen
and elastin fibers. It has only two layers: the papillary layer with papillae that extend into the epidermis and the lower,
reticular layer composed of loose connective tissue. The hypodermis, deep to the dermis of skin, is the connective tissue
that connects the dermis to underlying structures; it also harbors adipose tissue for fat storage and protection.


5.2 Accessory Structures of the Skin
Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. Hair is made of dead keratinized
cells, and gets its color from melanin pigments. Nails, also made of dead keratinized cells, protect the extremities of our
fingers and toes from mechanical damage. Sweat glands and sebaceous glands produce sweat and sebum, respectively. Each
of these fluids has a role to play in maintaining homeostasis. Sweat cools the body surface when it gets overheated and helps
excrete small amounts of metabolic waste. Sebum acts as a natural moisturizer and keeps the dead, flaky, outer keratin layer
healthy.


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5.3 Functions of the Integumentary System
The skin plays important roles in protection, sensing stimuli, thermoregulation, and vitamin D synthesis. It is the first layer
of defense to prevent dehydration, infection, and injury to the rest of the body. Sweat glands in the skin allow the skin
surface to cool when the body gets overheated. Thermoregulation is also accomplished by the dilation or constriction of
heat-carrying blood vessels in the skin. Immune cells present among the skin layers patrol the areas to keep them free of
foreign materials. Fat stores in the hypodermis aid in both thermoregulation and protection. Finally, the skin plays a role in
the synthesis of vitamin D, which is necessary for our well-being but not easily available in natural foods.


5.4 Diseases, Disorders, and Injuries of the Integumentary System
Skin cancer is a result of damage to the DNA of skin cells, often due to excessive exposure to UV radiation. Basal cell
carcinoma and squamous cell carcinoma are highly curable, and arise from cells in the stratum basale and stratum spinosum,
respectively. Melanoma is the most dangerous form of skin cancer, affecting melanocytes, which can spread/metastasize
to other organs. Burns are an injury to the skin that occur as a result of exposure to extreme heat, radiation, or chemicals.
First-degree and second-degree burns usually heal quickly, but third-degree burns can be fatal because they penetrate the
full thickness of the skin. Scars occur when there is repair of skin damage. Fibroblasts generate scar tissue in the form of
collagen, which forms a basket-weave pattern that looks different from normal skin.
Bedsores and stretch marks are the result of excessive pressure on the skin and underlying tissue. Bedsores are characterized
by necrosis of tissue due to immobility, whereas stretch marks result from rapid growth. Eczema is an allergic reaction that
manifests as a rash, and acne results from clogged sebaceous glands. Eczema and acne are usually long-term skin conditions
that may be treated successfully in mild cases. Calluses and corns are the result of abrasive pressure on the skin.


INTERACTIVE LINK QUESTIONS
1. The skin consists of two layers and a closely associated
layer. View this animation (http://openstaxcollege.org/l/
layers) to learn more about layers of the skin. What are the
basic functions of each of these layers?
2. Figure 5.4 If you zoom on the cells at the outermost
layer of this section of skin, what do you notice about the
cells?


3. Figure 5.6 If you zoom on the cells of the stratum
spinosum, what is distinctive about them?
4. This ABC video follows the story of a pair of fraternal
African-American twins, one of whom is albino. Watch this
video (http://openstaxcollege.org/l/albino) to learn about
the challenges these children and their family face. Which
ethnicities do you think are exempt from the possibility of
albinism?


REVIEW QUESTIONS
5. The papillary layer of the dermis is most closely
associated with which layer of the epidermis?


a. stratum spinosum
b. stratum corneum
c. stratum granulosum
d. stratum basale


6. Langerhans cells are commonly found in the ________.


a. stratum spinosum
b. stratum corneum
c. stratum granulosum
d. stratum basale


7. The papillary and reticular layers of the dermis are
composed mainly of ________.


a. melanocytes
b. keratinocytes
c. connective tissue
d. adipose tissue


8. Collagen lends ________ to the skin.
a. elasticity
b. structure
c. color
d. UV protection


9. Which of the following is not a function of the
hypodermis?


a. protects underlying organs
b. helps maintain body temperature
c. source of blood vessels in the epidermis
d. a site to long-term energy storage


10. In response to stimuli from the sympathetic nervous
system, the arrector pili ________.


a. are glands on the skin surface
b. can lead to excessive sweating
c. are responsible for goose bumps
d. secrete sebum


11. The hair matrix contains ________.
a. the hair follicle
b. the hair shaft
c. the glassy membrane
d. a layer of basal cells


12. Eccrine sweat glands ________.
a. are present on hair
b. are present in the skin throughout the body and
produce watery sweat


c. produce sebum
d. act as a moisturizer


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13. Sebaceous glands ________.
a. are a type of sweat gland
b. are associated with hair follicles
c. may function in response to touch
d. release a watery solution of salt and metabolic
waste


14. Similar to the hair, nails grow continuously throughout
our lives. Which of the following is furthest from the nail
growth center?


a. nail bed
b. hyponychium
c. nail root
d. eponychium


15. In humans, exposure of the skin to sunlight is required
for ________.


a. vitamin D synthesis
b. arteriole constriction
c. folate production
d. thermoregulation


16. One of the functions of the integumentary system is
protection. Which of the following does not directly
contribute to that function?


a. stratum lucidum
b. desmosomes
c. folic acid synthesis
d. Merkel cells


17. An individual using a sharp knife notices a small
amount of blood where he just cut himself. Which of the
following layers of skin did he have to cut into in order to
bleed?


a. stratum corneum
b. stratum basale
c. papillary dermis
d. stratum granulosum


18. As you are walking down the beach, you see a dead,
dry, shriveled-up fish. Which layer of your epidermis keeps
you from drying out?


a. stratum corneum
b. stratum basale
c. stratum spinosum
d. stratum granulosum


19. If you cut yourself and bacteria enter the wound, which
of the following cells would help get rid of the bacteria?


a. Merkel cells
b. keratinocytes
c. Langerhans cells
d. melanocytes


20. In general, skin cancers ________.
a. are easily treatable and not a major health
concern


b. occur due to poor hygiene
c. can be reduced by limiting exposure to the sun
d. affect only the epidermis


21. Bedsores ________.
a. can be treated with topical moisturizers
b. can result from deep massages
c. are preventable by eliminating pressure points
d. are caused by dry skin


22. An individual has spent too much time sun bathing.
Not only is his skin painful to touch, but small blisters have
appeared in the affected area. This indicates that he has
damaged which layers of his skin?


a. epidermis only
b. hypodermis only
c. epidermis and hypodermis
d. epidermis and dermis


23. After a skin injury, the body initiates a wound-healing
response. The first step of this response is the formation of
a blood clot to stop bleeding. Which of the following would
be the next response?


a. increased production of melanin by melanocytes
b. increased production of connective tissue
c. an increase in Pacinian corpuscles around the
wound


d. an increased activity in the stratum lucidum
24. Squamous cell carcinomas are the second most
common of the skin cancers and are capable of
metastasizing if not treated. This cancer affects which
cells?


a. basal cells of the stratum basale
b. melanocytes of the stratum basale
c. keratinocytes of the stratum spinosum
d. Langerhans cells of the stratum lucidum


CRITICAL THINKING QUESTIONS
25. What determines the color of skin, and what is the
process that darkens skin when it is exposed to UV light?
26. Cells of the epidermis derive from stem cells of the
stratum basale. Describe how the cells change as they
become integrated into the different layers of the epidermis.
27. Explain the differences between eccrine and apocrine
sweat glands.
28. Describe the structure and composition of nails.


29. Why do people sweat excessively when exercising
outside on a hot day?
30. Explain your skin’s response to a drop in body core
temperature.
31.Why do teenagers often experience acne?
32.Why do scars look different from surrounding skin?


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6 | BONE TISSUE AND THE
SKELETAL SYSTEM


Figure 6.1 Child Looking at Bones Bone is a living tissue. Unlike the bones of a fossil made inert by a process of
mineralization, a child’s bones will continue to grow and develop while contributing to the support and function of other
body systems. (credit: James Emery)


Introduction
Chapter Objectives


After studying this chapter, you will be able to:
• List and describe the functions of bones
• Describe the classes of bones
• Discuss the process of bone formation and development
• Explain how bone repairs itself after a fracture
• Discuss the effect of exercise, nutrition, and hormones on bone tissue
• Describe how an imbalance of calcium can affect bone tissue


Bones make good fossils. While the soft tissue of a once living organism will decay and fall away over time, bone tissue
will, under the right conditions, undergo a process of mineralization, effectively turning the bone to stone. A well-preserved


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 203




fossil skeleton can give us a good sense of the size and shape of an organism, just as your skeleton helps to define your size
and shape. Unlike a fossil skeleton, however, your skeleton is a structure of living tissue that grows, repairs, and renews
itself. The bones within it are dynamic and complex organs that serve a number of important functions, including some
necessary to maintain homeostasis.


6.1 | The Functions of the Skeletal System
By the end of this section, you will be able to:
• Define bone, cartilage, and the skeletal system
• List and describe the functions of the skeletal system


Bone, or osseous tissue, is a hard, dense connective tissue that forms most of the adult skeleton, the support structure
of the body. In the areas of the skeleton where bones move (for example, the ribcage and joints), cartilage, a semi-rigid
form of connective tissue, provides flexibility and smooth surfaces for movement. The skeletal system is the body system
composed of bones and cartilage and performs the following critical functions for the human body:
• supports the body
• facilitates movement
• protects internal organs
• produces blood cells
• stores and releases minerals and fat
Support, Movement, and Protection
The most apparent functions of the skeletal system are the gross functions—those visible by observation. Simply by looking
at a person, you can see how the bones support, facilitate movement, and protect the human body.
Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilage of your skeletal system
compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs,
muscle, and skin.
Bones also facilitate movement by serving as points of attachment for your muscles. While some bones only serve as a
support for the muscles, others also transmit the forces produced when your muscles contract. From a mechanical point of
view, bones act as levers and joints serve as fulcrums (Figure 6.2). Unless a muscle spans a joint and contracts, a bone is not
going to move. For information on the interaction of the skeletal and muscular systems, that is, the musculoskeletal system,
seek additional content.


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Figure 6.2 Bones Support Movement Bones act as levers when muscles span a joint and contract. (credit: Benjamin
J. DeLong)


Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs
and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect
your brain (Figure 6.3).


Figure 6.3 Bones Protect Brain The cranium completely surrounds and protects the brain from non-traumatic injury.


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 205




Orthopedist
An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the
musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other
devices, but others may be best treated with surgery (Figure 6.4).


Figure 6.4 Arm Brace An orthopedist will sometimes prescribe the use of a brace that reinforces the underlying
bone structure it is being used to support. (credit: Juhan Sonin)


While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the
child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even
performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus
fails to close completely during embryologic development.
Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature
of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing
it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and
thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or
indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column
injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see
in the elderly.
Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle,
and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery.


Mineral Storage, Energy Storage, and Hematopoiesis
On a metabolic level, bone tissue performs several critical functions. For one, the bone matrix acts as a reservoir for
a number of minerals important to the functioning of the body, especially calcium, and potassium. These minerals,
incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological
processes. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved
in the transmission of nerve impulses.
Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the interior of most
bone is referred to as bone marrow (Figure 6.5). There are two types of bone marrow: yellow marrow and red marrow.
Yellow marrow contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of
energy. Red marrow is where hematopoiesis—the production of blood cells—takes place. Red blood cells, white blood
cells, and platelets are all produced in the red marrow.


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Figure 6.5 Head of Femur Showing Red and Yellow Marrow The head of the femur contains both yellow and
red marrow. Yellow marrow stores fat. Red marrow is responsible for hematopoiesis. (credit: modification of work by
“stevenfruitsmaak”/Wikimedia Commons)


6.2 | Bone Classification
By the end of this section, you will be able to:
• Classify bones according to their shapes
• Describe the function of each category of bones


The 206 bones that compose the adult skeleton are divided into five categories based on their shapes (Figure 6.6). Their
shapes and their functions are related such that each categorical shape of bone has a distinct function.


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 207




Figure 6.6 Classifications of Bones Bones are classified according to their shape.


Long Bones
A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes
the shape of a bone, not its size. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as
well as in the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges). Long bones function as levers; they move
when muscles contract.


Short Bones
A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short
bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and
support as well as some limited motion.


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Flat Bones
The term “ flat bone” is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved.
Examples include the cranial (skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones
serve as points of attachment for muscles and often protect internal organs.


Irregular Bones
An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other
classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it
from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones.


Sesamoid Bones
A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form
in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint.
The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and
placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae
(singular = patella) are the only sesamoid bones found in common with every person. Table 6.1 reviews bone classifications
with their associated features, functions, and examples.


Bone Classifications
Bone


classification Features Function(s) Examples


Long Cylinder-like shape, longerthan it is wide Leverage
Femur, tibia, fibula, metatarsals,
humerus, ulna, radius,
metacarpals, phalanges


Short
Cube-like shape,
approximately equal in
length, width, and thickness


Provide stability, support,
while allowing for some
motion


Carpals, tarsals


Flat Thin and curved
Points of attachment for
muscles; protectors of
internal organs


Sternum, ribs, scapulae, cranial
bones


Irregular Complex shape Protect internal organs Vertebrae, facial bones


Sesamoid Small and round; embeddedin tendons
Protect tendons from
compressive forces Patellae


Table 6.1


6.3 | Bone Structure
By the end of this section, you will be able to:
• Identify the anatomical features of a bone
• Define and list examples of bone markings
• Describe the histology of bone tissue
• Compare and contrast compact and spongy bone
• Identify the structures that compose compact and spongy bone
• Describe how bones are nourished and innervated


Bone tissue (osseous tissue) differs greatly from other tissues in the body. Bone is hard and many of its functions depend on
that characteristic hardness. Later discussions in this chapter will show that bone is also dynamic in that its shape adjusts to
accommodate stresses. This section will examine the gross anatomy of bone first and then move on to its histology.


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 209




Gross Anatomy of Bone
The structure of a long bone allows for the best visualization of all of the parts of a bone (Figure 6.7). A long bone has two
parts: the diaphysis and the epiphysis. The diaphysis is the tubular shaft that runs between the proximal and distal ends of
the bone. The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow marrow. The walls
of the diaphysis are composed of dense and hard compact bone.


Figure 6.7 Anatomy of a Long Bone A typical long bone shows the gross anatomical characteristics of bone.


The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone.
Red marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that
contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) cartilage in a growing bone. When the bone
stops growing in early adulthood (approximately 18–21 years), the cartilage is replaced by osseous tissue and the epiphyseal
plate becomes an epiphyseal line.
The medullary cavity has a delicate membranous lining called the endosteum (end- = “inside”; oste- = “bone”), where
bone growth, repair, and remodeling occur. The outer surface of the bone is covered with a fibrous membrane called the
periosteum (peri- = “around” or “surrounding”). The periosteum contains blood vessels, nerves, and lymphatic vessels that
nourish compact bone. Tendons and ligaments also attach to bones at the periosteum. The periosteum covers the entire outer
surface except where the epiphyses meet other bones to form joints (Figure 6.8). In this region, the epiphyses are covered
with articular cartilage, a thin layer of cartilage that reduces friction and acts as a shock absorber.


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Figure 6.8 Periosteum and Endosteum The periosteum forms the outer surface of bone, and the endosteum lines
the medullary cavity.


Flat bones, like those of the cranium, consist of a layer of diploë (spongy bone), lined on either side by a layer of compact
bone (Figure 6.9). The two layers of compact bone and the interior spongy bone work together to protect the internal organs.
If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer.


Figure 6.9 Anatomy of a Flat Bone This cross-section of a flat bone shows the spongy bone (diploë) lined on either
side by a layer of compact bone.


Bone Markings
The surface features of bones vary considerably, depending on the function and location in the body. Table 6.2 describes the
bone markings, which are illustrated in (Figure 6.10). There are three general classes of bone markings: (1) articulations,
(2) projections, and (3) holes. As the name implies, an articulation is where two bone surfaces come together (articulus =
“joint”). These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the
function of the articulation. A projection is an area of a bone that projects above the surface of the bone. These are the
attachment points for tendons and ligaments. In general, their size and shape is an indication of the forces exerted through
the attachment to the bone. A hole is an opening or groove in the bone that allows blood vessels and nerves to enter the
bone. As with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at
these points.


Bone Markings
Marking Description Example


Articulations Where two bones meet Knee joint
Table 6.2


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Bone Markings
Marking Description Example


Head Prominent rounded surface Head of femur
Facet Flat surface Vertebrae
Condyle Rounded surface Occipital condyles
Projections Raised markings Spinous process of the vertebrae
Protuberance Protruding Chin
Process Prominence feature Transverse process of vertebra
Spine Sharp process Ischial spine
Tubercle Small, rounded process Tubercle of humerus
Tuberosity Rough surface Deltoid tuberosity
Line Slight, elongated ridge Temporal lines of the parietal bones
Crest Ridge Iliac crest
Holes Holes and depressions Foramen (holes through which blood vessels can pass through)
Fossa Elongated basin Mandibular fossa
Fovea Small pit Fovea capitis on the head of the femur
Sulcus Groove Sigmoid sulcus of the temporal bones
Canal Passage in bone Auditory canal
Fissure Slit through bone Auricular fissure
Foramen Hole through bone Foramen magnum in the occipital bone
Meatus Opening into canal External auditory meatus
Sinus Air-filled space in bone Nasal sinus
Table 6.2


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Figure 6.10 Bone Features The surface features of bones depend on their function, location, attachment of
ligaments and tendons, or the penetration of blood vessels and nerves.


Bone Cells and Tissue
Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide a surface for
inorganic salt crystals to adhere. These salt crystals form when calcium phosphate and calcium carbonate combine to create
hydroxyapatite, which incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes,
or calcifies, on the collagen fibers. The hydroxyapatite crystals give bones their hardness and strength, while the collagen
fibers give them flexibility so that they are not brittle.
Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of
cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts (Figure 6.11).


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 213




Figure 6.11 Bone Cells Four types of cells are found within bone tissue. Osteogenic cells are undifferentiated
and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function
changes, and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in
appearance from other bone cells.


The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the
periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts.
As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in
structure and becomes an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte
is located in a space called a lacuna and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of
the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each
other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular = canaliculus), channels
within the bone matrix.
If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in
the properties of a third category of bone cells—the osteogenic cell. These osteogenic cells are undifferentiated with high
mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the
periosteum and the marrow. They differentiate and develop into osteoblasts.
The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved
for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast. They are found
on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not
from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new
bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone.
Table 6.3 reviews the bone cells, their functions, and locations.


Bone Cells
Cell type Function Location


Osteogenic
cells Develop into osteoblasts Deep layers of the periosteum and the marrow


Osteoblasts Bone formation Growing portions of bone, including periosteum andendosteum


Osteocytes Maintain mineral concentration ofmatrix Entrapped in matrix


Osteoclasts Bone resorption Bone surfaces and at sites of old, injured, or unneededbone
Table 6.3


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Compact and Spongy Bone
The differences between compact and spongy bone are best explored via their histology. Most bones contain compact and
spongy osseous tissue, but their distribution and concentration vary based on the bone’s overall function. Compact bone is
dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in
weight distribution.
Compact Bone
Compact bone is the denser, stronger of the two types of bone tissue (Figure 6.12). It can be found under the periosteum
and in the diaphyses of long bones, where it provides support and protection.


Figure 6.12 Diagram of Compact Bone (a) This cross-sectional view of compact bone shows the basic structural
unit, the osteon. (b) In this micrograph of the osteon, you can clearly see the concentric lamellae and central canals.
LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)


The microscopic structural unit of compact bone is called an osteon, or Haversian system. Each osteon is composed of
concentric rings of calcified matrix called lamellae (singular = lamella). Running down the center of each osteon is the
central canal, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves
branch off at right angles through a perforating canal, also known as Volkmann’s canals, to extend to the periosteum and
endosteum.
The osteocytes are located inside spaces called lacunae (singular = lacuna), found at the borders of adjacent lamellae. As
described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system
allows nutrients to be transported to the osteocytes and wastes to be removed from them.


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Spongy (Cancellous) Bone
Like compact bone, spongy bone, also known as cancellous bone, contains osteocytes housed in lacunae, but they are not
arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called
trabeculae (singular = trabecula) (Figure 6.13). The trabeculae may appear to be a random network, but each trabecula
forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the
dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces
in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs.


Figure 6.13 Diagram of Spongy Bone Spongy bone is composed of trabeculae that contain the osteocytes. Red
marrow fills the spaces in some bones.


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Skeletal System: Paget’s Disease
Paget’s disease usually occurs in adults over age 40. It is a disorder of the bone remodeling process that begins with
overactive osteoclasts. This means more bone is resorbed than is laid down. The osteoblasts try to compensate but the
new bone they lay down is weak and brittle and therefore prone to fracture.
While some people with Paget’s disease have no symptoms, others experience pain, bone fractures, and bone
deformities (Figure 6.14). Bones of the pelvis, skull, spine, and legs are the most commonly affected. When occurring
in the skull, Paget’s disease can cause headaches and hearing loss.


Figure 6.14 Paget's Disease Normal leg bones are relatively straight, but those affected by Paget’s disease are
porous and curved.


What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a
role. Some scientists believe Paget’s disease is due to an as-yet-unidentified virus.
Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone
resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body.
Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In
addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease.
Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget’s disease.
However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures
because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most
doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh
the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or
fractures, which in turn reduces the risk of surgical repair and its associated risks and complications.


Blood and Nerve Supply
The spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries
enter through the nutrient foramen (plural = foramina), small openings in the diaphysis (Figure 6.15). The osteocytes in
spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the
marrow cavities. As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone
through the foramina.


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In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more
metabolically active regions of the bone. The nerves sense pain, and it appears the nerves also play roles in regulating blood
supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.


Figure 6.15 Diagram of Blood and Nerve Supply to Bone Blood vessels and nerves enter the bone through the
nutrient foramen.


Watch this video (http://openstaxcollege.org/l/microbone) to see the microscopic features of a bone.


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6.4 | Bone Formation and Development
By the end of this section, you will be able to:
• Explain the function of cartilage
• List the steps of intramembranous ossification
• List the steps of endochondral ossification
• Explain the growth activity at the epiphyseal plate
• Compare and contrast the processes of modeling and remodeling


In the early stages of embryonic development, the embryo’s skeleton consists of fibrous membranes and hyaline cartilage.
By the sixth or seventh week of embryonic life, the actual process of bone development, ossification (osteogenesis), begins.
There are two osteogenic pathways—intramembranous ossification and endochondral ossification—but bone is the same
regardless of the pathway that produces it.


Cartilage Templates
Bone is a replacement tissue; that is, it uses a model tissue on which to lay down its mineral matrix. For skeletal
development, the most common template is cartilage. During fetal development, a framework is laid down that determines
where bones will form. This framework is a flexible, semi-solid matrix produced by chondroblasts and consists of
hyaluronic acid, chondroitin sulfate, collagen fibers, and water. As the matrix surrounds and isolates chondroblasts, they are
called chondrocytes. Unlike most connective tissues, cartilage is avascular, meaning that it has no blood vessels supplying
nutrients and removing metabolic wastes. All of these functions are carried on by diffusion through the matrix. This is why
damaged cartilage does not repair itself as readily as most tissues do.
Throughout fetal development and into childhood growth and development, bone forms on the cartilaginous matrix. By the
time a fetus is born, most of the cartilage has been replaced with bone. Some additional cartilage will be replaced throughout
childhood, and some cartilage remains in the adult skeleton.


Intramembranous Ossification
During intramembranous ossification, compact and spongy bone develops directly from sheets of mesenchymal
(undifferentiated) connective tissue. The flat bones of the face, most of the cranial bones, and the clavicles (collarbones) are
formed via intramembranous ossification.
The process begins when mesenchymal cells in the embryonic skeleton gather together and begin to differentiate into
specialized cells (Figure 6.16a). Some of these cells will differentiate into capillaries, while others will become osteogenic
cells and then osteoblasts. Although they will ultimately be spread out by the formation of bone tissue, early osteoblasts
appear in a cluster called an ossification center.
The osteoblasts secrete osteoid, uncalcified matrix, which calcifies (hardens) within a few days as mineral salts are
deposited on it, thereby entrapping the osteoblasts within. Once entrapped, the osteoblasts become osteocytes (Figure
6.16b). As osteoblasts transform into osteocytes, osteogenic cells in the surrounding connective tissue differentiate into new
osteoblasts.
Osteoid (unmineralized bone matrix) secreted around the capillaries results in a trabecular matrix, while osteoblasts on
the surface of the spongy bone become the periosteum (Figure 6.16c). The periosteum then creates a protective layer
of compact bone superficial to the trabecular bone. The trabecular bone crowds nearby blood vessels, which eventually
condense into red marrow (Figure 6.16d).


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Figure 6.16 Intramembranous Ossification Intramembranous ossification follows four steps. (a) Mesenchymal
cells group into clusters, and ossification centers form. (b) Secreted osteoid traps osteoblasts, which then become
osteocytes. (c) Trabecular matrix and periosteum form. (d) Compact bone develops superficial to the trabecular bone,
and crowded blood vessels condense into red marrow.


Intramembranous ossification begins in utero during fetal development and continues on into adolescence. At birth, the
skull and clavicles are not fully ossified nor are the sutures of the skull closed. This allows the skull and shoulders to deform
during passage through the birth canal. The last bones to ossify via intramembranous ossification are the flat bones of the
face, which reach their adult size at the end of the adolescent growth spurt.


Endochondral Ossification
In endochondral ossification, bone develops by replacing hyaline cartilage. Cartilage does not become bone. Instead,
cartilage serves as a template to be completely replaced by new bone. Endochondral ossification takes much longer than
intramembranous ossification. Bones at the base of the skull and long bones form via endochondral ossification.
In a long bone, for example, at about 6 to 8 weeks after conception, some of the mesenchymal cells differentiate into
chondrocytes (cartilage cells) that form the cartilaginous skeletal precursor of the bones (Figure 6.17a). Soon after, the
perichondrium, a membrane that covers the cartilage, appears Figure 6.17b).


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Figure 6.17 Endochondral Ossification Endochondral ossification follows five steps. (a) Mesenchymal cells
differentiate into chondrocytes. (b) The cartilage model of the future bony skeleton and the perichondrium form.
(c) Capillaries penetrate cartilage. Perichondrium transforms into periosteum. Periosteal collar develops. Primary
ossification center develops. (d) Cartilage and chondrocytes continue to grow at ends of the bone. (e) Secondary
ossification centers develop. (f) Cartilage remains at epiphyseal (growth) plate and at joint surface as articular cartilage.


As more matrix is produced, the chondrocytes in the center of the cartilaginous model grow in size. As the matrix calcifies,
nutrients can no longer reach the chondrocytes. This results in their death and the disintegration of the surrounding cartilage.
Blood vessels invade the resulting spaces, not only enlarging the cavities but also carrying osteogenic cells with them, many
of which will become osteoblasts. These enlarging spaces eventually combine to become the medullary cavity.


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As the cartilage grows, capillaries penetrate it. This penetration initiates the transformation of the perichondrium into the
bone-producing periosteum. Here, the osteoblasts form a periosteal collar of compact bone around the cartilage of the
diaphysis. By the second or third month of fetal life, bone cell development and ossification ramps up and creates the
primary ossification center, a region deep in the periosteal collar where ossification begins (Figure 6.17c).
While these deep changes are occurring, chondrocytes and cartilage continue to grow at the ends of the bone (the future
epiphyses), which increases the bone’s length at the same time bone is replacing cartilage in the diaphyses. By the time the
fetal skeleton is fully formed, cartilage only remains at the joint surface as articular cartilage and between the diaphysis and
epiphysis as the epiphyseal plate, the latter of which is responsible for the longitudinal growth of bones. After birth, this
same sequence of events (matrix mineralization, death of chondrocytes, invasion of blood vessels from the periosteum, and
seeding with osteogenic cells that become osteoblasts) occurs in the epiphyseal regions, and each of these centers of activity
is referred to as a secondary ossification center (Figure 6.17e).


How Bones Grow in Length
The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in
immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is
ossified, and the diaphysis grows in length. The epiphyseal plate is composed of four zones of cells and activity (Figure
6.18). The reserve zone is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the
matrix. These chondrocytes do not participate in bone growth but secure the epiphyseal plate to the osseous tissue of the
epiphysis.


Figure 6.18 Longitudinal Bone Growth The epiphyseal plate is responsible for longitudinal bone growth.


The proliferative zone is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes. It makes
new chondrocytes (via mitosis) to replace those that die at the diaphyseal end of the plate. Chondrocytes in the next layer,
the zone of maturation and hypertrophy, are older and larger than those in the proliferative zone. The more mature cells


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are situated closer to the diaphyseal end of the plate. The longitudinal growth of bone is a result of cellular division in the
proliferative zone and the maturation of cells in the zone of maturation and hypertrophy.
Most of the chondrocytes in the zone of calcified matrix, the zone closest to the diaphysis, are dead because the matrix
around them has calcified. Capillaries and osteoblasts from the diaphysis penetrate this zone, and the osteoblasts secrete
bone tissue on the remaining calcified cartilage. Thus, the zone of calcified matrix connects the epiphyseal plate to the
diaphysis. A bone grows in length when osseous tissue is added to the diaphysis.
Bones continue to grow in length until early adulthood. The rate of growth is controlled by hormones, which will be
discussed later. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage,
longitudinal growth stops. All that remains of the epiphyseal plate is the epiphyseal line (Figure 6.19).


Figure 6.19 Progression from Epiphyseal Plate to Epiphyseal Line As a bone matures, the epiphyseal plate
progresses to an epiphyseal line. (a) Epiphyseal plates are visible in a growing bone. (b) Epiphyseal lines are the
remnants of epiphyseal plates in a mature bone.


How Bones Grow in Diameter
While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after
longitudinal growth ceases. This is called appositional growth. Osteoclasts resorb old bone that lines the medullary cavity,
while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum. The erosion of old
bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of
the diaphysis but also increase the diameter of the medullary cavity. This process is called modeling.


Bone Remodeling
The process in which matrix is resorbed on one surface of a bone and deposited on another is known as bone modeling.
Modeling primarily takes place during a bone’s growth. However, in adult life, bone undergoes remodeling, in which
resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which
is resorbed. Injury, exercise, and other activities lead to remodeling. Those influences are discussed later in the chapter, but
even without injury or exercise, about 5 to 10 percent of the skeleton is remodeled annually just by destroying old bone and
renewing it with fresh bone.


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Skeletal System
Osteogenesis imperfecta (OI) is a genetic disease in which bones do not form properly and therefore are fragile and
break easily. It is also called brittle bone disease. The disease is present from birth and affects a person throughout life.
The genetic mutation that causes OI affects the body’s production of collagen, one of the critical components of bone
matrix. The severity of the disease can range from mild to severe. Those with the most severe forms of the disease
sustain many more fractures than those with a mild form. Frequent and multiple fractures typically lead to bone
deformities and short stature. Bowing of the long bones and curvature of the spine are also common in people afflicted
with OI. Curvature of the spine makes breathing difficult because the lungs are compressed.
Because collagen is such an important structural protein in many parts of the body, people with OI may also experience
fragile skin, weak muscles, loose joints, easy bruising, frequent nosebleeds, brittle teeth, blue sclera, and hearing loss.
There is no known cure for OI. Treatment focuses on helping the person retain as much independence as possible while
minimizing fractures and maximizing mobility. Toward that end, safe exercises, like swimming, in which the body is
less likely to experience collisions or compressive forces, are recommended. Braces to support legs, ankles, knees, and
wrists are used as needed. Canes, walkers, or wheelchairs can also help compensate for weaknesses.
When bones do break, casts, splints, or wraps are used. In some cases, metal rods may be surgically implanted into the
long bones of the arms and legs. Research is currently being conducted on using bisphosphonates to treat OI. Smoking
and being overweight are especially risky in people with OI, since smoking is known to weaken bones, and extra body
weight puts additional stress on the bones.


Watch this video (http://openstaxcollege.org/l/bonegrows) to see how a bone grows.


6.5 | Fractures: Bone Repair
By the end of this section, you will be able to:
• Differentiate among the different types of fractures
• Describe the steps involved in bone repair


A fracture is a broken bone. It will heal whether or not a physician resets it in its anatomical position. If the bone is not
reset correctly, the healing process will keep the bone in its deformed position.
When a broken bone is manipulated and set into its natural position without surgery, the procedure is called a closed
reduction. Open reduction requires surgery to expose the fracture and reset the bone. While some fractures can be minor,
others are quite severe and result in grave complications. For example, a fractured diaphysis of the femur has the potential to
release fat globules into the bloodstream. These can become lodged in the capillary beds of the lungs, leading to respiratory
distress and if not treated quickly, death.


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Types of Fractures
Fractures are classified by their complexity, location, and other features (Figure 6.20). Table 6.4 outlines common types of
fractures. Some fractures may be described using more than one term because it may have the features of more than one
type (e.g., an open transverse fracture).


Figure 6.20 Types of Fractures Compare healthy bone with different types of fractures: (a) closed fracture, (b)
open fracture, (c) transverse fracture, (d) spiral fracture, (e) comminuted fracture, (f) impacted fracture, (g) greenstick
fracture, and (h) oblique fracture.


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Types of Fractures
Type of
fracture Description


Transverse Occurs straight across the long axis of the bone
Oblique Occurs at an angle that is not 90 degrees
Spiral Bone segments are pulled apart as a result of a twisting motion
Comminuted Several breaks result in many small pieces between two large segments
Impacted One fragment is driven into the other, usually as a result of compression
Greenstick A partial fracture in which only one side of the bone is broken
Open (or
compound)


A fracture in which at least one end of the broken bone tears through the skin; carries a
high risk of infection


Closed (or
simple) A fracture in which the skin remains intact


Table 6.4


Bone Repair
When a bone breaks, blood flows from any vessel torn by the fracture. These vessels could be in the periosteum, osteons,
and/or medullary cavity. The blood begins to clot, and about six to eight hours after the fracture, the clotting blood has
formed a fracture hematoma (Figure 6.21a). The disruption of blood flow to the bone results in the death of bone cells
around the fracture.


Figure 6.21 Stages in Fracture Repair The healing of a bone fracture follows a series of progressive steps: (a) A
fracture hematoma forms. (b) Internal and external calli form. (c) Cartilage of the calli is replaced by trabecular bone.
(d) Remodeling occurs.


Within about 48 hours after the fracture, chondrocytes from the endosteum have created an internal callus (plural = calli)
by secreting a fibrocartilaginous matrix between the two ends of the broken bone, while the periosteal chondrocytes and
osteoblasts create an external callus of hyaline cartilage and bone, respectively, around the outside of the break (Figure
6.21b). This stabilizes the fracture.
Over the next several weeks, osteoclasts resorb the dead bone; osteogenic cells become active, divide, and differentiate into
osteoblasts. The cartilage in the calli is replaced by trabecular bone via endochondral ossification (Figure 6.21c).
Eventually, the internal and external calli unite, compact bone replaces spongy bone at the outer margins of the fracture, and
healing is complete. A slight swelling may remain on the outer surface of the bone, but quite often, that region undergoes
remodeling (Figure 6.21d), and no external evidence of the fracture remains.


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Visit this website (http://openstaxcollege.org/l/fracturequiz) to review different types of fractures and then take a
short self-assessment quiz.


6.6 | Exercise, Nutrition, Hormones, and Bone Tissue
By the end of this section, you will be able to:
• Describe the effect exercise has on bone tissue
• List the nutrients that affect bone health
• Discuss the role those nutrients play in bone health
• Describe the effects of hormones on bone tissue


All of the organ systems of your body are interdependent, and the skeletal system is no exception. The food you take in via
your digestive system and the hormones secreted by your endocrine system affect your bones. Even using your muscles to
engage in exercise has an impact on your bones.


Exercise and Bone Tissue
During long space missions, astronauts can lose approximately 1 to 2 percent of their bone mass per month. This loss of
bone mass is thought to be caused by the lack of mechanical stress on astronauts’ bones due to the low gravitational forces
in space. Lack of mechanical stress causes bones to lose mineral salts and collagen fibers, and thus strength. Similarly,
mechanical stress stimulates the deposition of mineral salts and collagen fibers. The internal and external structure of a bone
will change as stress increases or decreases so that the bone is an ideal size and weight for the amount of activity it endures.
That is why people who exercise regularly have thicker bones than people who are more sedentary. It is also why a broken
bone in a cast atrophies while its contralateral mate maintains its concentration of mineral salts and collagen fibers. The
bones undergo remodeling as a result of forces (or lack of forces) placed on them.
Numerous, controlled studies have demonstrated that people who exercise regularly have greater bone density than those
who are more sedentary. Any type of exercise will stimulate the deposition of more bone tissue, but resistance training has a
greater effect than cardiovascular activities. Resistance training is especially important to slow down the eventual bone loss
due to aging and for preventing osteoporosis.


Nutrition and Bone Tissue
The vitamins and minerals contained in all of the food we consume are important for all of our organ systems. However,
there are certain nutrients that affect bone health.
Calcium and Vitamin D
You already know that calcium is a critical component of bone, especially in the form of calcium phosphate and calcium
carbonate. Since the body cannot make calcium, it must be obtained from the diet. However, calcium cannot be absorbed
from the small intestine without vitamin D. Therefore, intake of vitamin D is also critical to bone health. In addition to
vitamin D’s role in calcium absorption, it also plays a role, though not as clearly understood, in bone remodeling.
Milk and other dairy foods are not the only sources of calcium. This important nutrient is also found in green leafy
vegetables, broccoli, and intact salmon and canned sardines with their soft bones. Nuts, beans, seeds, and shellfish provide
calcium in smaller quantities.
Except for fatty fish like salmon and tuna, or fortified milk or cereal, vitamin D is not found naturally in many foods. The
action of sunlight on the skin triggers the body to produce its own vitamin D (Figure 6.22), but many people, especially


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those of darker complexion and those living in northern latitudes where the sun’s rays are not as strong, are deficient in
vitamin D. In cases of deficiency, a doctor can prescribe a vitamin D supplement.


Figure 6.22 Synthesis of Vitamin D Sunlight is one source of vitamin D.


Other Nutrients
Vitamin K also supports bone mineralization and may have a synergistic role with vitamin D in the regulation of bone
growth. Green leafy vegetables are a good source of vitamin K.
The minerals magnesium and fluoride may also play a role in supporting bone health. While magnesium is only found in
trace amounts in the human body, more than 60 percent of it is in the skeleton, suggesting it plays a role in the structure
of bone. Fluoride can displace the hydroxyl group in bone’s hydroxyapatite crystals and form fluorapatite. Similar to its
effect on dental enamel, fluorapatite helps stabilize and strengthen bone mineral. Fluoride can also enter spaces within
hydroxyapatite crystals, thus increasing their density.
Omega-3 fatty acids have long been known to reduce inflammation in various parts of the body. Inflammation can interfere
with the function of osteoblasts, so consuming omega-3 fatty acids, in the diet or in supplements, may also help enhance
production of new osseous tissue. Table 6.5 summarizes the role of nutrients in bone health.


Nutrients and Bone Health
Nutrient Role in bone health


Calcium Needed to make calcium phosphate and calcium carbonate, which form the hydroxyapatitecrystals that give bone its hardness
Vitamin D Needed for calcium absorption
Table 6.5


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Nutrients and Bone Health
Nutrient Role in bone health


Vitamin K Supports bone mineralization; may have synergistic effect with vitamin D
Magnesium Structural component of bone
Fluoride Structural component of bone
Omega-3 fatty
acids Reduces inflammation that may interfere with osteoblast function


Table 6.5


Hormones and Bone Tissue
The endocrine system produces and secretes hormones, many of which interact with the skeletal system. These hormones
are involved in controlling bone growth, maintaining bone once it is formed, and remodeling it.
Hormones That Influence Osteoblasts and/or Maintain the Matrix
Several hormones are necessary for controlling bone growth and maintaining the bone matrix. The pituitary gland
secretes growth hormone (GH), which, as its name implies, controls bone growth in several ways. It triggers chondrocyte
proliferation in epiphyseal plates, resulting in the increasing length of long bones. GH also increases calcium retention,
which enhances mineralization, and stimulates osteoblastic activity, which improves bone density.
GH is not alone in stimulating bone growth and maintaining osseous tissue. Thyroxine, a hormone secreted by the thyroid
gland promotes osteoblastic activity and the synthesis of bone matrix. During puberty, the sex hormones (estrogen in girls,
testosterone in boys) also come into play. They too promote osteoblastic activity and production of bone matrix, and in
addition, are responsible for the growth spurt that often occurs during adolescence. They also promote the conversion of the
epiphyseal plate to the epiphyseal line (i.e., cartilage to its bony remnant), thus bringing an end to the longitudinal growth
of bones. Additionally, calcitriol, the active form of vitamin D, is produced by the kidneys and stimulates the absorption of
calcium and phosphate from the digestive tract.


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Skeletal System
Osteoporosis is a disease characterized by a decrease in bone mass that occurs when the rate of bone resorption
exceeds the rate of bone formation, a common occurrence as the body ages. Notice how this is different from Paget’s
disease. In Paget’s disease, new bone is formed in an attempt to keep up with the resorption by the overactive
osteoclasts, but that new bone is produced haphazardly. In fact, when a physician is evaluating a patient with thinning
bone, he or she will test for osteoporosis and Paget’s disease (as well as other diseases). Osteoporosis does not have
the elevated blood levels of alkaline phosphatase found in Paget’s disease.


Figure 6.23 Graph Showing Relationship Between Age and Bone Mass Bone density peaks at about 30 years
of age. Women lose bone mass more rapidly than men.


While osteoporosis can involve any bone, it most commonly affects the proximal ends of the femur, vertebrae, and
wrist. As a result of the loss of bone density, the osseous tissue may not provide adequate support for everyday
functions, and something as simple as a sneeze can cause a vertebral fracture. When an elderly person falls and breaks a
hip (really, the femur), it is very likely the femur that broke first, which resulted in the fall. Histologically, osteoporosis
is characterized by a reduction in the thickness of compact bone and the number and size of trabeculae in cancellous
bone.
Figure 6.23 shows that women lose bone mass more quickly than men starting at about 50 years of age. This occurs
because 50 is the approximate age at which women go through menopause. Not only do their menstrual periods
lessen and eventually cease, but their ovaries reduce in size and then cease the production of estrogen, a hormone that
promotes osteoblastic activity and production of bone matrix. Thus, osteoporosis is more common in women than in
men, but men can develop it, too. Anyone with a family history of osteoporosis has a greater risk of developing the
disease, so the best treatment is prevention, which should start with a childhood diet that includes adequate intake of
calcium and vitamin D and a lifestyle that includes weight-bearing exercise. These actions, as discussed above, are
important in building bone mass. Promoting proper nutrition and weight-bearing exercise early in life can maximize
bone mass before the age of 30, thus reducing the risk of osteoporosis.
For many elderly people, a hip fracture can be life threatening. The fracture itself may not be serious, but the
immobility that comes during the healing process can lead to the formation of blood clots that can lodge in the
capillaries of the lungs, resulting in respiratory failure; pneumonia due to the lack of poor air exchange that
accompanies immobility; pressure sores (bed sores) that allow pathogens to enter the body and cause infections; and
urinary tract infections from catheterization.
Current treatments for managing osteoporosis include bisphosphonates (the same medications often used in Paget’s
disease), calcitonin, and estrogen (for women only). Minimizing the risk of falls, for example, by removing tripping
hazards, is also an important step in managing the potential outcomes from the disease.


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Hormones That Influence Osteoclasts
Bone modeling and remodeling require osteoclasts to resorb unneeded, damaged, or old bone, and osteoblasts to lay down
new bone. Two hormones that affect the osteoclasts are parathyroid hormone (PTH) and calcitonin.
PTH stimulates osteoclast proliferation and activity. As a result, calcium is released from the bones into the circulation, thus
increasing the calcium ion concentration in the blood. PTH also promotes the reabsorption of calcium by the kidney tubules,
which can affect calcium homeostasis (see below).
The small intestine is also affected by PTH, albeit indirectly. Because another function of PTH is to stimulate the synthesis
of vitamin D, and because vitamin D promotes intestinal absorption of calcium, PTH indirectly increases calcium uptake
by the small intestine. Calcitonin, a hormone secreted by the thyroid gland, has some effects that counteract those of
PTH. Calcitonin inhibits osteoclast activity and stimulates calcium uptake by the bones, thus reducing the concentration
of calcium ions in the blood. As evidenced by their opposing functions in maintaining calcium homeostasis, PTH and
calcitonin are generally not secreted at the same time. Table 6.6 summarizes the hormones that influence the skeletal
system.


Hormones That Affect the Skeletal System
Hormone Role


Growth
hormone Increases length of long bones, enhances mineralization, and improves bone density


Thyroxine Stimulates bone growth and promotes synthesis of bone matrix
Sex
hormones


Promote osteoblastic activity and production of bone matrix; responsible for adolescent growth
spurt; promote conversion of epiphyseal plate to epiphyseal line


Calcitriol Stimulates absorption of calcium and phosphate from digestive tract


Parathyroid
hormone


Stimulates osteoclast proliferation and resorption of bone by osteoclasts; promotes
reabsorption of calcium by kidney tubules; indirectly increases calcium absorption by small
intestine


Calcitonin Inhibits osteoclast activity and stimulates calcium uptake by bones
Table 6.6


6.7 | Calcium Homeostasis: Interactions of the Skeletal
System and Other Organ Systems
By the end of this section, you will be able to:
• Describe the effect of too much or too little calcium on the body
• Explain the process of calcium homeostasis


Calcium is not only the most abundant mineral in bone, it is also the most abundant mineral in the human body. Calcium
ions are needed not only for bone mineralization but for tooth health, regulation of the heart rate and strength of contraction,
blood coagulation, contraction of smooth and skeletal muscle cells, and regulation of nerve impulse conduction. The normal
level of calcium in the blood is about 10 mg/dL. When the body cannot maintain this level, a person will experience hypo-
or hypercalcemia.
Hypocalcemia, a condition characterized by abnormally low levels of calcium, can have an adverse effect on a number
of different body systems including circulation, muscles, nerves, and bone. Without adequate calcium, blood has difficulty
coagulating, the heart may skip beats or stop beating altogether, muscles may have difficulty contracting, nerves may have
difficulty functioning, and bones may become brittle. The causes of hypocalcemia can range from hormonal imbalances to
an improper diet. Treatments vary according to the cause, but prognoses are generally good.
Conversely, in hypercalcemia, a condition characterized by abnormally high levels of calcium, the nervous system is
underactive, which results in lethargy, sluggish reflexes, constipation and loss of appetite, confusion, and in severe cases,
coma.
Obviously, calcium homeostasis is critical. The skeletal, endocrine, and digestive systems play a role in this, but the kidneys
do, too. These body systems work together to maintain a normal calcium level in the blood (Figure 6.24).


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 231




Figure 6.24 Pathways in Calcium Homeostasis The body regulates calcium homeostasis with two pathways; one
is signaled to turn on when blood calcium levels drop below normal and one is the pathway that is signaled to turn on
when blood calcium levels are elevated.


Calcium is a chemical element that cannot be produced by any biological processes. The only way it can enter the body
is through the diet. The bones act as a storage site for calcium: The body deposits calcium in the bones when blood levels
get too high, and it releases calcium when blood levels drop too low. This process is regulated by PTH, vitamin D, and
calcitonin.
Cells of the parathyroid gland have plasma membrane receptors for calcium. When calcium is not binding to these receptors,
the cells release PTH, which stimulates osteoclast proliferation and resorption of bone by osteoclasts. This demineralization
process releases calcium into the blood. PTH promotes reabsorption of calcium from the urine by the kidneys, so that
the calcium returns to the blood. Finally, PTH stimulates the synthesis of vitamin D, which in turn, stimulates calcium
absorption from any digested food in the small intestine.
When all these processes return blood calcium levels to normal, there is enough calcium to bind with the receptors on the
surface of the cells of the parathyroid glands, and this cycle of events is turned off (Figure 6.24).
When blood levels of calcium get too high, the thyroid gland is stimulated to release calcitonin (Figure 6.24), which inhibits
osteoclast activity and stimulates calcium uptake by the bones, but also decreases reabsorption of calcium by the kidneys.
All of these actions lower blood levels of calcium. When blood calcium levels return to normal, the thyroid gland stops
secreting calcitonin.


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articular cartilage
articulation
bone
canaliculi


cartilage


central canal


closed reduction
compact bone
diaphysis
diploë
endochondral ossification
endosteum
epiphyseal line
epiphyseal plate


epiphysis
external callus
flat bone
fracture
fracture hematoma
hematopoiesis
hole
hypercalcemia
hypocalcemia
internal callus
intramembranous ossification
irregular bone
lacunae
long bone
medullary cavity
modeling


KEY TERMS
thin layer of cartilage covering an epiphysis; reduces friction and acts as a shock absorber


where two bone surfaces meet
hard, dense connective tissue that forms the structural elements of the skeleton


(singular = canaliculus) channels within the bone matrix that house one of an osteocyte’s many cytoplasmic
extensions that it uses to communicate and receive nutrients


semi-rigid connective tissue found on the skeleton in areas where flexibility and smooth surfaces support
movement


longitudinal channel in the center of each osteon; contains blood vessels, nerves, and lymphatic vessels;
also known as the Haversian canal


manual manipulation of a broken bone to set it into its natural position without surgery
dense osseous tissue that can withstand compressive forces


tubular shaft that runs between the proximal and distal ends of a long bone
layer of spongy bone, that is sandwiched between two the layers of compact bone found in flat bones


process in which bone forms by replacing hyaline cartilage
delicate membranous lining of a bone’s medullary cavity
completely ossified remnant of the epiphyseal plate
(also, growth plate) sheet of hyaline cartilage in the metaphysis of an immature bone; replaced by


bone tissue as the organ grows in length
wide section at each end of a long bone; filled with spongy bone and red marrow


collar of hyaline cartilage and bone that forms around the outside of a fracture
thin and curved bone; serves as a point of attachment for muscles and protects internal organs
broken bone


blood clot that forms at the site of a broken bone
production of blood cells, which occurs in the red marrow of the bones


opening or depression in a bone
condition characterized by abnormally high levels of calcium
condition characterized by abnormally low levels of calcium
fibrocartilaginous matrix, in the endosteal region, between the two ends of a broken bone


process by which bone forms directly from mesenchymal tissue
bone of complex shape; protects internal organs from compressive forces


(singular = lacuna) spaces in a bone that house an osteocyte
cylinder-shaped bone that is longer than it is wide; functions as a lever


hollow region of the diaphysis; filled with yellow marrow
process, during bone growth, by which bone is resorbed on one surface of a bone and deposited on another


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 233




nutrient foramen


open reduction
orthopedist
osseous tissue
ossification
ossification center
osteoblast
osteoclast
osteocyte
osteogenic cell


osteoid
osteon
osteoporosis


perforating canal


perichondrium
periosteum
primary ossification center


projection


proliferative zone


red marrow
remodeling


reserve zone
secondary ossification center
sesamoid bone
short bone
skeletal system
spongy bone
trabeculae
yellow marrow


small opening in the middle of the external surface of the diaphysis, through which an artery enters
the bone to provide nourishment


surgical exposure of a bone to reset a fracture
doctor who specializes in diagnosing and treating musculoskeletal disorders and injuries
bone tissue; a hard, dense connective tissue that forms the structural elements of the skeleton


(also, osteogenesis) bone formation
cluster of osteoblasts found in the early stages of intramembranous ossification


cell responsible for forming new bone
cell responsible for resorbing bone
primary cell in mature bone; responsible for maintaining the matrix


undifferentiated cell with high mitotic activity; the only bone cells that divide; they differentiate and
develop into osteoblasts
uncalcified bone matrix secreted by osteoblasts
(also, Haversian system) basic structural unit of compact bone; made of concentric layers of calcified matrix


disease characterized by a decrease in bone mass; occurs when the rate of bone resorption exceeds the
rate of bone formation, a common occurrence as the body ages


(also, Volkmann’s canal) channel that branches off from the central canal and houses vessels and
nerves that extend to the periosteum and endosteum


membrane that covers cartilage
fibrous membrane covering the outer surface of bone and continuous with ligaments


region, deep in the periosteal collar, where bone development starts during
endochondral ossification


bone markings where part of the surface sticks out above the rest of the surface, where tendons and
ligaments attach


region of the epiphyseal plate that makes new chondrocytes to replace those that die at the
diaphyseal end of the plate and contributes to longitudinal growth of the epiphyseal plate


connective tissue in the interior cavity of a bone where hematopoiesis takes place
process by which osteoclasts resorb old or damaged bone at the same time as and on the same surface


where osteoblasts form new bone to replace that which is resorbed
region of the epiphyseal plate that anchors the plate to the osseous tissue of the epiphysis


region of bone development in the epiphyses
small, round bone embedded in a tendon; protects the tendon from compressive forces


cube-shaped bone that is approximately equal in length, width, and thickness; provides limited motion
organ system composed of bones and cartilage that provides for movement, support, and protection


(also, cancellous bone) trabeculated osseous tissue that supports shifts in weight distribution
(singular = trabecula) spikes or sections of the lattice-like matrix in spongy bone


connective tissue in the interior cavity of a bone where fat is stored


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zone of calcified matrix


zone of maturation and hypertrophy


region of the epiphyseal plate closest to the diaphyseal end; functions to connect the
epiphyseal plate to the diaphysis


region of the epiphyseal plate where chondrocytes from the proliferative
zone grow and mature and contribute to the longitudinal growth of the epiphyseal plate


CHAPTER REVIEW
6.1 The Functions of the Skeletal System
The major functions of the bones are body support, facilitation of movement, protection of internal organs, storage of
minerals and fat, and hematopoiesis. Together, the muscular system and skeletal system are known as the musculoskeletal
system.


6.2 Bone Classification
Bones can be classified according to their shapes. Long bones, such as the femur, are longer than they are wide. Short bones,
such as the carpals, are approximately equal in length, width, and thickness. Flat bones are thin, but are often curved, such
as the ribs. Irregular bones such as those of the face have no characteristic shape. Sesamoid bones, such as the patellae, are
small and round, and are located in tendons.


6.3 Bone Structure
A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the
diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy
bone and red marrow. The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the organ grows in
length. The medullary cavity has a delicate membranous lining called the endosteum. The outer surface of bone, except in
regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum. Flat bones consist of two
layers of compact bone surrounding a layer of spongy bone. Bone markings depend on the function and location of bones.
Articulations are places where two bones meet. Projections stick out from the surface of the bone and provide attachment
points for tendons and ligaments. Holes are openings or depressions in the bones.
Bone matrix consists of collagen fibers and organic ground substance, primarily hydroxyapatite formed from calcium salts.
Osteogenic cells develop into osteoblasts. Osteoblasts are cells that make new bone. They become osteocytes, the cells
of mature bone, when they get trapped in the matrix. Osteoclasts engage in bone resorption. Compact bone is dense and
composed of osteons, while spongy bone is less dense and made up of trabeculae. Blood vessels and nerves enter the bone
through the nutrient foramina to nourish and innervate bones.


6.4 Bone Formation and Development
All bone formation is a replacement process. Embryos develop a cartilaginous skeleton and various membranes. During
development, these are replaced by bone during the ossification process. In intramembranous ossification, bone develops
directly from sheets of mesenchymal connective tissue. In endochondral ossification, bone develops by replacing hyaline
cartilage. Activity in the epiphyseal plate enables bones to grow in length. Modeling allows bones to grow in diameter.
Remodeling occurs as bone is resorbed and replaced by new bone. Osteogenesis imperfecta is a genetic disease in which
collagen production is altered, resulting in fragile, brittle bones.


6.5 Fractures: Bone Repair
Fractured bones may be repaired by closed reduction or open reduction. Fractures are classified by their complexity,
location, and other features. Common types of fractures are transverse, oblique, spiral, comminuted, impacted, greenstick,
open (or compound), and closed (or simple). Healing of fractures begins with the formation of a hematoma, followed by
internal and external calli. Osteoclasts resorb dead bone, while osteoblasts create new bone that replaces the cartilage in the
calli. The calli eventually unite, remodeling occurs, and healing is complete.


6.6 Exercise, Nutrition, Hormones, and Bone Tissue
Mechanical stress stimulates the deposition of mineral salts and collagen fibers within bones. Calcium, the predominant
mineral in bone, cannot be absorbed from the small intestine if vitamin D is lacking. Vitamin K supports bone mineralization
and may have a synergistic role with vitamin D. Magnesium and fluoride, as structural elements, play a supporting role
in bone health. Omega-3 fatty acids reduce inflammation and may promote production of new osseous tissue. Growth
hormone increases the length of long bones, enhances mineralization, and improves bone density. Thyroxine stimulates


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 235




bone growth and promotes the synthesis of bone matrix. The sex hormones (estrogen in women; testosterone in men)
promote osteoblastic activity and the production of bone matrix, are responsible for the adolescent growth spurt, and
promote closure of the epiphyseal plates. Osteoporosis is a disease characterized by decreased bone mass that is common
in aging adults. Calcitriol stimulates the digestive tract to absorb calcium and phosphate. Parathyroid hormone (PTH)
stimulates osteoclast proliferation and resorption of bone by osteoclasts. Vitamin D plays a synergistic role with PTH in
stimulating the osteoclasts. Additional functions of PTH include promoting reabsorption of calcium by kidney tubules
and indirectly increasing calcium absorption from the small intestine. Calcitonin inhibits osteoclast activity and stimulates
calcium uptake by bones.


6.7 Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems
Calcium homeostasis, i.e., maintaining a blood calcium level of about 10 mg/dL, is critical for normal body functions.
Hypocalcemia can result in problems with blood coagulation, muscle contraction, nerve functioning, and bone strength.
Hypercalcemia can result in lethargy, sluggish reflexes, constipation and loss of appetite, confusion, and coma. Calcium
homeostasis is controlled by PTH, vitamin D, and calcitonin and the interactions of the skeletal, endocrine, digestive, and
urinary systems.


REVIEW QUESTIONS
1. Which function of the skeletal system would be
especially important if you were in a car accident?


a. storage of minerals
b. protection of internal organs
c. facilitation of movement
d. fat storage


2. Bone tissue can be described as ________.
a. dead calcified tissue
b. cartilage
c. the skeletal system
d. dense, hard connective tissue


3. Without red marrow, bones would not be able to
________.


a. store phosphate
b. store calcium
c. make blood cells
d. move like levers


4. Yellow marrow has been identified as ________.


a. an area of fat storage
b. a point of attachment for muscles
c. the hard portion of bone
d. the cause of kyphosis


5. Which of the following can be found in areas of
movement?


a. hematopoiesis
b. cartilage
c. yellow marrow
d. red marrow


6. The skeletal system is made of ________.
a. muscles and tendons
b. bones and cartilage
c. vitreous humor
d. minerals and fat


7. Most of the bones of the arms and hands are long bones;
however, the bones in the wrist are categorized as
________.


a. flat bones
b. short bones


c. sesamoid bones
d. irregular bones


8. Sesamoid bones are found embedded in ________.


a. joints
b. muscles
c. ligaments
d. tendons


9. Bones that surround the spinal cord are classified as
________ bones.


a. irregular
b. sesamoid
c. flat
d. short


10. Which category of bone is among the most numerous
in the skeleton?


a. long bone
b. sesamoid bone
c. short bone
d. flat bone


11. Long bones enable body movement by acting as a
________.


a. counterweight
b. resistive force
c. lever
d. fulcrum


12. Which of the following occurs in the spongy bone of
the epiphysis?


a. bone growth
b. bone remodeling
c. hematopoiesis
d. shock absorption


13. The diaphysis contains ________.
a. the metaphysis
b. fat stores
c. spongy bone
d. compact bone


14. The fibrous membrane covering the outer surface of the
bone is the ________.


a. periosteum


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b. epiphysis
c. endosteum
d. diaphysis


15. Which of the following are incapable of undergoing
mitosis?


a. osteoblasts and osteoclasts
b. osteocytes and osteoclasts
c. osteoblasts and osteocytes
d. osteogenic cells and osteoclasts


16. Which cells do not originate from osteogenic cells?


a. osteoblasts
b. osteoclasts
c. osteocytes
d. osteoprogenitor cells


17. Which of the following are found in compact bone and
cancellous bone?


a. Haversian systems
b. Haversian canals
c. lamellae
d. lacunae


18. Which of the following are only found in cancellous
bone?


a. canaliculi
b. Volkmann’s canals
c. trabeculae
d. calcium salts


19. The area of a bone where the nutrient foramen passes
forms what kind of bone marking?


a. a hole
b. a facet
c. a canal
d. a fissure


20.Why is cartilage slow to heal?
a. because it eventually develops into bone
b. because it is semi-solid and flexible
c. because it does not have a blood supply
d. because endochondral ossification replaces all
cartilage with bone


21. Why are osteocytes spread out in bone tissue?


a. They develop from mesenchymal cells.
b. They are surrounded by osteoid.
c. They travel through the capillaries.
d. Formation of osteoid spreads out the osteoblasts
that formed the ossification centers.


22. In endochondral ossification, what happens to the
chondrocytes?


a. They develop into osteocytes.
b. They die in the calcified matrix that surrounds
them and form the medullary cavity.


c. They grow and form the periosteum.
d. They group together to form the primary
ossification center.


23. Which of the following bones is (are) formed by
intramembranous ossification?


a. the metatarsals
b. the femur


c. the ribs
d. the flat bones of the cranium


24. Bones grow in length due to activity in the ________.


a. epiphyseal plate
b. perichondrium
c. periosteum
d. medullary cavity


25. Bones grow in diameter due to bone formation
________.


a. in the medullary cavity
b. beneath the periosteum
c. in the epiphyseal plate
d. within the metaphysis


26.Which of the following represents the correct sequence
of zones in the epiphyseal plate?


a. proliferation, reserved, maturation, calcification
b. maturation, proliferation, reserved, calcification
c. calcification, maturation, proliferation, reserved
d. calcification, reserved, proliferation, maturation


27. A fracture can be both ________.
a. open and closed
b. open and transverse
c. transverse and greenstick
d. greenstick and comminuted


28. How can a fractured diaphysis release fat globules into
the bloodstream?


a. The bone pierces fat stores in the skin.
b. The yellow marrow in the diaphysis is exposed
and damaged.


c. The injury triggers the body to release fat from
healthy bones.


d. The red marrow in the fractured bone releases fat
to heal the fracture.


29. In a compound fracture, ________.
a. the break occurs at an angle to the bone
b. the broken bone does not tear the skin
c. one fragment of broken bone is compressed into
the other


d. broken bone pierces the skin
30. The internal and external calli are replaced by
________.


a. hyaline cartilage
b. trabecular bone
c. osteogenic cells
d. osteoclasts


31. The first type of bone to form during fracture repair is
________ bone.


a. compact
b. lamellar
c. spongy
d. dense


32. Wolff’s law, which describes the effect of mechanical
forces in bone modeling/remodeling, would predict that
________


a. a right-handed pitcher will have thicker bones in
his right arm compared to his left.


CHAPTER 6 | BONE TISSUE AND THE SKELETAL SYSTEM 237




b. a right-handed cyclist will have thicker bones in
her right leg compared to her left.


c. a broken bone will heal thicker than it was before
the fracture.


d. a bed-ridden patient will have thicker bones than
an athlete.


33. Calcium cannot be absorbed from the small intestine if
________ is lacking.


a. vitamin D
b. vitamin K
c. calcitonin
d. fluoride


34. Which one of the following foods is best for bone
health?


a. carrots
b. liver
c. leafy green vegetables
d. oranges


35. Which of the following hormones are responsible for
the adolescent growth spurt?


a. estrogen and testosterone
b. calcitonin and calcitriol
c. growth hormone and parathyroid hormone
d. thyroxine and progesterone


36. With respect to their direct effects on osseous tissue,
which pair of hormones has actions that oppose each other?


a. estrogen and testosterone
b. calcitonin and calcitriol
c. estrogen and progesterone
d. calcitonin and parathyroid hormone


37. When calcium levels are too high or too low, which
body system is primarily affected?


a. skeletal system
b. endocrine system
c. digestive system
d. nervous system


38. All of the following play a role in calcium homeostasis
except


a. thyroxine
b. calcitonin
c. parathyroid hormone
d. vitamin D


39. Which of the following is most likely to be released
when blood calcium levels are elevated?


a. thyroxine
b. calcitonin
c. parathyroid hormone
d. vitamin D


CRITICAL THINKING QUESTIONS
40. The skeletal system is composed of bone and cartilage
and has many functions. Choose three of these functions
and discuss what features of the skeletal system allow it to
accomplish these functions.
41. What are the structural and functional differences
between a tarsal and a metatarsal?
42. What are the structural and functional differences
between the femur and the patella?
43. If the articular cartilage at the end of one of your long
bones were to degenerate, what symptoms do you think you
would experience? Why?
44. In what ways is the structural makeup of compact and
spongy bone well suited to their respective functions?
45. In what ways do intramembranous and endochondral
ossification differ?
46. Considering how a long bone develops, what are the
similarities and differences between a primary and a
secondary ossification center?
47. What is the difference between closed reduction and
open reduction? In what type of fracture would closed


reduction most likely occur? In what type of fracture would
open reduction most likely occur?
48. In terms of origin and composition, what are the
differences between an internal callus and an external
callus?
49. If you were a dietician who had a young female patient
with a family history of osteoporosis, what foods would
you suggest she include in her diet? Why?
50. During the early years of space exploration our
astronauts, who had been floating in space, would return
to earth showing significant bone loss dependent on how
long they were in space. Discuss how this might happen
and what could be done to alleviate this condition.
51. An individual with very low levels of vitamin D
presents themselves to you complaining of seemingly
fragile bones. Explain how these might be connected.
52. Describe the effects caused when the parathyroid gland
fails to respond to calcium bound to its receptors.


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7 | AXIAL SKELETON


Figure 7.1 Lateral View of the Human Skull


Introduction
Chapter Objectives


After studying this chapter, you will be able to:
• Describe the functions of the skeletal system and define its two major subdivisions
• Identify the bones and bony structures of the skull, the cranial suture lines, the cranial fossae, and the
openings in the skull


• Discuss the vertebral column and regional variations in its bony components and curvatures
• Describe the components of the thoracic cage
• Discuss the embryonic development of the axial skeleton


CHAPTER 7 | AXIAL SKELETON 239




The skeletal system forms the rigid internal framework of the body. It consists of the bones, cartilages, and ligaments. Bones
support the weight of the body, allow for body movements, and protect internal organs. Cartilage provides flexible strength
and support for body structures such as the thoracic cage, the external ear, and the trachea and larynx. At joints of the body,
cartilage can also unite adjacent bones or provide cushioning between them. Ligaments are the strong connective tissue
bands that hold the bones at a moveable joint together and serve to prevent excessive movements of the joint that would
result in injury. Providing movement of the skeleton are the muscles of the body, which are firmly attached to the skeleton
via connective tissue structures called tendons. As muscles contract, they pull on the bones to produce movements of the
body. Thus, without a skeleton, you would not be able to stand, run, or even feed yourself!
Each bone of the body serves a particular function, and therefore bones vary in size, shape, and strength based on these
functions. For example, the bones of the lower back and lower limb are thick and strong to support your body weight.
Similarly, the size of a bony landmark that serves as a muscle attachment site on an individual bone is related to the strength
of this muscle. Muscles can apply very strong pulling forces to the bones of the skeleton. To resist these forces, bones have
enlarged bony landmarks at sites where powerful muscles attach. This means that not only the size of a bone, but also its
shape, is related to its function. For this reason, the identification of bony landmarks is important during your study of the
skeletal system.
Bones are also dynamic organs that can modify their strength and thickness in response to changes in muscle strength or
body weight. Thus, muscle attachment sites on bones will thicken if you begin a workout program that increases muscle
strength. Similarly, the walls of weight-bearing bones will thicken if you gain body weight or begin pounding the pavement
as part of a new running regimen. In contrast, a reduction in muscle strength or body weight will cause bones to become
thinner. This may happen during a prolonged hospital stay, following limb immobilization in a cast, or going into the
weightlessness of outer space. Even a change in diet, such as eating only soft food due to the loss of teeth, will result in a
noticeable decrease in the size and thickness of the jaw bones.


7.1 | Divisions of the Skeletal System
By the end of this section, you will be able to:
• Discuss the functions of the skeletal system
• Distinguish between the axial skeleton and appendicular skeleton
• Define the axial skeleton and its components
• Define the appendicular skeleton and its components


The skeletal system includes all of the bones, cartilages, and ligaments of the body that support and give shape to the body
and body structures. The skeleton consists of the bones of the body. For adults, there are 206 bones in the skeleton. Younger
individuals have higher numbers of bones because some bones fuse together during childhood and adolescence to form an
adult bone. The primary functions of the skeleton are to provide a rigid, internal structure that can support the weight of the
body against the force of gravity, and to provide a structure upon which muscles can act to produce movements of the body.
The lower portion of the skeleton is specialized for stability during walking or running. In contrast, the upper skeleton has
greater mobility and ranges of motion, features that allow you to lift and carry objects or turn your head and trunk.
In addition to providing for support and movements of the body, the skeleton has protective and storage functions. It protects
the internal organs, including the brain, spinal cord, heart, lungs, and pelvic organs. The bones of the skeleton serve as the
primary storage site for important minerals such as calcium and phosphate. The bone marrow found within bones stores fat
and houses the blood-cell producing tissue of the body.
The skeleton is subdivided into two major divisions—the axial and appendicular.


The Axial Skeleton
The skeleton is subdivided into two major divisions—the axial and appendicular. The axial skeleton forms the vertical,
central axis of the body and includes all bones of the head, neck, chest, and back (Figure 7.2). It serves to protect the brain,
spinal cord, heart, and lungs. It also serves as the attachment site for muscles that move the head, neck, and back, and for
muscles that act across the shoulder and hip joints to move their corresponding limbs.
The axial skeleton of the adult consists of 80 bones, including the skull, the vertebral column, and the thoracic cage. The
skull is formed by 22 bones. Also associated with the head are an additional seven bones, including the hyoid bone and
the ear ossicles (three small bones found in each middle ear). The vertebral column consists of 24 bones, each called a
vertebra, plus the sacrum and coccyx. The thoracic cage includes the 12 pairs of ribs, and the sternum, the flattened bone
of the anterior chest.


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Figure 7.2 Axial and Appendicular Skeleton The axial skeleton supports the head, neck, back, and chest and thus
forms the vertical axis of the body. It consists of the skull, vertebral column (including the sacrum and coccyx), and the
thoracic cage, formed by the ribs and sternum. The appendicular skeleton is made up of all bones of the upper and
lower limbs.


The Appendicular Skeleton
The appendicular skeleton includes all bones of the upper and lower limbs, plus the bones that attach each limb to the axial
skeleton. There are 126 bones in the appendicular skeleton of an adult. The bones of the appendicular skeleton are covered
in a separate chapter.


7.2 | The Skull
By the end of this section, you will be able to:
• List and identify the bones of the brain case and face
• Locate the major suture lines of the skull and name the bones associated with each
• Locate and define the boundaries of the anterior, middle, and posterior cranial fossae, the temporal fossa, and
infratemporal fossa


• Define the paranasal sinuses and identify the location of each
• Name the bones that make up the walls of the orbit and identify the openings associated with the orbit
• Identify the bones and structures that form the nasal septum and nasal conchae, and locate the hyoid bone
• Identify the bony openings of the skull


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The cranium (skull) is the skeletal structure of the head that supports the face and protects the brain. It is subdivided into
the facial bones and the brain case, or cranial vault (Figure 7.3). The facial bones underlie the facial structures, form the
nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws. The rounded brain case surrounds and
protects the brain and houses the middle and inner ear structures.
In the adult, the skull consists of 22 individual bones, 21 of which are immobile and united into a single unit. The 22nd bone
is the mandible (lower jaw), which is the only moveable bone of the skull.


Figure 7.3 Parts of the Skull The skull consists of the rounded brain case that houses the brain and the facial bones
that form the upper and lower jaws, nose, orbits, and other facial structures.


Watch this video (http://openstaxcollege.org/l/skull1) to view a rotating and exploded skull, with color-coded bones.
Which bone (yellow) is centrally located and joins with most of the other bones of the skull?


Anterior View of Skull
The anterior skull consists of the facial bones and provides the bony support for the eyes and structures of the face. This
view of the skull is dominated by the openings of the orbits and the nasal cavity. Also seen are the upper and lower jaws,
with their respective teeth (Figure 7.4).
The orbit is the bony socket that houses the eyeball and muscles that move the eyeball or open the upper eyelid. The upper
margin of the anterior orbit is the supraorbital margin. Located near the midpoint of the supraorbital margin is a small
opening called the supraorbital foramen. This provides for passage of a sensory nerve to the skin of the forehead. Below
the orbit is the infraorbital foramen, which is the point of emergence for a sensory nerve that supplies the anterior face
below the orbit.


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Figure 7.4 Anterior View of Skull An anterior view of the skull shows the bones that form the forehead, orbits (eye
sockets), nasal cavity, nasal septum, and upper and lower jaws.


Inside the nasal area of the skull, the nasal cavity is divided into halves by the nasal septum. The upper portion of the nasal
septum is formed by the perpendicular plate of the ethmoid bone and the lower portion is the vomer bone. Each side of
the nasal cavity is triangular in shape, with a broad inferior space that narrows superiorly. When looking into the nasal cavity
from the front of the skull, two bony plates are seen projecting from each lateral wall. The larger of these is the inferior
nasal concha, an independent bone of the skull. Located just above the inferior concha is the middle nasal concha, which
is part of the ethmoid bone. A third bony plate, also part of the ethmoid bone, is the superior nasal concha. It is much
smaller and out of sight, above the middle concha. The superior nasal concha is located just lateral to the perpendicular
plate, in the upper nasal cavity.


Lateral View of Skull
A view of the lateral skull is dominated by the large, rounded brain case above and the upper and lower jaws with their
teeth below (Figure 7.5). Separating these areas is the bridge of bone called the zygomatic arch. The zygomatic arch is the
bony arch on the side of skull that spans from the area of the cheek to just above the ear canal. It is formed by the junction
of two bony processes: a short anterior component, the temporal process of the zygomatic bone (the cheekbone) and a
longer posterior portion, the zygomatic process of the temporal bone, extending forward from the temporal bone. Thus
the temporal process (anteriorly) and the zygomatic process (posteriorly) join together, like the two ends of a drawbridge, to
form the zygomatic arch. One of the major muscles that pulls the mandible upward during biting and chewing arises from
the zygomatic arch.
On the lateral side of the brain case, above the level of the zygomatic arch, is a shallow space called the temporal
fossa. Below the level of the zygomatic arch and deep to the vertical portion of the mandible is another space called the
infratemporal fossa. Both the temporal fossa and infratemporal fossa contain muscles that act on the mandible during
chewing.


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Figure 7.5 Lateral View of Skull The lateral skull shows the large rounded brain case, zygomatic arch, and the upper
and lower jaws. The zygomatic arch is formed jointly by the zygomatic process of the temporal bone and the temporal
process of the zygomatic bone. The shallow space above the zygomatic arch is the temporal fossa. The space inferior
to the zygomatic arch and deep to the posterior mandible is the infratemporal fossa.


Bones of the Brain Case
The brain case contains and protects the brain. The interior space that is almost completely occupied by the brain is called
the cranial cavity. This cavity is bounded superiorly by the rounded top of the skull, which is called the calvaria (skullcap),
and the lateral and posterior sides of the skull. The bones that form the top and sides of the brain case are usually referred
to as the “flat” bones of the skull.
The floor of the brain case is referred to as the base of the skull. This is a complex area that varies in depth and has numerous
openings for the passage of cranial nerves, blood vessels, and the spinal cord. Inside the skull, the base is subdivided into
three large spaces, called the anterior cranial fossa,middle cranial fossa, and posterior cranial fossa (fossa = “trench or
ditch”) (Figure 7.6). From anterior to posterior, the fossae increase in depth. The shape and depth of each fossa corresponds
to the shape and size of the brain region that each houses. The boundaries and openings of the cranial fossae (singular =
fossa) will be described in a later section.


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Figure 7.6 Cranial Fossae The bones of the brain case surround and protect the brain, which occupies the cranial
cavity. The base of the brain case, which forms the floor of cranial cavity, is subdivided into the shallow anterior cranial
fossa, the middle cranial fossa, and the deep posterior cranial fossa.


The brain case consists of eight bones. These include the paired parietal and temporal bones, plus the unpaired frontal,
occipital, sphenoid, and ethmoid bones.
Parietal Bone
The parietal bone forms most of the upper lateral side of the skull (see Figure 7.5). These are paired bones, with the right
and left parietal bones joining together at the top of the skull. Each parietal bone is also bounded anteriorly by the frontal
bone, inferiorly by the temporal bone, and posteriorly by the occipital bone.
Temporal Bone
The temporal bone forms the lower lateral side of the skull (see Figure 7.5). Common wisdom has it that the temporal
bone (temporal = “time”) is so named because this area of the head (the temple) is where hair typically first turns gray,
indicating the passage of time.
The temporal bone is subdivided into several regions (Figure 7.7). The flattened, upper portion is the squamous portion of
the temporal bone. Below this area and projecting anteriorly is the zygomatic process of the temporal bone, which forms
the posterior portion of the zygomatic arch. Posteriorly is the mastoid portion of the temporal bone. Projecting inferiorly
from this region is a large prominence, themastoid process, which serves as a muscle attachment site. The mastoid process
can easily be felt on the side of the head just behind your earlobe. On the interior of the skull, the petrous portion of each
temporal bone forms the prominent, diagonally oriented petrous ridge in the floor of the cranial cavity. Located inside each
petrous ridge are small cavities that house the structures of the middle and inner ears.


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Figure 7.7 Temporal Bone A lateral view of the isolated temporal bone shows the squamous, mastoid, and
zygomatic portions of the temporal bone.


Important landmarks of the temporal bone, as shown in Figure 7.8, include the following:
• External acoustic meatus (ear canal)—This is the large opening on the lateral side of the skull that is associated with
the ear.


• Internal acoustic meatus—This opening is located inside the cranial cavity, on the medial side of the petrous ridge.
It connects to the middle and inner ear cavities of the temporal bone.


• Mandibular fossa—This is the deep, oval-shaped depression located on the external base of the skull, just in
front of the external acoustic meatus. The mandible (lower jaw) joins with the skull at this site as part of the
temporomandibular joint, which allows for movements of the mandible during opening and closing of the mouth.


• Articular tubercle—The smooth ridge located immediately anterior to the mandibular fossa. Both the articular
tubercle and mandibular fossa contribute to the temporomandibular joint, the joint that provides for movements
between the temporal bone of the skull and the mandible.


• Styloid process—Posterior to the mandibular fossa on the external base of the skull is an elongated, downward bony
projection called the styloid process, so named because of its resemblance to a stylus (a pen or writing tool). This
structure serves as an attachment site for several small muscles and for a ligament that supports the hyoid bone of the
neck. (See also Figure 7.7.)


• Stylomastoid foramen—This small opening is located between the styloid process and mastoid process. This is the
point of exit for the cranial nerve that supplies the facial muscles.


• Carotid canal—The carotid canal is a zig-zag shaped tunnel that provides passage through the base of the skull for
one of the major arteries that supplies the brain. Its entrance is located on the outside base of the skull, anteromedial to
the styloid process. The canal then runs anteromedially within the bony base of the skull, and then turns upward to its
exit in the floor of the middle cranial cavity, above the foramen lacerum.


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Figure 7.8 External and Internal Views of Base of Skull (a) The hard palate is formed anteriorly by the palatine
processes of the maxilla bones and posteriorly by the horizontal plate of the palatine bones. (b) The complex floor of
the cranial cavity is formed by the frontal, ethmoid, sphenoid, temporal, and occipital bones. The lesser wing of the
sphenoid bone separates the anterior and middle cranial fossae. The petrous ridge (petrous portion of temporal bone)
separates the middle and posterior cranial fossae.


Frontal Bone
The frontal bone is the single bone that forms the forehead. At its anterior midline, between the eyebrows, there is a slight
depression called the glabella (see Figure 7.5). The frontal bone also forms the supraorbital margin of the orbit. Near the
middle of this margin, is the supraorbital foramen, the opening that provides passage for a sensory nerve to the forehead.
The frontal bone is thickened just above each supraorbital margin, forming rounded brow ridges. These are located just


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behind your eyebrows and vary in size among individuals, although they are generally larger in males. Inside the cranial
cavity, the frontal bone extends posteriorly. This flattened region forms both the roof of the orbit below and the floor of the
anterior cranial cavity above (see Figure 7.8b).
Occipital Bone
The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity (Figure 7.9;
see also Figure 7.8). On its outside surface, at the posterior midline, is a small protrusion called the external occipital
protuberance, which serves as an attachment site for a ligament of the posterior neck. Lateral to either side of this bump is
a superior nuchal line (nuchal = “nape” or “posterior neck”). The nuchal lines represent the most superior point at which
muscles of the neck attach to the skull, with only the scalp covering the skull above these lines. On the base of the skull, the
occipital bone contains the large opening of the foramen magnum, which allows for passage of the spinal cord as it exits
the skull. On either side of the foramen magnum is an oval-shaped occipital condyle. These condyles form joints with the
first cervical vertebra and thus support the skull on top of the vertebral column.


Figure 7.9 Posterior View of Skull This view of the posterior skull shows attachment sites for muscles and joints
that support the skull.


Sphenoid Bone
The sphenoid bone is a single, complex bone of the central skull (Figure 7.10). It serves as a “keystone” bone, because
it joins with almost every other bone of the skull. The sphenoid forms much of the base of the central skull (see Figure
7.8) and also extends laterally to contribute to the sides of the skull (see Figure 7.5). Inside the cranial cavity, the right
and left lesser wings of the sphenoid bone, which resemble the wings of a flying bird, form the lip of a prominent ridge
that marks the boundary between the anterior and middle cranial fossae. The sella turcica (“Turkish saddle”) is located at
the midline of the middle cranial fossa. This bony region of the sphenoid bone is named for its resemblance to the horse
saddles used by the Ottoman Turks, with a high back and a tall front. The rounded depression in the floor of the sella turcica
is the hypophyseal (pituitary) fossa, which houses the pea-sized pituitary (hypophyseal) gland. The greater wings of the
sphenoid bone extend laterally to either side away from the sella turcica, where they form the anterior floor of the middle
cranial fossa. The greater wing is best seen on the outside of the lateral skull, where it forms a rectangular area immediately
anterior to the squamous portion of the temporal bone.
On the inferior aspect of the skull, each half of the sphenoid bone forms two thin, vertically oriented bony plates. These are
the medial pterygoid plate and lateral pterygoid plate (pterygoid = “wing-shaped”). The right and left medial pterygoid
plates form the posterior, lateral walls of the nasal cavity. The somewhat larger lateral pterygoid plates serve as attachment
sites for chewing muscles that fill the infratemporal space and act on the mandible.


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Figure 7.10 Sphenoid Bone Shown in isolation in (a) superior and (b) posterior views, the sphenoid bone is a single
midline bone that forms the anterior walls and floor of the middle cranial fossa. It has a pair of lesser wings and a pair
of greater wings. The sella turcica surrounds the hypophyseal fossa. Projecting downward are the medial and lateral
pterygoid plates. The sphenoid has multiple openings for the passage of nerves and blood vessels, including the optic
canal, superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum.


Ethmoid Bone
The ethmoid bone is a single, midline bone that forms the roof and lateral walls of the upper nasal cavity, the upper portion
of the nasal septum, and contributes to the medial wall of the orbit (Figure 7.11 and Figure 7.12). On the interior of the
skull, the ethmoid also forms a portion of the floor of the anterior cranial cavity (see Figure 7.8b).
Within the nasal cavity, the perpendicular plate of the ethmoid bone forms the upper portion of the nasal septum. The
ethmoid bone also forms the lateral walls of the upper nasal cavity. Extending from each lateral wall are the superior nasal
concha and middle nasal concha, which are thin, curved projections that extend into the nasal cavity (Figure 7.13).
In the cranial cavity, the ethmoid bone forms a small area at the midline in the floor of the anterior cranial fossa. This region
also forms the narrow roof of the underlying nasal cavity. This portion of the ethmoid bone consists of two parts, the crista
galli and cribriform plates. The crista galli (“rooster’s comb or crest”) is a small upward bony projection located at the
midline. It functions as an anterior attachment point for one of the covering layers of the brain. To either side of the crista
galli is the cribriform plate (cribrum = “sieve”), a small, flattened area with numerous small openings termed olfactory
foramina. Small nerve branches from the olfactory areas of the nasal cavity pass through these openings to enter the brain.
The lateral portions of the ethmoid bone are located between the orbit and upper nasal cavity, and thus form the lateral
nasal cavity wall and a portion of the medial orbit wall. Located inside this portion of the ethmoid bone are several small,
air-filled spaces that are part of the paranasal sinus system of the skull.


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Figure 7.11 Sagittal Section of Skull This midline view of the sagittally sectioned skull shows the nasal septum.


Figure 7.12 Ethmoid Bone The unpaired ethmoid bone is located at the midline within the central skull. It has an
upward projection, the crista galli, and a downward projection, the perpendicular plate, which forms the upper nasal
septum. The cribriform plates form both the roof of the nasal cavity and a portion of the anterior cranial fossa floor. The
lateral sides of the ethmoid bone form the lateral walls of the upper nasal cavity, part of the medial orbit wall, and give
rise to the superior and middle nasal conchae. The ethmoid bone also contains the ethmoid air cells.


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Figure 7.13 Lateral Wall of Nasal Cavity The three nasal conchae are curved bones that project from the lateral
walls of the nasal cavity. The superior nasal concha and middle nasal concha are parts of the ethmoid bone. The
inferior nasal concha is an independent bone of the skull.


Sutures of the Skull
A suture is an immobile joint between adjacent bones of the skull. The narrow gap between the bones is filled with dense,
fibrous connective tissue that unites the bones. The long sutures located between the bones of the brain case are not straight,
but instead follow irregular, tightly twisting paths. These twisting lines serve to tightly interlock the adjacent bones, thus
adding strength to the skull for brain protection.
The two suture lines seen on the top of the skull are the coronal and sagittal sutures. The coronal suture runs from side
to side across the skull, within the coronal plane of section (see Figure 7.5). It joins the frontal bone to the right and left
parietal bones. The sagittal suture extends posteriorly from the coronal suture, running along the midline at the top of the
skull in the sagittal plane of section (see Figure 7.9). It unites the right and left parietal bones. On the posterior skull, the
sagittal suture terminates by joining the lambdoid suture. The lambdoid suture extends downward and laterally to either
side away from its junction with the sagittal suture. The lambdoid suture joins the occipital bone to the right and left parietal
and temporal bones. This suture is named for its upside-down "V" shape, which resembles the capital letter version of the
Greek letter lambda (Λ). The squamous suture is located on the lateral skull. It unites the squamous portion of the temporal
bone with the parietal bone (see Figure 7.5). At the intersection of four bones is the pterion, a small, capital-H-shaped
suture line region that unites the frontal bone, parietal bone, squamous portion of the temporal bone, and greater wing of
the sphenoid bone. It is the weakest part of the skull. The pterion is located approximately two finger widths above the
zygomatic arch and a thumb’s width posterior to the upward portion of the zygomatic bone.


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Skeletal System
Head and traumatic brain injuries are major causes of immediate death and disability, with bleeding and infections as
possible additional complications. According to the Centers for Disease Control and Prevention (2010), approximately
30 percent of all injury-related deaths in the United States are caused by head injuries. The majority of head injuries
involve falls. They are most common among young children (ages 0–4 years), adolescents (15–19 years), and the
elderly (over 65 years). Additional causes vary, but prominent among these are automobile and motorcycle accidents.
Strong blows to the brain-case portion of the skull can produce fractures. These may result in bleeding inside the
skull with subsequent injury to the brain. The most common is a linear skull fracture, in which fracture lines radiate
from the point of impact. Other fracture types include a comminuted fracture, in which the bone is broken into several
pieces at the point of impact, or a depressed fracture, in which the fractured bone is pushed inward. In a contrecoup
(counterblow) fracture, the bone at the point of impact is not broken, but instead a fracture occurs on the opposite side
of the skull. Fractures of the occipital bone at the base of the skull can occur in this manner, producing a basilar fracture
that can damage the artery that passes through the carotid canal.
A blow to the lateral side of the head may fracture the bones of the pterion. The pterion is an important clinical
landmark because located immediately deep to it on the inside of the skull is a major branch of an artery that supplies
the skull and covering layers of the brain. A strong blow to this region can fracture the bones around the pterion.
If the underlying artery is damaged, bleeding can cause the formation of a hematoma (collection of blood) between
the brain and interior of the skull. As blood accumulates, it will put pressure on the brain. Symptoms associated with
a hematoma may not be apparent immediately following the injury, but if untreated, blood accumulation will exert
increasing pressure on the brain and can result in death within a few hours.


View this animation (http://openstaxcollege.org/l/headblow) to see how a blow to the head may produce a
contrecoup (counterblow) fracture of the basilar portion of the occipital bone on the base of the skull. Why may a
basilar fracture be life threatening?


Facial Bones of the Skull
The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The
facial bones include 14 bones, with six paired bones and two unpaired bones. The paired bones are the maxilla, palatine,
zygomatic, nasal, lacrimal, and inferior nasal conchae bones. The unpaired bones are the vomer and mandible bones.
Although classified with the brain-case bones, the ethmoid bone also contributes to the nasal septum and the walls of the
nasal cavity and orbit.
Maxillary Bone
Themaxillary bone, often referred to simply as the maxilla (plural = maxillae), is one of a pair that together form the upper
jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose (see Figure 7.4). The curved,
inferior margin of the maxillary bone that forms the upper jaw and contains the upper teeth is the alveolar process of the
maxilla (Figure 7.14). Each tooth is anchored into a deep socket called an alveolus. On the anterior maxilla, just below the
orbit, is the infraorbital foramen. This is the point of exit for a sensory nerve that supplies the nose, upper lip, and anterior
cheek. On the inferior skull, the palatine process from each maxillary bone can be seen joining together at the midline to
form the anterior three-quarters of the hard palate (see Figure 7.8a). The hard palate is the bony plate that forms the roof
of the mouth and floor of the nasal cavity, separating the oral and nasal cavities.


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Figure 7.14 Maxillary Bone The maxillary bone forms the upper jaw and supports the upper teeth. Each maxilla also
forms the lateral floor of each orbit and the majority of the hard palate.


Palatine Bone
The palatine bone is one of a pair of irregularly shaped bones that contribute small areas to the lateral walls of the nasal
cavity and the medial wall of each orbit. The largest region of each of the palatine bone is the horizontal plate. The plates
from the right and left palatine bones join together at the midline to form the posterior quarter of the hard palate (see Figure
7.8a). Thus, the palatine bones are best seen in an inferior view of the skull and hard palate.


Cleft Lip and Cleft Palate
During embryonic development, the right and left maxilla bones come together at the midline to form the upper jaw.
At the same time, the muscle and skin overlying these bones join together to form the upper lip. Inside the mouth,
the palatine processes of the maxilla bones, along with the horizontal plates of the right and left palatine bones, join
together to form the hard palate. If an error occurs in these developmental processes, a birth defect of cleft lip or cleft
palate may result.
Cleft lip is a common development defect that affects approximately 1:1000 births, most of which are male. This defect
involves a partial or complete failure of the right and left portions of the upper lip to fuse together, leaving a cleft (gap).
A more severe developmental defect is cleft palate, which affects the hard palate. The hard palate is the bony structure
that separates the nasal cavity from the oral cavity. It is formed during embryonic development by the midline fusion
of the horizontal plates from the right and left palatine bones and the palatine processes of the maxilla bones. Cleft
palate affects approximately 1:2500 births and is more common in females. It results from a failure of the two halves
of the hard palate to completely come together and fuse at the midline, thus leaving a gap between them. This gap
allows for communication between the nasal and oral cavities. In severe cases, the bony gap continues into the anterior
upper jaw where the alveolar processes of the maxilla bones also do not properly join together above the front teeth.
If this occurs, a cleft lip will also be seen. Because of the communication between the oral and nasal cavities, a cleft
palate makes it very difficult for an infant to generate the suckling needed for nursing, thus leaving the infant at risk
for malnutrition. Surgical repair is required to correct cleft palate defects.


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Zygomatic Bone
The zygomatic bone is also known as the cheekbone. Each of the paired zygomatic bones forms much of the lateral wall of
the orbit and the lateral-inferior margins of the anterior orbital opening (see Figure 7.4). The short temporal process of the
zygomatic bone projects posteriorly, where it forms the anterior portion of the zygomatic arch (see Figure 7.5).
Nasal Bone
The nasal bone is one of two small bones that articulate (join) with each other to form the bony base (bridge) of the nose.
They also support the cartilages that form the lateral walls of the nose (see Figure 7.11). These are the bones that are
damaged when the nose is broken.
Lacrimal Bone
Each lacrimal bone is a small, rectangular bone that forms the anterior, medial wall of the orbit (see Figure 7.4 and Figure
7.5). The anterior portion of the lacrimal bone forms a shallow depression called the lacrimal fossa, and extending inferiorly
from this is the nasolacrimal canal. The lacrimal fluid (tears of the eye), which serves to maintain the moist surface of the
eye, drains at the medial corner of the eye into the nasolacrimal canal. This duct then extends downward to open into the
nasal cavity, behind the inferior nasal concha. In the nasal cavity, the lacrimal fluid normally drains posteriorly, but with an
increased flow of tears due to crying or eye irritation, some fluid will also drain anteriorly, thus causing a runny nose.
Inferior Nasal Conchae
The right and left inferior nasal conchae form a curved bony plate that projects into the nasal cavity space from the lower
lateral wall (see Figure 7.13). The inferior concha is the largest of the nasal conchae and can easily be seen when looking
into the anterior opening of the nasal cavity.
Vomer Bone
The unpaired vomer bone, often referred to simply as the vomer, is triangular-shaped and forms the posterior-inferior part
of the nasal septum (see Figure 7.11). The vomer is best seen when looking from behind into the posterior openings of the
nasal cavity (see Figure 7.8a). In this view, the vomer is seen to form the entire height of the nasal septum. A much smaller
portion of the vomer can also be seen when looking into the anterior opening of the nasal cavity.
Mandible
The mandible forms the lower jaw and is the only moveable bone of the skull. At the time of birth, the mandible consists
of paired right and left bones, but these fuse together during the first year to form the single U-shaped mandible of the adult
skull. Each side of the mandible consists of a horizontal body and posteriorly, a vertically oriented ramus of the mandible
(ramus = “branch”). The outside margin of the mandible, where the body and ramus come together is called the angle of
the mandible (Figure 7.15).
The ramus on each side of the mandible has two upward-going bony projections. The more anterior projection is the
flattened coronoid process of the mandible, which provides attachment for one of the biting muscles. The posterior
projection is the condylar process of the mandible, which is topped by the oval-shaped condyle. The condyle of
the mandible articulates (joins) with the mandibular fossa and articular tubercle of the temporal bone. Together these
articulations form the temporomandibular joint, which allows for opening and closing of the mouth (see Figure 7.5). The
broad U-shaped curve located between the coronoid and condylar processes is the mandibular notch.
Important landmarks for the mandible include the following:
• Alveolar process of the mandible—This is the upper border of the mandibular body and serves to anchor the lower
teeth.


• Mental protuberance—The forward projection from the inferior margin of the anterior mandible that forms the chin
(mental = “chin”).


• Mental foramen—The opening located on each side of the anterior-lateral mandible, which is the exit site for a
sensory nerve that supplies the chin.


• Mylohyoid line—This bony ridge extends along the inner aspect of the mandibular body (see Figure 7.11). The
muscle that forms the floor of the oral cavity attaches to the mylohyoid lines on both sides of the mandible.


• Mandibular foramen—This opening is located on the medial side of the ramus of the mandible. The opening leads
into a tunnel that runs down the length of the mandibular body. The sensory nerve and blood vessels that supply the
lower teeth enter the mandibular foramen and then follow this tunnel. Thus, to numb the lower teeth prior to dental
work, the dentist must inject anesthesia into the lateral wall of the oral cavity at a point prior to where this sensory
nerve enters the mandibular foramen.


• Lingula—This small flap of bone is named for its shape (lingula = “little tongue”). It is located immediately next to
the mandibular foramen, on the medial side of the ramus. A ligament that anchors the mandible during opening and
closing of the mouth extends down from the base of the skull and attaches to the lingula.


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Figure 7.15 Isolated Mandible The mandible is the only moveable bone of the skull.


The Orbit
The orbit is the bony socket that houses the eyeball and contains the muscles that move the eyeball or open the upper eyelid.
Each orbit is cone-shaped, with a narrow posterior region that widens toward the large anterior opening. To help protect the
eye, the bony margins of the anterior opening are thickened and somewhat constricted. The medial walls of the two orbits
are parallel to each other but each lateral wall diverges away from the midline at a 45° angle. This divergence provides
greater lateral peripheral vision.
The walls of each orbit include contributions from seven skull bones (Figure 7.16). The frontal bone forms the roof and
the zygomatic bone forms the lateral wall and lateral floor. The medial floor is primarily formed by the maxilla, with a
small contribution from the palatine bone. The ethmoid bone and lacrimal bone make up much of the medial wall and the
sphenoid bone forms the posterior orbit.
At the posterior apex of the orbit is the opening of the optic canal, which allows for passage of the optic nerve from the
retina to the brain. Lateral to this is the elongated and irregularly shaped superior orbital fissure, which provides passage for
the artery that supplies the eyeball, sensory nerves, and the nerves that supply the muscles involved in eye movements.


Figure 7.16 Bones of the Orbit Seven skull bones contribute to the walls of the orbit. Opening into the posterior orbit
from the cranial cavity are the optic canal and superior orbital fissure.


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The Nasal Septum and Nasal Conchae
The nasal septum consists of both bone and cartilage components (Figure 7.17; see also Figure 7.11). The upper portion of
the septum is formed by the perpendicular plate of the ethmoid bone. The lower and posterior parts of the septum are formed
by the triangular-shaped vomer bone. In an anterior view of the skull, the perpendicular plate of the ethmoid bone is easily
seen inside the nasal opening as the upper nasal septum, but only a small portion of the vomer is seen as the inferior septum.
A better view of the vomer bone is seen when looking into the posterior nasal cavity with an inferior view of the skull,
where the vomer forms the full height of the nasal septum. The anterior nasal septum is formed by the septal cartilage,
a flexible plate that fills in the gap between the perpendicular plate of the ethmoid and vomer bones. This cartilage also
extends outward into the nose where it separates the right and left nostrils. The septal cartilage is not found in the dry skull.
Attached to the lateral wall on each side of the nasal cavity are the superior, middle, and inferior nasal conchae (singular =
concha), which are named for their positions (see Figure 7.13). These are bony plates that curve downward as they project
into the space of the nasal cavity. They serve to swirl the incoming air, which helps to warm and moisturize it before the air
moves into the delicate air sacs of the lungs. This also allows mucus, secreted by the tissue lining the nasal cavity, to trap
incoming dust, pollen, bacteria, and viruses. The largest of the conchae is the inferior nasal concha, which is an independent
bone of the skull. The middle concha and the superior conchae, which is the smallest, are both formed by the ethmoid bone.
When looking into the anterior nasal opening of the skull, only the inferior and middle conchae can be seen. The small
superior nasal concha is well hidden above and behind the middle concha.


Figure 7.17 Nasal Septum The nasal septum is formed by the perpendicular plate of the ethmoid bone and the
vomer bone. The septal cartilage fills the gap between these bones and extends into the nose.


Cranial Fossae
Inside the skull, the floor of the cranial cavity is subdivided into three cranial fossae (spaces), which increase in depth from
anterior to posterior (see Figure 7.6, Figure 7.8b, and Figure 7.11). Since the brain occupies these areas, the shape of each
conforms to the shape of the brain regions that it contains. Each cranial fossa has anterior and posterior boundaries and is
divided at the midline into right and left areas by a significant bony structure or opening.
Anterior Cranial Fossa
The anterior cranial fossa is the most anterior and the shallowest of the three cranial fossae. It overlies the orbits and contains
the frontal lobes of the brain. Anteriorly, the anterior fossa is bounded by the frontal bone, which also forms the majority of
the floor for this space. The lesser wings of the sphenoid bone form the prominent ledge that marks the boundary between
the anterior and middle cranial fossae. Located in the floor of the anterior cranial fossa at the midline is a portion of the
ethmoid bone, consisting of the upward projecting crista galli and to either side of this, the cribriform plates.


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Middle Cranial Fossa
The middle cranial fossa is deeper and situated posterior to the anterior fossa. It extends from the lesser wings of the
sphenoid bone anteriorly, to the petrous ridges (petrous portion of the temporal bones) posteriorly. The large, diagonally
positioned petrous ridges give the middle cranial fossa a butterfly shape, making it narrow at the midline and broad laterally.
The temporal lobes of the brain occupy this fossa. The middle cranial fossa is divided at the midline by the upward bony
prominence of the sella turcica, a part of the sphenoid bone. The middle cranial fossa has several openings for the passage
of blood vessels and cranial nerves (see Figure 7.8).
Openings in the middle cranial fossa are as follows:
• Optic canal—This opening is located at the anterior lateral corner of the sella turcica. It provides for passage of the
optic nerve into the orbit.


• Superior orbital fissure—This large, irregular opening into the posterior orbit is located on the anterior wall of the
middle cranial fossa, lateral to the optic canal and under the projecting margin of the lesser wing of the sphenoid bone.
Nerves to the eyeball and associated muscles, and sensory nerves to the forehead pass through this opening.


• Foramen rotundum—This rounded opening (rotundum = “round”) is located in the floor of the middle cranial fossa,
just inferior to the superior orbital fissure. It is the exit point for a major sensory nerve that supplies the cheek, nose,
and upper teeth.


• Foramen ovale of the middle cranial fossa—This large, oval-shaped opening in the floor of the middle cranial fossa
provides passage for a major sensory nerve to the lateral head, cheek, chin, and lower teeth.


• Foramen spinosum—This small opening, located posterior-lateral to the foramen ovale, is the entry point for an
important artery that supplies the covering layers surrounding the brain. The branching pattern of this artery forms
readily visible grooves on the internal surface of the skull and these grooves can be traced back to their origin at the
foramen spinosum.


• Carotid canal—This is the zig-zag passageway through which a major artery to the brain enters the skull. The
entrance to the carotid canal is located on the inferior aspect of the skull, anteromedial to the styloid process (see
Figure 7.8a). From here, the canal runs anteromedially within the bony base of the skull. Just above the foramen
lacerum, the carotid canal opens into the middle cranial cavity, near the posterior-lateral base of the sella turcica.


• Foramen lacerum—This irregular opening is located in the base of the skull, immediately inferior to the exit of the
carotid canal. This opening is an artifact of the dry skull, because in life it is completely filled with cartilage. All
the openings of the skull that provide for passage of nerves or blood vessels have smooth margins; the word lacerum
(“ragged” or “torn”) tells us that this opening has ragged edges and thus nothing passes through it.


Posterior Cranial Fossa
The posterior cranial fossa is the most posterior and deepest portion of the cranial cavity. It contains the cerebellum of the
brain. The posterior fossa is bounded anteriorly by the petrous ridges, while the occipital bone forms the floor and posterior
wall. It is divided at the midline by the large foramen magnum (“great aperture”), the opening that provides for passage of
the spinal cord.
Located on the medial wall of the petrous ridge in the posterior cranial fossa is the internal acoustic meatus (see Figure
7.11). This opening provides for passage of the nerve from the hearing and equilibrium organs of the inner ear, and the
nerve that supplies the muscles of the face. Located at the anterior-lateral margin of the foramen magnum is the hypoglossal
canal. These emerge on the inferior aspect of the skull at the base of the occipital condyle and provide passage for an
important nerve to the tongue.
Immediately inferior to the internal acoustic meatus is the large, irregularly shaped jugular foramen (see Figure 7.8a).
Several cranial nerves from the brain exit the skull via this opening. It is also the exit point through the base of the skull
for all the venous return blood leaving the brain. The venous structures that carry blood inside the skull form large, curved
grooves on the inner walls of the posterior cranial fossa, which terminate at each jugular foramen.


Paranasal Sinuses
The paranasal sinuses are hollow, air-filled spaces located within certain bones of the skull (Figure 7.18). All of the sinuses
communicate with the nasal cavity (paranasal = “next to nasal cavity”) and are lined with nasal mucosa. They serve to
reduce bone mass and thus lighten the skull, and they also add resonance to the voice. This second feature is most obvious
when you have a cold or sinus congestion. These produce swelling of the mucosa and excess mucus production, which
can obstruct the narrow passageways between the sinuses and the nasal cavity, causing your voice to sound different to
yourself and others. This blockage can also allow the sinuses to fill with fluid, with the resulting pressure producing pain
and discomfort.
The paranasal sinuses are named for the skull bone that each occupies. The frontal sinus is located just above the eyebrows,
within the frontal bone (see Figure 7.17). This irregular space may be divided at the midline into bilateral spaces, or these
may be fused into a single sinus space. The frontal sinus is the most anterior of the paranasal sinuses. The largest sinus is


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the maxillary sinus. These are paired and located within the right and left maxillary bones, where they occupy the area
just below the orbits. The maxillary sinuses are most commonly involved during sinus infections. Because their connection
to the nasal cavity is located high on their medial wall, they are difficult to drain. The sphenoid sinus is a single, midline
sinus. It is located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, thus making it the
most posterior of the paranasal sinuses. The lateral aspects of the ethmoid bone contain multiple small spaces separated by
very thin bony walls. Each of these spaces is called an ethmoid air cell. These are located on both sides of the ethmoid
bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae.


Figure 7.18 Paranasal Sinuses The paranasal sinuses are hollow, air-filled spaces named for the skull bone that
each occupies. The most anterior is the frontal sinus, located in the frontal bone above the eyebrows. The largest are
the maxillary sinuses, located in the right and left maxillary bones below the orbits. The most posterior is the sphenoid
sinus, located in the body of the sphenoid bone, under the sella turcica. The ethmoid air cells are multiple small spaces
located in the right and left sides of the ethmoid bone, between the medial wall of the orbit and lateral wall of the upper
nasal cavity.


Hyoid Bone
The hyoid bone is an independent bone that does not contact any other bone and thus is not part of the skull (Figure 7.19).
It is a small U-shaped bone located in the upper neck near the level of the inferior mandible, with the tips of the “U”
pointing posteriorly. The hyoid serves as the base for the tongue above, and is attached to the larynx below and the pharynx
posteriorly. The hyoid is held in position by a series of small muscles that attach to it either from above or below. These
muscles act to move the hyoid up/down or forward/back. Movements of the hyoid are coordinated with movements of the
tongue, larynx, and pharynx during swallowing and speaking.


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Figure 7.19 Hyoid Bone The hyoid bone is located in the upper neck and does not join with any other bone. It
provides attachments for muscles that act on the tongue, larynx, and pharynx.


7.3 | The Vertebral Column
By the end of this section, you will be able to:
• Describe each region of the vertebral column and the number of bones in each region
• Discuss the curves of the vertebral column and how these change after birth
• Describe a typical vertebra and determine the distinguishing characteristics for vertebrae in each vertebral region
and features of the sacrum and the coccyx


• Define the structure of an intervertebral disc
• Determine the location of the ligaments that provide support for the vertebral column


The vertebral column is also known as the spinal column or spine (Figure 7.20). It consists of a sequence of vertebrae
(singular = vertebra), each of which is separated and united by an intervertebral disc. Together, the vertebrae and
intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for
their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae.


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Figure 7.20 Vertebral Column The adult vertebral column consists of 24 vertebrae, plus the sacrum and coccyx.
The vertebrae are divided into three regions: cervical C1–C7 vertebrae, thoracic T1–T12 vertebrae, and lumbar L1–L5
vertebrae. The vertebral column is curved, with two primary curvatures (thoracic and sacrococcygeal curves) and two
secondary curvatures (cervical and lumbar curves).


Regions of the Vertebral Column
The vertebral column originally develops as a series of 33 vertebrae, but this number is eventually reduced to 24 vertebrae,
plus the sacrum and coccyx. The vertebral column is subdivided into five regions, with the vertebrae in each area named for
that region and numbered in descending order. In the neck, there are seven cervical vertebrae, each designated with the letter
“C” followed by its number. Superiorly, the C1 vertebra articulates (forms a joint) with the occipital condyles of the skull.
Inferiorly, C1 articulates with the C2 vertebra, and so on. Below these are the 12 thoracic vertebrae, designated T1–T12.
The lower back contains the L1–L5 lumbar vertebrae. The single sacrum, which is also part of the pelvis, is formed by the
fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results from the fusion of four small coccygeal vertebrae.
However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age.
An interesting anatomical fact is that almost all mammals have seven cervical vertebrae, regardless of body size. This means
that there are large variations in the size of cervical vertebrae, ranging from the very small cervical vertebrae of a shrew to
the greatly elongated vertebrae in the neck of a giraffe. In a full-grown giraffe, each cervical vertebra is 11 inches tall.


Curvatures of the Vertebral Column
The adult vertebral column does not form a straight line, but instead has four curvatures along its length (see Figure
7.20). These curves increase the vertebral column’s strength, flexibility, and ability to absorb shock. When the load on the
spine is increased, by carrying a heavy backpack for example, the curvatures increase in depth (become more curved) to
accommodate the extra weight. They then spring back when the weight is removed. The four adult curvatures are classified
as either primary or secondary curvatures. Primary curves are retained from the original fetal curvature, while secondary
curvatures develop after birth.
During fetal development, the body is flexed anteriorly into the fetal position, giving the entire vertebral column a single
curvature that is concave anteriorly. In the adult, this fetal curvature is retained in two regions of the vertebral column as the
thoracic curve, which involves the thoracic vertebrae, and the sacrococcygeal curve, formed by the sacrum and coccyx.


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Each of these is thus called a primary curve because they are retained from the original fetal curvature of the vertebral
column.
A secondary curve develops gradually after birth as the child learns to sit upright, stand, and walk. Secondary curves are
concave posteriorly, opposite in direction to the original fetal curvature. The cervical curve of the neck region develops as
the infant begins to hold their head upright when sitting. Later, as the child begins to stand and then to walk, the lumbar
curve of the lower back develops. In adults, the lumbar curve is generally deeper in females.
Disorders associated with the curvature of the spine include kyphosis (an excessive posterior curvature of the thoracic
region), lordosis (an excessive anterior curvature of the lumbar region), and scoliosis (an abnormal, lateral curvature,
accompanied by twisting of the vertebral column).


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Vertebral Column
Developmental anomalies, pathological changes, or obesity can enhance the normal vertebral column curves, resulting
in the development of abnormal or excessive curvatures (Figure 7.21). Kyphosis, also referred to as humpback or
hunchback, is an excessive posterior curvature of the thoracic region. This can develop when osteoporosis causes
weakening and erosion of the anterior portions of the upper thoracic vertebrae, resulting in their gradual collapse
(Figure 7.22). Lordosis, or swayback, is an excessive anterior curvature of the lumbar region and is most commonly
associated with obesity or late pregnancy. The accumulation of body weight in the abdominal region results an anterior
shift in the line of gravity that carries the weight of the body. This causes in an anterior tilt of the pelvis and a
pronounced enhancement of the lumbar curve.
Scoliosis is an abnormal, lateral curvature, accompanied by twisting of the vertebral column. Compensatory curves
may also develop in other areas of the vertebral column to help maintain the head positioned over the feet. Scoliosis is
the most common vertebral abnormality among girls. The cause is usually unknown, but it may result from weakness
of the back muscles, defects such as differential growth rates in the right and left sides of the vertebral column, or
differences in the length of the lower limbs. When present, scoliosis tends to get worse during adolescent growth
spurts. Although most individuals do not require treatment, a back brace may be recommended for growing children.
In extreme cases, surgery may be required.
Excessive vertebral curves can be identified while an individual stands in the anatomical position. Observe the
vertebral profile from the side and then from behind to check for kyphosis or lordosis. Then have the person bend
forward. If scoliosis is present, an individual will have difficulty in bending directly forward, and the right and left
sides of the back will not be level with each other in the bent position.


Figure 7.21 Abnormal Curvatures of the Vertebral Column (a) Scoliosis is an abnormal lateral bending of
the vertebral column. (b) An excessive curvature of the upper thoracic vertebral column is called kyphosis. (c)
Lordosis is an excessive curvature in the lumbar region of the vertebral column.


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Figure 7.22 Osteoporosis Osteoporosis is an age-related disorder that causes the gradual loss of bone density
and strength. When the thoracic vertebrae are affected, there can be a gradual collapse of the vertebrae. This
results in kyphosis, an excessive curvature of the thoracic region.


Osteoporosis is a common age-related bone disease in which bone density and strength is decreased. Watch this
video (http://openstaxcollege.org/l/osteoporosis) to get a better understanding of how thoracic vertebrae may become
weakened and may fracture due to this disease. How may vertebral osteoporosis contribute to kyphosis?


General Structure of a Vertebra
Within the different regions of the vertebral column, vertebrae vary in size and shape, but they all follow a similar structural
pattern. A typical vertebra will consist of a body, a vertebral arch, and seven processes (Figure 7.23).
The body is the anterior portion of each vertebra and is the part that supports the body weight. Because of this, the vertebral
bodies progressively increase in size and thickness going down the vertebral column. The bodies of adjacent vertebrae are
separated and strongly united by an intervertebral disc.
The vertebral arch forms the posterior portion of each vertebra. It consists of four parts, the right and left pedicles and the
right and left laminae. Each pedicle forms one of the lateral sides of the vertebral arch. The pedicles are anchored to the
posterior side of the vertebral body. Each lamina forms part of the posterior roof of the vertebral arch. The large opening
between the vertebral arch and body is the vertebral foramen, which contains the spinal cord. In the intact vertebral
column, the vertebral foramina of all of the vertebrae align to form the vertebral (spinal) canal, which serves as the bony
protection and passageway for the spinal cord down the back. When the vertebrae are aligned together in the vertebral
column, notches in the margins of the pedicles of adjacent vertebrae together form an intervertebral foramen, the opening
through which a spinal nerve exits from the vertebral column (Figure 7.24).
Seven processes arise from the vertebral arch. Each paired transverse process projects laterally and arises from the junction
point between the pedicle and lamina. The single spinous process (vertebral spine) projects posteriorly at the midline of
the back. The vertebral spines can easily be felt as a series of bumps just under the skin down the middle of the back.
The transverse and spinous processes serve as important muscle attachment sites. A superior articular process extends


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or faces upward, and an inferior articular process faces or projects downward on each side of a vertebrae. The paired
superior articular processes of one vertebra join with the corresponding paired inferior articular processes from the next
higher vertebra. These junctions form slightly moveable joints between the adjacent vertebrae. The shape and orientation of
the articular processes vary in different regions of the vertebral column and play a major role in determining the type and
range of motion available in each region.


Figure 7.23 Parts of a Typical Vertebra A typical vertebra consists of a body and a vertebral arch. The arch is
formed by the paired pedicles and paired laminae. Arising from the vertebral arch are the transverse, spinous, superior
articular, and inferior articular processes. The vertebral foramen provides for passage of the spinal cord. Each spinal
nerve exits through an intervertebral foramen, located between adjacent vertebrae. Intervertebral discs unite the
bodies of adjacent vertebrae.


Figure 7.24 Intervertebral Disc The bodies of adjacent vertebrae are separated and united by an intervertebral disc,
which provides padding and allows for movements between adjacent vertebrae. The disc consists of a fibrous outer
layer called the anulus fibrosus and a gel-like center called the nucleus pulposus. The intervertebral foramen is the
opening formed between adjacent vertebrae for the exit of a spinal nerve.


Regional Modifications of Vertebrae
In addition to the general characteristics of a typical vertebra described above, vertebrae also display characteristic size
and structural features that vary between the different vertebral column regions. Thus, cervical vertebrae are smaller than
lumbar vertebrae due to differences in the proportion of body weight that each supports. Thoracic vertebrae have sites for
rib attachment, and the vertebrae that give rise to the sacrum and coccyx have fused together into single bones.
Cervical Vertebrae
Typical cervical vertebrae, such as C4 or C5, have several characteristic features that differentiate them from thoracic or
lumbar vertebrae (Figure 7.25). Cervical vertebrae have a small body, reflecting the fact that they carry the least amount
of body weight. Cervical vertebrae usually have a bifid (Y-shaped) spinous process. The spinous processes of the C3–C6
vertebrae are short, but the spine of C7 is much longer. You can find these vertebrae by running your finger down the
midline of the posterior neck until you encounter the prominent C7 spine located at the base of the neck. The transverse


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processes of the cervical vertebrae are sharply curved (U-shaped) to allow for passage of the cervical spinal nerves. Each
transverse process also has an opening called the transverse foramen. An important artery that supplies the brain ascends
up the neck by passing through these openings. The superior and inferior articular processes of the cervical vertebrae are
flattened and largely face upward or downward, respectively.
The first and second cervical vertebrae are further modified, giving each a distinctive appearance. The first cervical (C1)
vertebra is also called the atlas, because this is the vertebra that supports the skull on top of the vertebral column (in Greek
mythology, Atlas was the god who supported the heavens on his shoulders). The C1 vertebra does not have a body or
spinous process. Instead, it is ring-shaped, consisting of an anterior arch and a posterior arch. The transverse processes of
the atlas are longer and extend more laterally than do the transverse processes of any other cervical vertebrae. The superior
articular processes face upward and are deeply curved for articulation with the occipital condyles on the base of the skull.
The inferior articular processes are flat and face downward to join with the superior articular processes of the C2 vertebra.
The second cervical (C2) vertebra is called the axis, because it serves as the axis for rotation when turning the head toward
the right or left. The axis resembles typical cervical vertebrae in most respects, but is easily distinguished by the dens
(odontoid process), a bony projection that extends upward from the vertebral body. The dens joins with the inner aspect of
the anterior arch of the atlas, where it is held in place by transverse ligament.


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Figure 7.25 Cervical Vertebrae A typical cervical vertebra has a small body, a bifid spinous process, transverse
processes that have a transverse foramen and are curved for spinal nerve passage. The atlas (C1 vertebra) does not
have a body or spinous process. It consists of an anterior and a posterior arch and elongated transverse processes.
The axis (C2 vertebra) has the upward projecting dens, which articulates with the anterior arch of the atlas.


Thoracic Vertebrae
The bodies of the thoracic vertebrae are larger than those of cervical vertebrae (Figure 7.26). The characteristic feature
for a typical midthoracic vertebra is the spinous process, which is long and has a pronounced downward angle that causes
it to overlap the next inferior vertebra. The superior articular processes of thoracic vertebrae face anteriorly and the inferior
processes face posteriorly. These orientations are important determinants for the type and range of movements available to
the thoracic region of the vertebral column.
Thoracic vertebrae have several additional articulation sites, each of which is called a facet, where a rib is attached. Most
thoracic vertebrae have two facets located on the lateral sides of the body, each of which is called a costal facet (costal =
“rib”). These are for articulation with the head (end) of a rib. An additional facet is located on the transverse process for
articulation with the tubercle of a rib.


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Figure 7.26 Thoracic Vertebrae A typical thoracic vertebra is distinguished by the spinous process, which is long and
projects downward to overlap the next inferior vertebra. It also has articulation sites (facets) on the vertebral body and
a transverse process for rib attachment.


Figure 7.27 Rib Articulation in Thoracic Vertebrae Thoracic vertebrae have superior and inferior articular facets on
the vertebral body for articulation with the head of a rib, and a transverse process facet for articulation with the rib
tubercle.


Lumbar Vertebrae
Lumbar vertebrae carry the greatest amount of body weight and are thus characterized by the large size and thickness
of the vertebral body (Figure 7.28). They have short transverse processes and a short, blunt spinous process that projects
posteriorly. The articular processes are large, with the superior process facing backward and the inferior facing forward.


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Figure 7.28 Lumbar Vertebrae Lumbar vertebrae are characterized by having a large, thick body and a short,
rounded spinous process.


Sacrum and Coccyx
The sacrum is a triangular-shaped bone that is thick and wide across its superior base where it is weight bearing and then
tapers down to an inferior, non-weight bearing apex (Figure 7.29). It is formed by the fusion of five sacral vertebrae, a
process that does not begin until after the age of 20. On the anterior surface of the older adult sacrum, the lines of vertebral
fusion can be seen as four transverse ridges. On the posterior surface, running down the midline, is themedian sacral crest,
a bumpy ridge that is the remnant of the fused spinous processes (median = “midline”; while medial = “toward, but not
necessarily at, the midline”). Similarly, the fused transverse processes of the sacral vertebrae form the lateral sacral crest.
The sacral promontory is the anterior lip of the superior base of the sacrum. Lateral to this is the roughened auricular
surface, which joins with the ilium portion of the hipbone to form the immobile sacroiliac joints of the pelvis. Passing
inferiorly through the sacrum is a bony tunnel called the sacral canal, which terminates at the sacral hiatus near the inferior
tip of the sacrum. The anterior and posterior surfaces of the sacrum have a series of paired openings called sacral foramina
(singular = foramen) that connect to the sacral canal. Each of these openings is called a posterior (dorsal) sacral foramen
or anterior (ventral) sacral foramen. These openings allow for the anterior and posterior branches of the sacral spinal
nerves to exit the sacrum. The superior articular process of the sacrum, one of which is found on either side of the
superior opening of the sacral canal, articulates with the inferior articular processes from the L5 vertebra.
The coccyx, or tailbone, is derived from the fusion of four very small coccygeal vertebrae (see Figure 7.29). It articulates
with the inferior tip of the sacrum. It is not weight bearing in the standing position, but may receive some body weight when
sitting.


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Figure 7.29 Sacrum and Coccyx The sacrum is formed from the fusion of five sacral vertebrae, whose lines of fusion
are indicated by the transverse ridges. The fused spinous processes form the median sacral crest, while the lateral
sacral crest arises from the fused transverse processes. The coccyx is formed by the fusion of four small coccygeal
vertebrae.


Intervertebral Discs and Ligaments of the Vertebral Column
The bodies of adjacent vertebrae are strongly anchored to each other by an intervertebral disc. This structure provides
padding between the bones during weight bearing, and because it can change shape, also allows for movement between
the vertebrae. Although the total amount of movement available between any two adjacent vertebrae is small, when these
movements are summed together along the entire length of the vertebral column, large body movements can be produced.
Ligaments that extend along the length of the vertebral column also contribute to its overall support and stability.
Intervertebral Disc
An intervertebral disc is a fibrocartilaginous pad that fills the gap between adjacent vertebral bodies (see Figure 7.24).
Each disc is anchored to the bodies of its adjacent vertebrae, thus strongly uniting these. The discs also provide padding
between vertebrae during weight bearing. Because of this, intervertebral discs are thin in the cervical region and thickest
in the lumbar region, which carries the most body weight. In total, the intervertebral discs account for approximately 25
percent of your body height between the top of the pelvis and the base of the skull. Intervertebral discs are also flexible and
can change shape to allow for movements of the vertebral column.
Each intervertebral disc consists of two parts. The anulus fibrosus is the tough, fibrous outer layer of the disc. It forms a
circle (anulus = “ring” or “circle”) and is firmly anchored to the outer margins of the adjacent vertebral bodies. Inside is the
nucleus pulposus, consisting of a softer, more gel-like material. It has a high water content that serves to resist compression
and thus is important for weight bearing. With increasing age, the water content of the nucleus pulposus gradually declines.
This causes the disc to become thinner, decreasing total body height somewhat, and reduces the flexibility and range of
motion of the disc, making bending more difficult.
The gel-like nature of the nucleus pulposus also allows the intervertebral disc to change shape as one vertebra rocks side
to side or forward and back in relation to its neighbors during movements of the vertebral column. Thus, bending forward
causes compression of the anterior portion of the disc but expansion of the posterior disc. If the posterior anulus fibrosus is
weakened due to injury or increasing age, the pressure exerted on the disc when bending forward and lifting a heavy object
can cause the nucleus pulposus to protrude posteriorly through the anulus fibrosus, resulting in a herniated disc (“ruptured”
or “slipped” disc) (Figure 7.30). The posterior bulging of the nucleus pulposus can cause compression of a spinal nerve
at the point where it exits through the intervertebral foramen, with resulting pain and/or muscle weakness in those body
regions supplied by that nerve. The most common sites for disc herniation are the L4/L5 or L5/S1 intervertebral discs, which
can cause sciatica, a widespread pain that radiates from the lower back down the thigh and into the leg. Similar injuries of
the C5/C6 or C6/C7 intervertebral discs, following forcible hyperflexion of the neck from a collision accident or football
injury, can produce pain in the neck, shoulder, and upper limb.


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Figure 7.30 Herniated Intervertebral Disc Weakening of the anulus fibrosus can result in herniation (protrusion) of
the nucleus pulposus and compression of a spinal nerve, resulting in pain and/or muscle weakness in the body regions
supplied by that nerve.


Watch this animation (http://openstaxcollege.org/l/diskslip) to see what it means to “slip” a disk. Watch this second
animation (http://openstaxcollege.org/l/herndisc) to see one possible treatment for a herniated disc, removing and
replacing the damaged disc with an artificial one that allows for movement between the adjacent certebrae. How could
lifting a heavy object produce pain in a lower limb?


Ligaments of the Vertebral Column
Adjacent vertebrae are united by ligaments that run the length of the vertebral column along both its posterior and anterior
aspects (Figure 7.31). These serve to resist excess forward or backward bending movements of the vertebral column,
respectively.
The anterior longitudinal ligament runs down the anterior side of the entire vertebral column, uniting the vertebral
bodies. It serves to resist excess backward bending of the vertebral column. Protection against this movement is particularly
important in the neck, where extreme posterior bending of the head and neck can stretch or tear this ligament, resulting in a
painful whiplash injury. Prior to the mandatory installation of seat headrests, whiplash injuries were common for passengers
involved in a rear-end automobile collision.
The supraspinous ligament is located on the posterior side of the vertebral column, where it interconnects the spinous
processes of the thoracic and lumbar vertebrae. This strong ligament supports the vertebral column during forward bending
motions. In the posterior neck, where the cervical spinous processes are short, the supraspinous ligament expands to become
the nuchal ligament (nuchae = “nape” or “back of the neck”). The nuchal ligament is attached to the cervical spinous
processes and extends upward and posteriorly to attach to the midline base of the skull, out to the external occipital
protuberance. It supports the skull and prevents it from falling forward. This ligament is much larger and stronger in four-
legged animals such as cows, where the large skull hangs off the front end of the vertebral column. You can easily feel this
ligament by first extending your head backward and pressing down on the posterior midline of your neck. Then tilt your
head forward and you will fill the nuchal ligament popping out as it tightens to limit anterior bending of the head and neck.
Additional ligaments are located inside the vertebral canal, next to the spinal cord, along the length of the vertebral column.
The posterior longitudinal ligament is found anterior to the spinal cord, where it is attached to the posterior sides of
the vertebral bodies. Posterior to the spinal cord is the ligamentum flavum (“yellow ligament”). This consists of a series


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of short, paired ligaments, each of which interconnects the lamina regions of adjacent vertebrae. The ligamentum flavum
has large numbers of elastic fibers, which have a yellowish color, allowing it to stretch and then pull back. Both of these
ligaments provide important support for the vertebral column when bending forward.


Figure 7.31 Ligaments of Vertebral Column The anterior longitudinal ligament runs the length of the vertebral
column, uniting the anterior sides of the vertebral bodies. The supraspinous ligament connects the spinous processes
of the thoracic and lumbar vertebrae. In the posterior neck, the supraspinous ligament enlarges to form the nuchal
ligament, which attaches to the cervical spinous processes and to the base of the skull.


Use this tool (http://openstaxcollege.org/l/vertcolumn) to identify the bones, intervertebral discs, and ligaments of
the vertebral column. The thickest portions of the anterior longitudinal ligament and the supraspinous ligament are
found in which regions of the vertebral column?


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Chiropractor
Chiropractors are health professionals who use nonsurgical techniques to help patients with musculoskeletal system
problems that involve the bones, muscles, ligaments, tendons, or nervous system. They treat problems such as neck
pain, back pain, joint pain, or headaches. Chiropractors focus on the patient’s overall health and can also provide
counseling related to lifestyle issues, such as diet, exercise, or sleep problems. If needed, they will refer the patient to
other medical specialists.
Chiropractors use a drug-free, hands-on approach for patient diagnosis and treatment. They will perform a physical
exam, assess the patient’s posture and spine, and may perform additional diagnostic tests, including taking X-ray
images. They primarily use manual techniques, such as spinal manipulation, to adjust the patient’s spine or other joints.
They can recommend therapeutic or rehabilitative exercises, and some also include acupuncture, massage therapy, or
ultrasound as part of the treatment program. In addition to those in general practice, some chiropractors specialize in
sport injuries, neurology, orthopaedics, pediatrics, nutrition, internal disorders, or diagnostic imaging.
To become a chiropractor, students must have 3–4 years of undergraduate education, attend an accredited, four-year
Doctor of Chiropractic (D.C.) degree program, and pass a licensure examination to be licensed for practice in their
state. With the aging of the baby-boom generation, employment for chiropractors is expected to increase.


7.4 | The Thoracic Cage
By the end of this section, you will be able to:
• Discuss the components that make up the thoracic cage
• Identify the parts of the sternum and define the sternal angle
• Discuss the parts of a rib and rib classifications


The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of the 12 pairs of ribs with their costal
cartilages and the sternum (Figure 7.32). The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1–T12). The
thoracic cage protects the heart and lungs.


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Figure 7.32 Thoracic Cage The thoracic cage is formed by the (a) sternum and (b) 12 pairs of ribs with their costal
cartilages. The ribs are anchored posteriorly to the 12 thoracic vertebrae. The sternum consists of the manubrium,
body, and xiphoid process. The ribs are classified as true ribs (1–7) and false ribs (8–12). The last two pairs of false
ribs are also known as floating ribs (11–12).


Sternum
The sternum is the elongated bony structure that anchors the anterior thoracic cage. It consists of three parts: the manubrium,
body, and xiphoid process. The manubrium is the wider, superior portion of the sternum. The top of the manubrium has
a shallow, U-shaped border called the jugular (suprasternal) notch. This can be easily felt at the anterior base of the
neck, between the medial ends of the clavicles. The clavicular notch is the shallow depression located on either side at the
superior-lateral margins of the manubrium. This is the site of the sternoclavicular joint, between the sternum and clavicle.
The first ribs also attach to the manubrium.
The elongated, central portion of the sternum is the body. The manubrium and body join together at the sternal angle, so
called because the junction between these two components is not flat, but forms a slight bend. The second rib attaches to
the sternum at the sternal angle. Since the first rib is hidden behind the clavicle, the second rib is the highest rib that can be
identified by palpation. Thus, the sternal angle and second rib are important landmarks for the identification and counting
of the lower ribs. Ribs 3–7 attach to the sternal body.
The inferior tip of the sternum is the xiphoid process. This small structure is cartilaginous early in life, but gradually
becomes ossified starting during middle age.


Ribs
Each rib is a curved, flattened bone that contributes to the wall of the thorax. The ribs articulate posteriorly with the T1–T12
thoracic vertebrae, and most attach anteriorly via their costal cartilages to the sternum. There are 12 pairs of ribs. The ribs
are numbered 1–12 in accordance with the thoracic vertebrae.
Parts of a Typical Rib
The posterior end of a typical rib is called the head of the rib (see Figure 7.27). This region articulates primarily with
the costal facet located on the body of the same numbered thoracic vertebra and to a lesser degree, with the costal facet
located on the body of the next higher vertebra. Lateral to the head is the narrowed neck of the rib. A small bump on the
posterior rib surface is the tubercle of the rib, which articulates with the facet located on the transverse process of the same
numbered vertebra. The remainder of the rib is the body of the rib (shaft). Just lateral to the tubercle is the angle of the rib,
the point at which the rib has its greatest degree of curvature. The angles of the ribs form the most posterior extent of the
thoracic cage. In the anatomical position, the angles align with the medial border of the scapula. A shallow costal groove
for the passage of blood vessels and a nerve is found along the inferior margin of each rib.


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Rib Classifications
The bony ribs do not extend anteriorly completely around to the sternum. Instead, each rib ends in a costal cartilage. These
cartilages are made of hyaline cartilage and can extend for several inches. Most ribs are then attached, either directly or
indirectly, to the sternum via their costal cartilage (see Figure 7.32). The ribs are classified into three groups based on their
relationship to the sternum.
Ribs 1–7 are classified as true ribs (vertebrosternal ribs). The costal cartilage from each of these ribs attaches directly to the
sternum. Ribs 8–12 are called false ribs (vertebrochondral ribs). The costal cartilages from these ribs do not attach directly
to the sternum. For ribs 8–10, the costal cartilages are attached to the cartilage of the next higher rib. Thus, the cartilage
of rib 10 attaches to the cartilage of rib 9, rib 9 then attaches to rib 8, and rib 8 is attached to rib 7. The last two false ribs
(11–12) are also called floating ribs (vertebral ribs). These are short ribs that do not attach to the sternum at all. Instead,
their small costal cartilages terminate within the musculature of the lateral abdominal wall.


7.5 | Embryonic Development of the Axial Skeleton
By the end of this section, you will be able to:
• Discuss the two types of embryonic bone development within the skull
• Describe the development of the vertebral column and thoracic cage


The axial skeleton begins to form during early embryonic development. However, growth, remodeling, and ossification
(bone formation) continue for several decades after birth before the adult skeleton is fully formed. Knowledge of the
developmental processes that give rise to the skeleton is important for understanding the abnormalities that may arise in
skeletal structures.


Development of the Skull
During the third week of embryonic development, a rod-like structure called the notochord develops dorsally along the
length of the embryo. The tissue overlying the notochord enlarges and forms the neural tube, which will give rise to the
brain and spinal cord. By the fourth week, mesoderm tissue located on either side of the notochord thickens and separates
into a repeating series of block-like tissue structures, each of which is called a somite. As the somites enlarge, each one
will split into several parts. The most medial of these parts is called a sclerotome. The sclerotomes consist of an embryonic
tissue called mesenchyme, which will give rise to the fibrous connective tissues, cartilages, and bones of the body.
The bones of the skull arise from mesenchyme during embryonic development in two different ways. The first mechanism
produces the bones that form the top and sides of the brain case. This involves the local accumulation of mesenchymal
cells at the site of the future bone. These cells then differentiate directly into bone producing cells, which form the skull
bones through the process of intramembranous ossification. As the brain case bones grow in the fetal skull, they remain
separated from each other by large areas of dense connective tissue, each of which is called a fontanelle (Figure 7.33).
The fontanelles are the soft spots on an infant’s head. They are important during birth because these areas allow the skull to
change shape as it squeezes through the birth canal. After birth, the fontanelles allow for continued growth and expansion
of the skull as the brain enlarges. The largest fontanelle is located on the anterior head, at the junction of the frontal and
parietal bones. The fontanelles decrease in size and disappear by age 2. However, the skull bones remained separated from
each other at the sutures, which contain dense fibrous connective tissue that unites the adjacent bones. The connective tissue
of the sutures allows for continued growth of the skull bones as the brain enlarges during childhood growth.
The second mechanism for bone development in the skull produces the facial bones and floor of the brain case. This also
begins with the localized accumulation of mesenchymal cells. However, these cells differentiate into cartilage cells, which
produce a hyaline cartilage model of the future bone. As this cartilage model grows, it is gradually converted into bone
through the process of endochondral ossification. This is a slow process and the cartilage is not completely converted to
bone until the skull achieves its full adult size.
At birth, the brain case and orbits of the skull are disproportionally large compared to the bones of the jaws and lower
face. This reflects the relative underdevelopment of the maxilla and mandible, which lack teeth, and the small sizes of the
paranasal sinuses and nasal cavity. During early childhood, the mastoid process enlarges, the two halves of the mandible
and frontal bone fuse together to form single bones, and the paranasal sinuses enlarge. The jaws also expand as the teeth
begin to appear. These changes all contribute to the rapid growth and enlargement of the face during childhood.


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Figure 7.33 Newborn Skull The bones of the newborn skull are not fully ossified and are separated by large areas
called fontanelles, which are filled with fibrous connective tissue. The fontanelles allow for continued growth of the skull
after birth. At the time of birth, the facial bones are small and underdeveloped, and the mastoid process has not yet
formed.


Development of the Vertebral Column and Thoracic cage
Development of the vertebrae begins with the accumulation of mesenchyme cells from each sclerotome around the
notochord. These cells differentiate into a hyaline cartilage model for each vertebra, which then grow and eventually
ossify into bone through the process of endochondral ossification. As the developing vertebrae grow, the notochord largely
disappears. However, small areas of notochord tissue persist between the adjacent vertebrae and this contributes to the
formation of each intervertebral disc.
The ribs and sternum also develop from mesenchyme. The ribs initially develop as part of the cartilage model for each
vertebra, but in the thorax region, the rib portion separates from the vertebra by the eighth week. The cartilage model of
the rib then ossifies, except for the anterior portion, which remains as the costal cartilage. The sternum initially forms as
paired hyaline cartilage models on either side of the anterior midline, beginning during the fifth week of development. The
cartilage models of the ribs become attached to the lateral sides of the developing sternum. Eventually, the two halves of the
cartilaginous sternum fuse together along the midline and then ossify into bone. The manubrium and body of the sternum
are converted into bone first, with the xiphoid process remaining as cartilage until late in life.


View this video (http://openstaxcollege.org/l/skullbones) to review the two processes that give rise to the bones of
the skull and body. What are the two mechanisms by which the bones of the body are formed and which bones are
formed by each mechanism?


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Craniosynostosis
The premature closure (fusion) of a suture line is a condition called craniosynostosis. This error in the normal
developmental process results in abnormal growth of the skull and deformity of the head. It is produced either by
defects in the ossification process of the skull bones or failure of the brain to properly enlarge. Genetic factors are
involved, but the underlying cause is unknown. It is a relatively common condition, occurring in approximately 1:2000
births, with males being more commonly affected. Primary craniosynostosis involves the early fusion of one cranial
suture, whereas complex craniosynostosis results from the premature fusion of several sutures.
The early fusion of a suture in primary craniosynostosis prevents any additional enlargement of the cranial bones and
skull along this line. Continued growth of the brain and skull is therefore diverted to other areas of the head, causing an
abnormal enlargement of these regions. For example, the early disappearance of the anterior fontanelle and premature
closure of the sagittal suture prevents growth across the top of the head. This is compensated by upward growth by the
bones of the lateral skull, resulting in a long, narrow, wedge-shaped head. This condition, known as scaphocephaly,
accounts for approximately 50 percent of craniosynostosis abnormalities. Although the skull is misshapen, the brain
still has adequate room to grow and thus there is no accompanying abnormal neurological development.
In cases of complex craniosynostosis, several sutures close prematurely. The amount and degree of skull deformity is
determined by the location and extent of the sutures involved. This results in more severe constraints on skull growth,
which can alter or impede proper brain growth and development.
Cases of craniosynostosis are usually treated with surgery. A team of physicians will open the skull along the fused
suture, which will then allow the skull bones to resume their growth in this area. In some cases, parts of the skull
will be removed and replaced with an artificial plate. The earlier after birth that surgery is performed, the better the
outcome. After treatment, most children continue to grow and develop normally and do not exhibit any neurological
problems.


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alveolar process of the mandible
alveolar process of the maxilla
angle of the mandible
angle of the rib


anterior (ventral) sacral foramen


anterior arch
anterior cranial fossa


anterior longitudinal ligament


anulus fibrosus


appendicular skeleton


articular tubercle
atlas
axial skeleton
axis
body of the rib
brain case


calvaria
carotid canal


cervical curve


cervical vertebrae
clavicular notch


coccyx


condylar process of the mandible
condyle
coronal suture
coronoid process of the mandible


KEY TERMS
upper border of mandibular body that contains the lower teeth


curved, inferior margin of the maxilla that supports and anchors the upper teeth
rounded corner located at outside margin of the body and ramus junction


portion of rib with greatest curvature; together, the rib angles form the most posterior extent of the
thoracic cage


one of the series of paired openings located on the anterior (ventral) side of the
sacrum


anterior portion of the ring-like C1 (atlas) vertebra
shallowest and most anterior cranial fossa of the cranial base that extends from the frontal bone


to the lesser wing of the sphenoid bone
ligament that runs the length of the vertebral column, uniting the anterior aspects of


the vertebral bodies
tough, fibrous outer portion of an intervertebral disc, which is strongly anchored to the bodies of the


adjacent vertebrae
all bones of the upper and lower limbs, plus the girdle bones that attach each limb to the axial


skeleton
smooth ridge located on the inferior skull, immediately anterior to the mandibular fossa


first cervical (C1) vertebra
central, vertical axis of the body, including the skull, vertebral column, and thoracic cage


second cervical (C2) vertebra
shaft portion of a rib


portion of the skull that contains and protects the brain, consisting of the eight bones that form the cranial
base and rounded upper skull


(also, skullcap) rounded top of the skull
zig-zag tunnel providing passage through the base of the skull for the internal carotid artery to the brain;


begins anteromedial to the styloid process and terminates in the middle cranial cavity, near the posterior-lateral base
of the sella turcica


posteriorly concave curvature of the cervical vertebral column region; a secondary curve of the
vertebral column


seven vertebrae numbered as C1–C7 that are located in the neck region of the vertebral column
paired notches located on the superior-lateral sides of the sternal manubrium, for articulation with the


clavicle
small bone located at inferior end of the adult vertebral column that is formed by the fusion of four coccygeal


vertebrae; also referred to as the “tailbone”
thickened upward projection from posterior margin of mandibular ramus


oval-shaped process located at the top of the condylar process of the mandible
joint that unites the frontal bone to the right and left parietal bones across the top of the skull


flattened upward projection from the anterior margin of the mandibular ramus


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costal cartilage


costal facet
costal groove
cranial cavity
cranium
cribriform plate


crista galli


dens
ear ossicles
ethmoid air cell


ethmoid bone


external acoustic meatus
external occipital protuberance
facet
facial bones
false ribs


floating ribs
fontanelle


foramen lacerum
foramen magnum


foramen ovale of the middle cranial fossa
foramen rotundum


foramen spinosum
frontal bone
frontal sinus
glabella
greater wings of sphenoid bone


hard palate


hyaline cartilage structure attached to the anterior end of each rib that provides for either direct or
indirect attachment of most ribs to the sternum


site on the lateral sides of a thoracic vertebra for articulation with the head of a rib
shallow groove along the inferior margin of a rib that provides passage for blood vessels and a nerve
interior space of the skull that houses the brain


skull
small, flattened areas with numerous small openings, located to either side of the midline in the floor


of the anterior cranial fossa; formed by the ethmoid bone
small upward projection located at the midline in the floor of the anterior cranial fossa; formed by the


ethmoid bone
bony projection (odontoid process) that extends upward from the body of the C2 (axis) vertebra


three small bones located in the middle ear cavity that serve to transmit sound vibrations to the inner ear
one of several small, air-filled spaces located within the lateral sides of the ethmoid bone, between the


orbit and upper nasal cavity
unpaired bone that forms the roof and upper, lateral walls of the nasal cavity, portions of the floor of the


anterior cranial fossa and medial wall of orbit, and the upper portion of the nasal septum
ear canal opening located on the lateral side of the skull


small bump located at the midline on the posterior skull
small, flattened area on a bone for an articulation (joint) with another bone, or for muscle attachment


fourteen bones that support the facial structures and form the upper and lower jaws and the hard palate
vertebrochondral ribs 8–12 whose costal cartilage either attaches indirectly to the sternum via the costal


cartilage of the next higher rib or does not attach to the sternum at all
vertebral ribs 11–12 that do not attach to the sternum or to the costal cartilage of another rib


expanded area of fibrous connective tissue that separates the brain case bones of the skull prior to birth and
during the first year after birth


irregular opening in the base of the skull, located inferior to the exit of carotid canal
large opening in the occipital bone of the skull through which the spinal cord emerges and the


vertebral arteries enter the cranium
oval-shaped opening in the floor of the middle cranial fossa


round opening in the floor of the middle cranial fossa, located between the superior orbital fissure
and foramen ovale


small opening in the floor of the middle cranial fossa, located lateral to the foramen ovale
unpaired bone that forms forehead, roof of orbit, and floor of anterior cranial fossa
air-filled space within the frontal bone; most anterior of the paranasal sinuses


slight depression of frontal bone, located at the midline between the eyebrows
lateral projections of the sphenoid bone that form the anterior wall of the middle


cranial fossa and an area of the lateral skull
bony structure that forms the roof of the mouth and floor of the nasal cavity, formed by the palatine process


of the maxillary bones and the horizontal plate of the palatine bones


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head of the rib
horizontal plate
hyoid bone
hypoglossal canal


hypophyseal (pituitary) fossa


inferior articular process


inferior nasal concha


infraorbital foramen
infratemporal fossa


internal acoustic meatus
intervertebral disc


intervertebral foramen
jugular (suprasternal) notch
jugular foramen
kyphosis
lacrimal bone
lacrimal fossa


lambdoid suture


lamina
lateral pterygoid plate


lateral sacral crest


lesser wings of the sphenoid bone


ligamentum flavum
lingula
lordosis
lumbar curve


lumbar vertebrae


posterior end of a rib that articulates with the bodies of thoracic vertebrae
medial extension from the palatine bone that forms the posterior quarter of the hard palate


small, U-shaped bone located in upper neck that does not contact any other bone
paired openings that pass anteriorly from the anterior-lateral margins of the foramen magnum deep


to the occipital condyles
shallow depression on top of the sella turcica that houses the pituitary (hypophyseal)


gland
bony process that extends downward from the vertebral arch of a vertebra that articulates


with the superior articular process of the next lower vertebra
one of the paired bones that project from the lateral walls of the nasal cavity to form the largest


and most inferior of the nasal conchae
opening located on anterior skull, below the orbit
space on lateral side of skull, below the level of the zygomatic arch and deep (medial) to the


ramus of the mandible
opening into petrous ridge, located on the lateral wall of the posterior cranial fossa


structure located between the bodies of adjacent vertebrae that strongly joins the vertebrae;
provides padding, weight bearing ability, and enables vertebral column movements


opening located between adjacent vertebrae for exit of a spinal nerve
shallow notch located on superior surface of sternal manubrium


irregularly shaped opening located in the lateral floor of the posterior cranial cavity
(also, humpback or hunchback) excessive posterior curvature of the thoracic vertebral column region


paired bones that contribute to the anterior-medial wall of each orbit
shallow depression in the anterior-medial wall of the orbit, formed by the lacrimal bone that gives rise


to the nasolacrimal canal
inverted V-shaped joint that unites the occipital bone to the right and left parietal bones on the


posterior skull
portion of the vertebral arch on each vertebra that extends between the transverse and spinous process


paired, flattened bony projections of the sphenoid bone located on the inferior skull, lateral to
the medial pterygoid plate


paired irregular ridges running down the lateral sides of the posterior sacrum that was formed by
the fusion of the transverse processes from the five sacral vertebrae


lateral extensions of the sphenoid bone that form the bony lip separating the
anterior and middle cranial fossae


series of short ligaments that unite the lamina of adjacent vertebrae
small flap of bone located on the inner (medial) surface of mandibular ramus, next to the mandibular foramen
(also, swayback) excessive anterior curvature of the lumbar vertebral column region


posteriorly concave curvature of the lumbar vertebral column region; a secondary curve of the vertebral
column


five vertebrae numbered as L1–L5 that are located in lumbar region (lower back) of the vertebral
column


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mandible
mandibular foramen
mandibular fossa
mandibular notch
manubrium
mastoid process
maxillary bone
maxillary sinus
medial pterygoid plate


median sacral crest


mental foramen
mental protuberance
middle cranial fossa


middle nasal concha


mylohyoid line
nasal bone
nasal cavity
nasal conchae


nasal septum


nasolacrimal canal


neck of the rib
notochord


nuchal ligament


nucleus pulposus


occipital bone
occipital condyle
optic canal
orbit
palatine bone


unpaired bone that forms the lower jaw bone; the only moveable bone of the skull
opening located on the inner (medial) surface of the mandibular ramus


oval depression located on the inferior surface of the skull
large U-shaped notch located between the condylar process and coronoid process of the mandible


expanded, superior portion of the sternum
large bony prominence on the inferior, lateral skull, just behind the earlobe


(also, maxilla) paired bones that form the upper jaw and anterior portion of the hard palate
air-filled space located with each maxillary bone; largest of the paranasal sinuses


paired, flattened bony projections of the sphenoid bone located on the inferior skull medial to
the lateral pterygoid plate; form the posterior portion of the nasal cavity lateral wall


irregular ridge running down the midline of the posterior sacrum that was formed from the fusion
of the spinous processes of the five sacral vertebrae


opening located on the anterior-lateral side of the mandibular body
inferior margin of anterior mandible that forms the chin
centrally located cranial fossa that extends from the lesser wings of the sphenoid bone to the


petrous ridge
nasal concha formed by the ethmoid bone that is located between the superior and inferior


conchae
bony ridge located along the inner (medial) surface of the mandibular body


paired bones that form the base of the nose
opening through skull for passage of air
curved bony plates that project from the lateral walls of the nasal cavity; include the superior and


middle nasal conchae, which are parts of the ethmoid bone, and the independent inferior nasal conchae bone
flat, midline structure that divides the nasal cavity into halves, formed by the perpendicular plate of the


ethmoid bone, vomer bone, and septal cartilage
passage for drainage of tears that extends downward from the medial-anterior orbit to the nasal


cavity, terminating behind the inferior nasal conchae
narrowed region of a rib, next to the rib head


rod-like structure along dorsal side of the early embryo; largely disappears during later development but
does contribute to formation of the intervertebral discs


expanded portion of the supraspinous ligament within the posterior neck; interconnects the spinous
processes of the cervical vertebrae and attaches to the base of the skull


gel-like central region of an intervertebral disc; provides for padding, weight-bearing, and
movement between adjacent vertebrae


unpaired bone that forms the posterior portions of the brain case and base of the skull
paired, oval-shaped bony knobs located on the inferior skull, to either side of the foramen magnum


opening spanning between middle cranial fossa and posterior orbit
bony socket that contains the eyeball and associated muscles


paired bones that form the posterior quarter of the hard palate and a small area in floor of the orbit


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palatine process
paranasal sinuses


parietal bone
pedicle
perpendicular plate of the ethmoid bone


petrous ridge


posterior (dorsal) sacral foramen


posterior arch
posterior cranial fossa
posterior longitudinal ligament


primary curve


pterion


ramus of the mandible
ribs
sacral canal
sacral foramina


sacral hiatus
sacral promontory
sacrococcygeal curve


sacrum


sagittal suture
sclerotome


scoliosis
secondary curve


sella turcica
septal cartilage
skeleton


medial projection from the maxilla bone that forms the anterior three quarters of the hard palate
cavities within the skull that are connected to the conchae that serve to warm and humidify


incoming air, produce mucus, and lighten the weight of the skull; consist of frontal, maxillary, sphenoidal, and
ethmoidal sinuses


paired bones that form the upper, lateral sides of the skull
portion of the vertebral arch that extends from the vertebral body to the transverse process


downward, midline extension of the ethmoid bone that forms the
superior portion of the nasal septum


petrous portion of the temporal bone that forms a large, triangular ridge in the floor of the cranial cavity,
separating the middle and posterior cranial fossae; houses the middle and inner ear structures


one of the series of paired openings located on the posterior (dorsal) side of the
sacrum


posterior portion of the ring-like C1 (atlas) vertebra
deepest and most posterior cranial fossa; extends from the petrous ridge to the occipital bone


ligament that runs the length of the vertebral column, uniting the posterior sides of
the vertebral bodies


anteriorly concave curvatures of the thoracic and sacrococcygeal regions that are retained from the
original fetal curvature of the vertebral column
H-shaped suture junction region that unites the frontal, parietal, temporal, and sphenoid bones on the lateral side


of the skull
vertical portion of the mandible


thin, curved bones of the chest wall
bony tunnel that runs through the sacrum
series of paired openings for nerve exit located on both the anterior (ventral) and posterior (dorsal)


aspects of the sacrum
inferior opening and termination of the sacral canal


anterior lip of the base (superior end) of the sacrum
anteriorly concave curvature formed by the sacrum and coccyx; a primary curve of the


vertebral column
single bone located near the inferior end of the adult vertebral column that is formed by the fusion of five sacral


vertebrae; forms the posterior portion of the pelvis
joint that unites the right and left parietal bones at the midline along the top of the skull


medial portion of a somite consisting of mesenchyme tissue that will give rise to bone, cartilage, and
fibrous connective tissues


abnormal lateral curvature of the vertebral column
posteriorly concave curvatures of the cervical and lumbar regions of the vertebral column that


develop after the time of birth
elevated area of sphenoid bone located at midline of the middle cranial fossa
flat cartilage structure that forms the anterior portion of the nasal septum


bones of the body


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skull
somite
sphenoid bone
sphenoid sinus
spinous process
squamous suture


sternal angle


sternum
styloid process
stylomastoid foramen
superior articular process


superior articular process of the sacrum


superior nasal concha
superior nuchal line


superior orbital fissure
supraorbital foramen
supraorbital margin
supraspinous ligament
suture
temporal bone


temporal fossa
temporal process of the zygomatic bone


thoracic cage
thoracic curve


thoracic vertebrae


transverse foramen
transverse process
true ribs
tubercle of the rib


bony structure that forms the head, face, and jaws, and protects the brain; consists of 22 bones
one of the paired, repeating blocks of tissue located on either side of the notochord in the early embryo


unpaired bone that forms the central base of skull
air-filled space located within the sphenoid bone; most posterior of the paranasal sinuses
unpaired bony process that extends posteriorly from the vertebral arch of a vertebra
joint that unites the parietal bone to the squamous portion of the temporal bone on the lateral side of


the skull
junction line between manubrium and body of the sternum and the site for attachment of the second rib to


the sternum
flattened bone located at the center of the anterior chest


downward projecting, elongated bony process located on the inferior aspect of the skull
opening located on inferior skull, between the styloid process and mastoid process


bony process that extends upward from the vertebral arch of a vertebra that articulates
with the inferior articular process of the next higher vertebra


paired processes that extend upward from the sacrum to articulate (join)
with the inferior articular processes from the L5 vertebra


smallest and most superiorly located of the nasal conchae; formed by the ethmoid bone
paired bony lines on the posterior skull that extend laterally from the external occipital


protuberance
irregularly shaped opening between the middle cranial fossa and the posterior orbit
opening located on anterior skull, at the superior margin of the orbit
superior margin of the orbit
ligament that interconnects the spinous processes of the thoracic and lumbar vertebrae


junction line at which adjacent bones of the skull are united by fibrous connective tissue
paired bones that form the lateral, inferior portions of the skull, with squamous, mastoid, and petrous


portions
shallow space on the lateral side of the skull, above the level of the zygomatic arch


short extension from the zygomatic bone that forms the anterior portion
of the zygomatic arch


consists of 12 pairs of ribs and sternum
anteriorly concave curvature of the thoracic vertebral column region; a primary curve of the vertebral


column
twelve vertebrae numbered as T1–T12 that are located in the thoracic region (upper back) of the


vertebral column
opening found only in the transverse processes of cervical vertebrae
paired bony processes that extends laterally from the vertebral arch of a vertebra


vertebrosternal ribs 1–7 that attach via their costal cartilage directly to the sternum
small bump on the posterior side of a rib for articulation with the transverse process of a thoracic


vertebra


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vertebra
vertebral (spinal) canal


vertebral arch
vertebral column
vertebral foramen


vomer bone
xiphoid process
zygomatic arch


zygomatic bone
zygomatic process of the temporal bone


individual bone in the neck and back regions of the vertebral column
bony passageway within the vertebral column for the spinal cord that is formed by the series


of individual vertebral foramina
bony arch formed by the posterior portion of each vertebra that surrounds and protects the spinal cord
entire sequence of bones that extend from the skull to the tailbone
opening associated with each vertebra defined by the vertebral arch that provides passage for the


spinal cord
unpaired bone that forms the inferior and posterior portions of the nasal septum


small process that forms the inferior tip of the sternum
elongated, free-standing arch on the lateral skull, formed anteriorly by the temporal process of the


zygomatic bone and posteriorly by the zygomatic process of the temporal bone
cheekbone; paired bones that contribute to the lateral orbit and anterior zygomatic arch


extension from the temporal bone that forms the posterior portion of the
zygomatic arch


CHAPTER REVIEW
7.1 Divisions of the Skeletal System
The skeletal system includes all of the bones, cartilages, and ligaments of the body. It serves to support the body, protect the
brain and other internal organs, and provides a rigid structure upon which muscles can pull to generate body movements.
It also stores fat and the tissue responsible for the production of blood cells. The skeleton is subdivided into two parts. The
axial skeleton forms a vertical axis that includes the head, neck, back, and chest. It has 80 bones and consists of the skull,
vertebral column, and thoracic cage. The adult vertebral column consists of 24 vertebrae plus the sacrum and coccyx. The
thoracic cage is formed by 12 pairs of ribs and the sternum. The appendicular skeleton consists of 126 bones in the adult
and includes all of the bones of the upper and lower limbs plus the bones that anchor each limb to the axial skeleton.


7.2 The Skull
The skull consists of the brain case and the facial bones. The brain case surrounds and protects the brain, which occupies
the cranial cavity inside the skull. It consists of the rounded calvaria and a complex base. The brain case is formed by eight
bones, the paired parietal and temporal bones plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The narrow
gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The sagittal suture joins the right
and left parietal bones. The coronal suture joins the parietal bones to the frontal bone, the lamboid suture joins them to the
occipital bone, and the squamous suture joins them to the temporal bone.
The facial bones support the facial structures and form the upper and lower jaws. These consist of 14 bones, with the paired
maxillary, palatine, zygomatic, nasal, lacrimal, and inferior conchae bones and the unpaired vomer and mandible bones.
The ethmoid bone also contributes to the formation of facial structures. The maxilla forms the upper jaw and the mandible
forms the lower jaw. The maxilla also forms the larger anterior portion of the hard palate, which is completed by the smaller
palatine bones that form the posterior portion of the hard palate.
The floor of the cranial cavity increases in depth from front to back and is divided into three cranial fossae. The anterior
cranial fossa is located between the frontal bone and lesser wing of the sphenoid bone. A small area of the ethmoid bone,
consisting of the crista galli and cribriform plates, is located at the midline of this fossa. The middle cranial fossa extends
from the lesser wing of the sphenoid bone to the petrous ridge (petrous portion of temporal bone). The right and left sides
are separated at the midline by the sella turcica, which surrounds the shallow hypophyseal fossa. Openings through the skull
in the floor of the middle fossa include the optic canal and superior orbital fissure, which open into the posterior orbit, the
foramen rotundum, foramen ovale, and foramen spinosum, and the exit of the carotid canal with its underlying foramen
lacerum. The deep posterior cranial fossa extends from the petrous ridge to the occipital bone. Openings here include the
large foramen magnum, plus the internal acoustic meatus, jugular foramina, and hypoglossal canals. Additional openings
located on the external base of the skull include the stylomastoid foramen and the entrance to the carotid canal.
The anterior skull has the orbits that house the eyeballs and associated muscles. The walls of the orbit are formed by
contributions from seven bones: the frontal, zygomatic, maxillary, palatine, ethmoid, lacrimal, and sphenoid. Located at the
superior margin of the orbit is the supraorbital foramen, and below the orbit is the infraorbital foramen. The mandible has


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two openings, the mandibular foramen on its inner surface and the mental foramen on its external surface near the chin.
The nasal conchae are bony projections from the lateral walls of the nasal cavity. The large inferior nasal concha is an
independent bone, while the middle and superior conchae are parts of the ethmoid bone. The nasal septum is formed by the
perpendicular plate of the ethmoid bone, the vomer bone, and the septal cartilage. The paranasal sinuses are air-filled spaces
located within the frontal, maxillary, sphenoid, and ethmoid bones.
On the lateral skull, the zygomatic arch consists of two parts, the temporal process of the zygomatic bone anteriorly and
the zygomatic process of the temporal bone posteriorly. The temporal fossa is the shallow space located on the lateral skull
above the level of the zygomatic arch. The infratemporal fossa is located below the zygomatic arch and deep to the ramus
of the mandible.
The hyoid bone is located in the upper neck and does not join with any other bone. It is held in position by muscles and
serves to support the tongue above, the larynx below, and the pharynx posteriorly.


7.3 The Vertebral Column
The vertebral column forms the neck and back. The vertebral column originally develops as 33 vertebrae, but is eventually
reduced to 24 vertebrae, plus the sacrum and coccyx. The vertebrae are divided into the cervical region (C1–C7 vertebrae),
the thoracic region (T1–T12 vertebrae), and the lumbar region (L1–L5 vertebrae). The sacrum arises from the fusion of
five sacral vertebrae and the coccyx from the fusion of four small coccygeal vertebrae. The vertebral column has four
curvatures, the cervical, thoracic, lumbar, and sacrococcygeal curves. The thoracic and sacrococcygeal curves are primary
curves retained from the original fetal curvature. The cervical and lumbar curves develop after birth and thus are secondary
curves. The cervical curve develops as the infant begins to hold up the head, and the lumbar curve appears with standing
and walking.
A typical vertebra consists of an enlarged anterior portion called the body, which provides weight-bearing support. Attached
posteriorly to the body is a vertebral arch, which surrounds and defines the vertebral foramen for passage of the spinal
cord. The vertebral arch consists of the pedicles, which attach to the vertebral body, and the laminae, which come
together to form the roof of the arch. Arising from the vertebral arch are the laterally projecting transverse processes and
the posteriorly oriented spinous process. The superior articular processes project upward, where they articulate with the
downward projecting inferior articular processes of the next higher vertebrae.
A typical cervical vertebra has a small body, a bifid (Y-shaped) spinous process, and U-shaped transverse processes with
a transverse foramen. In addition to these characteristics, the axis (C2 vertebra) also has the dens projecting upward from
the vertebral body. The atlas (C1 vertebra) differs from the other cervical vertebrae in that it does not have a body, but
instead consists of bony ring formed by the anterior and posterior arches. The atlas articulates with the dens from the axis.
A typical thoracic vertebra is distinguished by its long, downward projecting spinous process. Thoracic vertebrae also have
articulation facets on the body and transverse processes for attachment of the ribs. Lumbar vertebrae support the greatest
amount of body weight and thus have a large, thick body. They also have a short, blunt spinous process. The sacrum is
triangular in shape. The median sacral crest is formed by the fused vertebral spinous processes and the lateral sacral crest
is derived from the fused transverse processes. Anterior (ventral) and posterior (dorsal) sacral foramina allow branches of
the sacral spinal nerves to exit the sacrum. The auricular surfaces are articulation sites on the lateral sacrum that anchor the
sacrum to the hipbones to form the pelvis. The coccyx is small and derived from the fusion of four small vertebrae.
The intervertebral discs fill in the gaps between the bodies of adjacent vertebrae. They provide strong attachments and
padding between the vertebrae. The outer, fibrous layer of a disc is called the anulus fibrosus. The gel-like interior is
called the nucleus pulposus. The disc can change shape to allow for movement between vertebrae. If the anulus fibrosus is
weakened or damaged, the nucleus pulposus can protrude outward, resulting in a herniated disc.
The anterior longitudinal ligament runs along the full length of the anterior vertebral column, uniting the vertebral bodies.
The supraspinous ligament is located posteriorly and interconnects the spinous processes of the thoracic and lumbar
vertebrae. In the neck, this ligament expands to become the nuchal ligament. The nuchal ligament is attached to the
cervical spinous processes and superiorly to the base of the skull, out to the external occipital protuberance. The posterior
longitudinal ligament runs within the vertebral canal and unites the posterior sides of the vertebral bodies. The ligamentum
flavum unites the lamina of adjacent vertebrae.


7.4 The Thoracic Cage
The thoracic cage protects the heart and lungs. It is composed of 12 pairs of ribs with their costal cartilages and the sternum.
The ribs are anchored posteriorly to the 12 thoracic vertebrae. The sternum consists of the manubrium, body, and xiphoid
process. The manubrium and body are joined at the sternal angle, which is also the site for attachment of the second ribs.
Ribs are flattened, curved bones and are numbered 1–12. Posteriorly, the head of the rib articulates with the costal facets
located on the bodies of thoracic vertebrae and the rib tubercle articulates with the facet located on the vertebral transverse
process. The angle of the ribs forms the most posterior portion of the thoracic cage. The costal groove in the inferior margin
of each rib carries blood vessels and a nerve. Anteriorly, each rib ends in a costal cartilage. True ribs (1–7) attach directly


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to the sternum via their costal cartilage. The false ribs (8–12) either attach to the sternum indirectly or not at all. Ribs 8–10
have their costal cartilages attached to the cartilage of the next higher rib. The floating ribs (11–12) are short and do not
attach to the sternum or to another rib.


7.5 Embryonic Development of the Axial Skeleton
Formation of the axial skeleton begins during early embryonic development with the appearance of the rod-like notochord
along the dorsal length of the early embryo. Repeating, paired blocks of tissue called somites then appear along either side
of notochord. As the somites grow, they split into parts, one of which is called a sclerotome. This consists of mesenchyme,
the embryonic tissue that will become the bones, cartilages, and connective tissues of the body.
Mesenchyme in the head region will produce the bones of the skull via two different mechanisms. The bones of the brain
case arise via intramembranous ossification in which embryonic mesenchyme tissue converts directly into bone. At the time
of birth, these bones are separated by fontanelles, wide areas of fibrous connective tissue. As the bones grow, the fontanelles
are reduced to sutures, which allow for continued growth of the skull throughout childhood. In contrast, the cranial base
and facial bones are produced by the process of endochondral ossification, in which mesenchyme tissue initially produces
a hyaline cartilage model of the future bone. The cartilage model allows for growth of the bone and is gradually converted
into bone over a period of many years.
The vertebrae, ribs, and sternum also develop via endochondral ossification. Mesenchyme accumulates around the
notochord and produces hyaline cartilage models of the vertebrae. The notochord largely disappears, but remnants of the
notochord contribute to formation of the intervertebral discs. In the thorax region, a portion of the vertebral cartilage model
splits off to form the ribs. These then become attached anteriorly to the developing cartilage model of the sternum. Growth
of the cartilage models for the vertebrae, ribs, and sternum allow for enlargement of the thoracic cage during childhood and
adolescence. The cartilage models gradually undergo ossification and are converted into bone.


INTERACTIVE LINK QUESTIONS
1. Watch this video (http://openstaxcollege.org/l/skull1)
to view a rotating and exploded skull with color-coded
bones. Which bone (yellow) is centrally located and joins
with most of the other bones of the skull?
2. View this animation (http://openstaxcollege.org/l/
headblow) to see how a blow to the head may produce
a contrecoup (counterblow) fracture of the basilar portion
of the occipital bone on the base of the skull. Why may a
basilar fracture be life threatening?
3. Osteoporosis is a common age-related bone disease in
which bone density and strength is decreased. Watch this
video (http://openstaxcollege.org/l/osteoporosis) to get a
better understanding of how thoracic vertebrae may
become weakened and may fractured due to this disease.
How may vertebral osteoporosis contribute to kyphosis?
4. Watch this animation (http://openstaxcollege.org/l/
diskslip) to see what it means to “slip” a disk. Watch


this second animation (http://openstaxcollege.org/l/
herndisc) to see one possible treatment for a herniated disc,
removing and replacing the damaged disc with an artificial
one that allows for movement between the adjacent
certebrae. How could lifting a heavy object produce pain in
a lower limb?
5. Use this tool (http://openstaxcollege.org/l/
vertcolumn) to identify the bones, intervertebral discs, and
ligaments of the vertebral column. The thickest portions
of the anterior longitudinal ligament and the supraspinous
ligament are found in which regions of the vertebral
column?
6. View this video (http://openstaxcollege.org/l/
skullbones) to review the two processes that give rise to the
bones of the skull and body. What are the two mechanisms
by which the bones of the body are formed and which bones
are formed by each mechanism?


REVIEW QUESTIONS
7. Which of the following is part of the axial skeleton?


a. shoulder bones
b. thigh bone
c. foot bones
d. vertebral column


8. Which of the following is a function of the axial
skeleton?


a. allows for movement of the wrist and hand
b. protects nerves and blood vessels at the elbow
c. supports trunk of body
d. allows for movements of the ankle and foot


9. The axial skeleton ________.


a. consists of 126 bones
b. forms the vertical axis of the body
c. includes all bones of the body trunk and limbs
d. includes only the bones of the lower limbs


10. Which of the following is a bone of the brain case?


a. parietal bone
b. zygomatic bone
c. maxillary bone
d. lacrimal bone


11. The lambdoid suture joins the parietal bone to the
________.


a. frontal bone


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b. occipital bone
c. other parietal bone
d. temporal bone


12. The middle cranial fossa ________.
a. is bounded anteriorly by the petrous ridge
b. is bounded posteriorly by the lesser wing of the
sphenoid bone


c. is divided at the midline by a small area of the
ethmoid bone


d. has the foramen rotundum, foramen ovale, and
foramen spinosum


13. The paranasal sinuses are ________.
a. air-filled spaces found within the frontal, maxilla,
sphenoid, and ethmoid bones only


b. air-filled spaces found within all bones of the
skull


c. not connected to the nasal cavity
d. divided at the midline by the nasal septum


14. Parts of the sphenoid bone include the ________.


a. sella turcica
b. squamous portion
c. glabella
d. zygomatic process


15. The bony openings of the skull include the ________.


a. carotid canal, which is located in the anterior
cranial fossa


b. superior orbital fissure, which is located at the
superior margin of the anterior orbit


c. mental foramen, which is located just below the
orbit


d. hypoglossal canal, which is located in the
posterior cranial fossa


16. The cervical region of the vertebral column consists of
________.


a. seven vertebrae
b. 12 vertebrae
c. five vertebrae
d. a single bone derived from the fusion of five
vertebrae


17. The primary curvatures of the vertebral column
________.


a. include the lumbar curve
b. are remnants of the original fetal curvature
c. include the cervical curve
d. develop after the time of birth


18. A typical vertebra has ________.
a. a vertebral foramen that passes through the body
b. a superior articular process that projects
downward to articulate with the superior portion
of the next lower vertebra


c. lamina that spans between the transverse process
and spinous process


d. a pair of laterally projecting spinous processes
19. A typical lumbar vertebra has ________.


a. a short, rounded spinous process
b. a bifid spinous process
c. articulation sites for ribs
d. a transverse foramen


20. Which is found only in the cervical region of the
vertebral column?


a. nuchal ligament
b. ligamentum flavum
c. supraspinous ligament
d. anterior longitudinal ligament


21. The sternum ________.
a. consists of only two parts, the manubrium and
xiphoid process


b. has the sternal angle located between the
manubrium and body


c. receives direct attachments from the costal
cartilages of all 12 pairs of ribs


d. articulates directly with the thoracic vertebrae
22. The sternal angle is the ________.


a. junction between the body and xiphoid process
b. site for attachment of the clavicle
c. site for attachment of the floating ribs
d. junction between the manubrium and body


23. The tubercle of a rib ________.
a. is for articulation with the transverse process of a
thoracic vertebra


b. is for articulation with the body of a thoracic
vertebra


c. provides for passage of blood vessels and a nerve
d. is the area of greatest rib curvature


24. True ribs are ________.
a. ribs 8–12
b. attached via their costal cartilage to the next
higher rib


c. made entirely of bone, and thus do not have a
costal cartilage


d. attached via their costal cartilage directly to the
sternum


25. Embryonic development of the axial skeleton involves
________.


a. intramembranous ossification, which forms the
facial bones.


b. endochondral ossification, which forms the ribs
and sternum


c. the notochord, which produces the cartilage
models for the vertebrae


d. the formation of hyaline cartilage models, which
give rise to the flat bones of the skull


26. A fontanelle ________.
a. is the cartilage model for a vertebra that later is
converted into bone


b. gives rise to the facial bones and vertebrae
c. is the rod-like structure that runs the length of the
early embryo


d. is the area of fibrous connective tissue found at
birth between the brain case bones


CRITICAL THINKING QUESTIONS


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27. Define the two divisions of the skeleton.
28. Discuss the functions of the axial skeleton.
29. Define and list the bones that form the brain case or
support the facial structures.
30. Identify the major sutures of the skull, their locations,
and the bones united by each.
31. Describe the anterior, middle, and posterior cranial
fossae and their boundaries, and give the midline structure
that divides each into right and left areas.
32. Describe the parts of the nasal septum in both the dry
and living skull.
33. Describe the vertebral column and define each region.
34. Describe a typical vertebra.
35. Describe the sacrum.


36. Describe the structure and function of an intervertebral
disc.
37. Define the ligaments of the vertebral column.
38. Define the parts and functions of the thoracic cage.
39. Describe the parts of the sternum.
40. Discuss the parts of a typical rib.
41. Define the classes of ribs.
42. Discuss the processes by which the brain-case bones of
the skull are formed and grow during skull enlargement.
43. Discuss the process that gives rise to the base and facial
bones of the skull.
44. Discuss the development of the vertebrae, ribs, and
sternum.


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8 | THE APPENDICULAR
SKELETON


Figure 8.1 Dancer The appendicular skeleton consists of the upper and lower limb bones, the bones of the hands
and feet, and the bones that anchor the limbs to the axial skeleton. (credit: Melissa Dooley/flickr)


Introduction
Chapter Objectives


After studying this chapter, you will be able to:
• Discuss the bones of the pectoral and pelvic girdles, and describe how these unite the limbs with the axial
skeleton


• Describe the bones of the upper limb, including the bones of the arm, forearm, wrist, and hand
• Identify the features of the pelvis and explain how these differ between the adult male and female pelvis
• Describe the bones of the lower limb, including the bones of the thigh, leg, ankle, and foot
• Describe the embryonic formation and growth of the limb bones


Your skeleton provides the internal supporting structure of the body. The adult axial skeleton consists of 80 bones that form
the head and body trunk. Attached to this are the limbs, whose 126 bones constitute the appendicular skeleton. These bones
are divided into two groups: the bones that are located within the limbs themselves, and the girdle bones that attach the
limbs to the axial skeleton. The bones of the shoulder region form the pectoral girdle, which anchors the upper limb to the
thoracic cage of the axial skeleton. The lower limb is attached to the vertebral column by the pelvic girdle.


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Because of our upright stance, different functional demands are placed upon the upper and lower limbs. Thus, the bones
of the lower limbs are adapted for weight-bearing support and stability, as well as for body locomotion via walking or
running. In contrast, our upper limbs are not required for these functions. Instead, our upper limbs are highly mobile and
can be utilized for a wide variety of activities. The large range of upper limb movements, coupled with the ability to easily
manipulate objects with our hands and opposable thumbs, has allowed humans to construct the modern world in which we
live.


8.1 | The Pectoral Girdle
By the end of this section, you will be able to:
• Describe the bones that form the pectoral girdle
• List the functions of the pectoral girdle


The appendicular skeleton includes all of the limb bones, plus the bones that unite each limb with the axial skeleton (Figure
8.2). The bones that attach each upper limb to the axial skeleton form the pectoral girdle (shoulder girdle). This consists of
two bones, the scapula and clavicle (Figure 8.3). The clavicle (collarbone) is an S-shaped bone located on the anterior side
of the shoulder. It is attached on its medial end to the sternum of the thoracic cage, which is part of the axial skeleton. The
lateral end of the clavicle articulates (joins) with the scapula just above the shoulder joint. You can easily palpate, or feel
with your fingers, the entire length of your clavicle.


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Figure 8.2 Axial and Appendicular Skeletons The axial skeleton forms the central axis of the body and consists of
the skull, vertebral column, and thoracic cage. The appendicular skeleton consists of the pectoral and pelvic girdles,
the limb bones, and the bones of the hands and feet.


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Figure 8.3 Pectoral Girdle The pectoral girdle consists of the clavicle and the scapula, which serve to attach the
upper limb to the sternum of the axial skeleton.


The scapula (shoulder blade) lies on the posterior aspect of the shoulder. It is supported by the clavicle, which also
articulates with the humerus (arm bone) to form the shoulder joint. The scapula is a flat, triangular-shaped bone with a
prominent ridge running across its posterior surface. This ridge extends out laterally, where it forms the bony tip of the
shoulder and joins with the lateral end of the clavicle. By following along the clavicle, you can palpate out to the bony tip
of the shoulder, and from there, you can move back across your posterior shoulder to follow the ridge of the scapula. Move
your shoulder around and feel how the clavicle and scapula move together as a unit. Both of these bones serve as important
attachment sites for muscles that aid with movements of the shoulder and arm.
The right and left pectoral girdles are not joined to each other, allowing each to operate independently. In addition, the
clavicle of each pectoral girdle is anchored to the axial skeleton by a single, highly mobile joint. This allows for the
extensive mobility of the entire pectoral girdle, which in turn enhances movements of the shoulder and upper limb.


Clavicle
The clavicle is the only long bone that lies in a horizontal position in the body (see Figure 8.3). The clavicle has several
important functions. First, anchored by muscles from above, it serves as a strut that extends laterally to support the scapula.
This in turn holds the shoulder joint superiorly and laterally from the body trunk, allowing for maximal freedom of motion


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for the upper limb. The clavicle also transmits forces acting on the upper limb to the sternum and axial skeleton. Finally, it
serves to protect the underlying nerves and blood vessels as they pass between the trunk of the body and the upper limb.
The clavicle has three regions: the medial end, the lateral end, and the shaft. The medial end, known as the sternal
end of the clavicle, has a triangular shape and articulates with the manubrium portion of the sternum. This forms
the sternoclavicular joint, which is the only bony articulation between the pectoral girdle of the upper limb and the
axial skeleton. This joint allows considerable mobility, enabling the clavicle and scapula to move in upward/downward
and anterior/posterior directions during shoulder movements. The sternoclavicular joint is indirectly supported by the
costoclavicular ligament (costo- = “rib”), which spans the sternal end of the clavicle and the underlying first rib. The lateral
or acromial end of the clavicle articulates with the acromion of the scapula, the portion of the scapula that forms the bony
tip of the shoulder. There are some sex differences in the morphology of the clavicle. In women, the clavicle tends to be
shorter, thinner, and less curved. In men, the clavicle is heavier and longer, and has a greater curvature and rougher surfaces
where muscles attach, features that are more pronounced in manual workers.
The clavicle is the most commonly fractured bone in the body. Such breaks often occur because of the force exerted on
the clavicle when a person falls onto his or her outstretched arms, or when the lateral shoulder receives a strong blow.
Because the sternoclavicular joint is strong and rarely dislocated, excessive force results in the breaking of the clavicle,
usually between the middle and lateral portions of the bone. If the fracture is complete, the shoulder and lateral clavicle
fragment will drop due to the weight of the upper limb, causing the person to support the sagging limb with their other
hand. Muscles acting across the shoulder will also pull the shoulder and lateral clavicle anteriorly and medially, causing
the clavicle fragments to override. The clavicle overlies many important blood vessels and nerves for the upper limb, but
fortunately, due to the anterior displacement of a broken clavicle, these structures are rarely affected when the clavicle is
fractured.


Scapula
The scapula is also part of the pectoral girdle and thus plays an important role in anchoring the upper limb to the body. The
scapula is located on the posterior side of the shoulder. It is surrounded by muscles on both its anterior (deep) and posterior
(superficial) sides, and thus does not articulate with the ribs of the thoracic cage.
The scapula has several important landmarks (Figure 8.4). The three margins or borders of the scapula, named for their
positions within the body, are the superior border of the scapula, the medial border of the scapula, and the lateral
border of the scapula. The suprascapular notch is located lateral to the midpoint of the superior border. The corners
of the triangular scapula, at either end of the medial border, are the superior angle of the scapula, located between the
medial and superior borders, and the inferior angle of the scapula, located between the medial and lateral borders. The
inferior angle is the most inferior portion of the scapula, and is particularly important because it serves as the attachment
point for several powerful muscles involved in shoulder and upper limb movements. The remaining corner of the scapula,
between the superior and lateral borders, is the location of the glenoid cavity (glenoid fossa). This shallow depression
articulates with the humerus bone of the arm to form the glenohumeral joint (shoulder joint). The small bony bumps
located immediately above and below the glenoid cavity are the supraglenoid tubercle and the infraglenoid tubercle,
respectively. These provide attachments for muscles of the arm.


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Figure 8.4 Scapula The isolated scapula is shown here from its anterior (deep) side and its posterior (superficial)
side.


The scapula also has two prominent projections. Toward the lateral end of the superior border, between the suprascapular
notch and glenoid cavity, is the hook-like coracoid process (coracoid = “shaped like a crow’s beak”). This process projects
anteriorly and curves laterally. At the shoulder, the coracoid process is located inferior to the lateral end of the clavicle.
It is anchored to the clavicle by a strong ligament, and serves as the attachment site for muscles of the anterior chest and
arm. On the posterior aspect, the spine of the scapula is a long and prominent ridge that runs across its upper portion.
Extending laterally from the spine is a flattened and expanded region called the acromion or acromial process. The
acromion forms the bony tip of the superior shoulder region and articulates with the lateral end of the clavicle, forming the
acromioclavicular joint (see Figure 8.3). Together, the clavicle, acromion, and spine of the scapula form a V-shaped bony
line that provides for the attachment of neck and back muscles that act on the shoulder, as well as muscles that pass across
the shoulder joint to act on the arm.
The scapula has three depressions, each of which is called a fossa (plural = fossae). Two of these are found on the posterior
scapula, above and below the scapular spine. Superior to the spine is the narrow supraspinous fossa, and inferior to the
spine is the broad infraspinous fossa. The anterior (deep) surface of the scapula forms the broad subscapular fossa. All of
these fossae provide large surface areas for the attachment of muscles that cross the shoulder joint to act on the humerus.
The acromioclavicular joint transmits forces from the upper limb to the clavicle. The ligaments around this joint are
relatively weak. A hard fall onto the elbow or outstretched hand can stretch or tear the acromioclavicular ligaments,
resulting in a moderate injury to the joint. However, the primary support for the acromioclavicular joint comes from a very
strong ligament called the coracoclavicular ligament (see Figure 8.3). This connective tissue band anchors the coracoid
process of the scapula to the inferior surface of the acromial end of the clavicle and thus provides important indirect support
for the acromioclavicular joint. Following a strong blow to the lateral shoulder, such as when a hockey player is driven into
the boards, a complete dislocation of the acromioclavicular joint can result. In this case, the acromion is thrust under the
acromial end of the clavicle, resulting in ruptures of both the acromioclavicular and coracoclavicular ligaments. The scapula
then separates from the clavicle, with the weight of the upper limb pulling the shoulder downward. This dislocation injury
of the acromioclavicular joint is known as a “shoulder separation” and is common in contact sports such as hockey, football,
or martial arts.


8.2 | Bones of the Upper Limb
By the end of this section, you will be able to:
• Identify the divisions of the upper limb and describe the bones in each region
• List the bones and bony landmarks that articulate at each joint of the upper limb


The upper limb is divided into three regions. These consist of the arm, located between the shoulder and elbow joints; the
forearm, which is between the elbow and wrist joints; and the hand, which is located distal to the wrist. There are 30 bones


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in each upper limb (see Figure 8.2). The humerus is the single bone of the upper arm, and the ulna (medially) and the
radius (laterally) are the paired bones of the forearm. The base of the hand contains eight bones, each called a carpal bone,
and the palm of the hand is formed by five bones, each called a metacarpal bone. The fingers and thumb contain a total of
14 bones, each of which is a phalanx bone of the hand.


Humerus
The humerus is the single bone of the upper arm region (Figure 8.5). At its proximal end is the head of the humerus.
This is the large, round, smooth region that faces medially. The head articulates with the glenoid cavity of the scapula to
form the glenohumeral (shoulder) joint. The margin of the smooth area of the head is the anatomical neck of the humerus.
Located on the lateral side of the proximal humerus is an expanded bony area called the greater tubercle. The smaller
lesser tubercle of the humerus is found on the anterior aspect of the humerus. Both the greater and lesser tubercles serve as
attachment sites for muscles that act across the shoulder joint. Passing between the greater and lesser tubercles is the narrow
intertubercular groove (sulcus), which is also known as the bicipital groove because it provides passage for a tendon of
the biceps brachii muscle. The surgical neck is located at the base of the expanded, proximal end of the humerus, where
it joins the narrow shaft of the humerus. The surgical neck is a common site of arm fractures. The deltoid tuberosity is
a roughened, V-shaped region located on the lateral side in the middle of the humerus shaft. As its name indicates, it is the
site of attachment for the deltoid muscle.


Figure 8.5 Humerus and Elbow Joint The humerus is the single bone of the upper arm region. It articulates with the
radius and ulna bones of the forearm to form the elbow joint.


Distally, the humerus becomes flattened. The prominent bony projection on the medial side is the medial epicondyle of
the humerus. The much smaller lateral epicondyle of the humerus is found on the lateral side of the distal humerus. The
roughened ridge of bone above the lateral epicondyle is the lateral supracondylar ridge. All of these areas are attachment
points for muscles that act on the forearm, wrist, and hand. The powerful grasping muscles of the anterior forearm arise
from the medial epicondyle, which is thus larger and more robust than the lateral epicondyle that gives rise to the weaker
posterior forearm muscles.
The distal end of the humerus has two articulation areas, which join the ulna and radius bones of the forearm to form the
elbow joint. The more medial of these areas is the trochlea, a spindle- or pulley-shaped region (trochlea = “pulley”), which
articulates with the ulna bone. Immediately lateral to the trochlea is the capitulum (“small head”), a knob-like structure
located on the anterior surface of the distal humerus. The capitulum articulates with the radius bone of the forearm. Just


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above these bony areas are two small depressions. These spaces accommodate the forearm bones when the elbow is fully
bent (flexed). Superior to the trochlea is the coronoid fossa, which receives the coronoid process of the ulna, and above
the capitulum is the radial fossa, which receives the head of the radius when the elbow is flexed. Similarly, the posterior
humerus has the olecranon fossa, a larger depression that receives the olecranon process of the ulna when the forearm is
fully extended.


Ulna
The ulna is the medial bone of the forearm. It runs parallel to the radius, which is the lateral bone of the forearm (Figure
8.6). The proximal end of the ulna resembles a crescent wrench with its large, C-shaped trochlear notch. This region
articulates with the trochlea of the humerus as part of the elbow joint. The inferior margin of the trochlear notch is formed
by a prominent lip of bone called the coronoid process of the ulna. Just below this on the anterior ulna is a roughened
area called the ulnar tuberosity. To the lateral side and slightly inferior to the trochlear notch is a small, smooth area
called the radial notch of the ulna. This area is the site of articulation between the proximal radius and the ulna, forming
the proximal radioulnar joint. The posterior and superior portions of the proximal ulna make up the olecranon process,
which forms the bony tip of the elbow.


Figure 8.6 Ulna and Radius The ulna is located on the medial side of the forearm, and the radius is on the lateral
side. These bones are attached to each other by an interosseous membrane.


More distal is the shaft of the ulna. The lateral side of the shaft forms a ridge called the interosseous border of the ulna.
This is the line of attachment for the interosseous membrane of the forearm, a sheet of dense connective tissue that unites
the ulna and radius bones. The small, rounded area that forms the distal end is the head of the ulna. Projecting from the
posterior side of the ulnar head is the styloid process of the ulna, a short bony projection. This serves as an attachment
point for a connective tissue structure that unites the distal ends of the ulna and radius.
In the anatomical position, with the elbow fully extended and the palms facing forward, the arm and forearm do not form
a straight line. Instead, the forearm deviates laterally by 5–15 degrees from the line of the arm. This deviation is called the
carrying angle. It allows the forearm and hand to swing freely or to carry an object without hitting the hip. The carrying
angle is larger in females to accommodate their wider pelvis.


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Radius
The radius runs parallel to the ulna, on the lateral (thumb) side of the forearm (see Figure 8.6). The head of the radius
is a disc-shaped structure that forms the proximal end. The small depression on the surface of the head articulates with the
capitulum of the humerus as part of the elbow joint, whereas the smooth, outer margin of the head articulates with the radial
notch of the ulna at the proximal radioulnar joint. The neck of the radius is the narrowed region immediately below the
expanded head. Inferior to this point on the medial side is the radial tuberosity, an oval-shaped, bony protuberance that
serves as a muscle attachment point. The shaft of the radius is slightly curved and has a small ridge along its medial side.
This ridge forms the interosseous border of the radius, which, like the similar border of the ulna, is the line of attachment
for the interosseous membrane that unites the two forearm bones. The distal end of the radius has a smooth surface for
articulation with two carpal bones to form the radiocarpal joint or wrist joint (Figure 8.7 and Figure 8.8). On the medial
side of the distal radius is the ulnar notch of the radius. This shallow depression articulates with the head of the ulna,
which together form the distal radioulnar joint. The lateral end of the radius has a pointed projection called the styloid
process of the radius. This provides attachment for ligaments that support the lateral side of the wrist joint. Compared
to the styloid process of the ulna, the styloid process of the radius projects more distally, thereby limiting the range of
movement for lateral deviations of the hand at the wrist joint.


Watch this video (http://openstaxcollege.org/l/fractures) to see how fractures of the distal radius bone can affect
the wrist joint. Explain the problems that may occur if a fracture of the distal radius involves the joint surface of the
radiocarpal joint of the wrist.


Carpal Bones
The wrist and base of the hand are formed by a series of eight small carpal bones (see Figure 8.7). The carpal bones are
arranged in two rows, forming a proximal row of four carpal bones and a distal row of four carpal bones. The bones in the
proximal row, running from the lateral (thumb) side to the medial side, are the scaphoid (“boat-shaped”), lunate (“moon-
shaped”), triquetrum (“three-cornered”), and pisiform (“pea-shaped”) bones. The small, rounded pisiform bone articulates
with the anterior surface of the triquetrum bone. The pisiform thus projects anteriorly, where it forms the bony bump that
can be felt at the medial base of your hand. The distal bones (lateral to medial) are the trapezium (“table”), trapezoid
(“resembles a table”), capitate (“head-shaped”), and hamate (“hooked bone”) bones. The hamate bone is characterized by
a prominent bony extension on its anterior side called the hook of the hamate bone.
A helpful mnemonic for remembering the arrangement of the carpal bones is “So Long To Pinky, Here Comes The Thumb.”
This mnemonic starts on the lateral side and names the proximal bones from lateral to medial (scaphoid, lunate, triquetrum,
pisiform), then makes a U-turn to name the distal bones from medial to lateral (hamate, capitate, trapezoid, trapezium).
Thus, it starts and finishes on the lateral side.


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Figure 8.7 Bones of the Wrist and Hand The eight carpal bones form the base of the hand. These are arranged into
proximal and distal rows of four bones each. The metacarpal bones form the palm of the hand. The thumb and fingers
consist of the phalanx bones.


The carpal bones form the base of the hand. This can be seen in the radiograph (X-ray image) of the hand that shows
the relationships of the hand bones to the skin creases of the hand (see Figure 8.8). Within the carpal bones, the four
proximal bones are united to each other by ligaments to form a unit. Only three of these bones, the scaphoid, lunate, and
triquetrum, contribute to the radiocarpal joint. The scaphoid and lunate bones articulate directly with the distal end of the
radius, whereas the triquetrum bone articulates with a fibrocartilaginous pad that spans the radius and styloid process of the
ulna. The distal end of the ulna thus does not directly articulate with any of the carpal bones.
The four distal carpal bones are also held together as a group by ligaments. The proximal and distal rows of carpal bones
articulate with each other to form the midcarpal joint (see Figure 8.8). Together, the radiocarpal and midcarpal joints are
responsible for all movements of the hand at the wrist. The distal carpal bones also articulate with the metacarpal bones of
the hand.


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Figure 8.8 Bones of the Hand This radiograph shows the position of the bones within the hand. Note the carpal
bones that form the base of the hand. (credit: modification of work by Trace Meek)


In the articulated hand, the carpal bones form a U-shaped grouping. A strong ligament called the flexor retinaculum spans
the top of this U-shaped area to maintain this grouping of the carpal bones. The flexor retinaculum is attached laterally
to the trapezium and scaphoid bones, and medially to the hamate and pisiform bones. Together, the carpal bones and the
flexor retinaculum form a passageway called the carpal tunnel, with the carpal bones forming the walls and floor, and the
flexor retinaculum forming the roof of this space (Figure 8.9). The tendons of nine muscles of the anterior forearm and
an important nerve pass through this narrow tunnel to enter the hand. Overuse of the muscle tendons or wrist injury can
produce inflammation and swelling within this space. This produces compression of the nerve, resulting in carpal tunnel
syndrome, which is characterized by pain or numbness, and muscle weakness in those areas of the hand supplied by this
nerve.


Figure 8.9 Carpal Tunnel The carpal tunnel is the passageway by which nine muscle tendons and a major nerve
enter the hand from the anterior forearm. The walls and floor of the carpal tunnel are formed by the U-shaped grouping
of the carpal bones, and the roof is formed by the flexor retinaculum, a strong ligament that anteriorly unites the bones.


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Metacarpal Bones
The palm of the hand contains five elongated metacarpal bones. These bones lie between the carpal bones of the wrist and
the bones of the fingers and thumb (see Figure 8.7). The proximal end of each metacarpal bone articulates with one of the
distal carpal bones. Each of these articulations is a carpometacarpal joint (see Figure 8.8). The expanded distal end of
each metacarpal bone articulates at the metacarpophalangeal joint with the proximal phalanx bone of the thumb or one
of the fingers. The distal end also forms the knuckles of the hand, at the base of the fingers. The metacarpal bones are
numbered 1–5, beginning at the thumb.
The first metacarpal bone, at the base of the thumb, is separated from the other metacarpal bones. This allows it a
freedom of motion that is independent of the other metacarpal bones, which is very important for thumb mobility. The
remaining metacarpal bones are united together to form the palm of the hand. The second and third metacarpal bones are
firmly anchored in place and are immobile. However, the fourth and fifth metacarpal bones have limited anterior-posterior
mobility, a motion that is greater for the fifth bone. This mobility is important during power gripping with the hand (Figure
8.10). The anterior movement of these bones, particularly the fifth metacarpal bone, increases the strength of contact for the
medial hand during gripping actions.


Figure 8.10 Hand During Gripping During tight gripping—compare (b) to (a)—the fourth and, particularly, the fifth
metatarsal bones are pulled anteriorly. This increases the contact between the object and the medial side of the hand,
thus improving the firmness of the grip.


Phalanx Bones
The fingers and thumb contain 14 bones, each of which is called a phalanx bone (plural = phalanges), named after the
ancient Greek phalanx (a rectangular block of soldiers). The thumb ( pollex) is digit number 1 and has two phalanges, a
proximal phalanx, and a distal phalanx bone (see Figure 8.7). Digits 2 (index finger) through 5 (little finger) have three
phalanges each, called the proximal, middle, and distal phalanx bones. An interphalangeal joint is one of the articulations
between adjacent phalanges of the digits (see Figure 8.8).


Visit this site (http://openstaxcollege.org/l/handbone) to explore the bones and joints of the hand. What are the three
arches of the hand, and what is the importance of these during the gripping of an object?


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Appendicular System: Fractures of Upper Limb Bones
Due to our constant use of the hands and the rest of our upper limbs, an injury to any of these areas will cause a
significant loss of functional ability. Many fractures result from a hard fall onto an outstretched hand. The resulting
transmission of force up the limb may result in a fracture of the humerus, radius, or scaphoid bones. These injuries are
especially common in elderly people whose bones are weakened due to osteoporosis.
Falls onto the hand or elbow, or direct blows to the arm, can result in fractures of the humerus (Figure 8.11). Following
a fall, fractures at the surgical neck, the region at which the expanded proximal end of the humerus joins with the shaft,
can result in an impacted fracture, in which the distal portion of the humerus is driven into the proximal portion. Falls
or blows to the arm can also produce transverse or spiral fractures of the humeral shaft.
In children, a fall onto the tip of the elbow frequently results in a distal humerus fracture. In these, the olecranon of
the ulna is driven upward, resulting in a fracture across the distal humerus, above both epicondyles (supracondylar
fracture), or a fracture between the epicondyles, thus separating one or both of the epicondyles from the body of the
humerus (intercondylar fracture). With these injuries, the immediate concern is possible compression of the artery to
the forearm due to swelling of the surrounding tissues. If compression occurs, the resulting ischemia (lack of oxygen)
due to reduced blood flow can quickly produce irreparable damage to the forearm muscles. In addition, four major
nerves for shoulder and upper limb muscles are closely associated with different regions of the humerus, and thus,
humeral fractures may also damage these nerves.
Another frequent injury following a fall onto an outstretched hand is a Colles fracture (“col-lees”) of the distal radius
(see Figure 8.11). This involves a complete transverse fracture across the distal radius that drives the separated distal
fragment of the radius posteriorly and superiorly. This injury results in a characteristic “dinner fork” bend of the
forearm just above the wrist due to the posterior displacement of the hand. This is the most frequent forearm fracture
and is a common injury in persons over the age of 50, particularly in older women with osteoporosis. It also commonly
occurs following a high-speed fall onto the hand during activities such as snowboarding or skating.
The most commonly fractured carpal bone is the scaphoid, often resulting from a fall onto the hand. Deep pain at the
lateral wrist may yield an initial diagnosis of a wrist sprain, but a radiograph taken several weeks after the injury, after
tissue swelling has subsided, will reveal the fracture. Due to the poor blood supply to the scaphoid bone, healing will
be slow and there is the danger of bone necrosis and subsequent degenerative joint disease of the wrist.


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Figure 8.11 Fractures of the Humerus and Radius Falls or direct blows can result in fractures of the surgical neck
or shaft of the humerus. Falls onto the elbow can fracture the distal humerus. A Colles fracture of the distal radius is
the most common forearm fracture.


Watch this video (http://openstaxcollege.org/l/colles) to learn about a Colles fracture, a break of the distal radius,
usually caused by falling onto an outstretched hand. When would surgery be required and how would the fracture be
repaired in this case?


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8.3 | The Pelvic Girdle and Pelvis
By the end of this section, you will be able to:
• Define the pelvic girdle and describe the bones and ligaments of the pelvis
• Explain the three regions of the hip bone and identify their bony landmarks
• Describe the openings of the pelvis and the boundaries of the greater and lesser pelvis


The pelvic girdle (hip girdle) is formed by a single bone, the hip bone or coxal bone (coxal = “hip”), which serves as the
attachment point for each lower limb. Each hip bone, in turn, is firmly joined to the axial skeleton via its attachment to
the sacrum of the vertebral column. The right and left hip bones also converge anteriorly to attach to each other. The bony
pelvis is the entire structure formed by the two hip bones, the sacrum, and, attached inferiorly to the sacrum, the coccyx
(Figure 8.12).
Unlike the bones of the pectoral girdle, which are highly mobile to enhance the range of upper limb movements, the bones
of the pelvis are strongly united to each other to form a largely immobile, weight-bearing structure. This is important for
stability because it enables the weight of the body to be easily transferred laterally from the vertebral column, through the
pelvic girdle and hip joints, and into either lower limb whenever the other limb is not bearing weight. Thus, the immobility
of the pelvis provides a strong foundation for the upper body as it rests on top of the mobile lower limbs.


Figure 8.12 Pelvis The pelvic girdle is formed by a single hip bone. The hip bone attaches the lower limb to the axial
skeleton through its articulation with the sacrum. The right and left hip bones, plus the sacrum and the coccyx, together
form the pelvis.


Hip Bone
The hip bone, or coxal bone, forms the pelvic girdle portion of the pelvis. The paired hip bones are the large, curved bones
that form the lateral and anterior aspects of the pelvis. Each adult hip bone is formed by three separate bones that fuse
together during the late teenage years. These bony components are the ilium, ischium, and pubis (Figure 8.13). These names
are retained and used to define the three regions of the adult hip bone.


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Figure 8.13 The Hip Bone The adult hip bone consists of three regions. The ilium forms the large, fan-shaped
superior portion, the ischium forms the posteroinferior portion, and the pubis forms the anteromedial portion.


The ilium is the fan-like, superior region that forms the largest part of the hip bone. It is firmly united to the sacrum at
the largely immobile sacroiliac joint (see Figure 8.12). The ischium forms the posteroinferior region of each hip bone. It
supports the body when sitting. The pubis forms the anterior portion of the hip bone. The pubis curves medially, where it
joins to the pubis of the opposite hip bone at a specialized joint called the pubic symphysis.
Ilium
When you place your hands on your waist, you can feel the arching, superior margin of the ilium along your waistline
(see Figure 8.13). This curved, superior margin of the ilium is the iliac crest. The rounded, anterior termination of the
iliac crest is the anterior superior iliac spine. This important bony landmark can be felt at your anterolateral hip. Inferior
to the anterior superior iliac spine is a rounded protuberance called the anterior inferior iliac spine. Both of these iliac
spines serve as attachment points for muscles of the thigh. Posteriorly, the iliac crest curves downward to terminate as
the posterior superior iliac spine. Muscles and ligaments surround but do not cover this bony landmark, thus sometimes
producing a depression seen as a “dimple” located on the lower back. More inferiorly is the posterior inferior iliac spine.
This is located at the inferior end of a large, roughened area called the auricular surface of the ilium. The auricular surface
articulates with the auricular surface of the sacrum to form the sacroiliac joint. Both the posterior superior and posterior
inferior iliac spines serve as attachment points for the muscles and very strong ligaments that support the sacroiliac joint.
The shallow depression located on the anteromedial (internal) surface of the upper ilium is called the iliac fossa. The inferior
margin of this space is formed by the arcuate line of the ilium, the ridge formed by the pronounced change in curvature
between the upper and lower portions of the ilium. The large, inverted U-shaped indentation located on the posterior margin
of the lower ilium is called the greater sciatic notch.
Ischium
The ischium forms the posterolateral portion of the hip bone (see Figure 8.13). The large, roughened area of the inferior
ischium is the ischial tuberosity. This serves as the attachment for the posterior thigh muscles and also carries the weight
of the body when sitting. You can feel the ischial tuberosity if you wiggle your pelvis against the seat of a chair. Projecting
superiorly and anteriorly from the ischial tuberosity is a narrow segment of bone called the ischial ramus. The slightly
curved posterior margin of the ischium above the ischial tuberosity is the lesser sciatic notch. The bony projection
separating the lesser sciatic notch and greater sciatic notch is the ischial spine.
Pubis
The pubis forms the anterior portion of the hip bone (see Figure 8.13). The enlarged medial portion of the pubis is the pubic
body. Located superiorly on the pubic body is a small bump called the pubic tubercle. The superior pubic ramus is the
segment of bone that passes laterally from the pubic body to join the ilium. The narrow ridge running along the superior
margin of the superior pubic ramus is the pectineal line of the pubis.


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The pubic body is joined to the pubic body of the opposite hip bone by the pubic symphysis. Extending downward and
laterally from the body is the inferior pubic ramus. The pubic arch is the bony structure formed by the pubic symphysis,
and the bodies and inferior pubic rami of the adjacent pubic bones. The inferior pubic ramus extends downward to join
the ischial ramus. Together, these form the single ischiopubic ramus, which extends from the pubic body to the ischial
tuberosity. The inverted V-shape formed as the ischiopubic rami from both sides come together at the pubic symphysis is
called the subpubic angle.


Pelvis
The pelvis consists of four bones: the right and left hip bones, the sacrum, and the coccyx (see Figure 8.12). The pelvis
has several important functions. Its primary role is to support the weight of the upper body when sitting and to transfer
this weight to the lower limbs when standing. It serves as an attachment point for trunk and lower limb muscles, and also
protects the internal pelvic organs. When standing in the anatomical position, the pelvis is tilted anteriorly. In this position,
the anterior superior iliac spines and the pubic tubercles lie in the same vertical plane, and the anterior (internal) surface of
the sacrum faces forward and downward.
The three areas of each hip bone, the ilium, pubis, and ischium, converge centrally to form a deep, cup-shaped cavity called
the acetabulum. This is located on the lateral side of the hip bone and is part of the hip joint. The large opening in the
anteroinferior hip bone between the ischium and pubis is the obturator foramen. This space is largely filled in by a layer
of connective tissue and serves for the attachment of muscles on both its internal and external surfaces.
Several ligaments unite the bones of the pelvis (Figure 8.14). The largely immobile sacroiliac joint is supported by a pair of
strong ligaments that are attached between the sacrum and ilium portions of the hip bone. These are the anterior sacroiliac
ligament on the anterior side of the joint and the posterior sacroiliac ligament on the posterior side. Also spanning
the sacrum and hip bone are two additional ligaments. The sacrospinous ligament runs from the sacrum to the ischial
spine, and the sacrotuberous ligament runs from the sacrum to the ischial tuberosity. These ligaments help to support and
immobilize the sacrum as it carries the weight of the body.


Figure 8.14 Ligaments of the Pelvis The posterior sacroiliac ligament supports the sacroiliac joint. The sacrospinous
ligament spans the sacrum to the ischial spine, and the sacrotuberous ligament spans the sacrum to the ischial
tuberosity. The sacrospinous and sacrotuberous ligaments contribute to the formation of the greater and lesser sciatic
foramens.


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Watch this video (http://openstaxcollege.org/l/3Dpelvis) for a 3-D view of the pelvis and its associated ligaments.
What is the large opening in the bony pelvis, located between the ischium and pubic regions, and what two parts of the
pubis contribute to the formation of this opening?


The sacrospinous and sacrotuberous ligaments also help to define two openings on the posterolateral sides of the pelvis
through which muscles, nerves, and blood vessels for the lower limb exit. The superior opening is the greater sciatic
foramen. This large opening is formed by the greater sciatic notch of the hip bone, the sacrum, and the sacrospinous
ligament. The smaller, more inferior lesser sciatic foramen is formed by the lesser sciatic notch of the hip bone, together
with the sacrospinous and sacrotuberous ligaments.
The space enclosed by the bony pelvis is divided into two regions (Figure 8.15). The broad, superior region, defined
laterally by the large, fan-like portion of the upper hip bone, is called the greater pelvis (greater pelvic cavity; false pelvis).
This broad area is occupied by portions of the small and large intestines, and because it is more closely associated with the
abdominal cavity, it is sometimes referred to as the false pelvis. More inferiorly, the narrow, rounded space of the lesser
pelvis (lesser pelvic cavity; true pelvis) contains the bladder and other pelvic organs, and thus is also known as the true
pelvis. The pelvic brim (also known as the pelvic inlet) forms the superior margin of the lesser pelvis, separating it from
the greater pelvis. The pelvic brim is defined by a line formed by the upper margin of the pubic symphysis anteriorly, and
the pectineal line of the pubis, the arcuate line of the ilium, and the sacral promontory (the anterior margin of the superior
sacrum) posteriorly. The inferior limit of the lesser pelvic cavity is called the pelvic outlet. This large opening is defined by
the inferior margin of the pubic symphysis anteriorly, and the ischiopubic ramus, the ischial tuberosity, the sacrotuberous
ligament, and the inferior tip of the coccyx posteriorly. Because of the anterior tilt of the pelvis, the lesser pelvis is also
angled, giving it an anterosuperior (pelvic inlet) to posteroinferior (pelvic outlet) orientation.


Figure 8.15 Male and Female Pelvis The female pelvis is adapted for childbirth and is broader, with a larger subpubic
angle, a rounder pelvic brim, and a wider and more shallow lesser pelvic cavity than the male pelvis.


Comparison of the Female and Male Pelvis
The differences between the adult female and male pelvis relate to function and body size. In general, the bones of the male
pelvis are thicker and heavier, adapted for support of the male’s heavier physical build and stronger muscles. The greater
sciatic notch of the male hip bone is narrower and deeper than the broader notch of females. Because the female pelvis
is adapted for childbirth, it is wider than the male pelvis, as evidenced by the distance between the anterior superior iliac
spines (see Figure 8.15). The ischial tuberosities of females are also farther apart, which increases the size of the pelvic
outlet. Because of this increased pelvic width, the subpubic angle is larger in females (greater than 80 degrees) than it is in


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males (less than 70 degrees). The female sacrum is wider, shorter, and less curved, and the sacral promontory projects less
into the pelvic cavity, thus giving the female pelvic inlet (pelvic brim) a more rounded or oval shape compared to males.
The lesser pelvic cavity of females is also wider and more shallow than the narrower, deeper, and tapering lesser pelvis of
males. Because of the obvious differences between female and male hip bones, this is the one bone of the body that allows
for the most accurate sex determination. Table 8.1 provides an overview of the general differences between the female and
male pelvis.


Overview of Differences between the Female and Male Pelvis
Female pelvis Male pelvis


Pelvic weight Bones of the pelvis are lighter andthinner
Bones of the pelvis are thicker and
heavier


Pelvic inlet shape Pelvic inlet has a round or oval shape Pelvic inlet is heart-shaped
Lesser pelvic cavity
shape Lesser pelvic cavity is shorter and wider


Lesser pelvic cavity is longer and
narrower


Subpubic angle Subpubic angle is greater than 80degrees Subpubic angle is less than 70 degrees


Pelvic outlet shape Pelvic outlet is rounded and larger Pelvic outlet is smaller
Table 8.1


Forensic Pathology and Forensic Anthropology
A forensic pathologist (also known as a medical examiner) is a medically trained physician who has been specifically
trained in pathology to examine the bodies of the deceased to determine the cause of death. A forensic pathologist
applies his or her understanding of disease as well as toxins, blood and DNA analysis, firearms and ballistics, and
other factors to assess the cause and manner of death. At times, a forensic pathologist will be called to testify under
oath in situations that involve a possible crime. Forensic pathology is a field that has received much media attention
on television shows or following a high-profile death.
While forensic pathologists are responsible for determining whether the cause of someone’s death was natural,
a suicide, accidental, or a homicide, there are times when uncovering the cause of death is more complex, and
other skills are needed. Forensic anthropology brings the tools and knowledge of physical anthropology and human
osteology (the study of the skeleton) to the task of investigating a death. A forensic anthropologist assists medical
and legal professionals in identifying human remains. The science behind forensic anthropology involves the study of
archaeological excavation; the examination of hair; an understanding of plants, insects, and footprints; the ability to
determine how much time has elapsed since the person died; the analysis of past medical history and toxicology; the
ability to determine whether there are any postmortem injuries or alterations of the skeleton; and the identification of
the decedent (deceased person) using skeletal and dental evidence.
Due to the extensive knowledge and understanding of excavation techniques, a forensic anthropologist is an integral
and invaluable team member to have on-site when investigating a crime scene, especially when the recovery of human
skeletal remains is involved. When remains are bought to a forensic anthropologist for examination, he or she must first
determine whether the remains are in fact human. Once the remains have been identified as belonging to a person and
not to an animal, the next step is to approximate the individual’s age, sex, race, and height. The forensic anthropologist
does not determine the cause of death, but rather provides information to the forensic pathologist, who will use all of
the data collected to make a final determination regarding the cause of death.


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8.4 | Bones of the Lower Limb
By the end of this section, you will be able to:
• Identify the divisions of the lower limb and describe the bones of each region
• Describe the bones and bony landmarks that articulate at each joint of the lower limb


Like the upper limb, the lower limb is divided into three regions. The thigh is that portion of the lower limb located between
the hip joint and knee joint. The leg is specifically the region between the knee joint and the ankle joint. Distal to the ankle
is the foot. The lower limb contains 30 bones. These are the femur, patella, tibia, fibula, tarsal bones, metatarsal bones, and
phalanges (see Figure 8.2). The femur is the single bone of the thigh. The patella is the kneecap and articulates with the
distal femur. The tibia is the larger, weight-bearing bone located on the medial side of the leg, and the fibula is the thin
bone of the lateral leg. The bones of the foot are divided into three groups. The posterior portion of the foot is formed by a
group of seven bones, each of which is known as a tarsal bone, whereas the mid-foot contains five elongated bones, each
of which is a metatarsal bone. The toes contain 14 small bones, each of which is a phalanx bone of the foot.


Femur
The femur, or thigh bone, is the single bone of the thigh region (Figure 8.16). It is the longest and strongest bone of the
body, and accounts for approximately one-quarter of a person’s total height. The rounded, proximal end is the head of the
femur, which articulates with the acetabulum of the hip bone to form the hip joint. The fovea capitis is a minor indentation
on the medial side of the femoral head that serves as the site of attachment for the ligament of the head of the femur. This
ligament spans the femur and acetabulum, but is weak and provides little support for the hip joint. It does, however, carry
an important artery that supplies the head of the femur.


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Figure 8.16 Femur and Patella The femur is the single bone of the thigh region. It articulates superiorly with the hip
bone at the hip joint, and inferiorly with the tibia at the knee joint. The patella only articulates with the distal end of the
femur.


The narrowed region below the head is the neck of the femur. This is a common area for fractures of the femur. The greater
trochanter is the large, upward, bony projection located above the base of the neck. Multiple muscles that act across the
hip joint attach to the greater trochanter, which, because of its projection from the femur, gives additional leverage to these
muscles. The greater trochanter can be felt just under the skin on the lateral side of your upper thigh. The lesser trochanter
is a small, bony prominence that lies on the medial aspect of the femur, just below the neck. A single, powerful muscle
attaches to the lesser trochanter. Running between the greater and lesser trochanters on the anterior side of the femur is
the roughened intertrochanteric line. The trochanters are also connected on the posterior side of the femur by the larger
intertrochanteric crest.
The elongated shaft of the femur has a slight anterior bowing or curvature. At its proximal end, the posterior shaft has the
gluteal tuberosity, a roughened area extending inferiorly from the greater trochanter. More inferiorly, the gluteal tuberosity
becomes continuous with the linea aspera (“rough line”). This is the roughened ridge that passes distally along the posterior
side of the mid-femur. Multiple muscles of the hip and thigh regions make long, thin attachments to the femur along the
linea aspera.
The distal end of the femur has medial and lateral bony expansions. On the lateral side, the smooth portion that covers the
distal and posterior aspects of the lateral expansion is the lateral condyle of the femur. The roughened area on the outer,
lateral side of the condyle is the lateral epicondyle of the femur. Similarly, the smooth region of the distal and posterior
medial femur is the medial condyle of the femur, and the irregular outer, medial side of this is the medial epicondyle


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of the femur. The lateral and medial condyles articulate with the tibia to form the knee joint. The epicondyles provide
attachment for muscles and supporting ligaments of the knee. The adductor tubercle is a small bump located at the superior
margin of the medial epicondyle. Posteriorly, the medial and lateral condyles are separated by a deep depression called the
intercondylar fossa. Anteriorly, the smooth surfaces of the condyles join together to form a wide groove called the patellar
surface, which provides for articulation with the patella bone. The combination of the medial and lateral condyles with the
patellar surface gives the distal end of the femur a horseshoe (U) shape.


Watch this video (http://openstaxcollege.org/l/midfemur) to view how a fracture of the mid-femur is surgically
repaired. How are the two portions of the broken femur stabilized during surgical repair of a fractured femur?


Patella
The patella (kneecap) is largest sesamoid bone of the body (see Figure 8.16). A sesamoid bone is a bone that is incorporated
into the tendon of a muscle where that tendon crosses a joint. The sesamoid bone articulates with the underlying bones to
prevent damage to the muscle tendon due to rubbing against the bones during movements of the joint. The patella is found
in the tendon of the quadriceps femoris muscle, the large muscle of the anterior thigh that passes across the anterior knee
to attach to the tibia. The patella articulates with the patellar surface of the femur and thus prevents rubbing of the muscle
tendon against the distal femur. The patella also lifts the tendon away from the knee joint, which increases the leverage
power of the quadriceps femoris muscle as it acts across the knee. The patella does not articulate with the tibia.


Visit this site (http://openstaxcollege.org/l/kneesurgery) to perform a virtual knee replacement surgery. The
prosthetic knee components must be properly aligned to function properly. How is this alignment ensured?


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Runner’s Knee
Runner’s knee, also known as patellofemoral syndrome, is the most common overuse injury among runners. It is most
frequent in adolescents and young adults, and is more common in females. It often results from excessive running,
particularly downhill, but may also occur in athletes who do a lot of knee bending, such as jumpers, skiers, cyclists,
weight lifters, and soccer players. It is felt as a dull, aching pain around the front of the knee and deep to the patella.
The pain may be felt when walking or running, going up or down stairs, kneeling or squatting, or after sitting with the
knee bent for an extended period.
Patellofemoral syndrome may be initiated by a variety of causes, including individual variations in the shape and
movement of the patella, a direct blow to the patella, or flat feet or improper shoes that cause excessive turning in or
out of the feet or leg. These factors may cause in an imbalance in the muscle pull that acts on the patella, resulting in
an abnormal tracking of the patella that allows it to deviate too far toward the lateral side of the patellar surface on the
distal femur.
Because the hips are wider than the knee region, the femur has a diagonal orientation within the thigh, in contrast to
the vertically oriented tibia of the leg (Figure 8.17). The Q-angle is a measure of how far the femur is angled laterally
away from vertical. The Q-angle is normally 10–15 degrees, with females typically having a larger Q-angle due to
their wider pelvis. During extension of the knee, the quadriceps femoris muscle pulls the patella both superiorly and
laterally, with the lateral pull greater in women due to their large Q-angle. This makes women more vulnerable to
developing patellofemoral syndrome than men. Normally, the large lip on the lateral side of the patellar surface of the
femur compensates for the lateral pull on the patella, and thus helps to maintain its proper tracking.
However, if the pull produced by the medial and lateral sides of the quadriceps femoris muscle is not properly
balanced, abnormal tracking of the patella toward the lateral side may occur. With continued use, this produces pain
and could result in damage to the articulating surfaces of the patella and femur, and the possible future development of
arthritis. Treatment generally involves stopping the activity that produces knee pain for a period of time, followed by a
gradual resumption of activity. Proper strengthening of the quadriceps femoris muscle to correct for imbalances is also
important to help prevent reoccurrence.


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Figure 8.17 The Q-Angle The Q-angle is a measure of the amount of lateral deviation of the femur from the
vertical line of the tibia. Adult females have a larger Q-angle due to their wider pelvis than adult males.


Tibia
The tibia (shin bone) is the medial bone of the leg and is larger than the fibula, with which it is paired (Figure 8.18). The
tibia is the main weight-bearing bone of the lower leg and the second longest bone of the body, after the femur. The medial
side of the tibia is located immediately under the skin, allowing it to be easily palpated down the entire length of the medial
leg.


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Figure 8.18 Tibia and Fibula The tibia is the larger, weight-bearing bone located on the medial side of the leg. The
fibula is the slender bone of the lateral side of the leg and does not bear weight.


The proximal end of the tibia is greatly expanded. The two sides of this expansion form the medial condyle of the tibia
and the lateral condyle of the tibia. The tibia does not have epicondyles. The top surface of each condyle is smooth and
flattened. These areas articulate with the medial and lateral condyles of the femur to form the knee joint. Between the
articulating surfaces of the tibial condyles is the intercondylar eminence, an irregular, elevated area that serves as the
inferior attachment point for two supporting ligaments of the knee.
The tibial tuberosity is an elevated area on the anterior side of the tibia, near its proximal end. It is the final site of
attachment for the muscle tendon associated with the patella. More inferiorly, the shaft of the tibia becomes triangular in
shape. The anterior apex of
MH this triangle forms the anterior border of the tibia, which begins at the tibial tuberosity and runs inferiorly along the
length of the tibia. Both the anterior border and the medial side of the triangular shaft are located immediately under the
skin and can be easily palpated along the entire length of the tibia. A small ridge running down the lateral side of the tibial
shaft is the interosseous border of the tibia. This is for the attachment of the interosseous membrane of the leg, the sheet
of dense connective tissue that unites the tibia and fibula bones. Located on the posterior side of the tibia is the soleal line,
a diagonally running, roughened ridge that begins below the base of the lateral condyle, and runs down and medially across
the proximal third of the posterior tibia. Muscles of the posterior leg attach to this line.
The large expansion found on the medial side of the distal tibia is the medial malleolus (“little hammer”). This forms the
large bony bump found on the medial side of the ankle region. Both the smooth surface on the inside of the medial malleolus
and the smooth area at the distal end of the tibia articulate with the talus bone of the foot as part of the ankle joint. On the
lateral side of the distal tibia is a wide groove called the fibular notch. This area articulates with the distal end of the fibula,
forming the distal tibiofibular joint.


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Fibula
The fibula is the slender bone located on the lateral side of the leg (see Figure 8.18). The fibula does not bear weight. It
serves primarily for muscle attachments and thus is largely surrounded by muscles. Only the proximal and distal ends of the
fibula can be palpated.
The head of the fibula is the small, knob-like, proximal end of the fibula. It articulates with the inferior aspect of the lateral
tibial condyle, forming the proximal tibiofibular joint. The thin shaft of the fibula has the interosseous border of the
fibula, a narrow ridge running down its medial side for the attachment of the interosseous membrane that spans the fibula
and tibia. The distal end of the fibula forms the lateral malleolus, which forms the easily palpated bony bump on the lateral
side of the ankle. The deep (medial) side of the lateral malleolus articulates with the talus bone of the foot as part of the
ankle joint. The distal fibula also articulates with the fibular notch of the tibia.


Tarsal Bones
The posterior half of the foot is formed by seven tarsal bones (Figure 8.19). The most superior bone is the talus. This has
a relatively square-shaped, upper surface that articulates with the tibia and fibula to form the ankle joint. Three areas of
articulation form the ankle joint: The superomedial surface of the talus bone articulates with the medial malleolus of the
tibia, the top of the talus articulates with the distal end of the tibia, and the lateral side of the talus articulates with the lateral
malleolus of the fibula. Inferiorly, the talus articulates with the calcaneus (heel bone), the largest bone of the foot, which
forms the heel. Body weight is transferred from the tibia to the talus to the calcaneus, which rests on the ground. The medial
calcaneus has a prominent bony extension called the sustentaculum tali (“support for the talus”) that supports the medial
side of the talus bone.


Figure 8.19 Bones of the Foot The bones of the foot are divided into three groups. The posterior foot is formed by
the seven tarsal bones. The mid-foot has the five metatarsal bones. The toes contain the phalanges.


The cuboid bone articulates with the anterior end of the calcaneus bone. The cuboid has a deep groove running across
its inferior surface, which provides passage for a muscle tendon. The talus bone articulates anteriorly with the navicular
bone, which in turn articulates anteriorly with the three cuneiform (“wedge-shaped”) bones. These bones are the medial
cuneiform, the intermediate cuneiform, and the lateral cuneiform. Each of these bones has a broad superior surface and
a narrow inferior surface, which together produce the transverse (medial-lateral) curvature of the foot. The navicular and
lateral cuneiform bones also articulate with the medial side of the cuboid bone.


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Use this tutorial (http://openstaxcollege.org/l/footbones) to review the bones of the foot. Which tarsal bones are in
the proximal, intermediate, and distal groups?


Metatarsal Bones
The anterior half of the foot is formed by the five metatarsal bones, which are located between the tarsal bones of the
posterior foot and the phalanges of the toes (see Figure 8.19). These elongated bones are numbered 1–5, starting with
the medial side of the foot. The first metatarsal bone is shorter and thicker than the others. The second metatarsal is the
longest. The base of the metatarsal bone is the proximal end of each metatarsal bone. These articulate with the cuboid
or cuneiform bones. The base of the fifth metatarsal has a large, lateral expansion that provides for muscle attachments.
This expanded base of the fifth metatarsal can be felt as a bony bump at the midpoint along the lateral border of the foot.
The expanded distal end of each metatarsal is the head of the metatarsal bone. Each metatarsal bone articulates with the
proximal phalanx of a toe to form a metatarsophalangeal joint. The heads of the metatarsal bones also rest on the ground
and form the ball (anterior end) of the foot.


Phalanges
The toes contain a total of 14 phalanx bones (phalanges), arranged in a similar manner as the phalanges of the fingers
(see Figure 8.19). The toes are numbered 1–5, starting with the big toe ( hallux). The big toe has two phalanx bones, the
proximal and distal phalanges. The remaining toes all have proximal, middle, and distal phalanges. A joint between adjacent
phalanx bones is called an interphalangeal joint.


View this link (http://openstaxcollege.org/l/bunion) to learn about a bunion, a localized swelling on the medial side
of the foot, next to the first metatarsophalangeal joint, at the base of the big toe. What is a bunion and what type of
shoe is most likely to cause this to develop?


Arches of the Foot
When the foot comes into contact with the ground during walking, running, or jumping activities, the impact of the body
weight puts a tremendous amount of pressure and force on the foot. During running, the force applied to each foot as it
contacts the ground can be up to 2.5 times your body weight. The bones, joints, ligaments, and muscles of the foot absorb
this force, thus greatly reducing the amount of shock that is passed superiorly into the lower limb and body. The arches of
the foot play an important role in this shock-absorbing ability. When weight is applied to the foot, these arches will flatten
somewhat, thus absorbing energy. When the weight is removed, the arch rebounds, giving “spring” to the step. The arches
also serve to distribute body weight side to side and to either end of the foot.


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The foot has a transverse arch, a medial longitudinal arch, and a lateral longitudinal arch (see Figure 8.19). The transverse
arch forms the medial-lateral curvature of the mid-foot. It is formed by the wedge shapes of the cuneiform bones and bases
(proximal ends) of the first to fourth metatarsal bones. This arch helps to distribute body weight from side to side within the
foot, thus allowing the foot to accommodate uneven terrain.
The longitudinal arches run down the length of the foot. The lateral longitudinal arch is relatively flat, whereas the medial
longitudinal arch is larger (taller). The longitudinal arches are formed by the tarsal bones posteriorly and the metatarsal
bones anteriorly. These arches are supported at either end, where they contact the ground. Posteriorly, this support is
provided by the calcaneus bone and anteriorly by the heads (distal ends) of the metatarsal bones. The talus bone, which
receives the weight of the body, is located at the top of the longitudinal arches. Body weight is then conveyed from the talus
to the ground by the anterior and posterior ends of these arches. Strong ligaments unite the adjacent foot bones to prevent
disruption of the arches during weight bearing. On the bottom of the foot, additional ligaments tie together the anterior and
posterior ends of the arches. These ligaments have elasticity, which allows them to stretch somewhat during weight bearing,
thus allowing the longitudinal arches to spread. The stretching of these ligaments stores energy within the foot, rather than
passing these forces into the leg. Contraction of the foot muscles also plays an important role in this energy absorption.
When the weight is removed, the elastic ligaments recoil and pull the ends of the arches closer together. This recovery of
the arches releases the stored energy and improves the energy efficiency of walking.
Stretching of the ligaments that support the longitudinal arches can lead to pain. This can occur in overweight individuals,
with people who have jobs that involve standing for long periods of time (such as a waitress), or walking or running long
distances. If stretching of the ligaments is prolonged, excessive, or repeated, it can result in a gradual lengthening of the
supporting ligaments, with subsequent depression or collapse of the longitudinal arches, particularly on the medial side of
the foot. This condition is called pes planus (“flat foot” or “fallen arches”).


8.5 | Development of the Appendicular Skeleton
By the end of this section, you will be able to:
• Describe the growth and development of the embryonic limb buds
• Discuss the appearance of primary and secondary ossification centers


Embryologically, the appendicular skeleton arises from mesenchyme, a type of embryonic tissue that can differentiate into
many types of tissues, including bone or muscle tissue. Mesenchyme gives rise to the bones of the upper and lower limbs, as
well as to the pectoral and pelvic girdles. Development of the limbs begins near the end of the fourth embryonic week, with
the upper limbs appearing first. Thereafter, the development of the upper and lower limbs follows similar patterns, with the
lower limbs lagging behind the upper limbs by a few days.


Limb Growth
Each upper and lower limb initially develops as a small bulge called a limb bud, which appears on the lateral side of
the early embryo. The upper limb bud appears near the end of the fourth week of development, with the lower limb bud
appearing shortly after (Figure 8.20).


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Figure 8.20 Embryo at Seven Weeks Limb buds are visible in an embryo at the end of the seventh week of
development (embryo derived from an ectopic pregnancy). (credit: Ed Uthman/flickr)


Initially, the limb buds consist of a core of mesenchyme covered by a layer of ectoderm. The ectoderm at the end of the limb
bud thickens to form a narrow crest called the apical ectodermal ridge. This ridge stimulates the underlying mesenchyme
to rapidly proliferate, producing the outgrowth of the developing limb. As the limb bud elongates, cells located farther from
the apical ectodermal ridge slow their rates of cell division and begin to differentiate. In this way, the limb develops along a
proximal-to-distal axis.
During the sixth week of development, the distal ends of the upper and lower limb buds expand and flatten into a paddle
shape. This region will become the hand or foot. The wrist or ankle areas then appear as a constriction that develops at the
base of the paddle. Shortly after this, a second constriction on the limb bud appears at the future site of the elbow or knee.
Within the paddle, areas of tissue undergo cell death, producing separations between the growing fingers and toes. Also
during the sixth week of development, mesenchyme within the limb buds begins to differentiate into hyaline cartilage that
will form models of the future limb bones.
The early outgrowth of the upper and lower limb buds initially has the limbs positioned so that the regions that will become
the palm of the hand or the bottom of the foot are facing medially toward the body, with the future thumb or big toe both
oriented toward the head. During the seventh week of development, the upper limb rotates laterally by 90 degrees, so that
the palm of the hand faces anteriorly and the thumb points laterally. In contrast, the lower limb undergoes a 90-degree
medial rotation, thus bringing the big toe to the medial side of the foot.


Watch this animation (http://openstaxcollege.org/l/limbbuds) to follow the development and growth of the upper
and lower limb buds. On what days of embryonic development do these events occur: (a) first appearance of the upper
limb bud (limb ridge); (b) the flattening of the distal limb to form the handplate or footplate; and (c) the beginning of
limb rotation?


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Ossification of Appendicular Bones
All of the girdle and limb bones, except for the clavicle, develop by the process of endochondral ossification. This process
begins as the mesenchyme within the limb bud differentiates into hyaline cartilage to form cartilage models for future
bones. By the twelfth week, a primary ossification center will have appeared in the diaphysis (shaft) region of the long
bones, initiating the process that converts the cartilage model into bone. A secondary ossification center will appear in each
epiphysis (expanded end) of these bones at a later time, usually after birth. The primary and secondary ossification centers
are separated by the epiphyseal plate, a layer of growing hyaline cartilage. This plate is located between the diaphysis and
each epiphysis. It continues to grow and is responsible for the lengthening of the bone. The epiphyseal plate is retained for
many years, until the bone reaches its final, adult size, at which time the epiphyseal plate disappears and the epiphysis fuses
to the diaphysis. (Seek additional content on ossification in the chapter on bone tissue.)
Small bones, such as the phalanges, will develop only one secondary ossification center and will thus have only a single
epiphyseal plate. Large bones, such as the femur, will develop several secondary ossification centers, with an epiphyseal
plate associated with each secondary center. Thus, ossification of the femur begins at the end of the seventh week with the
appearance of the primary ossification center in the diaphysis, which rapidly expands to ossify the shaft of the bone prior to
birth. Secondary ossification centers develop at later times. Ossification of the distal end of the femur, to form the condyles
and epicondyles, begins shortly before birth. Secondary ossification centers also appear in the femoral head late in the first
year after birth, in the greater trochanter during the fourth year, and in the lesser trochanter between the ages of 9 and 10
years. Once these areas have ossified, their fusion to the diaphysis and the disappearance of each epiphyseal plate follow a
reversed sequence. Thus, the lesser trochanter is the first to fuse, doing so at the onset of puberty (around 11 years of age),
followed by the greater trochanter approximately 1 year later. The femoral head fuses between the ages of 14–17 years,
whereas the distal condyles of the femur are the last to fuse, between the ages of 16–19 years. Knowledge of the age at
which different epiphyseal plates disappear is important when interpreting radiographs taken of children. Since the cartilage
of an epiphyseal plate is less dense than bone, the plate will appear dark in a radiograph image. Thus, a normal epiphyseal
plate may be mistaken for a bone fracture.
The clavicle is the one appendicular skeleton bone that does not develop via endochondral ossification. Instead, the clavicle
develops through the process of intramembranous ossification. During this process, mesenchymal cells differentiate directly
into bone-producing cells, which produce the clavicle directly, without first making a cartilage model. Because of this early
production of bone, the clavicle is the first bone of the body to begin ossification, with ossification centers appearing during
the fifth week of development. However, ossification of the clavicle is not complete until age 25.


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Appendicular System: Congenital Clubfoot
Clubfoot, also known as talipes, is a congenital (present at birth) disorder of unknown cause and is the most common
deformity of the lower limb. It affects the foot and ankle, causing the foot to be twisted inward at a sharp angle, like
the head of a golf club (Figure 8.21). Clubfoot has a frequency of about 1 out of every 1,000 births, and is twice as
likely to occur in a male child as in a female child. In 50 percent of cases, both feet are affected.


Figure 8.21 Clubfoot Clubfoot is a common deformity of the ankle and foot that is present at birth. Most cases
are corrected without surgery, and affected individuals will grow up to lead normal, active lives. (credit: James W.
Hanson)


At birth, children with a clubfoot have the heel turned inward and the anterior foot twisted so that the lateral side
of the foot is facing inferiorly, commonly due to ligaments or leg muscles attached to the foot that are shortened or
abnormally tight. These pull the foot into an abnormal position, resulting in bone deformities. Other symptoms may
include bending of the ankle that lifts the heel of the foot and an extremely high foot arch. Due to the limited range of
motion in the affected foot, it is difficult to place the foot into the correct position. Additionally, the affected foot may
be shorter than normal, and the calf muscles are usually underdeveloped on the affected side. Despite the appearance,
this is not a painful condition for newborns. However, it must be treated early to avoid future pain and impaired
walking ability.
Although the cause of clubfoot is idiopathic (unknown), evidence indicates that fetal position within the uterus is not
a contributing factor. Genetic factors are involved, because clubfoot tends to run within families. Cigarette smoking
during pregnancy has been linked to the development of clubfoot, particularly in families with a history of clubfoot.
Previously, clubfoot required extensive surgery. Today, 90 percent of cases are successfully treated without surgery
using new corrective casting techniques. The best chance for a full recovery requires that clubfoot treatment begin
during the first 2 weeks after birth. Corrective casting gently stretches the foot, which is followed by the application of
a holding cast to keep the foot in the proper position. This stretching and casting is repeated weekly for several weeks.
In severe cases, surgery may also be required, after which the foot typically remains in a cast for 6 to 8 weeks. After the
cast is removed following either surgical or nonsurgical treatment, the child will be required to wear a brace part-time
(at night) for up to 4 years. In addition, special exercises will be prescribed, and the child must also wear special shoes.
Close monitoring by the parents and adherence to postoperative instructions are imperative in minimizing the risk of
relapse.
Despite these difficulties, treatment for clubfoot is usually successful, and the child will grow up to lead a normal,
active life. Numerous examples of individuals born with a clubfoot who went on to successful careers include Dudley
Moore (comedian and actor), Damon Wayans (comedian and actor), Troy Aikman (three-time Super Bowl-winning
quarterback), Kristi Yamaguchi (Olympic gold medalist in figure skating), Mia Hamm (two-time Olympic gold
medalist in soccer), and Charles Woodson (Heisman trophy and Super Bowl winner).


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acetabulum


acromial end of the clavicle
acromial process
acromioclavicular joint
acromion
adductor tubercle
anatomical neck
ankle joint


anterior border of the tibia
anterior inferior iliac spine


anterior sacroiliac ligament


anterior superior iliac spine
apical ectodermal ridge


arcuate line of the ilium


arm
auricular surface of the ilium


base of the metatarsal bone
bicipital groove
calcaneus
capitate


capitulum
carpal bone


carpal tunnel
carpometacarpal joint


clavicle


KEY TERMS
large, cup-shaped cavity located on the lateral side of the hip bone; formed by the junction of the ilium,


pubis, and ischium portions of the hip bone
lateral end of the clavicle that articulates with the acromion of the scapula


acromion of the scapula
articulation between the acromion of the scapula and the acromial end of the clavicle


flattened bony process that extends laterally from the scapular spine to form the bony tip of the shoulder
small, bony bump located on the superior aspect of the medial epicondyle of the femur
line on the humerus located around the outside margin of the humeral head


joint that separates the leg and foot portions of the lower limb; formed by the articulations between the talus
bone of the foot inferiorly, and the distal end of the tibia, medial malleolus of the tibia, and lateral malleolus of the
fibula superiorly


narrow, anterior margin of the tibia that extends inferiorly from the tibial tuberosity
small, bony projection located on the anterior margin of the ilium, below the anterior


superior iliac spine
strong ligament between the sacrum and the ilium portions of the hip bone that supports


the anterior side of the sacroiliac joint
rounded, anterior end of the iliac crest


enlarged ridge of ectoderm at the distal end of a limb bud that stimulates growth and
elongation of the limb


smooth ridge located at the inferior margin of the iliac fossa; forms the lateral portion of the
pelvic brim
region of the upper limb located between the shoulder and elbow joints; contains the humerus bone


roughened area located on the posterior, medial side of the ilium of the hip bone;
articulates with the auricular surface of the sacrum to form the sacroiliac joint


expanded, proximal end of each metatarsal bone
intertubercular groove; narrow groove located between the greater and lesser tubercles of the humerus


heel bone; posterior, inferior tarsal bone that forms the heel of the foot
from the lateral side, the third of the four distal carpal bones; articulates with the scaphoid and lunate


proximally, the trapezoid laterally, the hamate medially, and primarily with the third metacarpal distally
knob-like bony structure located anteriorly on the lateral, distal end of the humerus
one of the eight small bones that form the wrist and base of the hand; these are grouped as a proximal row


consisting of (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones, and a distal row
containing (from lateral to medial) the trapezium, trapezoid, capitate, and hamate bones


passageway between the anterior forearm and hand formed by the carpal bones and flexor retinaculum
articulation between one of the carpal bones in the distal row and a metacarpal bone of the


hand
collarbone; elongated bone that articulates with the manubrium of the sternum medially and the acromion of


the scapula laterally


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coracoclavicular ligament


coracoid process


coronoid fossa


coronoid process of the ulna


costoclavicular ligament
coxal bone
cuboid


deltoid tuberosity
distal radioulnar joint
distal tibiofibular joint
elbow joint


femur
fibula
fibular notch


flexor retinaculum


foot
forearm
fossa
fovea capitis


glenohumeral joint


glenoid cavity


gluteal tuberosity


greater pelvis


greater sciatic foramen


greater sciatic notch


greater trochanter


strong band of connective tissue that anchors the coracoid process of the scapula to the
lateral clavicle; provides important indirect support for the acromioclavicular joint


short, hook-like process that projects anteriorly and laterally from the superior margin of the
scapula


depression on the anterior surface of the humerus above the trochlea; this space receives the coronoid
process of the ulna when the elbow is maximally flexed


projecting bony lip located on the anterior, proximal ulna; forms the inferior margin
of the trochlear notch


band of connective tissue that unites the medial clavicle with the first rib
hip bone


tarsal bone that articulates posteriorly with the calcaneus bone, medially with the lateral cuneiform bone, and
anteriorly with the fourth and fifth metatarsal bones


roughened, V-shaped region located laterally on the mid-shaft of the humerus
articulation between the head of the ulna and the ulnar notch of the radius
articulation between the distal fibula and the fibular notch of the tibia


joint located between the upper arm and forearm regions of the upper limb; formed by the articulations
between the trochlea of the humerus and the trochlear notch of the ulna, and the capitulum of the humerus and the
head of the radius
thigh bone; the single bone of the thigh
thin, non-weight-bearing bone found on the lateral side of the leg


wide groove on the lateral side of the distal tibia for articulation with the fibula at the distal tibiofibular
joint


strong band of connective tissue at the anterior wrist that spans the top of the U-shaped grouping of
the carpal bones to form the roof of the carpal tunnel
portion of the lower limb located distal to the ankle joint
region of the upper limb located between the elbow and wrist joints; contains the radius and ulna bones


(plural = fossae) shallow depression on the surface of a bone
minor indentation on the head of the femur that serves as the site of attachment for the ligament to the


head of the femur
shoulder joint; formed by the articulation between the glenoid cavity of the scapula and the head


of the humerus
(also, glenoid fossa) shallow depression located on the lateral scapula, between the superior and lateral


borders
roughened area on the posterior side of the proximal femur, extending inferiorly from the base of


the greater trochanter
(also, greater pelvic cavity or false pelvis) broad space above the pelvic brim defined laterally by the


fan-like portion of the upper ilium
pelvic opening formed by the greater sciatic notch of the hip bone, the sacrum, and the


sacrospinous ligament
large, U-shaped indentation located on the posterior margin of the ilium, superior to the ischial


spine
large, bony expansion of the femur that projects superiorly from the base of the femoral neck


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greater tubercle
hallux
hamate


hand
head of the femur


head of the fibula


head of the humerus


head of the metatarsal bone
head of the radius


head of the ulna


hip bone
hip joint


hook of the hamate bone
humerus
iliac crest
iliac fossa
ilium
inferior angle of the scapula
inferior pubic ramus


infraglenoid tubercle


infraspinous fossa
intercondylar eminence


intercondylar fossa


intermediate cuneiform


interosseous border of the fibula


interosseous border of the radius


enlarged prominence located on the lateral side of the proximal humerus
big toe; digit 1 of the foot
from the lateral side, the fourth of the four distal carpal bones; articulates with the lunate and triquetrum


proximally, the fourth and fifth metacarpals distally, and the capitate laterally
region of the upper limb distal to the wrist joint


rounded, proximal end of the femur that articulates with the acetabulum of the hip bone to form the
hip joint


small, knob-like, proximal end of the fibula; articulates with the inferior aspect of the lateral
condyle of the tibia


smooth, rounded region on the medial side of the proximal humerus; articulates with the
glenoid fossa of the scapula to form the glenohumeral (shoulder) joint


expanded, distal end of each metatarsal bone
disc-shaped structure that forms the proximal end of the radius; articulates with the capitulum of


the humerus as part of the elbow joint, and with the radial notch of the ulna as part of the proximal radioulnar joint
small, rounded distal end of the ulna; articulates with the ulnar notch of the distal radius, forming the


distal radioulnar joint
coxal bone; single bone that forms the pelvic girdle; consists of three areas, the ilium, ischium, and pubis
joint located at the proximal end of the lower limb; formed by the articulation between the acetabulum of the


hip bone and the head of the femur
bony extension located on the anterior side of the hamate carpal bone


single bone of the upper arm
curved, superior margin of the ilium
shallow depression found on the anterior and medial surfaces of the upper ilium


superior portion of the hip bone
inferior corner of the scapula located where the medial and lateral borders meet


narrow segment of bone that passes inferiorly and laterally from the pubic body; joins with the
ischial ramus to form the ischiopubic ramus


small bump or roughened area located on the lateral border of the scapula, near the inferior
margin of the glenoid cavity


broad depression located on the posterior scapula, inferior to the spine
irregular elevation on the superior end of the tibia, between the articulating surfaces of the


medial and lateral condyles
deep depression on the posterior side of the distal femur that separates the medial and lateral


condyles
middle of the three cuneiform tarsal bones; articulates posteriorly with the navicular bone,


medially with the medial cuneiform bone, laterally with the lateral cuneiform bone, and anteriorly with the second
metatarsal bone


small ridge running down the medial side of the fibular shaft; for attachment of
the interosseous membrane between the fibula and tibia


narrow ridge located on the medial side of the radial shaft; for attachment of the
interosseous membrane between the ulna and radius bones


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interosseous border of the tibia


interosseous border of the ulna


interosseous membrane of the forearm
interosseous membrane of the leg


interphalangeal joint
intertrochanteric crest


intertrochanteric line


intertubercular groove (sulcus)


ischial ramus


ischial spine


ischial tuberosity


ischiopubic ramus


ischium
knee joint


lateral border of the scapula
lateral condyle of the femur


lateral condyle of the tibia


lateral cuneiform


lateral epicondyle of the femur
lateral epicondyle of the humerus
lateral malleolus
lateral supracondylar ridge


leg
lesser pelvis


lesser sciatic foramen


small ridge running down the lateral side of the tibial shaft; for attachment of the
interosseous membrane between the tibia and fibula


narrow ridge located on the lateral side of the ulnar shaft; for attachment of the
interosseous membrane between the ulna and radius


sheet of dense connective tissue that unites the radius and ulna bones
sheet of dense connective tissue that unites the shafts of the tibia and fibula


bones
articulation between adjacent phalanx bones of the hand or foot digits
short, prominent ridge running between the greater and lesser trochanters on the posterior side


of the proximal femur
small ridge running between the greater and lesser trochanters on the anterior side of the


proximal femur
bicipital groove; narrow groove located between the greater and lesser tubercles of


the humerus
bony extension projecting anteriorly and superiorly from the ischial tuberosity; joins with the inferior


pubic ramus to form the ischiopubic ramus
pointed, bony projection from the posterior margin of the ischium that separates the greater sciatic notch


and lesser sciatic notch
large, roughened protuberance that forms the posteroinferior portion of the hip bone; weight-


bearing region of the pelvis when sitting
narrow extension of bone that connects the ischial tuberosity to the pubic body; formed by the


junction of the ischial ramus and inferior pubic ramus
posteroinferior portion of the hip bone
joint that separates the thigh and leg portions of the lower limb; formed by the articulations between the


medial and lateral condyles of the femur, and the medial and lateral condyles of the tibia
diagonally oriented lateral margin of the scapula
smooth, articulating surface that forms the distal and posterior sides of the lateral


expansion of the distal femur
lateral, expanded region of the proximal tibia that includes the smooth surface that


articulates with the lateral condyle of the femur as part of the knee joint
most lateral of the three cuneiform tarsal bones; articulates posteriorly with the navicular bone,


medially with the intermediate cuneiform bone, laterally with the cuboid bone, and anteriorly with the third
metatarsal bone


roughened area of the femur located on the lateral side of the lateral condyle
small projection located on the lateral side of the distal humerus


expanded distal end of the fibula
narrow, bony ridge located along the lateral side of the distal humerus, superior to the


lateral epicondyle
portion of the lower limb located between the knee and ankle joints


(also, lesser pelvic cavity or true pelvis) narrow space located within the pelvis, defined superiorly by the
pelvic brim (pelvic inlet) and inferiorly by the pelvic outlet


pelvic opening formed by the lesser sciatic notch of the hip bone, the sacrospinous ligament,
and the sacrotuberous ligament


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lesser sciatic notch
lesser trochanter
lesser tubercle
ligament of the head of the femur


limb bud


linea aspera
lunate


medial border of the scapula
medial condyle of the femur


medial condyle of the tibia


medial cuneiform


medial epicondyle of the femur
medial epicondyle of the humerus
medial malleolus
metacarpal bone


metacarpophalangeal joint


metatarsal bone


metatarsophalangeal joint
midcarpal joint


navicular


neck of the femur
neck of the radius
obturator foramen
olecranon fossa


olecranon process
patella
patellar surface


shallow indentation along the posterior margin of the ischium, inferior to the ischial spine
small, bony projection on the medial side of the proximal femur, at the base of the femoral neck


small, bony prominence located on anterior side of the proximal humerus
ligament that spans the acetabulum of the hip bone and the fovea capitis of the


femoral head
small elevation that appears on the lateral side of the embryo during the fourth or fifth week of development,


which gives rise to an upper or lower limb
longitudinally running bony ridge located in the middle third of the posterior femur


from the lateral side, the second of the four proximal carpal bones; articulates with the radius proximally, the
capitate and hamate distally, the scaphoid laterally, and the triquetrum medially


elongated, medial margin of the scapula
smooth, articulating surface that forms the distal and posterior sides of the medial


expansion of the distal femur
medial, expanded region of the proximal tibia that includes the smooth surface that


articulates with the medial condyle of the femur as part of the knee joint
most medial of the three cuneiform tarsal bones; articulates posteriorly with the navicular bone,


laterally with the intermediate cuneiform bone, and anteriorly with the first and second metatarsal bones
roughened area of the distal femur located on the medial side of the medial condyle
enlarged projection located on the medial side of the distal humerus


bony expansion located on the medial side of the distal tibia
one of the five long bones that form the palm of the hand; numbered 1–5, starting on the lateral


(thumb) side of the hand
articulation between the distal end of a metacarpal bone of the hand and a proximal


phalanx bone of the thumb or a finger
one of the five elongated bones that forms the anterior half of the foot; numbered 1–5, starting on the


medial side of the foot
articulation between a metatarsal bone of the foot and the proximal phalanx bone of a toe


articulation between the proximal and distal rows of the carpal bones; contributes to movements of the
hand at the wrist


tarsal bone that articulates posteriorly with the talus bone, laterally with the cuboid bone, and anteriorly with
the medial, intermediate, and lateral cuneiform bones


narrowed region located inferior to the head of the femur
narrowed region immediately distal to the head of the radius
large opening located in the anterior hip bone, between the pubis and ischium regions


large depression located on the posterior side of the distal humerus; this space receives the olecranon
process of the ulna when the elbow is fully extended


expanded posterior and superior portions of the proximal ulna; forms the bony tip of the elbow
kneecap; the largest sesamoid bone of the body; articulates with the distal femur


smooth groove located on the anterior side of the distal femur, between the medial and lateral
condyles; site of articulation for the patella


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pectineal line
pectoral girdle


pelvic brim


pelvic girdle
pelvic inlet
pelvic outlet


pelvis
phalanx bone of the foot


phalanx bone of the hand


pisiform


pollex
posterior inferior iliac spine


posterior sacroiliac ligament


posterior superior iliac spine
proximal radioulnar joint
proximal tibiofibular joint


pubic arch


pubic body
pubic symphysis
pubic tubercle
pubis
radial fossa


radial notch of the ulna


radial tuberosity
radiocarpal joint


radius


narrow ridge located on the superior surface of the superior pubic ramus
shoulder girdle; the set of bones, consisting of the scapula and clavicle, which attaches each upper limb


to the axial skeleton
pelvic inlet; the dividing line between the greater and lesser pelvic regions; formed by the superior margin


of the pubic symphysis, the pectineal lines of each pubis, the arcuate lines of each ilium, and the sacral promontory
hip girdle; consists of a single hip bone, which attaches a lower limb to the sacrum of the axial skeleton
pelvic brim
inferior opening of the lesser pelvis; formed by the inferior margin of the pubic symphysis, right and left


ischiopubic rami and sacrotuberous ligaments, and the tip of the coccyx
ring of bone consisting of the right and left hip bones, the sacrum, and the coccyx


(plural = phalanges) one of the 14 bones that form the toes; these include the proximal and
distal phalanges of the big toe, and the proximal, middle, and distal phalanx bones of toes two through five


(plural = phalanges) one of the 14 bones that form the thumb and fingers; these include
the proximal and distal phalanges of the thumb, and the proximal, middle, and distal phalanx bones of the fingers
two through five


from the lateral side, the fourth of the four proximal carpal bones; articulates with the anterior surface of the
triquetrum
(also, thumb) digit 1 of the hand


small, bony projection located at the inferior margin of the auricular surface on the
posterior ilium


strong ligament spanning the sacrum and ilium of the hip bone that supports the
posterior side of the sacroiliac joint


rounded, posterior end of the iliac crest
articulation formed by the radial notch of the ulna and the head of the radius
articulation between the head of the fibula and the inferior aspect of the lateral condyle of


the tibia
bony structure formed by the pubic symphysis, and the bodies and inferior pubic rami of the right and left


pubic bones
enlarged, medial portion of the pubis region of the hip bone


joint formed by the articulation between the pubic bodies of the right and left hip bones
small bump located on the superior aspect of the pubic body


anterior portion of the hip bone
small depression located on the anterior humerus above the capitulum; this space receives the head of the


radius when the elbow is maximally flexed
small, smooth area on the lateral side of the proximal ulna; articulates with the head of the


radius as part of the proximal radioulnar joint
oval-shaped, roughened protuberance located on the medial side of the proximal radius
wrist joint, located between the forearm and hand regions of the upper limb; articulation formed


proximally by the distal end of the radius and the fibrocartilaginous pad that unites the distal radius and ulna bone,
and distally by the scaphoid, lunate, and triquetrum carpal bones
bone located on the lateral side of the forearm


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sacroiliac joint
sacrospinous ligament
sacrotuberous ligament
scaphoid


scapula
shaft of the femur
shaft of the fibula
shaft of the humerus
shaft of the radius
shaft of the tibia
shaft of the ulna
soleal line
spine of the scapula


sternal end of the clavicle
sternoclavicular joint


styloid process of the radius
styloid process of the ulna
subpubic angle


subscapular fossa
superior angle of the scapula
superior border of the scapula
superior pubic ramus
supraglenoid tubercle
suprascapular notch
supraspinous fossa
surgical neck
sustentaculum tali
talus


tarsal bone


thigh
tibia


joint formed by the articulation between the auricular surfaces of the sacrum and ilium
ligament that spans the sacrum to the ischial spine of the hip bone
ligament that spans the sacrum to the ischial tuberosity of the hip bone


from the lateral side, the first of the four proximal carpal bones; articulates with the radius proximally, the
trapezoid, trapezium, and capitate distally, and the lunate medially


shoulder blade bone located on the posterior side of the shoulder
cylindrically shaped region that forms the central portion of the femur
elongated, slender portion located between the expanded ends of the fibula
narrow, elongated, central region of the humerus


narrow, elongated, central region of the radius
triangular-shaped, central portion of the tibia
narrow, elongated, central region of the ulna


small, diagonally running ridge located on the posterior side of the proximal tibia
prominent ridge passing mediolaterally across the upper portion of the posterior scapular


surface
medial end of the clavicle that articulates with the manubrium of the sternum


articulation between the manubrium of the sternum and the sternal end of the clavicle; forms
the only bony attachment between the pectoral girdle of the upper limb and the axial skeleton


pointed projection located on the lateral end of the distal radius
short, bony projection located on the medial end of the distal ulna


inverted V-shape formed by the convergence of the right and left ischiopubic rami; this angle is
greater than 80 degrees in females and less than 70 degrees in males


broad depression located on the anterior (deep) surface of the scapula
corner of the scapula between the superior and medial borders of the scapula
superior margin of the scapula


narrow segment of bone that passes laterally from the pubic body to join the ilium
small bump located at the superior margin of the glenoid cavity
small notch located along the superior border of the scapula, medial to the coracoid process
narrow depression located on the posterior scapula, superior to the spine


region of the humerus where the expanded, proximal end joins with the narrower shaft
bony ledge extending from the medial side of the calcaneus bone


tarsal bone that articulates superiorly with the tibia and fibula at the ankle joint; also articulates inferiorly with the
calcaneus bone and anteriorly with the navicular bone


one of the seven bones that make up the posterior foot; includes the calcaneus, talus, navicular, cuboid,
medial cuneiform, intermediate cuneiform, and lateral cuneiform bones
portion of the lower limb located between the hip and knee joints
shin bone; the large, weight-bearing bone located on the medial side of the leg


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tibial tuberosity
trapezium


trapezoid


triquetrum


trochlea


trochlear notch


ulna
ulnar notch of the radius


ulnar tuberosity


elevated area on the anterior surface of the proximal tibia
from the lateral side, the first of the four distal carpal bones; articulates with the scaphoid proximally, the


first and second metacarpals distally, and the trapezoid medially
from the lateral side, the second of the four distal carpal bones; articulates with the scaphoid proximally, the


second metacarpal distally, the trapezium laterally, and the capitate medially
from the lateral side, the third of the four proximal carpal bones; articulates with the lunate laterally, the


hamate distally, and has a facet for the pisiform
pulley-shaped region located medially at the distal end of the humerus; articulates at the elbow with the


trochlear notch of the ulna
large, C-shaped depression located on the anterior side of the proximal ulna; articulates at the elbow


with the trochlea of the humerus
bone located on the medial side of the forearm


shallow, smooth area located on the medial side of the distal radius; articulates with the
head of the ulna at the distal radioulnar joint


roughened area located on the anterior, proximal ulna inferior to the coronoid process


CHAPTER REVIEW
8.1 The Pectoral Girdle
The pectoral girdle, consisting of the clavicle and the scapula, attaches each upper limb to the axial skeleton. The clavicle is
an anterior bone whose sternal end articulates with the manubrium of the sternum at the sternoclavicular joint. The sternal
end is also anchored to the first rib by the costoclavicular ligament. The acromial end of the clavicle articulates with the
acromion of the scapula at the acromioclavicular joint. This end is also anchored to the coracoid process of the scapula by
the coracoclavicular ligament, which provides indirect support for the acromioclavicular joint. The clavicle supports the
scapula, transmits the weight and forces from the upper limb to the body trunk, and protects the underlying nerves and blood
vessels.
The scapula lies on the posterior aspect of the pectoral girdle. It mediates the attachment of the upper limb to the clavicle,
and contributes to the formation of the glenohumeral (shoulder) joint. This triangular bone has three sides called the medial,
lateral, and superior borders. The suprascapular notch is located on the superior border. The scapula also has three corners,
two of which are the superior and inferior angles. The third corner is occupied by the glenoid cavity. Posteriorly, the spine
separates the supraspinous and infraspinous fossae, and then extends laterally as the acromion. The subscapular fossa is
located on the anterior surface of the scapula. The coracoid process projects anteriorly, passing inferior to the lateral end of
the clavicle.


8.2 Bones of the Upper Limb
Each upper limb is divided into three regions and contains a total of 30 bones. The upper arm is the region located between
the shoulder and elbow joints. This area contains the humerus. The proximal humerus consists of the head, which articulates
with the scapula at the glenohumeral joint, the greater and lesser tubercles separated by the intertubercular (bicipital) groove,
and the anatomical and surgical necks. The humeral shaft has the roughened area of the deltoid tuberosity on its lateral side.
The distal humerus is flattened, forming a lateral supracondylar ridge that terminates at the small lateral epicondyle. The
medial side of the distal humerus has the large, medial epicondyle. The articulating surfaces of the distal humerus consist of
the trochlea medially and the capitulum laterally. Depressions on the humerus that accommodate the forearm bones during
bending (flexing) and straightening (extending) of the elbow include the coronoid fossa, the radial fossa, and the olecranon
fossa.
The forearm is the region of the upper limb located between the elbow and wrist joints. This region contains two bones, the
ulna medially and the radius on the lateral (thumb) side. The elbow joint is formed by the articulation between the trochlea
of the humerus and the trochlear notch of the ulna, plus the articulation between the capitulum of the humerus and the head
of the radius. The proximal radioulnar joint is the articulation between the head of the radius and the radial notch of the
ulna. The proximal ulna also has the olecranon process, forming an expanded posterior region, and the coronoid process
and ulnar tuberosity on its anterior aspect. On the proximal radius, the narrowed region below the head is the neck; distal
to this is the radial tuberosity. The shaft portions of both the ulna and radius have an interosseous border, whereas the distal


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ends of each bone have a pointed styloid process. The distal radioulnar joint is found between the head of the ulna and the
ulnar notch of the radius. The distal end of the radius articulates with the proximal carpal bones, but the ulna does not.
The base of the hand is formed by eight carpal bones. The carpal bones are united into two rows of bones. The proximal row
contains (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform bones. The scaphoid, lunate, and triquetrum
bones contribute to the formation of the radiocarpal joint. The distal row of carpal bones contains (from medial to lateral)
the hamate, capitate, trapezoid, and trapezium bones (“So Long To Pinky, Here Comes The Thumb”). The anterior hamate
has a prominent bony hook. The proximal and distal carpal rows articulate with each other at the midcarpal joint. The carpal
bones, together with the flexor retinaculum, also form the carpal tunnel of the wrist.
The five metacarpal bones form the palm of the hand. The metacarpal bones are numbered 1–5, starting with the thumb
side. The first metacarpal bone is freely mobile, but the other bones are united as a group. The digits are also numbered
1–5, with the thumb being number 1. The fingers and thumb contain a total of 14 phalanges (phalanx bones). The thumb
contains a proximal and a distal phalanx, whereas the remaining digits each contain proximal, middle, and distal phalanges.


8.3 The Pelvic Girdle and Pelvis
The pelvic girdle, consisting of a hip bone, serves to attach a lower limb to the axial skeleton. The hip bone articulates
posteriorly at the sacroiliac joint with the sacrum, which is part of the axial skeleton. The right and left hip bones converge
anteriorly and articulate with each other at the pubic symphysis. The combination of the hip bone, the sacrum, and the
coccyx forms the pelvis. The pelvis has a pronounced anterior tilt. The primary function of the pelvis is to support the upper
body and transfer body weight to the lower limbs. It also serves as the site of attachment for multiple muscles.
The hip bone consists of three regions: the ilium, ischium, and pubis. The ilium forms the large, fan-like region of the hip
bone. The superior margin of this area is the iliac crest. Located at either end of the iliac crest are the anterior superior
and posterior superior iliac spines. Inferior to these are the anterior inferior and posterior inferior iliac spines. The auricular
surface of the ilium articulates with the sacrum to form the sacroiliac joint. The medial surface of the upper ilium forms
the iliac fossa, with the arcuate line marking the inferior limit of this area. The posterior margin of the ilium has the large
greater sciatic notch.
The posterolateral portion of the hip bone is the ischium. It has the expanded ischial tuberosity, which supports body weight
when sitting. The ischial ramus projects anteriorly and superiorly. The posterior margin of the ischium has the shallow lesser
sciatic notch and the ischial spine, which separates the greater and lesser sciatic notches.
The pubis forms the anterior portion of the hip bone. The body of the pubis articulates with the pubis of the opposite hip
bone at the pubic symphysis. The superior margin of the pubic body has the pubic tubercle. The pubis is joined to the
ilium by the superior pubic ramus, the superior surface of which forms the pectineal line. The inferior pubic ramus projects
inferiorly and laterally. The pubic arch is formed by the pubic symphysis, the bodies of the adjacent pubic bones, and the
two inferior pubic rami. The inferior pubic ramus joins the ischial ramus to form the ischiopubic ramus. The subpubic angle
is formed by the medial convergence of the right and left ischiopubic rami.
The lateral side of the hip bone has the cup-like acetabulum, which is part of the hip joint. The large anterior opening is the
obturator foramen. The sacroiliac joint is supported by the anterior and posterior sacroiliac ligaments. The sacrum is also
joined to the hip bone by the sacrospinous ligament, which attaches to the ischial spine, and the sacrotuberous ligament,
which attaches to the ischial tuberosity. The sacrospinous and sacrotuberous ligaments contribute to the formation of the
greater and lesser sciatic foramina.
The broad space of the upper pelvis is the greater pelvis, and the narrow, inferior space is the lesser pelvis. These areas are
separated by the pelvic brim (pelvic inlet). The inferior opening of the pelvis is the pelvic outlet. Compared to the male, the
female pelvis is wider to accommodate childbirth, has a larger subpubic angle, and a broader greater sciatic notch.


8.4 Bones of the Lower Limb
The lower limb is divided into three regions. These are the thigh, located between the hip and knee joints; the leg, located
between the knee and ankle joints; and distal to the ankle, the foot. There are 30 bones in each lower limb. These are the
femur, patella, tibia, fibula, seven tarsal bones, five metatarsal bones, and 14 phalanges.
The femur is the single bone of the thigh. Its rounded head articulates with the acetabulum of the hip bone to form the
hip joint. The head has the fovea capitis for attachment of the ligament of the head of the femur. The narrow neck joins
inferiorly with the greater and lesser trochanters. Passing between these bony expansions are the intertrochanteric line on the
anterior femur and the larger intertrochanteric crest on the posterior femur. On the posterior shaft of the femur is the gluteal
tuberosity proximally and the linea aspera in the mid-shaft region. The expanded distal end consists of three articulating
surfaces: the medial and lateral condyles, and the patellar surface. The outside margins of the condyles are the medial and
lateral epicondyles. The adductor tubercle is on the superior aspect of the medial epicondyle.
The patella is a sesamoid bone located within a muscle tendon. It articulates with the patellar surface on the anterior side of
the distal femur, thereby protecting the muscle tendon from rubbing against the femur.


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The leg contains the large tibia on the medial side and the slender fibula on the lateral side. The tibia bears the weight
of the body, whereas the fibula does not bear weight. The interosseous border of each bone is the attachment site for the
interosseous membrane of the leg, the connective tissue sheet that unites the tibia and fibula.
The proximal tibia consists of the expanded medial and lateral condyles, which articulate with the medial and lateral
condyles of the femur to form the knee joint. Between the tibial condyles is the intercondylar eminence. On the anterior
side of the proximal tibia is the tibial tuberosity, which is continuous inferiorly with the anterior border of the tibia. On the
posterior side, the proximal tibia has the curved soleal line. The bony expansion on the medial side of the distal tibia is the
medial malleolus. The groove on the lateral side of the distal tibia is the fibular notch.
The head of the fibula forms the proximal end and articulates with the underside of the lateral condyle of the tibia. The
distal fibula articulates with the fibular notch of the tibia. The expanded distal end of the fibula is the lateral malleolus.
The posterior foot is formed by the seven tarsal bones. The talus articulates superiorly with the distal tibia, the medial
malleolus of the tibia, and the lateral malleolus of the fibula to form the ankle joint. The talus articulates inferiorly with the
calcaneus bone. The sustentaculum tali of the calcaneus helps to support the talus. Anterior to the talus is the navicular bone,
and anterior to this are the medial, intermediate, and lateral cuneiform bones. The cuboid bone is anterior to the calcaneus.
The five metatarsal bones form the anterior foot. The base of these bones articulate with the cuboid or cuneiform bones.
The metatarsal heads, at their distal ends, articulate with the proximal phalanges of the toes. The big toe (toe number 1) has
proximal and distal phalanx bones. The remaining toes have proximal, middle, and distal phalanges.


8.5 Development of the Appendicular Skeleton
The bones of the appendicular skeleton arise from embryonic mesenchyme. Limb buds appear at the end of the fourth week.
The apical ectodermal ridge, located at the end of the limb bud, stimulates growth and elongation of the limb. During the
sixth week, the distal end of the limb bud becomes paddle-shaped, and selective cell death separates the developing fingers
and toes. At the same time, mesenchyme within the limb bud begins to differentiate into hyaline cartilage, forming models
for future bones. During the seventh week, the upper limbs rotate laterally and the lower limbs rotate medially, bringing the
limbs into their final positions.
Endochondral ossification, the process that converts the hyaline cartilage model into bone, begins in most appendicular
bones by the twelfth fetal week. This begins as a primary ossification center in the diaphysis, followed by the later
appearance of one or more secondary ossifications centers in the regions of the epiphyses. Each secondary ossification
center is separated from the primary ossification center by an epiphyseal plate. Continued growth of the epiphyseal
plate cartilage provides for bone lengthening. Disappearance of the epiphyseal plate is followed by fusion of the bony
components to form a single, adult bone.
The clavicle develops via intramembranous ossification, in which mesenchyme is converted directly into bone tissue.
Ossification within the clavicle begins during the fifth week of development and continues until 25 years of age.


INTERACTIVE LINK QUESTIONS
1. Watch this video (http://openstaxcollege.org/l/
fractures) to see how fractures of the distal radius bone can
affect the wrist joint. Explain the problems that may occur
if a fracture of the distal radius involves the joint surface of
the radiocarpal joint of the wrist.
2. Visit this site (http://openstaxcollege.org/l/handbone)
to explore the bones and joints of the hand. What are the
three arches of the hand, and what is the importance of
these during the gripping of an object?
3. Watch this video (http://openstaxcollege.org/l/colles)
to learn about a Colles fracture, a break of the distal radius,
usually caused by falling onto an outstretched hand. When
would surgery be required and how would the fracture be
repaired in this case?
4. Watch this video (http://openstaxcollege.org/l/
3Dpelvis) for a 3-D view of the pelvis and its associated
ligaments. What is the large opening in the bony pelvis,
located between the ischium and pubic regions, and what
two parts of the pubis contribute to the formation of this
opening?


5. Watch this video (http://openstaxcollege.org/l/
midfemur) to view how a fracture of the mid-femur is
surgically repaired. How are the two portions of the broken
femur stabilized during surgical repair of a fractured
femur?
6. Visit this site (http://openstaxcollege.org/l/
kneesurgery) to perform a virtual knee replacement
surgery. The prosthetic knee components must be properly
aligned to function properly. How is this alignment
ensured?
7. Use this tutorial (http://openstaxcollege.org/l/
footbones) to review the bones of the foot. Which tarsal
bones are in the proximal, intermediate, and distal groups?
8. View this link (http://openstaxcollege.org/l/bunion) to
learn about a bunion, a localized swelling on the medial
side of the foot, next to the first metatarsophalangeal joint,
at the base of the big toe. What is a bunion and what type
of shoe is most likely to cause this to develop?
9. Watch this animation (http://openstaxcollege.org/l/
limbbuds) to follow the development and growth of the
upper and lower limb buds. On what days of embryonic


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development do these events occur: (a) first appearance of
the upper limb bud (limb ridge); (b) the flattening of the


distal limb to form the handplate or footplate; and (c) the
beginning of limb rotation?


REVIEW QUESTIONS
10. Which part of the clavicle articulates with the
manubrium?


a. shaft
b. sternal end
c. acromial end
d. coracoid process


11. A shoulder separation results from injury to the
________.


a. glenohumeral joint
b. costoclavicular joint
c. acromioclavicular joint
d. sternoclavicular joint


12. Which feature lies between the spine and superior
border of the scapula?


a. suprascapular notch
b. glenoid cavity
c. superior angle
d. supraspinous fossa


13. What structure is an extension of the spine of the
scapula?


a. acromion
b. coracoid process
c. supraglenoid tubercle
d. glenoid cavity


14. Name the short, hook-like bony process of the scapula
that projects anteriorly.


a. acromial process
b. clavicle
c. coracoid process
d. glenoid fossa


15. How many bones are there in the upper limbs
combined?


a. 20
b. 30
c. 40
d. 60


16. Which bony landmark is located on the lateral side of
the proximal humerus?


a. greater tubercle
b. trochlea
c. lateral epicondyle
d. lesser tubercle


17.Which region of the humerus articulates with the radius
as part of the elbow joint?


a. trochlea
b. styloid process
c. capitulum
d. olecranon process


18. Which is the lateral-most carpal bone of the proximal
row?


a. trapezium
b. hamate
c. pisiform
d. scaphoid


19. The radius bone ________.
a. is found on the medial side of the forearm
b. has a head that articulates with the radial notch of
the ulna


c. does not articulate with any of the carpal bones
d. has the radial tuberosity located near its distal end


20. How many bones fuse in adulthood to form the hip
bone?


a. 2
b. 3
c. 4
d. 5


21. Which component forms the superior part of the hip
bone?


a. ilium
b. pubis
c. ischium
d. sacrum


22. Which of the following supports body weight when
sitting?


a. iliac crest
b. ischial tuberosity
c. ischiopubic ramus
d. pubic body


23. The ischial spine is found between which of the
following structures?


a. inferior pubic ramus and ischial ramus
b. pectineal line and arcuate line
c. lesser sciatic notch and greater sciatic notch
d. anterior superior iliac spine and posterior
superior iliac spine


24. The pelvis ________.
a. has a subpubic angle that is larger in females
b. consists of the two hip bones, but does not
include the sacrum or coccyx


c. has an obturator foramen, an opening that is
defined in part by the sacrospinous and
sacrotuberous ligaments


d. has a space located inferior to the pelvic brim
called the greater pelvis


25. Which bony landmark of the femur serves as a site for
muscle attachments?


a. fovea capitis
b. lesser trochanter
c. head
d. medial condyle


26.What structure contributes to the knee joint?
a. lateral malleolus of the fibula
b. tibial tuberosity
c. medial condyle of the tibia
d. lateral epicondyle of the femur


27. Which tarsal bone articulates with the tibia and fibula?


a. calcaneus


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b. cuboid
c. navicular
d. talus


28.What is the total number of bones found in the foot and
toes?


a. 7
b. 14
c. 26
d. 30


29. The tibia ________.
a. has an expanded distal end called the lateral
malleolus


b. is not a weight-bearing bone
c. is firmly anchored to the fibula by an
interosseous membrane


d. can be palpated (felt) under the skin only at its
proximal and distal ends


30. Which event takes place during the seventh week of
development?


a. appearance of the upper and lower limb buds


b. flattening of the distal limb bud into a paddle
shape


c. the first appearance of hyaline cartilage models
of future bones


d. the rotation of the limbs
31. During endochondral ossification of a long bone,
________.


a. a primary ossification center will develop within
the epiphysis


b. mesenchyme will differentiate directly into bone
tissue


c. growth of the epiphyseal plate will produce bone
lengthening


d. all epiphyseal plates will disappear before birth
32. The clavicle ________.


a. develops via intramembranous ossification
b. develops via endochondral ossification
c. is the last bone of the body to begin ossification
d. is fully ossified at the time of birth


CRITICAL THINKING QUESTIONS
33. Describe the shape and palpable line formed by the
clavicle and scapula.
34. Discuss two possible injuries of the pectoral girdle that
may occur following a strong blow to the shoulder or a hard
fall onto an outstretched hand.
35. Your friend runs out of gas and you have to help
push his car. Discuss the sequence of bones and joints
that convey the forces passing from your hand, through
your upper limb and your pectoral girdle, and to your axial
skeleton.
36. Name the bones in the wrist and hand, and describe or
sketch out their locations and articulations.
37. Describe the articulations and ligaments that unite the
four bones of the pelvis to each other.
38. Discuss the ways in which the female pelvis is adapted
for childbirth.


39. Define the regions of the lower limb, name the bones
found in each region, and describe the bony landmarks that
articulate together to form the hip, knee, and ankle joints.
40. The talus bone of the foot receives the weight of the
body from the tibia. The talus bone then distributes this
weight toward the ground in two directions: one-half of the
body weight is passed in a posterior direction and one-half
of the weight is passed in an anterior direction. Describe
the arrangement of the tarsal and metatarsal bones that are
involved in both the posterior and anterior distribution of
body weight.
41. How can a radiograph of a child’s femur be used to
determine the approximate age of that child?
42. How does the development of the clavicle differ from
the development of other appendicular skeleton bones?


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9 | JOINTS


Figure 9.1 Girl Kayaking Without joints, body movements would be impossible. (credit: Graham Richardson/
flickr.com)


Introduction
Chapter Objectives


After this chapter, you will be able to:
• Discuss both functional and structural classifications for body joints
• Describe the characteristic features for fibrous, cartilaginous, and synovial joints and give examples of each
• Define and identify the different body movements
• Discuss the structure of specific body joints and the movements allowed by each
• Explain the development of body joints


The adult human body has 206 bones, and with the exception of the hyoid bone in the neck, each bone is connected to
at least one other bone. Joints are the location where bones come together. Many joints allow for movement between the
bones. At these joints, the articulating surfaces of the adjacent bones can move smoothly against each other. However, the
bones of other joints may be joined to each other by connective tissue or cartilage. These joints are designed for stability
and provide for little or no movement. Importantly, joint stability and movement are related to each other. This means that
stable joints allow for little or no mobility between the adjacent bones. Conversely, joints that provide the most movement
between bones are the least stable. Understanding the relationship between joint structure and function will help to explain
why particular types of joints are found in certain areas of the body.


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The articulating surfaces of bones at stable types of joints, with little or no mobility, are strongly united to each other.
For example, most of the joints of the skull are held together by fibrous connective tissue and do not allow for movement
between the adjacent bones. This lack of mobility is important, because the skull bones serve to protect the brain. Similarly,
other joints united by fibrous connective tissue allow for very little movement, which provides stability and weight-bearing
support for the body. For example, the tibia and fibula of the leg are tightly united to give stability to the body when
standing. At other joints, the bones are held together by cartilage, which permits limited movements between the bones.
Thus, the joints of the vertebral column only allow for small movements between adjacent vertebrae, but when added
together, these movements provide the flexibility that allows your body to twist, or bend to the front, back, or side. In
contrast, at joints that allow for wide ranges of motion, the articulating surfaces of the bones are not directly united to
each other. Instead, these surfaces are enclosed within a space filled with lubricating fluid, which allows the bones to move
smoothly against each other. These joints provide greater mobility, but since the bones are free to move in relation to each
other, the joint is less stable. Most of the joints between the bones of the appendicular skeleton are this freely moveable type
of joint. These joints allow the muscles of the body to pull on a bone and thereby produce movement of that body region.
Your ability to kick a soccer ball, pick up a fork, and dance the tango depend on mobility at these types of joints.


9.1 | Classification of Joints
By the end of this section, you will be able to:
• Distinguish between the functional and structural classifications for joints
• Describe the three functional types of joints and give an example of each
• List the three types of diarthrodial joints


A joint, also called an articulation, is any place where adjacent bones or bone and cartilage come together (articulate
with each other) to form a connection. Joints are classified both structurally and functionally. Structural classifications of
joints take into account whether the adjacent bones are strongly anchored to each other by fibrous connective tissue or
cartilage, or whether the adjacent bones articulate with each other within a fluid-filled space called a joint cavity. Functional
classifications describe the degree of movement available between the bones, ranging from immobile, to slightly mobile,
to freely moveable joints. The amount of movement available at a particular joint of the body is related to the functional
requirements for that joint. Thus immobile or slightly moveable joints serve to protect internal organs, give stability to the
body, and allow for limited body movement. In contrast, freely moveable joints allow for much more extensive movements
of the body and limbs.


Structural Classification of Joints
The structural classification of joints is based on whether the articulating surfaces of the adjacent bones are directly
connected by fibrous conne