H

HUMAN PHYSIOLOGY
by Wikibooks contributors


From Wikibooks,
the open-content textbooks collection




© Copyright 2006–2007, Wikibooks contributors. This book is published by Wikibooks contributors.


Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free
Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License".


Image licenses are listed in the section entitled "Image Credits."


Main authors: Provophys (C) Whiteknight (C) RiRi82 (C) Jcran69 (C) Scout21972 (C) · Jtervortn (C) ·
DorothyD (C) · VWilkes (C) · Jacquel (C) · Danyellmarie (C) · Keith davis (C) · Mperkins (C) · Never2late
(C) · Shellybird2 (C) · BriannaLenford (C) · Jen A (C) · Pwoodson (C) · Nataliehaveron (C) · Melissasmith
(C) · Brentwaldrop (C)


Cover: Der Mensch als Industriepalast (Man as Industrial Palace) by Fritz Kahn.


The current version of this Wikibook may be found at:
http://en.wikibooks.org/wiki/Human_Physiology




Contents
Introduction........................................................................................................................................... 3


CHAPTERS..............................................................................................................................4
01. Homeostasis.....................................................................................................................................4
02. Cell Physiology............................................................................................................................. 14
03. The Integumentary System............................................................................................................35
04. The Nervous System..................................................................................................................... 54
05. Senses............................................................................................................................................ 81
06. The Muscular System..................................................................................................................107
07. Blood Physiology........................................................................................................................ 122
08. The Cardiovascular System.........................................................................................................137
09. The Immune System....................................................................................................................162
10. The Urinary System.....................................................................................................................186
11. The Respiratory System.............................................................................................................. 201
12. The Gastrointestinal System........................................................................................................217
13. Nutrition...................................................................................................................................... 244
14. The Endocrine System.................................................................................................................262
15. The Male Reproductive System.................................................................................................. 281
16. The Female Reproductive System...............................................................................................301
17. Pregnancy and Birth.................................................................................................................... 326
18. Genetics and Inheritance............................................................................................................. 351
19. Development: Birth Through Death............................................................................................370


APPENDICES....................................................................................................................... 397
A. Answers to Review Questions......................................................................................................397


ABOUT THE BOOK.............................................................................................................. 424
History & Document Notes...............................................................................................................424
Authors & Image Credits.................................................................................................................. 425
GNU Free Documentation License................................................................................................... 426


Introduction
Human physiology is the study of the functioning of the normal body, and is responsible for
describing how various systems of the human body work. Explanations often begin at a macroscopic
level and proceed to a molecular level. In 1926, Fritz Kahn portrayed the body as a complex chemical
plant, as seen in the painting on the right. This textbook provides an introductory explanation of the
workings of the human body, with an effort to draw connections between the body systems and explain
their interdependencies. A framework for the book is homeostasis and how the body maintains balance
within each system. This is intended as a first introduction to physiology for a college-level course. As
such, some material is deliberately left out (but references will be provided within chapters for students
wishing to learn more).




Chapter 1


1 HOMEOSTASIS
live version • discussion • edit lesson • comment • report an error


Overview
he human body consists of trillions of cells all working together for the maintenance of the entire
organism. While cells may perform very different functions, all the cells are quite similar in their


metabolic requirements. Maintaining a constant internal environment with all that the cells need to
survive (oxygen, glucose, mineral ions, waste removal, and so forth) is necessary for the well-being of
individual cells and the well-being of the entire body. The varied processes by which the body
regulates its internal environment are collectively referred to as homeostasis.


T


What is Homeostasis?
Homeostasis in a general sense refers to stability, balance or equilibrium. Maintaining a stable


internal environment requires constant monitoring and adjustments as conditions change. This
adjusting of physiological systems within the body is called homeostatic regulation.


Homeostatic regulation involves three parts or mechanisms: 1) the receptor, 2) the control center
and 3) the effector.


The receptor receives information that something in the environment is changing. The control
center or integration center receives and processes information from the receptor. And lastly, the
effector responds to the commands of the control center by either opposing or enhancing the stimulus.


A metaphor to help us understand this process is the operation of a thermostat. The thermostat
monitors and controls room temperature. The thermostat is set at a certain temperature that is
considered ideal, the set point. The function of the thermostat is to keep the temperature in the room
within a few degrees of the set point. If the room is colder than the set point, the thermostat receives
information from the thermometer (the receptor) that it is too cold. The effectors within the thermostat
then will turn on the heat to warm up the room. When the room temperature reaches the set point, the
receptor receives the information, and the thermostat "tells" the heater to turn off. This also works
when it is too hot in the room. The thermostat receives the information and turns on the air conditioner.
When the set point temperature is reached, the thermostat turns off the air conditioner.


Our bodies control body temperature in a similar way. The brain is the control center, the receptor
is our body's temperature sensors, and the effector is our blood vessels and sweat glands in our skin.
When we feel heat, the temperature sensors in our skin send the message to our brain. Our brain then
sends the message to the sweat glands to increase sweating and increase blood flow to our skin. When
we feel cold, the opposite happens. Our brain sends a message to our sweat glands to decrease
sweating, decrease blood flow, and begin shivering. This is an ongoing process that continually works
to restore and maintain homeostasis.


Because the internal and external environment of the body are constantly changing and
adjustments must be made continuously to stay at or near the set point, homeostasis can be thought of
as a dynamic equilibrium.
4 | Human Physiology




Homeostasis


Positive and Negative Feedback
When a change of variable occurs, there are two main types of feedback to which the system


reacts:
• Negative feedback: a reaction in which the system responds in such a way as to reverse the


direction of change. Since this tends to keep things constant, it allows the maintenance of
homeostasis. For instance, when the concentration of carbon dioxide in the human body
increases, the lungs are signaled to increase their activity and expel more carbon dioxide.
Thermoregulation is another example of negative feedback. When body temperature rises (or
falls), receptors in the skin and the hypothalamus sense a change, triggering a command from
the brain. This command, in turn, effects the correct response, in this case a decrease in body
temperature.


• Home Heating System Vs. Negative Feedback: When you are home, you set your
thermostat to a desired temperature. Let's say today you set it at 70 degrees. The thermometer in
the thermostat waits to sense a temperature change either too high above or too far below the 70
degree set point. When this change happens the thermometer will send a message to the
"Control Center", or thermostat, Which in turn will then send a message to the furnace to either
shut off if the temperature is too high or kick back on if the temperature is too low. In the home-
heating example the air temperature is the "NEGATIVE FEEDBACK." When the Control
Center receives negative feedback it triggers a chain reaction in order to maintain room
temperature.


• Positive feedback: a response is to amplify the change in the variable. This has a
destabilizing effect, so does not result in homeostasis. Positive feedback is less common in
naturally occurring systems than negative feedback, but it has its applications. For example, in
nerves, a threshold electric potential triggers the generation of a much larger action potential.
Blood clotting and events in childbirth are other types of positive feedback.


'*Harmful Positive Feedback' Although Positive Feedback is needed within Homeostasis it also
can be harmful at times. When you have a high fever it causes a metabolic change that can push the
fever higher and higher. In rare occurrences the the body temperature reaches 113 degrees the cellular
proteins stop working and the metabolism stops, resulting in death.


Summary: Sustainable systems require combinations of both kinds of feedback. Generally with
the recognition of divergence from the homeostatic condition, positive feedbacks are called into play,
whereas once the homeostatic condition is approached, negative feedback is used for "fine tuning"
responses. This creates a situation of "metastability," in which homeostatic conditions are maintained
within fixed limits, but once these limits are exceeded, the system can shift wildly to a wholly new (and
possibly less desirable) situation of homeostasis.


Homeostatic systems have several properties
• They are ultra-stable, meaning the system is capable of testing which way its variables


should be adjusted.
• Their whole organization (internal, structural, and functional) contributes to the


Wikibooks | 5




Chapter 1
maintenance of balance.


• Physiology is largely a study of processes related to homeostasis. Some of the functions
you will learn about in this book are not specifically about homeostasis (e.g. how muscles
contract), but in order for all bodily processes to function there must be a suitable internal
environment. Homeostasis is, therefore, a fitting framework for the introductory study of
physiology.


Where did the term "Homeostasis" come from?
The concept of homeostasis was first articulated by the French scientist Claude Bernard (1813-


1878) in his studies of the maintenance of stability in the "milieu interior." He said, "All the vital
mechanisms, varied as they are, have only one object, that of preserving constant the conditions of life
in the internal environment" (from Leçons sur les Phénonèmes de la Vie Commune aux Animaux et aux
Végétaux, 1879). The term itself was coined by American physiologist Walter Cannon, author of The
Wisdom of the Body (1932). The word comes from the Greek homoios (same, like, resembling) and
stasis (to stand, posture).


Cruise Control on a car as a simple metaphor for homeostasis
When a car is put on cruise control it has a set speed limit that it will travel. At times this speed


may vary by a few miles per hour but in general the system will maintain the set speed. If the car starts
to go up a hill, the systems will automatically increase the amount of fuel given to maintain the set
speed. If the car starts to come down a hill, the car will automatically decrease the amount of fuel given
in order to maintain the set speed. It is the same with homeostasis- the body has a set limit on each
environment. If one of these limits increases or decreases, the body will sense and automatically try to
fix the problem in order to maintain the pre-set limits This is a simple metaphor of how the body
operates--constant monitoring of levels, and automatic small adjustments when those levels fall below
(or rise above) a set point.


Pathways That Alter Homeostasis
A variety of homeostatic mechanisms maintain the internal environment within tolerable limits.


Either homeostasis is maintained through a series of control mechanisms, or the body suffers various
illnesses or disease. When the cells in your body begin to malfunction, the homeostatic balance
becomes disrupted. Eventually this leads to disease or cell malfunction. Disease and cellular
malfunction can be caused in two basic ways: either, deficiency (cells not getting all they need) or
toxicity (cells being poisoned by things they do not need). When homeostasis is interrupted in your
cells, there are pathways to correct or worsen the problem. In addition to the internal control
mechanisms, there are external influences based primarily on lifestyle choices and environmental
exposures that influence our body's ability to maintain cellular health.


• Nutrition: If your diet is lacking in a specific vitamin or mineral your cells will function
poorly, possibly resulting in a disease condition. For example, a menstruating woman with
inadequate dietary intake of iron will become anemic. Lack of hemoglobin, a molecule that
requires iron, will result in reduced oxygen-carrying capacity. In mild cases symptoms may be


6 | Human Physiology




Homeostasis


vague (e.g. fatigue), but if the anemia is severe the body will try to compensate by increasing
cardiac output, leading to palpitations and sweatiness, and possibly to heart failure.


• Toxins: Any substance that interferes with cellular function, causing cellular malfunction.
This is done through a variety of ways; chemical, plant, insecticides, and or bites. A commonly
seen example of this is drug overdoses. When a person takes too much of a drug their vital signs
begin to waver; either increasing or decreasing, these vital signs can cause problems including
coma, brain damage and even death.


• Psychological: Your physical health and mental health are inseparable. Our thoughts and
emotions cause chemical changes to take place either for better as with meditation, or worse as
with stress.


• Physical: Physical maintenance is essential for our cells and bodies. Adequate rest,
sunlight, and exercise are examples of physical mechanisms for influencing homeostasis. Lack
of sleep is related to a number of ailments such as irregular cardiac rhythms, fatigue, anxiety
and headaches.


• Genetic/Reproductive: Inheriting strengths and weaknesses can be part of our genetic
makeup. Genes are sometimes turned off or on due to external factors which we can have some
control over, but at other times little can be done to correct or improve genetic diseases.
Beginning at the cellular level a variety of diseases come from mutated genes. For example,
cancer can be genetically inherited or can be caused due to a mutation from an external source
such as radiation or genes altered in a fetus when the mother uses drugs.


• Medical: Because of genetic differences some bodies need help in gaining or maintaining
homeostasis. Through modern medicine our bodies can be given different aids -from anti-bodies
to help fight infections or chemotherapy to kill harmful cancer cells. Traditional and alternative
medical practices have many benefits, but the potential for harmful effects is also present.
Whether by nosocomial infections, or wrong dosage of medication, homeostasis can be altered
by that which is trying to fix it. Trial and error with medications can cause potential harmful
reactions and possibly death if not caught soon enough.


The factors listed above all have their effects at the cellular level, whether harmful or beneficial.
Inadequate beneficial pathways (deficiency) will almost always result in a harmful waiver in
homeostasis. Too much toxicity also causes homeostatic imbalance, resulting in cellular malfunction.
By removing negative health influences, and providing adequate positive health influences, your body
is better able to self-regulate and self-repair, thus maintaining homeostasis.


Homeostasis Throughout the Body
Each body system contributes to the homeostasis of other systems and of the entire organism. No


system of the body works in isolation, and the well-being of the person depends upon the well-being of
all the interacting body systems. A disruption within one system generally has consequences for several
additional body systems. Here are some brief explanations of how various body systems contribute to
the maintenance of homeostasis:


Wikibooks | 7




Chapter 1


Nervous System
The nervous system, along with the endocrine system, serves as the primary control center of the


body working below the level of consciousness. For example, the hypothalamus of the brain is where
the body's "thermostat" is found. The hypothalamus also stimulates the pituitary gland to release
various hormones that control metabolism and development of the body. The sympathetic and
parasympathetic divisions of the nervous system alternatively stimulate or inhibit various bodily
responses (such as heart rate, breathing rate, etc) to help maintain proper levels. It also controls
contractions like the arrector pili muscles (involved in thermoregulation) and skeletal muscles, which in
addition to moving the body, also cause bone thickening and maintenance, which affects bone
composition. The nervous system also regulates various systems such as respiratory (controls pace and
depth of breathing), cardiovascular system (controls heart rate and blood pressure), endocrine organs
(causes secretion of ADH and oxytocin), the digestive system (regulates the digestive tract movement
and secretion), and the urinary system (it helps adjust renal blood pressure and also controls voiding the
bladder). The nervous system is also involved in our sexual behaviors and functions.


Endocrine System
The endocrine system consists of glands which secrete hormones into the bloodstream. Each


hormone has an effect on one or more target tissues. In this way the endocrine system regulates the
metabolism and development of most body cells and body systems. To be more specific, the Endocrine
system has sex hormones that can activate sebaceous glands, development of mammary glands, alter
dermal blood flow and release lipids from adipocytes and MSH can stimulate melanocytes on our skin.
Our bone growth is regulated by several hormones, and the endocrine system helps with the
mobilization of calcitonin and calcium. In the muscular system hormones adjust muscle metabolism,
energy production, and growth. In the nervous system hormones affect neural metabolism, regulate
fluid/electrolyte balance and help with reproductive hormones that influence CNS development and
behaviors. In the Cardiovascular system we need hormones that regulate the production of RBC's,
elevate and lower blood pressure. Hormones also have anti-inflammatory affects as well as stimulates
the lymphatic system. In summary, the endocrine system has a regulatory effect on basically every
other body system.


Integumentary System
The integumentary system is involved in protecting the body from invading microbes (mainly by


forming a thick impenetrable layer), regulating body temperature through sweating and vasodilation, or
shivering and piloerection (goose bumps), and regulating ion balances in the blood. Stimulation of mast
cells also produce changes in blood flow and capillary permeability which can effect the blood flow in
the body and how it is regulated. It also helps synthesize vitamin D which interacts with calcium and
phosphorus absorption needed for bone growth and maintenance for example if we ever broke a bone
and it needs repair. Hair on the skin guards entrance into the nasal cavity or other orifices preventing
invaders of getting further into our bodies. Our skin also helps maintain balance by excretion of water
and other solutes (i.e.) the keratinized epidermis limits fluid loss through skin. Thus saying it provides
mechanical protection against environmental hazards. We need to remember that our skin is
integumentary, it is our first line of defense and to keep it balanced takes work.


8 | Human Physiology




Homeostasis


Skeletal System
The skeletal system serves as an important mineral reserve. For example, if blood levels of calcium


or magnesium are low and the minerals are not available in the diet, they will be taken from the bones.
On the other hand the skeletal system provides Calcium needed for all muscle contractions.
Lymphocytes and other cells relating to the immune response are produced and stored in the bone
marrow. The skeletal system aids in protection of the nervous system, endocrine organs, chest and
pelvic regions - all of these are vital organs.


Muscular System
The muscular system is largely responsible for maintaining body temperature through heat


production. It also contributes to blood glucose balance by storing energy as glycogen. Indirectly it
contributes to the well-being of the organism by simply allowing a person to move about (to find and
consume food, find shelter from weather extremes, etc.) by having our skin intact to muscles help us
with our facial expressions. In the nervous system it helps monitor body position. Muscles provide us
with protecting our endocrine glands and digestive organs. They also contol contractions during sex
producing sensations, and controls muscles that allow you to hold your urine when you are thousands
of miles away from the nearest bathroom or without toilet paper. Muscles also aid in moving blood
through veins, protect deep blood vessels and help the lymphatic system move lymph.


Cardiovascular System
The cardiovascular system, in addition to needing to maintain itself within certain levels, plays a


role in maintenance of other body systems by transporting hormones (heart secretes ANP and BNP)
and nutrients (oxygen, EPO to bones,etc.), taking away waste products, and providing all living body
cells with a fresh supply of oxygen and removing carbon dioxide. Homeostasis is disturbed if the
cardiovascular or lymphatic systems are not functioning correctly. Our skin, bones, muscles, nervous
system, endocrine, lymphatic system, lungs, digestive tract, urinary system and reproductive use the
cardiovascular system as its "road" or "highway" as far as distribution of things that go on in our body.
There are many risk factors for an unhealthy cardiovascular system. Some diseases associated are
typically labeled "uncontrollable" or "controllable." The main uncontrollable risk factors are age,
gender, and a family history of heart disease, especially at an early age.


Lymphatic System
The lymphatic system has three principal roles. First is the maintenance of blood and tissue


volume. Excess fluid that leaves the capillaries when under pressure would build up and cause edema.
Secondly, the lymphatic system absorbs fatty acids and triglycerides from fat digestion so that these
components of digestion do not enter directly into the blood stream. Third, the lymphatic system is
involved in defending the body against invading microbes, and the immune response. This system
assists in maintenance such as bone repair after injuries and muscle repair after an injury. Another
defense is maintaining the acid pH of urine to fight infections in the urinary system. The tonsils are our
bodies helpers to defend us against infections and toxins absorbed from the digestive tract. The tonsils
also protect against infections entering into our lungs.


Wikibooks | 9




Chapter 1


Respiratory System
The respiratory system works in conjunction with the cardiovascular system to provide oxygen to


cells within every body system for cellular metabolism. The respiratory system also removes carbon
dioxide. Since CO2 is mainly transported in the plasma as bicarbonate ions, which act as a chemical
buffer, the respiratory system also helps maintain proper blood pH levels a fact that is very important
for homeostasis. As a result of hyperventilation, CO2 is decreased in blood levels. This causes the pH
of body fluids to increase. If acid levels rise above 7.45, the result is respiratory alkalosis. On the other
hand, too much CO2 causes pH to fall below 7.35 which results in respiratory acidosis. The respiratory
system also helps the lymphatic system by trapping pathogens and protecting deeper tissues within.
Note that when you have increased thoracic space it can provide abdominal pressure through the
contraction of respiratory muscles. This can assist in defecation. Remember the lungs are the gateway
for our breath of life.


Digestive System
Without a regular supply of energy and nutrients from the digestive system all body systems would


soon suffer. The digestive system absorbs organic substances, vitamins, ions, and water that are needed
all over the body. In the skin the digestive tract provides lipids for storage in the subcutaneous layer.
Note that food undergoes three types of processes in the body: digestion, absorption, and elimination. If
one of these is not working, you will have problems that will be extremely noticeable. Mechanics of
digestion can be chemical digestion, movements, ingestion absorption, and elimination. In order to
maintain a healthy and efficient digestive system we have to remember the components involved. If
these are disturbed, digestive health may be compromised.


Urinary System
Toxic nitrogenous wastes accumulate as proteins and nucleic acids are broken down and used for


other purposes. The urinary system rids the body of these wastes. The urinary system is also directly
involved in maintaining proper blood volume (and indirectly blood pressure) and ion concentration
within the blood. One other contribution is that the kidneys produce a hormone (erythropoietin) that
stimulates red blood cell production. The kidneys also play an important role in maintaining the correct
water content to of the body and the correct salt composition of extracellular fluid. External changes
that lead to excess fluid loss trigger feedback mechanisms than they act to maintain the body's fluid
content by inhibiting fluid loss.


Reproductive System
The Reproductive System is unique in that it does little to contribute to the homeostasis of the


organism. Rather than being tied to the maintenance of the organism, the reproductive system relates to
the maintenance of the species. Having said that, the sex hormones do have an effect on other body
systems, and an imbalance can lead to various disorders (e.g. a woman whose ovaries are removed
early in life is at much higher risk of osteoporosis).


10 | Human Physiology




Homeostasis


Case Study
Heat stroke and Heat exhaustion


If you have ever tried to do some sort of heavy manual labor on a hot day or competed in a
physical competition you may have experienced dizziness and weakness. In some cases when it is
severe enough you may have even gone as far as to collapsing and loss of consciousness. This is known
as heat exhaustion. Heat exhaust happens when your body is trying to get rid of excessive heat and
keep its temperature at a optimal place. When the body is trying to get rid of a lot of heat you will start
to sweat in large amounts which will lead to a significant reduction of blood volume. The body also
diverts the blood to the skin from other areas of the body. With both of these changes the body
produces a reduction in blood pressure which will reduce the blood flow to the brain and give you the
symptoms described above. heat stroke is a far more serious condition. This happens when the body's
temperature rises out of control due to the failure of the thermoregulating system. If the body is unable
to reduce its temperature due to outside or physical influences the brain will start to malfunction.
Delirium and loss of consciences set in. The center of the brain controlling the sweat glands will stop
functioning halting the production of sweat. This causes the body's temperature to rise even faster.
furthermore with the increase of the body's temperature the metabolic process will speed up causing
even more heat in the body. If left untreated this will result in death. One of the easiest ways to spot
heat stroke is the skin. If it is flushed due to the increase of blood flow but dry because the sweat glands
have stopped secreting the individual will need medical attention fast.


Other Examples
• Thermoregulation


• The skeletal muscles can shiver to produce heat if the body temperature is too low.
• Non-shivering thermogenesis involves the decomposition of fat to produce heat.
• Sweating cools the body with the use of evaporation.
• Chemical regulation


• The pancreas produces insulin and glucagon to control blood-sugar concentration.
• The lungs take in oxygen and give off carbon dioxide, which regulates pH in the blood.
• The kidneys remove urea, and adjust the concentrations of water and a wide variety of ions.


Main examples of homeostasis in mammals are as follows:
• The regulation of the amounts of water and minerals in the body. This is known as


osmoregulation. This happens primarily in the kidneys.
• The removal of metabolic waste. This is known as excretion. This is done by the excretory


organs such as the kidneys and lungs.
• The regulation of body temperature. This is mainly done by the skin.
• The regulation of blood glucose level. This is mainly done by the liver and the insulin and


glucagon secreted by the pancreas in the body.
Most of these organs are controlled by hormones secreted from the pituitary gland, which in turn is


directed by the hypothalamus.


Wikibooks | 11




Chapter 1


Review Questions
1.Meaning of Homeostasis:


A) contributor and provider
B) expand
C) same or constant
D) receiver


2.What is the normal pH value for body fluid?
A) 7.15-7.25
B) 7.35-7.45
C) 7.55- 7.65
D) 7.00-7.35
E) 6.5-7.5


3.An example of the urinary system working with the respiratory system to regulate blood pH would be
A) When you hold your breath the kidneys will remove CO2 from your blood
B) If you exercise a lot your urine will become more acidic
C) If you have smoke and develop emphysema the kidneys will remove fewer bicarbonate ions
from circulation
D) If you hyperventilate the kidneys will counteract the alkalinity by adding hydrogen ions into the
blood stream
E) None of the above-the urinary system never works with the respiratory system


4.The urge to breathe comes in direct response to:
A) How long it has been since you last took a breath
B) The oxygen concentration of your surrounding environment
C) The buildup of nitrogen within your blood stream
D) The pH of your blood
E) The buildup of blood pressure that occurs when you don't breathe


5.In response to a bacterial infection my body's thermostat is raised. I start to shiver and produce more
body heat. When my body temperature reaches 101 degrees, I stop shivering and my body temperature
stops going up. This is an example of:


A) Negative feedback
B) A malfunctioning control system
C) Positive feedback
D) A negative impact


6.Which of the follow is an example of a positive feedback?
A) Shivering to warm up in a cold winter storm
B) A cruise control set on your car applies more gas when going up a hill
C) You sweat on a hot summer's day and the blood vessels in your skin vasodilate


12 | Human Physiology




Homeostasis


D) You get cut and platelets form a clot. This in turn activates the fibrin clotting system and more
blood forms clots


7.Where is the body's "thermostat" found?
A) Within the nervous system, in the Hypothalamus
B) Within the integumentary system, in the skin
C) Within the brain, in the corpus callosum
D) Within the Urinary system, in the kidneys


Glossary
Control Center or Integration Center: receives and processes information from the receptor
Effector: responds to the commands of the control center by either opposing or enhancing the


stimulus
Homeostasis: refers to stability, balance or equilibrium
Negative Feedback: a reaction in which the system responds in such a way as to reverse the


direction of change
Positive Feedback: a response is to amplify the change in the variable
Receptor: receives information that something in the environment is changing


live version • discussion • edit lesson • comment • report an error


Wikibooks | 13




Chapter 2


2 CELL PHYSIOLOGY
live version • discussion • edit lesson • comment • report an error


Cell Structure and Function
What is a Cell?


ells are the microscopic fundamental units of all living things. Every living thing has cells:
bacteria, protozoans, fungi, plants, and animals are the main groups (Kingdoms) of living things.


Some organisms are made up of just one cell (e.g. bacteria and protozoans), but animals, including
human beings, are multicellular. An adult human body is composed of about 100 trillion cells! Each
cell has basic requirements to sustain it, and the body's organ systems are largely built around
providing the many trillions of cells with those basic needs (such as oxygen, food, and waste removal).


C
There are about 200 different kinds of specialized cells in the human body. When many identical


cells are organized together it is called a tissue (such as muscle tissue, nervous tissue, etc). Various
tissues organized together for a common purpose are called organs (e.g. the stomach is an organ, and so
is the skin, the brain, and the uterus).


Ideas about cell structure have changed considerably over the years. Early biologists saw cells as
simple membranous sacs containing fluid and a few floating particles. Today's biologists know that
cells are infinitely more complex than this. Therefore, a strong knowledge of the various cellular
organelles and their functions is important to any physiologist. If a person's cells are healthy, then that
person is healthy. All physiological processes, growth and development, and disease can be described
at the cellular level.


Specialized Cells of the Human Body
Although there are specialized cells - both in structure and function - within the body, all cells


have similarities in their structural organization and metabolic needs (such as maintaining energy levels
via conversion of carbohydrate to ATP and using genes to create and maintain proteins).


Here are some of the different types of specialized cells within the human body.
• Nerve Cells: Also called Neurons, these cells are in the nervous system and function to


process and transmit information. They are the core components of the brain, spinal cord and
peripheral nerves. They use chemical and electrical synapses to relay signals throughout the
body.


• Epithelial cells: Functions of epithelial cells include secretion, absorption, protection,
transcellular transport, sensation detection, and selective permeability. Epithelium lines both the
outside (skin) and the inside cavities and lumen of bodies.


• Exocrine cells: These cells secrete products through ducts, such as mucus, sweat, or
digestive enzymes.


14 | Human Physiology




Cell Physiology


• Endocrine cells: These cells are similar to exocrine cells, but secrete their products directly
into the bloodstream instead of through a duct. Endocrine cells are found throughout the body
but are concentrated in hormone-secreting glands such as the pituitary.


• Blood Cells: The most common types of blood cells are:
• red blood cells (erythrocytes). The main function of red blood cells is to collect


oxygen in the lungs and deliver it through the blood to the body tissues. Gas exchange is
carried out by simple diffusion (To see this in action please click here).


• various types of white blood cells (leukocytes). They are produced in the bone marrow and
help the body to fight infectious disease and foreign objects in the immune system. White cells
are found in the circulatory system, lymphatic system, spleen, and other body tissues.


Cell Size
Cells are the smallest living units within our body, but play a big role in making our body function


properly. Many cells never have a large increase in size after they are first formed from a parental cell.
Typical stem cells reproduce, double in size, then reproduce again. Most Cytosolic contents such as the
endomembrane system and the cytoplasm easily scale to larger sizes in larger cells. If a cell becomes
too large, the normal cellular amount of DNA may not be adequate to keep the cell supplied with RNA.
Large cells often replicate their chromosomes to an abnormally high amount or become multinucleated.
Large cells that are primarily for nutrient storage can have a smooth surface membrane, but
metabolically active large cells often have some sort of folding of the cell surface membrane in order to
increase the surface area available for transport functions.


Cellular Organization
Several different molecules interact to form organelles with our body. Each type of organelle has a


specific function. Organelles perform the vital functions that keep our cells alive.


Cell Membranes
The boundary of the cell, sometimes called the plasma membrane, separates internal metabolic


events from the external environment and controls the movement of materials into and out of the cell.
This membrane is very selective about what it allows to pass through; this characteristic is referred to
as "selective permeability." For example, it allows oxygen and nutrients to enter the cell while keeping
toxins and waste products out. The plasma membrane is a double phospholipid membrane, or a lipid
bilayer, with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar
hydrophilic heads forming the inner and outer surfaces of the membrane.


Protein and Cholesterol
Proteins and cholesterol molecules are scattered throughout the flexible phospholipid membrane.


Peripheral proteins attach loosely to the inner or outer surface of the plasma membrane. Integral
proteins lie across the membrane, extending from inside to outside. A variety of proteins are scattered
throughout the flexible matrix of phospholipid molecules, somewhat like icebergs floating in the ocean,


Wikibooks | 15




Chapter 2
and this is termed the fluid mosaic model of the cell membrane.


The phospholipid bilayer is selectively permeable. Only small, uncharged polar molecules can pass
freely across the membrane. Some of these molecules are H2O and CO2, hydrophobic (nonpolar)
molecules like O2, and lipid soluble molecules such as hydrocarbons. Other molecules need the help of
a membrane protein to get across. There are a variety of membrane proteins that serve various
functions:


• Channel proteins: Proteins that provide passageways through the membranes for certain
hydrophilic or water-soluble substances such as polar and charged molecules. No energy is used
during transport, hence this type of movement is called facilitated diffusion.


• Transport proteins: Proteins that spend energy (ATP) to transfer materials across the
membrane. When energy is used to provide passageway for materials, the process is called
active transport.


• Recognition proteins: Proteins that distinguish the identity of neighboring cells. These
proteins have oligosaccharide or short polysaccharide chains extending out from their cell
surface.


• Adhesion proteins: Proteins that attach cells to neighboring cells or provide anchors for
the internal filaments and tubules that give stability to the cell.


• Receptor proteins: Proteins that initiate specific cell responses once hormones or other
trigger molecules bind to them.


• Electron transfer proteins: Proteins that are involved in moving electrons from one
molecule to another during chemical reactions.


Passive Transport Across the Cell Membrane
Passive transport describes the movement of substances down a concentration gradient and does


not require energy use.
• Bulk flow is the collective movement of substances in the same direction in response to a


force, such as pressure. Blood moving through a vessel is an example of bulk flow.
• Simple diffusion, or diffusion, is the net movement of substances from an area of higher


concentration to an area of lower concentration. This movement occurs as a result of the random
and constant motion characteristic of all molecules, (atoms or ions) and is independent from the
motion of other molecules. Since, at any one time, some molecules may be moving against the
gradient and some molecules may be moving down the gradient, although the motion is random,
the word "net" is used to indicate the overall, eventual end result of the movement.


• Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma
membrane. Water can pass freely through the plasma membrane without the aid of specialized
proteins.


• Osmosis is the diffusion of water molecules across a selectively permeable membrane.
When water moves into a body by osmosis, hydrostatic pressure or osmotic pressure may build
up inside the body.


• Dialysis is the diffusion of solutes across a selectively permeable membrane.


16 | Human Physiology




Cell Physiology


Active Transport Across the Cell Membrane
Active transport is the movement of solutes against a gradient and requires the expenditure of


energy, usually in the form of ATP. Active transport is achieved through one of these two mechanisms:


Protein Pumps


• Transport proteins in the plasma membrane transfer solutes such as small ions (Na+, K+,
Cl-, H+), amino acids, and monosaccharides.


• The proteins involved with active transport are also known as ion pumps.
• The protein binds to a molecule of the substance to be transported on one side of the


membrane, then it uses the released energy (ATP) to change its shape, and releases it on the
other side.


• The protein pumps are specific, there is a different pump for each molecule to be
transported.


• Protein pumps are catalysts in the splitting of ATP → ADP + phosphate, so they are called
ATPase enzymes.


Cystic fibrosis is a genetic disorder that results in a misshapen chloride ion pump. By not
regulating chloride levels properly the cells produce thick mucus.


Vesicular Transport
• Vesicles or other bodies in the cytoplasm move macromolecules or large particles across


the plasma membrane. Types of vesicular transport include:
1. Exocytosis, which describes the process of vesicles fusing with the plasma membrane and


releasing their contents to the outside of the cell. This process is common when a cell produces
substances for export.
2. Endocytosis, which describes the capture of a substance outside the cell when the plasma


membrane merges to engulf it. The substance subsequently enters the cytoplasm enclosed in a
vesicle.
There are three kinds of endocytosis:


• Phagocytosis or cellular eating, occurs when the dissolved materials enter the cell. The
plasma membrane engulfs the solid material, forming a phagocytic vesicle.


• Pinocytosis or cellular drinking occurs when the plasma membrane folds inward to form
a channel allowing dissolved substances to enter the cell. When the channel is closed, the
liquid is encircled within a pinocytic vesicle.


• Receptor-mediated endocytosis occurs when specific molecules in the fluid surrounding
the cell bind to specialized receptors in the plasma membrane. As in pinocytosis, the
plasma membrane folds inward and the formation of a vesicle follows.


Note: Certain hormones are able to target specific cells by receptor-mediated endocytosis.


Wikibooks | 17




Chapter 2


Parts of the Cell


Cytoplasm
The gel-like material within the cell membrane is referred to as the cytoplasm. It is a fluid matrix,


the cytosol, which consists of 80% to 90% water, salts, organic molecules and many enzymes that
catalyze reactions, along with dissolved substances such as proteins and nutrients. The cytoplasm plays
an important role in a cell, serving as a "molecular soup" in which organelles are suspended and held
together by a fatty membrane.


Within the plasma membrane of a cell, the cytoplasm surrounds the nuclear envelope and the
cytoplasmic organelles. It plays a mechanical role by moving around inside the membrane and pushing
against the cell membrane helping to maintain the shape and consistency of the cell and again, to
provide suspension to the organelles. It is also a storage space for chemical substances indispensable to
life, which are involved in vital metabolic reactions, such as anaerobic glycolysis and protein synthesis.


The cell membrane keeps the cytoplasm from leaking out. It contains many different organelles
which are considered the insoluble constituents of the cytoplasm, such as the mitochondria, lysosomes,
peroxysomes, ribosomes, several vacuoles and cytoskeletons, as well as complex cell membrane
structures such as the endoplasmic reticulum and the Golgi apparatus that each have specific functions
within the cell.


• Cytoskeleton
Threadlike proteins that make up the cytoskeleton continually reconstruct to adapt to the cells


constantly changing needs. It helps cells maintain their shape and allows cells and their contents to


18 | Human Physiology




Cell Physiology


move. The cytoskeleton allows certain cells such as neutrophils and macrophages to make amoeboid
movements.


The network is composed of three elements:
microtubules, actin filaments, and intermediate fibers.


• Microtubules
Microtubules function as the framework along


which organelles and vesicles move within a cell. They
are the thickest of the cytoskeleton structures. They are
long hollow cylinders, composed of protein subunits,
called tubulin. Microtubules form mitotic spindles, the
machinery that partitions chromosomes between two
cells in the process of cell division. Without mitotic
spindles cells could not reproduce.


Microtubules, intermediate filaments, and
microfilaments are three protein fibers of decreasing
diameter, respectively. All are involved in establishing
the shape or movements of the cytoskeleton, the
internal structure of the cell.


• Microfilaments
Microfilaments provide mechanical


support for the cell, determine the cell
shape, and in some cases enable cell
movements. They have an arrow-like
appearance, with a fast growing plus or
barbed end and a slow growing minus or
pointed end. They are made of the protein
actin and are involved in cell motility. They
are found in almost every cell, but are
predominant in muscle cells and in the cells
that move by changing shape, such as
phagocytes (white blood cells that scour the
body for bacteria and other foreign
invaders).


Organelles
Organelles are bodies embedded in the cytoplasm that serve to physically separate the various


metabolic activities that occur within cells. The organelles are each like separate little factories, each
organelle is responsible for producing a certain product that is used elsewhere in the cell or body.


Cells of all living things are divided into two broad categories: prokaryotes and eukaryotes.
Bacteria (and archea) are prokaryotes, which means they lack a nucleus or other membrane-bound


Wikibooks | 19


A photograph of microtubules.


A photograph of microfilaments. (GFDL - Y tambe)




Chapter 2
organelles. Eukaryotes include all protozoans, fungi, plants, and animals (including humans), and these
cells are characterized by a nucleus (which houses the chromosomes) as well as a variety of other
organelles. Human cells vary considerably (consider the differences between a bone cell, a blood cell,
and a nerve cell), but most cells have the features described below.


• Nucleus
Controls the cell;


houses the genetic material
(DNA). The nucleus is the
largest of the cells
organelles. Cells can have
more than one nucleus or
lack a nucleus all together.
Skeletal muscle cells
contain more than one
nucleus whereas red blood
cells do not contain a
nucleus at all. The nucleus
is bounded by the nuclear envelope, a phospholipid bilayer similar to the plasma membrane. The space
between these two layers is the nucleolemma Cisterna.


The nucleus contains the DNA,
as mentioned above, the hereditary
information in the cell. Normally
the DNA is spread out within the
nucleus as a threadlike matrix
called chromatin. When the cell
begins to divide, the chromatin
condenses into rod-shaped bodies
called chromosomes, each of
which, before dividing, is made up
of two long DNA molecules and
various histone molecules. The
histones serve to organize the
lengthy DNA, coiling it into
bundles called nucleosomes. Also
visible within the nucleus are one
or more nucleoli, each consisting of
DNA in the process of
manufacturing the components of
ribosomes. Ribosomes are shipped
to the cytoplasm where they
assemble amino acids into proteins. The nucleus also serves as the site for the separation of the
chromosomes during cell division.


• Chromosomes


20 | Human Physiology


A comparison and Eukaryote and Prokaryote cells.


A cross-sectional diagram of a cell.




Cell Physiology


Inside each cell nucleus are chromosomes. Chromosomes are
made up of chromatin, which is made up of protein and
deoxyribonucleic acid strands. Deoxyribonucleic acid is DNA, the
genetic material that is in the shape of a twisted ladder, also called
the double helix. Humans have 23 pairs of chromosomes. Down
Syndrome and Cri du Chat Syndrome result from having an
abnormal number of chromosomes.


• Centrioles
Centrioles are rod like structures composed of 9 bundles which


contain three microtubules each. Two perpendicularly placed
centrioles surrounded by proteins make up the centrosome.
Centrioles are very important in cellular division, where they arrange
the mitotic spindles that pull the chromosome apart.


Centrioles and basal bodies act as microtubule organizing
centers. A pair of centrioles (enclosed in a centrosome) located
outside the nuclear envelope gives rise to the microtubules that make
up the spindle apparatus used during cell division. Basal bodies are at the base of each flagellum and
cilium and appear to organize their development.


• Ribosomes
Ribosomes play an active role in the


complex process of protein synthesis, where
they serve as the structures that facilitate the
joining of amino acids. Each ribosome is
composed of a large and small subunit
which are made up of ribosomal proteins
and ribosomal RNAs. They can either be
found in groups called polyribosomes within
the cytoplasm or found alone. Occasionally
they are attached to the endoplasmic
reticulum.


Wikibooks | 21


A rough sketch of a chromosome.
(GFDL - Magnus Manske)


Ribosome structure (GFDL - Dominus, MaxPower)




Chapter 2
• Mitochondria


Mitochondria are the
organelles that function as the cell
"powerhouse", generating ATP,
the universal form of energy used
by all cells. It converts food
nutrients such as glucose, to a fuel
(ATP) that the cells of the body
can use. Mitochondria are tiny
saclike structures found near the
nucleus. Little shelves called
cristae are formed from folds in
the inner membrane. Cells that
are metabolically active such as
muscle, liver and kidney cells
have high energy requirements
and therefore have more
mitochondria.


Mitochondria are unique in that they have their own mitochondrial DNA (separate from the DNA
that is in the nucleus). It is believed that eukaryotes evolved from one cell living inside another cell,
and mitochondria share many traits with free-living bacteria (similar chromosome, similar ribosomes,
etc).


• Endoplasmic Reticulum
Endoplasmic means "within the plasm" and reticulum means "network".
A complex three dimensional internal membrane system of flattened sheets, sacs and tubes, that


play an important role in making proteins and shuttling cellular products; also involved in metabolisms
of fats, and the production of various materials. In cross-section, they appear as a series of maze-like
channels, often closely associated with the nucleus. When ribosomes are present, the rough ER attaches
polysaccharide groups to the polypeptides as they are assembled by the ribosomes. Smooth ER, without
ribosomes, is responsible for various activities, including the synthesis of lipids and hormones,
especially in cells that produce these substances for export from the cell.


Rough endoplasmic reticulum has characteristic bumpy appearance due to the multitude of
ribosomes coating it. It is the site where proteins not destined for the cytoplasm are synthesized.


Smooth endoplasmic reticulum provides a variety of functions, including lipid synthesis and
degradation, and calcium ion storage. In liver cells, the smooth ER is involved in the breakdown of
toxins, drugs, and toxic byproducts from cellular reactions.


• Golgi Apparatus
"Packages" cellular products in sacs called vesicles so that the products can cross the cell


22 | Human Physiology


A cutaway view inside a mitochondria. (GFDL - pschemp)




Cell Physiology


membrane and exit the cell. The Golgi apparatus is the central delivery system for the cell. It is a
group of flattened sacs arranged much like a stack of bowls. They function to modify and package
proteins and lipids into vesicles, small spherically shaped sacs that bud from the ends of a Golgi
apparatus. Vesicles often migrate to and merge with the plasma membrane, releasing their contents
outside the cell. The Golgi apparatus also transports lipids and creates lysosomes and organelles
involved in digestion.


• Vacuoles
Spaces in the cytoplasm that sometimes serve to carry materials to the cell membrane for discharge


to the outside of the cell. Vacuoles are formed during endocytosis when portions of the cell membrane
are pinched off.


• Lysosomes
Lysosomes are sac-like compartments that contain a number of powerful degradative enzymes.


They are built in the Golgi apparatus. They break down harmful cell products and waste materials,
cellular debris, and foreign invaders such as bacteria, and then force them out of the cell. Tay-Sachs
disease and Pompe's disease are just two of the malfunctions of lysosomes or their digestive proteins.


• Peroxisomes
Organelles in which oxygen is used to oxidize substances, breaking down lipids and detoxifying


certain chemicals. Peroxisomes self replicate by enlarging and then dividing. They are common in liver
and kidney cells that break down potentially harmful substances. Peroxisomes can convert hydrogen
peroxide, a toxin made of H2O2 to H2O.


Extracellular structures
• Extracellular matrix Human cells, like other animal cells, do not have a rigid cell wall.


Human cells do have an important and variable structure outside of their cell membrane called
the extracellular matrix. Sometimes this matrix can be extensive and solid (examples = calcified
bone matrix, cartilage matrix), while other times it consists of a layer of extracellular proteins
and carbohydrates. This matrix is responsible for cells binding to each other and is incredibly
important in how cells physically and physiologically interact with each other.


• Flagella Many prokaryotes have flagella, allowing, for example, an E. coli bacteria to
propel its way up the urethra to cause a UTI (Urinary Tract Infection). Human cells, however
(and in fact most eukaryotic cells) lack flagella. This makes sense since humans are
multicellular, and individual cells do not need to swim around. The obvious exception to this is
with sperm, and indeed each sperm is propelled by a single flagellum. The flagellum of sperm is
composed of microtubules.


• Cilia Cilia are especially notable on the single-celled protozoans, where they beat in
synchrony to move the cells nimbly through the water. They are composed of extensions of the
cell membrane that contain microtubules. When present in humans they are typically found in
large numbers on a single surface of the cells, where rather than moving cells, they move
materials. The mucociliary escalator of the respiratory system consists of mucus-secreting cells


Wikibooks | 23




Chapter 2
lining the trachea and bronchi, and
ciliated epithelial cells that move the
mucus ever-upward. In this manner
mold spores, bacteria, and debris are
caught in the mucus, removed from the
trachea, and pushed into the esophagus
(to be swallowed into a pit of acid). In
the oviducts cilia move the ovum from
the ovary to the uterus, a journey which
takes a few days.


Cell Junctions
The plasma membranes of adjacent cells


are usually separated by extracellular fluids
that allow transport of nutrients and wastes to and from the bloodstream. In certain tissues, however,
the membranes of adjacent cells may join and form a junction. Three kinds of cell junctions are
recognized:


• Desmosomes are protein attachments between adjacent cells. Inside the plasma membrane,
a desmosome bears a disk shaped structure from which protein fibers extend into the cytoplasm.
Desmosomes act like spot welds to hold together tissues that undergo considerable stress, such
as our skin or heart muscle.


• Tight junctions are tightly stitched seams between cells. The junction completely encircles
each cell, preventing the movement of material between the cell. Tight junctions are
characteristic of cells lining the digestive tract, where materials are required to pass through
cells,rather than intercellular spaces, to penetrate the bloodstream.


• Gap junctions are narrow tunnels between cells that consist of proteins called connexons.
The proteins allow only the passage of ions and small molecules. In this manner, gap junctions
allow communication between cells through the exchange of materials or the transmission of
electrical impulses.


Cell Metabolism
Cell metabolism is the total energy released and consumed by a cell. Metabolism describes all of


the chemical reactions that are happening in the body. Some reactions, called anabolic reactions, create
needed products. Other reactions, called catabolic reactions, break down products. Your body is
performing both anabolic and catabolic reactions at the same time and around the clock, twenty four
hours a day, to keep your body alive and functioning. Even while you sleep, your cells are busy
metabolizing.


• Catabolism: The energy releasing process in which a chemical or food is used (broken
down) by degredation or decomposition, into smaller pieces.


• Anabolism: Anabolism is just the opposite of catabolism. In this portion of metabolism, the
24 | Human Physiology


A magnified view of several cells, with visible cilia.




Cell Physiology


cell consumes energy to produce larger molecules via smaller ones.


Wikibooks | 25




Chapter 2


Energy Rich Molecules


Adenosine Triphosphate (ATP)
ATP is the currency of the cell. When the


cell needs to use energy such as when it needs
to move substances across the cell membrane
via the active transport system, it "pays" with
molecules of ATP. The total quantity of ATP
in the human body at any one time is about 0.1
Mole. The energy used by human cells
requires the hydrolysis of 200 to 300 moles of
ATP daily. This means that each ATP
molecule is recycled 2000 to 3000 times
during a single day. ATP cannot be stored,
hence its consumption must closely follow its
synthesis. On a per-hour basis, 1 kilogram of ATP is created, processed and then recycled in the body.
Looking at it another way, a single cell uses about 10 million ATP molecules per second to meet its
metabolic needs, and recycles all of its ATP molecules about every 20-30 seconds.


Flavin Adenine Dinucleotide (FAD)
When two hydrogen atoms are bonded, FAD is reduced to FADH2 and is turned into an energy-


carrying molecule. FAD accommodates two equivalents of Hydrogen; both the hydride and the proton
ions. This is used by organisms to carry out energy requiring processes. FAD is reduced in the citric
acid cycle during aerobic respiration


Nicotinamide Adenine Dinucleotide (NADH)
Nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate


(NADP) are two important cofactors found in cells. NADH is the reduced form of NAD+, and NAD+ is
the oxidized form of NADH. It forms NADP with the addition of a phosphate group to the 2' position
of the adenosyl nucleotide through an ester linkage.


Space-filling model of NADHNAD is used extensively in glycolysis and the citric acid cycle of
cellular respiration. The reducing potential stored in NADH can be converted to ATP through the
electron transport chain or used for anabolic metabolism. ATP "energy" is necessary for an organism to
live. Green plants obtain ATP through photosynthesis, while other organisms obtain it by cellular
respiration.


Nicotinamide adenine dinucleotide phosphate (NADP+)NADP is used in anabolic reactions, such
as fat acid and nucleic acid synthesis, that require NADPH as a reducing agent. In chloroplasts, NADP
is an oxidising agent important in the preliminary reactions of photosynthesis. The NADPH produced
by photosynthesis is then used as reducing power for the biosynthetic reactions in the Calvin cycle of
photosynthesis.


26 | Human Physiology


Chemical diagram of an ATP molecule.




Cell Physiology


MH2 + NAD+ → NADH + H+ + M: + energy,
where M is a metabolite. Two hydrogen ions (a
hydride ion and an H+ ion) are transferred from the
metabolite. One electron is transferred to the
positively-charged nitrogen, and one hydrogen
attaches to the carbon atom opposite to the nitrogen.


The change upon nicotinamide group when
NAD+ is reduced The human body synthesizes
NAD from the vitamin niacin in the form of
nicotinic acid or nicotinamide.


Cellular Respiration
Cellular respiration is the energy releasing process by which sugar molecules are broken down by


a series of reactions and the chemical energy gets converted to energy stored in ATP molecules. The
reactions that convert the fuel (glucose) to usable energy (ATP) are glycolysis, the Krebs cycle
(sometimes called the citric acid cycle), and the electron transport chain. Altogether these reactions are
referred to as "cellular respiration" or "aerobic respiration." Oxygen is needed as the final electron
acceptor, and carrying out cellular respiration is the very reason we breathe and the reason we eat.


Glycolysis
The glycolytic pathway (glycolysis) is where glucose, the smallest molecule that a carbohydrate


can be broken into during digestion, gets oxidized and broken into two 3-carbon molecules (pyruvates),
which are then fed into the Kreb's Cycle. Glycolysis is the beginning of cellular respiration and takes
place in the cytoplasm. Two molecules of ATP are required for glycolysis, but four are produced so
there is a net gain of two ATP per glucose molecule. Two NADH molecules transfer electrons (in the
form of hydrogen ions) to the electron transport chain in the mitochondria, where they will be used to
generate additional ATP. During physical exertion when the mitochondria are already producing the
maximum ATP possible with the amount of oxygen available, glycolysis can continue to produce an
additional 2 ATP per glucose molecule without sending the electrons to the mitochondria. However,
during this anaerobic respiration lactic acid is produced, which may accumulate and lead to temporary
muscle cramping.


Krebs Cycle
The Krebs cycle was named after Sir Hans Krebs (1900-1981), who proposed the key elements of


this pathway in 1937 and was awarded the Nobel Prize in Medicine for its discovery in 1953.
Two molecules of pyruvate enter the Krebs cycle, which is called the aerobic pathway because it


requires the presence of oxygen in order to occur. This cycle is a major biological pathway that occurs
in humans and every plant and animal.


After glycolysis takes place in the cell's cytoplasm, the pyruvic acid molecules travel into the


Wikibooks | 27


Chemical diagram of an NADH molecule.




Chapter 2
interior of the mitochondrion. Once the pyruvic acid is inside, carbon dioxide is enzymatically removed
from each three-carbon pyruvic acid molecule to form acetic acid. The enzyme then combines the
acetic acid with an enzyme, coenzyme A, to produce acetyl coenzyme A, also known as acetyl CoA.


Once acetyl CoA is formed, the Krebs cycle begins. The cycle is split into eight steps, each of
which will be explained below.


• Step 1: The acetic acid subunit of acetyl CoA is combined with oxaloacetate to form a
molecule of citrate. The acetyl coenzyme A acts only as a transporter of acetic acid from one
enzyme to another. After Step 1, the coenzyme is released by hydrolysis so that it may combine
with another acetic acid molecule to begin the Krebs cycle again.


• Step 2: The citric acid molecule undergoes an isomerization. A hydroxyl group and a
hydrogen molecule are removed from the citrate structure in the form of water. The two carbons
form a double bond until the water molecule is added back. Only now, the hydroxyl group and
hydrogen molecule are reversed with respect to the original structure of the citrate molecule.
Thus, isocitrate is formed.


• Step 3: In this step, the isocitrate molecule is oxidized by a NAD molecule. The NAD
molecule is reduced by the hydrogen atom and the hydroxyl group. The NAD binds with a
hydrogen atom and carries off the other hydrogen atom leaving a carbonyl group. This structure
is very unstable, so a molecule of CO2 is released creating alpha-ketoglutarate.


• Step 4: In this step, our friend, coenzyme A, returns to oxidize the alpha-ketoglutarate
molecule. A molecule of NAD is reduced again to form NADH and leaves with another
hydrogen. This instability causes a carbonyl group to be released as carbon dioxide and a
thioester bond is formed in its place between the former alpha-ketoglutarate and coenzyme A to
create a molecule of succinyl-coenzyme A complex.


• Step 5: A water molecule sheds its hydrogen atoms to coenzyme A. Then, a free-floating
phosphate group displaces coenzyme A and forms a bond with the succinyl complex. The
phosphate is then transferred to a molecule of GDP to produce an energy molecule of GTP. It
leaves behind a molecule of succinate.


• Step 6: In this step, succinate is oxidized by a molecule of FAD (Flavin adenine
dinucleotide). The FAD removes two hydrogen atoms from the succinate and forces a double
bond to form between the two carbon atoms, thus creating fumarate.


• Step 7: An enzyme adds water to the fumarate molecule to form malate. The malate is
created by adding one hydrogen atom to a carbon atom and then adding a hydroxyl group to a
carbon next to a terminal carbonyl group.


• Step 8: In this final step, the malate molecule is oxidized by a NAD molecule. The carbon
that carried the hydroxyl group is now converted into a carbonyl group. The end product is
oxaloacetate which can then combine with acetyl-coenzyme A and begin the Krebs cycle all
over again.


• Summary: In summary, three major events occur during the Krebs cycle. One GTP
(guanosine triphosphate) is produced which eventually donates a phosphate group to ADP to


28 | Human Physiology




Cell Physiology


form one ATP; three molecules of NAD are reduced; and one molecule of FAD is reduced.
Although one molecule of GTP leads to the production of one ATP, the production of the
reduced NAD and FAD are far more significant in the cell's energy-generating process. This is
because NADH and FADH2 donate their electrons to an electron transport system that generates
large amounts of energy by forming many molecules of ATP.


To see a visual summary of "Kreb Cycle" please click here.


Electron Transport System
The most complicated system of all. In the respiration chain, oxidation and reduction reactions


occur repeatedly as a way of transporting energy. The respiratory chain is also called the electron
transport chain. At the end of the chain, oxygen accepts the electron and water is produced.


Redox Reaction
This is a simultaneous oxidation-reduction process whereby cellular metabolism occurs, such as


the oxidation of sugar in the human body, through a series of very complex electron transfer processes.
The chemical way to look at redox processes is that the substance being oxidized transfers


electrons to the substance being reduced. Thus, in the reaction, the substance being oxidized (aka. the
reducing agent) loses electrons, while the substance being reduced (aka. the oxidizing agent) gains
electrons. Remember: LEO (Losing Electrons is Oxidation) the lion says GER (Gaining Electrons is
Reduction).


The term redox state is often used to describe the balance of NAD+/NADH and NADP+/NADPH
in a biological system such as a cell or organ. The redox state is reflected in the balance of several sets
of metabolites (e.g., lactate and pyruvate, beta-hydroxybutyrate and acetoacetate) whose
interconversion is dependent on these ratios. An abnormal redox state can develop in a variety of
deleterious situations, such as hypoxia, shock, and sepsis.


Carbohydrates
Carbohydrate molecules consist of carbon, hydrogen, and oxygen. They have a general formula


Cn(H2O)n. There are several sub-families based on molecular size.


Carbohydrates are chemical compounds that contain oxygen, hydrogen, and carbon atoms, and no
other elements. They consist of monosaccharide sugars of varying chain lengths.


Certain carbohydrates are an important storage and transport form of energy in most organisms,
including plants and animals. Carbohydrates are classified by their number of sugar units:
monosaccharides (such as glucose and fructose), disaccharides (such as sucrose and lactose),
oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).


The simplest carbohydrates are monosaccharides, which are small straight-chain aldehydes and


Wikibooks | 29




Chapter 2
ketones with many hydroxyl groups added, usually one on each carbon except the functional group.
Other carbohydrates are composed of monosaccharide units and break down under hydrolysis. These
may be classified as disaccharides, oligosaccharides, or polysaccharides, depending on whether they
have two, several, or many monosaccharide units.


Proteins
All proteins contain carbon, hydrogen, oxygen and nitrogen. Some also contain phosphorus and


sulfur. The building blocks of proteins are amino acids. There are 20 different kinds of amino acids
used by the human body. They unite by peptide bonds to form long molecules called polypeptides.
Polypeptides are assembled into proteins. Proteins have four levels of structure


• Primary Primary structure is the sequence of amino acids bonded in the polypeptide.
• Secondary The secondary structure is formed by hydrogen bonds between amino acids.


The polypeptide can coil into a helix or form a pleated sheet.
• Tertiary The tertiary structure refers to the three-dimensional folding of the helix or


pleated sheet.


30 | Human Physiology




Cell Physiology


• Quaternary The quaternary structure refers to the spatial relationship among the
polypeptide in the protein.


Enzymes
Enzymes are essential for life because most chemical reactions in living cells would occur too


slowly or would lead to different products without enzymes. A biological molecule that catalyzes a
chemical reaction. Most enzymes are proteins and the word "enzyme" is often used to mean a protein
enzyme. Some RNA molecules also have a catalytic activity, and to differentiate them from protein
enzymes, they are referred to as RNA enzymes or ribozymes.


Review Questions
1. List 2 functions of the cell membrane.
Questions 2 - 6 Match the following organelles with their function: 2. Mitochondria 3. Vacuoles 4.
Cilia 5. Smooth ER 6. Golgi Apparatus


A. Movement of the cell
B. Lipid synthesis and transport
C. "Powerhouse" of the cell, makes ATP
D. Storage areas, mainly found in plant cells
E. Packages and distributes cellular products


7. The diffusion of H2O across a semi permeable or selectively permeable membrane is termed
A. Active transport
B. Diffusion
C. Osmosis
D. Endocytosis


8. Oxygen enters a cell via?
a. Diffusion
b. Filtration
c. Osmosis
d. Active transport


9. The term used to describe, "cell eating" is?
a. Exocytosis
b. Phagocytosis
c. Pinocytosis
d. Diffusion


10. Which of the following requires energy?


Wikibooks | 31




Chapter 2
a. Diffusion
b. Osmosis
c. Active transport
d. Facilitated diffusion


11. Protein synthesis occurs at the
a. Mitochondria
b. Lysosomes
c. Within the nucleus
d. Ribosomes


12. Which of the following is not found in the cell membrane?
a. Cholesterol
b. Phospholipids
c. Proteins
d. Galactose
e. Nucleic acids


Glossary
Active Transport: the movement of solutes against a gradient and requires the expenditure of


energy
Adenosine Triphosphate (ATP): a cell’s source of energy
Bulk Flow: the collective movement of substances in the same direction in response to a force
Cells: the microscopic fundamental unit that makes up all living things
Cell Membrane: boundary of the cell, sometimes called the plasma membrane
Cytoplasm: a water-like substance that fills cells. The cytoplasm consists of cytosol and the


cellular organelles, except the cell nucleus. The cytosol is made up of water, salts, organic molecules
and many enzymes that catalyze reactions. The cytoplasm holds all of the cellular organelles outside of
the nucleus, maintains the shape and consistency of the cell, and serves as a storage place for chemical
substances.


Cytoskeleton: made of threadlike proteins, helps cells maintain their shape and allows cells and
their contents to move


Dialysis: the diffusion of solutes across a selectively permeable membrane. Most commonly heard
of when a patient has had renal failure. In medicine, dialysis is a type of renal replacement therapy
which is used to provide an artificial replacement for lost kidney function due to renal failure. It is a life
support treatment and does not treat any kidney diseases.


Endocrine cells: similar to exocrine cells, but secrete their products directly into the bloodstream


32 | Human Physiology




Cell Physiology


instead of through a duct
Endocytosis: the capture of a substance outside the cell when the plasma membrane merges to


engulf it
Endoplasmic Reticulum: organelle that play an important role in making proteins and shuttling


cellular products; also involved in metabolisms of fats, and the production of various materials
Epithelial Cells: cells that aid in secretion, absorption, protection, trans-cellular transport,


sensation detection, and selective permeability
Exocrine Cells: cells that secrete products through ducts, such as mucus, sweat, or digestive


enzymes
Exocytosis: the process of vesicles fusing with the plasma membrane and releasing their contents


to the outside of the cell
Facilitated Diffusion: the diffusion of solutes through channel proteins in the plasma membrane
Golgi Apparatus: "packages" cellular products in sacs called vesicles so that the products can


cross the cell membrane and exit the cell
Glycolysis: process in which sugars (glucose) are converted to acid
Lysosomes: sac-like compartments that contain a number of powerful degradative enzymes
Microfilaments: provide mechanical support for the cell, determine the cell shape, and in some


cases enable cell movements
Microtubules: function as the framework along which organelles and vesicles move within a cell
Mitochondria: the organelles that function as the cell "powerhouse", generating ATP
Nucleus: controls the cell; houses the genetic material
Organelles: bodies embedded in the cytoplasm that serve to physically separate the various


metabolic activities that occur within cells
Osmosis: the diffusion of water molecules across a selectively permeable membrane from an area


of high solute concentration to an area of low solute concentration.
Passive Transport: the movement of substances down a concentration gradient and does not


require energy use
Peroxisomes: organelles in which oxygen is used to oxidize substances, breaking down lipids and


detoxifying certain chemicals
Phagocytosis: a form of endocytosis wherein large particles are enveloped by the cell membrane


of a (usually larger) cell and internalized to form a phagosome, or "food vacuole." In animals,


Wikibooks | 33




Chapter 2
phagocytosis is performed by specialized cells called phagocytes, which serve to remove foreign bodies
and thus fight infection. In vertebrates, these include larger macrophages and smaller granulocytes,
types of blood cells. Bacteria, dead tissue cells, and small mineral particles are all examples of objects
that may be phagocytosed.


Pinocytosis: also called cellular drinking, is a form of endocytosis, a process in which small
particles are taken in by a cell by splitting into smaller particles. The particles then form small vesicles
which subsequently fuse with lysosomes to hydrolyze, or to break down, the particles. This process
requires adenosine triphosphate (ATP).


Receptor-mediated Endocytosis: occurs when specific molecules in the fluid surrounding the cell
bind to specialized receptors in the plasma membrane


Red Blood Cells (erythrocytes): cells that collect oxygen in the lungs and deliver it through the
blood to the body tissues


Ribosomes: play an active role in the complex process of protein synthesis, where they serve as
the structures that facilitate the joining of amino acids


Simple Diffusion: the net movement of substances from an area of higher concentration to an area
of lower concentration


Vacuoles: spaces in the cytoplasm that sometimes serve to carry materials to the cell membrane
for discharge to the outside of the cell


White Blood Cells (leukocytes): produced in the bone marrow and help the body to fight
infectious disease and foreign objects in the immune system


live version • discussion • edit lesson • comment • report an error


34 | Human Physiology




The Integumentary System


3 THE INTEGUMENTARY SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction
he integumentary system consists of the skin, the subcutaneous tissue below the skin, hair, nails,
and assorted glands. The most obvious function of the integumentary system is the protection that


the skin gives to underlying tissues. The skin not only keeps most harmful substances out, but also
prevents the loss of fluids.
T


A major function of the subcutaneous tissue is to connect the skin to underlying tissues such as
muscles. Hair on the scalp provides insulation from cold for the head. The hair of eyelashes and
eyebrows helps keep dust and perspiration out of the eyes, and the hair in our nostrils helps keep dust
out of the nasal cavities. Any other hair on our bodies no longer serves a function, but is an
evolutionary remnant. Nails protect the tips of fingers and toes from mechanical injury. Fingernails
give the fingers greater ability to pick up small objects.


There are four types of glands in the integumentary system: Sudoriferous glands, Sebaceous
glands, Ceruminous glands, and Mammary glands. Sudoriferous glands are sweat producing glands.
These are important to help maintain body temperature. Sebaceous glands are oil producing glands
which help inhibit bacteria, keep us waterproof and prevent our hair and skin from drying out.
Ceruminous glands produce earwax which keeps the outer surface of the eardrum pliable and prevents
drying. Mammary glands produce milk.


Skin
In zoologyy and dermatology, skin is an organ of the integumentary system made up of a layer of


tissues that guard underlying muscles and organs. As the interface with the surroundings, it plays the
most important role in protecting against pathogens. Its other main functions are insulation and
temperature regulation, sensation and vitamin D and B synthesis. Skin is considered one of the most
important parts of the body.


Skin has pigmentation, melanin, provided by melanocytes, which absorbs some of the potentially
dangerous radiation in sunlight. It also contains DNA repair enzymes which reverse UV damage, and
people who lack the genes for these enzymes suffer high rates of skin cancer. One form predominantly
produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and
can often be deadly. Human skin pigmentation varies among populations in a striking manner. This has
sometimes led to the classification of people(s) on the basis of skin color.


Damaged skin will try to heal by forming scar tissue, often giving rise to discoloration and
depigmentation of the skin.


The skin is often known as "the largest organ in the human body". This applies to exterior surface,
as it covers the body, appearing to have the largest surface area of all the organs. Moreover, it applies
to weight, as it weighs more than any single internal organ, accounting for about 15 percent of body


Wikibooks | 35




Chapter 3
weight. For the average adult human, the skin has a surface area of between 1.5-2.0 square meters,
most of it is between 2-3 mm thick. The average square inch of skin holds 650 sweat glands, 20 blood
vessels, 60,000 melanocytes, and more than a thousand nerve endings.


The use of natural or synthetic cosmetics to treat the appearance of the face and condition of the
skin (such as pore control and black head cleansing) is common among many cultures.


36 | Human Physiology




The Integumentary System


Layers
The skin has two major layer which


are made of different tissues and have
very different functions.


Skin is composed of the epidermis
and the dermis. Below these layers lies
the hypodermis or subcutaneous
adipose layer, which is not usually
classified as a layer of skin.


The outermost epidermis consists
of stratified squamous keratinizing
epithelium with an underlying basement
membrane. It contains no blood vessels,
and is nourished by diffusion from the
dermis. The main type of cells which
make up the epidermis are
keratinocytes, with melanocytes and
Langerhans cells also present. The
epidermis can be further subdivided into
the following strata (beginning with the
outermost layer): corneum, lucidum,
granulosum, spinosum, basale. Cells are
formed through mitosis at the innermost
layers. They move up the strata changing shape and composition as they differentiate, inducing
expression of new types of keratin genes. They eventually reach the corneum and become sloughed off
(desquamation). This process is called keratinization and takes place within about 30 days. This layer
of skin is responsible for keeping water in the body and keeping other harmful chemicals and
pathogens out.


Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule.
Arterial shunt vessels may bypass the network in ears, the nose and fingertips.


The dermis lies below the epidermis and contains a number of structures including blood vessels,
nerves, hair follicles, smooth muscle, glands and lymphatic tissue. It consists of loose connective tissue
otherwise called areolar connective tissue - collagen, elastin and reticular fibers are present. Erector
muscles, attached between the hair papilla and epidermis, can contract, resulting in the hair fiber pulled
upright and consequentially goose bumps. The main cell types are fibroblasts, adipocytes (fat storage)
and macrophages. Sebaceous glands are exocrine glands which produce, a mixture of lipids and waxy
substances: lubrication, water-proofing, softening and antibactericidal actions are among the many
functions of sebum. Sweat Glands open up via a duct onto the skin by a pore.


The dermis is made of an irregular type of fibrous connective tissue consisting of collagen and
elastin fibers. It can be split into the papillary and reticular layers. The papillary layer is outermost and
extends into the epidermis to supply it with vessels. It is composed of loosely arranged fibers. Papillary
ridges make up the lines of the hands giving us fingerprints. The reticular layer is more dense and is
continuous with the hypodermis. It contains the bulk of the structures (such as sweat glands). The


Wikibooks | 37


Diagram of the layers of human skin




Chapter 3
reticular layer is composed of irregularly arranged fibers and resists stretching.


The hypodermis is not part of the skin, and lies below the
dermis. Its purpose is to attach the skin to underlying bone and
muscle as well as supplying it with blood vessels and nerves. It
consists of loose connective tissue and elastin. The main cell
types are fibroblasts, macrophages and adipocytes (the
hypodermis contains 50% of body fat). Fat serves as padding
and insulation for the body.


Functions
1. Protection: Skin gives an anatomical barrier between


the internal and external environment in bodily defense;
Langerhans cells in the skin are part of the immune system
2. Sensation: Skin contains a variety of nerve endings that


react to heat, cold, touch, pressure, vibration, and tissue
injury; see somatosensory system and touch.
3. Heat regulation: The skin contains a blood supply far


greater than its requirements which allows precise control of
energy loss by radiation, convection and conduction. Dilated
blood vessels increase perfusion and heat loss while
constricted vessels greatly reduce cutaneous blood flow and
conserve heat. Erector pili muscles are significant in
animals.
4. Control of evaporation: The skin provides a relatively


dry and impermeable barrier to fluid loss. Loss of this
function contributes to the massive fluid loss in burns.
5. Excretion: The concentration of urea is 1/130th that of


urine. Excretion by sweating is at most a secondary function
to temperature regulation.


In medicine, the branch concerned with the skin is called dermatology.
Tumors:


• Benign tumors of the skin: Squamous cell papilloma
• Skin cancer
• Acne
• Keratosis pilaris
• Fungal infections such as athlete's foot
• microbial infections
• calcinosis cutis
• ulcer


38 | Human Physiology


Clinical Application:
The patch drug delivery system. The
transdermal patch is an increasingly
popular drug delivery system. These
patches are designed so that the drug


molecules diffuse through the
epidermis to the blood vessels in the
dermis layer. A typical patch works
well for small lipid-soluble molecules
(for example, estrogen, nitroglycerin,
and nicotine) that can make their way


between epidermal cells.




The Integumentary System


Hair
Types of hair


Humans have three different types of hair:
• Lanugo, the fine hair that covers nearly the entire body of embryos
• Vellus hair, the short, fine, "peach fuzz" body hair that grows in most places on the human


body in both sexes
• Terminal hair, the fully developed hair, which is generally longer, coarser, thicker, and


darker than vellus hair


Pathological impacts on hair
Drugs used in cancer chemotherapy frequently cause a temporary loss of hair, noticeable on the


head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones. Other
diseases and traumas can cause temporary or permanent loss of hair, either generally or in patches.


The hair shafts may also store certain poisons for years, even decades, after death. In the case of
Col. Lafayette Baker, who died July 3, 1868, use of an atomic absorption spectrophotometer showed
the man was killed by white arsenic. The prime suspect was Wallace Pollock, Baker's brother-in-law.
According to Dr. Ray A. Neff, Pollack had laced Baker's beer with it over a period of months, and a
century or so later minute traces of arsenic showed up in the dead man's hair. Mrs. Baker's diary seems
to confirm that it was indeed arsenic, as she writes of how she found some vials of it inside her
brother's suit coat one day.


Wikibooks | 39




Chapter 3


Nails
Parts of the fingernail


The fingernail is an important structure made of
keratin. The fingernail generally serve two purposes. It
serves as a protective plate and enhances sensation of
the fingertip. The protection function of the fingernail
is commonly known, but the sensation function is
equally important. The fingertip has many nerve
endings in it allowing us to receive volumes of
information about objects we touch. The nail acts as a
counterforce to the fingertip providing even more
sensory input when an object is touched.


Nail Structure
The structure we know of as the nail is divided into six specific parts - the root, nail bed, nail plate,


eponychium (cuticle), perionychium, and hyponychium.
Root The root of the fingernail is also known as the germinal matrix. This portion of the nail is


actually beneath the skin behind the fingernail and extends several millimeters into the finger. The
fingernail root produces most of the volume of the nail and the nail bed. This portion of the nail does
not have any melanocytes, or melanin producing cells. The edge of the germinal matrix is seen as a
white, crescent shaped structure called the lunula.


Nail Bed The nail bed is part of the nail matrix called the sterile matrix. It extends from the edge
of the germinal matrix, or lunula, to the hyponychium. The nail bed contains the blood vessels, nerves,
and melanocytes, or melanin-producing cells. As the nail is produced by the root, it streams down
along the nail bed, which adds material to the undersurface of the nail making it thicker. It is important
for normal nail growth that the nail bed be smooth. If it is not, the nail may split or develop grooves
that can be cosmetically unappealing.


Nail Plate The nail plate is the actual fingernail, made of translucent keratin. The pink appearance
of the nail comes from the blood vessels underneath the nail. The underneath surface of the nail plate
has grooves along the length of the nail that help anchor it to the nail bed.


Eponychium The cuticle of the fingernail is also called the eponychium. The cuticle is situated
between the skin of the finger and the nail plate fusing these structures together and providing a
waterproof barrier.


Perionychium The perioncyhium is the skin that overlies the nail plate on its sides. It is also
known as the paronychial edge. The perionychium is the site of hangnails, ingrown nails, and an
infection of the skin called paronychia.


Hyponychium The hyponychium is the area between the nail plate and the fingertip. It is the
junction between the free edge of the nail and the skin of the fingertip, also providing a waterproof


40 | Human Physiology


The parts of a finger nail




The Integumentary System


barrier.


Nail Diseases
Nail diseases are in a separate category from diseases of


the skin. Although nails are a skin appendage, they have
their own signs and symptoms which may relate to other
medical conditions. Nail conditions that show signs of
infection or inflammation require medical assistance and
cannot be treated at a beauty parlor. Deformity or disease of
the nails may be referred to as onychosis.


There are many disease that can occur with the fingernails and toenails. The most common of these
diseases are ingrown nails and fungal infections.


Ingrown Nails
Onychocryptosis, commonly known as "ingrown nails" (unguis incarnatus), can affect either the


fingers or the toes. In this condition, the nail cuts into one or both sides of the nail bed, resulting in
inflammation and possibly infection. The relative rarity of this condition in the fingers suggests that
pressure from the ground or shoe against the toe is a prime factor. The movements involved in walking
or other physical disturbances can contribute to the problem. Mild onychocryptosis, particularly in the
absence of infection, can be treated by trimming and rounding the nail. More advanced cases, which
usually include infection, are treated by surgically excising the ingrowing portion of the nail down to
its bony origin and cauterizing the matrix, or 'root', to prevent recurrence. This surgery is called
matricectomy. The best results are achieved by cauterizing the matrix with phenol. Another method,
which is much less effective, is excision of the matrix, sometimes called a 'cold steel procedure'


Nail Fungus
An infection of nail fungus (onychomycosis) occurs when fungi infect one or more of your nails.


Onychomycosis generally begins as a white or yellow spot under the tip of the fingernail or toenail. As
the nail fungus spreads deeper into the nail, it may cause the nail to discolor, thicken and develop
crumbling edges — an unsightly and potentially painful problem.


Infections of nail fungus account for about half of all nail disorders. These infections usually
develop on nails continually exposed to warm, moist environments, such as sweaty shoes or shower
floors. Nail fungus isn't the same as athlete's foot, which primarily affects the skin of the feet, but at
times the two may coexist and can be caused by the same type of fungus.


An infection with nail fungus may be difficult to treat, and infections may recur. But medications
are available to help clear up nail fungus permanently.


Wikibooks | 41


Nails: left hand, adult human male




Chapter 3


Clinical Application
Nail inspection can give a great deal of information about the internal working of the body as well,


and like tongue or iris inspection, has a long history of diagnostic use in cantraditional medical
practices such as Chinese medicine.


Pliability: Brittleness is associated with iron deficiency, thyroid problems, impaired kidney
function, circulation problems[2], and biotin deficiency[3] Splitting and fraying are associated with
psoriasis, folic acid, protein and/or Vitamin C deficiency. Unusual thickness is associated with
circulation problems. Thinning nails and itchy skin are associated with lichen planus[4].


Shape and texture: Clubbing, or nails that curve down around the fingertips with nail beds that
bulge is associated with oxygen deprivation and lung, heart, or liver disease. Spooning, or nails that
grow upwards is associated with iron or B12 deficiency. Flatness can indicate a B12 vitamin
deficiency[5] or Raynaud's disease[6] Pitting of the nails is associated with Psoriasis. Horizontal ridges
indicate stress, and Beau's lines are associated with many serious conditions. Vertical ridges are
associated with arthritis[7]. Vertical grooves are associated with kidney disorders, aging, and iron
deficiency[8]. Beading is associated with rheumatoid arthritis[9]. Nails that resemble hammered brass
are associated with (or portend) hair loss[10]. Short small beds are associated with heart disease[11].
Coloration of the nail bed:


Mee's lines are associated with arsenic or thallium poisoning, and renal failure. White lines across
the nail are associated with heart disease, liver disease, or a history of a recent high fever[12]. Opaque
white nails with a dark band at the fingertip are associated with cancer, cirrhosis, congestive heart
failure, diabetes and aging[13]. Paleness or whitening is associated with liver or kidney disease and
anemia[14]. Yellowing of the nail bed is associated with chronic bronchitis, lymphatic problems,
diabetes, and liver disorders. Brown or copper nail beds are associated with arsenic or copper
poisoning, and local fungal infection. Grey nail beds are associated with arthritis, edema, malnutrition,
post-operative effects, glaucoma and cardio-pulmonary disease[15]. redness is associated with heart
conditions. dark nails are associated with B12 deficiency. Stains of the nail plate (not the nail bed) are
associated with nail polish[16], smoking, and henna use.


Markings: Pink and white nails are associated with kidney disease[17]. Parallel white lines in the
nails are associated with hypoalbuminemia. red skin at the base of the nail is associated with
connective tissue disorders[18]. blue lunulae are associated with silver poisoning or lung disorder[19].
blue nail beds are (much like blue skin) associated with poor oxygenation of the blood (asthma,
emphysema, etc)[20]. small white patches are associated with zinc or calcium deficiency or
malabsorption, parasites, or local injury[21]. receded lunulae (fewer than 8) are associated with poor
circulation[22], shallow breathing habits or thyroid mysfunction[23]. large lunulae (more than 25% of
the thumb nail) is associated with high blood pressure.


Myths
It is a myth that nails and hair will continue growing for several days after death. The appearance


of growth is actually caused by the retraction of skin as the surrounding tissue dehydrates (desiccation),
making nails and hair more prominent.


42 | Human Physiology




The Integumentary System


Glands


Sweat Glands
In humans, there are two kinds of sweat glands which differ greatly in both the composition of the


sweat and its purpose: Also "click" here"How are body Sweats" to see a short movie on sweat glands.


Eccrine
Eccrine sweat glands are distributed over the entire body surface but are particularly abundant on


the palms of hands, soles of feet, and on the forehead. These produce sweat that is composed chiefly of
water with various salts. These glands are used for body temperature regulation.


Eccrine sweat glands are coiled tubular glands derived from the outer layer of skin but extending


Wikibooks | 43


A diagrammatic sectional view of the skin (magnified). Sweat gland labeled as "sudoriferous
gland" at center right.




Chapter 3
into the inner layer. They are distributed over almost the entire surface of the body in humans and
many other species, but are lacking in some marine and fur-bearing species. The sweat glands are
controlled by sympathetic cholinergic nerves which are controlled by a center in the hypothalamus. The
hypothalamus senses core temperature directly, and also has input from temperature receptors in the
skin and modifies the sweat output, along with other thermoregulatory processes.


Human eccrine sweat is composed chiefly of water with various salts and organic compounds in
solution. It contains minute amounts of fatty materials, urea, and other wastes. The concentration of
sodium varies from 35–65 mmol/l and is lower in people acclimatised to a hot environment. The sweat
of other species generally differ in composition.


Apocrine
Apocrine glands occur during the early to mid puberty ages approximately around the age of 15


and release more than normal amounts of sweat for approximately a month and subsequently regulate
and release normal amounts of sweat after a certain period of time. Apocrine sweat glands produce
sweat that contains fatty materials. These glands are mainly present in the armpits and around the
genital area and their activity is the main cause of sweat odor, due to the bacteria that break down the
organic compounds in the sweat from these glands. Emotional stress increases the production of sweat
from the apocrine glands, or more precisely: the sweat already present in the tubule is squeezed out.
Apocrine sweat glands essentially serve as scent glands.


In some areas of the body, these sweat glands are modified to produce wholly different secretions,
however, including the cerumen ("wax") of the outer ear. Other glands, such as Mammary glands, are
greatly enlarged and modified to produce milk.


44 | Human Physiology




The Integumentary System


Sebaceous Glands
The sebaceous glands are glands found in the skin


of mammals. They secrete an oily substance called
sebum (Latin, meaning fat or tallow) that is made of
fat (lipids) and the debris of dead fat-producing cells.
These glands exist in humans throughout the skin
except in the palms of the hands and soles of the feet.
Sebum acts to protect and waterproof hair and skin,
and keep them from becoming dry, brittle, and
cracked. It can also inhibit the growth of
microorganisms on skin.


Sebaceous glands can usually be found in hair-
covered areas where they are connected to hair
follicles to deposit sebum on the hairs, and bring it to
the skin surface along the hair shaft. The structure
consisting of hair, hair follicle and sebaceous gland is also known as pilosebaceous unit. Sebaceous
glands are also found in non haired areas of lips, eyelids, penis, labia minora and nipples; here the
sebum reaches the surface through ducts. In the glands, sebum is produced within specialized cells and
is released as these cells burst; sebaceous glands are thus classified as holocrine glands.


Sebum is odorless, but its bacterial breakdown can produce odors. Sebum is the cause of some
people experiencing "oily" hair if it is not washed for several days. Earwax is partly sebum, as is
mucopurulent discharge, the dry substance accumulating in the corners of the eye after sleeping.


Wikibooks | 45


Schematic view of a hair follicle with sebaceous gland.




Chapter 3


The composition of sebum varies from species to species; in humans, the lipid content consists of
about 25% wax monoesters, 41% triglycerides, 16% free fatty acids, and 12% squalene.


The activity of the sebaceous glands increases during puberty because of heightened levels of
androgens.


Sebaceous glands are involved in skin problems such as acne and keratosis pilaris. A blocked
sebaceous gland can result in a sebaceous cyst. The prescription drug isotretinoin significantly reduces
the amount of sebum produced by the sebaceous glands, and is used to treat acne. The extreme use (up
to 10 times doctor prescribed amounts) of anabolic steroids by bodybuilders to prevent weight loss tend
to stimulate the sebaceous glands which can cause acne.


The sebaceous glands of a human fetus in utero secrete a substance called Vernix caseosa, a
"waxy" or "cheesy" white substance coating the skin of newborns.


The preputial glands of mice and rats are large modified sebaceous glands that produce
pheromones.


46 | Human Physiology


A hair follicle with associated structures.




The Integumentary System


Ceruminous glands
Earwax, also known by the medical term cerumen, is a yellowish, waxy substance secreted in the


ear canal of humans and many other mammals. It plays a vital role in the human ear canal, assisting in
cleaning and lubrication, and also provides some protection from bacteria, fungus, and insects. A
comprehensive review of the physiology and pathophysiology of cerumen can be found in Roeser and
Ballachanda. Excess or impacted cerumen can press against the eardrum and/or occlude the external
auditory canal and impair hearing.


Production, composition, and different types
Cerumen is produced in the outer third of the cartilaginous portion of the human ear canal. It is a


mixture of viscous secretions from sebaceous glands and less-viscous ones from modified apocrine
sweat glands.


Two distinct genetically determined types of earwax are distinguished -- the wet-type which is
dominant, and the dry type which is recessive. Asians and Native Americans are more likely to have
the dry type of cerumen (grey and flaky), whereas Caucasians and Africans are more likely to have the
wet type (honey-brown to dark-brown and moist). Cerumen type has been used by anthropologists to
track human migratory patterns, such as those of the Inuit.


The difference in cerumen type has been tracked to a single base change (an single nucleotide
polymorphism) in a gene known as "ATP-binding cassette C11 gene". In addition to affecting cerumen
type, this mutation also reduces sweat production. The researchers conjecture that the reduction in
sweat was beneficial to the ancestors of East Asians and Native Americans who are thought to have
lived in cold climates.


Function
Cleaning. Cleaning of the ear canal occurs as a result of the "conveyor belt" process of epithelial


migration, aided by jaw movement. Cells formed in the center of the tympanic membrane migrate
outwards from the umbo (at a rate equivalent to that of fingernail growth) to the walls of the ear canal,
and accelerate towards the entrance of the ear canal. The cerumen in the canal is also carried outwards,
taking with it any dirt, dust, and particulate matter that may have gathered in the canal. Jaw movement
assists this process by dislodging debris attached to the walls of the ear canal, increasing the likelihood
of its extrusion.


Lubrication. Lubrication prevents desiccation and itching of the skin within the ear canal (known
as asteatosis). The lubricative properties arise from the high lipid content of the sebum produced by the
sebaceous glands. In wet-type cerumen at least, these lipids include cholesterol, squalene, and many
long-chain fatty acids and alcohols.


Antibacterial and antifungal roles. While studies conducted up until the 1960s found little
evidence supporting an antibacterial role for cerumen, more recent studies have found that cerumen
provides some bactericidal protection against some strains of bacteria. Cerumen has been found to be
effective in reducing the viability of a wide range of bacteria (sometimes by up to 99%), including
Haemophilus influenzae, Staphylococcus aureus, and many variants of Escherichia coli. The growth of


Wikibooks | 47




Chapter 3
two fungi commonly present in otomycosis was also significantly inhibited by human cerumen. These
antimicrobial properties are due principally to the presence of saturated fatty acids, lysozyme and,
especially, to the relatively low pH of cerumen (typically around 6.1 in normal individuals.


48 | Human Physiology




The Integumentary System


Mammary Glands
Mammary glands are the organs that, in the female


mammal, produce milk for the sustenance of the young.
These exocrine glands are enlarged and modified sweat
glands and are the characteristic of mammals which gave
the class its name.


Structure
The basic components of the mammary gland are the


alveoli (hollow cavities, a few millimetres large) lined with
milk-secreting epithelial cells and surrounded by
myoepithelial cells. These alveoli join up to form groups
known as lobules, and each lobule has a lactiferous duct
that drains into openings in the nipple. The myoepithelial
cells can contract, similar to muscle cells, and thereby push
the milk from the alveoli through the lactiferous ducts
towards the nipple, where it collects in widenings (sinuses)
of the ducts. A suckling baby essentially squeezes the milk
out of these sinuses.


One distinguishes between a simple mammary gland, which consists of all the milk-secreting
tissue leading to a single lactiferous duct, and a complex mammary gland, which consists of all the
simple mammary glands serving one nipple.


Humans normally have two complex mammary glands, one in each breast, and each complex
mammary gland consists of 10-20 simple glands. (The presence of more than two nipples is known as
polythelia and the presence of more than two complex mammary glands as polymastia.)


Wikibooks | 49


Cross section of the breast of a human female.




Chapter 3


Also, "click" this;"Breast tissue", to this a movie visiual of the breast.


Development and hormonal control
The development of mammary glands is controlled by hormones. The mammary glands exist in


both sexes, but they are rudimentary until puberty when in response to ovarian hormones, they begin to
develop in the female. Click this [1]to see what breast tissue does in a female during menustration.
Estrogen promotes formation, while testosterone inhibits it.


At the time of birth, the baby has lactiferous ducts but no alveoli. Little branching occurs before
puberty when ovarian estrogens stimulate branching differentiation of the ducts into spherical masses
of cells that will become alveoli. True secretory alveoli only develop in pregnancy, where rising levels
of estrogen and progesterone cause further branching and differentiation of the duct cells, together with
an increase in adipose tissue and a richer blood flow.


Colostrum is secreted in late pregnancy and for the first few days after giving birth. True milk
secretion (lactation) begins a few days later due to a reduction in circulating progesterone and the
presence of the hormone prolactin. The suckling of the baby causes the release of the hormone
oxytocin which stimulates contraction of the myoepithelial cells.


Breast cancer
As described above, the cells of mammary glands can easily be induced to grow and multiply by


50 | Human Physiology


Dissection of a lactating breast. 1 - Fat; 2 - Lactiferous duct/lobule; 3 - Lobule; 4 - Connective
tissue; 5 - Sinus of lactiferous duct; 6 - Lactiferous duct




The Integumentary System


hormones. If this growth runs out of control, cancer results. Almost all instances of breast cancer
originate in the lobules or ducts of the mammary
glands.


Types of breast cancer
• DCIS: Ductal Carcinoma in Situ
• LCIS: Lobular Carcinoma in Situ
• Invasive ductal carcinoma
• Invasive lobular carcinoma
• Inflammatory breast cancer
• Paget's disease


Other mammals
The number of complex and simple mammary


glands varies widely in different mammals. The
nipples and glands can occur anywhere along the two
milk lines, two roughly-parallel lines along the front of
the body. They are easy to visualize on dogs or cats,
where there are from 3 to 5 pairs of nipples following
the milk lines. In general most mammals develop
mammary glands in pairs along these lines, with a
number approximating the number of young typically
birthed at a time.


Male mammals typically have rudimentary
mammary glands and nipples, with a few exceptions: male mice don't have nipples, and male horses
lack nipples and mammary glands.


Mammary glands are true protein factories, and several companies have constructed transgenic
animals, mainly goats and cows, in order to produce proteins for pharmaceutical use. Complex
glycoproteins such as monoclonal antibodies or antithrombin cannot be produced by genetically
engineered bacteria, and the production in live mammals is much cheaper than the use of mammalian
cell cultures.


Homeostasis
As a whole, the integumentary system plays a big part in maintaining homeostasis. The


integumentary system is the outermost organ system of the body and many of its functions are related
to this location. The skin protects the body against pathogens and chemicals, minimizes loss or entry of
water, and blocks the harmful effects of sunlight. Sensory receptors in the skin provide information
about the external environment, helping the skin regulate body temperature in response to
environmental changes and helping the body react to pain and other tactile stimuli. The large surface
area of the skin makes it ideal for temperature regulation. The rate of heat loss can be regulated by the
amount of blood flowing through the the blood vessels in the dermis close to the surface of the skin.
When the body temperature rises, as for example during exercise, sympathetic tone is reduced and this


Wikibooks | 51


Early Signs of Breast Cancer




Chapter 3
brings about dilation of the blood vessels supplying the skin. The increase in skin blood flow allows
heat to be lost more rapidly so that body temperature does not rise above the normal homeostatic range.
The rate of heat loss can also be boosted by the production of sweat, which takes up additional heat as
it evaporates. Conversely, if heat production is less than required, the dermal vessels constrict, sweating
stops, and heat is conserved by the body.


Glossary
Areolar


Areolar connective tissue is a pliable, mesh-like tissue with a fluid matrix and functions to
cushion and protect body organs. It acts as a packaging tissue holding the internal organs together
and in correct placement.


Basal lamina
Basal lamina (often erroneously called basement membrane) is a layer on which epithelium sits.
This layer is composed of an electron-dense layer (lamina densa) between two electron-lucid
layers (lamina lucida), and is approximately 40-50 nm thick (with exceptions such as the 100-200
nm glomerular basement membrane).


Dermis
The dermis is the layer of skin beneath the epidermis that consists of connective tissue and
cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a
basement membrane.


Epidermis
The epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the
body's surface and is made up of stratified squamous epithelium with an underlying basal lamina.


Fibroblasts
A fibroblast is a cell that makes the structural fibers and ground substance of connective tissue.


Hair follicle
A hair follicle is part of the skin that grows hair by packing old cells together.


Hypodermis
The hypodermis (also called the hypoderm), is the lowermost layer of the integumentary system
in vertebrates. It is derived from the mesoderm, but unlike the dermis, it is not derived from the
dermatome region of the mesoderm.


Impetigo
This is a superficial skin infection most common among children age 2–6 years. People who play
close contact sports such as rugby, American football and wrestling are also susceptible,
regardless of age. The name derives from the Latin impetere ("assail"). It is also known as school
sores.


Melanocytes
These are cells located in the bottom layer of the skin's epidermis and in the middle layer of the
eye, the uvea. Through a process called melanogenesis, these cells produce melanin, a pigment in


52 | Human Physiology




The Integumentary System


the skin, eyes, and hair.
Melanoma


A melanoma is a malignant tumor that originates in melanocytes. It is a highly malignant form of
skin cancer, and, though rare, is responsible for the majority of skin cancer-related deaths.


Onychosis
Deformity or disease of the nails


Papillary
The papillery layer is outermost and extends into the epidermis to supply it with vessels. It is
composed of loosely arranged fibres. Papillary ridges make up the lines of the hands.


Recticular Layer
The reticular layer is more dense and is continuous with the hypodermis. It contains the bulk of
the structures (such as sweat glands). The reticular layer is composed of irregularly arranged
fibres and resists stretching.


For more fun pictures of other skin diseases and skin problems "click" to this cool website
"Dermatology Image Database". Note: From this link then click "Clinical Skin Diseases Images".


Review Questions
1. Name all of the parts of the integumentary system.
2. Name the cells that produce melanin and describe its function.
3. Name and describe the importance of the cutaneous senses.
4. Explain how sweating helps maintain normal body temperature.
5. Explain where on the body hair has important functions and describe these functions.


References
Brannon, Heather (2006). "Nail Anatomy" About, Inc., A part of The New York Times
Company.
American Academy of Dermatology - Nail Health
Cobb, Judith. Fingernails, Jewels or Tools? Nature's Field - Nail diagnosis
Graaff, Van De (2002). Human Anatomy, Sixth Edition. New York: McGraw-Hill.
Mader, Sylvia S. (2004). Human Biology. New York: McGraw-Hill.
Sorrentino, Sheila A. (2004). Mosby's textbook for Nursing Assistants, 6th Edition. St. Louis,
Missouri: Mosby.


live version • discussion • edit lesson • comment • report an error


Wikibooks | 53




Chapter 4


4 THE NERVOUS SYSTEM
live version • discussion • edit lesson • comment • report an error


he central nervous system includes the brain and spinal cord. The brain and spinal cord are
protected by bony structures, membranes, and fluid. The brain is held in the cranial cavity of the


skull and it consists of the cerebrum, cerebellum, and the brain stem. The nerves involved are cranial
nerves and spinal nerves.
T


54 | Human Physiology




The Nervous System


Overview of the entire nervous system
The nervous system has three main functions, sensory input, integration of data and motor output.


Sensory input is when the body gathers information or data, by way of neurons, glia and synapses. The
nervous system is composed of excitable nerve cells and synapses connecting the cells to one another,
to centers throughout the body or to other neurons. These neurons operate on excitation or inhibition
and although nerve cells can vary in size and location their communication with one another
determines their function. These nerves conduct impulses from sensory receptors to the brain and
spinal cord. The data is then processed by way of integration of data, which occurs only in the brain.
After the brain has processed the information, impulses are then conducted from the brain and spinal
cord to muscles and glands, which is called motor output. Glia cells are found within tissues and are not
excitable but help with myelination, ionic regulation and extracellular fluid.


The nervous system is comprised of two major parts, or subdivisions, the central nervous
system(CNS) and the peripheral nervous system(PNS). The CNS includes the brain and spinal cord.
The brain is the body's "control center". The CNS has various centers located within it that carry out
the sensory, motor and intergration of data. These centers can be subdivided to Lower Centers
(including the spinal cord and brain stem) and Higher centers communicating with the brain via
effectors. The PNS is a vast network of spinal and cranial nerves that are linked to the brain and the
spinal cord. It contains sensory receptors which help in processing changes in the internal and external
environment. This information is sent to the CNS via afferent sensory nerves. The PNS is then
subdivided into the autonomic nervous system and the somatic nervous system. The autonomic has
involuntary control of internal organs, blood vessels, smooth and cardiac muscles. The somatic has
voluntary control of skin, bones, joints, and skeletal muscle. The two systems function together, by way
of nerves from the PNS entering and becoming part of the CNS, and vice versa.


General functions of the CNS
The central nervous system (CNS) represents the largest


part of the nervous system, including the brain and the spinal
cord. Together, with the peripheral nervous system (PNS), it has
a fundamental role in the control of behavior.


The CNS is conceived as a system devoted to information processing, where an appropriate motor
output is computed as a response to a sensory input. Many threads of research suggest that motor
activity exists well before the maturation of the sensory systems, and senses only influence behavior
without dictating it. This has brought the conception of the CNS as an autonomous system.


Wikibooks | 55


CNS:
The "Central Nervous System",


comprised of brain, brainstem, and
spinal cord.




Chapter 4


Structure and function of neurons
Structure


Neurons are highly specialized for the processing and transmission of
cellular signals. Given the diversity of functions performed by neurons in
different parts of the nervous system, there is, as expected, a wide variety in
the shape, size, and electrochemical properties of neurons. For instance, the
soma of a neuron can vary in size from 4 to 100 micrometers in diameter.[1]


The soma (cell body) is the central part of the neuron. It contains the
nucleus of the cell, and therefore is where most protein synthesis occurs.
The nucleus ranges from 3 to 18 micrometers in diameter.[2] The dendrites
of a neuron are cellular extensions with many branches, and metaphorically
this overall shape and structure is referred to as a dendritic tree. This is
where the majority of input to the neuron occurs. However, information
outflow (i.e. from dendrites to other neurons) can also occur (except in
chemical synapse in which backflow of impulse is inhibited by the fact that axon do not possess
chemoreceptors and dendrites cannot secrete neurotransmitter chemical). This explains one way
conduction of nerve impulse. The axon is a finer, cable-like projection which can extend tens,
hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve
signals away from the soma (and also carry some types of information back to it). Many neurons have
only one axon, but this axon may - and usually will - undergo extensive branching, enabling
communication with many target cells. The part of the axon where it emerges from the soma is called
the 'axon hillock'. Besides being an anatomical structure, the axon hillock is also the part of the neuron
that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-
excited part of the neuron and the spike initiation zone for the axon: in neurological terms it has the
greatest hyperpolarized action potential threshold. While the axon and axon hillock are generally
involved in information outflow, this region can also receive input from other neurons as well. The
axon terminal is a specialized structure at the end of the axon that is used to release neurotransmitter
chemicals and communicate with target neurons. Although the canonical view of the neuron attributes
dedicated functions to its various anatomical components, dendrites and axons often act in ways
contrary to their so-called main function.


Axons and dendrites in the central nervous system are typically only about a micrometer thick,
while some in the peripheral nervous system are much thicker. The soma is usually about 10–25
micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon
of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes.
Sensory neurons have axons that run from the toes to the dorsal columns, over 1.5 meters in adults.
Giraffes have single axons several meters in length running along the entire length of their necks. Much
of what is known about axonal function comes from studying the squids giant axon, an ideal
experimental preparation because of its relatively immense size (0.5–1 millimeters thick, several
centimeters long).


Function
Sensory afferent neurons convey information from tissues and organs into the central nervous


56 | Human Physiology


Brain, brain stem, and spinal
chord.




The Nervous System


system. Efferent neurons transmit signals from the central nervous system to the effector cells and are
sometimes called motor neurons. Interneurons connect neurons within specific regions of the central
nervous system. Afferent and efferent can also refer generally to neurons which, respectively, bring
information to or send information from brain region.


Classification by action on other neurons
Excitatory neurons excite their target postsynaptic neurons or target cells causing it to function.


Motor neurons and somatic neurons are all excitatory neurons. Excitatory neurons in the brain are often
glutamatergic. Spinal motor neurons, which synapse on muscle cells, use acetylcholine as their
neurotransmitter. Inhibitory neurons inhibit their target neurons. Inhibitory neurons are also known as
short axon neurons, interneurons or microneurons. The output of some brain structures (neostriatum,
globus pallidus, cerebellum) are inhibitory. The primary inhibitory neurotransmitters are GABA and
glycine. Modulatory neurons evoke more complex effects termed neuromodulation. These neurons use
such neurotransmitters as dopamine, acetylcholine, serotonin and others. Each synapses can receive
both excitatory and inhibitory signals and the outcome is determined by the adding up of summation.


Excitatory and inhibitory process
The release of a excitatory


neurotransmitter (ACHe) at the synapses will
cause an inflow of positively charged sodium
ions (Na+) making a localized depolarization
of the membrane. The current then flows to
the resting (polarized) segment of the axon.


Inhibitory synapse causes an inflow of
Cl- (chlorine) or K+ (potassium) making the
synaptic membrane hyperpolarized. This
increase prevents depolarization, causing a
decrease in the possibility of an axon
discharge. If they are both equal to their
charges, then the operation will cancel itself
out. There are two types of summation:
spatial and temporal. Spatial summation
requires several excitatory synapses (firing
several times) to add up,thus causing an axon discharge. It also occurs within inhibitory synapses,
where just the opposite will occur. In temporal summation, it causes an increase of the frequency at the
same synapses until it is large enough to cause a discharge. Spatial and temporal summation can occur
at the same time as well.


Summation
When excitatory synapses exceed the amount of inhibitory synapses there are, then the excitatory


synapses will prevail over the other. The same goes with inhibitory synapses, if there are more


Wikibooks | 57


Nerve Synapse




Chapter 4
inhibitory synapses than excitatory, the synapses will be inhibited. To determine all of this is called
summation.


Classification by discharge patterns:
Neurons can be classified according to their electrophysiological characteristics (note that a single


action potential is not enough to move a large muscle, and instead will cause a twitch).
Tonic or regular spiking: Some neurons are typically constantly (or tonically) active. Example:


interneurons in neurostriatum.
Phasic or bursting: Neurons that fire in bursts are called phasic.
Fast spiking: Some neurons are notable for their fast firing rates. For example, some types of


cortical inhibitory interneurons, cells in globus pallidus.
Thin-spike: Action potentials of some neurons are more narrow compared to the others. For


example, interneurons in prefrontal cortex are thin-spike neurons.
Classification by neurotransmitter released:
Some examples are cholinergic, GABAergic, glutamatergic and dopaminergic neurons.


Central Nervous System
The central nervous system is the control center for the body. It regulates organ function, higher


thought, and movement of the body. The central nervous system consists of the brain and spinal cord.


58 | Human Physiology




The Nervous System


Brain
The brain is found in the cranial


cavity. Within it are found the higher nerve
centers responsible for coordinating the
sensory and motor systems of the body
(forebrain). The brain stem houses the
lower nerve centers (consisting of
midbrain, pons, and medulla),


Medulla
The medulla is the control center for


respiratory, cardiovascular and digestive
functions.


Pons
The pons houses the control centers for respiration and inhibitory functions. Here it will interact


with the cerebellum.


Cerebrum
The cerebrum, or top portion of the brain, is divided by a deep crevice, called the longitudinal


sulcus. The longitudinal sulcus separates the cerebrum in to the right and left hemispheres. In the
hemispheres you will find the cerebral cortex, basal ganglia and the limbic system. The two
hemispheres are connected by a bundle of nerve fibers called the corpus callosum. The right
hemisphere is responsible for the left side of the body while the opposite is true of the left hemisphere.
Each of the two hemispheres are divided into four separated lobes: the frontal in control of specialized
motor control, learning,planning and speech; parietal in control of somatic sensory functions; occipital
in control of vision; and temporal lobes which consists of hearing centers and some speech. Located
deep to the temporal lobe of the cerebrum is the insula.


Cerebellum
The cerebellum is the part of the brain that is located posterior to the medulla oblongata and pons.


It coordinates skeletal muscles to produce smooth, graceful motions. The cerebellum receives
information from our eyes, ears, muscles, and joints about what position our body is currently in. It also
receives output from the cerebral cortex about where these parts should be. After processing this
information, the cerebellum sends motor impulses from the brainstem to the skeletal muscles. The main
function of the cerebellum is coordination. The cerebellum is also responsible for balance and posture.
It also assists us when we are learning a new motor skill, such as playing a sport or musical instrument.


Wikibooks | 59


A color-coated image of the brain, showing the main sections.




Chapter 4


The Limbic System and Higher Mental Functions


The Limbic System
The Limbic System is a complex set of structures found just


beneath the cerebrum and on both sides of the thalamus. It combines
higher mental functions, and primitive emotion, into one system. It is
often referred to as the emotional nervous system. It is not only
responsible for our emotional lives, but also our higher mental
functions, such as learning and formation of memories. The Limbic
system explains why some things seem so pleasurable to us, such as
eating and why some medical conditions are caused by mental stress,
such as high blood pressure. There are two significant structures
within the limbic system and several smaller structures that are
important as well. They are:


1. The Hippocampus
2. The Amygdala
3. The Thalamus
4. The Hypothalamus
5. The Fornix and Parahippocampus
6. The Cingulate Gyrus


Structures of the Limbic System
Hippocampus


The Hippocampus is found deep in the temporal lobe, shaped like a seahorse. It consists of two
horns that curve back from the amygdala. It is situated in the brain so as to make the prefrontal
area aware of our past experiences stored in that area. The prefrontal area of the brain consults
this structure to use memories to modify our behavior. The hippocampus is responsible for
memory.


Amygdala
The Amygdala is a little almond shaped structure, deep inside the anteroinferior region of the
temporal lobe, connects with the hippocampus, the septi nuclei, the prefrontal area and the medial
dorsal nucleus of the thalamus. These connections make it possible for the amigdala to play its
important role on the mediation and control of such activities and feelings as love, friendship,
affection, and expression of mood. The amygdala is the center for identification of danger and is
fundamental for self preservation. The amygdala is the nucleus responsible for fear.


Thalamus
Lesions or stimulation of the medial, dorsal, and anterior nuclei of the thalamus are associated
with changes in emotional reactivity. However, the importance of these nuclei on the regulation
of emotional behavior is not due to the thalamus itself, but to the connections of these nuclei with
other limbic system structures. The medial dorsal nucleus makes connections with cortical zones
of the prefrontal area and with the hypothalamus. The anterior nuclei connect with the mamillary
bodies and through them, via fornix, with the hippocampus and the cingulated gyrus, thus taking


60 | Human Physiology


Image of the brain, showing the
Limbic system.




The Nervous System


part in what is known as the Papez's circuit.


Wikibooks | 61




Chapter 4


Case Study
Central Pain Syndrome


I was 42 years old when my life changed forever. I had a
stroke. As an avid viewer of medical programs on television I
assumed that I would have physical therapy for my paralyzed left
side and get on with my life. No one ever mentioned pain or the
possibility of pain, as a result of the stroke. I did experience
unusual sensitivity to touch while still in the hospital, but nothing
to prepare me for what was to come.


The part of my brain that is damaged is the Thalamus. This
turns out to be the pain center and what I have now is an out of
control Thalamus, resulting in Thalamic Pain syndrome, also
called Central Pain Syndrome. This means that 24 hours a day, seven days a week, my brain sends
messages of pain and it never goes away. I am under the care of physicians, who not only understand
chronic pain, but are also willing to treat it with whatever medications offer some help. None of the
medications, not even narcotic medications, take the pain away. They just allow me to manage it so I
can function.
Hypothalamus


The Hypothalamus is a small part of the brain located just below the thalamus on both sides of
the third ventricle. Lesions of the hypothalamus interfere with several vegetative functions and
some so called motivated behaviors like sexuality, combativeness, and hunger. The hypothalamus
also plays a role in emotion. Specifically, the lateral parts seem to be involved with pleasure and
rage, while the medial part is linked to aversion, displeasure, and a tendency to uncontrollable
and loud laughing. However, in general the hypothalamus has more to do with the expression of
emotions. When the physical symptoms of emotion appear, the threat they pose returns, via the
hypothalamus, to the limbic centers and then the prefrontal nuclei, increasing anxiety.


The Fornix and Parahippocampal
These small structures are important connecting pathways for the limbic system.


The Cingulate Gyrus
The Cingulate Gyrus is located in the medial side of the brain between the cingulated sulcus and
the corpus callosum. There is still much to be learned about this gyrus, but it is already known
that its frontal part coordinates smells and sights, with pleasant memories of previous emotions.
The region participates in the emotional reaction to pain and in the regulation of aggressive
behavior.


Memory and Learning
Memory is defined as : The mental faculty of retaining and recalling past experiences, the act or


instance of remembering recollection. Learning takes place when we retain and utilize past memories.
There are three basic types of memory:


62 | Human Physiology


Image of the brain showing the location of
the hypothalamus.




The Nervous System


1. Sensory Memory
2. Short Term Memory
3. Long Term Memory


Sensory Memory
The sensory memories act as a buffer for stimuli through senses. A sensory memory retains an
exact copy of what is seen or heard: iconic memory for visual, echoic memory for aural and
haptic memory for touch. Information is passed from sensory memory into short term memory.
Some believe it lasts only 300 milliseconds, it has unlimited capacity. Selective attention
determines what information moves from sensory memory to short term memory.


Short Term Memory
Short Term Memory acts as a scratch pad for temporary recall of the information under process.
For instance, in order to understand this sentence you need to hold in your mind the beginning of
the sentence as you read the rest. Short term memory decays rapidly and also has a limited
capacity. Chunking of information can lead to an increase in the short term memory capacity, this
is the reason why a hyphenated phone number is easier to remember than a single long number.
The successful formation of a chunk is known as closure. Interference often causes disturbance in
short term memory retention. This accounts for the desire to complete a task held in short term
memory as soon as possible.
Within short term memory there are three basic operations:


1. Iconic memory - the ability to hold visual images
2. Acoustic memory - the ability to hold sounds. Can be held longer than iconic.
3. Working memory - an active process to keep it until it is put to use. Note that the goal is not


really to move the information from short term memory to long term memory, but merely to put
it to immediate use.


The process of transferring information from short term to long term memory involves the
encoding or consolidation of information. This is not a function of time, that is, the longer the memory
stays in the short term the more likely it is to be placed in the long term memory. On organizing
complex information in short term before it can be encoded into the long term memory, in this process
the meaningfulness or emotional content of an item may play a greater role in its retention in the long
term memory. The limbic system sets up local reverberating circuits such as the Papaz's Circuit.


Long Term Memory
Long Term Memory is used for storage of information over a long time. Information from short
to long term memory is transferred after a short period. Unlike short term memory, long term
memory has little decay. Long term potential is an enhanced response at the synapse within the
hippocampus. It is essential to memory storage. The limbic system isn't directly involved in long
term memory necessarily but it selects them from short term memory, consolidates these
memories by playing them like a continuous tape, and involves the hippocampus and amygdala.
There are two types of long term memory:


1. Episodic Memory


Wikibooks | 63




Chapter 4
2. Semantic Memory


Episodic memory represents our memory of events and experiences in a serial form. It is from this
memory that we can reconstruct the actual events that took place at a given point in our lives. Semantic
memory, on the other hand, is a structured record of facts, concepts, and skills that we have acquired.
The information in the semantic memory is derived from our own episode memory, such as that we can
learn new facts or concepts from experiences.


There are three main activities that are related to long term memory:
1. Storage
2. Deletion
3. Retrieval


Information for short term memory is stored in long term memory by rehearsal. The repeated
exposure to a stimulus or the rehearsal of a piece of information transfers it into long term memory.
Experiments also suggest that learning is most effective if it is distributed over time. Deletion is mainly
caused by decay and interference. Emotional factors also affect long term memory. However, it is
debatable whether we actually ever forget anything or whether it just sometimes becomes increasingly
difficult to retrieve it. Information may not be recalled sometimes but may be recognized, or may be
recalled only with prompting. This leads us to the third operation of memory, information retrieval.


There are two types of information retrieval:
1. Recall
2. Recognition


In recall, the information is reproduced from memory. In recognition the presentation of the
information provides the knowledge that the information has been seen before. Recognition is of lesser
complexity, as the information is provided as a cue. However, the recall may be assisted by the
provision of retrieval cues which enable the subject to quickly access the information in memory.


Language and Speech
Language depends on semantic memory so some of the same areas in the brain are involved in


both memory and language. Articulation, the forming of speech, is represented bilaterally in the motor
areas. However, language analysis and speech formation take place in most individuals in regions of
the left hemisphere only. The two regions involved are:


1. Broca's Area
2. Wernicke's Area


Broca's area is located just in front of the voice control area of the left motor cortex. This region
assembles the motor of speech and writing. For example, patients with lesions in this area:


1. Understand language perfectly
2. May be able to write perfectly
3. Seldom speak spontaneously


64 | Human Physiology




The Nervous System


Wernicke's area is part of the auditory and visual associations cortex. This region is responsible for
the analysis and formation of language content. For example, patients with lesions in this area:


1. Are unable to name objects
2. Are unable to understand the meaning of words
3. Articulate speech readily but usually nonsensically


Diseases of the Limbic System
There are several well known diseases that are disorders of the limbic system. A few are:


1. Psychosis
2. Schizophrenia
3. Depression


An increased DA response in the limbic system results in schizophrenia. DA may be synthesized
or secreted in excess, DA receptors may be supersensitive, and DA regulatory mechanism may be
defective. Symptoms are decreased by drugs which block DA receptors. Symptoms of schizophrenia
are:


1. Loss of touch with reality
2. Decreased ability to think and reason
3. Decreased ability to concentrate
4. Decreased memory
5. Regress in child-like behavior
6. Altered mood and impulsive behavior
7. Auditory hallucinations


Symptoms may be so severe that the individual cannot function.
Depression is caused by decreased levels of NE and/or serotonin in the limbic system. Drugs


which increase NE and/or serotonin decrease the symptoms of depression. Depression is the most
common major mental illness and is characterized by both emotional and physical symptoms.
Symptoms of depression are:


1. Intense sadness and despair
2. Anxiety
3. Loss of ability to concentrate
4. Pessimism
5. Feelings of low self esteem
6. Insomnia or hypersomnia
7. Increased or decreased appetite
8. Changes in body temperature and endocrine gland function


10 to 15% of depressed individuals display suicidal behavior during their lifetime.
Another common form of depression is manic depression. Manic is an acute state characterized by:


Wikibooks | 65




Chapter 4
1. Excessive elation and impaired judgment
2. Insomnia and irritability
3. Hyperactivity
4. Uncontrolled speech


Manic depression, also known as bipolar disorder, displays mood swings between manic and
depression. The limbic system receptors are unregulated. Drugs used are unique mood stabilizers.


The hippocampus is particularly vulnerable to several disease processes, including ischemia, which
is any obstruction of blood flow or oxygen deprivation, Alzheimer’s disease, and epilepsy. These
diseases selectively attack CA1, which effectively cuts through the hippocampal circuit.


A connection between autism and the limbic system has also been noted as well. URL:
http://www.autism.org/limbic.html


The Peripheral Nervous System
The peripheral nervous system includes 12 cranial nerves 31 pairs of spinal nerves. It can be


subdivided into the somatic and autonomic systems. It is a way of communication from the central
nervous system to the rest of the body by nerve impulses that regulate the functions of the human body.


66 | Human Physiology




The Nervous System


The twelve cranial nerves are
I Olfactory Nerve for smell
II Optic Nerve for vision
III Oculomotor for looking around
IV Trochlear for moving eye
V Trigeminal for feeling touch on face
VI Abducens to move eye muscles
VII Facial to smile, wink, and help us taste
VIII Vestibulocochlear to help with balance, equilibrium, and hearing
IX Glossopharengeal for swallowing and gagging
X Vagus for swallowing, talking, and parasympathetic actions of digestion
XI Spinal accessory for shrugging shoulders
XII Hypoglossal for tongue more divided into different regions as muscles


Wikibooks | 67


The Cranial Nerves




Chapter 4
The 10 out of the 12 cranial nerves originate from the brainstem, and mainly control the functions


of the anatomic structures of the head with some exceptions. CN X receives visceral sensory
information from the thorax and abdomen, and CN XI is responsible for innervating the
sternocleidomastoid and trapezius muscles, neither of which is exclusively in the head.


Spinal nerves take their origins from the spinal cord. They control the functions of the rest of the
body. In humans, there are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1
coccygeal. The naming convention for spinal nerves is to name it after the vertebra immediately above
it. Thus the fourth thoracic nerve originates just below the fourth thoracic vertebra. This convention
breaks down in the cervical spine. The first spinal nerve originates above the first cervical vertebra and
is called C1. This continues down to the last cervical spinal nerve, C8. There are only 7 cervical
vertebrae and 8 cervical spinal nerves.


Lateral cord
The lateral cord gives rise to the following nerves:


• The lateral pectoral nerve, C5, C6 and C7 to the pectoralis major muscle, or musculus
pectoralis major.


• The musculocutaneous nerve which innervates the biceps muscle
• The median nerve, partly. The other part comes from the


medial cord. See below for details.


Posterior cord
The posterior cord gives rise to the following nerves:


• The upper subscapular nerve, C7 and C8, to the
subscapularis muscle, or musculus supca of the rotator cuff.


• The lower subscapular nerve, C5 and C6, to the teres major
muscle, or the musculus teres major, also of the rotator cuff.


• The thoracodorsal nerve, C6, C7 and C8, to the latissimus
dorsi muscle, or musculus latissimus dorsi.


• The axillary nerve, which supplies sensation to the shoulder
and motor to the deltoid muscle or musculus deltoideus, and the
teres minor muscle, or musculus teres minor.


• The radial nerve, or nervus radialis, which innervates the
triceps brachii muscle, the brachioradialis muscle, or musculus
brachioradialis,, the extensor muscles of the fingers and wrist
(extensor carpi radialis muscle), and the extensor and abductor
muscles of the thumb. See radial nerve injuries.


Medial cord
The medial cord gives rise to the following nerves:


68 | Human Physiology


Diagram showing human
dermatoms, i.e., skin regions with
respect to the routing of their
nerve connection of their afferent
nerves through the spinal cord.




The Nervous System


• The median pectoral nerve, C8 and T1, to the pectoralis muscle
• The medial brachial cutaneous nerve, T1
• The medial antebrachial cutaneous nerve, C8 and T1
• The median nerve, partly. The other part comes from the lateral cord. C7, C8 and T1 nerve


roots. The first branch of the median nerve is to the pronator teres muscle, then the flexor carpi
radialis, the palmaris longus and the flexor digitorum superficialis. The median nerve provides
sensation to the anterior palm, the anterior thumb, index finger and middle finger. It is the nerve
compressed in carpal tunnel syndrome.


• The ulnar nerve originates in nerve roots C7, C8 and T1. It provides sensation to the ring
and pinky fingers. It innervates the flexor carpi ulnaris muscle, the flexor digitorum profundus
muscle to the ring and pinky fingers, and the intrinsic muscles of the hand (the interosseous
muscle, the lumbrical muscles and the flexor pollicus brevis muscle). This nerve traverses a
groove on the elbow called the cubital tunnel, also known as the funny bone. Striking the nerve
at this point produces an unpleasant sensation in the ring and little fingers.


Other thoracic spinal nerves (T3-T12)
The remainder of the thoracic spinal nerves, T3 through T12, do little recombining. They form the


intercostal nerves, so named because they run between the ribs. For points of reference, the 7th
intercostal nerve terminates at the lower end of the sternum, also known as the xyphoid process. The
10th intercostal nerve terminates at the umbilicus, or the belly button.


The somatic nervous system is that part of the peripheral nervous system associated with the
voluntary control of body movements through the action of skeletal muscles, and also reception of
external stimuli. The somatic nervous system consists of afferent fibers that receive information from
external sources, and efferent fibers that are responsible for muscle contraction. The somatic system
includes the pathways from the skin and skeletal muscles to the Central Nervous System. It is also
described as involved with activities that involve consciousness.


The basic route of the efferent somatic nervous system includes a two neuron sequence. The first is
the upper motor neuron, whose cell body is located in the precentral gyrus (Brodman Area 4) of the
brain. It receives stimuli from this area to control skeletal (voluntary) muscle. The upper motor neuron
carries this stimulus down the corticospinal tract and synapses in the ventral horn of the spinal cord
with the alpha motor neuron, a lower motor neuron. The upper motor neuron releases acetylcholine
from its axon terminal knobs and these are received by nicotinic receptors on the alpha motor neuron.
The alpha motor neurons cell body sends the stimulus down its axon via the ventral root of the spinal
cord and proceeds to its neuromuscular junction of its skeletal muscle. There, it releases acetylcholine
from its axon terminal knobs to the muscles nicotinic receptors, resulting in stimulus to contract the
muscle.


The somatic system includes all the neurons connected with the muscles, sense organs and skin. It
deals with sensory information and controls the movement of the body.


The Autonomic System
The Autonomic system deals with the visceral organs, like the heart, stomach, gland, and the


intestines. It regulates systems that are unconsciously carried out to keep our body alive and well, such


Wikibooks | 69




Chapter 4
as breathing, digestion (peristalsis), and regulation of the heartbeat. The Autonomic system consists of
the sympathetic and the parasympathetic divisions. Both divisions work without conscious effort,
and they have similar nerve pathways, but the sympathetic and parasympathetic systems generally have
opposite effects on target tissues (they are antagonistic). By controlling the relative input from each
division, the autonomic system regulates many aspects of homeostasis. One of the main nerves for the
parasympathetic autonomic system is Cranial Nerve X, the Vegas nerve.


The Sympathetic System
The sympathetic nervous system activates what is often termed the fight or flight response, as it is


most active under sudden stressful circumstances (such as being attacked). This response is also known
as sympathetico-adrenal response of the body, as the pre-ganglionic sympathetic fibers that end in the
adrenal medulla (but also all other sympathetic fibers) secrete acetylcholine, which activates the
secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it.
Therefore, this response that acts primarily on the cardiovascular system is mediated directly via
impulses transmitted through the sympathetic nervous system and indirectly via catecholamines
secreted from the adrenal medulla.


Western science typically looks at the SNS as an automatic regulation system, that is, one that
operates without the intervention of conscious thought. Some evolutionary theorists suggest that the
sympathetic nervous system operated in early organisms to maintain survival (Origins of
Consciousness, Robert Ornstein; et al.), as the sympathetic nervous system is responsible for priming
the body for action. One example of this priming is in the moments before waking, in which
sympathetic outflow spontaneously increases in preparation for action.


The parasympathetic nervous system is part of the autonomic nervous system. Sometimes called
the rest and digest system or feed and breed. The parasympathetic system conserves energy as it slows
the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the
gastrointestinal tract.


Organization
Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in


the intermediolateral cell column (or lateral horn), beginning at the first thoracic segment of the spinal
cord and extending into the second or third lumbar segments. Because its cells begin in the thoracic and
lumbar regions of the spinal cord, the SNS is said to have a thoracolumbar outflow. Axons of these
nerves leave the spinal cord in the ventral branches (rami) of the spinal nerves, and then separate out as
'white rami' (so called from the shiny white sheaths of myelin around each axon) which connect to two
chain ganglia extending alongside the vertebral column on the left and right. These elongated ganglia
are also known as paravertebral ganglia or sympathetic trunks. In these hubs, connections (synapses)
are made which then distribute the nerves to major organs, glands, and other parts of the body.


70 | Human Physiology




The Nervous System


In order to reach the target organs and glands, the axons must travel long distances in the body,
and, to accomplish this, many axons link up with the axon of a second cell. The ends of the axons do
not make direct contact, but rather link across a space, the synapse.


Wikibooks | 71


The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic
plexuses. (After Schwalbe.)




Chapter 4
In the SNS and other components of the peripheral nervous system, these synapses are made at


sites called ganglia. The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber
leaves the ganglion is called a postganglionic cell. As mentioned previously, the preganglionic cells of
the SNS are located between the first thoracic segment and the second or third lumbar segments of the
spinal cord. Postganglionic cells have their cell bodies in the ganglia and send their axons to target
organs or glands.


The ganglia include not just the sympathetic trunks but also the superior cervical ganglion (which
sends sympathetic nerve fibers to the head), and the celiac and mesenteric ganglia (which send
sympathetic fibers to the gut).


Information transmission
Messages travel through the SNS in a bidirectional flow. Efferent messages can trigger changes in


different parts of the body simultaneously. For example, the sympathetic nervous system can accelerate
heart rate; widen bronchial passages; decrease motility (movement) of the large intestine; constrict
blood vessels; increase peristalsis in the esophagus; cause pupil dilation, piloerection (goose bumps)
and perspiration (sweating); and raise blood pressure. Afferent messages carry sensations such as heat,
cold, or pain.


The first synapse (in the sympathetic chain) is mediated by nicotinic receptors physiologically
activated by acetylcholine, and the target synapse is mediated by adrenergic receptors physiologically
activated by either noradrenaline or adrenaline. An exception is with sweat glands which receive
sympathetic innervation but have muscarinic acetylcholine receptors which are normally characteristic
of PNS. Another exception is with certain deep muscle blood vessels, which have acetylcholine
receptors and which dilate (rather than constrict) with an increase in sympathetic tone. The sympathetic
system cell bodies are located on the spinal cord excluding the cranial and sacral regions. The
preganglonic neurons exit from the vertebral column and synapse with the postganglonic nerouns in the
sympathetic trunk.


The parasympathetic nervous system is one of three divisions of the autonomic nervous system.
Sometimes called the rest and digest system, the parasympathetic system conserves energy as it slows
the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the
gastrointestinal tract.


Relationship to sympathetic
While an oversimplification, it is said that the parasympathetic system acts in a reciprocal manner


to the effects of the sympathetic nervous system; in fact, in some tissues innervated by both systems,
the effects are synergistic.


Receptors
The parasympathetic nervous system uses only acetylcholine (ACh) as its neurotransmitter. The


ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors. Most
transmissions occur in two stages: When stimulated, the preganglionic nerve releases ACh at the


72 | Human Physiology




The Nervous System


ganglion, which acts on nicotinic receptors of the postganglionic nerve. The postganglionic nerve then
releases ACh to stimulate the muscarinic receptors of the target organ.


The three main types of muscarinic receptors that are well characterised are:
• The M1 muscarinic receptors are located in the neural system.
• The M2 muscarinic receptors are located in the heart, and act to bring the heart back to


normal after the actions of the sympathetic nervous system: slowing down the heart rate,
reducing contractile forces of the atrial cardiac muscle, and reducing conduction velocity of the
atrioventricular node (AV node). Note, they have no effect on the contractile forces of the
ventricular muscle.


• The M3 muscarinic receptors are located at many places in the body, such as the smooth
muscles of the blood vessels, as well as the lungs, which means that they cause vasoconstriction
and bronchoconstriction. They are also in the smooth muscles of the gastrointestinal tract (GIT),
which help in increasing intestinal motility and dilating sphincters. The M3 receptors are also
located in many glands that help to stimulate secretion in salivary glands and other glands of the
body.


Nervous Tissue
The nervous system coordinates the activity of the muscles, monitors the organs, constructs and


also stops input from the senses, and initiates actions. Prominent participants in a nervous system
include neurons and nerves, which play roles in such coordination.Our nervous tissue only consists of
two types of cells. These cells are neurons and neuroglia cells. The neurons are responsible for
transmitting nerve impulses. Neuroglia cells are responsible for supporting and nourishing the neuron
cells.


Types of Neurons
There are three types of neurons in the


body. We have sensory neurons, interneurons,
and motor neurons. Neurons are a major class of
cells in the nervous system. Neurons are
sometimes called nerve cells, though this term is
technically imprecise, as many neurons do not
form nerves. In vertebrates, neurons are found in
the brain, the spinal cord and in the nerves and
ganglia of the peripheral nervous system. Their
main role is to process and transmit information.
Neurons have excitable membranes, which
allow them to generate and propagate electrical
impulses. Sensory neuron takes nerve impulses
or messages right from the sensory receptor and
delivers it to the central nervous system. A sensory receptor is a structure that can find any kind of
change in it's surroundings or environment.


Wikibooks | 73




Chapter 4


Structure of a neuron
Neurons have three different parts to them. They all have an axon, a cell body and dendrites. The


axon is the part of the neuron that conducts nerve impulses. Axons can get to be quite long. When an
axon is present in nerves, it is called a nerve fiber. A cell body has a nucleous and it also has other
organelles. The dendrites are the short pieces that come off of the cell body that receive the signals
from sensory receptors and other neurons.


Myelin Sheath
Schwann cells contain a lipid substance called myelin in their plasma membranes. When schwann


cells wrap around axons, a myelin sheath forms. There are gaps that have no myelin sheath around
them; these gaps are called nodes of Ranvier. Myelin sheathes make excellent insulators. Axons that
are longer have a myelin sheath, while shorter axons do not. The disease multiple sclerosis is an
autoimune disease where the body attacks the myelin sheath of the central nervous system.


Case Study
A 35-year-old male in 1986 had been admitted to a hospital in Florida three weeks previous to


being diagnosed, with complaints of weakness and spasticity in the right leg, difficulties with balance,
and fatigue and malaise. Tests performed at the Florida hospital had revealed abnormalities in spinal
fluid and MRI brain scan. The patient complained of being severely depressed and anxious. He had
anger at his circumstances and frequent crying spells. One month previously he had noticed aching and
loss of vision in the left eye that had since improved.


This man was diagnosed with Multiple Sclerosis. MS is a chronic, degenerative, and progressive
disorder that affects the nerve fibers in the brain and spinal cord. Myelin is a fatty substance that
surrounds and insulates the nerve fibers and facilitates the conduction of the nerve impulse
transmissions. MS is characterized by intermittent damage to myelin (called demyelination) caused by
the destruction of specialized cells (oligodendrocytes) that form the substance. Demyelination causes
scarring and hardening (sclerosis) of nerve fibers usually in the spinal cord, brain stem, and optic
nerves, which slows nerve impulses and results in weakness, numbness, pain, and vision loss. Because
different nerves are affected at different times, MS symptoms often worsen (exacerbate), improve, and
develop in different areas of the body. Early symptoms of the disorder may include vision changes
(blurred vision, blind spots) and muscle weakness. MS can progress steadily or cause acute attacks
(exacerbations) followed by partial or complete reduction in symptoms (remission). Most patients with
the disease have a normal lifespan.
There are different types of MS


Multiple sclerosis is classified according to frequency and severity of neurological symptoms, the
ability of the CNS to recover, and the accumulation of damage.


74 | Human Physiology




The Nervous System


Treating Depression
Every now and then we all feel a little blue, these feelings can be caused by losing a loved one.


Clinical depression goes much further than just feeling down. Depression has many symptoms,
including lack of energy, abnormal eating habits (either too much or too little)and sleeping problems
(also too much or too little). Often a person can feel worthless and have thoughts of committing
suicide. The cause of depression and its symptoms are a mystery but we do understand that it is an
illness associated with biochemical changes in the brain. A lot of research goes on to explain that it is
associated with a lack of amines serotonin and norephinephrine. Therefore pharmacological treatment
strategies often try to increase amine concentrations in the brain.


One class of antidepressants is monoamine oxidase inhibitors. Mono amine oxidase is a enzyme
that breaks down your amines like norephinephrine and serotonin. Because the antidepressants inhibit
their degradation they will remain in the synaptic cleft for a longer period of time making the effect just
as if you had increased theses types of neurotransmitters.


A newer class of antidepressants is selective serotonin reuptake inhibitors (SSRI's). With SSRI's
decreasing the uptake of serotonin back into the cell that will increase the amount of serotonin present
in the synaptic cleft. SSRI's are more specific than the monoamine oxidase inhibitors because they only
affect serotonergic synapses. You might recognize these SSRI's by name as Prozac and Paxil.


Wikibooks | 75




Chapter 4


Drug Abuse
Scientists have long accepted that there is a biological basis for


drug addiction, though the exact mechanisms responsible are only
now being identified. It is believed that addictive substances create
dependence in the user by changing the brain's reward functions,
located in the mesolimbic dopamine system—the part of the brain
that reinforces certain behaviors such as eating, sexual intercourse,
exercise, and social interaction. Addictive substances, through
various means and to different degrees, cause the synapses of this
system to flood with excessive amounts of dopamine, creating a brief
rush of euphoria more commonly called a "high”. Some say that
abuse begins when the user begins shirking responsibility in order to
afford drugs or to have enough time to use them. Some say it begins when a person uses "excessive"
amounts, while others draw the line at the point of legality, and others believe it amounts to chronic use
despite degenerating mental and physical health in the user. Some think that any intoxicant
consumption is an inappropriate activity. Here are some drugs that are abused frequently: Acid/LSD,
Alcohol, Club Drugs, Cocaine, Ecstasy/MDMA, Heroin, Inhalants, Marijuana, Methamphetamine,
PCP/Phencyclidine, Prescription Medications, Smoking/Nicotine and Steroids.


76 | Human Physiology




The Nervous System


Methamphetamine
In the US, medically prescribed


methamphetamine is distributed in tablet
form under the brand name Desoxyn®.


Illicit methamphetamine comes in a
variety of forms. Most commonly it is
found as a colorless crystalline solid, sold
on the street under a variety of names, such
as: crystal meth or crystal. Crystal
methamphetamine may also be referred to
as shards, rock, P, upside-down b, pony,
crissie, crystal, glass, ice, devil's dandruff, chimichanga, Jib, critter, Tina, Crawford, Working Man's
Cocaine, Pook, tik, or "broken glass". People may confuse crack cocaine with methamphetamine.


It is also sold as a less-pure crystalline powder called crank or speed, or in crystalline rock form
called dope, shit, tina, or tweak; both "dope" and "speed" are often used to refer to other drugs.
Colorful flavored pills containing methamphetamine and caffeine are known as yaba (Thai for "crazy
medicine"). At its most impure, it is sold as a crumbly brown or off-white rock commonly referred to
as peanut butter crank. See the list of street names for a more comprehensive list of common street
names for methamphetamine.


Methamphetamine found on the street may be pure, or adulterated with chemicals that were used
to synthesize it. In some instances, it may be diluted or cut with non-psychoactive substances like
inositol. In other instances, it may be mixed with other psychoactive drugs.


Marijuana
The drug cannabis is produced from parts of the cannabis plant, primarily the cured flowers and


gathered trichomes of the female plant. The major active chemical compound tetrahydrocannabinol,
commonly referred to as THC, has psychoactive and medicinal effects when consumed, usually by
smoking or ingestion. Cannabis has been consumed by humans for thousands of years; in the 20th
century there was an upswing in the use of cannabis for recreational and religious purposes.


The possession, use, or sale of psychoactive cannabis products became illegal in many parts of the
world in the early 20th century. Since then, while some countries have intensified the enforcement of
cannabis prohibition, others have reduced the priority of enforcement to the point of de facto legality.
Cannabis remains illegal in the vast majority of the world's countries.


The nature and intensity of the immediate effects of cannabis consumption vary according to the
dose, the species or hybridization of the source plant, the method of consumption, the user's mental and
physical characteristics (such as possible tolerance), and the environment of consumption. This is
sometimes referred to as set and setting. Smoking the same cannabis either in a different frame of mind
(set) or in a different location (setting) can alter the effects or perception of the effects by the
individual. Effects of cannabis consumption may be loosely classified as cognitive and physical.
Anecdotal evidence suggests that the Cannabis sativa species tends to produce more of the cognitive or
perceptual effects, while Cannabis indica tends to produce more of the physical effects.


Wikibooks | 77


Illegal methlab.




Chapter 4


Review Questions
1. The junction between one neuron and the next, or between a neuron and an effector is called:


A ) A synapse
B ) A dendrite
C ) A neuotransmitter
D ) A ventricle
E ) None of the above


2. A fast excitatory synapses follows this order:
A ) (1) neurotransmitter released (2) diffused across the synaptic cleft to a receptor protein (3)
binding of the transmitter opens pores in the ion channels and positive ions move in.
B ) (1) neurotransmitter released (2) diffused across the synaptic cleft to a receptor protein (3)
binding of the transmitter opens pores in the ion channels and negative ions move in.
C ) (1) neurotransmitter released (2) diffused across the synaptic cleft to a receptor amino acid (3)
binding of the transmitter opens pores in the ion channels and positive ions move in.
D ) (1) diffused across the synaptic cleft to a receptor protein (2) neurotransmitter released (3)
binding of the transmitter opens pores in the ion channels and positive ions move in.
E ) None of the above


3. Resting potential is
A ) excess positive ions accumulate inside the plasma membrane
B ) excess negative ions accumulate inside the plasma membrane
C ) excess positive ions accumulate outside the plasma membrane
D ) excess positive ions accumulate outside the plasma membrane
E ) both b & d
F ) both a & b


4. Sensory neurons have:
A ) A short dendrite and a long axon
B ) A short dendrite and a short axon
C ) A long dendrite and a short axon
D ) A long dendrite and a long axon
E ) Their axons and dendrites may be either long or short


5. ________blocks Acetylcholine receptor sites causing muscle relaxation.
A ) Novocain
B ) curare
C ) Nicotine
D ) Nerve gases


6. Transmission across a synapse is dependent on the release of _______?
A ) neurotransmitters


78 | Human Physiology




The Nervous System


B ) synaptic vesicle
C ) neurons
D ) receptor proteins


7. Motor neurons take messages
A ) from the muscle fiber to the central nervous system
B ) away from the central nervous system to the central nervous system
C ) that are classified
D ) away from the central nervous system to muscle fiber


8. The medulla oblongata helps to regulate which of the following:
A ) Breathing
B ) Heartbeat
C ) Sneezing
D ) Vomiting
E ) All of the above


Glossary
Afferent Messages: carry sensations such as heat, cold, or pain
Autonomic System: deals with the visceral organs, like the heart, stomach, gland, and the


intestines
Axon: the part of the neuron that conducts nerve impulses
Cannabis: drug produced from parts of the cannabis plant, causes paralyzation
Central Nervous System: the system that includes the brain and the spinal cord
Cerebellum: part of the brain that is located posterior to the medulla oblongata and pons,


coordinates skeletal muscles to produce smooth, graceful motions
Cerebrospinal Fluid: acts a shock absorber for the central nervous system, protecting the brain


and spinal cord from injury; it also has a high glucose content which serves as a nutritional factor
Dendrites: short pieces that come off of the cell body that receive the signals from sensory


receptors and other neurons
Episodic Memory: represents our memory of events and experiences in a serial form
Excitatory Neurotransmitter: a neurotransmitter that acts to elicit an action potential by opening


chloride ion channels
Longitudinal Sulcus: separates the cerebrum in to the right and left hemispheres


Wikibooks | 79




Chapter 4
Long Term Memory: used for storage of information over a long time
Myelin: a fatty substance that surrounds and insulates the nerve fibers and facilitates the


conduction of the nerve impulse transmissions
Multiple Sclerosis: disease that affects the CNS by causing hardening and scaring of the myelin
Nodes of Ranvier: unmyelinated gaps between sections of myelin
Peripheral Nervous System: a way of communication from the central nervous system to the rest


of the body by nerve impulses that regulate the functions of the human body
Postganglionic Cells: have their cell bodies in the ganglia and send their axons to target organs or


glands
Sensory Receptor: structure that can find any kind of change in it's surroundings or environment
Somatic Nervous System: the part of the peripheral nervous system associated with the voluntary


control of body movements through the action of skeletal muscles, and also reception of external
stimuli


Synapses: the gap between two neurons; new synapses lead to learning


References
http://action.painfoundation.org/site/News2?page=NewsArticle&id=5135&security=1&news_iv_c


trl=1061 Esther Wednesday, October 19, 2005
http://www.neurologychannel.com/multiplesclerosis/
http://www.theraj.com/ms/casestudy.html


live version • discussion • edit lesson • comment • report an error


80 | Human Physiology




Senses


5 SENSES
live version • discussion • edit lesson • comment • report an error


enses are the physiological methods of perception. The senses and their operation, classification,
and theory are overlapping topics studied by a variety of fields. Sense is a faculty by which outside


stimuli are perceived.
S


We experience reality through our senses. A sense is a faculty by which outside stimuli are
perceived. Many neurologists disagree about how many senses there actually are due to a broad
interpretation of the definition of a sense. Our senses are split into two different groups. Our
Exteroceptors detect stimulation from the outsides of our body. For example smell,taste,and
equilibrium. The Interoceptors receive stimulation from the inside of our bodies. For instance, blood
pressure dropping, changes in the gluclose and Ph levels. Children are generally taught that there are
five senses (sight, hearing, touch, smell, taste). However, it is generally agreed that there are at least
seven different senses in humans, and a minimum of two more observed in other organisms. Sense can
also differ from one person to the next. Take taste for an example, what may taste great to me will taste
awful to someone else. This all has to do with how our brains interpret the stimuli that is given.


Chemoreception
The senses of Gustation (taste) and Olfaction (smell) fall under the category of Chemoreception.


Specialized cells act as receptors for certain chemical compounds. As these compounds react with the
receptors, an impulse is sent to the brain and is registered as a certain taste or smell. Gustation and
Olfaction are chemical senses because the receptors they contain are sensitive to the molecules in the
food we eat, along with the air we breath.


Gustatory System
In humans, the sense of taste is transduced by taste buds and is conveyed via three of the twelve


cranial nerves. Cranial nerve VII, the facial nerve, carries taste sensations from the anterior two thirds
of the tongue (excluding the circumvallate papillae, see lingual papilla) and soft palate. Cranial nerve
IX the glossopharyngeal nerve carries taste sensations from the posterior one third of the tongue
(including the circumvallate papillae). Also a branch of the vagus nerve carries some taste sensations
from the back of the oral cavity (i.e. pharynx and epiglottis). Information from these cranial nerves is
processed by the gustatory system. Though there are small differences in sensation, which can be
measured with highly specific instruments, all taste buds can respond to all types of taste. Sensitivity to
all tastes is distributed across the whole tongue and indeed to other regions of the mouth where there
are taste buds (epiglottis, soft palate).


Papilla
Papilla are specialized epithelial cells. There are four types of papillae: filiform (thread-shape),


fungiform (mushroom-shape), foliate (leaf-shape), and circumvallate (ringed-circle). All papillae
except the filiform have taste buds on their surface. Some act directly by ion channels, others act
indirectly.


Wikibooks | 81




Chapter 5
• Fungiform papillae - as the name suggests, are slightly mushroom shaped if looked at in


section. These are present mostly at the apex (tip) of the tongue.
• Filiform papillae - these are thin, longer papillae that don't contain taste buds but are the


most numerous. These papillae are mechanical and not involved in gustation.
• Foliate papillae - these are ridges and grooves towards the posterior part of the tongue.
• Circumvallate papillae - there are only about 3-14 of these papillae on most people and


they are present at the back of the oral part of the tongue. They are arranged in a circular-shaped
row just in front of the sulcus terminalis of the tongue.


Structure of Taste Buds


Each taste bud is flask-like in shape, its broad base resting on the corium, and its neck opening by
an orifice, the gustatory pore, between the cells of the epithelium.


The bud is formed by two kinds of cells: supporting cells and gustatory cells.


82 | Human Physiology


The mouth cavity. The cheeks have been slit transversely and the tongue pulled
forward.




Senses


The supporting cells are mostly arranged like the
staves of a cask, and form an outer envelope for the
bud. Some, however, are found in the interior of the
bud between the gustatory cells. The gustatory cells
occupy the central portion of the bud; they are spindle-
shaped, and each possesses a large spherical nucleus
near the middle of the cell. The peripheral end of the
cell terminates at the gustatory pore in a fine hair-like
filament, the gustatory hair.


The central process passes toward the deep
extremity of the bud, and there ends in single or
bifurcated varicosities.


The nerve fibrils after losing their medullary
sheaths enter the taste bud, and end in fine extremities
between the gustatory cells; other nerve fibrils ramify
between the supporting cells and terminate in fine
extremities; these, however, are believed to be nerves
of ordinary sensation and not gustatory.


Types of Taste
Salt


Arguably the simplest receptor found in the mouth is the salt (NaCl) receptor. An ion channel in
the taste cell wall allows Na+ ions to enter the cell. This on its own depolarizes the cell, and
opens voltage-regulated Ca2+ gates, flooding the cell with ions and leading to neurotransmitter
release. This sodium channel is known as EnAC and is composed of three subunits. EnAC can be
blocked by the drug amiloride in many mammals, especially rats. The sensitivity of the salt taste
to amiloride in humans, however, is much less pronounced, leading to conjecture that there may
be additional receptor proteins besides EnAC that may not have been discovered yet.


Sour
Sour taste signals the presence of acidic compounds (H+ ions in solution). There are three
different receptor proteins at work in sour taste. The first is a simple ion channel which allows
hydrogen ions to flow directly into the cell. The protein for this is EnAC, the same protein
involved in the distinction of salt taste (this implies a relationship between salt and sour receptors
and could explain why salty taste is reduced when a sour taste is present). There are also H+
gated channels present. The first is a K+ channel, which ordinarily allows K+ ions to escape from
the cell. H+ ions block these, trapping the potassium ions inside the cell (this receptor is
classified as MDEG1 of the EnAC/Deg Family). A third protein opens to Na+ ions when a
hydrogen ion attaches to it, allowing the sodium ions to flow down the concentration gradient
into the cell. The influx of ions leads to the opening of a voltage regulated Ca2+ gate. These
receptors work together and lead to depolarization of the cell and neurotransmitter release.


Bitter
There are many classes of bitter compounds which can be chemically very different. It is
interesting that the human body has evolved a very sophisticated sense for bitter substances: we


Wikibooks | 83


Semidiagrammatic view of a portion of the mucous
membrane of the tongue. Two fungiform papillæ are
shown. On some of the filiform papillæ the epithelial
prolongations stand erect, in one they are spread out,
and in three they are folded in.




Chapter 5
can distinguish between the many radically different compounds which produce a generally
“bitter” response. This may be because the sense of bitter taste is so important to survival, as
ingesting a bitter compound may lead to injury or death. Bitter compounds act through structures
in the taste cell walls called G-protein coupled receptors (GPCR’s). Recently, a new group of
GPCR’s was discovered, known as the T2R’s, which is thought to only respond to bitter stimuli.
When the bitter compound activates the GPCR, it in turn releases gustducin, the G-protein it was
coupled to. Gustducin is made of three subunits. When it is activated by the GPCR, its subunits
break apart and activate phosphodiesterase, a nearby enzyme. It then converts a precursor within
the cell into a secondary messenger, which closes potassium ion channels. This secondary
messenger can stimulate the endoplasmic reticulum to release Ca2+, which contributes to
depolarization. This leads to a build-up of potassium ions in the cell, depolarization, and
neurotransmitter release. It is also possible for some bitter tastants to interact directly with the G-
protein, because of a structural similarity to the relevant GPCR.


Sweet
Like bitter tastes, sweet taste transduction involves GPCR’s. The specific mechanism depends on
the specific molecule. “Natural” sweeteners such as saccharides activate the GPCR, which
releases gustducin. The gustducin then activates the molecule adenylate cyclase, which is already
inside the cell. This molecule increases concentration of the molecule cAMP, or adenosine 3',
5'-cyclic monophosphate. This protein will either directly or indirectly close potassium ion
channels, leading to depolarization and neurotransmitter release. Synthetic sweeteners such as
saccharin activate different GPCR’s, initiating a similar process of protein transitions, starting
with the protein phospholipase A, which ultimately leads to the blocking of potassium ion
channels.


Umami
Umami is a Japanese word meaning "savory" or "meaty". It is thought that umami receptors act
much the same way as bitter and sweet receptors (they involve GPCR’s), but not much is known
about their specific function. We do know that umami detects glutamates that are common in
meats, cheese and other protein-heavy foods. Umami receptors react to foods treated with
monosodium glutamate (MSG). This explains why eating foods that have MSG in them often
give a sense of fullness. It is thought that the amino acid L-glutamate bonds to a type of GPCR
known as a metabotropic glutamate receptor (mGluR4). This causes the G-protein complex to
activate a secondary receptor, which ultimately leads to neurotransmitter release. The
intermediate steps are not known.


Disorders of the Tongue
Loss of taste
You may lose your sense of taste if the facial nerve is damaged. Then there is also Sjogren's
Syndrome where the saliva production is reduced. In most cases the loss of taste is typically a
symptom of anosmia - a loss of the sense of smell.
Sore tongue
It is usually caused by some form of trauma, such as biting your tongue, or eating piping-hot or
highly acidic food or drink.
If your top and bottom teeth don’t fit neatly together, tongue trauma is more likely.
Some people may experience a sore tongue from grinding their teeth (bruxism).
Disorders such as diabetes, anemia, some types of vitamin deficiency and certain skin diseases


84 | Human Physiology




Senses


can include a sore tongue among the range of symptoms.
Glossodynia
A condition characterized by a burning sensation on the tongue.
Benign migratory glossitis
This condition is characterized by irregular and inflamed patches on the tongue surface that often
have white borders. The tongue may be generally swollen, red and sore. Another name for this
condition is geographic tongue. The cause of benign migratory glossitis is unknown.
Risk factors are thought to include:


• Mineral or vitamin deficiencies
• Local irritants, such as strong mouthwashes, cigarettes or alcohol
• Certain forms of anemia
• Infection
• Certain medications
• Stress


Olfactory System
Olfaction is the sense of smell. In humans the sence of Smell is received in nasopharynx.


Airborne molecules go into solution on moist epithelial surface of nasal passage. An olfactory receptors
neuron sends an impulse via Cranial nerve I the olfactory nerve. Although 80-90% of what we think is
"taste" actually is due to smell. This is why when we have a head cold or stuffed up nose we have a
harder time tasting our foods.


Receptors
Humans have 347 functional odor receptor genes; the other genes have nonsense mutations. This


number was determined by analyzing the genome in the Human Genome Project; the number may vary
among ethnic groups, and does vary among individuals. For example, not all people can smell
androstenone, a component of male sweat.


Each olfactory receptor neuron in the nose expresses only one functional odor receptor. Odor
receptor nerve cells may function like a key-lock system: if the odor molecules can fit into the lock the
nerve cell will respond. According to shape theory, each receptor detects a feature of the odor
molecule. Weak-shape theory, known as odotope theory, suggests that different receptors detect only
small pieces of molecules, and these minimal inputs are combined to create a larger olfactory
perception (similar to the way visual perception is built up of smaller, information-poor sensations,
combined and refined to create a detailed overall perception). An alternative theory, the vibration
theory proposed by Luca Turin (1996, 2002), posits that odor receptors detect the frequencies of
vibrations of odor molecules in the infrared range by electron tunneling. However, the behavioral
predictions of this theory have been found lacking (Keller and Vosshall, 2004).


An olfactory receptor neuron, also called an olfactory sensory neuron, is the primary transduction
cell in the olfactory system. Humans have about 40 million olfactory receptor neurons. In vertebrates,
olfactory receptor neurons reside on the olfactory epithelium in the nasal cavity. These cells are bipolar
neurons with a dendrite facing the interior space of the nasal cavity and an axon that travels along the


Wikibooks | 85




Chapter 5
olfactory nerve to the olfactory bulb.


Many tiny hair-like cilia protrude from the olfactory receptor cell's dendrite and into the mucus
covering the surface of the olfactory epithelium. These cilia contain olfactory receptors, a type of G
protein-coupled receptor. Each olfactory receptor cell contains only one type of olfactory receptor, but
many separate olfactory receptor cells contain the same type of olfactory receptor. The axons of
olfactory receptor cells of the same type converge to form glomeruli in the olfactory bulb.


Olfactory receptors can bind to a variety of odor molecules. The activated olfactory receptor in
turn activates the intracellular G-protein GOLF, and adenylate cyclase and production of Cyclic AMP
opens ion channels in the cell membrane, resulting in an influx of sodium and calcium ions into the
cell. This influx of positive ions causes the neuron to depolarize, generating an action potential.


Individual olfactory receptor neurons are replaced approximately every 40 days by neural stem
cells residing in the olfactory epithelium. The regeneration of olfactory receptor cells, as one of the
only few instances of adult neurogenesis in the central nervous system, has raised considerable interest
in dissecting the pathways for neural development and differentiation in adult organisms.


In the brain
The axons from all the thousands of cells expressing the same odor receptor converge in the


olfactory bulb. Mitral cells in the olfactory bulb send the information about the individual features to
other parts of the olfactory system in the brain, which puts together the features into a representation of
the odor. Since most odor molecules have many individual features, the combination of features gives
the olfactory system a broad range of odors that it can detect.


Odor information is easily stored in long term memory and has strong connections to emotional
memory. This is possibly due to the olfactory system's close anatomical ties to the limbic system and
hippocampus, areas of the brain that have long been known to be involved in emotion and place
memory, respectively.


Pheromonal olfaction
Some pheromones are detected by the olfactory system, although in many vertebrates pheromones


are also detected by the vomeronasal organ, located in the vomer, between the nose and the mouth.
Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make
a face called flehmen to direct air to this organ. In humans, it is unknown whether or not pheromones
exist.


Olfaction and Gustation
Olfaction, taste and trigeminal receptors together contribute to flavor. It should be emphasized that


there are no more than 5 distinctive tastes: salty, sour, sweet, bitter, and umami. The 10,000 different
scents which humans usually recognize as 'tastes' are often lost or severely diminished with the loss of
olfaction. This is the reason why food has little flavor when your nose is blocked, as from a cold.


The key nutrition players in our taste is the olfactory function, 80-90% of what we consider taste is


86 | Human Physiology




Senses


dependent on our senses of smell. With aging our olfactory function declines. In the elderly careful
monitoring of appetite is necessary due to the alterations in the olfactory function. Nutritionist suggest
giving a dual approach of supplementation of the trace minerals zinc and iron to enhance the smell and
taste senses.


Disorders of Olfaction
Anosmia


Anosmia is the lack of olfaction, or a loss of the sense of smell. It can be either temporary or
permanent. A related term, hyposmia refers to a decrease in the ability to smell. Some people
may be anosmic for one particular odor. This is called "specific anosmia" and may be genetically
based. Anosmia can have a number of detrimental effects. Patients with anosmia may find food
less appetizing. Loss of smell can also be dangerous because it hinders the detection of gas leaks,
fire, body odor, and spoiled food. The common view of anosmia as trivial can make it more
difficult for a patient to receive the same types of medical aid as someone who has lost other
senses, such as hearing or sight. A temporary loss of smell can be caused by a stuffy nose or
infection. In contrast, a permanent loss of smell may be caused by death of olfactory receptor
neurons in the nose, or by brain injury in which there is damage to the olfactory nerve or damage
to brain areas that process smell. The lack of the sense of smell at birth, usually due to genetic
factors, is referred as congenital anosmia. Anosmia may be an early sign of degenerative brain
diseases such as Parkinson's disease and Alzheimer's disease. Another specific cause of
permanent loss could be from damage to olfactory receptor neurons due to use of nasal sprays. To
avoid loss of smell from nasal sprays, use them for only a short amount of time. Nasal sprays that
are used to treat allergy related congestion are the only nasal sprays that are safe to use for
extended periods of time.


Phantosmia
Phantosmia is the phenomenon of smelling odors that aren't really present. (AKA Phantom odors)
The most common odors are unpleasant smells such as rotting flesh, vomit, feces, smoke etc.
Phantosmia often results from damage to the nervous tissue in the olfactory system. The damage
can be caused by viral infection, trauma, surgery, and possibly exposure to toxins or drugs. It can
also be induced by epilepsy affecting the olfactory cortex. It is also thought the condition can
have psychiatric origins.


Dysosmia
When things smell differently than they should.


The Sense of Vision
Vision needs to have the work of both the eyes and the brain to process any information. The


majority of the stimuli is done in the eyes and then the information is sent to the brain by the way of
nerve impulses. At least one-third of the information of what the eye sees is processed in the cerebral
cortex of the brain.


Wikibooks | 87




Chapter 5


Anatomy of the Eye


The human eye is a elongated ball about 1-inch (2.5 cm) in diameter and is protected by a bony
socket in the skull. The eye has three layers or coats that make up the exterior wall of the eyeball,
which are the sclera, choroid, and retina.
Sclera


The outer layer of the eye is the sclera, which
is a tough white fibrous layer that maintains,
protects and supports the shape of the eye.
The front of the sclera is transparent and is
called the cornea. The cornea refracts light
rays and acts like the outer window of the
eye.


Choroid
The middle thin layer of the eye is the
choroid, also known as the choroidea or choroid coat, it is the vascular layer of the eye lying
between the retina and the sclera. The choroid provides oxygen and nourishment to the outer
layers of the retina. It also contains a nonreflective pigment that acts as a light shield and prevents
light from scattering. Light enters the front of the eye through a hole in the choroid coat called the
pupil. The iris contracts and dilates to compensate for the changes in light intensity. If the light is
bright the iris then contracts making the pupil smaller, and if the light is dim, the iris dilates
making the pupil bigger. Just posterior to the iris is the lens, which is composed mainly of
proteins called crystallins. The lens is attached by the zonules to the ciliary body that contains the
ciliary muscles that control the shape of the lens for accommodation. Along with the ciliary body
and iris, the choroid forms the uveal tract. The uvea is the middle of the three concentric layers
that make up an eye. The name is possibly a reference to its almost black color, wrinkled
appearance and grape-like size and shape when stripped intact from a cadaveric eye.


88 | Human Physiology


Cross-section of the human eye.




Senses


Retina
The third or the innermost layer of the eye is call the retina. In adult humans the entire retina is
72% of a sphere about 22 mm in diameter. The retina lays over the back two thirds of the choroid
coat, which is located in the posterior compartment. The compartment is filled with vitreous
humor which is a clear, gelatinous material. Within the retina there are cells called rod cells and
cone cells also known as photoreceptors. The rod cells are very sensitive to light and do not see
color, that is why when we are in a darkened room we see only shades of gray. The cone cells are
sensitive to different wavelengths of light, and that is how we are able to tell different colors. It is
a lack of cones sensitive to red, blue, or green light that causes individuals to have deficiencies in
color vision or various kinds of color blindness. At the center of the retina is the optic disc,
sometimes known as "the blind spot" because it lacks photoreceptors. It is where the optic nerve
leaves the eye and takes the nerve impulses to the brain. The cornea and the lens of the eye
focuses the light onto a small area of the retina called the fovea centralis where the cone cells are
densely packed. The fovea is a pit that has the highest visual acuity and is responsible for our
sharp central vision - there are no rods in the fovea.


Photoreceptors
A photoreceptor, or photoreceptor cell, is a specialized type of neuron found in the eye's retina
that is capable of phototransduction. More specifically, the photoreceptor sends signals to other
neurons by a change in its membrane potential when it absorbs photons. Eventually, this
information will be used by the visual system to form a complete representation of the visual
world. There are 2 types of photoreceptors: rods are responsible for scotopic, or night vision,
whereas cones are responsible for photopic, or daytime vision as well as color perception.


Extraocular muscles
Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the
inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the
muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an
almost pure rotation, with only about one millimeter of translation, thus, the eye can be
considered as undergoing rotations about a single point in the center of the eye. Five of the


Wikibooks | 89


Retina's simplified axial organization. The retina is a stack of several neuronal layers. Light is concentrated from
the eye and passes across these layers (from left to right) to hit the photoreceptors (right layer). This elicits
chemical transformation mediating a propagation of signal to the bipolar and horizontal cells (middle yellow
layer). The signal is then propagated to the amacrine and ganglion cells. These neurons ultimately may produce
action potentials on their axons. This spatiotemporal pattern of spikes determines the raw input from the eyes to
the brain.




Chapter 5
extraocular muscles have their origin in the back of the orbit in a fibrous ring called the annulus
of Zinn. Four of these then course forward through the orbit and insert onto the globe on its
anterior half (i.e., in front of the eye's equator). These muscles are named after their straight
paths, and are called the four rectus muscles, or four recti. They insert on the globe at 12, 3, 6,
and 9 o'clock, and are called the superior, lateral, inferior and medial rectus muscles. (Note that
lateral and medial are relative to the subject, with lateral toward the side and medial toward the
midline, thus the medial rectus is the muscle closest to the nose).


Eye Movement
The visual system in the brain is too slow to process that information if the images are slipping


across the retina at more than a few degrees per second, thus, for humans to be able to see while
moving, the brain must compensate for the motion of the head by turning the eyes. To get a clear view
of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea.
Eye movements are thus very important for visual perception, and any failure to make them correctly
can lead to serious visual disabilities. Having two eyes is an added complication, because the brain
must point both of them accurately enough that the object of regard falls on corresponding points of the
two retinas; otherwise, double vision would occur. The movements of different body parts are
controlled by striated muscles acting around joints. The movements of the eye are no exception, but
they have special advantages not shared by skeletal muscles and joints, and so are considerably
different.
Try This Experiment


Hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake
your hand from side to side, slowly at first, and then faster and faster. At first you will be able to
see your fingers quite clearly. But as the frequency of shaking passes about one hertz, the fingers
will become a blur. Now, keep your hand still, and shake your head (up and down or left and
right). No matter how fast you shake your head, the image of your fingers remains clear. This
demonstrates that the brain can move the eyes opposite to head motion much better than it can
follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving
hand, images slip on the retina and you see a blurred hand.


How we see an object
• The light rays enter the eye through the cornea (transparent front portion of eye to focus the


light rays)
• Then, light rays move through the pupil, which is surrounded by Iris to keep out extra light
• Then, light rays move through the crystalline lens (Clear lens to further focus the light


rays )
• Then, light rays move through the vitreous humor (clear jelly like substance)
• Then, light rays fall on the retina, which processes and converts incident light to neuron


signals using special pigments in rod and cone cells.
• These neuron signals are transmitted through the optic nerve,
• Then, the neuron signals move through the visual pathway - Optic nerve > Optic Chiasm >


Optic Tract > Optic Radiations > Cortex
• Then, the neuron signals reach the occipital (visual) cortex and its radiations for the brain's


processing.


90 | Human Physiology




Senses


• The visual cortex interprets the signals as images and along with other parts of the brain,
interpret the images to extract form, meaning, memory and context of the images.


Depth Perception
Depth perception is the visual ability to perceive the world in three dimensions. It is a trait


common to many higher animals. Depth perception allows the beholder to accurately gauge the
distance to an object.


Depth perception is often confused with binocular vision, also known as Stereopsis. Depth
perception does rely on binocular vision, but it also uses many other monocular cues.


Diseases, disorders, and age-related changes
There are many diseases, disorders, and age-related changes that may affect the eyes and


surrounding structures. As the eye ages certain changes occur that can be attributed solely to the aging
process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the
quality of vision worsens due to reasons independent of aging eye diseases. While there are many
changes of significance in the non-diseased eye, the most functionally important changes seem to be a
reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). The area of
the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates
also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the
retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses
in bright light and extremely dark glasses in dim light. Therefore, for any detailed visually guided tasks
on which performance varies with illumination, older persons require extra lighting.
Color Blindness


Color Blindness or color vision deficiency, in humans is the
inability to perceive differences between some or all colors that
other people can distinguish. It is most often of genetic nature,
but may also occur because of eye, nerve, or brain damage, or
due to exposure to certain chemicals. There are many types of
color blindness. The most common variety are hereditary
(genetic) photoreceptor disorders, but it is also possible to
acquire color blindness through damage to the retina, optic
nerve, or higher brain areas. There is generally no treatment to
cure color deficiencies, however, certain types of tinted filters
and contact lenses may help an individual to distinguish different
colors better.


Night Blindness
Also known as Nyctalopia, is a condition making it difficult or
impossible to see in the dark. It is a symptom of several eye
diseases. Night blindness may exist from birth, or be caused by
injury or malnutrition (for example, a lack of vitamin A). The most common cause of nyctalopia
is retinitis pigmentosa, a disorder in which the rod cells in the retina gradually lose their ability to
respond to the light. Patients suffering from this genetic condition have progressive nyctalopia


Wikibooks | 91


This image contains a two digit
number similar to the sample above.
Someone who is protanopic might
not see this number. (GFDL - Zxc,
Cyp)




Chapter 5
and eventually their day-time vision may also be affected. In congenital stationary night
blindness the rods do not work from birth, but as the name implies, sufferers do not get worse.
Another cause of night blindness is a deficiency of retinol, or vitamin A, found in fish oils, liver
and dairy products.


Day Blindness
Also known as Hemeralopia is the inability to see clearly in bright light. The daytime vision gets
worse and worse. Nighttime vision remains unchanged due to the use of rods as opposed to cones
(during the day), which get affected by hemeralopia and in turn degrade the daytime optical
response.


Floater
Also known as "Muscae Volitantes" are deposits of
various size, shape, consistency, refractive index, and
motility within the eye's normally transparent vitreous
humour. Floaters are suspended in the vitreous humour,
the thick fluid or gel that fills the eye. Thus, they
generally follow the rapid motions of the eye, while
drifting slowly within the fluid. Floaters are visible only
because they do not remain perfectly fixed within the
eye. The shapes are shadows projected onto the retina by
tiny structures of protein or other cell debris discarded
over the years and trapped in the vitreous humour. They
are also common after cataract operations or after
trauma. In some cases, floaters are congenital.


Glaucoma
A group of diseases of the optic nerve involving loss of
retinal ganglion cells in a characteristic pattern of optic
neuropathy. Although raised intraocular pressure is a
significant risk factor for developing glaucoma, there is no set
threshold for intraocular pressure that causes glaucoma. One
person may develop nerve damage at a relatively low
pressure, while another person may have high eye pressures
for years and yet never develop damage. Untreated glaucoma
leads to permanent damage of the optic nerve and resultant
visual field loss, which can progress to blindness.


Visual Agnosia
Visual agnosia is the inability of the brain to make sense of or
make use of some part of otherwise normal visual stimulus,
and is typified by the inability to recognize familiar objects or faces. This is distinct from
blindness, which is a lack of sensory input to the brain due to damage to the eye or optic nerve.
Visual agnosia is often due to damage, such as stroke, in posterior parietal lobe in the right
hemisphere of the brain. Careful analysis of the nature of visual agnosia has led to improved
understanding of the brain's role in normal vision.


Deadly Nightshade
Deadly Nightshade is a plant oil that can potentially kill you. Atrophine taken from this plant


92 | Human Physiology


Impression of floaters, as seen against a blue
sky. (GFDL - Acdx)


Picture of children holding a ball as
seen by someone with glaucoma.




Senses


causes your eyes to dilate. This was used in the middle ages by women who wanted to look more
attractive for men. To this day, it is still used by opthamologists. How this works is that the
atrophine is a competitor with acetylcholine. The Nightshadow goes into your receptors on the
postsynaptic membrane of an action potential. This makes it so that the acetylcholine doesn’t
have any receptor site so the Na ion is not able to be released.


Critical Thinking
1. Explain why you are normally unaware of your blind spot.
2. Stare at a bright light for 10 seconds and then stare at a white sheet of paper. What do you


observe and why?
3. What is it that makes things "dissapear" when you are staring at them at night, and how do


you make them reappear?
4. Name what rods are sensitive to and also what cones are sensitive to.
5. Explain how Deadly Nightshade works.


The Senses Of Hearing
The ear is the sense organ that detects sound and plays a major role in the sense of balance and


body position. It is also a device used for collecting and funneling sound waves recieved by the ear.
The sensory receptors for both hearing and equilibrium are found in the inner ear, consisting of hair
cells that have stereocilia (long mircovilli). The stereocillia are extremely sensitive to mechanical
stimulations, which are known as mechanoreceptors.


Wikibooks | 93




Chapter 5


Anatomy of the Ear
The ear has three divisions: the outer ear, middle ear, and the inner ear.


Outer Ear (Auricle, Ear Canal, Surface of Ear Drum)
The outer ear is the most external portion of the ear. The outer ear includes the pinna (also called
auricle), the ear canal, and the very most superficial layer of the ear drum (also called the
tympanic membrane). Although the word "ear" may properly refer to the pinna (the flesh covered
cartilage appendage on either side of the head), this portion of the ear is not vital for hearing. The
complicated design of the human outer ear does help capture sound, but the most important
functional aspect of the human outer ear is the ear canal itself. This outer ear canal skin is applied
to cartilage; the thinner skin of the deep canal lies on the bone of the skull. If the ear canal is not
open, hearing will be dampened. Ear wax (medical name - cerumen) is produced by glands in the
skin of the outer portion of the ear canal. Only the thicker cerumen-producing ear canal skin has
hairs. The outer ear ends at the most superficial layer of the tympanic membrane. The tympanic
membrane is commonly called the ear drum.


Middle Ear (Air Filled Cavity behind the Ear Drum, includes most of the Ear Drum, and Ear Bones)
The middle ear includes most of the ear drum (tympanic membrane) and the 3 ear bones ossicles:
malleus (or hammer), incus (or anvil), and stapes (or stirrup). The opening of the Eustachian tube
is also within the middle ear. The malleus has a long process (the handle) that is attached to the
mobile portion of the ear drum. The incus is the bridge between the malleus and stapes. The
stapes is the smallest named bone in the human body. The stapes transfers the vibrations of the
incus to the oval window, a portion of the inner ear to which it is connected. It is the final bone in


94 | Human Physiology




Senses


the chain to transfer vibrations from the eardrum to the inner ear. The arrangement of these 3
bones is a sort of Rube Goldberg device: movement of the tympanic membrane causes movement
of the first bone, which causes movement of the second, which causes movement of the third.
When this third bone pushes down, it causes movement of fluid within the cochlea (a portion of
the inner ear). This particular fluid only moves when the stapes footplate is depressed into the
inner ear. Unlike the open ear canal, however, the air of the middle ear is not in direct contact
with the atmosphere outside the body. The Eustachian tube connects from the chamber of the
middle ear to the back of the pharynx. The middle ear in humans is very much like a specialized
paranasal sinus, called the tympanic cavity, it, like the paranasal sinuses, is a hollow mucosa
lined cavity in the skull that is ventilated through the nose. The mastoid portion of the temporal
bone, which can be felt as a bump in the skull behind the pinna, also contains air, which
ventilates through the middle ear.


Inner Ear (Cochlea, Vestibule, and Semi-Circular Canals)
The inner ear includes both the organ of hearing (the cochlea) and a sense organ that is attuned to
the effects of both gravity and motion labyrinth or vestibular apparatus. The balance portion of
the inner ear consists of three semi-circular canals and the vestibule. The inner ear is encased in
the hardest bone of the body. Within this ivory hard bone, there are fluid-filled hollows. Within
the cochlea are three fluid filled spaces: the tympanic canal, the vestibular canal, and the middle
canal. The eighth cranial nerve comes from the brain stem to enter the inner ear. When sound
strikes the ear drum, the movement is transferred to the footplate of the stapes, which presses into
one of the fluid-filled ducts of the cochlea. The hair cells in the organ of Corti are tuned to certain
sound frequencies, being responsive to high frequencies near the oval window and to low
frequencies near the apex of the cochlea.
The fluid inside this duct is moved, flowing against the receptor cells of the organ of Corti, which
fire. These stimulate the Spiral Ganglion, which sends information through the auditory portion
of the eighth cranial nerve to the brain.


Hair Cell
Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at the top, for which
they are named. These cilia are the mechanosensors for hearing. Lightly resting atop the longest
cilia is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the
cilia and allowing electric current into the hair cell. Hair cells, like the photoreceptors of the eye,
show a graded response, instead of the spikes typical of other neurons. One may ask how such a
wiggle of a hair bundle triggers a difference in membrane potential. Special hair cells are the
actual sensory receptors which will fire off action potentials when they are disturbed.
Immediately over the hair cells of the organ of Corti is an overhanging “tectorial membrane.”
When the Bones of the Middle Ear vibrate the oval window, these vibrations are transmitted to
the fluid within the cochlea and eventually cause the round window on the cochlea to bulge
outward. These vibrations disturb the membrane on which the Organ of Corti is located, causing
the hair cells to “rub” against the overhanging tectorial membrane. The disturbed hair cells will
then fire action potentials. The current model is that cilia are attached to one another by “tip
links”, structures which link the tips of one cilium to another. Stretching and compressing the tip
links may open an ion channel and produce the receptor potential in the hair cell. These graded
potentials are not bound by the “all or none” properties of an action potential. There are far fewer
hair cells than afferent nerve fibers in the cochlea. The nerve that innervates the cochlea is the
vestibulocochlear nerve, or cranial nerve number VIII. Neuronal dendrites innervate cochlear hair
cells. The neurotransmitter itself is thought to be glutamate. At the presynaptic juncture, there is a
distinct “presynaptic dense body” or ribbon. This dense body is surrounded by synaptic vesicles


Wikibooks | 95




Chapter 5
and is thought to aid in the fast release of neurotransmitter. Efferent projections from the brain to
the cochlea also play a role in the perception of sound. Efferent synapses occur on outer hair cells
and on afferent dendrites under inner hair cells.


Process of Hearing
Detection of sound motion is associated with the right posterior superior temporal gyrus. The


superior temporal gyrus contains several important structures of the brain, including: (1)marking the
location of the primary auditory cortex, the cortical region responsible for the sensation of sound.
Sections 41 and 42 are called the primary auditory area of the cerebrum, and processes the basic
characteristics of sound such as pitch and rhythm. The auditory association area is located within the
temporal lobe of the brain, in an area called the Wernicke's area, or area 22. This area, near the lateral
cerebral sulcus, is an important region for the processing of acoustic energy so that it can be
distinguished as speech, music, or noise. It also interprets words that are heard into an associated
thought pattern of understanding. The gnostic area of the cerebrum, (areas 5, 7, 39 and 40) helps to
integrate all incoming sense patterns so that a common thought can be formed (correlated) using all
arriving sensory information.


Hearing Under Water
Hearing threshold and the ability to localize sound sources are reduced underwater. in which the


speed of sound is faster than in air. Underwater, hearing is by bone conduction and localization of
sound appears to depend on differences in amplitude detected by bone conduction.


Localization of Sound by Humans
Humans are normally able to hear a variety of sound frequencies, from about 20Hz to 20kHz. Our


ability to estimate just where the sound is coming from, sound localization, is dependent on both
hearing ability of each of the two ears, and the exact quality of the sound. Since each ear lies on an
opposite side of the head, a sound will reach the closest ear first, and its amplitide will be loudest in
that ear. Much of the brain's ability to localize sound depends on interaural (between ears) intensity
differences and interaural temporal or phase differences.


Two mechanisms are known to be used.
Bushy neurons can resolve time differences as small as the time it takes sound to pass one ear and


reach the other (10 milliseconds). For high frequencies, frequencies with a wavelength shorter than the
listener's head, more sound reaches the nearer ear. Human echolocation is a technique involving
echolocation used by some blind humans to navigate within their environment.


Process of Equilibrium
Equilibrioception or sense of balance is one of the physiological senses. It allows humans and


animals to walk without falling. Some animals are better in this than humans, for example allowing a
cat (as a quadruped using its inner ear and tail) to walk on a thin fence. All forms of equilibrioception


96 | Human Physiology




Senses


can be described as the detection of acceleration.
It is determined by the level of fluid properly called endolymph in the labyrinth - a complex set of


tubing in the inner ear.
When the sense of balance is interrupted it causes dizziness, disorientation and nausea.
You can temporarily disturb your sense of balance by closing your eyes and turning rapidly in


circles five or six times. This starts the fluid swirling in circles inside your ear canal. When you stop
turning it takes a few seconds for the fluid to lose momentum, and until then the sense from your inner
ear conflicts with the information coming from your vision, causing dizziness and disorientation. Most
astronauts find that their sense of balance is impaired when in orbit, because there is not enough gravity
to keep the ear's fluid in balance. This causes a form of motion sickness called space sickness.


Disorders with the Ear


Case Study
A 45-year-old woman wakes up not feeling well. She believes that she may be coming down with


the flu due to nausea that she is feeling, so she continues with her day. As the day progresses so does
the feeling of nausea. While watching a movie with members of her family, the sick feeling seems to
intensify and so they leave the movie. In the lobby of the movie theater she becomes very unbalanced
and collapses. The fear is that she is experiencing a stroke. After being taken to the hospital via
ambulance, the ER doctors also feel that it may be a stroke and do CAT scans to verify. Nothing shows
up on the scans but the feeling of nausea and vertigo are intense. The woman is later diagnosed with an
inner ear infection. The next 6-9 months of her life are filled with antibiotics, balance therapy and
continued nausea and vertigo. Nothing seems to help so the doctors go into her inner ear surgically
through her skull. They cut the vestibular nerve that is linked to the balance center on the left side. The
right inner ear will eventually compensate for this loss of balance however it will take months of
balance therapy. After a year from the onset on the inner ear infection, the woman has had three inner
ear surgeries, loss of hearing in the left ear and problems with her balance. Doctors have told her they
have done everything that they can and that she will now have to live with these conditions on a daily
basis.
Deafness


The word deaf can have at least two different meanings. The first term is used to indicate the
presence of enough hearing loss such that an individual is not sensitive to sound. Someone with a
partial loss of hearing is more likely to be referred to as hearing impaired or the qualified partially
deaf. The second term is used to indicate someone who was born without the sense of hearing or
total deafness.


Otitis Media
An inflammation of the middle ear segment. It is usually associated with a buildup of fluid and
frequently causes an earache. The fluid may or may not be infected. The typical progress of otitis
media is: the tissues surrounding the Eustachian tube swell due to an infection and/or severe
congestion. The Eustachian tube remains blocked most of the time. The air present in the middle
ear is slowly absorbed into the surrounding tissues. A strong negative pressure creates a vacuum


Wikibooks | 97




Chapter 5
in the middle ear. The vacuum reaches a point where fluid from the surrounding tissues
accumulates in the middle ear. Streptococcus pneumoniae and Haemophilus influenzae are the
most common bacterial causes of otitis media. As well as being caused by Streptococcus
pneumoniae and Haemophilus influenzae it can also be caused by the common cold.


Vertigo (dizziness)
Vertigo, sometimes called a headrush, is a major symptom of a balance disorder. It is the
sensation of spinning while the body is stationary with respect to the earth or surroundings. With
the eyes shut, there will be a sensation that the body is in movement, called subjective vertigo; if
the eyes are open, the surroundings will appear to move past the field of vision, called objective
vertigo. The effects may be slight. It may cause nausea or, if severe, may give rise to difficulty
with standing and walking. Vertigo is usually associated with a problem in the inner ear balance
mechanisms (vestibular system), in the brain, or with the nerve connections between these two
organs. The most common cause is benign paroxysmal positional vertigo, or BPPV. Vertigo can
be a symptom of an underlying harmless cause, such as in BPPV or it can suggest more serious
problems. These include drug toxicities, strokes or tumors (though these are much less common
than BPPV).


Motion sickness
Motion sickness is a condition in which the endolymph (the fluid found in the semicircular canals
of the inner ears) becomes 'stirred up', causing confusion between the difference between
apparent perceived movement (none or very little), and actual movement. Depending on the
cause, it is also referred to as seasickness, carsickness, airsickness, or spacesickness. Nausea is
the most common symptom of motion sickness. If the motion causing nausea is not resolved, the
sufferer will frequently vomit within twenty minutes. Unlike ordinary sickness, vomiting in
motion sickness tends not to relieve the nausea. If you don't want to consult a doctor, one
common form of relief is to eat mints.


Dysacusis
Dysacusis is a hearing impairment characterized by difficulty in processing details of sound, but
not primarily a loss of the ability to perceive sound. May also refer to pain or discomfort due to
sound.


Critical Thinking
1. Explain how the pitch of sound is coded. How is the loudness of sound coded?
2. What do the three semicircular canals in the inner ear enable us to do? How do they


accomplish this?
3. What does the eustachian tube do? What does the eustachian tube have to do with a middle


ear infection?
4. What is the advantage of having a oval window?


Touch
Touch is the first sense developed in the womb and the last sense used before death. With 50 touch


receptors for every square centimeter and about 5 million sensory cells overall the skin is very sensitive
and one of the largest and most complex organs in our bodies. These touch receptors are grouped by


98 | Human Physiology




Senses


type and include Mechanoreceptors (sensitive to pressure, vibration and slip), Thermoreceptors
(sensitive to changes in temperature), and Nocioreceptors (responsible for pain).


Pacinian Corpuscles
Pacinian corpuscles detect gross pressure changes and vibrations. They are the largest of the


receptors. Any deformation in the corpuscle causes action potentials to be generated, by opening
pressure-sensitive sodium ion channels in the axon membrane. This allows sodium ions to influx in,
creating a receptor potential. Pacinian corpuscles cause action potentials when the skin is rapidly
indented but not when the pressure is steady, due to the layers of connective tissue that cover the nerve
ending (Kandel et al., 2000). It is thought that they respond to high velocity changes in joint position.


Meissner's Corpuscle
Meissner's corpuscles are distributed throughout the skin, but concentrated in areas especially


sensitive to light touch, such as the fingertips, palms, soles, lips, tongue, face, nipples and the external
skin of the male and female genitals. They are primarily located just beneath the epidermis within the
dermal papillae. Any physical deformation in the Meissner’s corpuscle will cause an action potential in
the nerve. Since they are rapidly adapting or phasic, the action potentials generated quickly decrease
and eventually cease. If the stimulus is removed, the corpuscle regains its shape and while doing so (ie:
while physically reforming) causes another volley of action potentials to be generated. (This is the
reason one stops "feeling" one's clothes.) This process is called sensory adaption. Because of their
superficial location in the dermis, these corpuscles are particularly sensitive to touch and vibrations, but
for the same reasons, they are limited in their detection because they can only signal that something is
touching the skin. Meissner's corpuscles do not detect pain; this is signaled exclusively by free nerve
endings.


Merkel’s Discs
Merkel’s Discs are Mechanoreceptors, making them sensitive to pressure and vibration. In


humans, Merkel cells occur in the superficial skin layers, and are found clustered beneath the ridges of
the fingertips that make up fingerprints. They’re somewhat rigid in structure, and the fact that they are
not encapsulated, causes them to have a sustained response (in the form of action potentials or spikes)
to mechanical deflection of the tissue. Merkel nerve endings are extremely sensitive to tissue
displacement, and may respond to displacements of less than 1 _m. Several studies indicate that they
mediate high-resolution tactile discrimination, and are responsible for the ability of our fingertips to
feel fine detailed surface patterns (e.g. for reading Braille).


Wikibooks | 99




Chapter 5


Ruffini corpuscles
Ruffini corpuscles are Thermoreceptors, aiding in the detection of temperature changes. Named


after Angelo Ruffini, the Ruffini ending is a class of slowly adapting mechanoreceptor thought to exist
only in the glabrous dermis and subcutaneous tissue of humans. This spindle-shaped receptor is
sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and
movement.


Disorders of Touch
Sensory Processing Disorder


In most people sensory integration occurs naturally without a thought process. But in some
people the sensory integration does not develop properly and becomes distorted. In these people,
the brain and central nervous system misinterprets everyday sensory information such as touch,
sound and movement. Research is still being done on this disorder but they are finding direct
links to SPD with other disorders like ADD/ADHD, premature birth, Autism, Down’s Syndrome
and Fragile X.


Tactile defensiveness
Considered a category of SPD, tactile defensiveness is an overreaction to the sense of touch.
Identified by Dr. Jean Ayers in the 1960’s. A person with tactile defensiveness will react with a
“flight or fight” reaction to touch stimuli that a normal person would interpret as harmless. Most
cases are noticed in children or babies due to the fact that they do not want to be touched or
cuddled as a normal child would. A child with this disorder will probably have these sign or
symptoms:


• Does not like to go barefoot or have feet touched


100 | Human Physiology


Layers of the skin, showing the Merkel's Cell. (GFDL - Crystal Mason)




Senses


• Does not enjoy baths, haircuts, nail clipping
• Requires tags to be removed from all clothing
• Does not want their face touched
• Hard time eating because of textures, temperatures of the food
• Does not want to touch anything that is messy or has a sticky texture


Congenital insensitivity to pain with anhidrosis or CIPA
Exceedingly rare disease. There are only about 35 known cases in the United States. CIPA is a
severe autosomal recessive condition in which the peripheral nerves demonstrate a loss of
unmyelinated and small myelinated fibers. The actual physiopathological mechanism is still
unknown and being studied- this is an extremely hard disease to study due to the rarity of cases.
Most people with the disease will not live long due to injuries received that go untreated because
they are unknown and severe


Case Study


Insensitivity to pain
Wouldn't it wonderful if you could no longer feel pain. Is that not something we all would like to


have? Or do we have pain for a good reason? Although it is rare there is a disease known as congenital
insensitivity to pain. This genetic abnormality cause some people to lack certain components of the
sensory system to receive pain. The exact reason for the problem is unknown and varies between
people. Sadly people who have the disease die in childhood. Injuries are very common with people who
have congenital insensitivity to pain. They often will lose digits, may suffer from burns and their knees
often have sores from kneeling to long. Clearly pain has a purpose, it is our warning signal when things
are awry.


The newborn's senses
Newborns can feel all different sensations, but respond most enthusiastically to soft stroking,


cuddling and caressing. Gentle rocking back and forth will oftentimes calm a crying infant, as will
massages and warm baths. Newborns may comfort themselves by sucking their thumbs, or a pacifier.
The need to suckle is instinctive and allows newborns to feed.
Vision


Newborn infants have unremarkable vision, being able to focus on objects only about 18 inches
(45 cm) directly in front of their face. While this may not be much, it is all that is needed for the
infant to look at the mother’s face when breastfeeding. When a newborn is not sleeping, or
feeding, or crying, he or she may spend a lot of time staring at random objects. Usually anything
that is shiny, has sharp contrasting colors, or has complex patterns will catch an infant's eye.
However, the newborn has a preference for looking at other human faces above all else.


Hearing
While still inside the mother, the infant can hear many internal noises, such as the mother's
heartbeat, as well as many external noises including human voices, music and most other sounds.
Therefore, although a newborn's ears may have some fluid present, he or she can hear sound from


Wikibooks | 101




Chapter 5
birth. Newborns usually respond to a female's voice over a male's. This may explain why people
will unknowingly raise the pitch of their voice when talking to newborns. The sound of other
human voices, especially the mother's, can have a calming or soothing effect on the newborn.
Conversely, loud or sudden noises will startle and scare a newborn.


Taste
Newborns can respond to different tastes, including sweet, sour, bitter, and salty substances, with
preference toward sweets.


Smell
A newborn has a developed sense of smell at birth, and within the first week of life can already
distinguish the differences between the mother's own breast milk and the breast milk of another
female.


Reflex Stimulation Response Age of disappearance Function


Eye blink
Bright light
shinning in eyes or
clap hands by eyes.


Closes eyelids
quickly. Permanent


This reflex
protects the
infant from an
excessive
amount of
stimulation.


Withdrawal
Stick sole of foot
with a stimulus like
a pin.


This causes the foot to
withdraw. Flexing of
the knee to hip occurs.


Decreases after the
10th day of birth


This protects the
infant from
excessive
unpleasant
tactile
stimulation.


Rooting
Touch cheek near
the corner of the
mouth.


The infant's head will
turn towards the site
of stimulation.


3 weeks (due to the
voluntary response
that is now capable
for infant to do at this
time)


This reflex
helps baby to
find the
mothers' nipple.


Sucking Place fingers in infant's mouth.
The infant will suck
finger rhythmically.


4 months (voluntary
sucking will come
about)


This helps with
feeding.


Swimming
Place the baby in
pool of water face
down.


The baby paddles and
kicks in swimming
movements.


4 to 6 month
This helps baby
to survive if
dropped into the
water.


Moro Hold infant in a
cradling horizontal
position and slightly
lower the baby in a
fast motion toward


The baby will make a
embracing motion and
arch its back
extending it's legs and
throwing it's arms


6 months In the
evolutionary
past this may
have helped the
baby cling to the


102 | Human Physiology




Senses


the ground while
making a loud
sound.


outward. Finally it
will bring the arms in
toward its body


mother.


Palmar
grasp


Place the finger in
baby's palm and
press against the
palm.


The baby will
immediately grasp the
finger.


3 to 4 months
This prepares
infant for
voluntary
grasping.


Tonic neck
Turn the baby's
head to one side
while the baby is
awake.


This will cause the
baby to extend one
arm in front of its eye
or to the side to which
the head has been
turned.


4 months
This may
prepare for
voluntary
reaching.


Stepping
Hold the baby under
the arm and permit
the bare feet of the
baby to touch a flat
surface.


The baby will lift one
foot after the other in
a stepping fashion.


2 months (this applies
to a baby who has
gained weight. For
baby who is not as
heavy, this reflex may
be submissive.)


This prepares
the baby for
voluntary
walking.


Babinski
Touch the foot in a
stroking manner
form the toe toward
the heel.


The baby's toes will
fan out and curl as the
foot twists inward.


8 to 12 months Unknown


Review Questions
1. Located under the hardest bone in the body, these control not only hearing but also a sense of


gravity and motion:
A) The incus and the stapes
B) The pinna and the ear drum
C) the vestibular nerve and the semi circular canals
D) The eustachian tube and the stapes


2. The retina does the following;
A) allows vision in light and dark, using cones and rods
B) Gives depth perception using binocular vision
C) Contains the ciliary muscles that control the shape of the lens
D) Protects and supports the shape of the eye


3. This is the reason that we stop feeling the clothes that we are wearing
A) Merkel’s Discs are somewhat rigid in structure, and the fact that they are not encapsulated,
causes them to have a sustained response
B) Meissner’s corpuscle are rapidly adapting or phasic, the action potentials generated quickly


Wikibooks | 103




Chapter 5
decrease and eventually cease
C) Ruffini corpuscles is a class of slowly adapting mechanoreceptor
D) Pacinian corpuscles allow sodium ions to influx in, creating a receptor potential


4. When eating a piece of candy, I will use the following to sense that it is sweet
A) Fungiform papillae
B) Filiform papillae
C) Foliate papillae
D) Circumvallate papillae
E) All of the above


5. If I have a cold, food may not taste as good to me because
A) The nerve fibrils are not functioning properly
B) My food will taste the same; taste and smell have nothing in common
C) Papilla become blocked by mucus and are unable to function
D) Olfaction, taste and trigeminal receptors together contribute to the flavor of my food


6. Walking from a well lit room into a dark room would cause the following to occur
A) The sclera in the eye to open and eventually allow me to see in the dark
B) The extraocular muscles in the eye to open and eventually allow me to see in the dark
C) The cones in the eye to open and eventually allow me to see in the dark
D) the rods in the eye to open and eventually allow me to see in the dark


7. Hair cells in the ear
A) Are the actual sensory receptors that will fire off action potentials when they are disturbed
B) Show a graded response, instead of the spikes typical of other neurons
C) “Rub” against the overhanging tectorial membrane
D) All of the above


8. Eyesight decreases with age because
A) Older eyes receive much less light at the retina
B) There are numerous eye diseases that can affect an older eye
C) The extent to which the pupil dilates decreases with age
D) all of the above


9. Teens walking off of a roller coaster in Magic Mountain seem to have vertigo because
A) The fluid in the auricle has not stopped moving causing conflicts with the information coming
from your vision
B) the fluid in the cochlea has not stopped moving causing conflicts with the information coming
from your vision
C) The fluid in the tympanic membrane has not stopped moving causing conflicts with the
information coming from your vision
D) The fluid in the stirrup has not stopped moving causing conflicts with the information coming


104 | Human Physiology




Senses


from your vision
10. These receptors react to foods treated with monosodium glutamate


A) Salt
B) Sour
C) Bitter
D) Sweet
E) Umami


Glossary
Anosmia: Lack of olfaction, or a loss of the sense of smell
Auditory Canal: Tube from the auditory meatus or opening of the ear to the tympanic membrane
Auditory Tube: Either of the paired tubes connecting the middle ears to the nasopharynx;
equalizes air pressure on the two sides of the eardrum
Chemoreception: Physiological response of a sense organ to a chemical stimulus
Choroid: Vascular layer of the eye lying between the retina and the sclera
Circumvallate papillae: Papillae that are present on the back of the oral part of the tongue
Cochlea: Is concerned with hearing, resembling a shell of a snail
Dysosmia: When things smell differently than they should
Equilibrium: Sense of balance
Extraocular muscles: Six muscles that control eye movements: lateral rectus, medial rectus,
inferior rectus, superior rectus, inferior oblique and superior oblique
Filiform papillae: Thin, longer papillae that don't contain taste buds but are the most numerous
Foliate papillae: Ridges and grooves towards the posterior part of the tongue
Fungiform papillae: These are present mostly at the apex (tip) of the tongue- slightly mushroom
shaped
Gustation: The sense of taste
Hair Cell: Mechanosensors for hearing, columnar cells each with a bundle of 100-200 specialized
cilia at the top
Haptic: From the Greek Haphe, means pertaining to the sense of touch
Hyposmia: Decreased ability to smell
Inner Ear: Innermost part of the ear, contains the cochlea, westibule and semi-circular canals
Mechanoreceptor: Sensory receptor that responds to mechanical pressure or distortion
Meissner's Corpuscle: Encapsulated unmyelinated nerve endings, usually found in areas sensitive
to light touch
Middle Ear: Air Filled Cavity behind the Ear Drum, includes most of the ear Drum and ear Bones
Nasopharynx: Nasal part of the pharynx that lies behind the nose and above the level of the soft
palate
Nociception: The perception of pain
Olfaction: The sense of smell
Otitis Media: An inflammation of the middle ear
Outer Ear: External portion of the ear, includes the auricle, ear canal and surface of the ear drum
Oval Window: Fenestra that has the base of the stapes attached to it
Pacinian Corpuscles; Detect gross pressure changes and vibrations
Papilla: Specialized epithelial cells that are small projections on the top of the tongue
Perception: The brain’s interpretation of a sensation
Phantosmia: Phenomenon of smelling odors that aren't really present (AKA Phantom odors)


Wikibooks | 105




Chapter 5
Photoreceptors: Specialized type of neuron found in the eye's retina that is capable of
phototransduction
Pinna: Auricle of the ear
Retina: Thin layer of neural cells that lines the back of the eyeball of vertebrates and some
cephalopods
Round Window: Fenestra leading into the cochlea
Sclera: White outer coating of the eye- gives the eye its shape and helps to protect the delicate
inner parts
Semicircular Canals: Certain canals of the inner ear
Sensation: Occurs when nerve impulses arrive in the brain
Sensory adaptation: A decrease in response to stimuli
Stapes: One of the small bones in the tympanum of the ear; the stirrups bone
Tactition: The sense of pressure perception, generally in the skin
Tympanic Membrane: The membrane in the ear that vibrates to produce sound
Umami: Japanese word meaning savory or meaty- type of taste signal


References
• Hänig, D.P., 1901. Zur Psychophysik des Geschmackssinnes. Philosophische Studien, 17:


576-623.
• Collings, V.B., 1974. Human Taste Response as a Function of Locus of Stimulation on the


Tongue and Soft Palate. Perception & Psychophysics, 16: 169-174.
• Buck, Linda and Richard Axel. (1991). A Novel Multigene Family May Encode Odorant


Receptors: A Molecular Basis for Odor Recognition. Cell 65:175-183.
live version • discussion • edit lesson • comment • report an error


106 | Human Physiology




The Muscular System


6 THE MUSCULAR SYSTEM
live version • discussion • edit lesson • comment • report an error


Overview


he muscular system is the biological system of humans that allows them to move. The muscular
system, in vertebrates, is controlled through the nervous system, although some muscles, like


cardiac muscle, can be completely autonomous.
T


Muscle is contractile tissue and is derived from the mesodermal layer of embryonic germ cells. Its
function is to produce force and cause motion, either locomotion or movement within internal organs.
Much of muscle contraction occurs without conscious thought and is necessary for survival, like the
contraction of the heart or peristalsis, which pushes food through the digestive system. Voluntary
muscle contraction is used to move the body and can be finely controlled, such as movements of the
finger or gross movements that of the biceps and triceps.


Muscle is composed of muscle cells (sometimes known as "muscle fibers"). Within the cells are
myofibrils; myofibrils contain sarcomeres which are composed of actin and myosin. Individual muscle
cells are lined with endomysium. Muscle cells are bound together by perimysium into bundles called
fascicles. These bundles are then grouped together to form muscle, and is lined by epimysium. Muscle
spindles are distributed throughout the muscles, and provide sensory feedback information to the
central nervous system.


Skeletal muscle, which involves muscles from the skeletal tissue, is arranged in discrete groups.
An example of which includes the biceps brachii. It is connected by tendons to processes of the
skeleton. In contrast, smooth muscle occurs at various scales in almost every organ, from the skin (in
which it controls erection of body hair) to the blood vessels and digestive tract (in which it controls the
caliber of a lumen and peristalsis).


There are approximately 650 skeletal muscles in the human body (see list of muscles of the human


Wikibooks | 107




Chapter 6
body). Contrary to popular belief, the number of muscle fibers cannot be increased through exercise;
instead the muscle cells simply get bigger. It is however believed that myofibrils have a limited
capacity for growth through hypertrophy and will split if subject to increased demand.


The ten types of muscle have significant differences. However, all but three use the movement of
actin against myosin to create Muscle contraction|contraction and relaxation. In skeletal muscle,
contraction is stimulated by action potential|electrical impulses transmitted by the nerves, the motor
nerves and motoneurons in particular. All skeletal muscle and many smooth muscle contractions are
facilitated by the neurotransmitter acetylcholine.


Muscular activity accounts for most of the body's energy consumption. Muscles store energy for
their own use in the form of glycogen, which represents about 1% of their mass. This can be rapidly
converted to glucose when more energy is necessary.


Types
There are three types of muscle:


• Smooth muscle or "involuntary muscle" is a spindle shaped muscle found within the walls
of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, ureters,
bladder, and blood vessels. Smooth muscle contains only one nucleus, and no striations.


• Cardiac muscle is also an "involuntary muscle" but it is striated in structure. Like Smooth
muscle, cardiac muscle contains only one nucleus. Cardiac muscle is found only within the
heart.


• Skeletal muscle or "voluntary muscle" is anchored by tendons to the bone and is used to
affect skeletal movement such as locomotion. Skeletal muscle is multinucleated the nucleus is
peripherally located, and the skeletal muscle is striated. Functions of the skeletal muscle
include:


• Support of the body
• Aids in bone movement
• Helps maintain a constant temperature throughout the body
• Assists with the movement of cardiovascular and lymphatic vessels through contractions
• Protection of internal organs and helps to stabilize joints


Cardiac and skeletal muscle are striated in that they contain sarcomere and are packed into highly-
regular arrangements of bundles; smooth muscle has neither. Striated muscle is often used in short,
intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.


108 | Human Physiology




The Muscular System


Skeletal muscle is further divided into several subtypes:
• Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in


mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry
more oxygen and sustain aerobic activity.


• Type II, fast twitch, muscle has three major kinds that are, in order of increasing contractile
speed:


• a) Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries
and appears red.


• b) Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin.
This is the fastest muscle type in humans. It can contract more quickly and with a greater
amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity
before muscle contraction becomes painful (often attributed to a build-up of lactic acid). N.B. in
some books and articles this muscle in humans was, confusingly, called type IIB


• c) Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in
mitochondria and myoglobin. In small animals like rodents or rabbits this is the major fast
muscle type, explaining the pale color of their meat.


For most muscles, contraction occurs as a result of conscious effort originating in the brain. The
brain sends signals, in the form of action potentials, through the nervous system to the motor neuron
that innervates the muscle fiber. However, some muscles (such as the heart) do not contract as a result
of conscious effort. These are said to be autonomic. Also, it is not always necessary for the signals to
originate from the brain. Reflexes are fast, unconscious muscular reactions that occur due to
unexpected physical stimuli. The action potentials for reflexes originate in the spinal cord instead of the
brain.


There are three general types of muscle contractions, skeletal muscle contractions, heart muscle
contractions, and smooth muscle contractions.


Muscular System Working With Other Body Systems
1. Homeostasis
2. Protection
3. Calcium Metabolism
4. Maintaining Body Temperature


Skeletal Muscle Contractions
Steps of a skeletal muscle contraction:


• An action potential reaches the axon of the motor neuron.
• The action potential activates voltage gated calcium ion channels on the axon, and calcium


rushes in.
• The calcium causes acetylcholine vesicles in the axon to fuse with the membrane, releasing


the acetylcholine into the cleft between the axon and the motor end plate of the muscle fiber.
• The skeletal muscle fiber is excited my large mylenated nerve fibers which attach to the


Wikibooks | 109




Chapter 6
neuromuscular junction. There is one neuromuscular junction for each fiber.


• The acetylcholine diffuses across the cleft and binds to nicotinic receptors on the motor end
plate, opening channels in the membrane for sodium and potassium. Sodium rushes in, and
potassium rushes out. However, because sodium is more permeable, the muscle fiber membrane
becomes more positively charged, triggering an action potential.


• The action potential on the muscle fiber causes the sarcoplasmic reticulum to release
calcium ions(Ca++).


• The calcium binds to the troponin present on the thin filaments of the myofibrils. The
troponin then allosterically modulates the tropomyosin. Normally the tropomyosin physically
obstructs binding sites for cross-bridge; once calcium binds to the troponin, the troponin forces
the tropomyosin to move out of the way, unblocking the binding sites.


• The cross-bridge (which is already in a ready-state) binds to the newly uncovered binding
sites. It then delivers a power stroke.


• ATP binds the cross-bridge, forcing it to conform in such a way as to break the actin-
myosin bond. Another ATP is split to energize the cross bridge again.


• Steps 7 and 8 repeat as long as calcium is present on thin filament.
• Throughout this process, the calcium is actively pumped back into the sarcoplasmic


reticulum. When no longer present on the thin filament, the tropomyosin changes back to its
previous state, so as to block the binding sites again. The cross-bridge then ceases binding to the
thin filament, and the contractions cease as well.


• Muscle contraction remains as long as Ca++ is abundant in sarcoplasm.
Types of Contractions:


• Isometric contraction--muscle does not shorten during contraction and does not require the
sliding of myofibrils but muscles are stiff.


• Isotonic contraction--inertia is used to move or work. More energy is used by the muscle
and contraction lasts longer than isometric contraction.


• Twitch--exciting the nerve to a muscle or by passing electrical stimulus through muscle
itself. Some fibers contract quickly while others contract slowly.


The Efficiency of Muscle Contraction:
• Only about 20% of input energy converts into muscular work. The rest of the energy is


heat.
• 50% of energy from food is used in ATP formation.
• If a muscle contraction is slow or without movement, energy is lost as maintenance heat.
• If muscle contraction is rapid, energy is used to reduce friction.


Summation of Muscle Contraction: It is the adding together of individual muscle twitches to make
strong muscle movements.


• Multiple motor unit summation--increasing number of motor units contracting
simultaneously.


• Wave summation--increasing rapidity of contraction of individual motor units.
• Tetanization--higher frequency successive contractions fuse together and cannot be


distinguished from one another.


110 | Human Physiology




The Muscular System


Sliding Filament theory
When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until


the actin and myosin filaments are completely overlapped. The H zone becomes smaller and smaller
due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the
muscle is fully contracted, the H zone is no longer visible (as in the bottom diagram, left). Note that the
actin and myosin filaments themselves do not change length, but instead slide past each other.


Cellular Action of Skeletal Muscles
During cellular respiration the mitochondria, within skeletal muscle cells, convert glucose from the


blood to carbon dioxide and water in the process of producing ATP (see cell physiology). ATP is
needed for all muscular movement. When the need of ATP in the muscle is higher than the cells can
produce with aerobic respiration, the cells will produce extra ATP in a process called anaerobic
respiration. The first step of aerobic respiration(glycolysis) produces two ATP per glucose molecule.
When the rest of the aerobic respiration pathway is occupied the pyruvate molecule can be converted to
lactic acid. This method produces much less ATP than the aerobic method, but it does it faster and
allows the muscles to do a bit more than if they relied solely on ATP production from aerobic


Wikibooks | 111


Sliding myofibril (GFDL - Slashme)




Chapter 6
respiration. The drawback to this method is that lactic acid accumulates and causes the muscles to
fatigue. They will eventually stop contracting until the breakdown of lactic acid is sufficient to allow
for movement once again. People experience this most noticeably when they repeatedly lift heavy
things such as weights or sprint for a long distance. Muscle soreness sometimes occurs after vigorous
activity, and is often misunderstood by the general public to be the result of lactic acid buildup. This is
a misconception because the muscle does fatigue from lactic acid buildup, but it does not stay in the
muscle tissue long enough to cause tissue breakdown or soreness. During heavy breathing, following
exercise, the cells are converting the lactic acid either back into glucose or converting it to pyruvate and
sending it through the additional steps of aerobic respiration. By the time a person is breathing
normally again the lactic acid has been removed. The soreness is actually from small tears in the fibers
themselves. After the fibers heal they will increase in size. The number of mitochondria will also
increase if there is continued demand for additional ATP. Hence, through exercise the muscles can
increase in both strength and endurance.


Another misconception is that as the muscle increases in size it also gains more fibers. This is not
true. The fibers themselves increase in size rather than in quantity. The same holds true for adipose
tissue--fat cells do not increase in number, but rather the amount of lipids (oil) in the cells increase.


Muscle fibers are also genetically programmed to reach a certain size and stop growing from there,
so after awhile even the hardest working weightlifter will only reach a certain level of strength and
endurance. Some people will get around this by taking steroids. The artificial steroids cause all sorts of
trouble for the person. They can cause the adrenal glands to stop producing corticosteroids and
glucosteroids. This leads to the atrophy of the gland's medulla and causes permanent loss of the
production of these hormones. The testicles may also atrophy in response to steroids. Eventually the
testes will stop making testosterone and sperm, rendering the male infertile.


One of the more serious problems associated with abnormal gain of muscle mass is heart failure.
While for most people gaining muscle and losing fat is desirable, a body builder is at risk of producing
more muscle mass than the heart can handle. One pound of fat contains about 3.5 miles of blood
vessels, but one pound of muscle has about 6.5 miles. Hence, additional muscle causes the heart to
pump more blood. Some people that have too much muscle will be very strong but will not have a
healthy aerobic endurance, in part because of the difficulty of providing oxygenated blood to so much
tissue.
Sliding filament theory


This link shows the animation of the sliding filament theory.
explanation and image of sliding filament theory


this link gives a better demonstration of the theory with the explanation.


Involuntary Muscle Movement
Spasms


When Smooth and skeletal muscles go through multiple spasms it is referred either as seizure or
convulsion.


Cramps
Strenuous activities can cause painful spasms that are long, this is referred to as cramps.


112 | Human Physiology




The Muscular System


Injury
Sprain


A injury to a joint that involves a stretched or torn ligament.
Muscle Strain


A strain occurs when a muscle or the tendon that attaches it to the bone is overstretched or torn.
Muscle strains are also called pulled muscles.


Who gets it?
Anyone can strain a muscle. However, people involved in sports or other forms of strenuous


exercise are more likely to strain a muscle.


What causes it?
Muscles are bunches of fibers that can contract. Muscle strains usually occur during activities that


require the muscle to tighten forcefully. The muscle is strained either because it is not properly
stretched, or warmed up, before the activity; it is too weak; or because the muscle is already injured and
not allowed time to recover. So, many muscle strains occur during exercise or sports activities. They
can also occur when lifting heavy objects. What are the symptoms?


When a muscle is strained, it hurts and is difficult to move. You may also feel a burning sensation
in the area of the injured muscle, or feel as though something has "popped." Sometimes the area of the
strained muscle looks bruised or swells. A strained muscle might spasm, which means it contracts
suddenly and involuntarily, causing severe pain. How is it diagnosed?


To diagnose a muscle strain, your doctor will examine the painful area, and ask how and when the
injury happened. He or she may order other diagnostic tests, such as x-rays, to rule out any injury to the
bone.


What is the treatment?
Muscle strains are treated with rest, ice, compression, and elevation, or RICE. You will be told to


rest the injured area to reduce pain and swelling. If the strain is in the leg or foot area, you may need to
use crutches. Ice packs are recommended at regular intervals (as recommended by your doctor) over
the first few days after the injury. Ice causes the blood vessels to constrict, which reduces inflammation
and pain. Anti-inflammatory medications might also be used to relieve pain. Compression and
elevation help to reduce swelling. Your doctor may also recommend physical therapy to speed your
recovery. You should avoid the type of activity that caused the injury until the muscle is completely
healed. Self-care tips


You can prevent muscle strains by warming up for at least 10 minutes before participating in any
strenuous exercise or heavy lifting. When you warm up, you increase the blood circulation to the
muscle and prepare it for exercise. When starting any new exercise program or sport, it's important to
begin gradually so your muscles are conditioned for the activity.


Wikibooks | 113




Chapter 6


Steroids
Anabolic steroids, which are synthetic versions of the primary male sex hormone testosterone, can


be injected, taken orally, or used transdermally. These drugs are Controlled Substances that can be
prescribed to treat conditions such as body wasting in patients with AIDS, and other diseases that occur
when the body produces abnormally low amounts of testosterone. However, the doses prescribed to
treat these medical conditions are 10 to 100 times lower than the doses that are abused for performance
enhancement.


Let me be clear:- while anabolic steroids can enhance certain types of performance or
appearance,they are dangerous drugs, and when used inappropriately, they can cause a host of severe,
long-lasting, and often irreversible negative health consequences. These drugs can stunt the height of
growing adolescents, masculinize women, and alter sex characteristics of men. Anabolic steroids can
lead to premature heart attacks, strokes, liver tumors, kidney failure and serious psychiatric problems.
In addition, because steroids are often injected, users risk contracting or transmitting HIV or hepatitis.


Abuse of anabolic steroids differs from the abuse of other illicit substances because the initial use
of anabolic steroids is not driven by the immediate euphoria that accompanies most drugs of abuse,
such as cocaine, heroin, and marijuana, but by the desire of the abuser to change their appearance and
performance, characteristics of great importance to adolescents. These effects of steroids can boost
confidence and strength leading the abuser to overlook the potential serious long-term damage that
these substances can cause.....


Government agencies such as NIDA support research that increases our understanding of the
impact of steroid abuse and improves our ability to prevent abuse of these drugs. For example, NIDA
funding led to the development of two highly effective programs that not only prevent anabolic steroid
abuse among male and female high school athletes, but also promote other healthy behaviors and
attitudes. The ATLAS (targeting male athletes) and ATHENA (targeting female athletes) programs
have been adopted by schools in 29 states and Puerto Rico. Both Congress and the Substance Abuse
and Mental Health Services Administration have endorsed ATLAS and ATHENA as model prevention
programs, which could and should be implemented in more communities throughout the country.


In addition to these prevention programs and other research efforts, also has invested in public
education efforts to increase awareness about the dangers of steroid abuse. We have material on our
website about steroid abuse at www.steroidabuse.gov and in April 2005 we again will distribute a
"Game Plan" public service announcement designed to bring attention to abuse of anabolic steroids.


Research has shown that the inappropriate use of anabolic steroids can have catastrophic medical,
psychiatric and behavioral consequences.


I hope that students, parents, teachers, coaches and others will take advantage of the information
on our website about anabolic steroids abuse and join us in our prevention and education efforts.
Participating in sports offers many benefits, but young people and adults shouldn't take unnecessary
health risks in an effort to win.(Nora D. Volkow, M.D.)


-Human-made substances related to male sex hormones. Some athletes abuse anabolic steroids to
enhance performance. Abuse of anabolic steroids can lead to serious health problems, some of which
are irreversible.


114 | Human Physiology




The Muscular System


Major side effects can include liver tumors and cancer, jaundice, high blood pressure, kidney
tumors, severe acne, and trembling. In males, side effects may include shrinking of the testicles and
breast development. In females, side effects may include growth of facial hair, menstrual changes, and
deepened voice. In teenagers, growth may be halted prematurely and permanently.


The therapeutic use of steroids can be realized by patients and their doctors by using them in a
manner that is beneficial to the person.


Smooth Muscle Contraction
• Contractions are initiated by an influx of calcium which binds to calmodulin.
• The calcium-calmodulin complex binds to and activates myosin light-chain kinase.
• Myosin light-chain kinase phosphorylates myosin light-chains, causing them to interact


with actin filaments. This causes contraction.
The calcium ions leave the troponin molecule in order to maintain the calcium ion concentration in


the sacoplasm. As the calcium ions are being actively pumped by the calcium pumps present in the
membrane of the sarcoplasmic reticulum creating a deficiency in the fluid around the myofibrils.This
causes the removal of calcium ions from the troponin.Thus the tropomyosin-troponin complex again
covers the binding sites on the actin fiaments and contraction ceases.


Cardiac Muscle
Cardiac muscle cells within the myocardium are arranged


in layers that completely encase the chambers of the heart. The
contraction of these cells pressurizes the blood inside the
chambers of the heart so that the blood can travel out of the
heart to the rest of the body. Cardiac muscle contains elements
of both striated skeletal muscle and smooth muscle. The
striated structure of cardiac muscle is due to the arrangement
of thick myosin and thin actin filaments which are similar to
the arrangement of skeletal muscle. Compared to skeletal muscle, cardiac muscle is much shorter and
has several branching processes. Intercalated disks join the ends of cardiac muscle cells to each other.
Gap junctions are situated adjacent to intercalated disks which is similar to those found in many
smooth muscle cells. One percent of cardiac cells do not contract because the non-contracting cells
have specialized features that aid in heart excitation. The non-contracting cells form a network known
as the conducting system and contact other cardiac muscle cells at gap junctions. The conduction
system begins the heartbeat and assists in spreading the contraction impulse rapidly through out the
heart. Certain cardiac muscle cells also secrete atrial natriuretic peptide (ANP)[1] which inhibits
sodium reabsorbtion in the kidneys.


ATP in the Human Body
Muscles cells, like all cells, use ATP as an energy source. The total quantity of ATP in the human


body at any one time is about 0.1 Mole. The energy used by human cells requires the hydrolysis of 200


Wikibooks | 115




Chapter 6
to 300 moles of ATP daily. This means that each ATP molecule is recycled 2000 to 3000 times during
a single day. ATP cannot be stored, hence its consumption must closely follow its synthesis. On a per-
hour basis, 1 kilogram of ATP is created, processed and then recycled in the body. Looking at it
another way, a single cell uses about 10 million ATP molecules per second to meet its metabolic needs,
and recycles all of its ATP molecules about every 20-30 seconds.


Lactic Acid
Catabolized carbohydrates is known as glycolysis. The end product of glycolysis, pyruvate can go


into different directions depending on aerobic or anaerobic conditions. In aerobic it goes through the
Krebs cycle and is anaerobic it goes through the Cori cycle. In the Cori cycle pyruvate is converted to
lactate, this forms lactic acid, lactic acid causes muscle fatigue. In the aerobic conditions pyruvate goes
through the Krebs cycle. For more about Krebs cycle refer to chapter 2 Cell Physiology.


Muscle Disorders
DERMATOMYOSITIS AND POLYMYOSITIS — Dermatomyositis and polymyositis cause


inflammation of the muscles. They are rare disorders, affecting only about one in 100,000 people per
year. More women than men are affected. Although the peak age of onset is in the 50s, the disorders
can occur at any age.


Signs and symptoms — Patients complain of muscle weakness that usually worsens over several
months, though in some cases symptoms come on suddenly. The affected muscles are close to the trunk
(as opposed to in the wrists or feet), involving for example the hip, shoulder, or neck muscles. Muscles
on both sides of the body are equally affected. In some cases, muscles are sore or tender. Some patients
have involvement of the muscles of the pharynx (throat) or the esophagus (the tube leading from the
throat to the stomach), causing problems with swallowing. In some cases, this leads to food being
misdirected from the esophagus to the lungs, causing severe pneumonia.


In dermatomyositis, there is a rash, though sometimes the rash resolves before muscle problems
occur. A number of different types of rash can occur, including rashes on the fingers, the chest and
shoulders, or on the upper eyelids (show picture 1-3). In rare cases, the rash of dermatomyositis
appears but myopathy never develops.


Other problems sometimes associated with these diseases include fever, weight loss, arthritis, cold-
induced color changes in the fingers or toes (Raynaud phenomenon), and heart or lung problems.


Muscle Atrophy
Alternative names : Atrophy of the muscles, Muscle wasting, Wasting
The majority of muscle atrophy in the general population results from disuse. People with


sedentary jobs and senior citizens with decreased activity can lose muscle tone and develop significant
atrophy. This type of atrophy is reversible with vigorous exercise. Bed-ridden people can undergo
significant muscle wasting. Astronauts, free of the gravitational pull of Earth, can develop decreased
muscle tone and loss of calcium from their bones following just a few days of weightlessness.
116 | Human Physiology




The Muscular System


Muscle atrophy resulting from disease rather than disuse is generally one of two types, that
resulting from damage to the nerves that supply the muscles, and disease of the muscle itself. Examples
of diseases affecting the nerves that control muscles would be poliomyelitis, amyotrophic lateral
sclerosis (ALS or Lou Gehrig's disease), and Guillain-Barre syndrome. Examples of diseases affecting
primarily the muscles would include muscular dystrophy, myotonia congenita, and myotonic dystrophy
as well as other congenital, inflammatory or metabolic myopathies.


Even minor muscle atrophy usually results in some loss of mobility or power.


Common Causes
• some atrophy that occurs normally with aging
• cerebrovascular accident (stroke)
• spinal cord injury
• peripheral nerve injury (peripheral neuropathy)
• other injury
• prolonged immobilization
• osteoarthritis
• rheumatoid arthritis
• prolonged corticosteroid therapy
• diabetes (diabetic neuropathy)
• burns
• poliomyelitis
• amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease)
• Guillain-Barre syndrome
• muscular dystrophy
• myotonia congenita
• myotonic dystrophy
• myopathy


Muscular Dystrophy
Muscular dystrophy (MD) is a group of rare inherited muscle diseases in which muscle fibers are


unusually susceptible to damage. Muscles, primarily voluntary muscles, become progressively weaker.
In the late stages of muscular dystrophy, fat and connective tissue are often replaced by muscle fibers.
In some types of muscular dystrophy, heart muscles, other involuntary muscles and other organs are
affected.


The most common types of muscular dystrophy appear to be due to a genetic deficiency of the
muscle protein dystrophin. There's no cure for muscular dystrophy, but medications and therapy can
slow the course of the disease.


Wikibooks | 117




Chapter 6


Medical Mysteries
Sleep Twitches


The twitching phenomenon that happens in the early stage of sleep is called a hypnagogic massive
jerk, or simply a hypnic jerk. It has also been referred to as a sleep start. There has been little research
on this topic, but there have been some theories put forth. When the body drifts off into sleep, it
undergoes physiological changes related to body temperature, breathing rate and muscular tone. Hypnic
jerks may be the result of muscle changes. Another theory suggests that the transition from the waking
to the sleeping state signals the body to relax. But the brain may interpret the relaxation as a sign of
falling and then signal the arms and legs to wake up. Electroencephalogram studies have shown sleep
starts affect almost 10 percent of the population regularly, 80 percent occasionally, and another 10
percent rarely.


Muscle movement or twitching also may take place during the Rapid Eye Movement, or REM,
phase of sleep. This also is the time when dreams occur. During the REM phase, all voluntary muscular
activity stops with a drop in muscle tone, but some individuals may experience slight eyelid or ear
twitching or slight jerks. Some people with REM behavioral disorder, or RBD, may experience more
violent muscular twitching and full-fledged activity during sleep. This is because they do not achieve
muscle paralysis, and as a result, act out their dreams. Researchers think that people with RBD lack
neurological barriers that define the different stages of sleep. New research done by the Mayo Clinic
and published in the July 2003 issue of Sleep Medicine shows that melatonin can help lessen RBD
symptoms.


Resources:
Sleep twitches, or myoclonic jerks, as they are sometimes called, are explained in easily
understood language on this website.
Learn more about REM Behavior Disorder, or RBD, and treatment for sufferers.
View information about various sleep disorders such as insomnia, apnea, and narcolepsy.


Microbiology
Clostridium tetani


Tetanus
Normally a nerve impulse initiates contraction of a muscle. At the same time, an opposing muscle
receives the signal to relax so as not to oppose the contraction. Tetanus toxin blocks the
relaxation, so both sets of muscle contract. The usual cause of tetany is lack of calcium, but
excess of phosphate (high phosphate-to-calcium ratio) can also trigger the spasms.


Clostridium botulinum
Infant botulism (floppy baby syndrome) the most common form of botulism in the U.S. of the
four forms of botulism.
If ingested, the toxin is absorbed in the intestine, goes to the blood, and on to the nervous system.
It acts on the peripheral nervous system by blocking the impulse that is normally passes along to
the nervous system. By clocking the impulse that is normally passed along to motor end plates so
the muscle contraction can be released, resulting in paralysis.


118 | Human Physiology




The Muscular System


Glossary
Actin


A protien that forms a long polymer rods called microfilaments; Interacts with myosin to cause
movement in muscles.


ATP
"Adenosine Triphosphate" is a nucleotide that comes from adenosine that takes place in muscle
tissue: This provides a large source of energy for cellular reactions.


Cardiac muscle
is also an "involuntary muscle" but it's a specialized kind of muscle found only within the heart.


Clostridium botulinum
A pathogen that causes botulism, gram stain positive, morphology is rod shaped, grows in
anaerobic conditions, and produces spores.


Clostridium tetani
A pathogen that causes lock jaw, gram stain positive, morphology is tennis racket shaped rod,
grows in anaerobic conditions, and produces spores.


Cori cycle
In anaerobic conditions produces lactic acid.


Cramp
A localized muscle spasm that happens after strenuous activity.


Glycogen
Glucose that has been converted for energy storage. Muscles store energy for their own use in
this form.


Lactic acid
Causes muscle fatigue.


Muscle
Contractile tissue that is derived from the mesodermal layer of embryonic germ cells.


Muscular Dystrophy
A hereditary disease characterized by progressive atrophy of muscle fibers


Myosin
The fibrous motor protein that uses ATP to drive movements along actin filaments.


Sarcoplasmic Reticulum
Smooth-surfaced tubules forming a plexus around each myofibril that function as a storage and
release area for calcium ions (CA+2).


Skeletal muscle
this "voluntary muscle" is anchored by tendons to the bone and is used to affect skeletal


Wikibooks | 119




Chapter 6
movement such as locomotion.


Smooth muscle
this "involuntary muscle" is found within the walls of organs and structures such as the
esophagus, stomach, intestines, bronchi, uterus, ureters, bladder, and blood vessels.


Sprain
Injuries that involves a stretched or torn ligament.


Strain
A injury to the muscle or tendon attachment


Review Questions
1. Smooth Muscle is


A) Voluntary and Spindle Shaped
B) Voluntary and Striated
C) Involuntary and Spindle Shaped
D) Involuntary and Striated


2. Skeletal Muscle is
A) Voluntary and Spindle Shaped
B) Voluntary and Striated
C) Involuntary and Spindle Shaped
D) Involuntary and Striated


3. Cardiac Muscle is
A) Voluntary and Spindle Shaped
B) Voluntary and Striated
C) Involuntary and Spindle Shaped
D) Involuntary and Striated


4. Which type of muscle cell is multinucleated?
A) Cardiac
B) Smooth
C) Skeletal
D) All of the Above


5. What is an example of a smooth muscle?
A) Masseter (Face)
B) Bladder
C) Heart
D) Pronator Teres (Forearm)


120 | Human Physiology




The Muscular System


E) Rectus Abdominis (belly)
6. Each myosin filament is surrounded by ____ actin filaments.


A) Two
B) Four
C) Six
D) Eight
E) Seven


References
• Van De Graaff (2002) Human Anatomy 6th ed. McGraw-Hill Higher Education
• Windmaier, P.W. Raff, H. Strang, T.S. (2004) Vander, Sherman, & Luciano's Human


Physiology, the Mechanisms of Body Function 9th ed. Mcgraw-Hill
live version • discussion • edit lesson • comment • report an error


Wikibooks | 121




Chapter 7


7 BLOOD PHYSIOLOGY
live version • discussion • edit lesson • comment • report an error


Overview of Blood
he primary function of blood is to supply nutrients and constitutional elements to tissues and to
remove waste products. Blood also enables cells and different substances to be transported


between tissues and organs. Problems with blood composition or circulation can lead to downstream
tissue malfunction. Blood is also involved in maintaining homeostasis by acting as a medium for
transferring heat to the skin and by acting as a buffer system for bodily pH.


T


The blood is circulated around the lungs and body by the pumping action of the heart. Additional
return pressure may be generated by gravity and the actions of skeletal muscles.
122 | Human Physiology


Blood circulation from the heart to the lungs.




Blood Physiology


Transport of Oxygen
Oxygen (O2) Is carried throughout the body by the blood circulation. Pulmonary circulation


happens when blood leaves the heart, enters the lungs, and becomes saturated with oxygen. Once this
saturated blood exits the heart, it delivers the oxygen to all organs throughout the body. This oxygen
depleted blood is then delivered back to the lungs to be renewed with fresh oxygen.


Blood Composition
Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells,


platelets). Anatomically, blood is considered a connective tissue, due to its origin in the bones and its
function. Blood is the means and transport system of the body used in carrying elements (e.g. nutrition,
waste, heat) from one location in the body to another, by way of blood vessels.


Blood is made of two parts:
1. Plasma which makes up 45-55% of blood volume.
2. Formed cellular elements (red and white blood cells, and platelets) which combine to make


the remaining blood volume.


Plasma makeup
Plasma is made up of 90% water, 7-8% soluble proteins (albumin maintains bloods osmotic


integrity, others clot, etc), 1% electrolytes, and 1% elements in transit. One percent of the plasma is
salt, which helps with the pH of the blood. The largest group of solutes in plasma contains three
important proteins to be discussed. There are: albumins, globulins, and clotting proteins.


Albumins are the most common group of proteins in plasma and consist of nearly two-thirds of
them (60-80%). They are produced in the liver. The main function of albumins is to maintain the
osmotic balance between the blood and tissue fluids and is called colloid osmotic pressure. In addition,
albumins assist in transport of different materials, such as vitamins and certain molecules and drugs
(e.g. bilirubin, fatty acids, and penicillin).


Globulins are a diverse group of proteins, designated into three groups: gamma, alpha, and beta.
Their main function is to transport various substances in the blood. Gamma gobulins assist the body's
immune system in defense against infections and illness.


Clotting proteins are mainly produced in the liver as well. There are at least 12 substances, known
as "clotting factors" that participate in the clotting process. One important clotting protein that is part of
this group is fibrinogen, one of the main component in the formation of blood clots. In response to
tissue damage, fibrinogen makes fibrin threads, which serve as adhesive in binding platelets, red blood
cells, and other molecules together, to stop the blood flow. (This will be discussed in more detail later
on in the chapter.)


Wikibooks | 123




Chapter 7


Red Blood Cells


Overview
Red blood cell (erythrocyte) also known as "RBC's". RBC’s are formed


in the myeloid tissue or most commonly known as red bone marrow, although
when the body is under severe conditions the yellow bone marrow, which is
also in the fatty places of the marrow in the body will also make RBC’s. The
formation of RBC’s is called erythropoiesis ( erythro / red; poiesis /
formation). Red blood cells lose nuclei upon maturation, and take on a biconcave, dimpled, shape.
They are about 7-8 micrometers in diameter. There are about 1000x more red blood cells than white
blood cells. RBC's live about 120 days and do not self repair. RBC's contain hemoglobin which
transports oxygen from the lungs to the rest of the body, such as to the muscles, where it releases the
oxygen load.The hemoglobin gets it's red color from their respiratory pigments.


Shape
RBC'S have a shape of a disk that appears to be “caved in” or almost flattened in the middle; this is


called bi-concave. This bi-concave shape allows the RBC to carry oxygen and pass through even the
smallest capillaries in the lungs. This shape also allows RBCs to stack like dinner plates and bend as
they flow smoothly through the narrow blood vessels in the body. RBC's lack a nucleus (no DNA) and
no organelles, meaning that these cells cannot divide or replicate themselves like the cells in our skin
and muscles. RBC’s have a short life span of about 120 days, however, as long as our myeloid tissue is
working correctly, we will produce about 2-3 million RBC's per second. That is about 200 billion a
day! This allows us to have more to replace the ones we lose.


Main Component
The main component of the RBC is hemoglobin protein which is about 25 million per cell. The


word hemoglobin comes from hemo meaning blood and globin meaning protein. This is the protein
substance of four different proteins: polypeptide globin chains that contain anywhere from 141 to 146
amino acids. Hemoglobin also is responsible for the cell’s ability to transport oxygen and carbon
dioxide. This hemoglobin + iron + oxygen interact with each other forming the RBC's bright red color.
You can call this interaction by product oxyhemoglobin. Carbon Monoxide forms with hemoglobin
faster that oxygen, and stays formed for several hours making hemoglobin unavailable for oxygen
transport right away. Also a red blood cell contains about 200 million hemoglobin molecules. If all this
hemoglobin was in the plasma rather than inside the cells, your blood would be so "thick" that the heart
would have a difficult time pumping it through. The thickness of blood is called viscosity. The greater
the viscosity of blood, the more friction there is and more pressure is needed to force blood through.


Functions
The main function is the transportation of oxygen throughout the body and the ability of the blood


to carry out carbon dioxide which is called carbamino – hemoglobin. Maintaining the balance of
blood is important. The balance can be measured by the acid and base levels in the blood. This is called
pH. Normal pH of blood ranges between 7.35-7.45; this normal blood is called Alkaline (less acidic


124 | Human Physiology




Blood Physiology


then water). A drop in pH is called Acidic. This condition is also called Acidosis. A jump in pH higher
then 7.45 is called "Alkalis". To maintain the homeostasis (or balance,) the blood has tiny molecules
within the RBC that help prevent drops or increases from happening.


Destruction
Red blood cells are broken down and hemoglobin is released. The globin part of the hemoglobin is


broken down into amino acid componants, which in turn is recycled by the body. The iron is recovered
and returned to the bone marrow to be reused. The heme portion of the molecule experiences a
chemical change and then gets excreted as bile pigment (bilirubin) by the liver. Heme portion after
being broken down contributes to the color of feces and your skin color changing after being bruised.


White Blood Cells


Shape
White blood cells are different from red cells


in the fact that they are usually larger in size 10-
14 micrometers in diameter. White blood cells do
not contain hemoglobin which in turn makes them
translucent. Many times in diagrams or pictures
white blood cells are represented in a blue color,
mainly because blue is the color of the stain used
to see the cells. White blood cells also have
nucleii, that are some what segmented and are
surrounded by electrons inside the membrane.


Functions
White blood cells (leukocytes) are also known as "WBC's". White blood cells are made in the


bone marrow but they also divide in the blood and lymphatic systems. They are commonly amoeboid
(cells that move or feed by means of temporary projections, called pseudopods (false feet), and escape
the circulatory system through the capillary beds. The different types of WBC's are Basophils,
Eosinophils, Eeutrophils, Monocytes, B- and T-cell lymphocytes. Neutrophils, Eosinophils, and
Basophils are all granular leukocytes. Lymphocytes and Monocytes are agranular leukocytes.
Basophils store and synthesize histamine which is important in allergic reactions. They enter the tissues
and become "mass cells" which help blood flow to injured tissues by the release of histamine.
Eosinophils are chemotaxic and kill parasites. Neutrophils are the first to act when there is an infection
and are also the most abundant white blood cells. Neutrophils fight bacteria and viruses by
phagocytosis which mean they engulf pathogens that may cause infection. The life span of a of
Neutrophil is only about 12-48 hours. Monocytes are the biggest of the white blood cells and are
responsible for rallying the cells to defend the body. Monocytes carry out phagocytosis and are also
called macrophages. Lymphocytes help with our immune response. There are two Lymphocytes: the B-
and T- cell. B-Lymphocytes produce antibodies that find and mark pathogens for destruction. T-
Lymphocytes kill anything that they deem abnormal to the body.


Wikibooks | 125


From left to right diagram of Erythrocyte, Thrombocyte, and
Leukocyte




Chapter 7
WBCs are classified by phenotype which can be identified by looking at the WBCs under a


microscope. The Granular phenotype are able to stain blue. The Agranular phenotype are able to stain
red. Neutrophils make up 50-70% of Granular cells Eosinophils make up 2-4%, and Basophils 0-1%.
Monocytes make up 2-8% of Agranular cells. B and T Lymphocytes make up 20-30%. As you can see,
there is a great deal of differentiation between WBCs. These special cells help our bodies defend
themselves against pathogens. Not only do they help our immune system but they remove toxins,
wastes, and abnormal or damaged cells. Thus, we can say that WBCs' main function is being
Phagocytic which means to engulf or swallow cells.


Platelets
Platelets, also called thrombocytes, are membrane-bound cell


fragments. Platelets have no nucleus, are between one to two
micrometers in diameter, and are about 1/10th to 1/20th as abundant
as white blood cells. Less than 1% of whole blood consists of
platelets. They result from fragmentation of large cells called
Megakaryocytes - which are cells derived from stem cells in the bone
marrow. Platelets are produced at a rate of 200 billion per day. Their
production is regulated by the hormone called Thrombopoietin. The
circulating life of a platelet is 8–10 days. The sticky surface of the
platelets allow them to accumulate at the site of broken blood vessels
to form a clot. This aids in the process of hemostasis ("blood
stopping"). Platelets secrete factors that increase local platelet
aggregation (e.g., Thromboxane A), enhance vasoconstriction (e.g.,
Serotonin), and promote blood coagulation (e.g., Thromboplastin).


Hemostasis (Coagulation or Clotting)
Hemostasis is the natural process of stopping blood flow or loss of blood following an injury.


(hemo = blood; stasis = standing). It has three stages: (1) vascular spasm, vasoconstriction, or intense
contraction of blood vessels, (2) formation of a platelet plug and (3) blood clotting or coagulation.
Once the flow of blood has been stopped, tissue repair can begin.


Vascular spasm or Vasoconsriction: In a normal individual, immediately after a blood vessel has
been cut and endothelial cells are damaged, vasoconstriction occurs, thus slowing blood flow to the
area. Smooth muscle in the vessel wall goes through spasms or intense contractions that constrict the
vessel. If the vessels are small, spasms compress the inner walls together and may be able to stop the
bleeding completely. If the vessels are medium to large-sized, the spasms slow down immediate
outflow of blood, lessening the damage but still preparing the vessel for the later steps of hemostasis.
These vascular spasms usually last for about 30 minutes, long enough for the next two stages of
hemostasis to take place.


Formation of a Platelet Plug: Within 20 seconds of an injury, coagulation is initiated. Contrary to
popular belief, clotting of a cut on the skin is not initiated by air or drying out, but by platelets adhering
to and activated by collagen in the blood vessels endothelium. The activated platelets then release the
contents of their granules, which contain a variety of substances that stimulate further platelet
activation and enhance the hemostatic process.


126 | Human Physiology


A 250 ml bag of newly collected
platelets.




Blood Physiology


When the lining of a blood vessel breaks and endothelial cells are damaged, revealing collagen
proteins in the vessel wall, platelets swell, grow spikey extensions, and start clumping together. They
start to stick to each other and the walls of the vessel. This continues as more platelets congregate and
undergo these same transformations. This process results in a platelet plug that seals the injured area. If
the injury is small, a platelet plug may be able to form and close it within several seconds. If the
damage is more serious, the next step of blood clotting will take place. Platelets contain secretory
granules. When they stick to the proteins in the vessel walls, they degranulate, thus releasing their
products, which include ADP (adenosine diphosphate), serotonin, and thromboxane A2.


A Blood Clot Forms: If the platelet plug is not enough to stop the bleeding, the third stage of
hemostasis begins: the formation of a blood clot. First, blood changes from a liquid to a gel. At least 12
substances called clotting factors take part in a series of chemical reactions that eventually create a
mesh of protein fibers within the blood. Each of the clotting factors has a very specific function. We
will discuss just three of the substances here: prothrombin, thrombin, and fibrinogen. Prothrombin and
fibrinogin are proteins that are produced and deposited in the blood by the liver.


• Prothrombin: When blood vessels are damaged, vessels and nearby platelets are stimulated
to release a substance called prothrombin activator, which in turn activates the conversion of
prothrombin, a plasma protein, into an enzyme called thrombin. This reaction requires calcium
ions.


• Thrombin: Thrombin facilitates the conversion of a soluble plasma protein called
fibrinogen into long insoluble fibers or threads of the protein fibrin.


• Fibrin: Fibrin threads wind around the platelet plug at the damaged area of the blood
vessel, forming an interlocking network of fibers and a framework for the clot. This net of fibers
traps and helps hold platelets, blood cells and other molecules tight to the site of injury,
functioning as the initial clot. This temporary fibrin clot can form in less than a minute, and
usually does a good job of reducing the blood flow. Next, platelets in the clot begin to shrink,
tightening the clot and drawing together the vessel walls. Usually, this whole process of clot
formation and tightening takes less than a half hour.


The use of adsorbent chemicals, such as zeolites, and other hemostatic agents, are also being
explored for use in sealing severe injuries quickly.


ABO Group System
The ABO blood group is represented by substances on the surface of red blood cells (RBCs).


These substances are important because they contain specific sequences of amino acid and
carbohydrates which are antigenic. As well as being on the surface of RBCs, some of these antigens are
also present on the cells of other tissues. A complete blood type describes the set of 29 substances on
the surface of RBCs, and an individual's blood type is one of the many possible combinations of blood
group antigens. Usually only the ABO blood group system and the presence or absence of the Rhesus
D antigen (also known as the Rhesus factor or RH factor) are determined and used to describe the
blood type. Over 400 different blood group antigens have been found, many of these being very rare. If
an individual is exposed to a blood group antigen that is not recognized as self, the individual can
become sensitized to that antigen; the immune system makes specific antibodies which binds
specifically to a particular blood group antigen and an immunological memory against that particular


Wikibooks | 127




Chapter 7
antigen is formed. These antibodies can bind to antigens on the surface of transfused red blood cells (or
other tissue cells) often leading to destruction of the cells by recruitment of other components of the
immune system. Knowledge of a individual's blood type is important to identify appropriate blood for
transfusion or tissue for organ transplantation.


Surface Antigens
Several different RBC surface antigens


stemming from one allele (or very closely linked
genes) are collectively labeled as a blood group
system (or blood group). The two most important
blood group systems were discovered during early
experiments with blood transfusion, the ABO
group in 1901 and the Rhesus group in 1937 .
These two blood groups are reflected in the
common nomenclature A positive, O negative,
etc. with letters referring to the ABO group and
positive/negative to the presence/absence of the
RhD antigen of the Rhesus group. Development
of the Coombs test in 1945 and the advent of
transfusion medicine led to discovery of more
blood groups.


Blood Group AB individuals have both A
and B antigens on the surface of their RBCs, and
their blood serum does not contain any antibodies against either A or B antigen. Therefore, a individual
with type AB blood can receive blood from any group (with AB being preferable), but can only donate
blood to another group AB individual.


Blood Group A individuals have the A antigen on the surface of their RBCs, and blood serum
containing IgM antibodies against the B antigen. Therefore, a group A individual can only receive
blood from individuals of groups A or O (with A being preferable), and can donate blood to individuals
of groups A or AB.


Blood Group B individuals have the B antigen on their surface of their RBCs, and blood serum
containing IgM antibodies against the A antigen. Therefore, a group B individual can only receive
blood from individuals of groups B or O (with B being preferabe), and can donate blood to individuals
of groups B or AB.


Blood group O individuals do not have either A or B antigens on the surface of their RBCs, but
their blood serum contains IgM antibodies against both A and B antigens. Therefore, a group O
individual can only receive blood from a group O individual, but they can donate blood to individuals
of any ABO blood group (ie A, B, O or AB).


Inheritance
Blood types are inherited and represent contributions from both parents. The ABO blood type is


128 | Human Physiology


Compatibility of blood types.




Blood Physiology


controlled by a single gene with three alleles: i, IA, and IB. The gene encodes an enzyme that modifies
the carbohydrate content of the red blood cell antigens.


IA gives type A,
IB gives type B,
i give types O
IA and IB are dominant over i, so ii people have type O, IAIA or IAi have A, and IBIB or IBi have


type B. IAIB people have both phenotypes because A and B are codominant, which means that type A
and B parents can have an AB child. Thus, it is extremely unlikely for a type AB parent to have a type
O child (it is not, however, direct proof of illegitimacy): the cis-AB phenotype has a single enzyme that
creates both A and B antigens. The resulting red blood cells do not usually express A or B antigen at
the same level that would be expected on common group A or B red blood cells, which can help solve
the problem of an apparently genetically impossible blood group.


Rh Factor
Many people have the Rh Factor on the red blood cell. Rh carriers do not have the antibodies for


the Rh Factor, but can make them if exposed to Rh. Most commonly Rh is seen when anti-Rh
antibodies cross from the mothers placenta into the child before birth. The Rh Factor enters the child
destroying the child's red blood cells. This is called Hemolytic Disease.


Compatibility in Blood Transfusions
Blood transfusions between donor and recipient of incompatible blood types can cause severe


acute immunological reactions, hemolysis (RBC destruction), renal failure, shock, and sometimes
death. Antibodies can be highly active and can attack RBCs and bind components of the complement
system to cause massive hemolysis of the transfused blood.


A patient should ideally receive their own blood or type-specific blood products to minimize the
chance of a transfusion reaction. If time allows, the risk will further be reduced by cross-matching
blood, in addition to blood typing both recipient and donor. Cross-matching involves mixing a sample
of the recipient's blood with a sample of the donor's blood and checking to see if the mixture
agglutinates, or forms clumps. Blood bank technicians usually check for agglutination with a
microscope, and if it occurs, that particular donor's blood cannot be transfused to that particular
recipient. Blood transfusion is a potentially risky medical procedure and it is vital that all blood
specimens are correctly identified, so in cross-matching labeling is standardized using a barcode system
known as ISBT 128.


Hemolytic Disease of the Newborn
Often a pregnant woman carries a fetus with a different blood type to herself, and sometimes the


mother forms antibodies against the red blood cells of the fetus, leading to low fetal blood counts, a
condition known as hemolytic disease of the newborn.


Hemolytic disease of the newborn, (also known as HDN) is an alloimmune condition that develops
in a fetus when the IgG antibodies produced by the mother and passing through the placenta include


Wikibooks | 129




Chapter 7
ones which attack the red blood cells in the fetal circulation. The red cells are broken down and the
fetus can develop reticulocytosis and anemia. The fetal disease ranges from mild to very severe and
fetal death from heart failure - hydrops fetalis - can occur. When the disease is moderate or severe
many erythroblasts are present in the fetal blood and so these forms of the disease can be called
erythroblastosis fetalis.


Before birth, options for treatment include intrauterine transfusion or early induction of labor when
pulmonary maturity has been attained, fetal distress is present, or 35 to 37 weeks of gestation have
passed. The mother may also undergo plasma exchange to reduce the circulating levels of antibody by
as much as 75%.


After birth, treatment depends on the severity of the condition, but could include temperature
stabilization and monitoring, phototherapy, transfusion with compatible packed red blood, exchange
transfusion with a blood type compatible with both the infant and the mother, sodium bicarbonate for
correction of acidosis and/or assisted ventilation.


Rh negative mothers who have had a pregnancy with or are pregnant with a Rh positive infant, are
given Rh immune globulin (RhIG) also known as Rhogam, during pregnancy and after delivery to
prevent sensitization to the D antigen. It works by binding any fetal red cells with the D antigen before
the mother is able to produce an immune response and form anti-D IgG. A drawback to pre-partum
administration of RhIG is that it causes a positive antibody screen when the mother is tested which is
indistinguishable from immune reasons for antibody production.


Diseases of the Blood
Disseminated Intravascular Coagulation


Disseminated intravascular coagulation (DIC), also called consumptive coagulopathy, is a
pathological process in the body where the blood starts to coagulate throughout the whole body. This
depletes the body of its platelets and coagulation factors, and there is a paradoxically increased risk of
hemorrhage. It occurs in critically ill patients, especially those with Gram-negative sepsis (particularly
meningococcal sepsis) and acute promyelocytic leukemia.


Hemophilia
Hemophilia is a disease where there is low or no blood protein, causing an inability to produce


blood clots. There are two types of Hemophilia: Type A, which is a deficiency in factor VIII and Type
B, (Christmas disease) a deficiency on factor IX. Because people with hemophilia do not have the
ability to make blood clots, even a little cut may kill them, or the smallest bump or jar to the body
could cause severe bruising that doesn't get better for months.


Hemophilia is passed down from mothers to their sons. Hemophilia is sometimes known as the
"Royal Disease". This is because Queen Victoria, Queen of England (1837-1901), was a carrier of
hemophilia. The hemophilia disease was passed down to her son Leopold who ended up dying at age
31. Queen Victoria also had two daughters who were carriers. These daughters passed hemophilia into
the Spanish, German, and Russian royal families. One of the most famous stories is that of the Russian


130 | Human Physiology




Blood Physiology


royal family. Alexandra, granddaughter to Queen Victoria, married Nicholas (Tsar of Russia in the
1900s). Alexandra was a carrier of the disease and passed the disease to their first son, Tsarevich Alexi,
who was heir to the throne of Russia. The family tried to keep their son's secret from the people, but
Alexi suffered with serious bruises and extreme pain. The family found help from a monk named
Rasputin. He kept their secret and gained a great deal of power over the family, making them think he
was their only hope. During this time of great turmoil in Russia, Nicholas and Alexandra spent most of
their attentions on their son, and not on the people. It wasn't long before the Bolshevik Revolution of
1917 began.


Factor V Leiden
The opposite of Hemophilia, Factor V Leiden is the name given to a variant of human factor V that


causes a hypercoagulability disorder. In this disorder the Leiden variant of factor V, cannot be
inactivated by activated protein C. Factor V Leiden is the most common hereditary hypercoagulability
disorder amongst Eurasians. It is named after the city Leiden (The Netherlands), where it was first
identified in 1994 by Prof R. Bertina et al. Those that have it are at a slightly higher risk of developing
blood clotts than those without. Those that test positive for factor V should avoid (oral contreseptives,
obesity, smoking, and high blood pressure.)


Anemia
Anemia (AmE) or anaemia (BrE), from the Greek ( ναιμία) meaning "without blood", refers to aἈ


deficiency of red blood cells (RBCs) and/or hemoglobin. This results in a reduced ability of blood to
transfer oxygen to the tissues, causing hypoxia. Snce all human cells depend on oxygen for survival,
varying degrees of anemia can have a wide range of clinical consequences. Hemoglobin (the oxygen-
carrying protein in the red blood cells) has to be present to ensure adequate oxygenation of all body
tissues and organs.


The three main classes of anemia include excessive blood loss (acutely such as a hemorrhage or
chronically through low-volume loss), excessive blood cell destruction (hemolysis) or deficient red
blood cell production (ineffective hematopoiesis). In menstruating women, dietary iron deficiency is a
common cause of deficient red blood cell production.


Sickle cell
Sickle-cell disease is a general term for a group of genetic disorders caused


by sickle hemoglobin (Hgb S or Hb S). In many forms of the disease, the red
blood cells change shape upon deoxygenation because of polymerization of the
abnormal sickle hemoglobin. This process damages the red blood cell
membrane, and can cause the cells to become stuck in blood vessels. This
deprives the downstream tissues of oxygen and causes ischemia and infarction.
The disease is chronic and lifelong. Individuals are most often well, but their
lives are punctuated by periodic painful attacks. In addition to periodic pain,
there may be damage of internal organs, and/or stroke. Lifespan is often
shortened with sufferers living to an average of 40 years. It is common in people from parts of the
world where malaria is or was common, especially in sub-Saharan Africa or in descendants of those


Wikibooks | 131


Image of RBC's with
Sickle Cell mutations.




Chapter 7
peoples.
Genetics


Sickle-cell disease is inherited in the autosomal recessive pattern, depicted above. The allele
responsible for sickle cell anemia is autosomal recessive. A person who receives the defective gene
from both father and mother develops the disease; a person who receives one defective and one healthy
allele remains healthy, but can pass on the disease and is known as a carrier. If two parents who are
carriers have a child, there is a 1-in-4 chance of their child developing the illness and a 1-in-2 chance of
their child just being a carrier.


Polycythemia
Polycythemia is a condition in which there is a net increase in the total circulating erythrocyte (red


blood cell) mass of the body. There are several types of polycythemia.


Primary Polycythemia
In primary polycythemia, there may be 8 to 9 million and occasionally 11 million erythrocytes per


cubic millimeter of blood (a normal range for adults is 4-5 million), and the hematocrit may be as high
as 70 to 80%. In addition, the total blood volume can increase to as much as twice as normal. The
entire vascular system can become markedly engorged with blood, and circulation times for blood
throughout the body can increase up to twice the normal value. The increased numbers of erythrocytes
can increase the viscosity of the blood to as much as five times normal. Capillaries can become plugged
by the very viscous blood, and the flow of blood through the vessels tends to be extremely sluggish.


As a consequence of the above, people with untreated Polycythemia are at a risk of various
thrombotic events (deep venous thrombosis, pulmonary embolism), heart attack and stroke, and have a
substantial risk of Budd-Chiari syndrome (hepatic vein thrombosis). The condition is considered
chronic; no cure exists. Symptomatic treatment (see below) can normalize the blood count and most
patients can live a normal life for years.


Secondary polycythemia
Secondary polycythemia is caused by either appropriate or inappropriate increases in the


production of erythropoietin that result in an increased production of erythrocytes. In secondary
polycythemia, there may be 6 to 8 million and occasionally 9 million erythrocytes per cubic millimeter
of blood. A type of secondary polycythemia in which the production of erythropoietin increases
appropriately is called physiologic polycythemia. Physiologic polycythemia occurs in individuals
living at high altitudes (4275 to 5200 meters), where oxygen availability is less than at sea level. Many
athletes train at higher altitudes to take advantage of this effect — a legal form of blood doping. Actual
polychthemia sufferers have been known to use their condition as an athletic advantage for greater
stamina.


Other causes of secondary polycythemia include smoking, renal or liver tumors, or heart or lung
diseases that result in hypoxia. Endocrine abnormalities, prominently including pheochromocytoma
and adrenal adenoma with Cushing's Syndrome, are also secondary causes. Athletes and bodybuilders
132 | Human Physiology




Blood Physiology


who abuse anabolic steroids or erythropoietin may develop secondary polycythemia.
Relative polycythemia


Relative polycythemia is an apparent rise of the erythrocyte level in the blood; however, the
underlying cause is reduced blood plasma. Relative polycythemia is often caused by fluid loss i.e.
burns, dehydration and stress polycythemia.


Leukemia
Leukemia is a cancer of the blood or bone marrow characterized by an abnormal proliferation of


blood cells, usually white blood cells (leukocytes). It is part of the broad group of diseases called
hematological neoplasms. Damage to the bone marrow, by way of displacing the normal marrow cells
with increasing numbers of malignant cells, results in a lack of blood platelets, which are important in
the blood clotting process. This means people with leukemia may become bruised, bleed excessively,
or develop pin-prick bleeds (petechiae).


White blood cells, which are involved in fighting pathogens, may be suppressed or dysfunctional,
putting the patient at the risk of developing infections. The red blood cell deficiency leads to anaemia,
which may cause dyspnea. All symptoms may also be attributable to other diseases; for diagnosis,
blood tests and a bone marrow biopsy are required.


Glossary
Albumin: a major blood protein responsible for the maintenance of osmotic (water) pressure in the


blood
Anemia: a deficiency of red blood cells or hemoglobin caused by lack of iron, folic acid or


vitamin B12 in the diet, or by red blood cell destruction; associated with decreased ability of blood to
carry oxygen


B-Cell: cell responsible for the distribution of antibodies
Basophil: this white blood cell enters damaged tissues and releases a histamine and other


chemicals that promote inflammation in the body to fight pathogens
Blood: the means and transport system of the body used in carrying elements - nutrition, waste,


heat - from one location in the body to another by way of blood vessels
Eosinophil: white blood cell that is involved in the immune response against parasitic worms


(such as tapeworms and roundworms). Named because it stains with the red dye "eosin."
Factor V Leiden most common genetic hypercoagulability disorder.
Formed Elements: the red blood cells, white blood cells and platelets found in blood
Hematocrit: measurement of the % of red blood cells found in blood


Wikibooks | 133




Chapter 7
Hemoglobin (Hb): iron-containing pigment in red blood cells that combines with and transports


oxygen
Hemophilia: genetic disorder in which the affected individual may have uncontrollable bleeding;


blood does not clot
Hemostasis: the process by which blood flow is stopped; also describes the clotting of blood
Lymphocytes: cells of the Lymphatic system, provide defense against specific pathogen or toxins
Monocytes: The largest white blood cell. Becomes a macrophage when activated. Engulfs


pathogens and debris through phagocytosis, also involved in presenting antigens to B and T
lymphocytes.


Neutrophils: the most common white blood cell; they are phagocytic and engulf pathogens or
debris in the tissues; also release cytotoxic enzymes and chemicals to kill pathogens


NK-Cells: also known as "Natural Killer Cells", these T lymphocytes are responsible for
surveillance and detection of abnormal tissue cells; important in preventing cancer


Phagocytosis: process by which amoeboid-like cells engulf and ingest, and thereby destroy,
foreign matter or material


T-Cell: cells that mediate by coordinating the immune system and enter the peripheral tissues.
They can attack foreign cells directly and control the activities of other lymphocytes


Review Questions
1.Taking aspirin every day can reduce the risk of heart disease because:


A) it is a powerful vasodilator
B) it blocks pain receptors in heart tissue
C) it stops ventricular fibrillation
D) it loosens plaque on arterial walls
E) it prevents platelet clumping


2. A hematocrit measures percentage of:
A) White blood cells
B) Plasma
C) Platelets
D) Red blood cells


3. Fred's blood type is O- and Ginger's is B+. Fred and Ginger have a son who is AB+. What do you
conclude?


A) If they have a second child Ginger needs to have RhoGam shot
B) There is no risk to a second child, unless it has a negative blood type


134 | Human Physiology




Blood Physiology


C) If the child needs a blood transfusion Fred could provide it safely, but not Ginger
D) Fred is not the boy’s father


4. Which blood component plays the largest role in maintaining the osmotic pressure of blood?
A) albumin
B) carbon dioxide
C) white blood cells
D) fibrinogen
E) globulins


5. If you hold your breath for one minute
A) The kidneys will increase sodium ion reabsorption
B) Hydrogen-ion concentration in the blood will increase
C) Your heart rate will greatly slow
D) Hemoglobin will bind to oxygen more strongly


6. Most of the carbon dioxide produced by tissues is transported to the lungs as:
A) Small gas bubbles in the plasma
B) Gas bound to hemoglobin in the red blood cells
C) Bicarbonate ions in the plasma
D) Gas bound to white blood cells and albumin
E) Gas transported through the lymphatic system


7. To prevent blood loss after a tissue injury, blood vessels first
A) Form a platelet plug
B) Form a clot
C) Initiate the coagulation cascade
D) Constrict and form barriers


8. You take a blood sample from a male cyclist at the end of a long race. The hematocrit is 60%. The
most likely conclusion is:


A) This is within normal range for most adult males
B) This cyclist is anemic
C) This low of a hematocrit could indicate liver damage or leukemia
D) The cyclist is dehydrated
E) The cyclist has been taking pharmaceutical erythropoietin


9. In a normal blood sample, which of the following cells will be the most abundant?
A) Neutrophils
B) Basophils
C) Eosinophils
D) Monocytes
E) Lymphocytes


Wikibooks | 135




Chapter 7
10. A bag of donated blood does not clot because


A) There is not enough oxygen
B) It cannot dry out
C) It is kept refrigerated
D) There is no free calcium
E) All of the above


live version • discussion • edit lesson • comment • report an error


136 | Human Physiology




The Cardiovascular System


8 THE CARDIOVASCULAR SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction
he heart is the life-beating, always-thumping muscle in your chest. From inside the womb until
death, the thump goes on. The heart for the average human will contract about 3 billion times;


never resting, never stopping to take a break except for a fraction of a second between beats. If a person
lives to be 80 years old, his or her heart will continue to beat an average of 100,000 times a day. Many
believe that the heart is the first organ to become functional. Within weeks of conception the heart
starts its mission of supplying the body with nutrients even though the embryo is no bigger than a
capital letter on this page. The primary function of the heart is to pump blood through the arteries,
capillaries, and veins. There is an estimated 60,000 miles of vessels throughout an adult body. Blood
then transports oxygen, nutrients, disease causing viruses, bacteria, hormones and has other important
functions as well. The heart is the pump that keeps blood circulating properly. Americans today have
many options to take care of the heart and its counterparts. Due to expanding medical technology, it
makes it much easier to do so. This chapter is dedicated to the heart and its many parts.


T


The Heart
The heart is a hollow, muscular organ about the size of a fist. It is responsible for pumping blood


through the blood vessels by repeated, rhythmic contractions. The heart is composed of cardiac muscle,
an involuntary muscle tissue that is found only within this organ. The term "cardiac" (as in cardiology)
means "related to the heart” and comes from the Greek word kardia, for "heart." It has a four
chambered, double pump and is located in the thoracic cavity between the lungs.


The cardiac muscle is self-exciting, meaning it has its own conduction system. This is in contrast
with skeletal muscle, which requires either conscious or reflex nervous stimuli. The heart's rhythmic
contractions occur spontaneously, although the frequency or heart rate can be changed by nervous or
hormonal influence such as exercise or the perception of danger.


Myocardium
The myocardium is the muscular tissue of the heart. The myocardium is composed of specialized


cardiac muscle cells with an ability not possessed by muscle tissue elsewhere in the body. Cardiac
muscle, like other muscles, can contract, but it can also conduct electricity, like nerves. The blood to
the myocardium is supplied by the coronary arteries. If these arteries are occluded by atherosclerosis
and/or thrombosis, this can lead to angina pectoris or myocardial infarction due to ischemia (lack of
oxygen). Failure of the heart to contract properly (for various reasons) is termed heart failure, generally
leading to fluid retention, edema, pulmonary edema, renal insufficiency, hepatomegaly, a shortened life
expectancy and decreased quality of life


Wikibooks | 137




Chapter 8


Pericardium
The pericardium is the thick, membranous sac that surrounds the heart. It protects and lubricates


the heart. There are two layers to the pericardium: the fibrous pericardium and the serous pericardium.
The serous pericardium is divided into two layers; in between these two layers there is a space called
the pericardial cavity.


Epicardium
The layer next to the heart is the visceral layer, also known as the Epicardium. This is the inner


most layer and consists of connective tissue.


Heart Chambers
The heart has four chambers, two atrium and two ventricles. The atriums are smaller with thin


walls, while the ventricles are larger and much stronger.
Atrium
There are two atria on either side of the heart. On the right side is the atrium that holds blood that


needs oxygen. The left atrium holds that blood that has been oxygenated and is ready to be sent to the
body. The right atrium receives de-oxygenated blood from the superior vena cava and inferior vena
cava. The left atrium receives oxygenated blood from the left and right pulmonary veins.


Ventricles
The ventricle is a heart chamber which collects blood from an atrium and pumps it out of the heart.


There are two ventricles: the right ventricle pumps blood into the pulmonary circulation for the lungs,
and the left ventricle pumps blood into the systemic circulation for the rest of the body. Ventricles have
thicker walls than the atria, and thus can create the higher blood pressure. Comparing the left and right
ventricle, the left ventricle have thicker walls because it needs to pump blood to the whole body. This
leads to the common misconception that the heart lies on the left side of the body.


Septum
The interventricular septum (ventricular septum, or during development septum inferious) is the


stout wall separating the lower chambers (the ventricles) of the heart from one another. The ventricular
septum is directed backward and to the right, and is curved toward the right ventricle. The greater
portion of it is thick and muscular and constitutes the muscular ventricular septum. Its upper and
posterior part, which separates the aortic vestibule from the lower part of the right atrium and upper
part of the right ventricle, is thin and fibrous, and is termed the membranous ventricular septum.


Valves
The two atrioventricular (AV) valves are a one-way valve that ensures blood flows from the atria


138 | Human Physiology




The Cardiovascular System


to the ventricles, and not the other way. The two semilunar (SL) valves are present in the arteries
leaving the heart; they prevent blood from flowing back into the ventricles. The sound heard in a heart
beat is the heart valves shutting. The right AV valve is also called the tricuspid valve because it has
three flaps. It is located between the right atrium and the right ventricle. The tricuspid valve allows
blood to flow from the right atrium into the right ventricle when the heart is relaxed during diastole.
When the heart begins to contract, the heart enters a phase called systole, and the atrium pushes blood
into the ventricle. Then, the ventricle begins to contract and blood pressure inside the heart rises. When
the ventricular pressure exceeds the pressure in the atrium, the tricuspid valve snaps shut. The left AV
valve is also called the bicuspid valve because it has two flaps. It is also known as the mitral valve due
to the resemblance to a bishop's mitre (a type of hat). This valve prevents blood in the left ventricle
from flowing into the left atrium. As it is on the left side of the heart, it must cope with a lot of strain
and pressure; this is why it is made of only two cusps, as there is less to go wrong. There are two
remaining valves called the Semilunar Valves. They have flaps that resemble half moons. The
pulmonary semilunar valve sits between the right ventricle and the pulmonary trunk. The aortic
semilunar valve sits between the ventricle and the aorta.


Subvalvular Apparatus
The chordae tendinae are attached to papillary muscles that cause tension to better hold the valve.


Together, the papillary muscles and the chordae tendinae are known as the subvalvular apparatus. The
function of the subvalvular apparatus is to keep the valves from prolapsing into the atria when they
close. The subvalvular apparatus have no effect on the opening and closing of the valves. This is
caused entirely by the pressure gradient across the valve.


Complications With The Heart
The most common congenital abnormality of the heart is the bicuspid aortic valve. In this


condition, instead of three cusps, the aortic valve has two cusps. This condition is often undiagnosed
until the person develops calcific aortic stenosis. Aortic stenosis occurs in this condition usually in
patients in their 40s or 50s, an average of 10 years earlier than in people with normal aortic valves.
Another common complication of rheumatic fever is thickening and stenosis (partial blocking) of the
mitral valve. For patients who have had rheumatic fever dentist are advised to prophylactally
administer antibiotics prior to dental work to prevent bacterial endocarditis that occurs when bacteria
from the teeth enter the circulation and attach to damaged heart valves.


Passage of Blood Through the Heart
While it is convenient to describe the flow of the blood through the right side of the heart and then


through the left side, it is important to realize that both atria contract at the same time and that both
ventricles contract at the same time. The heart works as two pumps, one on the right and one on the left
that works simultaneously. The right pump pumps the blood to the lungs or the pulmonary circulation
at the same time that the left pump pumps blood to the rest of the body or the systemic circulation.
Venous blood from systemic circulation (deoxygenated) enters the right atrium through the superior
and inferior vena cava. The right atrium contracts and forces the blood through the tricuspid valve
(right atrioventricular valve) and into the right ventricles. The right ventricles contract and force the
blood through the pulmonary semilunar valve into the pulmonary trunk and out the pulmonary artery.


Wikibooks | 139




Chapter 8
This takes the blood to the lungs where the blood releases carbon dioxide and receives a new supply of
oxygen. The new blood is carried in the pulmonary veins that take it to the left atrium. The left atrium
then contracts and forces blood through the left atrioventricular, bicuspid, or mitral, valve into the left
ventricle. The left ventricle contracts forcing blood through the aortic semilunar valve into the
ascending aorta. It then branches to arteries carrying oxygen rich blood to all parts of the body.


Blood Flow After The Heart
Aorta-Arteries-Arterioles-Capillaries-Venules-Veins-Vena Cava


Blood Flow Through Capillaries
From the arterioles, the blood then enters one or more capillaries. The walls of capillaries are so


thin and fragile that blood cells can only pass in single file. Inside the capillaries, exchange of oxygen
and carbon dioxide takes place. Red blood cells inside the capillary releases their oxygen which passes
through the wall and into the surrounding tissue. The tissue then releases waste, such as carbon dioxide,
which then passes through the wall and into the red blood cells.


The Circulatory System
The circulatory system is extremely important in sustaining life. It’s proper functioning is


responsible for the delivery of oxygen and nutrients to all cells, as well as the removal of carbon
dioxide, waste products, maintenance of optimum pH, and the mobility of the elements, proteins and
cells, of the immune system. In developed countries, the two leading causes of death, myocardial
infarction and stroke are each direct results of an arterial system that has been slowly and progressively
compromised by years of deterioration.


Arteries
Arteries are muscular blood vessels that carry oxygenated blood away from the heart to the body.


The only exception being the pulmonary artery that carries deoxygenated blood to the lungs. Arteries
have a thick wall that consists of three layers. The inside layer is called the endothelium, the middle
layer is mostly smooth muscle and the outside layer is connective tissue. The artery walls are thick so
that when blood enters under pressure the walls can expand.


Arterioles
An arteriole is a small artery that extends and leads to capillaries. Arterioles have thick smooth


muscular walls. These smooth muscles are able to contract (causing vessel constriction) and relax
(causing vessel dilation). This contracting and relaxing affects blood pressure; the higher number of
vessels dilated, the lower blood pressure will be. Arterioles are just visible to the naked eye.


140 | Human Physiology




The Cardiovascular System


Capillaries
Capillaries are the smallest of a body’s vessels; they connect arteries and veins, and most closely


interact with tissues. They are very prevalent in the body; total surface area is about 6,300 square
meters. Because of this, no cell is ever very far from a capillary.


The walls of capillaries are composed of a single layer of cells, the endothelium. This layer is so
thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the
tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried
away for removal from the body.


The "capillary bed" is the network of capillaries present throughout the body. These beds are able
to be “opened” and “closed” at any given time, according to need. This process is called autoregulation
and capillary beds usually carry no more than 25% of the amount of blood it could hold at any time.
The more metabolically active the cells, the more capillaries it will require to supply nutrients.


Capillaries are the smallest of a body’s vessels; they connect arteries and veins, and most closely
interact with tissues. They are very prevalent in the body; total surface area is about 6,300 square
meters. Because of this, no cell is ever very far from a capillary. The walls of capillaries are composed
of a single layer of cells, the endothelium. This layer is so thin that molecules such as oxygen, water
and lipids can pass through them by diffusion and enter the tissues. Waste products such as carbon
dioxide and urea can diffuse back into the blood to be carried away for removal from the body. The
"capillary bed" is the network of capillaries present throughout the body. These beds are able to be
“opened” and “closed” at any given time, according to need. This process is called autoregulation and
capillary beds usually carry no more than 25% of the amount of blood it could hold at any time. The
more metabolically active the cells, the more capillaries it will require to supply nutrients. Capillaries
come in three types:


• Continuous: Continuous capillaries have a sealed epithelium and only allow small
molecules, water and ions to diffuse.


• Fenestrated: Fenestrated capillaries (as their name implies "fenster") have openings that
allow larger molecules to diffuse.


• Sinusoidal: Sinusoidal capillaries are special forms of fenestrated capillaries that have
larger opening allowing RBCs and serum proteins to enter.


Veins
Veins carry deoxygenated blood to the heart. The only exception to this is in the pulmonary vein


that carries oxygenated blood to the heart. Most of the blood volume is found in the venous system;
about 70% at any given time.The veins outer walls have the same three layers as the artery, differing
only because there is a lack of smooth muscle in the inner layer and less connective tissue on the outer
layer. Veins have low blood pressure compared to arteries and need the help of skeletal muscles to
bring blood back to the heart. Most veins have one-way valves called venous valves to prevent
backflow caused by gravity. They also have a thick collagen outer layer, which helps maintain blood
pressure and stop blood pooling. If a person is standing still for long periods or is bedridden, blood can
accumulates in veins and can cause varicose veins. The hollow internal cavity in which the blood flows
is called the lumen. A muscular layer allows veins to contract, which puts more blood into circulation.
Veins are used medically as points of access to the blood stream, permitting the withdrawal of blood


Wikibooks | 141




Chapter 8
specimens (venipuncture) for testing purposes, and enabling the infusion of fluid, electrolytes,
nutrition, and medications (intravenous delivery).


Venules
A venule is a small vein that allows deoxygenated blood to return from the capillary beds to the


larger blood veins. Venules have three layers; they have the same makeup as arteries with less smooth
muscle, making them thinner.


The Cardiovascular Pathways
The double circulatory system of blood flow refers to the separate systems of pulmonary


circulation and the systemic circulation in amphibians, birds and mammals (including humans.) In
contrast, fishes have a single circulation system. For instance, the adult human heart consists of two
separated pumps, the right side with the right atrium and ventricle (which pumps deoxygenated blood
into the pulmonary circulation), and the left side with the left atrium and ventricle (which pumps
oxygenated blood into the systemic circulation). Blood in one circuit has to go through the heart to
enter the other circuit. Blood circulates through the body two to three times every minute. In one day,
the blood travels a total of 19,000 km (12,000 miles), or four times the distance across the U.S. from
coast to coast.


The Pulmonary Circuit
In the pulmonary circuit, blood is pumped to the lungs from the right ventricle of the heart. It is


carried to the lungs via pulmonary arteries. At lungs, oxygen in the alveolae diffuses to the capillaries
surrounding the alveolae and carbon dioxide inside the blood diffuses to the alveolae. As a result, blood
is oxygenated which is then carried to the heart's left half -to the left atrium via pulmonary veins.
Oxygen rich blood is prepared for the whole organs and tissues of the body. This is important because
mitochondria inside the cells should use oxygen to produce energy from the organic compounds.


The Systemic Circuit
The systemic circuit supplies oxygenated blood to the organ system. Oxygenated blood from the


lungs is returned to the left atrium, then the ventricle contracts and pumps blood into the aorta.
Systemic arteries split from the aorta and direct blood into the capillaries. Cells consume the oxygen
and nutrients and add carbon dioxide, wastes, enzymes and hormones. The veins drain the
deoxygenated blood from the capillaries and return the blood to the right atrium.


Aorta
The aorta is the largest of the arteries in the systemic circuit. The blood is pumped from the left


ventricle into the aorta and from there it branches to all parts of the body. The aorta is an elastic artery,
and as such is able to distend. When the left ventricle contracts to force blood into the aorta, the aorta
expands. This stretching gives the potential energy that will help maintain blood pressure during


142 | Human Physiology




The Cardiovascular System


diastole, as during this time the aorta contracts passively.


Superior Venae Cavae
The superior vena cava (SVC) is a large but short vein that carries de-oxygenated blood from the


upper half of the body to the heart's right atrium. It is formed by the left and right brachiocephalic veins
(also referred to as the innominate veins) which receive blood from the upper limbs and the head and
neck. The azygous vein (which receives blood from the ribcage) joins it just before it enters the right
atrium.


Inferior Venae Cavae
The inferior vena cava (or IVC) is a large vein that carries de-oxygenated blood from the lower


half of the body into the heart. It is formed by the left and right common iliac veins and transports
blood to the right atrium of the heart. It is posterior to the abdominal cavity, and runs along side of the
vertebral column on its right side.


Coronary Arteries
Heart showing the Coronary Arteries The coronary circulation consists of the blood vessels that


supply blood to, and remove blood from, the heart muscle itself. Although blood fills the chambers of
the heart, the muscle tissue of the heart, or myocardium, is so thick that it requires coronary blood
vessels to deliver blood deep into the myocardium. The vessels that supply blood high in oxygen to the
myocardium are known as coronary arteries. The vessels that remove the deoxygenated blood from the
heart muscle are known as cardiac veins. The coronary arteries that run on the surface of the heart are
called epicardial coronary arteries. These arteries, when healthy, are capable of auto regulation to
maintain coronary blood flow at levels appropriate to the needs of the heart muscle. These relatively
narrow vessels are commonly affected by atherosclerosis and can become blocked, causing angina or a
heart attack. The coronary arteries are classified as "end circulation", since they represent the only
source of blood supply to the myocardium: there is very little redundant blood supply, which is why
blockage of these vessels can be so critical. In general there are two main coronary arteries, the left and
right.


• Right coronary artery
• Left coronary artery


Both of these arteries originate from the beginning (root) of the aorta, immediately above the aortic
valve. As discussed below, the left coronary artery originates from the left aortic sinus, while the right
coronary artery originates from the right aortic sinus. Four percent of people have a third, the posterior
coronary artery. In rare cases, a patient will have one coronary artery that runs around the root of the
aorta.


Hepatic Veins
In human anatomy, the hepatic veins are the blood vessels that drain de-oxygenated blood from the


Wikibooks | 143




Chapter 8
liver and blood cleaned by the liver (from the stomach, pancreas, small intestine and colon) into the
inferior vena cava.


They arise from the substance of the liver, more specifically the central vein of the liver lobule.
They can be differentiated into two groups, the upper group and lower group.


The upper group of three typically arises from the posterior aspect of the liver and drain the
quadrate lobe and left lobe. The lower group rise from the right lobe and caudate lobe, are variable in
number, and are typically smaller than those in the upper group. None of the hepatic veins have valves.


Cardiac Cycle
Cardiac cycle is the term used to describe the relaxation and contraction that occur, as a heart


works to pump blood through the body. Heart rate is a term used to describe the frequency of the
cardiac cycle. It is considered one of the four vital signs. Usually it is calculated as the number of
contractions (heart beats) of the heart in one minute and expressed as "beats per minute" (bpm). When
resting, the adult human heart beats at about 70 bpm (males) and 75 bpm (females), but this rate varies
between people. However, the reference range is nominally between 60 bpm (if less termed
bradycardia) and 100 bpm (if greater, termed tachycardia). Resting heart rates can be significantly
lower in athletes, and significantly higher in the obese.


The body can increase the heart rate in response to a wide variety of conditions in order to increase
the cardiac output (the amount of blood ejected by the heart per unit time). Exercise, environmental
stressors or psychological stress can cause the heart rate to increase above the resting rate.


The pulse is the most straightforward way of measuring the heart rate, but it can be deceptive when
some strokes do not lead to much cardiac output. In these cases (as happens in some arrhythmias), the
heart rate may be considerably higher than the pulse. Every single 'beat' of the heart involves three
major stages: atrial systole, ventricular systole and complete cardiac diastole. Throughout the cardiac
cycle, the blood pressure increases and decreases. As ventricles contract the pressure rise, causing the
AV valves to slam shut.


Systole
The heart in the systole phase. Systole, or contraction, of the heart is initiated by the electrical cells


of the sinoatrial node, which is the heart's natural pacemaker. These cells are activated spontaneously
by depolarization of their membranes beyond a certain threshold for excitation. At this point, voltage-
gated calcium channels on the cell membrane open and allow calcium ions to pass through, into the
sarcoplasm, or interior, of the muscle cell. Some calcium ions bind to receptors on the sarcoplasmic
reticulum causing an influx of calcium ions into the sarcoplasm. The calcium ions bind to the troponin,
causing a conformation change, breaking the bond between the protein tropomyosin, to which the
troponin is attached, and the myosin binding sites. This allows the myosin heads to bind to the myosin
binding sites on the actin protein filament and contraction results as the myosin heads draw the actin
filaments along, are bound by ATP, causing them to release the actin, and return to their original
position, breaking down the ATP into ADP and a phosphate group. The action potential spreads via the
passage of sodium ions through the gap junctions that connect the sarcoplasm of adjacent myocardial
cells.


144 | Human Physiology




The Cardiovascular System


Norepinephrine (noradrenaline) is released by the terminal boutons of depolarized sympathetic
fibers, at the sinoatrial and atrioventricular nodes. Norepinephrine diffuses across the synaptic cleft
binds to the β1-adrenoreceptors – G-protein linked receptors, consisting of seven transmembrane
domains – shifting their equilibrium towards the active state. The receptor changes its conformation
and mechanically activates the G-protein which is released. The G-protein is involved in the production
of cyclic adenyl monophosphate (camp) from adenosine triphosphate (ATP) and this in turn activates
the protein kinase (β-adrenoreceptor kinase). β-adrenoreceptor kinase phosphorylates the calcium ion
channels in the sarcolema, so that calcium ion influx is increased when they are activated by the
appropriate transmembrane voltage. This will of course, cause more of the calcium receptors in the
sarcoplasmic reticulum to be activated, creating a larger flow of calcium ions into the sarcoplasm.
More troponin will be bound and more myosin binding sites cleared [of tropomyosin] so that more
myosin heads can be recruited for the contraction and a greater force and speed of contraction results.
[Phosphodiesterase catalyses the decomposition of cAMP to AMP so that it is no longer able to activate
the protein kinase. AMP will of course, go on to be phosphorylated to ATP and may be recycled.]
Noradrenaline also affects the atrioventricular node, reducing the delay before continuing conduction
of the action potential via the bundle of HIS.


Diastole
The heart in the diastole phase. Cardiac Diastole is the period of time when the heart relaxes after


contraction in preparation for refilling with circulating blood. Ventricular diastole is when the
ventricles are relaxing, while atrial diastole is when the atria are relaxing. Together they are known as
complete cardiac diastole. During ventricular diastole, the pressure in the (left and right) ventricles
drops from the peak that it reaches in systole. When the pressure in the left ventricle drops to below the
pressure in the left atrium, the mitral valve opens, and the left ventricle fills with blood that was
accumulating in the left atrium. Likewise, when the pressure in the right ventricle drops below that in
the right atrium, the tricuspid valve opens and the right ventricle fills with blood that was in the right
atrium


"Lub-Dub"
The first heart tone, or S1, "Lub" is caused by the closure of the atrioventricular valves, mitral and


tricuspid, at the beginning of ventricular contraction, or systole. When the pressure in the ventricles
rises above the pressure in the atria, these valves close to prevent regurgitation of blood from the
ventricles into the atria. The second heart tone, or S2 (A2 and P2), "Dub" is caused by the closure of
the aortic valve and pulmonic valve at the end of ventricular systole. As the left ventricle empties, its
pressure falls below the pressure in the aorta, and the aortic valve closes. Similarly, as the pressure in
the right ventricle falls below the pressure in the pulmonary artery, the pulmonic valve closes. During
inspiration, negative intrathoracic pressure causes increased blood return into the right side of the heart.
The increased blood volume in the right ventricle causes the pulmonic valve to stay open longer during
ventricular systole. This causes an increased delay in the P2 component of S2. During expiration, the
positive intrathoracic pressure causes decreased blood return to the right side of the heart. The reduced
volume in the right ventricle allows the pulmonic valve to close earlier at the end of ventricular systole,
causing P2 to occur earlier, and "closer" to A2. It is physiological to hear the splitting of the second
heart tone by younger people and during inspiration. During expiration normally the interval between
the two components shortens and the tone becomes merged.


Wikibooks | 145




Chapter 8


The Heart's Electrical Conduction System
The heart is primarily made up of muscle tissue. A network of nerve fibers coordinates the


contraction and relaxation of the cardiac muscle tissue to obtain an efficient, wave-like pumping action
of the heart


Control of Heartbeat
The heart contains two cardiac pacemakers that spontaneously cause the heart to beat. These can


be controlled by the autonomic nervous system and circulating adrenaline. If the cardiac muscles just
contracted and relaxed randomly at a natural rhythm the cycle would become disordered and the heart
would become unable to carry on its function of being a pump. Sometimes when the heart undergoes
great damage to one part of the cardiac muscle or the person incurs an electric shock, the cardiac cycle
can become uncoordinated and chaotic. Some parts of the heart will contract whilst others will relax so
that instead of contracting and relaxing as a whole, the heart will flutter abnormally. This is called
fibrillation and can be fatal if not treated within 1 minute.


SA Node
The sinoatrial node (abbreviated SA node or SAN, also called the sinus node) is the impulse generating
(pacemaker) tissue located in the right atrium of the heart. Although all of the heart's cells possess the
ability to generate the electrical impulses (or action potentials) that trigger cardiac contraction, the
sinoatrial node is what normally initiates it, simply because it generates impulses slightly faster than the
other areas with pacemaker potential. Because cardiac myocytes, like all nerve cells, have refractory
periods following contraction during which additional contractions cannot be triggered, their
pacemaker potential is overridden by the sinoatrial node. The SA node emits a new impulse before
either the AV or purkinje fibers reach threshold. The sinoatrial node (SA node) is a group of cells
positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are
modified cardiac myocytes. They possess some contractile filaments, though they do not contract. Cells
in the SA node will naturally discharge (create action potentials) at about 70-80 times/minute. Because
the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the
primary pacemaker. If the SA node doesn't function, or the impulse generated in the SA node is
blocked before it travels down the electrical conduction system, a group of cells further down the heart
will become the heart's pacemaker. These cells form the atrioventricular node (AV node), which is an
area between the right atrium and ventricle, within the atrial septum. The impulses from the AV node
will maintain a slower heart rate (about 40-60 beats per a minute). When there is a pathology in the AV
node or purkinje fibers, an ectopic pacemaker can occur in different parts of the heart. The ectopic
pacemaker typically discharges faster than the SA node and causes an abnormal sequence of
contraction. The SA node is richly innervated by vagal and sympathetic fibers. This makes the SA node
susceptible to autonomic influences. Stimulation of the vagus nerve causes decrease in the SA node
rate (thereby causing decrease in the heart rate). Stimulation via sympathetic fibers causes increase in
the SA node rate (thereby increasing the heart rate). The sympathetic nerves are distributed to all parts
of the heart, especially in ventricular muscles. The parasympathetic nerves mainly control SA and AV
nodes, some atrial muscle and ventricular muscle. Parasympathetic stimulation from the vagal nerves
decreases the rate of the AV node by causing the release of acetylcholine at vagal endings which in turn
increases the K+ permeability of the cardiac muscle fiber. Vagal stimulation can block transmission
through AV junction or stop SA node contraction which is called "ventricular escape." When this
happens, the purkinje fibers in the AV bundle develops a rhythm of their own. In the majority of
patients, the SA node receives blood from the right coronary artery, meaning that a myocardial


146 | Human Physiology




The Cardiovascular System


infarction occluding it will cause ischemia in the SA node unless there is a sufficiently good
anastomosis from the left coronary artery. If not, death of the affected cells will stop the SA node from
triggering the heartbeat


AV Node
The atrioventricular node (abbreviated AV node) is the tissue between the atria and the ventricles of the
heart, which conducts the normal electrical impulse from the atria to the ventricles. The AV node
receives two inputs from the atria: posteriorly via the crista terminalis, and anteriorly via the interatrial
septum. [1] An important property that is unique to the AV node is decremental conduction. This is the
property of the AV node that prevents rapid conduction to the ventricle in cases of rapid atrial rhythms,
such as atrial fibrillation or atrial flutter. The atrioventricular node delays impulses for 0.1 second
before spreading to the ventricle walls. The reason it is so important to delay the cardiac impulse is to
ensure that the atria are empty completely before the ventricles contract (Campbell et al, 2002). The
blood supply of the AV node is from a branch of the right coronary artery in 85% to 90% of
individuals, and from a branch of the left circumflex artery in 10% to 15% of individuals. In certain
types of supraventricular tachycardia, a person could have two AV Nodes; this will cause a loop in
electrical current and uncontrollably-rapid heart beat. When this electricity catches up with itself, it will
dissipate and return to normal heart-beat speed.


AV Bundle
The bundle of HIS is a collection of heart muscle cells specialized for electrical conduction that
transmits the electrical impulses from the AV node (located between the atria and the ventricles) to the
point of the apex of the fascicular branches. The fascicular branches then lead to the Purkinje fibers
which innervate the ventricles, causing the cardiac muscle of the ventricles to contract at a paced
interval. These specialized muscle fibers in the heart were named after the Swiss cardiologist Wilhelm
His, Jr., who discovered them in 1893. Cardiac muscle is very specialized, as it is the only type of
muscle that has an internal rhythm; i.e., it is myogenic which means that it can naturally contract and
relax without receiving electrical impulses from nerves. When a cell of cardiac muscle is placed next to
another, they will beat in unison. The fibers of the Bundle of HIS allow electrical conduction to occur
more easily and quickly than typical cardiac muscle. They are an important part of the electrical
conduction system of the heart as they transmit the impulse from the AV node (the ventricular
pacemaker) to the rest of the heart. The bundle of HIS branches into the three bundle branches: the
right left anterior and left posterior bundle branches that run along the intraventricular septum. The
bundles give rise to thin filaments known as Purkinje fibers. These fibers distribute the impulse to the
ventricular muscle. Together, the bundle branches and purkinje network comprise the ventricular
conduction system. It takes about 0.03-0.04s for the impulse to travel from the bundle of HIS to the
ventricular muscle. It is extremely important for these nodes to exist as they ensure the correct control
and co-ordination of the heart and cardiac cycle and make sure all the contractions remain within the
correct sequence and in sync.


Purkinje Fibers
Purkinje fibers (or Purkyne tissue) are located in the inner ventricular walls of the heart, just beneath
the endocardium. These fibers are specialized myocardial fibers that conduct an electrical stimulus or
impulse that enables the heart to contract in a coordinated fashion. Purkinje fibers work with the
sinoatrial node (SA node) and the atrioventricular node (AV node) to control the heart rate. During the
ventricular contraction portion of the cardiac cycle, the Purkinje fibers carry the contraction impulse
from the left and right bundle branches to the myocardium of the ventricles. This causes the muscle
tissue of the ventricles to contract and force blood out of the heart — either to the pulmonary
circulation (from the right ventricle) or to the systemic circulation (from the left ventricle). They were


Wikibooks | 147




Chapter 8
discovered in 1839 by Jan Evangelista Purkinje, who gave them his name.


Pacemaker
The contractions of the heart are controlled by electrical impulses, these fire at a rate which controls the
beat of the heart. The cells that create these rhythmical impulses are called pacemaker cells, and they
directly control the heart rate. Artificial devices also called pacemakers can be used after damage to the
body's intrinsic conduction system to produce these impulses synthetically.


Fibrillation
Fibrillation is when the heart flutters abnormally. This can be detected by an electrocardiogram


which measures the waves of excitation passing through the heart and plotting a graph of potential
difference (voltage) against time. If the heart and cardiac cycle is functioning properly the
electrocardiogram shows a regular, repeating pattern. However if there is fibrillation there will be no
apparent pattern. In a hospital the monitor would make a sound and alert the doctors to treat the
fibrillation by passing a huge current through the chest wall and shocking the heart out of its
fibrillation. This causes the cardiac muscle to stop completely for 5 seconds and when it begins to beat
again the cardiac cycle would have resumed to normal and the heart will be beating in a controlled
manner again. Fibrillation is an example of "circus movement" of impulses through the heart muscle.


Circus movement occurs when an impulse begins in one part of the heart muscle and spreads in a
circuitous pathway through the heart then returns to the originally excited muscle and "re-enters" it to
stimulate it once more. The signal never stops. A cause of circus movement is long length pathway in
which the muscle is no longer in a refractatory state when the stimulus returns to it. A "flutter" is a
circus movement in coordinated, low frequency waves that cause rapid heart rate. If the Bundle of HIS
is blocked, it will result in dissociation between the activity of the atria and that of the ventricles,
otherwise called a third degree heart block. The other cause of a third degree block would be a block of
the right, left anterior, and left posterior bundle branches. A third degree block is very serious medical
condition that will most likely require an artificial pacemaker.


The EKG
Also know as the Electrocardiogram. Cardiac electrophysiology is the science of the mechanisms,


functions, and performance of the electrical activities of specific regions of the heart. The EKG is the
recording of the heart's electrical activity as a graph. The graph can show the heart's rate and rhythm, it
can detect enlargement of the heart, decreased blood flow, or the presence of current or past heart
attacks. EKG's are inexpensive, Non-invasive, quick, and painless. Depending of the results, the
patient’s medical history, and a physical exam; further tests or a combination of medications and
lifestyle changes may be ordered.


How To Read An EKG
EKG Waveform


148 | Human Physiology




The Cardiovascular System


P
P wave- indicates that the
atria are electrically
stimulated to pump blood
into the ventricles.


QRS
QRS complex- indicates that
the ventricles are electrically
stimulated to pump blood
out.


ST
ST segment- indicates the
amount of time from the end
of the contraction of the
ventricles to the beginning of
the T wave.


T
T wave- indicates the
recovery period of the
ventricles.


Cardiac Muscle Contraction
After an action potential excites the plasma membrane of the cardiac muscle cell the contraction is


due to an increase in the cytoplasmic concentration of Calcium ions. Similar to skeletal muscle, the
release of Ca+ ions from the sarcoplasmic reticulum binds to troponin which allows actin to bind with
myosin. The difference between skeletal muscle and cardiac muscle is that when the action potential
opens voltage gated calcium ion channels in the T-tubules. The increase in cytosolic calcium causes
calcium ions to bind to receptors on the surface of the sarcoplasmic reticulum. The binding of calcium
ions to these receptors causes the opening of more calcium ion channels in the SR membrane. Calcium
ions then rush out of the SR and bind to troponin and allow the myosin and actin to bind together which
causes contraction. This sequence is called calcium-induced calcium release. Contraction ends when
the level of cytosolic calcium returns to normal resting levels.


Blood Pressure
Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless


indicated otherwise, blood pressure refers to systemic arterial blood pressure, i.e., the pressure in the
large arteries delivering blood to body parts other than the lungs, such as the brachial artery (in the
arm). The pressure of the blood in other vessels is lower than the arterial pressure. Blood pressure
values are universally stated in millimeters of mercury (mm Hg). The systolic pressure is defined as the
peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the lowest pressure (at
the resting phase of the cardiac cycle). The mean arterial pressure and pulse pressure are other
important quantities. Typical values for a resting, healthy adult are approximately 120 mm Hg systolic
and 80 mm Hg diastolic (written as 120/80 mm Hg), with large individual variations. These measures
of blood pressure are not static, but undergo natural variations from one heartbeat to another or
throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors,


Wikibooks | 149




Chapter 8
drugs, or disease.


Systolic Pressure
Systolic Pressure is the highest when the blood is being pumped out of the left ventricle into the


aorta during ventricular systole. The average high during systole is 120 mm Hg.


Diastolic Pressure
Diastolic blood pressure lowers steadily lowers to a low of 80 mm Hg during ventricular diastole.


Cardiovascular Disease
Cardiovascular disease refers to the class of diseases that involve the heart and/or blood vessels


(arteries and veins). While the term technically refers to any disease that affects the cardiovascular
system, it is usually used to refer to those related to atherosclerosis (arterial disease). These conditions
have similar causes, mechanisms, and treatments. Over 50 million Americans have cardiovascular
problems, and most other Western countries face high and increasing rates of cardiovascular disease. It
is the number 1 cause of death and disability in the United States and most European countries. By the
time that heart problems are detected, the underlying cause (atherosclerosis) is usually quite advanced,
having progressed for decades. There is therefore increased emphasis on preventing atherosclerosis by
modifying risk factors, such as healthy eating, exercise and avoidance of smoking.


Hypertension
Hypertension or high blood pressure is a medical condition wherein the blood pressure is


chronically elevated. Persistent hypertension is one of the risk factors for strokes, heart attacks, heart
failure and arterial aneurysm, and is a leading cause of chronic renal failure


Atherosclerosis
Atherosclerosis is a disease affecting the arterial blood vessel. It is commonly referred to as a


"hardening" or "furring" of the arteries. It is caused by the formation of multiple plaques within the
arteries. Arteriosclerosis ("hardening of the artery") results from a deposition of tough, rigid collagen
inside the vessel wall and around the atheroma. This increases the stiffness, decreases the elasticity of
the artery wall. Atherosclerosis typically begins in early adolescence, is usually found in most major
arteries, and yet is asymptomatic and not detected by most diagnostic methods during life. It most
commonly becomes seriously symptomatic when interfering with the coronary circulation supplying
the heart or cerebral circulation supplying the brain, and is considered the most important underlying
cause of strokes, heart attacks, various heart diseases including congestive heart failure and most
cardiovascular diseases in general.


Plaque
Plaque Atheroma or commonly known as plaque is an abnormal inflammatory accumulation of


macrophage white blood cells within the walls of arteries.


150 | Human Physiology




The Cardiovascular System


Circulatory Shock
Circulatory Shock is a severe condition that results from reduced blood circulation.
Thrombus
A thrombus, or blood clot, is the final product of the blood coagulation step in hemostasis. It is


achieved via the aggregation of platelets that form a platelet plug, and the activation of the humoral
coagulation system (i.e. clotting factors). A thrombus is physiologic in cases of injury, but pathologic
in case of thrombosis.


Preventing blood clots reduces the risk of stroke, heart attack and pulmonary embolism. Heparin
and warfarin are often used to inhibit the formation and growth of existing blood clots, thereby
allowing the body to shrink and dissolve the blood clots through normal methods.


Embolism
An embolism occurs when an object (the embolus) migrates from one part of the body (through


circulation) and causes a blockage (occlusion) of a blood vessel in another part of the body. Blood clots
form the most common embolic material by far: other possible embolic materials include fat globules
(a fat embolism), air bubbles (an air embolism), septic emboli (containing pus and bacteria), or
amniotic fluid.


Stroke
A stroke, also known as cerebrovascular accident (CVA), is an acute neurological injury whereby


the blood supply to a part of the brain is interrupted. Strokes can be classified into two major
categories: ischemic and hemorrhagic. ~80% of strokes are due to ischemia.


• Ischemic Stroke: In ischemic stroke, which occurs in approximately 85-90% of strokes, a
blood vessel becomes occluded and the blood supply to part of the brain is totally or partially
blocked. Ischemic stroke is commonly divided into thrombotic stroke, embolic stroke, systemic
hypoperfusion (Watershed or Border Zone stroke), or venous thrombosis


• Hemorrhagic Stroke: A hemorrhagic stroke, or cerebral hemorrhage, is a form of stroke
that occurs when a blood vessel in the brain ruptures or bleeds. Like ischemic strokes,
hemorrhagic strokes interrupt the brain's blood supply because the bleeding vessel can no longer
carry the blood to its target tissue. In addition, blood irritates brain tissue, disrupting the delicate
chemical balance, and, if the bleeding continues, it can cause increased intracranial pressure
which physically impinges on brain tissue and restricts blood flow into the brain. In this respect,
hemorrhagic strokes are more dangerous than their more common counterpart, ischemic strokes.
There are two types of hemorrhagic stroke: intracerebral hemorrhage, and subarachnoid
hemorrhage.


The term "brain attack" is starting to come into use in the United States for stroke, just as the term
"heart attack" is used for myocardial infarction, where a cutoff of blood causes necrosis to the tissue of
the heart. Many hospitals have "brain attack" teams within their neurology departments specifically for
swift treatment of stroke.


Heart Attack


Wikibooks | 151




Chapter 8
Acute myocardial infarction (AMI or MI), commonly known as a heart attack, A heart attack


occurs when the supply of blood and oxygen to an area of heart muscle is blocked, usually by a clot in
a coronary artery. Often, this blockage leads to arrhythmias (irregular heartbeat or rhythm) that cause a
severe decrease in the pumping function of the heart and may bring about sudden death. If the blockage
is not treated within a few hours, the affected heart muscle will die and be replaced by scar tissue. It is
the leading cause of death for both men and women all over the world


Angina Pectoris
Angina Pectoris is chest pain due to ischemia (a lack of blood and hence oxygen supply) of the


heart muscle, generally due to obstruction or spasm of the coronary arteries (the heart's blood vessels).
Coronary Bypass
Coronary artery bypass surgery, coronary artery bypass graft surgery and heart bypass are surgical


procedures performed on patients with coronary artery disease for the relief of angina and possible
improved heart muscle function. Veins or arteries from elsewhere in the patient's body are grafted from
the aorta to the coronary arteries, bypassing coronary artery narrowing caused by atherosclerosis and
improves the blood supply to the myocardium (heart muscle).


Congestive Heart Failure
Congestive heart failure (CHF), also called congestive cardiac failure (CCF) or just heart failure, is


a condition that can result from any structural or functional cardiac disorder that impairs the ability of
the heart to fill with or pump a sufficient amount of blood throughout the body. It is not to be confused
with "cessation of heartbeat", which is known as asystole, or with cardiac arrest, which is the cessation
of normal cardiac function in the face of heart disease. Because not all patients have volume overload
at the time of initial or subsequent evaluation, the term "heart failure" is preferred over the older term
"congestive heart failure". Congestive heart failure is often undiagnosed due to a lack of a universally
agreed definition and difficulties in diagnosis, particularly when the condition is considered "mild".


Aneurysm
An aneurysm (or aneurism) is a localized dilation or ballooning of a blood vessel by more than


50% of the diameter of the vessel and can lead to instant death at anytime. Aneurysms most commonly
occur in arteries at the base of the brain (the circle of Willis) and in the aorta (the main artery coming
out of the heart) - this is an aortic aneurysm. This bulge in a blood vessel, much like a bulge on an
over-inflated inner tube, can lead to death at anytime. The larger an aneurysm becomes, the more likely
it is to burst. Aneurysms are also described according to their shape: Saccular or fusiform. A saccular
aneurysm resembles a small sack; a fusiform aneurysm is shaped like a spindle.


Dissolving Blood Clots
To dissolve blood clots you would use a drug that converts plasminogen (molecule found in


blood), to plasmin, (enzyme that dissolves blood clots).
Clearing Clogged Arteries
One way to unblock a coronary artery (or other blood vessel) is percutaneous transluminal


152 | Human Physiology




The Cardiovascular System


coronary angioplasty (PTCA), which was first performed in 1977. A wire is passed from the femoral
artery in the leg or the radial artery in the arm up to the diseased coronary artery, to beyond the area of
the coronary artery that is being worked upon. Over this wire, a balloon catheter is passed into the
segment that is to be opened up. The end of the catheter contains a small folded balloon. When the
balloon is hydraulically inflated, it compresses the atheromatous plaque and stretches the artery wall to
expand. At the same time, if an expandable wire mesh tube (stent) was on the balloon, then the stent
will be implanted (left behind) to support the new stretched open position of the artery from the inside.


Dilated and Inflamed Veins
Varicose Veins
Varicose veins are veins on the leg which are large, twisted, and ropelike, and can cause pain,


swelling, or itching. They are an extreme form of telangiectasia, or spider veins. Varicose veins result
due to insufficiency of the valves in the communicating veins. These are veins which link the
superficial and deep veins of the lower limb. Normally, blood flows from the superficial to the deep
veins, facilitating return of blood to the heart. However, when the valve becomes defective, blood is
forced into the superficial veins by the action of the muscle pump (which normally aids return of blood
to the heart by compressing the deep veins). People who have varicose veins are more at risk of getting
a Deep Vein Thrombosis (DVT) and pulmonary embolisms.


Phlebitis
Phlebitis is an inflammation of a vein, usually in the legs. This is usually the most serious if found


in a deep vein. However, most people with the condition, perhaps 80 to 90 percent, are women. The
disease may also have a genetic component, as it is known to run in families.


Congenital Heart Defects
Heart defects present at birth are called congenital heart defects. Slightly less than 1% of all


newborn infants have congenital heart disease. Eight defects are more common than all others and
make up 80% of all congenital heart diseases, whereas the remaining 20% consist of many
independently infrequent conditions or combinations of several defects.


Acyanotic Defects
Acyanotic heart defects are those in which there is a normal amount of oxygen in the bloodstream.


The most common congenital heart defect is a ventral septal defect, which occurs in about 20% of all
children with congenital heart disease. In VSD blood from the left ventricle is shunted to the right
ventricle, resulting in oxygenated blood returning into pulmonic circulation. One of the potential
problems of VSD is pulmonary hypertension.


Cyanotic Defects
Cyanotic heart defects refer to defects that result in decreased amounts of oxygen in the blood. In


Wikibooks | 153




Chapter 8
cyanotic heart defects deoxygenated blood from the right ventricle flows into the systemic circulation.
Cyanotic defects include tetrogy of fallot and transposition of the great arteries.


Homeostasis
Homeostasis in the body is only possible if the cardiovascular system is working properly. This


means that the system needs to deliver oxygen and nutrients to the tissue fluid that surrounds the cells
and also take away the metabolic waste. The heart is composed of arteries that take blood from the
heart, and vessels that return blood to the heart. Blood is pumped by the heart into two circuits: the
pulmonary and systemic circuits. The pulmonary circuit carries blood through the lungs where gas
exchange occurs and the systemic system transports blood to all parts of the body where exchange with
tissue fluid takes place. The cardiovascular system works together with all other systems to maintain
homeostasis.


The Lymphatic System
The lymphatic system is closely related to the cardiovascular system. There are three main ways


that they work together to maintain homeostasis: the lymphatic system receives the excess tissue fluid
and returns it to the bloodstream, lacteals take fat molecules from the intestinal villi and transport them
to the bloodstream and both systems work together to defend the body against disease.


Interesting Facts
• Heart Disease is the number one killer in American women.
• 16.7 million deaths are result forms of cardiovascular disease, heart disease and stroke.
• Stress, birth control pills and alcohol are just some risk factors of developing heart disease.
• Recent research suggests that taking a small dose of aspirin daily may help prevent a heart attack
(because aspirin inhibits platelet clumping).
• The length of all your blood vessels lined up is about 60,000 miles long! Farther than the distance
between California and Asia which is about 13,000 miles! blah


Ways To A Healthy Heart
• Eating healthy, good nutrition
• Fitness and Exercise
• Having a healthy lifestyle, don't smoke
• Watch cholesterol and blood pressure
• Reduce the fat, sodium, and calories in your diet.
• The total length of capillaries in an average adult human is approximately 25,000 mi (42,000
km).


Aging
The heart muscle becomes less efficient with age, and there is a decrease in both maximum cardiac


output and heart rate, although resting levels may be more than adequate. The health of the


154 | Human Physiology




The Cardiovascular System


myocardium depends on its blood supply, and with age there is greater likelihood that arthrosclerosis
will narrow the coronary arteries. Arthrosclerosis is the deposition of cholesterol on and in the walls of
the arteries, which decreases blood flow and forms rough surfaces that may cause intravascular clot
formation High blood pressure (hypertension) causes the left ventricle to work harder. It may enlarge
and outgrow its blood supply, thus becoming weaker. A weak ventricle is not an efficient pump, and
may progress to congestive heart failure. This process may be slow or rapid. The heart valves may
become thickened by fibrosis, leading to heart murmurs and less efficient pumping. Arrhythmias are
also more common with age, as the cells of the conduction pathway become less efficient.


Shock
Physiological stress can be any kind of injury form burns, to broken bones; the body's response to


stress is categorized in two phases the ebb phase (early phase) begins immediately after the injury. And
the second phase is about 36 to 48 hours after injury is called the flow phase. In the ebb (shock) phase
there is Inadequate circulation, decreased insulin level, decreased oxygen consumption, hypothermia
(low body temperature), hypovolemia (low blood volume), and hypotension (low blood pressure). In
the flow phase there is increased levels of catecholamine, glucocorticoids, and glucagons, normal or
elevated insulin levels, catabolic (breakdown), hyperglycemic (high blood sugar), increased oxygen
consumption/respiratory rate, hyperthermia (high body temperature) fever sets in, hypermetabolism,
increased insulin resistance, increased cardiac output.


Premature ventricular contractions (PVC's)
Excitation occurs through the SA node to the AV node if there are abnormalities or drug


interference that malfunctions the AV node the ventricles will not receive the initiating stimuli and the
autorhythmic cells in the bundle branches begin to initiate actions on their own rate becoming the
pacemakers for the ventricles. This in turn will cause conduction disorder. With conduction that causes
problems with the bundle branches there is the right and the left premature ventricular contractions.
Right is most common and may go untreated. Left is always a serious problem and must be treated.


Intrinsic Control of heartbeat
• SA node (located in the right atrium near the entrance of the superior vena cava)
• AV node (located at the base of right atrium)
• AV bundle (located in the intraventricular septum between the two ventricles that go in two directions
right and left bundle branches that leave the septum to enter the walls of both ventricle)
• Bundle Branches (the branching off the septum to the walls of the ventricles that run into the purkinje
fibers that then make contact with ventricular myocardial cells to spread the impulse to the rest of the
ventricles)


Electrocardiogram
• The P is the atrial depolarization


Wikibooks | 155




Chapter 8
• QRS is the ventricular depolarization
• T is the ventricular repolarization


Extrinsic Control of Heartbeat
Autonomic system with two subdivisions: the sympathetic division and the parasympathetic


division. Hormonal control of blood pressure
• Epinephrine
• Norepinephrine
• ANP : Atrial natriuretic peptide
• ADH: Antidiuretic hormone
• Renin-Angiotension system


Case Study
An example of the ever expanding technology for the heart is best described in this story:
In 1955, when I was five years old, I first learned by my family physician that I had a heart


murmur and that it would eventually need attention. By the time I was 15 in 1965, I had two cardiac
catherizations at Rhode Island Hospital. The tests were inconclusive and I was told to go on with my
life and wait and see if I had a problem.


It wasn't until 1975 that I was told by my family physician that I should have my heart checked
again. Dr. David Kitzes of Mariam Hospital performed another catherization. This time, unlike the
others, I was told that because of new machine technology, Dr. Kitzes found that I had aortic stenosis,
which is a narrowing of the valve passage by build-up of plaque due to the valve being malformed at
birth. Dr. Kitzes informed me that I could lead a normal life until I was in my fifties or sixties before I
would need corrective surgery.


In 1996, I had an echocardiogram and it was determined that my heart was enlarged. My family
physician said that I should see a cardiologist. I down played the visit as not being serious after hearing
the same thing many times. This time I entered the office of Jon Lambrecht, I had never met him
before. Within a few minutes my whole life was turned around. After asking me about my symptoms,
which were fatigue, weakness, asthmatic symptoms, as well as ashen skin color and dizziness, he
informed me of how serious my condition was and the only salvation was immediate open-heart
surgery to replace the aortic valve. I began to cry as I thought my life was over. Dr. Lambrecht studied
my reaction and told me that this condition is repairable and that I don't have a terminal illness. I didn't
have a lot of time to think about it. Within 10 days from that visit, I was the recipient of a Meditronic
Hall Prosthetic heart valve. The operation was performed by Dr. Robert Indeglia at Miriam Hospital in
Providence, R.I. on March 20th, 1996.


It has been almost 3 years since the surgery and I am doing better than I could have expected. In
1977 my son Kevin was born with Hypoplastic Left-heart Syndrome and only lived for 2 days because
heart surgery wasn't performed like today. I am thankful that I lived at a time when medical technology
paved the way for a second chance because of my new aortic heart valve. Our goal in this chapter is to
take you by the hand and lead you through each part of the cardiovascular system, so that you too may
156 | Human Physiology




The Cardiovascular System


learn and come to respect the greatness of this blood pumping machine we all call the heart.
Stroke
Cerebrovascular disease are those that affect blood vessels in the brain and happen to be the third


cause of death in the United States only behind heart disease and cancer. Stroke (also called
cerebrovascular accident or CVR) is a cerebrovascular disorder caused by a sudden decrease or
stoppage of blood flow to a part of the brain. Decreased blood flow also known as ischemia is
dangerous to any tissue but brain tissue is even more vulnerable, mainly due to the high rate of its
metabolic reactions. In fact if you stopped blood flow for no more than three minutes it may be
sufficient enough to cause death of most brain cells. For this reason a stroke can kill people within
minutes or leave them with severe brain damage.


Strokes may be classified as either occlusive or hemorrhagic and may happen either in the interior
of the brain or on its surface. In a occlusive stroke blood flow through a vessel is blocked. In a
hemorrhagic stroke a blood vessel ruptures causing a hemorrhage.


Summary
As with all of the body systems, the cardiovascular system plays a part in maintaining homeostasis.


The nervous system regulates the functioning of the heart based on what the heart is supposed to do.
The pumping of the heart maintains normal blood pressure and proper oxygenation of tissues. The
vascular system forms passageways for the blood, but they aren't simply just a pipeline system. The
vessels are not passive tubes, but rather active contributors to homeostasis. The arteries and veins help
maintain blood pressure, and the capillaries provide sites for the necessary exchanges of materials
between the blood and the tissues.


Review Questions
1. This conducts electricity like nerves


A) Epicardium
B) Pericardium
C) Myocardium
D) Subvalaular Apparatus
E) None of these, only nerves conduct electricity


2. This carries the most blood at any given time in the body
A) Veins
B) Capillary Beds
C) Veins
D) Aorta
E) Vena Cava


3. The following contract together to pump blood


Wikibooks | 157




Chapter 8
A) Right atrium with the right ventricle and left atrium with the left ventricle
B) Right atrium with left atrium and right ventricles with left ventricle
C) Tricuspid valve and mitral valve
D) Aorta and pulmonary artery
E) Aorta, pulmonary artery and pulmonary vein


4. This is the pacemaker of the heart
A) AV node
B) Purkinje fibers
C) AV Bundle
D) SA node
E) None of these, a pacemaker is surgically inserted


5. When reading an EKG, this letter shows the depolarization from the AV node down to the AV
bundle


A) S
B) P
C) U
D) T
E) Q


6. The T wave in an EKG shows
A) Resting potential
B) Atrial depolarization
C) SA node excitation
D) Ventricle repolarization
E) Purkinje Excitation


7. Blood pressure is the measure of
A) Pressure exerted by the blood on the walls of the blood vessels
B) Pressure exerted by the blood on the arteries
C) Pressure exerted by the blood on the veins
D) Pressure exerted by the blood on the aorta


8. Systolic Pressure is
A) An average of 120 mm Hg
B) Lowers steadily during ventricle systole
C) The highest when blood is being pumped out of the left ventricle into the aorta
D) An average of 80 mm Hg
E) Both A and C
F) Both B and D


158 | Human Physiology




The Cardiovascular System


Glossary
Acute myocardial infarction (AMI or MI) commonly known as a heart attack, is a disease state that
occurs when the blood supply to a part of the heart is interrupted. The resulting ischemia or oxygen
shortage causes damage and potential death of heart tissue. Aorta: the largest of the arteries in the
systemic circuit
Aortic Valve: lies between the left ventricle and the aorta
Antidiuretic hormone: Produced in the posterior pituitary ADH (vasopressin), major function is to
regulate blood pressure by water retention by the kidneys.
Arteriole: a small diameter blood vessel that extends and branches out from an artery and leads to
capillaries
Atrial natriuretic peptide: Produced in the atria of the heart, it increases urinary excretion of sodium
which causes water loss which in turn the viscosity of the blood is lowered and in turn lowers the blood
pressure.
Atrioventricular Node (abbreviated AV node): the tissue between the atria and the ventricles of the
heart, which conducts the normal electrical impulse from the atria to the ventricles
Atrioventricular valves: large, multi-cusped valves that prevent backflow from the ventricles into the
atria during systole
AV Bundle: collection of heart muscle cells specialized for electrical conduction that transmits the
electrical impulses from the AV node
Barbiturates: CNS depressants, sedative-hypnotics
Blood Pressure: the pressure exerted by the blood on the walls of the blood vessels
Capillaries: the smallest of a body’s vessels, they connect arteries and veins
Cardiac Cycle: term used to describe the sequence of events that occur as a heart works to pump blood
through the body
Cerebral Vascular Accident (CVA): Also known as a stroke, is a rapidly developing loss of a part of
brain function or loss of conciousness due to an interruption in the blood supply to all or part of the
brain. That is, a stroke involves the sudden loss of neuronal function due to a disturbance in cerebral
perfusion. There are many different causes for the interruption of blood supply, and different parts of
the brain can be affected. Because of this, a stroke can be quite heterogeneous. Patients with the same
cause of stroke can have widely differing handicaps. Similarly, patients with the same clinical handicap
can in fact have different causes of their stroke.
Chordae Tendinae: cord-like tendons that connect the papillary muscles to the tricuspid valve and the
mitral valve in the heart
Coronary Arteries: blood vessels that supply blood to, and remove blood from, the heart muscle itself
Continuous Capillaries: have a sealed epithelium and only allow small molecules, water and ions to
diffuse
Deep-vein thrombosis (DVT): is the formation of a blood clot ("thrombus") in a deep vein. It
commonly affects the leg veins, such as the femoral vein or the popliteal vein or the deep veins of the
pelvis. Occasionally the veins of the arm are affected
Diastole: period of time when the heart relaxes after contraction in preparation for refilling with
circulating blood
Diastolic Pressure: lowest point in blood pressure where the heart relaxes
Edema: The swelling that forms when too much tissue fluid forms or not enough taken away
Electrocardiogram: the recording of the heart's electrical activity as a graph
Epinephrine: Produced in the adrenal medulla of the adrenal glands, major function is
vasoconstriction that will in turn increase respiratory rate and increase cardiac out put.
Fenestrated Capillaries: have openings that allow larger molecules to diffuse


Wikibooks | 159




Chapter 8
Fibrous Pericardium: a dense connective tissue that protects the heart, anchoring it to the surrounding
walls, and preventing it from overfilling with blood
Heart Rate: term used to describe the frequency of the cardiac cycle
Hepatic Veins: blood vessels that drain de-oxygenated blood from the liver and blood cleaned by the
liver (from the stomach, pancreas, small intestine and colon) into the inferior vena cava
Hypertension or High Blood Pressure: medical condition wherein the blood pressure is chronically
elevated
Inferior Vena Cava (or IVC): a large vein that carries de-oxygenated blood from the lower half of the
body into the heart
Intraventricular Septum: the stout wall separating the lower chambers (the ventricles) of the heart
from one another
Left Atrium:receives oxygenated blood from the left and right pulmonary veins
Lub-Dub: first heart tone, or S1; caused by the closure of the atrioventricular valves, mitral and
tricuspid, at the beginning of ventricular contraction, or systole
Lumen: hollow internal cavity in which the blood flows
Lymph: originates as blood plasma that leaks from the capillaries of the circulatory system, becoming
interstitial fluid, filling the space between individual cells of tissue
Mitral valve: also known as the bicuspid valve; prevents blood flowing from the left ventricle into the
left atrium
Myocardium: the muscular tissue of the heart.
Norepinephrine: Produced in the adrenal medulla of the adrenal glands, major function is a strong
vasoconstrictor that will in turn increase respiratory rate.
Pacemaker Cells: cells that create these rhythmical impulses of the heart
Plaque: an abnormal inflammatory accumulation of macrophage white blood cells within the
walls of arteries
Pulmonary Valve: lies between the right ventricle and the pulmonary artery; prevents back-flow of
blood into the ventricle
Pulse: the number of heartbeats per minute
Purkinje Fibers (or Purkinje tissue): located in the inner ventricular walls of the heart, just beneath
the endocardium; specialized myocardial fibers that conduct an electrical stimulus or impulse that
enables the heart to contract in a coordinated fashion
Renin-Angiotension system:
Right Atrium: receives de-oxygenated blood from the superior vena cava and inferior vena cava
Serous Pericardium: functions in lubricating the heart to prevent friction from occurring during heart
activity
Semilunar Valves: positioned on the pulmonary artery and the aorta
Sinoatrial Node: (abbreviated SA node or SAN, also called the sinus node): the impulse generating
(pacemaker) tissue located in the right atrium of the heart
Sinusoidal Capillaries: special forms of fenestrated capillaries that have larger opening allowing
RBCs and serum proteins to enter
Systole: contraction of the heart
Systolic Pressure:' the highest point in blood pressure when the blood is being pumped out of the left
ventricle into the aorta during ventricular systole
Superior Vena Cava (SVC): a large but short vein that carries de-oxygenated blood from the upper
half of the body to the heart's right atrium
Thrombus: a blood clot in an intact blood vessel
Tricuspid Valve: on the right side of the heart, between the right atrium and the right ventricle; allows
blood to flow from the right atrium into the right ventricle when the heart is relaxed during diastole
Vasoconstriction: the constriction of blood vessels
Vasodilation: the dilation of blood vessels
160 | Human Physiology




The Cardiovascular System


Veins:carry de-oxygenated blood from the capillary blood vessels to the right part of the heart
Ventricle: a heart chamber which collects blood from an atrium
Venule: a small blood vessel that allows deoxygenated blood to return from the capillary beds to the
larger blood vessels called


References
1. Van De Graaff, Kent M. Human Anatomy. McGraw Hill Publishing, Burr Ridge, IL. 2002.
2. www.health.howstuffworks.com
3. www.americanheart.org
4. www.heartcenteronline.com
5. Essentials of Anatomy and Physiology, Valerie C. Scanlon and Tina Sanders


live version • discussion • edit lesson • comment • report an error


Wikibooks | 161




Chapter 9


9 THE IMMUNE SYSTEM
live version • discussion • edit lesson • comment • report an error


Overview
he immune system is a complex system that is responsible for protecting us against infections and
foreign substances. There are three lines of defense: the first is to keep invaders out (through skin,


mucus membranes, etc), the second line of defense consists of non-specific ways to defend against
pathogens that have broken through the first line of defense (such as with inflammatory response and
fever). The third line of defense is mounted against specific pathogens that are causing disease (B cells
produce antibodies against bacteria or viruses in the extracellular fluid, while T cells kill cells that have
become infected). The immune system is closely tied to the lymphatic system, with B and T
lymphocytes being found primarily within lymph nodes. Tonsils and the thymus gland are also
considered lymph organs and are involved in immunity. We often don't realize how effective the
immune system is until it fails or malfunctions, such as when the lymphocytes are attacked by HIV in
an AIDS patient.


T


The Immune System as a Castle
The immune system is a silent wonder. While we are very aware of our heart beating and the


breaths we take, we are much less aware of our immune system that protects us from thousands of
potentially deadly attacks every day.


In this chapter we will discuss the immune system we each possess that is working around the
clock, protecting us from disease and death.


A good way to start understanding the immune system is to liken it to a castle. A castle, like our
bodies, is a fortress. A castle has three lines of defense:


• First, A moat and drawbridge. The first line of defense in our bodies are physical and
chemical barriers - our skin, stomach acids, mucous, tears, vaginal opening, of which the last
three mostly produce lysozyme to destroy harmful incoming pathogens.


• Second, Sentries and archers who stand on the castle wall. In our bodies the second line of
defense is non-specific immune responses - marcrophages, neutrophils, interferons, and
complement proteins. This line of defense also includes fever and inflammatory response as
nonspecific defenses.


• Third, Soldiers within the castle.Our third line of defense is specific immune responses - T
Cells and B Cells. There are many types of each which work like a close knit team to destroy
pathogens.


If pathogens (invaders) try and succeed in penetrating the first line of defense, then the second line
of defense is ready to act. If both the first and second line of defense fail, then the third line of defense
will act. It is when all three lines of defense are breached that we get sick and are subject to disease.


So what we are trying to say is that the immune system is a set of mechanisms of defense,


162 | Human Physiology




The Immune System


protecting an organism from infection by identifying and attacking pathogens. This is a difficult task,
since pathogens range from viruses to parasitic worms and must be detected with absolute specificity as
they are "hidden" amongst normal cells and tissues. Pathogens are also constantly changing themselves
to avoid detection and successfully infect and destroy their hosts.


Lymphatic System
The lymphatic system and the immune system are terms that are used interchangeably to refer to


the body's ability to defend against pathogens. The lymphatic system is comprised of three interrelated
functions: (1) Removal of excess fluids, lymph, from body tissues, (2) Absorption of fatty acids and
subsequent transport of fat, chyle, to the circulatory system and (3) Formation of white blood cells
(WBCs), and initiation of immunity through the formation of antibodies, lending specific resistance to
pathogens.


Lymphatic Pathways
The lymphatic system acts as a secondary circulatory system, except it collaborates with white


blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or
bacteria. Unlike the circulatory system, the lymphatic system is not closed and has no central pump; the
lymph moves slowly and under low pressure due to peristalsis, the operation of semilunar valves in the
lymph veins, and the milking action of skeletal muscles. Like veins, lymph vessels have one-way,
semilunar valves and depend mainly on the movement of skeletal muscles to squeeze fluid through
them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic capillaries.
This fluid is then transported to progressively larger lymphatic vessels culminating in the right
lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body);
these ducts drain into the circulatory system at the right and left subclavian veins.


Lymph
Lymph originates as blood plasma that leaks from the capillaries of the circulatory system,


becoming interstitial fluid, filling the space between individual cells of tissue. Plasma is forced out of
the capillaries by hydrostatic pressure, and as it mixes with the interstitial fluid, the volume of fluid
accumulates slowly. Most of the fluid is returned to the capillaries by osmosis. The proportion of
interstitial fluid that is returned to the circulatory system by osmosis is about 90% of the former
plasma, with about 10% accumulating as overfill. The excess interstitial fluid is collected by the
lymphatic system by diffusion into lymph capillaries, and is processed by lymph nodes prior to being
returned to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has
almost the same composition as the original interstitial fluid.


Edema
Edema is the swelling that forms when too much tissue fluid forms or not enough taken away. It


can be caused by a variety of conditions such as allergic responses (too much vasodilation), starvation
(lack of albumin in blood lowers osmotic pressure and decreases amount of fluid returning to
capillaries), and lymphatic disorders (e.g. blockage due to parasite in elephantiasis, or removal of


Wikibooks | 163




Chapter 9
lymph nodes due to a radical mastectomy). Edema is common in the lower extremities when people
spend a lot of time sitting, because the fluid return is based largely on the massaging action of skeletal
muscles.


Lymphatic Vessels and Ducts
The lymphatic vessels are similar in structure to the cardiovascular veins, meaning they also have


valves. They are dependent upon the contraction of skeletal muscle, respiratory movements and valves
that do not allow backward flow. The vessels merge before entering one of two ducts.


• Thoracic Duct: This duct is much larger than the lymphatic duct. It serves the abdomen,
lower extremities and the left side of the upper body (head, neck, and arm)


• Right Lymphatic Duct: This duct serves all of the right side of the upper body and thoracic
area (head, neck).


Organs, Tissues and Cells of the Immune System
The immune system consists of a network of lymphatic organs, tissues, and cells. These structures


are supported by the reticuloendothelial system: loose connective tissue with a network of reticular
fibers. Phagocytic cells, including monocytes and macrophages, are located in the reticular connective
tissue. When micro-organisms invade the body, or the body encounters antigens (such as pollen),
antigens are transported to the lymph. Lymph is carried through the lymph vessels to regional lymph
nodes. In the lymph nodes, the macrophages and dendritic cells phagocytose the antigens, process
them, and present the antigens to lymphocytes, which can then start producing antibodies or serve as
memory cells. The function of memory cells is to recognize specific antigens in the future.


Primary Lymphatic Organs The primary lymphatic organs are the red bone marrow and the
thymus. They and are the site of production and maturation of lymphocytes, the type of white blood
cell that carries out the most important work of the immune system.


• Red Bone Marrow Red bone marrow, the soft, spongy, nutrient rich tissue in the cavities
of certain long bones, is the organ that is the site of blood cell production.


Some of the white blood cells produced in the marrow are: neutrophils, basophils, eosinophils,
monocytes, and lymphocytes. Lymphocytes differentiate into B lymphocytes and T lymphocytes. Red
bone marrow is also the site of maturation of B lymphocytes. T lymphocytes mature in the thymus.


• Thymus Gland The thymus gland is located in the upper thoracic cavity posterior to the
sternum and anterior to the ascending aorta. The thymus is an organ that is more active in
children, and shrinks as we get older. Connective tissue separates the thymus into lobules,
which contain lymphocytes. Thymic hormones such as thymosin are produced in the thymus
gland. Thymosin is thought to aid in the maturation of T lymphocytes. The Thymus is critical
to the immune system. Without a thymus, a person has no ability to reject foreign substances,
blood lymphocyte level is very poor, and the body’s response to most antigens is either absent
or very weak


Immature T lymphocytes travel from the bone marrow through the bloodstream to reach the


164 | Human Physiology




The Immune System


thymus. Here they mature and for the most part, stay in the thymus. Only 5% of T lymphocytes ever
leave the thymus. They only leave if they are able to pass the test: if they react with “self” cells, they
die. If they have the potential to attack a foreign cell, they leave the thymus.


Secondary Lymphatic OrgansThe secondary lymphatic organs also play an important role in the
immune system as they are places where lymphocytes find and bind with antigens This is followed by
the proliferation and activation of lymphocytes. The secondary organs include the spleen, lymph nodes,
tonsils, Preyer’s patches, and the appendix.


• The spleen, The spleen is a ductless, vertebrate gland that is closely associated with the
circulatory system, where it functions in the destruction of old red blood cells in holding a
reservoir of blood.located in the upper left region of the abdominal cavity, is divided into partial
compartments. Each compartment contains tissue known as white pulp and red pulp. The white
pulp contains lymphocytes and the red pulp acts in blood filtration. When blood enters the
spleen and flows through the sinuses for filtration, lymphocytes react to pathogens ,
macrophages engulf debris, and also remove old, worn out red blood cells. A person without a
spleen is more susceptible to infections and may need supplementary antibiotic therapy for the
rest of their life.


• Lymph Nodes are small oval shaped structures located along the lymphatic vessels. They
are about 1-25 mm in diameter. Lymph nodes act as filters, with an internal honeycomb of
connective tissue filled with lymphocytes that collect and destroy bacteria and viruses. They are
divided into compartments, each packed with B lymphocytes and a sinus. As lymph flows
through the sinuses, it is filtered by macrophages whose function is to engulf pathogens and
debris. Also present in the sinuses are T lymphocytes, whose functions are to fight infections
and attack cancer cells. Lymph nodes are in each cavity of the body except the dorsal cavity.
Physicians can often detect the body’s reaction to infection by feeling for swollen, tender lymph
nodes under the arm pits and in the neck, because when the body is fighting an infection, these
lymphocytes multiply rapidly and produce a characteristic swelling of the lymph nodes.


• Tonsils are often the first organs to encounter pathogens and antigens that come into the
body by mouth or nose. There are 3 pairs of tonsils in a ring about the pharynx.


• Peyer’s patches, located in the wall of the intestine and the appendix, attached to the
cecum of the large intestine, intercept pathogens that come into the body through the intestinal
tract.


Leukocytes
The primary cells of the immune system are the leukocytes or white blood cells(WBC). Most


leukocytes are much larger then red blood cells, but they are not nearly as numerous. A microliter of
whole blood contains about 5 million red blood cells but only about 7000 leukocytes.


Although most leukocytes circulate through the blood, they usually leave the capillaries and
function extravascularly (outside the vessels). Some types of leukocytes can live out in the tissue for
several months, but others may live for only hours or days. Leukocytes can be distinguished from one
another in stained tissue samples by the shape and size of the nucleus, the staining characterisitcs of the
cytoplasm and the cytoplasmic inclusions, and the regularity of the cell border.


Wikibooks | 165




Chapter 9
Leukocytes are are divided into six basic types eosinophils, basophils, neutrophils, monocytes,


lymphocytes, and dendritic cells.
One functional group of leukocytes is the phagocytes, WBC that engulf and ingest their targets by


phagocytosis. This group includes the neutrophils, macrophages, monocytes (which are macrophage
precursors), and eosinophils. A second functional group is the cytotoxic cells, so named because they
kill the cells they attack. This group includes eosinophils and some types of lymphocytes.


Lets take a closer look at the six basic types of leukocytes.
Eosinophils
Eosinophils fight parasites and contribute to allergic reactions. They are easily recognized by the


bright pink staining granules in their cytoplasm. Normally, there are only a few eosinophils found in
the peripheral circulatory. They account for only 1-3% of all leukocytes. The life span of a typical
eosinophil in the blood is about 6-12 hours. Eosinophils are known to attach to large parasites and
release substances from their granules that damage or kill the parasite. Because eosinophils kill
pathogens, they are classified as cytotoxic cells. Eosinophils also participate in allergic reactions, by
contributing to inflammation and tissue damage by releasing toxic enzymes.


Basophils
Basophils release histamine and other chemicals. Basophils are rare in circulation but are easily


recognized in a stained blood smear by the large, dark blue granules in their cytoplasm. They also
release mediators that contribute to inflammation. The granules contain histamine, heparin(an
anticoagulant), cytokines, and other chemicals involved in allergic and immune responses.


Neutrophils
Neutrophils "eat" bacteria and release cytokines. Neutrophils are the most abundant WBC, 50-70%


of the total. They are easily identified by a segmented neucleus. Neutrophils, like other leukocytes are
formed in the bone marrow. They are phagocytic cells that typically ingest and kill bacteria. Most
neutrophils remain in the blood but can leave the circulation if attracted to an extravascular site of
damage or infection. In addition to ingesting bacteria and foreign particles, neutrophils release a variety
of cytokines.


Monocytes
Monocytes are the precursor cells of tissue macrophages. Monocytes are not that common in the


blood 1-6% of WBC. Once out of the blood, monocytes enlarge and differentiate into macrophages.
Some tissue macrophages patrol the tissues, creeping along by amoeboid motion. Others find a location
and remain fixed in place. Macrophages are the primary scavengers within tissues. Macrophages also
remove larger particles, such as old RBC and dead neutrophils. Macrophages play an important role in
the development of acquired immunity. After they ingest and digest molecular or cellular antigens,
fragments of processed antigen are inserted into the macrophage membrane as part of surface protein
complexes.


Lymphocytes Lymphocytes are the key cells that mediate the acquired immune response of the
body. Only about 5% of lyphocytes are found in circulation. They constitute 20-30% of all WBC. Most


166 | Human Physiology




The Immune System


lymphocytes are found in lymphoid tissues, where they are more likely to encounter invaders. By one
estimate,the adult body contains a trillion lymphocytes at any one time.


Dendritic Cells
Dendritic cells activate lymphocytes. They are antigen-presenting cells characterized by long, thin


processes that resemble neuronal dendrites. Dendritic cells are found in the skin called Langerhans
cells and also in various organs. When dendritic cells recognize and capture antigens, they migrate to
secondary lymphoid tissues, where they present the antigens to lymphocytes.


Defenses Against Infection
Innate Defense – first line of defense


Physical and chemical barriers are the body's first line of defense.


Physical or Mechanical barriers
• Skin


One of the bodies first line of defenses against bacteria and other harmful organisms is the skin.
Our skin is a barrier which stops infection from entering the body. Millions of microorganisms live
harmlessly on the skin and in the air around us. Sebaceous glands in the skin produce sweat and sebum,
which, combined help to protect the skin. Both substances contain antiseptic properties, of lysozome is
the main property. Although our skin is a good defense, it isn’t perfect. The skin itself can also become
infected by bacteria, viruses, fungi or tiny parasites. Some examples of these are: boils, impetigo;
ringworm, athletes foot; cold sore, wart, verucca; and scabies.


• Mucus membranes
Another very important first line of defense is our mucus membranes. The mucous membranes (or


mucosae; singular: mucosa) line various body cavities that are exposed to the external environment and
internal organs. It is at several places continuous with skin: at the nostrils, the lips, the ears, the genital
area, and the anus. The nose and mouth serve as passageways for air going to and from the lungs. As
we inhale and exhale, the mucus membranes that line these passageways warm and humidify the air. It
has been said that there is more bacteria contained in a human mouth than the the sum of all the people
that have ever lived on the earth. Mucus membranes serve different functions, however, their more
important job is to secrete mucus that traps bacteria and other foreign debris that irritates the lining of
the respiratory tract. This mucus is produced and stored in the sinuses by other mucus membranes. We
get congested when there is excessive fluid in the sinus cavities. This is a result of an increase in mucus
secretions, as well as an increase in the amount of fluids that passes across the blood vessels of the
mucus membranes that line the nose and sinus. There are also many chemicals, such as pesticides and
anthrax that are absorbed through the skin. All mucous membranes are ciliated. Cilia are thin, tail-like
projections extending approximately 5–10 micrometers outwards from the cell body. Their main
function is to move things across their surface.


Wikibooks | 167




Chapter 9
• Mucociliary escalator


The mucociliary clearance of the respiratory tract is an important defense mechanism against
foreign debris and inhaled pathogens. The cilia that lines the upper and lower airways are lined with a
thin layer of mucus. These beat rapidly to propel particles that are trapped in the mucus layer to the
pharynx. Defective mucociliary clearance predisposes our respiratory tracts to recurrent infections.
These cilial defects may be either congenital or acquired by infection, toxins or drugs.


Chemical Defenses
• Tears, saliva


Tears and saliva contain lysozyme, an antiseptic enzyme that attacks cell walls of bacteria and
breaks them down.


• Stomach acids
Glands in the stomach lining produce hydrocloric acid. This acid kills most invading organisms


that are swallowed and take up residence there.


Non-specific responses to infection - 2nd line of defense
We are born with built in nonspecific defenses that all respond in the same way to invading


pathogens. The outermost defense our body has is our skin. The sebaceous glands produce sweat and
sebum, which contain ANTISEPTIC properties which protect. This bacteria-killing substance called
LYZOSOME is also found in tears and saliva. Acidic urine in the urinary tract and friendly bacteria in
the genital tract prevent the multiplying of harmful organisms in these areas. Most invading organisms
in the stomach are killed by gland production of hydrochloric acid. These are a few examples of how
the outer defenses protect us. All outer defenses work together as the body's first line of defense.


Inflammatory response
Any break in the skin will allow bacteria to enter the body. These foreign microbes will cause


swelling and reddening at the site of injury. This reaction by the body is called an inflammatory
reaction or inflammatory response.


Swelling, redness, heat, and pain
Inflammation is characterized by the following quintet: swelling (tumor), redness (rubor), heat


(calor), pain (dolor) and dysfunction of the organs involved (functio laesa). When an injury occurs, a
capillary and several tissue cells are apt to rupture, releasing histamine and kinins. These cause the
capillaries to dilate, become more permeable, and leak fluid into these tissues. Dilation and fluid
leaking into the tissues causes swelling, redness, and heat. The swelling and kinins stimulate nerve
endings, causing pain. If there has been a break in the skin due to the injury, invading microbes may
enter. A common cause of inflammation after surgery is serous fluid. This is a mixture of plamsa,
lymph and interstitial fluids .seeping from the damaged cells and vessels. If enough serous fluid


168 | Human Physiology




The Immune System


accumulates a mass called a seroma may form. Treatment of a seroma may involve the removal of the
fluid with a needle into a syringe, a process called aspiration.


Phagocytosis by neutrophils and macrophages
In the event of a break in the skin, neutrophils, monocytes (and macrophages) arrive and attempt to


engulf and destroy the invaders. Phagocytosis is receptor-mediated event, which ensures that only
unwanted particles are ingested. Stimulated macrophages can bring about an explosive increase in the
number of leukocytes by producing Colony Stimulating Factors (CSFs). The CSFs pass by way of the
blood to the bone marrow, where they stimulate the production and the release of white blood cells
(WBCs), primarily neutrophils. Lymphocytes in nearby lymph nodes produce specific antibodies to
attack the microbes. During the conflict, some neutrophils die and become mixed with dead tissue,
bacteria, living white cells, etc. This thick yellow-white fluid is called pus. When a person has an
illness, an examination of the numbers and types of WBC's in their blood can be very useful.


Complement System
The complement system is a biochemical cascade of the immune system that helps clear pathogens


from an organism, and promote healing. It is derived from many small plasma proteins that work
together to form the primary end result of cytolysis by disrupting the target cell's plasma membrane.


Complement is activated by antigen-antibody complexes and causes holes to form in the plasma
membrane of foreign microbes or cells (lysis). The complement system is considered a nonspecific
defense, but it can be activated against specific microbes that have been marked with antibodies.
Hemolytic transfusion reactions are caused by complement activation after a person expresses
antibodies against the antigens found on the inappropriately donated blood. Hemolytic Disease of the
Newborn (HDN) is due to maternal antibodies against the Rh factor crossing the placenta, binding to
the baby's red blood cells, and stimulating the baby's own complement system to lyse its red blood
cells.


Interferon in response to viral infection
Interferon (IFNs) are naturally occurring glycoproteins involved in non-specific immune


responses. Interferons do just as their name states they "interfere" with viral growth. Interferons are
initiated from a cell that has been infected by a virus. When a cell has been infected by a virus the virus
will then cause the cell to make viral nucleic acid. This nucleic acid acts as a signal and it causes the
cell to realize that it has been infected with a virus. So the cell will start making and sending out
interferons. The IFN's that the cell sends out go to nearby healthy cells and warns them of a virus. The
healthy cells then start intracellular changes that help the cells to be more resistant to the virus.


Adaptive Defense (Specific Defense--third line of defense)
This part of the immune system directly targets invading microbes. Our specific immune defenses


respond to antigens. An antigen is a protein (or polysaccharide) molecule, typically on the cell
membrane, that the body recognizes as nonself. They are found on microbes, foreign cells, or on cancer
cells. Normally our immune system does not respond to our own antigens (if it does, then this is an


Wikibooks | 169




Chapter 9
autoimmune disease). Sometimes we develop an immune response to a harmless antigen, such as
pollen or cat dander (this is an allergic response).


Lymphocytes
Specific immunity is dependent upon two types of lymphocytes, the B cells and the T cells. Their


names are based on where in the body they mature. B cells mature in the bone marrow, and T cells
mature in the thymus gland. In comparison, both B and T cells can recognize and target antigen-bearing
cells, although they go about this in different ways. B and T cell lymphocytes are capable of
recognizing an antigen because they have specific receptor molecules on their surface which exactly fit
individual antigens (like a lock and a key). Any B or T cell can only respond to one type of antigen.
The body does not know ahead of time which antigens it will encounter, but rather makes receptor sites
for a huge number of possible antigens. It is estimated that for the million or so antigens we encounter
in our lifetime we have an equal number of specific lymphocytes for each possible antigen.


B Cells Produce Antibodies
B cell lymphocytes are responsible for antibody-mediated immunity (humoral immunity). They


produce antibodies, which are proteins that bind with and neutralize specific antigens. Antibodies do
not directly kill bacteria, but mark them for destruction. When antibodies bind to viruses they can
prevent the viruses from infecting cells. When antibodies bind to toxins they can neutralize the toxin
(why we get immunized against the tetanus toxin). Humoral immunity works best fighting against
target viruses, bacteria, and foreign molecules that are soluble in blood and lymph before the bacteria
or viruses have entered into cells (extracellular bacteria and extracellular viruses).


B cells produce two different types of cells:
• plasma cells
• memory cells


Plasma cells
As B cells mature during embryonic development, they develop surface receptors that allow them


to recognize specific antigens. Then they travel in the bloodstream, distributing throughout the lymph
nodes, spleen, and tonsils. Once B cells reach their destination, they remain inactive until they
encounter a foreign cell with an antigen that matches their particular receptor site (most B cells remain
inactive for your entire life). The foreign antigen can be presented to the B cell directly, but usually
macrophages and T cell lymphocytes (helper T cells) interact with B cells as Antigen Presenting Cells
to bring about antibody production. Upon such an encounter, the B cell's receptors will bind to the
antigen. The appropriate B cell is turned on or stimulated. It then grows bigger, and rapidly multiplies
into a large homogenous group (clone). Most of these cells are plasma cells, which actively secrete
antibody that will bind with the original stimulating antigen . While most of the B cells remain in the
lymphatic system, the antibodies are secreted into the lymph fluid which then enters into the blood
plasma to circulate throughout the body. Although the clone cells only live a few days, their antibodies
remain and circulate in the blood and lymph, gradually decreasing in number.


Antibody Structure and Function


170 | Human Physiology




The Immune System


There are different classes of antibodies, or immunoglobulins (Ig), such as IgA, IgG, IgE, and IgM.
They can attach to the surface of a microbe and make it more easily phagocytized by neutrophils,
monocytes and macrophages. Anything that simplifies phagocytosis is called an opsonin. The process
of antibodies attaching to invaders can be termed 'opsonization.' Some antibodies can bind and
inactivate certain poisons or toxins and are called antitoxins (tetanus immunizations stimulate your
body to produce antibodies against the tetanus toxin rather than against the bacteria that produces the
toxin). Still other antibodies can bind to the surface of microbes and prevent their attachment to the
body's cells (thus preventing viruses from entering host cells). Also, some of them can stimulate nine
proteins found in plasma, called complement.


Memory B cells
At the time of activation some of the clones become memory B cells. These cells are long lived


and have recorded the information about the foreign antigen so antibodies can be made more quickly,
and in greater amount, in case a second exposure should occur. Since the second response is much
stronger than the first and puts more antibodies into circulation, we often receive "booster shots" for
immunizations.


T Cells Attack Infected Cells
Defending the body against intracellular pathogens is the role of T lymphocytes, which carry out


cell-mediated immunity(CMI). Macrophages phagocytize invading microbes and present parts of the
microbe (antigens) to the T cell lymphocytes. The appropriate T cell is turned on or stimulated. The
activated T cell rapidly multiplies into a large homogenous group (clone) of cytotoxic T cells (Tc cells).


• (a) Attack organisms directly, Also kill infected cells
These cytotoxic T cells migrate to the site of infection (or disease) and produce chemicals which


directly kill the invader. Cytotoxic T cells release “perforin” that causes pores to form in the plasma
membrane of the target cell, resulting in lysis.


• (b) T cells develop in the thymus gland from immature precursor cells that migrate there
from the bone marrow.


• (c) Killer and helper T cells
• (d) Memory T Cells


A portion of these activated T cells become memory T cells (Tm). These cells record the
information about the foreign antigen so T cells can respond more quickly, and more strongly, if a
second exposure occurs. A portion of the T cells become T helper cells (TH) or T suppressor cells (Ts).
TH cell stimulate other T cells and B cells by releasing cytokines and other stimulatory chemicals. Ts
cells suppress the immune response. Experience has shown that cell mediated immunity is most useful
to the body by: Protecting against microbes which exist inside of our body's cells (intracellular bacteria
and intracellular viruses). Protecting against fungal infections. Protecting against protozoan parasites.
Protecting against cancer cells.


Wikibooks | 171




Chapter 9


IMMUNE RESPONSE PATHWAYS
The innate response starts first, and it is reinforced by the more specific acquired response. The


two pathways are interconnected, so cooperation and communication is essential.


INFLAMMATION
What happens when bacteria invade? If the first line of defense fails, bacteria can reach the


extracellular fluid. There they usually cause an inflammatory response. This response coats antigens on
the bacterial surface, with antibodies. Then in return the antibodies will ingest the antigens with
phagocytic cells. This is characterized by a red, swollen warm area that is tender or painful. In addition
to the nonspecific inflammatory response, lymphocytes attracted to the area produce antibodies keyed
to the specific type of bacteria. If the infection continues it will produce a fever.


• What causes a fever?
During an infection macrophages may release cytokines (see glossary), such as interleukin-1, that


travel to the hypothalamus and induce a change in the thermostat setting. When the thermostat is raised
to a new normal temperature, the previous body temperature now registers as too cold. To increase the
temperature to the new level, our body shunts blood away from the skin (leaving it feeling cold and
clammy), the heart rate increases, and we shiver to generate heat until we reach the new set point. The
hypothalamus may subsequently lower the thermostat, in which case we suddenly feel hot and start to
sweat as our body attempts to cool off. A person may cycle between chills and sweats during the course
of an infection. While a fever can be dangerous if it gets too high, or if a patient is weak or has heart
trouble, there is some evidence suggesting that the body may overcome an infection faster if a fever is
allowed to run its course.


INTRACELLULAR DEFENSE
What happens when virus's invade the body?
First they encounter an extracellular phase just like the bacteria did. In the early stages of a viral


infection, innate immune responses and antibodies can help control the invasion of the virus. Once the
virus enters the body's host cells cytotoxic T lymphocytes are the main defense against intracellular
viruses. These cells look for infected host cells, then destroy them.


ACQUIRED IMMUNITY: ANTIGEN-SPECIFIC RESPONSES
Acquired immunity responses are antigen-specific responses in which the body recognizes a


foreign substance and selectively reacts to it. This is mediated primarily by lymphocytes. Acquired
immunity overlaps with the process of innate immunity. Acquired immunity can be subdivided into
active immunity and passive immunity.


Active Immunity occurs when the body is exposed to a pathogen and produces its own antibodies.
Active immunity is active because it is the "activation" of your immune system. Active immunity can
occur naturally, when a pathogen invades the body, or artificially, like when we are given vaccinations


172 | Human Physiology




The Immune System


containing disabled or killed pathogens. The body does require prior exposure to an antigen to develop
an active immunity. Some parents expose their children to some antigens so they will have immunity to
these diseases later in life.


Passive Immunity occurs when we acquire antibodies made by another human or animal. Passive
immunity is passive because it requires no response from the person's immune system. In passive
immunity you are not presenting the body with foreign antigens. Therefore your immune system will
not need to use B cells, and we know that if the B cells are never introduced your body isn't making
antibodies and it isn't making memory B cells. The transfer of antibodies from mother to fetus across
the placenta is one example. Injections containing antibodies are another. Sometimes travelers going
abroad may be injected with gamma globulin, but this passive immunity last only about three months.
Passive immunizations are used to protect people who have been exposed to infections or toxins, like
snake venom or tetanus.


ALLERGIC RESPONSES/INFLAMMATORY RESPONSES
An allergy is an inflammatory immune response to a nonpathogenic antigen. Left alone, the


antigen is not harmful to the body, but if someone is sensitive to the antigen, the body produces an
inflammatory response designed to get rid of it. Allergic inflammatory responses can range from from
mild tissue damage to fatal reactions. The immune response in allergies is called sensitivity or
hypersensitivity to the antigen. Immediate hypersensitivity reactions are mediated (immune
destruction) by antibodies and occur within minutes of exposure to antigens, which are called allergens.
Delayed hypersensitivity reactions are mediated by helper T cells and macrophages and may take
several days to develop.


What happens during a immediate hypersensitivity reaction?
1-Foreign protein or antigen is introduced
2-Macrophage cell ingests (phagpcytosis)
3- Activation of Th lymphocyte
4-Th (helper) lymphocyte
5-Foreign protein bound by membrane antibodies
6-B lymphocyte
7-Antigen processing (MHC II type)
8-Antigen-MHC II complex (antigen presentation)
9-Production of antigen-specific antibodies
10-Activation of B lymphocyte with active Th
2. Upon reexposure, the body reacts more strongly and rapidly. The allergen binds to IgE already


present on mast cells, triggering the immediate release of histamine, cytokines, and other mediators that
cause allergic symptoms. The severity of the reaction varies, ranging from localized reactions near the
site of where the allergen entered, such as a rash. To the most severe allergic reaction called
anaphylaxis. In an anaphylactic reaction, massive release of histamine and other cytokines cause
widespread vasodilation, circulatory collapse, and severe bronchoconstriction. Unless treated promptly,
anaphylaxis can result in death.


Skin tests for allergies of certain allergens can be injected into the skin. This is a good way to find
out what one might be allergic to so they can eliminate further exposure. Allergens that can cause


Wikibooks | 173




Chapter 9
immediate hypersensitivity include bee stings, pollen and certain foods. Allergies that cause chronic
allergic rhinitis and asthma are highly due to dust mites (dermatophgoides). It is not their bodies that
cause the reaction, but rather it's feces. Allergic attacks usually stop when the histamine has been
depleted. This can be stopped faster with an antihistamine drug or nasal spray.


What happens in a delayed hypersensitivity? It could take hours or days for symptoms to occur in a
delayed hypersensitivity. Delayed hypersensitivity is cell mediated with a T lymphocyte response.
Secretion of lymphokines, instead of histamine, happens in a delayed hypersensitivity. So, the
treatment would be a corticosteroid instead of an antihistamine. Examples of a delayed hypersensitivity
would be, poison sumac, poison oak and poison ivy. Skin tests for certain diseases are also considered
examples like TB test and the Mantoux test.


Infectious Organisms and Immunization
Beneficial Organisms


Intestinal bacteria
• Bacteria are prokaryotic (before nucleus) cells that we see usually as bacilli (rods) or cocci


(spheres). While they are the major cause of many diseases both fatal and mild, bacteria are also
our friends and can be of great service to us. Many bacteria in our bodies help prevent
pathogens from becoming established. "Good bacteria" helps protect us from "bad bacteria".
The large intestine is packed with normal microflora that digest substances otherwise
indigestible. This process provides our bodies with additional vitamins, fatty acids and
nutrients. Another example is the microflora that is in the vagina that helps maintain an acidic
pH, which discourages the growth of infectious organisms. These are examples of our immune
system's first line of defense.


Harmful Organisms
Viruses


• Viruses are non-living particles consisting of protein and nucleic acid that infect cells in
biological organisms. They can reproduce only by invading and taking over other cells as they
lack the cellular machinery for self reproduction. A virus is about ten times smaller than a
bacteria. Some viruses you will recognize are: influenza, herpes, measles, and the common cold.
Some viruses are particularly dangerous because they can undergo a period of latency, during
which they are hidden in the cell and do not reproduce. Influenza and HIV are examples of
viruses that frequently mutate, thus making it nearly impossible to achieve a long-lasting
immunity.


Bacteria
• Bacteria can be deadly. They are the major cause of preventable infections and death. Some


well known illnesses are caused by bacteria: staph infections, strep infections, tuberculosis,
food poisoning, tetanus, leprosy, and pneumonia. Because bacterial cells are different from


174 | Human Physiology




The Immune System


human cells, compounds can be found that can kill specific bacterial targets while leaving the
human patient unharmed. Antibacterial agents can be successful in wiping out a bacterial
infection. The problem with antibiotics is that many strains of bacteria are growing resistant to
them. Plus, our bodies are not getting the chance to develop immunity to certain bacteria. It may
be better to use probiotics (new supplements that promote the growth of healthy and helpful
bacteria) rather than depend on antibiotics so much.


Protozoans
• The protozoans are mostly eukaryotic unicellular organisms with organelles and a nucleus.
• Malaria is the most dangerous disease caused by protozoans and is endemic in about 50%


of the populations on Earth. Two to four million people die each year from malaria, a million of
these are under the age of five. malaria is caused by one of the Plasmodium species of
mosquitoes.


Fungi
• Fungi are more like animals and humans than they are like bacteria because of their


eukaryotic cells. Though they produce large, visible colonies on old bread, molds and yeasts are
in the category of microscopic fungi. Yeasts are one-celled and reproduce by budding. Molds
exist as cell chains, called hyphae.


• Mycoses are diseases caused by fungi. Because of the similarity between human cells and
fungal cells, it has been difficult for scientists to design antibiotics that are effective against
fungi and do not harm humans. Some of the diseases caused by fungi are: tineas, vaginal
infection (candidiasis), and histoplasmosis.


Diagnosis Infectious diseases are diagnosed by laboratory techniques such as microscopy and
culture. Since many bacteria have no color, scientists have developed special staining procedures to
more accurately diagnose.


• Culture
Bacteria and fungi can be identified by growing them on plates until colonies are visible. Viruses


are cultured on eggs or live cells.
• Antibiotic sensitivity


After colonies of bacteria are grown on plates, discs are placed on the plates that contain different
antibiotics. Bacteria will not grow around the most effective antibiotic.


• Tests for viruses
Since viruses are too small to be seen with a light microscope, viral infections can be diagnosed


indirectly by their effects on cells. Some viruses cause changes to the surface of cultured cells, causing
them to stick together.


Wikibooks | 175




Chapter 9


Immunization
While some infectious diseases are common and can occur many times in the same person, others


can only occur once in a lifetime thanks to the immune system and it's ability to remember the
organism and prevent following infections. To avoid an epidemic of a grave disease such as polio,
before the disease can be acquired, an immunization can create a man-made "memory".


• Active immunization
A person receives an injection (vaccine) that contains dead or harmless living forms of an


organism. The vaccine stimulates the immune system to produce antibodies and memorize the
organism. If there is a later exposure to this organism and subsequent infection, the antibodies will stop
the infection.


• Passive immunization
Blood containing antibodies is taken from animals or humans who have recently had an infection.


Blood serum is made that contains the antibodies, and then injected into the person. The antibodies
either attack an infection that is present or provide short-term protection.


• Genetically engineered viruses
Genetic engineering is a technique that alters or changes the DNA of a plant or animal by inserting


new genetic information from another organism. After these organisms replicate, vaccines and
hormones are made that can help fight disease.


• Hepatitis B vaccine
The gene of the surface antigen of Hepatitis B virus is implanted into the DNA of a single


bacterium. The bacteria produces viral antigens which are then implanted to stimulate the immune
system.


IMMUNE SYSTEM DISORDERS
The immune system is a very complex and highly developed system, yet it has a very simple


mission, seek and destroy invaders. When the immune system does not function properly it leaves the
body open for attacks from an array of diseases. We classify these into three broad categories;
autoimmunity, immunodeficiencies, and hypersensitivities.


Anything that can trigger the immune response is called an antigen. An antigen can be a microbe
such as a virus, or even a part of a microbe. Tissues of cells from another person also carry nonself
markers and act as antigens. This explains why tissue transplants can be rejected. In abnormal
situations, the immune system can mistake self for nonself and launch an attack against the body's own
cells or tissues. The result is called an autoimmune disease. Some forms of arthritis and diabetes are
autoimmune diseases. In other cases, the immune system responds to a seemingly harmless foreign
substance such as a dust mite. The result is allergy, and this kind of antigen is called an allergen.


176 | Human Physiology




The Immune System


The Allergic response
Type 1 hypersensitivity is an allergic reaction provoked by reexposure to a specific antigen.


Exposure may be by ingestion, inhalation, injection, or direct contact. The reaction is mediated by IgE
antibodies and produced by the immediate release of histamine, tryptase, arachidonate and derivatives
by basophils and mast cells. This causes an inflammatory response leading to an immediate (within
seconds to minutes) reaction.


The reaction may be either local or systemic. Symptoms vary from mild irritation to sudden death
from anaphylactic shock. Treatment usually involves epinephrine, antihistamines, and corticosteroids.


Hay Fever
Hay fever involves an allergic reaction to pollen and results in allergic rhinitis (inflammation of the


nasal mucosa). It is most common in the haying season, which is why the ailment was named hay
fever. A virtually identical reaction occurs with allergy to mold, animal dander, dust, and similar
inhaled allergens. Particulate matter in polluted air and chemicals such as chlorine and detergents,
which can normally be tolerated, can greatly aggravate the condition. The pollens that cause hay fever
vary from person to person and from region to region; generally speaking, the tiny, hardly visible
pollens of wind-pollinated plants are the predominant culprits.


Autoimmune Disorders
For reasons we do not fully understand, sometimes the immune system attacks the body the way it


normally would attack a germ or foreign substance. The genes some people inherit can contribute to
their susceptibility to develop an autoimmune disease. Most autoimmune diseases effect woman more
than men.


• In Juvenile-onset diabetes the immune system starts attacking and eliminating the cells in
the pancreas that make insulin.


• Multiple Sclerosis is a chronic degenerative disorder of the central nervous system where
the immune system starts attacking and destroying vital myelin in the brain and spinal cord.
This causes multiple sclerosis (scars) on the myelin sheath resulting in loss of nerve function.


• Another fairly known disorder is Rheumatoid Arthritis this is when the immune system
starts attacking the tissue inside your joints.


• There is another disorder, Organ and Tissue Transplants, that is classified under
immuno-deficiencies but in reality is not a failure of the immune system. In transplants, foreign
tissue is placed inside the body. These tissues do not perfectly match the surrounding cells. The
body sees this as something that should not be there and sends messages to attack and kill it.
This can make transplanting nearly impossible. This problem can not be completely prevented
but it can be diminished by making sure the donor tissue is a close match to the recipient tissue.
In addition, the recipient is placed on immuno-suppressing drugs to try and prevent the immune
system from attacking and rejecting the new organ or tissue.


Wikibooks | 177




Chapter 9
• Vitiligo is an autoimmune disorder in which the immune system destroys pigment-making


cells called melanocytes. This results in irregularly shaped milky-white patches of skin on
different parts of the body. This is the condition which Michael Jackson claims to have.


Immunodeficiency Diseases
When the immune system is presented with foreign antigens in association with dendritic cells, a


vigorous immune response ensues. (Antigens are the molecules on the surface of invader cells that
announce them as different from the body's cells.). Alternatively, dendritic cells can be exploited
during the development of many immune based diseases.


AIDS and HIV
Acquired immunodeficiency disease (AIDS) is a well-known immune system disease. Acquired


Immune Deficiency Syndrome or acquired immunodeficiency syndrome (AIDS or Aids) is a collection
of symptoms and infections resulting from the specific damage to the immune system caused by the
human immunodeficiency virus (HIV).The late stage of the condition leaves individuals prone to
opportunistic infections and tumors. Although treatments for AIDS and HIV exist to slow the virus's
progression, there is no known cure. HIV is transmitted through direct contact of a mucous membrane
or the bloodstream with a bodily fluid containing HIV, such as blood, semen, vaginal fluid, preseminal
fluid, and breast milk.This transmission can come in the form of anal, vaginal or oral sex, blood
transfusion, contaminated hypodermic needles, exchange between mother and baby during pregnancy,
childbirth, or breastfeeding, or other exposure to one of the above bodily fluids. AIDS is the most
severe manifestation of infection with HIV. HIV is a retrovirus that primarily infects vital components
of the human immune system such as CD4+ T cells (a subset of T cells), macrophages and dendritic
cells. It directly and indirectly destroys CD4+ T cells. CD4+ T cells are required for the proper
functioning of the immune system. When HIV kills CD4+ T cells so that there are fewer than 200
CD4+ T cells per microliter (µL) of blood, cellular immunity is lost, leading to the condition known as
AIDS. Acute HIV infection progresses over time to clinical latent HIV infection and then to early
symptomatic HIV infection and later to AIDS, which is identified on the basis of the amount of CD4+
T cells in the blood and the presence of certain infections.


In the absence of antiretroviral therapy, the median time of progression from HIV infection to
AIDS is nine to ten years, and the median survival time after developing AIDS is only 9.2 months.
However, the rate of clinical disease progression varies widely between individuals, from two weeks up
to 20 years. Many factors affect the rate of progression. These include factors that influence the body's
ability to defend against HIV such as the infected person's general immune function. Older people have
weaker immune systems, and therefore have a greater risk of rapid disease progression than younger
people. Poor access to health care and the existence of coexisting infections such as tuberculosis also
may predispose people to faster disease progression. The infected person's genetic inheritance plays an
important role and some people are resistant to certain strains of HIV.


Different Types of T Lymphocyte Cells
Several different subsets of T cells have been described, each with a distinct function.


178 | Human Physiology




The Immune System


Cytotoxic T cells (Tc cells) destroy virally infected cells and tumor cells, and are also implicated in
transplant rejection. These cells are also known as CD8+ T cells, since they express the CD8
glycoprotein at their surface.


Helper T cells, (Th cells) are the "middlemen" of the adaptive immune system. Once activated,
they divide rapidly and secrete small proteins called cytokines that regulate or "help" the immune
response. These cells (also called CD4+ T cells) are a target of HIV infection; the virus infects the cell
by using the CD4 protein to gain entry. The loss of Th cells as a result of HIV infection leads to the
symptoms of AIDS.


Memory T cells are a subset of antigen-specific T cells that persist long-term after an infection has
resolved. They quickly expand to large numbers of effector T cells upon re-exposure to their cognate
antigen, thus providing the immune system with "memory" against past infections. Memory cells may
be either CD4+ or CD8+.


Regulatory T cells (Treg cells), formerly known as suppressor T cells, are crucial for the
maintenance of immunological tolerance. Their major role is to shut down T cell mediated immunity
towards the end of an immune reaction and to suppress auto-reactive T cells that escaped the process of
negative selection in the thymus. Two major classes of regulatory T cells have been described,
including the naturally occurring Treg cells and the adaptive Treg cells.


Treg cells (also known as CD4+CD25+FoxP3+ Treg cells) arise in the thymus, whereas the
adaptive Treg cells (also known as Tr1 cells or Th3 cells) may originate during a normal immune
response. Naturally occurring Treg cells can be distinguished from other T cells by the presence of an
intracellular molecule called FoxP3. Mutations of the FOXP3 gene can prevent regulatory T cell
development, causing the fatal autoimmune disease IPEX.


'Natural Killer T cells''' (NKT cells) are a special kind of lymphocyte that bridges the adaptive
immune system with the innate immune system. Unlike conventional T cells that recognize peptide
antigen presented by major histocompatibility complex (MHC) molecules, NKT cells recognize
glycolipid antigen presented by a molecule called CD1d. Once activated, these cells can perform
functions ascribed to both Th and Tc cells (i.e. cytokine production and release of cytolytic/cell killing
molecules).


THE FUNCTIONS OF T LYMPHOCYTES T lymphocytes cells help with all components of
the immune system, including cell elimination by killer T cells and maintaining roles by helper and
suppressor T cells. Although the specific mechanisms of activation vary slightly between different
types of T cells, the "two-signal model" in CD4+ T cells holds true for most.


The Immune System Pioneers
Ilya Mechnikov and the Phagocyte Cells


In 1882, a Russian scientist named Ilya Mechnikov was experimenting with the larvae of the sea
star. He stuck a thorn in the larvae and then he noticed that something really weird happened. Strange
cells started gathering near the point of insertion. The cells that were surrounding the thorn were eating
any foreign substance that was entering through the ruptured skin. Mechnikov decided to name these


Wikibooks | 179




Chapter 9
new cells "phagocytes", in Greek meaning "devouring cells."


This discovery was very important since it helped scientists how the body defends itself against
disease. If the phagocyte encountered anything foreign, they absorb and destroy it. Phagocytes also
play an important part in activating the immune response in the rest of the body.


Paul Ehrlich and the Side-chain Theory
Ehrlich supposed that living cells have side-chains. These side chains can link with a particular


toxin, just as Emil Fisher said enzymes must bind to their receptors "like a key in a lock."
He theorized that a cell under threat grew additional side-chains to bind the toxin, and that these


additional side chains broke off to become the antibodies that circulate through the body. It was these
antibodies that Ehrlich first described as "magic bullets" in search of toxins.


Ehrlich figured that if a compound could be made that selectively targeted a disease causing
organism, then a toxin for that organism could be delivered along with the agent of selectivity. Hence, a
"magic bullet" would be created that killed only the organism targeted.


Ehrlich predicted autoimmunity calling it "horror autotoxicus".
In 1908, Ehrlich and Mechnikov received the Nobel Prize.


Review Questions
1-When neutrophils and macrophages squeeze out of capillaries to fight off infection it is called:


A) phagocytosis
B) hemolysis
C) interleukin
D) diapedesis
E) folliculitis


2-During a great battle between your WBC's and an aggressive microbe, an inflammatory response has
been initiated. Reddness and edema has kicked in what else does the body do to protect itself?


A) Histamine cause vasodilation
B) Hypothalmus raises the thermostat
C) Neutrophils engulf and destroy the microbe
D) Living and dead WBC and bacteria accumulate
E) All of the above


3-Specificity and memory are associated with which body defense mechanism?
A) inflammatory response
B) phagocytosis by macrophages and neutrophils
C) interferon


180 | Human Physiology




The Immune System


D) T cell and B cell responses
E) anatomical barriers in the body


4-An additional chemical defense found in tears and saliva?
A) T lymphocytes
B) saline
C) lysozyme
D) EFC


5-Which of the following does complement protein perform
A) They cause antibody release
B) T cell development
C) The release if histamine
D) Promotes tissue repair
E) Mast cell degranulation


6-Which substance induces fever?
A) Pyrogen
B) Pus
C) Monocytes
D) Edema
E) Interferon


7-Major function(s) of the lymphatic system is/are?
A) provide route for return of extracellular fluid
B) act as drain off for inflammatory response
C) render surveillance, recognition , and protection against foreign materials via lymphocytes,
phagocytes, and antibodies.
D) a and c
E) all of the above


8-An antigen is:
A) a chemical messenger that is released by virus infected cells
B) a lymphocyte responsible for cell-mediated immunity
C) something that coats the inside of lungs, causing infection
D) a protein or other molecule that is recognized as non-self
E) a thick yellow-white fluid


9-A foreign substance, usually a protein, that stimulates the immune system to react, such as by
producing antibodies is a ______________.


A) allergen
B) antigen
C) histamine


Wikibooks | 181




Chapter 9
D) mast cell
E) interferon


10-When a macrophage ingests an invading bacteria and takes the antigen to a lymph node, what
happens next?


A) the macrophage will present it to the first B-cell it encounters, and the B-cell will in turn
change its surface receptors to match the antigen
B) a B-cell will only become activated if it already has a match for the antigen
C) a matching B-cell will become activated into a cytotoxic T-cell
D) the cells of the lymph node will release histamine
E) the lymph node will increase production of neutrophils


11-What is the most common portal of entry for diseases, into the body?
A) Respiratory system
B) Endocrine system
C) Hematacrit system
D) Any opening into the body.


12-This gland shrinks in size during adulthood, and has hormones that function in maturation of T-
lymphocytes:


A) lymph nodes
B) thymus
C) spleen
D) GALT
E) tonsils


13-Which of the following is not a mechanical factor to protect the skin and mucous membranes from
infection?


A) Layers of cells
B) Tears
C) Saliva
D) Lysozyme
E) None of the above


14-Where is the site of maturation for a B cell?
A) thymus
B) bone marrow
C) pancreas
D) cortex


15-Nonspecific resistance is
A) The body's ability to ward off diseases.
B) The body's defenses against any kind of pathogen.


182 | Human Physiology




The Immune System


C) The body's defense against a particular pathogen.
D) The lack of resistance.
E) None of the above.


GLOSSARY
Antibody: Antibody or (immunoglobulin) is a protein generated by the immune system (B cells)


in response to a foreign substance (antigen).
Antibody titer: A test done to check the immunity of vaccination, when identification of a low


immunity to a vaccine a booster shot can be given to increase the immunity.
Antigen: Protein (or polysaccharide) molecule that the body recognizes as nonself. Substance


body recognizes as foreign such as, fungi, viruses, protozoans, parasitic worms, pollen, poison ivy plant
resin, insect venom, and transplanted organs.


Antiseptic: Antimicrobial substance applied to living tissue or skin to prevent infection.
Apoptosis: Programmed cell death
B Cell: Lymphocytes that are responsible for antibody-mediated immunity
Basophils: WBC that release histamine and other chemicals
Chemotaxis: Movement of cells, phagocytes especially, they move in a specific direction in a


tumbling fashion like rolling this is all due to a chemical stimulant.
Complement System: Biochemical cascade of the immune system that helps clear pathogens from


an organism, and promote healing
Cytokines: Regulatory peptides that control cell development,differentiation, and the immune


response
Dendritic: cells that activate lymphocytes
Diapedesis: The movement of WBC's for the blood to the surrounding tissue. A mechanism of the


kind phagocyte that will walk or crawl out of the blood stream to site of infection.
Edema: Swelling that forms when too much tissue fluid forms or not enough taken away
Eosinophils: WBC that fight parasites and contribute to allergic reactions
Histamine: Histamine is a chemical involved in inflammation, this chemical makes capillaries


leaky, in this it will move more fluid out into the tissue spaces.
IgA: Found in breast milk, mucus, saliva, and tears. This immunoglobulin functions to stop the


pathogens before entry to the internal environment.


Wikibooks | 183




Chapter 9
IgD: This immunoglobulin is found on B-cells and function is not known.
IgE: This immunoglobulin is combined with mast cells that in turn release histamine, this kind of


globulin is released in the presence of an allergic response or parasitic infection.
IgG: This immunoglobulin is the majority of the specific immunity against bacteria and viruses in


the extracellular fluid.
IgM: This immunoglobulin is associated to antibodies that react to incompatibility of ABO and Rh


factor grouping.
Immunoglobulins: Proteins that are antibodies receptors on the surface of B-cells, there are five


classes.
Kinins: Kinins is a chemical involved in inflammation, it is inactive in blood plasma but become


activated by tissue damage and in turn stimulate pain receptors in skin.
Leukocytes: primary cells of the immune system; also called white blood cells
Lymph: fluid of the lymph system; originates as blood plasma that leaks from the capillaries of the


circulatory system, becoming interstitial fluid, filling the space between individual cells of tissue
Lymphocytes: The key cells that mediate the acquired immune response of the body
Lymph Nodes: Small oval shaped structures located along the lymphatic vessels
Lysosome: Organelle containing digestive enzymes (acid hydrolases) that digest viruses, bacteria,


food particles and worn out organelles
Lysozyme: Enzyme that attacks cell walls of bacteria and breaks them down; found and used as an


antiseptic property in the body's first line of defense (ie. saliva, tears, sweat, etc)
Macrophages: WBC that are the primary scavengers within tissues
Membrane Attack Complex (MAC): Work in the same way as the perforins of the NK cells that


is it punches holes in the membrane that causes lysis.
Neutrophils: WBC that "eat" bacteria and release cytokines
Opsonin: Any substance that promotes a phagocytosis by binding a microbe to a phagocyte.
Perforin: Protein secreted by cytotoxic T cells, causes pores to form in the plasma membrane of


the target cell resulting in lysis.
Peyer’s Patches: located in the wall of the intestine and the appendix, attached to the cecum of the


large intestine, intercept pathogens that come into the body through the intestinal tract
Phagocytes: WBC that engulf and ingest their targets by phagocytosis


184 | Human Physiology




The Immune System


Pyrogens: Foreign substances and or microorganisms that causes hypothalamic thermoregulatory
center to increase and causes fever (pyrexia), are called pyrogens.


Right Lymphatic Duct: Lymphatic duct that serves all of the right side of the upper body and
thorasic area(head,neck)


Spleen: Ductless, vertebrate gland that is closely associated with the circulatory system, where it
functions in the destruction of old red blood cells in holding a reservoir of blood


T Cell: cells that carry out cell-mediated immunity
Thoracic Duct: Lymphatic duct that serves the abdomen, lower extremities and the left side of the


upper body(head,neck, and arm)
Thymus Gland: Gland that contains lymphocytes; produces thymosin that is thought to aid in the


maturation of T lymphocytes
live version • discussion • edit lesson • comment • report an error


Wikibooks | 185




Chapter 10


10 THE URINARY SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction
he Urinary System is a group of organs in the body concerned with filtering out excess fluid and
other substances from the bloodstream. The substances are filtered out from the body in the form


of urine. Urine is a liquid produced by the kidneys, collected in the bladder and excreted through the
urethra. Urine is used to extract excess minerals or vitamins as well as blood corpuscles from the body.
The Urinary organs include the kidneys, ureters, bladder, and urethra. The Urinary system works with
the other systems of the body to help maintain homeostasis. The kidneys are the main organs of
homeostasis because they maintain the acid base balance and the water salt balance of the blood.


T


Functions of the Urinary System
One of the major functions of the Urinary system is the process of excretion. Excretion is the


process of eliminating, from an organism, waste products of metabolism and other materials that are of
no use. The urinary system maintains an appropriate fluid volume by regulating the amount of water
that is excreted in the urine. Other aspects of its function include regulating the concentrations of
various electrolytes in the body fluids and maintaining normal pH of the blood. Several body organs
carry out excretion, but the kidneys are the most important excretory organ. The primary function of
the kidneys are to maintain a stable internal environment (homeostasis) for optimal cell and tissue
metabolism. They do this by separating urea, mineral salts, toxins, and other waste products from the
blood. They also do the job of conserving water, salts, and electrolytes. At least one kidney must
function properly for life to be maintained. Six important roles of the kidneys are:


Regulation of plasma ionic composition. Ions such as sodium, potassium, calcium, magnesium,
chloride, bicarbonate, and phosphates are regulated by the amount that the kidney excretes.


Regulation of plasma osmolarity. The kidneys regulate osmolarity because they have direct
control over how many ions and how much water a person excretes.


Regulation of plasma volume. Your kidneys are so important they even have an effect on your
blood pressure. The kidneys control plasma volume by controlling how much water a person excretes.
The plasma volume has a direct effect on the total blood volume, which has a direct effect on your
blood pressure. Salts such as NaCI can cause osmosis, the diffusion of water into the blood.


Regulation of plasma hydrogen ion concentration (pH). The kidneys partner up with the lungs
and they together control the pH. The kidneys have a major role because they control the amount of
bicarbonate excreted or held onto. The kidneys help maintain the blood Ph mainly by excreting
hydrogen ions and reabsorbing bicarbonate ions as needed.


Removal of metabolic waste products and foreign substances from the plasma. One of the
most important things the kidneys excrete is nitrogenous waste. As the liver breaks down amino acids it
also releases ammonia. The liver then quickly combines that ammonia with carbon dioxide, creating


186 | Human Physiology




The Urinary System


urea which is the primary nitrogenous end product of metabolism in humans. The liver turns the
ammonia into urea because it is much less toxic. We can also excrete some ammonia, creatinine and
uric acid. The creatinine comes from the metabolic breakdown of creatine phospate (a high-energy
phosphate in muscles). Uric acid comes from the break down of necloetides. Uric acid is insoluble and
too much uric acid in the blood will build up and form crystals that can collect in the joints and cause
gout.


Secretion of Hormones The endocrine system has assistance from the kidney's when releasing
hormones. Renin is released by the kidneys. Renin leads to the secretion of aldosterone which is
released from the adrenal cortex. Aldosterone promotes the kidneys to reabsorb the sodium (Na+) ions.
The kidneys also secrete erythropoietin when the blood doesn't have the capacity to carry oxygen.
Erythropoietin stimulates red blood cell production. The Vitamin D from the skin is also activated with
help from the kidneys. Calcium (Ca+) absorption from the digestive tract is promoted by vitamin D.


CC: Chapter Check: Name the role of the kidneys and how they work?


Organs in the Urinary System
Kidneys And Their Structure


The kidneys are a pair of bean shaped, reddish brown organs about the size of your fist. They are
covered by the renal capsule, which is a tough capsule of fibrous connective tissue. Adhering to the
surface of each kidney is two layers of fat to help cushion them. There is a concaved side of the kidney
that has a depression where a renal artery enters, and a renal vein and a ureter exit the kidney. The
kidneys are located at the rear wall of the abdominal cavity just above the waistline, and are protected
by the ribcage. They are considered retroperitoneal, which means they lie behind the peritoneum. There
are three major regions of the kidney, renal cortex, renal medulla and the renal pelvis. The outer,
granulated layer is the renal cortex. The cortex stretches down in between a radially striated inner layer.
The inner radially striated layer is the renal medulla. This contains pyramid shaped tissue called the
renal pyramids, separated by renal columns. The ureters are continuous with the renal pelvis and is the
very center of the kidney.


1. Renal pyramid
2. Interlobar artery
3. Renal artery
4. Renal vein
5. Renal hylum
6. Renal pelvis
7. Ureter
8. Minor calyx
9. Renal capsule
10.Inferior renal capsule
11.Superior renal capsule
12.Interlobar vein
13.Nephron
14.Minor calyx
15.Major calyx


Wikibooks | 187




Chapter 10
16.Renal papilla
17.Renal column


Renal Vein
The renal veins are veins that drain the kidney. They connect the kidney to the inferior vena cava.


Because the inferior vena cava is on the right half of the body, the left renal vein is generally the longer
of the two. Unlike the right renal vein, the left renal vein often receives the left gonadal vein (left
testicular vein in males, left ovarian vein in females). It frequently receives the left suprarenal vein as
well.


Renal Artery
The renal arteries normally arise off the abdominal aorta and supply the kidneys with blood. The


arterial supply of the kidneys are variable and there may be one or more renal arteries supplying each
kidney. Due to the position of the aorta, the inferior vena cava and the kidneys in the body, the right
renal artery is normally longer than the left renal artery. The right renal artery normally crosses
posteriorly to the inferior vena cava. The renal arteries carry a large portion of the total blood flow to
the kidneys. Up to a third of the total cardiac output can pass through the renal arteries to be filtered by
the kidneys.


Ureters
The ureters are two tubes that drain urine from the kidneys to the bladder. Each ureter is a


muscular tube about 10 inches (25 cm) long. Muscles in the walls of the ureters send the urine in small
spurts into the bladder, (a collapsible sac found on the forward part of the cavity of the bony pelvis that
allows temporary storage of urine). After the urine enters the bladder from the ureters, small folds in
the bladder mucosa act like valves peventing backward flow of the urine. The outlet of the bladder is
controlled by a sphincter muscle. A full bladder stimulates sensory nerves in the bladder wall that relax
the sphincter and allow release of the urine. However, relaxation of the sphincter is also in part a
learned response under voluntary control. The released urine enters the urethra.


Urinary Bladder
The urinary bladder is a hollow, muscular and distendible or elastic organ that sits on the pelvic


floor (superior to the prostate in males). On its anterior border lies the pubic symphysis and, on its
posterior border, the vagina (in females) and rectum (in males). The urinary bladder can hold
approximately 17 to 18 ounces (500 to 530 ml) of urine, however the desire to micturate is usually
experienced when it contains about 150 to 200 ml. When the bladder fills with urine (about half full),
stretch receptors send nerve impulses to the spinal cord, which then sends a reflex nerve impulse back
to the sphincter (muscular valve) at the neck of the bladder, causing it to relax and allow the flow of
urine into the urethra. The Internal urethral sphincter is involuntary. The ureters enter the bladder
diagonally from its dorsolateral floor in an area called the trigone. The trigone is a triangular shaped
area on the postero-inferior wall of the bladder. The urethra exits at the lowest point of the triangle of
the trigone. The urine in the bladder also helps regulate body temperature. If the bladder becomes
completely void of fluid, it causes the patient to chill.
188 | Human Physiology




The Urinary System


Urethra
The urethra is a muscular tube that connects the bladder with the outside of the body. The


function of the urethra is to remove urine from the body. It measures about 1.5 inches (3.8 cm) in a
woman but up to 8 inches (20 cm) in a man. Because the urethra is so much shorter in a woman it
makes it much easier for a woman to get harmful bacteria in her bladder this is commonly called a
bladder infection or a UTI. The most common bacteria of a UTI is E-coli from the large intestines that
have been excreted in fecal matter. Female urethra


In the human female, the urethra is about 1-2 inches long and opens in the vulva between the
clitoris and the vaginal opening.


Men have a longer urethra than women. This means that women tend to be more susceptible to
infections of the bladder (cystitis) and the urinary tract.


Male urethra
In the human male, the urethra is about 8 inches (20 cm) long and opens at the end of the penis.
The length of a male's urethra, and the fact it contains a number of bends, makes catheterisation


more difficult.
The urethral sphincter is a collective name for the muscles used to control the flow of urine from


the urinary bladder. These muscles surround the urethra, so that when they contract, the urethra is
closed.


• There are two distinct areas of muscle: the internal sphincter, at the bladder neck and
• the external, or distal, sphincter.


Human males have much stronger sphincter muscles than females, meaning that they can retain a
large amount of urine for twice as long, as much as 800mL, i.e. "hold it".


Nephrons
A nephron is the basic structural and functional unit of the kidney. The name nephron comes from


the Greek word (nephros) meaning kidney. Its chief function is to regulate water and soluble substances
by filtering the blood, reabsorbing what is needed and excreting the rest as urine. Nephrons eliminate
wastes from the body, regulate blood volume and pressure, control levels of electrolytes and
metabolites, and regulate blood pH. Its functions are vital to life and are regulated by the endocrine
system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.


Each nephron has its own supply of blood from two capillary regions from the renal artery. Each
nephron is composed of an initial filtering component (the renal corpuscle) and a tubule specialized for
reabsorption and secretion (the renal tubule). The renal corpuscle filters out large solutes from the
blood, delivering water and small solutes to the renal tubule for modification.


Wikibooks | 189




Chapter 10
Glomerulus


The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the
renal circulation. The glomerular blood pressure provides the driving force for fluid and solutes to be
filtered out of the blood and into the space made by Bowman's capsule. The remainder of the blood not
filtered into the glomerulus passes into the narrower efferent arteriole. It then moves into the vasa recta,
which are collecting capillaries intertwined with the convoluted tubules through the interstitial space,
where the reabsorbed substances will also enter. This then combines with efferent venules from other
nephrons into the renal vein, and rejoins with the main bloodstream.


Afferent/Efferent Arterioles
The afferent arteriole supplies blood to the glomerulus. A group of specialized cells known as


juxtaglomerular cells are located around the afferent arteriole where it enters the renal corpuscle. The
efferent arteriole drains the glomerulus. Between the two arterioles lies specialized cells called the
macula densa. The juxtaglomerular cells and the macula densa collectively form the juxtaglomerular
apparatus. It is in the juxtaglomerular apparatus cells that the enzyme renin is formed and stored.
Renin is released in response to decreased blood pressure in the afferent arterioles, decreased sodium
chloride in the distal convoluted tubule and sympathetic nerve stimulation of receptors (beta-adrenic)
on the juxtaglomerular cells. Renin is needed to form Angiotensin I and Angiotensin II which stimulate
the secretion of aldosterone by the adrenal cortex.


Glomerular Capsule or Bowman's Capsule
Bowman's capsule (also called the glomerular capsule) surrounds the glomerulus and is


composed of visceral (simple squamous epithelial cells) (inner) and parietal (simple squamous
epithelial cells) (outer) layers. The visceral layer lies just beneath the thickened glomerular basement
membrane and is made of podocytes which send foot processes over the length of the glomerulus. Foot
processes interdigitate with one another forming filtration slits that, in contrast to those in the
glomeruluar endothelium, are spanned by diaphragms. The size of the filtration slits restricts the
passage of large molecules (eg, albumin) and cells (eg, red blood cells and platelets). In addition, foot
processes have a negatively-charged coat (glycocalyx) that limits the filtration of negatively-charged
molecules, such as albumin. This action is called electrostatic repulsion.


The parietal layer of Bowman's capsule is lined by a single layer of squamous epithelium. Between
the visceral and parietal layers is Bowman's space, into which the filtrate enters after passing through
the podocytes' filtration slits. It is here that smooth muscle cells and macrophages lie between the
capillaries and provide support for them. Unlike the visceral layer, the parietal layer does not function
in filtration. Rather, the filtration barrier is formed by three components: the diaphragms of the
filtration slits, the thick glomerular basement membrane, and the glycocalyx secreted by podocytes.
99% of glomerular filtrate will ultimately be reabsorbed.


The process of filtration of the blood in the Bowman's capsule is ultrafiltration (or glomerular
filtration), and the normal rate of filtration is 125 ml/min, equivalent to ten times the blood volume
daily. Measuring the glomerular filtration rate (GFR) is a diagnostic test of kidney function. A
decreased GFR may be a sign of renal failure. Conditions that can effect GFR include: arterial pressure,
afferent arteriole constriction, efferent arteriole constriction, plasma protein concentration and colloid
osmotic pressure.


190 | Human Physiology




The Urinary System


Any proteins that are roughly 30 kilodaltons or under can pass freely through the membrane.
Although, there is some extra hindrance for negatively charged molecules due to the negative charge of
the basement membrane and the podocytes. Any small molecules such as water, glucose, salt (NaCl),
amino acids, and urea pass freely into Bowman's space, but cells, platelets and large proteins do not. As
a result, the filtrate leaving the Bowman's capsule is very similar to blood plasma in composition as it
passes into the proximal convoluted tubule. Together, the glomerulus and Bowman's capsule are called
the renal corpuscle.


Proximal Convoluted Tubule (PCT)
The proximal tubule can be anatomically divided into two segments: the proximal convoluted


tubule and the proximal straight tubule. The proximal convoluted tubule can be divided further into S1
and S2 segments based on the histological appearance of it's cells. Following this naming convention,
the proximal straight tubule is commonly called the S3 segment. The proximal convoluted tubule has
one layer of cuboidal cells in the lumen. This is the only place in the nephron that contains cuboidal
cells. These cells are covered with millions of microvilli. The microvilli serve to increase surface area
for reabsorption.


Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular
capillaries, including approximately two-thirds of the filtered salt and water and all filtered organic
solutes (primarily glucose and amino acids). This is driven by sodium transport from the lumen into the
blood by the Na+/K+ ATPase in the basolateral membrane of the epithelial cells. Much of the mass
movement of water and solutes occurs in between the cells through the tight junctions, which in this
case are not selective.


The solutes are absorbed isotonically, in that the osmotic potential of the fluid leaving the proximal
tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids, inorganic
phosphate, and some other solutes are reabsorbed via secondary active transport through cotransport
channels driven by the sodium gradient out of the nephron.


Loop of the Nephron or Loop of Henle
The loop of Henle (sometimes known as the nephron loop) is a U-shaped tube that consists of a


descending limb and ascending limb. It begins in the cortex, receiving filtrate from the proximal
convoluted tubule, extends into the medulla, and then returns to the cortex to empty into the distal
convoluted tubule. Its primary role is to concentrate the salt in the interstitium, the tissue surrounding
the loop.
Descending limb


Its descending limb is permeable to water but completely impermeable to salt, and thus only
indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into
the hypertonic interstitium of the renal medulla, water flows freely out of the descending limb by
osmosis until the tonicity of the filtrate and interstitium equilibrate. Longer descending limbs
allow more time for water to flow out of the filtrate, so longer limbs make the filtrate more
hypertonic than shorter limbs.


Ascending limb


Wikibooks | 191




Chapter 10
Unlike the descending limb, the ascending limb of Henle's loop is impermeable to water, a
critical feature of the countercurrent exchange mechanism employed by the loop. The ascending
limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives
countercurrent exchange. In passing through the ascending limb, the filtrate grows hypotonic
since it has lost much of its sodium content. This hypotonic filtrate is passed to the distal
convoluted tubule in the renal cortex.


Distal Convoluted Tubule (DCT)
The distal convoluted tubule is similar to the proximal convoluted tubule in structure and function.


Cells lining the tubule have numerous mitochondria, enabling active transport to take place by the
energy supplied by ATP. Much of the ion transport taking place in the distal convoluted tubule is
regulated by the endocrine system. In the presence of parathyroid hormone, the distal convoluted tubule
reabsorbs more calcium and excretes more phosphate. When aldosterone is present, more sodium is
reabsorbed and more potassium excreted. Atrial natriuretic peptide causes the distal convoluted tubule
to excrete more sodium. In addition, the tubule also secretes hydrogen and ammonium to regulate pH.
After traveling the length of the distal convoluted tubule, only 3% of water remains, and the remaining
salt content is negligible. 97.9% of the water in the glomerular filtrate enters the convoluted tubules and
collecting ducts by osmosis.


Collecting ducts
Each distal convoluted tubule delivers its filtrate to a system of collecting ducts, the first segment


of which is the connecting tubule. The collecting duct system begins in the renal cortex and extends
deep into the medulla. As the urine travels down the collecting duct system, it passes by the medullary
interstitium which has a high sodium concentration as a result of the loop of Henle's countercurrent
multiplier system. Though the collecting duct is normally impermeable to water, it becomes permeable
in the presence of antidiuretic hormone (ADH). As much as three-fourths of the water from urine can
be reabsorbed as it leaves the collecting duct by osmosis. Thus the levels of ADH determine whether
urine will be concentrated or dilute. Dehydration results in an increase in ADH, while water sufficiency
results in low ADH allowing for diluted urine. Lower portions of the collecting duct are also permeable
to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high ion
concentration (which is very important for the nephron).


Urine leaves the medullary collecting ducts through the renal papilla, emptying into the renal
calyces, the renal pelvis, and finally into the bladder via the ureter. Because it has a different embryonic
origin than the rest of the nephron (the collecting duct is from endoderm whereas the nephron is from
mesoderm), the collecting duct is usually not considered a part of the nephron proper.


Renal Hormones
1. Vitamin D- Becomes metabolically active in the kidney. Patients with renal disease have


symptoms of disturbed calcium and phosphate balance.
2. Erythropoietin- Released by the kidneys in response to decreased tissue oxygen levels


(hypoxia).
3. Natriuretic Hormone- Released from cardiocyte granules located in the right atria of the


heart in response to increased atrial stretch. It inhibits ADH secretions which can contribute to
the loss of sodium and water.


192 | Human Physiology




The Urinary System


Formation of Urine
Urine is formed in three steps: Filtration, Reabsorption, and Secretion.


Filtration
Blood enters the afferent arteriole and flows into the glomerulus. Blood in the glomerulus has both


filterable blood components and non-filterable blood components. Filterable blood components move
toward the inside of the glomerulus while non-filterable blood components bypass the filtration process
by exiting through the efferent arteriole. Filterable Blood components now take on plasma like form
called glomerular filtrate. A few of the filterable blood components are water, nitrogenous waste,
nutrients and salts (ions). Nonfilterable blood components include formed elements such as blood cells
and platelets along with plasma proteins. The glomerular filtrate is not the same consistency as urine, as
much of it is reabsorbed into the blood as the filtrate passes through the tubules of the nephron.


Reabsorption
Within the peritubular capillary network, molecules and ions are reabsorbed back into the blood.


Sodium Chloride reabsorbed into the system increases the osmolarity of blood in comparison to the
glomerular filtrate. This reabsorption process allows water (H2O) to pass from the glomerular filtrate
back into the circulatory system.


Glucose and various amino acids also are reabsorbed into the circulatory system. These nutrients
have carrier molecules that claim the glomerular molecule and release it back into the circulatory
system. If all of the carrier molecules are used up, excess glucose or amino acids are set free into the
urine. A complication of diabetes is the inability of the body to reabsorb glucose. If too much glucose
appears in the glomerular filtrate it increases the osmolarity of the filtrate, causing water to be released
into the urine rather than reabsorbed by the circulatory system. Frequent urination and unexplained
thirst are warning signs of diabetes, due to water not being reabsorbed.


Glomerular filtrate has now been separated into two forms: Reabsorbed Filtrate and Non-
reabsorbed Filtrate. Non-reabsorbed filtrate is now known as tubular fluid as it passes through the
collecting duct to be processed into urine.


Secretion
Some substances are removed from blood through the peritubular capillary network into the distal


convoluted tubule or collecting duct. These substances are Hydrogen ions, creatinine, and drugs. Urine
is a collection of substances that have not been reabsorbed during glomerular filtration or tubular
secretion.


Maintaining Water-Salt Balance
It is the job of the kidneys to maintain the water-salt balance of the blood. They also maintain


Wikibooks | 193




Chapter 10
blood volume as well as blood pressure. Simple examples of ways that this balance can be changed
include ingestion of water, dehydration, blood loss and salt ingestion.


Reabsorption of water
Direct control of water excretion in the kidneys is exercised by the anti-diuretic hormone (ADH),


released by the posterior lobe of the pituitary gland. ADH causes the insertion of water channels into
the membranes of cells lining the collecting ducts, allowing water reabsorption to occur. Without ADH,
little water is reabsorbed in the collecting ducts and dilute urine is excreted. There are several factors
that influence the secretion of ADH. The first of these happen when the blood plasma gets too
concentrated. When this occurs, special receptors in the hypothalamus release ADH. When blood
pressure falls, stretch receptors in the aorta and carotid arteries stimulate ADH secretion to increase
volume of the blood.


Reabsorption of Salt
The Kidneys also regulate the salt balance in the blood by controlling the excretion and the


reabsorption of various ions. As noted above, ADH plays a role in increasing water reabsorption in the
kidneys, thus helping to dilute bodily fluids. The kidneys also have a regulated mechanism for
reabsorbing sodium in the distal nephron. This mechanism is controlled by aldosterone, a steroid
hormone produced by the adrenal cortex. Aldosterone promotes the excretion of potassium ions and the
reabsorption of sodium ions. The release of Aldosterone is initiated by the kidneys. The
juxtaglomerular apparatus is a renal structure consisting of the macula densa, mesangial cells, and
juxtaglomerular cells. Juxtaglomerular cells (JG cells, also known as granular cells) are the site of renin
secretion. Renin is an enzyme that angiotensinogen (a large plasma protein produced by the liver) into
Angiotensin I and eventually into Angiotensin II which stimulates the adrenal cortex to produce
aldosterone. The reabsorption of sodium ions is followed by the reapsorption of water. This causes
blood pressure as well as blood volume to increase.


Atrial natriuretic hormone (ANH) is released by the atria of the heart when cardiac cells are
streatched due to increased blood volume. ANH inhibits the secretion of renin by the juxtaglomerular
apparatus and the secretion of the aldosterone by the adrenal cortex. This promotes the excretion of
sodium. When sodium is excreted so is water. This causes blood pressure and volume to decrease.


Hypernatremia
An increase in plasma sodium levels above normal is hypernatremia. Sodium is the primary


solute in the extracellular fluid. Sodium levels have a major role in osmolarity regulation. For excitable
cells the electrochemical gradient for sodium across the plasma membrane is critical for life. Water
retention and an increased blood pressure usually are signs of hypernatremia. If the plasma sodium
levels are below normal it is called hyponatremia. Signs of this are low plasma volume and
hypotension.


Diuretics
A diuretic (colloquially called a water pill) is any drug that elevates the rate of bodily urine


194 | Human Physiology




The Urinary System


excretion (diuresis). Diuretics also decrease the extracellular fluid (ECF) volume, and are primarily
used to produce a negative extracellular fluid balance. Caffeine, cranberry juice and alcohol are all
weak diuretics. In medicine, diuretics are used to treat heart failure, liver cirrhosis, hypertension and
certain kidney diseases. Diuretics alleviate the symptoms of these diseases by causing sodium and
water loss through the urine. As urine is produced by the kidney, sodium and water – which cause
edema related to the disease – move into the blood to replace the volume lost as urine, thereby reducing
the pathological edema. Some diuretics, such as acetazolamide, help to make the urine more alkaline
and are helpful in increasing excretion of substances such as aspirin in cases of overdose or poisoning.
The antihypertensive actions of some diuretics (thiazides and loop diuretics in particular) are
independent of their diuretic effect. That is, the reduction in blood pressure is not due to decreased
blood volume resulting from increased urine production, but occurs through other mechanisms and at
lower doses than that required to produce diuresis. Indapamide was specifically designed with this is
mind, and has a larger therapeutic window for hypertension (without pronounced diuresis) than most
other diuretics. Chemically, diuretics are a diverse group of compounds that either stimulate or inhibit
various hormones that naturally occur in the body to regulate urine production by the kidneys. Alcohol
produces diuresis through modulation of the vasopressin system.


Diseases of the Kidney
Diabetic nephropathy (nephropatia diabetica), also known as Kimmelstiel-Wilson syndrome and


intercapillary glomerulonephritis, is a progressive kidney disease caused by angiopathy of capillaries in
the kidney glomeruli. It is characterized by nodular glomerulosclerosis. It is due to longstanding
diabetes mellitus, and is a prime cause for dialysis in many Western countries.


In medicine, hematuria (or "haematuria") is the presence of blood in the urine. It is a sign of a
large number of diseases of the kidneys and the urinary tract, ranging from trivial to lethal.


Kidney stones, also known as nephrolithiases, urolithiases or renal calculi, are solid accretions
(crystals) of dissolved minerals in urine found inside the kidneys or ureters. They vary in size from as
small as a grain of sand to as large as a golf ball. Kidney stones typically leave the body in the urine
stream; if they grow relatively large before passing (on the order of millimeters), obstruction of a ureter
and distention with urine can cause severe pain most commonly felt in the flank, lower abdomen and
groin. Kidney stones are unrelated to gallstones.


Case Study I was 34 weeks pregnant when I noticed blood in my urine. I immediately went to my
OBGYN where I was told that I had a bladder infection and given an antibiotic. The next morning I
experienced the most intense pain. I was rushed to the ER where I was told that I had kidney stones.
The doctors explained that there was nothing they could do as long as I was pregnant. The next 3 weeks
of my life were filled with intense pain and multiple painkillers. After I delivered my baby, CAT scans
were done and I was informed that I had 6 kidney stones. It took three more weeks for me to pass all of
the stones the largest measuring 5 mm. The stones were tested and I was informed that my body had
been building up calcium due to my pregnancy and this was the cause of the kidney stones. I continued
to have kidney pain for 6 months after passing the stones. I now live my life on a low calcium diet and
the hope that my body will not develop more kidney stones.


Pyelonephritis When an infection of the renal pelvis and calices, called pyelitis, spreads to
involve the rest of the kidney as well, the result is pyelonephritis. It usually results from the spread of
fecal bacterium Escherichia coli from the anal region superiorly through the urinary tract. In severe


Wikibooks | 195




Chapter 10
cases, the kidney swells and scars, abscesses form, and the renal pelvis fills with pus. Left untreated,
the infected kidney may be severely damaged, but administration of antibiotics usually achieve a total
cure.


Glomerulonephritis Inflammation of the glomerular can be caused by immunologic
abnormalities, drugs or toxins, vascular disorders, and systemic diseases. Gloerulonephritis can be
acute, chronic or progressive. Two major changes in the urine are distinctive of glomerulonephritis:
hematuria and proteinuria with albumin as the major protein. There is also a decrease in urine as there
is a decrease in GFR (glomerular filtration rate). Renal failure is associated with oliguria (less than 400
ml of urine output per day).


Renal Failure Uremia is a syndrome of renal failure and includes elevated blood urea and
creatinine levels. Acute renal failure can be reversed if diagnosed early. Acute renal failure can be
caused by severe hypotension or severe glomerular disease. Diagnostic tests include BUN and plasma
creatinine level tests. It is considered to be chronic renal failure if the decline of renal function to less
than 25%.


Diabetes Insipidus
This is caused by the deficiency of or decrease of ADH. The person with (DI) has the inability to


concentrate their urine in water restriction, in turn they will void up 3 to 20 liters/day. There are two
forms of (DI), neurogenic, and nephrogenic. In nephrogenic (DI) the kidneys do not respond to ADH.
Usually the nephrogenic (DI) is characterized by the impairment of the urine concentrating capability
of the kidney along with concentration of water. The cause may be a genetic trait, electrolyte disorder,
or side effect of drugs such as, lithium. In the neurogenic (DI), it is usually caused by head injury near
the hypophysisal tract.


Urinary tract infections (UTI's)
The second most common type of bacterial infections seen by health care providers is UTI's. Out


of all the bacterias that colonize and cause urinary tract infections the big gun is Escherichia coli. In the
hospital indwelling catheters and straight catheterizing predispose the opportunity for urinary tract
infections. In females there are three stages in life that predispose urinary tract infections, that is
menarche, manipulation between intercourse, and menopause. However, a small percentage of men and
children will get urinary tract infections. In men it is usually due to the prostate gland growth which
usually occurs in older age men. In children it can occur 3% to 5% in girls and 1% in boys,
uncircumcised boys it is more common than circumcised ones to have a urinary tract infection, in girls
it may be the result of onset of toilet training, some predispositions for getting urinary tract infection
include family history and urinary tract anomalies. In neonates urinary tract infections is most common
when bacteremia is present.


Dialysis and Kidney Transplant
Generally, humans can live normally with just one kidney. Only when the amount of functioning


kidney tissue is greatly diminished will renal failure develop. If renal function is impaired, various


196 | Human Physiology




The Urinary System


forms of medications are used, while others are contraindicated. Provided that treatment is begun early,
it may be possible to reverse chronic kidney failure due to diabetes or high blood pressure. If creatinine
clearance (a measure of renal function) has fallen very low ("end-stage renal failure"), or if the renal
dysfunction leads to severe symptoms, dialysis is commenced. Dialysis is a medical procedure,
performed in various different forms, where the blood is filtered outside of the body.


Kidney transplantation is the only cure for end stage renal failure; dialysis, is a supportive
treatment; a form of "buying time" to bridge the inevitable wait for a suitable organ.


The first successful kidney transplant was announced on March 4, 1954 at Peter Bent Brigham
Hospital in Boston. The surgery was performed by Dr. Joseph E. Murray, who was awarded the Nobel
Prize in Medicine in 1990 for this feat.


There are two types of kidney transplants: living donor transplant and a cadaveric (dead donor)
transplant. When a kidney from a living donor, usually a blood relative, is transplanted into the patient's
body, the donor's blood group and tissue type must be judged compatible with the patient's, and
extensive medical tests are done to determine the health of the donor. Before a cadaveric donor's organs
can be transplanted, a series of medical tests have to be done to determine if the organs are healthy.
Also, in some countries, the family of the donor must give its consent for the organ donation. In both
cases, the recipient of the new organ needs to take drugs to suppress their immune system to help
prevent their body from rejecting the new kidney.


Review Questions
1.While reading a blood test I notice a high level of creatinine, I could assume from this that


A) There is a possibility of a UTI
B) There is a possibility of diabetes
C) There is a possibility of kidney failure
D) There is nothing wrong, this is normal


2.Direct control of water excretion in the kidneys is controlled by
A) Anti-diuretic hormone
B) The medulla oblongata
C) Blood plasma
D) Sodium amounts in the blood


3. Nephrons
A) Eliminate wastes from the body
B) Regulate blood volume and pressure
C) Control levels of electrolytes and metabolites
D) Regulate blood pH
E) All of the above


4. If I am dehydrated, my body will increase


Wikibooks | 197




Chapter 10
A) ATP
B) ADP
C) Diluted urine
D) ADH


5.Which part of the nephron removes water, ions and nutrients from the blood?
A ) vasa recta
B ) loop of henle
C ) proximal convuluted tubule
D ) peritubular capillaries
E ) glomerulus


6.Kidneys have a direct effect on which of the following
A ) Blood pressure
B ) How much water a person excretes
C ) Total blood volume
D ) pH
E ) all of the above


7. Why do substances in the glomerulus enter the Bowman's capsule?
A ) the magnetic charge of the Bowman's capsule attracts the substances
B ) the substances are actively transported into the Bowman's capsule
C ) blood pressure of the glomerulus is so great that most substances in blood move into capsule
D ) little green men force it in with their ray guns


8. What happens in tubular excretion?
A ) urine bonds are formed between the wastes
B ) wastes are diffused from the tubule
C ) wastes move into the distal convoluted tubule from the blood
D ) blood pressure forces wastes away from the kidney


9. The countercurrent exchange system includes_________and_________.
A ) glomerulus and macula densa
B ) proximal convoluted tubule and distal convoluted tubule
C ) loop of Henle and collecting tubule
D ) afferent arteriole and efferent arteriole
E ) ureters and bladder


10. The function of the loop of the nephron in the process of urine formation is:
A ) reabsorption of water
B ) production of filtrate
C ) reabsorption of solutes
D ) secretion of solutes


198 | Human Physiology




The Urinary System


Glossary
Antidiuretic: lessening or decreasing of urine production or an agent that decreases the release of


urine.
Catheterisation: a catheter is a tube that can be inserted into a body cavity, duct or vessel.


Catheters thereby allow drainage or injection of fluids or access by surgical instruments. The process of
inserting a catheter is catheterisation. In most uses a catheter is a thin, flexible tube: a "soft" catheter; in
some uses, it is a larger, solid tube: a "hard" catheter.


Dehydration: condition resulting from excessive loss of body fluid.
Diabetes: a general term for a disease characterized by the begining stages and onset of renal


failure. It is derived from the Greek word diabaínein, that literally means "passing through," or
"siphon", a reference to one of diabetes' major symptoms—excessive urine production.


Diuresis: secretion and passage of large amounts of urine.
Diuretic: increasing of urine production, or an agent that increases the production of urine.
Erythropoietin: hormone that stimulates stem cells in the bone marrow to produce red blood cells
Fibrous Capsule: the kidney's loose connective tissue
Glomerulus: capillary tuft that receives its blood supply from an afferent arteriole of the renal


circulation.
Gluconeogenesis: the cycle of producing a glucose form fat or protein; preformed by the kidney in


times of long fasting, initially gluconeogenesis is preformed by the liver
Juxtaglomerular (JG) cells: Renin-secreting cells that are in contact with the macula densa and


the afferent arterioles of the renal nephron.
Juxtaglomerular apparatus (JGA): A site of juxtaglomerular cells connecting with the macula


densa where renin is secreted and sensor for control of secretion of golmerular filtration rate.
Loop of Henle/ Nephron Loop: u-shaped tube that consists of a descending limb and ascending


limb; primary role is to concentrate the salt in the interstitium, the tissue surrounding the loop
Medullary Pyramids or Renal Pyramids: the cone shaped masses in the kidney
Micturition: another name for excretions
Nephron: basic structural and functional unit of the kidney; chief function is to regulate water and


soluble substances by filtering the blood, reabsorbing what is needed and excreting the rest as urine
Podocytes: filtration membrane, in the visceral layer of the bowman's capsule


Wikibooks | 199




Chapter 10
Renal Calculi: kidney stones, solid crystals of dissolved minerals in urine found inside the


kidneys
Renal Cortex: outer portion of the kidney
Renal Lobe: each pyramid together with the associated overlying cortex
Renal Pelvis: a central space, or cavity that transmits urine to the urinary bladder via the ureter
Renin: hormone released by the Juxtaglomerular (JG) cells of the kidneys when blood pressure


falls
TURP: transurethral resection of the prostate. During TURP, an instrument is inserted up the


urethra to remove the section of the prostate that is blocking urine flow. This is most commonly caused
by benign prostatic hyperplasia (BPH). A TURP usually requires hospitalization and is done using a
general or spinal anesthetic. It is now the most common surgery used to remove part of an enlarged
prostate.


Urethra: a muscular tube that connects the bladder with the outside of the body
Ureters: two tubes that drain urine from the kidneys to the bladder
Urine: liquid produced by the kidneys, collected in the bladder and excreted through the urethra
Urinary Bladder: a hollow, muscular and distensible or elastic organ that sits on the pelvic floor
Urinary System: a group of organs in the body concerned with filtering out excess fluid and other


substances from the bloodstream


References
Graaff, Van De (2002). "Human Anatomy, Sixth Edition". New York: McGraw-Hill.
Mader, Sylvia S. (2004). Human Biology. New York: McGraw-Hill.
Smith, Peter (1998). Internet reference, The Role of the Kidney. Department of Clinical Dental
Sciences,The University of Liverpool.
McCance, Katherine L., Heuther, Sue E. (1994). "Pathophysiology: The Biological Basis for
Disease In Adults and Children, Second Edition". Mosby-Year Book, Inc.


live version • discussion • edit lesson • comment • report an error


200 | Human Physiology




The Respiratory System


11 THE RESPIRATORY SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction
he Respiratory System is crucial to every human being. Without it, we would cease to live
outside of the womb. Let us begin by taking a look at the structure of the respiratory system and


how vital it is to life. During inhalation or exhalation air is pulled towards or away from the lungs, by
several cavities, tubes, and openings.
T


The organs of the respiratory system make sure that oxygen enters our bodies and carbon dioxide
leaves our bodies.


The respiratory tract is the path of air from the nose to the lungs. It is divided into two sections:
Upper Respiratory Tract and the Lower Respiratory Tract. Included in the upper respiratory tract
are the Nostrils, Nasal Cavities, Pharynx, Epiglottis, and the Larynx. The lower respiratory tract
consists of the Trachea, Bronchi, Bronchioles, and the Lungs.


As air moves along the respiratory tract it is warmed, moistened and filtered.


Functions
In this chaper we will discuss the four processes of respiration. They are:


1. BREATHING or ventilation
2. EXTERNAL RESPIRATION, which is the exchange of gases (oxygen and carbon


dioxide) between inhaled air and the blood.
3. INTERNAL RESPIRATION, which is the exchange of gases between the blood and


tissue fluids.
4. CELLULAR RESPIRATION


In addition to these main processes, the respiratory system serves for:
REGULATION OF BLOOD pH, which occurs in coordination with the kidneys, and as a
DEFENSE AGAINST MICROBES


Breathing and Lung Mechanics
Ventilation is the exchange of air between the external environment and the alveoli. Air moves by


bulk flow from an area of high pressure to low pressure. All pressures in the respiratory system are
relative to atmospheric pressure (760mmHg at sea level). Air will move in or out of the lungs
depending on the pressure in the alveoli. The body changes the pressure in the alveoli by changing the
volume of the lungs. As volume increases pressure decreases and as volume decreases pressure


Wikibooks | 201




Chapter 11
increases. There are two phases of ventilation; inspiration and expiration. During each phase the body
changes the lung dimensions to produce a flow of air either in or out of the lungs.


The body is able to change the dimensions of the lungs because of the relationship of the lungs to
the thoracic wall. Each lung is completely enclosed in a sac called the pleural sac. 2 structures
contribute to the formation of this sac. The parietal pleura is attached to the thoracic wall where as the
visceral pleura is attached to the lung itself. In-between these two membranes is a thin layer of
intrapleural fluid. The intrapleural fluid completely surrounds the lungs and lubricates the two surfaces
so that they can slide across each other. Changing the pressure of this fluid also allows the lungs and
the thoracic wall to move together during normal breathing. Much the way two glass slides with water
in-between them are difficult to pull apart, such is the relationship of the lungs to the thoracic wall.


The rhythm of ventilation is also controlled by the "Respiratory Center" which is located largely in
the medulla oblongota of the brain stem. This is part of the autonomic system and as such is not
controlled voluntarily (one can increase or decrease breathing rate voluntarily, but that involves a
different part of the brain). While resting, the respiratory center sends out action potentials that travel
along the phrenic nerves into the diaphragm and the external intercostal muscles of the rib cage,
causing inhalation. Relaxed exhalation occurs between impulses when the muscles relax. Normal adults
have a breathing rate of 12-20 respirations per minute.


The Pathway of Air
When one breathes air in at sea level, the inhalation is composed of different gases. These gases


and their quantities are Oxygen which makes up 21%, Nitrogen which is 78%, Carbon Dioxide with
0.04% and others with significantly smaller portions.


In the process of breathing, air enters into the nasal cavity through the nostrils and is filtered by
coarse hairs (vibrissae) and mucous that are found there. The vibrissae filter macroparticles, which are
particles of large size. Dust, pollen, smoke, and fine particles are trapped in the mucous that lines the
nasal cavities (hollow spaces within the bones of the skull that warm, moisten, and filter the air). There
are three bony projections inside the nasal cavity. The superior, middle, and inferior nasal conchae.
Air passes between these chonchae via the nasal meatuses.


Air then travels past the nasopharynx, oropharynx, and laryngopharynx, which are the three
portions that make up the pharynx. The pharynx is a funnel-shaped tube that connects our nasal and
oral cavities to the larynx. The tonsils which are part of the lymphatic system, form a ring at the
connection of the oral cavity and the pharynx. Here, they protect against foreign invasion of antigens.
Therefore the respiratory tract aids the immune system through this protection. Then the air travels
through the larynx. The larynx closes at the epiglottis to prevent the passage of food or drink as a
protection to our trachea and lungs. The larynx is also our voicebox; it contains vocal cords, in which it
produces sound. Sound is produced from the vibration of the vocal cords when air passes through them.


The trachea, which is also known as our windpipe, has ciliated cells and mucous secreting cells
lining it, and is held open by C-shaped cartilage rings. One of its functions is similar to the larynx and
nasal cavity, by way of protection from dust and other particles. The dust will adhere to the sticky
mucous and the cilia helps propel it back up the trachea, to where it is either swallowed or coughed up.
The mucociliary escalator extends from the top of the trachea all the way down to the bronchioles,
which we will discuss later. Through the trachea, the air is now able to pass into the bronchi.


202 | Human Physiology




The Respiratory System


Inspiration
Inspiration is initiated by contraction of the diaphragm and in some cases the intercostals muscles


when they receive nervous impulses. During normal quiet breathing, the phrenic nerves stimulate the
diaphragm to contract and move downward into the abdomen. This downward movement of the
diaphragm enlarges the thorax. When necessary, the intercostal muscles also increase the thorax by
contacting and drawing the ribs upward and outward.


The active increase of the thorax changes the stability set up in a resting lung. As the thoracic wall
moves away from lung which increases the space between the thoracic wall and lung and decreases the
pressure in the intrapleural cavity. This decrease in pressure causes the pressure in the alveoli to
become greater than the elastic recoil that is inherent in lung tissue. Thus, when contraction of the
diaphragm and the intercostal muscles actively increase the size of the thorax, the lungs are passively
forced to expand. This expansion increases the size of the alveoli which decreases pressure in the
alveoli. Pressure within the alveoli is now lower than atmospheric pressure which allows air to move
into the lungs through the structures discussed above.


Expiration
During quiet breathing, expiration is normally a passive process and does not require muscles to


work. When the lungs are stretched and expanded, stretchy receptors within the alveoli send inhibitory
nerve impulses to the medulla oblongata, causing it to stop sending signals to the rib cage and
diaphragm to relax and rise. This elastic recoil causes the lungs and chest cavity to shrink and increase
the air pressure within the lungs. This increased positive air pressure pushes the air out of the lungs.
Expiration happens as the diaphragm relaxes. Although the respiratory system is primarily under
involuntary control, and regulated by the medulla oblongata, we have some voluntary control over it
also. This is due to the higher brain function of the cerebral cortex.


When under physical or emotional stress, more frequent and deep breathing is needed, and both
inspiration and expiration will work as active processes. Additional muscles in the rib cage forcefully
contract and push more air out of the lungs. (This cannot occur during rest.) In addition to deeper
breathing, when coughing or sneezing we exhale forcibly. Our abdominal muscles will contract
suddenly (when there is an urge to cough or sneeze), raising the abdominal pressure. The rapid increase
in pressure pushes the relaxed diaphragm up against the pleural cavity. This causes air to be forced out
of the lungs.


Another function of the respiratory system is to sing and to speak. Our exert of conscious control
over our breathing is what allows us to speak and sing.


Lung Compliance
Lung Compliance is the magnitude of the change in lung volume produced by a change in


pulmonary pressure. Compliance can be considered the opposite of stiffness. A low lung compliance
would mean that the lungs would need a greater than average change in intrapleural pressure to change
the volume of the lungs. A high lung compliance would indicate that little pressure difference in
intrapleural pressure is needed to change the volume of the lungs. More energy is required to breathe
normally in a person with low lung compliance. Persons with low lung compliance due to disease


Wikibooks | 203




Chapter 11
therefore tend to take shallow breaths and breathe more frequently.


Determination of Lung Compliance Two major things determine lung compliance. The first is
the elasticity of the lung tissue. Any thickening of lung tissues due to disease will decrease lung
compliance. The second is surface tensions at air water interfaces in the alveoli. The surface of the
alveoli cells is moist. The attractive force, between the water cells on the alveoli, is called surface
tension. Thus, energy is required not only to expand the tissues of the lung but also to overcome the
surface tension of the water that lines the alveoli.


To overcome the forces of surface tension, certain alveoli cells secret a protein and lipid complex
called ""Surfactant””.


Respiratory System: Upper and Lower Respiratory Tracts
For the sake of convenience, we will divide the respiratory system in to the upper and lower


respiratory tracts:


Upper Respiratory Tract
The upper respiratory tract consists of the nose and the pharynx. Its primary function is to receive


the air from the external environment and filter, warm, and humidify it before it reaches the delicate
lungs where gas exchange will occur.


Air enters through the nostrils of the nose and is partially filtered by the nose hairs, then flows into
the nasal cavity. The nasal cavity is lined with epithelial tissue, containing blood vessels, which help
warm the air; and secrete mucous, which further filters the air. The endothelial lining of the nasal
cavity also contains tiny hairlike projections, called cilia. The cilia serve to transport dust and other
foreign particles, trapped in mucous, to the back of the nasal cavity and to the pharynx. There the
mucus is either coughed out, or swallowed and digested by powerful stomach acids. After passing
through the nasal cavity, the air flows down the pharynx to the larynx.


Lower Respiratory Tract
The lower respiratory tract starts with the larynx, and includes the trachea, the two bronchi that


branch from the trachea, and the lungs themselves. This is where gas exchange actually takes place.


Larynx
The larynx (plural larynges), colloquially known as the voice box, is an organ in our neck involved


in protection of the trachea and sound production. The larynx houses the vocal cords, and is situated
just below where the tract of the pharynx splits into the trachea and the esophagus. The larynx contains
two important structures: the epiglottis and the vocal cords.


The epiglottis is a flap of cartilage located at the opening to the larynx. During swallowing, the
larynx (at the epiglottis and at the glottis) closes to prevent swallowed material from entering the lungs;


204 | Human Physiology




The Respiratory System


the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter
produces a strong cough reflex to protect the lungs. Note: choking occurs when the esophagus fails to
cover the trachea, and food becomes lodged in our windpipe.


The vocal cords consist of two folds of connective tissue that stretch and vibrate when air passes
through them, causing vocalization. The length the vocal cords are stretched determines what pitch the
sound will have. The strength of expiration from the lungs also contributes to the loudness of the
sound. Our ability to have some voluntary control over the respiratory system enables us to sing and to
speak. In order for the larynx to function and produce sound, we need air. That is why we can't talk
when we're swallowing.


Homeostasis and Gas Exchange
Homeostasis is maintained by the respiratory system in two ways: gas exchange and regulation of


blood pH. Gas exchange is performed by the lungs by eliminating carbon dioxide, a waste product
given off by cellular respiration. As carbon dioxide exits the body, oxygen needed for cellular
respiration enters the body through the lungs. ATP, produced by cellular respiration, provides the
energy for the body to perform many functions, including nerve conduction and muscle contraction.
Lack of oxygen affects brain function, sense of judgment, and a host of other problems.


Gas Exchange
Gas exchange in the lungs is between the alveolar air and the blood in the pulmonary capillaries.


This exchange is a result of increased concentration of oxygen, and a decrease of C02.


External Respiration
External respiration is the exchange of gas between the air in the alveoli and the blood within the


pulmonary capillaries. A normal rate of respiration is 10-20 breaths per minute. In external respiration,
gases diffuse in either direction across the walls of the alveoli. Oxygen diffuses from the air into the
blood and carbon dioxide diffuses out of the blood into the air. Most of the carbon dioxide is carried to
the lungs in plasma as bicarbonate ions (HCO3-). When blood enters the pulmonary capillaries, the
bicarbonate ions and hydrogen ions are converted to carbonic acid (H2CO3) and then back into carbon
dioxide (CO2) and water. This chemical reaction also uses up hydrogen ions. The removal of these ions
gives the blood a more neutral pH, allowing hemoglobin to bind up more oxygen.


Internal Respiration
Internal respiration is the exchanging of gases at the cellular level.


The Passage Way From the Trachea to the Bronchioles
There is a point at the inferior portion of the trachea where it branches into two directions that


form the right and left primary bronchus. This point is called the Carina which is the keel-like cartilage


Wikibooks | 205




Chapter 11
plate at the division point. We are now at the Bronchial Tree. It is named so because it has a series of
respiratory tubes that branch off into smaller and smaller tubes as they run throughout the lungs.


Right and Left Lungs
The Right Primary Bronchus is the first portion we come to, it then branches off into the Lobar


(secondary) Bronchi, Segmental (tertiary) Bronchi, then to the Bronchioles which have little
cartilage and are lined by simple cuboidal epithelium (See fig. 1). The bronchi are lined by
pseudostratified columnar epithelium. Objects will likely lodge here at the junction of the Carina and
the Right Primary Bronchus because of the vertical structure. Items have a tendency to fall in it, where
as the Left Primary Bronchus has more of a curve to it which would make it hard to have things lodge
there.


The Left Primary Bronchus has the same setup as the right with the lobar, semental bronchi and
the bronchioles.


The lungs are attached to the heart and trachea through structures that are called the roots of the
lungs. The roots of the lungs are the bronchi, pulmonary vessels, bronchial vessels, lymphatic vessels,
and nerves. These structures enter and leave at the hilus of the lung which is "the depression in the
medial surface of a lung that forms the opening through which the bronchus, blood vessels, and nerves
pass" (medlineplus.gov).


There are a number of terminal bronchioles connected to respiratory bronchioles which then
advance into the alveolar ducts that then become alveolar sacs. Each bronchiole terminates in an
elongated space enclosed by many air sacs called alveoli which are surrounded by blood capillaries.
Present there as well, are Alveolar Macrophages, they ingest any microbes that reach the alveoli. The
Pulmonary Alveoli are microscopic, which means they can only be seen through a microscope,
membranous air sacs within the lungs. They are units of respiration and the site of gas exchange
between the respiratory and circulatory systems.


Cellular Respiration
First the oxygen must diffuse from the alveolus into the capillaries. It is able to do this because the


capillaries are permeable to oxygen. After it is in the capillary, about 5% will be dissolved in the blood
plasma. The other oxygen will bind to red blood cells. The red blood cells contain hemoglobin that
carries oxygen. Blood with hemoglobin is able to transport 26 times more oxygen than plasma without
hemoglobin. Our bodies would have to work much harder pumping more blood to supply our cells with
oxygen without the help of hemoglobin. Once it diffuses by osmosis it combines with the hemoglobin
to form oxyhemoglobin.


Now the blood carrying oxygen is pumped through the heart to the rest of the body. Oxygen will
travel in the blood into arteries, arterioles, and eventually capillaries where it will be very close to body
cells. Now with different conditions in temperature and pH (warmer and more acidic than in the lungs),
and with pressure being exerted on the cells, the hemoglobin will give up the oxygen where it will
diffuse to the cells to be used for cellular respiration, also called aerobic respiration. Cellular
respiration is the process of moving energy from one chemical form (glucose) into another (ATP),
since all cells use ATP for all metabolic reactions.


206 | Human Physiology




The Respiratory System


It is in the mitochondria of the cells where oxygen is actually consumed and carbon dioxide
produced. Oxygen is produced as it combines with hydrogen ions to form water at the end of the
electron transport chain (see chapter on cells). As cells take apart the carbon molecules from glucose,
these get released as carbon dioxide. Each body cell releases carbon dioxide into nearby capillaries by
diffusion, because the level of carbon dioxide is higher in the body cells than in the blood. In the
capillaries, some of the carbon dioxide is dissolved in plasma and some is taken by the hemoglobin, but
most enters the red blood cells where it binds with water to form carbonic acid. It travels to the
capillaries surrounding the lung where a water molecule leaves, causing it to turn back into carbon
dioxide. It then enters the lungs where it is exhaled into the atmosphere.


Lung Capacity
The normal volume moved in or out of the lungs during quiet breathing is called tidal volume.


When we are in a relaxed state, only a small amount of air is brought in and out, about 500 mL. You
can increase both the amount you inhale, and the amount you exhale, by breathing deeply. Breathing in
very deeply is Inspiratory Reserve Volume and can increase lung volume by 2900 mL, which is quite
a bit more than the tidal volume of 500 mL. We can also increase expiration by contracting our thoracic
and abdominal muscles. This is called expiratory reserve volume and is about 1400 ml of air. Vital
capacity is the total of tidal, inspiratory reserve and expiratory reserve volumes; it is called vital
capacity because it is vital for life, and the more air you can move, the better off you are. There are a
number of illnesses that we will discuss later in the chapter that decrease vital capacity. Vital Capacity
can vary a little depending on how much we can increase inspiration by expanding our chest and lungs.
Some air that we breathe never even reaches the lungs! Instead it fills our nasal cavities, trachea,
bronchi, and bronchioles. These passages aren't used in gas exchange so they are considered to be dead
air space. To make sure that the inhaled air gets to the lungs, we need to breathe slowly and deeply.
Even when we exhale deeply some air is still in the lungs,(about 1000 ml) and is called residual
volume. This air isn't useful for gas exchange. There are certain types of diseases of the lung where
residual volume builds up because the person cannot fully empty the lungs. This means that the vital
capacity is also reduced because their lungs are filled with useless air.


Stimulation of Breathing
There are two pathways of motor neuron stimulation of the respiratory muscles. The first is the


control of voluntary breathing by the cerebral cortex. The second is involuntary breathing controlled by
the medulla oblongata.


There are chemoreceptors in the aorta, the carotid arteries, and in the medulla oblongata of the
brainstem that are sensitive to pH. As carbon dioxide levels increase there is a buildup of carbonic acid,
which releases hydrogen ions and lowers pH. Thus, the chemoreceptors do not respond to changes in
oxygen levels (which actually change much more slowly), but to pH, which is an indirect measure of
carbon dioxide levels. In other words, CO2 is the driving force for breathing. The receptors in the
aorta and the carotid arteries stimulate an immediate increase in breathing rate and the receptors in the
medulla stimulate a sustained increase in breathing until blood pH returns to normal.


This response can be experienced by running a 100 meter dash. During this exertion (or any other
sustained exercise) your muscle cells must metabolize ATP at a much faster rate than usual, and thus
will produce much higher quantities of CO2. The blood pH drops as CO2 levels increase, and you will


Wikibooks | 207




Chapter 11
involuntarily increase breathing rate very soon after beginning the sprint. You will continue to breathe
heavily after the race, thus expelling more carbon dioxide, until pH has returned to normal.


Regulation of Blood pH
Many of us are not aware of the importance of maintaining the acid/base balance of our blood. It is


vital to our survival. Normal blood pH is set at 7.4, which is slightly alkaline or "basic". If the pH of
our blood drops below 7.2 or rises above 7.6 then very soon our brains would cease functioning
normally and we would be in big trouble. Blood pH levels below 6.9 or above 7.9 are usually fatal if
they last for more than a short time. Another wonder of our amazing bodies is the ability to cope with
every pH change – large or small. There are three factors in this process: the lungs, the kidneys and
buffers.


So what exactly is pH? pH is the concentration of hydrogen ions (H+). Buffers are molecules
which take in or release ions in order to maintain the H+ ion concentration at a certain level. When
blood pH is too low and the blood becomes too acidic (acidosis), the presence of too many H+ ions is
to blame. Buffers help to soak up those extra H+ ions. On the other hand, the lack of H+ ions causes
the blood to be too basic (alkalosis). In this situation, buffers release H+ ions. Buffers function to
maintain the pH of our blood by either donating or grabbing H+ ions as necessary to keep the number
of H+ ions floating around the blood at just the right amount.


The most important buffer we have in our bodies is a mixture of carbon dioxide (CO2) and
bicarbonate ion (HCO3). CO2 forms carbonic acid (H2CO3) when it dissolves in water and acts as an
acid giving up hydrogen ions (H+) when needed. HCO3 is a base and soaks up hydrogen ions (H+)
when there are too many of them. In a nutshell, blood pH is determined by a balance between
bicarbonate and carbon dioxide.


Bicarbonate Buffer System. With this important system our bodies maintain homeostasis. (Note
that H2CO3 is Carbonic Acid and HCO3 is Bicarbonate)


CO2 + H2O <---> H2CO3 <---> (H+) + HCO3
• If pH is too high, carbonic acid will donate hydrogen ions (H+) and pH will drop.
• If pH is too low, bicarbonate will bond with hydrogen ions (H+) and pH will rise.


Too much CO2 or too little HCO3 in the blood will cause acidosis. The CO2 level is increased
when hypoventilation or slow breathing occurs, such as if you have emphysema or pneumonia.
Bicarbonate will be lowered by ketoacidosis, a condition caused by excess fat metabolism (diabetes
mellitus).


Too much HCO3 or too little CO2 in the blood will cause alkalosis. This condition is less common
than acidosis. CO2 can be lowered by hyperventilation.


So, in summary, if you are going into respiratory acidosis the above equation will move to the
right. The body's H+ and CO2 levels will rise and the pH will drop. To counteract this the body will
breathe more and release H+. In contrast, if you are going into respiratory alkalosis the equation will
move to the left. The body's H+ and CO2 levels will fall and the pH will rise. So the body will try to
breathe less to release HCO3. You can think of it like a leak in a pipe: where ever there is a leak, the


208 | Human Physiology




The Respiratory System


body will "fill the hole".


Problems Associated With the Respiratory Tract and
Breathing


The environment of the lung is very moist, which makes it a hospitable environment for bacteria.
Many respiratory illnesses are the result of bacterial or viral infection of the lungs. Because we are
constantly being exposed to harmful bacteria and viruses in our environment, our respiratory health can
be adversely affected. There are a number of illnesses and diseases that can cause problems with
breathing. Some are simple infections, and others are disorders that can be quite serious.


Carbon Monoxide Poisoning: caused when carbon monoxide binds to hemoglobin in place of
oxygen. Carbon monoxide binds much tighter, without releasing, causing the hemoglobin to become
unavailable to oxygen. The result can be fatal in a very short amount of time.


Mild Symptoms: flu like symptoms, dizziness, fatigue, headaches, nausea, and irregular breathing
Moderate Symptoms: chest pain, rapid heart beat, difficulty thinking, blurred vision, shortness of
breath and unsteadiness
Severe Symptoms: seizures, palpitations, disorientation, irregular heart beat, low blood pressure,
coma and death.


Pulmonary Embolism: blockage of the pulmonary artery (or one of its branches) by a blood clot,
fat, air or clumped tumor cells. By far the most common form of pulmonary embolism is a
thromboembolism, which occurs when a blood clot, generally a venous thrombus, becomes dislodged
from its site of formation and embolizes to the arterial blood supply of one of the lungs.


Symptoms may include difficulty breathing, pain during breathing, and more rarely circulatory
instability and death. Treatment, usually, is with anticoagulant medication.


Upper Respiratory Tract Infections
The upper respiratory tract consists of our nasal cavities, pharynx, and larynx. Upper respiratory


infections (URI) can spread from our nasal cavities to our sinuses, ears, and larynx. Sometimes a viral
infection can lead to what is called a secondary bacterial infection. "Strep throat" is a primary
bacterial infection and can lead to an upper respiratory infection that can be generalized or even
systemic (affects the body as a whole). Antibiotics aren't used to treat viral infections, but are
successful in treating most bacterial infections, including strep throat. The symptoms of strep throat can
be a high fever, severe sore throat, white patches on a dark red throat, and stomach ache.
Sinusitis


An infection of the cranial sinuses is called sinusitis. Only about 1-3% of URI's are accompanied
by sinusitis. This "sinus infection" develops when nasal congestion blocks off the tiny openings
that lead to the sinuses. Some symptoms include: post nasal discharge, facial pain that worsens
when bending forward, and sometimes even tooth pain can be a symptom. Successful treatment
depends on restoring the proper drainage of the sinuses. Taking a hot shower or sleeping upright
can be very helpful. Otherwise, using a spray decongestant or sometimes a prescribed antibiotic


Wikibooks | 209




Chapter 11
will be necessary.


Otitis Media
Otitis media in an infection of the middle ear. Even though the middle ear is not part of the
respiratory tract, it is discussed here because it is often a complication seen in children who has a
nasal infection. The infection can be spread by way of the 'auditory (Eustachian) tube that leads
form the nasopharynx to the middle ear. The main symptom is usually pain. Sometimes though,
vertigo, hearing loss, and dizziness may be present. Antibiotics can be prescribed and tubes are
placed in the eardrum to prevent the buildup of pressure in the middle ear and the possibility of
hearing loss.


Tonsillitis
Tonsillitis occurs when the tonsils become swollen and inflamed. The tonsils located in the
posterior wall of the nasopharynx are often referred to as adenoids. If you suffer from tonsillitis
frequently and breathing becomes difficult, they can be removed surgically in a procedure called
a tonsillectomy.


Laryngitis
An infection of the larynx is called laryngitis. It is accompanied by hoarseness and being unable
to speak in an audible voice. Usually, laryngitis disappears with treatment of the URI. Persistent
hoarseness without a URI is a warning sign of cancer, and should be checked into by your
physician.


Lower Respiratory Tract Disorders
Lower respiratory tract disorders include infections, restrictive pulmonary disorders, obstructive


pulmonary disorders, and lung cancer.


Lower Respiratory Infections
Acute bronchitis


An infection that is located in the primary and secondary bronchi is called bronchitis. Most of the
time, it is preceded by a viral URI that led to a secondary bacterial infection. Usually, a
nonproductive cough turns into a deep cough that will expectorate mucus and sometimes pus.


Pneumonia
A bacterial or viral infection in the lungs where the bronchi and the alveoli fill with a thick fluid.
Usually it is preceded by influenza. Symptoms of pneumonia include high fever & chills, with
headache and chest pain. Pneumonia can be located in several lobules of the lung and obviously,
the more lobules involved, the more serious the infection. It can be caused by a bacteria that is
usually held in check, but due to stress or reduced immunity has gained the upper hand.


Restrictive Pulmonary Disorders
Pulmonary Fibrosis


Vital capacity is reduced in these types of disorders because the lungs have lost their elasticity.
Inhaling particles such as sand, asbestos, coal dust, or fiberglass can lead to pulmonary fibrosis,


210 | Human Physiology




The Respiratory System


a condition where fibrous tissue builds up in the lungs. This makes it so our lungs cannot inflate
properly and are always tending toward deflation.


Asthma
Asthma is a respiratory disease of the bronchi and bronchioles. The symptoms include wheezing,
shortness of breath, and sometimes a cough that will expel mucus. The airways are very sensitive
to irritants which can include pollen, dust, animal dander, and tobacco. Even being out in cold air
can be an irritant. When exposed to an irritant, the smooth muscle in the bronchioles undergoes
spasms. Most asthma patients have at least some degree of bronchial inflammation that reduces
the diameter of the airways and contributes to the seriousness of the attack. While asthma is not
curable, it IS treatable.


Respiratory Distress Syndrome
• Pathophysiology


At birth the pressure needed to expand the lungs requires high inspiratory pressure. In the presence
of normal surfactant levels the lungs retain as much as 40% of the residual volume after the first breath
and thereafter will only require far lower inspiratory pressures. In the case of deficiency of surfactant
the lungs will collapse between breaths, this makes the infant work hard and each breath is as hard as
the first breath. If this goes on further the pulmonary capillary membranes become more permeable,
letting in fibrin rich fluids between the alveolar spaces and in turn forms a hyaline membrane. The
hyaline membrane is a barrier to gas exchange, this hyaline membrane then causes hypoxemia and
carbon dioxide retention that in turn will further impair surfactant production.


• Etiology
Type two alveolar cells produce surfactant and do not develop until the 25th to the 28th week of


gestation, in this, respiratory distress syndrome is one of the most common respiratory disease in
premature infants. Furthermore, surfactant deficiency and pulmonary immaturity together leads to
alveolar collapse. Predisposing factors that contribute to poorly functioning type II alveolar cells in a
premature baby are if the child is a preterm male, white infants, infants of mothers with diabetes,
precipitous deliveries, cesarean section performed before the 38th week of gestation. Surfactant
synthesis is influenced by hormones, this ranges form insulin and cortisol. Insulin inhibits surfactant
production, explaining why infants of mothers with diabetes type 1 are at risk of development of
respiratory distress syndrome. Cortisol can speed up maturation of type II cells and therefore
production of surfactant. Finally, in the baby delivered by cesarean section are at greater risk of
developing respiratory distress syndrome because the reduction of cortisol produced because the lack
of stress that happens during vaginal delivery, hence cortisol increases in high stress and helps in the
maturation of type II cells of the alveoli that cause surfactant.


• Treatment
Today to prevent respiratory distress syndrome are animal sources and synthetic surfactants, and


administrated through the airways by an endotracheal tube and the surfactant is suspended in a saline
solution. Treatment is initiated post birth and in infants who are at high risk for respiratory distress
syndrome.


Wikibooks | 211




Chapter 11


Nutrition for COPD (Chronic Obstructive Pulmonary
Disease) Patients


Nutrition is particularly important for ventilator-dependent patient. When metabolizing
macronutrients carbon dioxide and water are produced. The respiratory quotient (RQ) is a ratio of
produced carbon dioxide to amount consumed. Carbohydrates metabolism produces the most amount
of carbon dioxide so they have the highest (RQ). Fats produce the least amount of carbon dioxide along
with proteins. Protein has a slightly higher RQ ratio. It is recommended that this kind of patient not
exceed a 1.0 respiratory quotient (RQ). Lowering carbohydrates and supplementing fat or protein in the
diet might not result in maintaining the desired outcome because, excess amounts fat or protein may
also result in a respiratory quotient (RQ) higher than 1.0.


Case Study
Cystic Fibrosis


This disease is most common in Caucasians and will happen to 1 in every 2500 people. It is most
known for its effects on the respiratory tract although it does effect other systems as well. The
respiratory passages become clogged with a thick mucus that is difficult to expel even with vigorous
coughing. Breathing becomes difficult and affected individuals run the risk of choking to death on their
own secretions unless strenuous effort is made to clear the lungs multiple times every day. Victims
frequently will die in the 20's of pneumonia All of us secret mucus by certain cells in the epithelium
that line the respiratory passage ways. In normal cases the cells also secrete a watery fluid that will
dilute the mucus making it easier to pass through the airways. In cystic fibrosis that secretion of watery
fluid is impaired. This makes the mucus thicker and difficult to clear from the passageways. A recent
discovery found that in cystic fibrosis is caused by a defect in a type of chloride protein found in apical
membranes of epithelial calls in the respiratory system and elsewhere. This defect directly impedes the
chlorine ions transport, which will then indirectly effect the transport of potassium ions. This causes the
epithelium, to not create its osmotic gradient necessary for water secretion. It has been known for a
long time that cystic fibrosis is caused by a single abnormal gene. This gene codes for a portion of the
chloride channel protein.


Glossary
Acidosis: A fall in blood pH levels below 7.35.
Acute Bronchitis: an infection that is located in the primary and secondary bronchi, it is preceded


by a viral URI that lead to a secondary bacterial infection
Alkalosis: A rise in blood ph levels above 7.45.
Asthma: respiratory disease of the bronchi and bronchioles that symptoms include wheezing,


shortness of breath, and sometimes a cough that will expel mucus
Bronchial Tree: named because it has a series of respiratory tubes that branch off into smaller and


212 | Human Physiology




The Respiratory System


smaller tubes as they run throughout the lungs
Cellular Respiration: takes place at the mitochondria of the cells where the oxygen is actually


consumed and carbon dioxide produced
Chronic Bronchitis: an obstructive pulmonary disease characterized by inflammation of the


bronchi of the lungs
COPD: Chronic Obstructive Pulmonary Disease.
Cystic Fibrosis (CF): disease that causes the formation of a thick mucus substance that affects the


lungs, intestines, pancreas and liver. It can be test for with a sweat test.
Emphysema: a chronic lung disease, often caused by exposure to toxic chemicals or long-term


exposure to tobacco smoke
External Respiration: the exchange of gases between the air in the alveoli and the blood within


the pulmonary capillaries
Hyperventilation: excessive rate and depth of breathing causing the blood pH to increase
Laryngitis: inflammation of the larynx. It causes hoarse voice or the complete loss of the voice


because of irritation to the vocal folds (vocal cords).
Lung Cancer: cancer of the tissue of the lung, often caused by smoking
Nasal Cavities: hollow spaces within the bones of the skull that warm, moisten, and filter the air
Otitis Media: infection of the middle ear, it is often a complication seen in children who have a


nasal infection
Pneumonia: bacterial or viral infection in the lungs where the bronchi and the alveoli fill with a


thick fluid
Pulmonary Alveoli: microscopic membranous air sacs within the lungs, they are units of


respiration and the site of gas exchange between the respiratory and circulatory systems
Pulmonary Embolism Pulmonary embolism is blockage of the pulmonary artery (or one of its


branches) by a blood clot, fat, air or clumped tumor cells.
Pulmonary Fibrosis: a condition where fibrous tissue builds up in the lungs
Pulmonary Tuberculosis (TB): infectious disease caused by the bacterium mycobacterium


tuberculosis
Respiratory Illnesses: the result of bacterial or viral infection of the lungs
Sinusitis: an infection of the cranial sinuses "sinus infection", develops when nasal congestion


blocks off the tiny openings that lead to the sinuses


Wikibooks | 213




Chapter 11
Strep Throat: a primary bacterial infection that can lead to an upper respiratory infection that can


be generalized or even systemic(affects the body as a whole)
Surfactant: Is a detergent like substance that is produced in the type II alveolar cells in the alveoli


in the lungs.
Tidal Volume: the volume moved in or out of the lungs during quiet breathing
Tonsillitis: when the tonsils become swollen and inflamed, the tonsils are located in the posterior


wall of the nasopharynx are often referred to as adenoids
Trachea: also known as our windpipe, has ciliated cells and mucous secreting cells lining it, and is


held open by C-shaped cartilage rings
Vibrissae: coarse hair that in the process of breathing, filters air that enters into the nasal cavity


through the nostrils
Vital Capacity: the total of tidal, inspiratory reserve and expiratory reserve volumes


Review Questions
1. This is total lung capacity


A) Vital capacity
B) Tidal volume
C) Expiratory reserve volume
D) Inspiratory reserve volume


2. Involuntary breathing is caused by the
A) Pituitary gland
B) Exocrine gland
C) Cerebral cortex
D) Medulla oblongata
E) Endocrine gland


3. Carbon monoxide is dangerous because
A) It binds strongly to hemoglobin, making it unavailable to oxygen
B) It binds strongly to plasma, making it unavailable to carbon dioxide
C) It raises the blood’s pH level, causing a person to hyperventilate
D) Carbon monoxide is not harmful, we have it in our bodies normally


4. Clubbing of the fingers could be a sign of
A) A viral infection
B) An upper respiratory infection
C) Chronic Obstructive Pulmonary Disease


214 | Human Physiology




The Respiratory System


D) Nothing, it’s inherited
5. The need to breathe is caused by


A) A decrease in blood pH
B) An increase in blood pH
C) A decrease in blood oxygen levels
D) A decrease in carbon dioxide levels


6. A person more susceptible to Chronic Obstructive Pulmonary Disease would be
A) A long time smoker
B) A long time fireman
C) A child whose parents smoke
D) A farmer that deals with pesticides
E) All of the above


7. The exchange of gases between the blood within the capillaries and tissue fluid surrounding the
body's cells is called?


A) external respiration
B) cell metabolism
C) cellular respiration
D) internal respiration


8. The medulla oblongata and pons regulate and measure what?
A) The pH level of your blood
B) Your body temperature
C) The amount of O2 in your blood
D) The amount of air in your lungs


9. About how many alveoli are there in the lungs?
A) 300 million
B) 300 billion
C) 300 trillion
D) 300 thousand
E) None of the above


10. In relation to atmospheric pressure, intrapleural pressure is:
A) more pressurized
B) less pressurized
C) about the same


Wikibooks | 215




Chapter 11


References
• Mader, Sylvia S. Human Biology. McGraw Hill Publishing, Burr Ridge, IL. 2004.
• Van De Graaff, Kent M. Human Anatomy. McGraw Hill Publishing, Burr Ridge, IL. 2002.
• Department of Environmental Biology, University of Adelaide, Adelaide, South Australia
• Wikipedia:Lung
• Medlineplus.gov. Hilus. http://www2.merriam-webster.com/cgi-


bin/mwmednlm?book=Medical&va=hilum+
• http://www.an-attorney-for-you.com/legal/carbon-monoxide.htm?gg&gclid=CIbc3P_r-


YYCFQwpNAodgmLHJA
• www.ineedtoknow.com
• "The respiratory system"Authors Mary Kitteredge,intro. by C. Everett Koop, M.D.,SC.D.,


foreword by Sandra Thurman
live version • discussion • edit lesson • comment • report an error


216 | Human Physiology




The Gastrointestinal System


12 THE GASTROINTESTINAL SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction
hich organ is the most important organ in the body? Most people would say the heart or the
brain, completely overlooking the gastrointestinal tract (GI tract). Though definitely not the


most attractive organs in the body, they are certainly among the most important. The 30+ foot long tube
that goes from the mouth to the anus is responsible for the many different body functions which will be
reviewed in this chapter. The GI tract is imperative for our well being and our life-long health. A non-
functioning or poorly functioning GI tract can be the source of many chronic health problems that can
interfere with your quality of life. In many instances the death of a person begins in the intestines.


W


The old saying "you are what you eat" perhaps would be more accurate if worded "you are what
you absorb and digest". Here we will be looking at the importance of these two functions of the
digestive system: absorption and digestion.


The Gastrointestinal System is responsible for the breakdown and absorption of various foods
and liquids needed to sustain life. Many different organs have essential roles in the digestion of food,
from the mechanical disrupting of the teeth to the creation of bile (an emulsifier) by the liver. Bile
production of the liver plays a important role in digestion: from being stored and concentrated in the
gallbladder during fasting stages to being discharged to the small intestine.


In order to understand the interactions of the different components we shall follow the food on its
journey through the human body. During digestion two main processes occur at the same time.


• Mechanical digestion: larger pieces of food get broken down into smaller pieces while
being prepared for chemical digestion. Mechanical digestion starts in the mouth and continues
into the stomach.


• Chemical digestion: starts in the stomach and continues into the intestines. Several different
enzymes break down macromolecules into smaller molecules that can be absorbed.


The GI tract starts with the mouth and proceeds to the esophagus, stomach, small intestine
(duodenum, jejunum, ileum), and then to the large intestine (colon), rectum, and terminates at the anus.
You could probably say the human body is just like a big donut. The GI tract is the donut hole. We will
also be discussing the pancreas and liver, and accessory organs of the gastrointestinal system that
contribute materials to the small intestine.


Layers of the GI Tract
The GI tract is composed of four layers, or tunics. Each layer has different tissues and functions.


From the inside out they are called: mucosa, submucosa, muscularis, and serosa.
Mucosa: The mucosa is the absorptive and secretory layer. It is composed of simple epithelium


cells and a thin connective tissue. There are specialized goblet cells that secrete mucus throughout the


Wikibooks | 217




Chapter 12
GI tract located within the mucosa. On the mucosa layer there are Villi and micro villi.


Submucosa: The submucosa is relatively thick, is highly vascular and serves the mucosa. The
absorbed elements that pass through the mucosa are picked up from the blood vessels of the
submucosa. The submucosa also has glands and nerve plexuses.


Muscularis: The muscularis is responsible for segmental contractions and peristaltic movement in
the GI tract. The muscularis is composed of two layers of muscle: an inner circular and outer
longitudinal layer of smooth muscle. These muscles cause food to move and churn with digestive
enzymes down the GI tract.


Serosa: The last layer is a protective layer. It is composed of avascular connective tissue and
simple squamous epithelium. It secretes lubricating serous fluid. This is the visible layer on the outside
of the organs.


Accessory Organs
1. Salivary glands


• a. Parotid gland
• b. Submandibular gland
• c. Sublingual gland


2. Tongue
3. Teeth
4. Liver


• Produces and excretes bile required for emulsifying fats. Some of the bile drains directly
into the duodenum and some is stored in the gall bladder.


• Helps metabolize proteins, lipids, and carbohydrates.
• Urea, chief end product of mammalian metabolism, is formed in liver from amino acids and


compounds of ammonia.
• Breaks down insulin and other hormones.
• Produces coagulation factors.


5. Gallbladder
• Bile storage.


6. Pancreas
• Endocrine functions: Digestive enzyme secretion.


• Stores zymogens (inactive enzymes) that will be activated by the brush boarder
membrane in the small intestine when a person eats protein (amino acids).


• Trypsinogen – Trypsin: digests protein.
• Chymotypsinogen – Chymotrypsin: digests proteins.
• Carboxypeptidases: digests proteins.


218 | Human Physiology




The Gastrointestinal System


• Lipase-lipid: digests fats.
• Amylase: digests carbohydrates.
• Endocrine functions: Hormone secretion.


• Somatostatin: inhibits the function of insulin. Produced if the body is getting too
much glucose.


• Glucagon: stimulates the stored glycogen in the liver to convert to glucose. Produced if the
body does not have enough glucose.


• Insulin: made in the beta cells of the Islets of Langerhans of the pancreas. Insulin is a
hormone that regulates blood glucose.


7. Vermiform appendix


The Digestive System
The first step in the digestive system can actually begin before the food is even in your mouth.


When you smell or see something that you just have to eat, you start to salivate in anticipation of
eating, thus beginning the digestive process.


Food is the body's source of fuel. Nutrients in food give the body's cells the energy they need to
operate. Before food can be used it has to be broken down into tiny little pieces so it can be absorbed
and used by the body. In humans, proteins need to be broken down into amino acids, starches into
sugars, and fats into fatty acids and glycerol.


During digestion two main processes occur at the same time.
• Mechanical digestion: larger pieces of food get broken down into smaller pieces while


being prepared for chemical digestion. Mechanical digestion starts in the mouth and continues
in to the stomach.


• Chemical digestion: several different enzymes break down macromolecules into smaller
molecules that can be more efficiently absorbed. Chemical digestion starts with saliva and
continues into the intestines.


The digestive system is made up by the alimentary canal, or the digestive tract, and other
abdominal organs that play a part in digestion such as the liver and the pancreas. The alimentary canal
is the long tube of organs that runs from the mouth (where the food enters) to the anus (where
indigestible waste leaves). The organs in the alimentary canal include the esophagus, stomach and the
intestines. The average adult digestive tract is about thirty feet (30') long. While in the digestive tract
the food is really passing through the body rather than being in the body. The smooth muscles of the
tubular digestive organs move the food efficiently along as it is broken down into absorbable atoms and
molecules. During absorption, the nutrients that come from food (such as proteins, fats, carbohydrates,
vitamins, and minerals) pass through the wall of the small intestine and into the bloodstream and
lymph. In this way nutrients can be distributed throughout the rest of the body. In the large intestine
there is reabsorption of water and absorption of some minerals as feces are formed. The parts of the
food that the body passes out through the anus is known as feces.


Mastication
Digestion begins in the mouth. A brain reflex triggers the flow of saliva when we see or even think


Wikibooks | 219




Chapter 12
about food. Saliva moistens the food while the teeth chew it up and make it easier to swallow.
Amylase, which is the digestive enzyme found in saliva, starts to break down starch into simpler sugars
before the food even leaves the mouth. The nervous pathway involved in salivary excretion requires
stimulation of receptors in the mouth, sensory impulses to the brain stem, and parasympathetic
impulses to salivary glands.


Swallowing your food happens when the muscles in your tongue and mouth move the food into
your pharynx. The pharynx, which is the passageway for food and air, is about five inches (5") long. A
small flap of skin called the epiglottis closes over the pharynx to prevent food from entering the trachea
and thus choking. For swallowing to happen correctly a combination of 25 muscles must all work
together at the same time. Salivary glands also produce an estimated three liters of saliva per day.


Esophageal Sphincter
After passing through the throat, the food moves down a muscular tube in the chest called the


esophagus. Peristalsis (involuntary wavelike muscle contractions along the G.I. tract) moves the food
from the esophagus and pushes it down into the stomach. At the end of the esophagus there is a
sphincter that allows food into the stomach then closes back up so the food cannot travel back up into
the esophagus.


Stomach
The stomach is a thick walled organ that lies of the left side of the diaphragm. It stores food and


acid for digestion. Stomach muscles mix up the food with enzymes and acids to make smaller
digestible pieces. Acid is needed for digestion in the stomach and is secreted by chief cells. The gastric
glands begin secreting before food enters the stomach due to the parasympathetic impulses of the vagas
nerve. The stomach lining has glands that produce up to three quarts of this digestive fluid daily. The
secretion of gastric juices occurs in three phases: cephalic, gastric, and intestinal. The cephalic phase is
activated by the smell and taste of food and swallowing. The gastric phase is activated by the chemical
effects of food and the distension of the stomach. The intestinal phase blocks the effect of the cephalic
and gastric phases. Gastric juice also contains an enzyme named pepsin, which digests proteins,
hydrochloric acid and mucus. Hydrochloric acid causes the stomach to maintain a pH of about 2, which
helps kill off bacteria that comes into the digestive system via food.


Water, alcohol, salt, and simple sugars can be absorbed directly through the stomach wall.
However, most substances in our food need a little more digestion and must travel into the intestines
before they can be absorbed. When the stomach is empty it is about the size of one fifth of a cup of
fluid. When stretched and expanded, it can hold up to eight cups of food after a big meal.


Once mixed with digestive juices in the stomach the food is called chyme. The pyloric sphincter, a
walnut shaped muscular tube at the stomach outlet, keeps chyme in the stomach until it reaches the
right consistency to pass into the small intestine. The food leaves the stomach in small squirts rather
than all at once.


Food that has not been digested then travels from the small intestine to the large intestine. When
the food reaches the large intestine, the work to absorb the nutrients is nearly done. The main function
of the large intestine is to excrete the fluid from the undigested food and produce solid waste and
finally to be excreted through the anus.


220 | Human Physiology




The Gastrointestinal System


Enzyme Produced In Site of Release pH Level
Carbohydrate Digestion:
Salivary amylase Salivary glands Mouth Neutral
Pancreatic amylase Pancreas Small intestine Basic
Maltase Small intestine Small intestine Basic
Protein Digestion:
Pepsin Gastric glands Stomach Acidic
Trypsin Pancreas Small intestine Basic
Peptidases Small intestine Small intestine Basic
Nucleic Acid Digestion:
Nuclease Pancreas Small intestine Basic
Nucleosidases Pancreas Small intestine Basic
Fat Digestion:
Lipase Pancreas Small intestine Basic


Esophagus
The esophagus (also spelled oesophagus/esophagus) or gullet is the muscular tube in vertebrates


through which ingested food passes from the mouth area to the stomach. The esophagus is continuous
with the laryngeal part of the pharynx at the level of the C6 vertebra.


Food is passed through the esophagus by using the process of peristalsis. It connects the pharynx,
which is the body cavity that is common to both the digestive and respiratory systems behind the
mouth, with the stomach, where the second stage of digestion is initiated (the first stage is in the mouth
with teeth and tongue masticating food and mixing it with saliva).


Histology
The esophagus is lined with mucus membranes, and uses peristaltic action to move swallowed


food down to the stomach.
The esophagus is lined by a stratified squamous epithelium, which is rapidly turned over, and


serves a protective effect due to the high volume transit of food, saliva, and mucus into the stomach.
The lamina propria of the esophagus is sparse. The mucus secreting glands are located in the
submucosa, and are connective structures called papillae.


The muscularis propria of the esophagus consists of straited muscle in the upper third (superior)
part of the esophagus. The middle third consists of a combination of smooth muscle and straited
muscle, and the bottom (inferior) third is only smooth muscle. The distal end of the esophagus is
slightly narrowed because of the thickened circular muscles. This part of the esophagus is called the
lower esophageal sphincter. This aids in keeping food down and not being regurgitated.


The esophagus has a rich lymphatic drainage as well.


Wikibooks | 221




Chapter 12


Stomach
The stomach lies between the esophagus and the first part of the small intestine (the duodenum). It


is on the left side of the abdominal cavity; the fundus of the stomach lying against the diaphragm.
Lying beneath the stomach is the pancreas. The greater omentum hangs from the greater curvature. A
mucous membrane lines the stomach which contains glands that secrete gastric juices. When food is
mixed and churned with gastric juices it forms a semi-liquid substance called chyme, which then,
through peristalsis, is pushed to the stomach and on to the small intestine.


The stomach is divided into four sections, each of which has different cells and functions. The
sections are: 1) Cardiac region, where the contents of the esophagus empty into the stomach, 2)
Fundus, formed by the upper curvature of the organ, 3) Body, the main central region, and 4) Pylorus
or atrium, the lower section of the organ that facilitates emptying the contents into the small intestine.
Two smooth muscle valves, or sphincters, keep the contents of the stomach contained. They are the: 1)
Cardiac or esophageal sphincter, dividing the tract above, and 2) Pyloric sphincter, dividing the
stomach from the small intestine. The gastric juice, which is in the stomach, is highly acidic with a pH
of 1-3. Gastric acid may cause or compound damage to the stomach wall or its layer of mucus, causing
a peptic ulcer. On the inside of the stomach there are folds of skin call the gastric rugae. Gastric rugae
make the stomach very extendable, especially after a very big meal.


Gastric Glands
There are many different gastric glands and they secret many different chemicals. Parietal cells


secrete hydrochloric acid; chief cells secrete pepsinogen; goblet cells secrete mucus; argentaffin cells
secrete serotonin and histamine; and G cells secrete the hormone gastrin.


Vessels and nerves
Arteries: The arteries supplying the stomach are the left gastric, the right gastric and right
gastroepiploic branches of the hepatic, and the left gastroepiploic and short gastric branches of
the lineal. They supply the muscular coat, ramify in the submucous coat, and are finally
distributed to the mucous membrane.
Capillaries: The arteries break up at the base of the gastric tubules into a plexus of fine
capillaries, which run upward between the tubules, anatomizing with each other, and ending in a
plexus of larger capillaries, which surround the mouths of the tubes, and also form hexagonal
meshes around the ducts.
Veins: From these the veins arise, and pursue a straight course downward, between the tubules,
to the submucous tissue; they end either in the lineal and superior mesenteric veins, or directly in
the portal vein.
Lymphatics: The lymphatics are numerous: They consist of a superficial and a deep set, and pass
to the lymph glands found along the two curvatures of the organ.
Nerves: The nerves are the terminal branches of the right and left urethra and other parts, the
former being distributed upon the back, and the latter upon the front part of the organ. A great
number of branches from the celiac plexus of the sympathetic are also distributed to it. Nerve
plexuses are found in the submucous coat and between the layers of the muscular coat as in the


222 | Human Physiology




The Gastrointestinal System


intestine. From these plexuses fibrils are distributed to the muscular tissue and the mucous
membrane.


Disorders of the Stomach
Disorders of the stomach are common. There can be a lot of different causes with a variety of


symptoms. The strength of the inner lining of the stomach needs a careful balance of acid and mucus. If
there is not enough mucus in the stomach, ulcers, abdominal pain, indigestion, heartburn, nausea and
vomiting could all be caused by the extra acid.


Erosions, ulcers, and tumors can cause bleeding. When blood is in the stomach it starts the
digestive process and turns black. When this happens, the person can have black stool or vomit. Some
ulcers can bleed very slowly so the person won't recognize the loss of blood. Over time, the iron in
your body will run out, which in turn, will cause anemia.


There isn't a known diet to prevent against getting ulcers. A balanced, healthy diet is always
recommended. Smoking can also be a cause of problems in the stomach. Tobacco increases acid
production and damages the lining of the stomach. It is not a proven fact that stress alone can cause an
ulcer.


Histology of the human stomach
Like the other parts of the gastrointestinal tract, the stomach walls are made of a number of layers.
From the inside to the outside, the first main layer is the mucosa. This consists of an epithelium,


the lamina propria underneath, and a thin bit of smooth muscle called the muscularis mucosa.
The submucosa lies under this and consists of fibrous connective tissue, separating the mucosa


from the next layer, the muscularis externa. The muscularis in the stomach differs from that of other GI
organs in that it has three layers of muscle instead of two. Under these muscle layers is the adventitia,
layers of connective tissue continuous with the omenta.


The epithelium of the stomach forms deep pits, called fundic or oxyntic glands. Different types of
cells are at different locations down the pits. The cells at the base of these pits are chief cells,
responsible for production of pepsinogen, an inactive precursor of pepsin, which degrades proteins. The
secretion of pepsinogen prevents self-digestion of the stomach cells.


Further up the pits, parietal cells produce gastric acid and a vital substance, intrinsic factor. The
function of gastric acid is two fold 1) it kills most of the bacteria in food, stimulates hunger, and
activates pepsinogen into pepsin, and 2) denatures the complex protein molecule as a precursor to
protein digestion through enzyme action in the stomach and small intestines. Near the top of the pits,
closest to the contents of the stomach, there are mucous-producing cells called goblet cells that help
protect the stomach from self-digestion.


The muscularis externa is made up of three layers of smooth muscle. The innermost layer is
obliquely-oriented: this is not seen in other parts of the digestive system: this layer is responsible for
creating the motion that churns and physically breaks down the food. The next layers are the square and
then the longitudinal, which are present as in other parts of the GI tract. The pyloric antrum which has
thicker skin cells in its walls and performs more forceful contractions than the fundus. The pylorus is


Wikibooks | 223




Chapter 12
surrounded by a thick circular muscular wall which is normally tonically constricted forming a
functional (if not anatomically discrete) pyloric sphincter, which controls the movement of chyme.


Control of secretion and motility
The movement and the flow of chemicals into the stomach are controlled by both the nervous


system and by the various digestive system hormones.
The hormone gastrin causes an increase in the secretion of HCL, pepsinogen and intrinsic factor


from parietal cells in the stomach. It also causes increased motility in the stomach. Gastrin is released
by G-cells into the stomach. It is inhibited by pH normally less than 4 (high acid), as well as the
hormone somatostatin.


Cholecystokinin (CCK) has most effect on the gall bladder, but it also decreases gastric emptying.
In a different and rare manner, secretin, produced in the small intestine, has most effects on the
pancreas, but will also diminish acid secretion in the stomach.


Gastric inhibitory peptide (GIP) and enteroglucagon decrease both gastric motility and secretion of
pepsin. Other than gastrin, these hormones act to turn off the stomach action. This is in response to
food products in the liver and gall bladder, which have not yet been absorbed. The stomach needs only
to push food into the small intestine when the intestine is not busy. While the intestine is full and still
digesting food, the stomach acts as a storage for food.


Small Intestine
The small intestine is the site where most of the chemical and mechanical digestion is carried out.


Tiny projections called villi line the small intestine which absorbs digested food into the capillaries.
Most of the food absorption takes place in the jejunum and the ileum.


The functions of a small intestine is, the digestion of proteins into peptides and amino acids
principally occurs in the stomach but some also occurs in the small intestine. Peptides are degraded into
amino acids; lipids (fats) are degraded into fatty acids and glycerol; and carbohydrates are degraded
into simple sugars.


The three main sections of the small intestine is The Duodenum, The Jejunum, The Ileum.
The Duodenum
In anatomy of the digestive system, the duodenum is a hollow jointed tube connecting the


stomach to the jejunum. It is the first and shortest part of the small intestine. It begins with the
duodenal bulb and ends at the ligament of Treitz. The duodenum is almost entirely retro peritoneal. The
duodenum is also where the bile and pancreatic juices enter the intestine.


The Jejunum
The Jejunum is a part of the small bowel, located between the distal end of duodenum and the


proximal part of ileum. The jejunum and the ileum are suspended by an extensive mesentery giving the
bowel great mobility within the abdomen. The inner surface of the jejunum, its mucous membrane, is


224 | Human Physiology




The Gastrointestinal System


covered in projections called villi, which increase the surface area of tissue available to absorb nutrients
from the gut contents. It is different from the ileum due to fewer goblet cells and generally lacks
Preyer's patches.


The Ileum
Its function is to absorb vitamin B12 and bile salts. Thee wall itself is made up of folds, each of


which has many tiny finger-like projections known as villi, on its surface. In turn, the epithelial cells
which line these villi possess even larger numbers of micro villi. The cells that line the ileum contain
the protease and carbohydrate enzymes responsible for the final stages of protein and carbohydrate
digestion. These enzymes are present in the cytoplasm of the epithelial cells. The villi contain large
numbers of capillaries which take the amino acids and glucose produced by digestion to the hepatic
portal vein and the liver.


Large Intestine
The large intestine (colon) extends from the end of the ileum to the anus. It is about 5 feet long,


being one-fifth of the whole extent of the intestinal canal. It's caliber is largest at the commencement at
the cecum, and gradually diminishes as far as the rectum, where there is a dilatation of considerable
size just above the anal canal. It differs from the small intestine in by the greater caliber, more fixed
position, sacculated form, and in possessing certain appendages to its external coat, the appendices
epiploicæ. Further, its longitudinal muscular fibers do not form a continuous layer around the gut, but
are arranged in three longitudinal bands or tæniæ.


The large intestine is divided into the cecum, colon, rectum, and anal canal. In its course, describes
an arch which surrounds the convolutions of the small intestine. It commences in the right iliac region,
in a dilated part, the cecum. It ascends through the right lumbar and hypochondriac regions to the under
surface of the liver; here it takes a bend, the right colic flexure, to the left and passes transversely
across the abdomen on the confines of the epigastric and umbilical regions, to the left hypochondriac
region; it then bends again, the left colic flexure, and descends through the left lumbar and iliac regions
to the pelvis, where it forms a bend called the sigmoid flexure; from this it is continued along the
posterior wall of the pelvis to the anus.


There are trillions of bacteria, yeasts, and parasites living in our intestines, mostly in the colon.
Over 400 species of organisms live in the colon. Most of these are very helpful to our health, while the
minority are harmful. Helpful organisms synthesize vitamins, like B12, biotin, and vitamin K. They
breakdown toxins and stop proliferation of harmful organisms. They stimulate the immune system and
produce short chain fatty acids (SCFAs) that are required for the health of colon cells and help prevent
colon cancer. There are many beneficial bacteria but some of the most common and important are
Lactobacillus Acidophilus and various species of Bifidobacterium. These are available as "probiotics"
from many sources.


Pancreas, Liver, and Gallbladder
The pancreas, liver, and gallbladder are essential for digestion. The pancreas produces enzymes


that help digest proteins, fats, and carbohydrates, the liver produces bile that helps the body absorb fat,


Wikibooks | 225




Chapter 12
and the gallbladder stores the bile until it is needed. The enzymes and bile travel through special
channels called ducts and into the small intestine where they help break down the food.


Pancreas
The pancreas is located posterior to the stomach and in close association with the duodenum.
In humans, the pancreas is a 6-10 inch elongated organ in the abdomen located retro peritoneal. It


is often described as having three regions: a head, body and tail. The pancreatic head abuts the second
part of the duodenum while the tail extends towards the spleen. The pancreatic duct runs the length of
the pancreas and empties into the second part of the duodenum at the ampulla of Vater. The common
bile duct commonly joins the pancreatic duct at or near this point.


The pancreas is supplied arterially by the pancreaticoduodenal arteries, themselves branches of the
superior mesenteric artery of the hepatic artery (branch of celiac trunk from the abdominal aorta). The
superior mesenteric artery provides the inferior pancreaticoduodenal arteries while the gastroduodenal
artery (one of the terminal branches of the hepatic artery) provides the superior pancreaticoduodenal
artery. Venous drainage is via the pancreatic duodenal veins which end up in the portal vein. The
splenic vein passed posterior to the pancreas but is said to not drain the pancreas itself. The portal vein
is formed by the union of the superior mesenteric vein and splenic vein posterior to the body of the
pancreas. In some people (as many as 40%) the inferior mesenteric vein also joins with the splenic vein
behind the pancreas, in others it simply joins with the superior mesenteric vein instead.


The function of the pancreas is to produce enzymes that break down all categories of digestible
foods (exocrine pancreas) and secrete hormones that affect carbohydrates metabolism (endocrine
pancreas).


• Exocrine
The pancreas is composed of pancreatic exocrine cells, whose ducts are arranged in clusters called


acini (singular acinus). The cells are filled with secretory granules containing the precursor digestive
enzymes (mainly trypsinogen, chymotrypsinogen, pancreatic lipase, and amylase) that are secreted into
the lumen of the acinus. These granules are termed zymogen granules (zymogen referring to the
inactive precursor enzymes.) It is important to synthesize inactive enzymes in the pancreas to avoid
auto degradation, which can lead to pancreatitis.


The pancreas is near the liver, and is the main source of enzymes for digesting fats (lipids) and
proteins - the intestinal walls have enzymes that will digest polysaccharides. Pancreatic secretions from
ductal cells contain bicarbonate ions and are alkaline in order to neutralize the acidic chyme that the
stomach churns out. Control of the exocrine function of the pancreas are via the hormone gastrin,
cholecystokinin and secretin, which are hormones secreted by cells in the stomach and duodenum, in
response to distension and/or food and which causes secretion of pancreatic juices.


The two major proteases which the pancreas are trypsinogen and chymotrypsinogen. These
zymogens are inactivated forms of trypsin and chymotrypsin. Once released in the intestine, the
enzyme enterokinase present in the intestinal mucosa activates trypsinogen by cleaving it to form
trypsin. The free trypsin then cleaves the rest of the trypsinogen and chymotrypsinogen to their active
forms.


226 | Human Physiology




The Gastrointestinal System


Pancreatic secretions accumulate in intralobular ducts that drain the main pancreatic duct, which
drains directly into the duodenum.


Due to the importance of its enzyme contents, injuring the pancreas is a very dangerous situation.
A puncture of the pancreas tends to require careful medical intervention.


• Endocrine
Scattered among the acini are the endocrine cells of the pancreas, in groups called the islets of


Langerhans. They are:
• Insulin-producing beta cells (50-80% of the islet cells)
• Glucagon-releasing alpha cells (15-20%)
• Somatostatin-producing delta cells (3-10%)
• Pancreatic polypeptide-containing PP cells (remaining %)


The islets are a compact collection of endocrine cells arranged in clusters and cords and are
crisscrossed by a dense network of capillaries. The capillaries of the islets are lined by layers of
endocrine cells in direct contact with vessels, and most endocrine cells are in direct contact with blood
vessels, by either cytoplasmic processes or by direct apposition.


Liver
The liver is an organ in vertebrates, including human. It plays a major role in metabolism and has a


number of functions in the body including glycogen storage, plasma protein synthesis, and drug
detoxification. It also produces bile, which is important in digestion. It performs and regulates a wide
variety of high-volume biochemical reaction requiring specialized tissues.


The liver normally weighs between 1.3 - 3.0 kilograms and is a soft, pinkish-brown "boomerang
shaped" organ. It is the second largest organ (the largest being the skin) and the largest gland within the
human body. its anatomical position in the body is immediately under the diaphragm on the right side
of the upper abdomen, The liver lies on the right side of the stomach and makes a kind of bed for the
gallbladder.


The liver is supplied by two main blood vessels on its right lobe: the hepatic artery and the portal
vein. The hepatic artery normally comes off the celiac trunk. The portal vein brings venous blood from
the spleen, pancreas, and small intestine, so that the liver can process the nutrients and byproducts of
food digestion. The hepatic veins drain directly into the inferior vena cava.


The bile produced in the liver is collected in bile canaliculi, which merge from bile ducts. These
eventually drain into the right and left hepatic ducts, which in turn merge to form the common hepatic
duct. The cystic duct (from the gallbladder) joins with the common hepatic duct to form the common
bile duct. Bile can either drain directly into the duodenum via the common bile duct or be temporarily
stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the
duodenum together at the ampulla of Vater. The branching's of the bile ducts resemble those of a tree,
and indeed term "biliary tree" is commonly used in this setting.


The liver is among the few internal human organs capable of natural regeneration of lost tissue: as
little as 25% of remaining liver can regenerate into a whole liver again. This is predominantly due to


Wikibooks | 227




Chapter 12
hepatocytes acting as unipotential stem cells. There is also some evidence of bio potential stem cells,
called oval cell, which can differentiate into either hepatocytes or cholangiocytes (cells that line bile
ducts).


The various functions of the liver are carried out by the liver cells or hepatocytes.
• The liver produces and excretes bile requires for dissolving fats. Some of the bile drains


directly into the duodenum, and some is stored in the gallbladder
• The liver performs several roles in carbohydrate metabolism:
• gluconeogenesis (the formation of glucose from certain amino acids, lactate or glycerol)
• Glycogenolysis (the formation of glucose from glycogen)
• Glycogenesis (the formation of glycogen from glucose)
• The breakdown of insulin and other hormones
• The liver is responsible for the mainstay of protein metabolism.
• The liver also performs several roles in lipid metabolism:
• cholesterol synthesis
• The production of triglycerides (fats)
• The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, X and


XI, as well as protein C, Protein S and antithrombin.
• The liver breaks down hemoglobin, creating metabolites that are added to bile as pigment
• The liver breaks down toxic substances and most medicinal products in a process called


drug metabolism. This sometimes results in toxication, when the metabolite is more toxic than
its precursor.


• The liver converts ammonia to urea.
• The liver stores a multitude of substances, including glucose in the form of glycogen,


vitamin B12, iron, and copper
• In the first trimester fetus, the liver is the main site of red blood cell production. By the


32nd weeks of gestation, the bone marrow has almost completely taken over that task.
• The liver is responsible for immunological effects the reticuloendothelial system if the liver


contains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the
portal system.


Gallbladder
The gallbladder is a pear shaped organ that stores about 50 ml of bile (or "gall") until the body


needs it for digestion. The gallbladder is about 7-10cm long in humans and is dark green in appearance
due to its contents (bile), not its tissue. It is connected to the liver and the duodenum by biliary tract.


The gallbladder is connected to the main bile duct through the gallbladder duct (cystic duct). The
main biliary tract runs from the liver to the duodenum, and the cystic duct is effectively a "cul de sac",
serving as entrance and exit to the gallbladder. The surface marking of the gallbladder is the
intersection of the midclavicular line (MCL) and the trans pyloric plane, at the tip of the ninth rib. The
blood supply is by the cystic artery and vein, which runs parallel to the cystic duct. The cystic artery is
highly variable, and this is of clinical relevance since it must be clipped and cut during a
cholecystectomy.


The gallbladder has a epithelial lining characterized by recesses called Aschoff's recesses, which
are pouches inside the lining. Under epithelium there is a layer of connective tissue, followed by a
muscular wall that contracts in response to cholecystokinin, a peptide hormone by the duodenum.


228 | Human Physiology




The Gastrointestinal System


The gallbladder stores bile, which is released when food containing fat enters the digestive tract,
stimulating the secretion of cholecystokinin (CCK). The bile emulsifies fats and neutralizes acids in
partly digested food. After being stored in the gallbladder, the bile becomes more concentrated than
when it left the liver, increasing its potency and intensifying its effect in fats.


Anus
The human anus is situated between the buttocks, posterior to the perineum. It has two anal


sphincters, one internal, the other external. These hold the anus closed until defecation occurs. One
sphincter consists of smooth muscle and its action is involuntary; the other consists of striated muscle
and its action is voluntary. In many animals, the anus is surrounded by anal sacs. Role of the anus is
when the rectum is full, the increase in intra-rectal pressure forces the walls of the anal canal apart
allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal
canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincters of
the anus allow the feces to be passed by muscles pulling the anus up over the exiting feces.


Conditions Affecting the Esophagus
There are two different types of conditions that may affect the esophagus. The first type is called


congenital: meaning a person is born with it. The second type is called non-congenital: meaning the
person develops it after birth. Some examples of these are:


Tracheoesophageal fistula and esophageal atresia
Both of these conditions are congenital. In Tracheoesophageal fistula there is a connection


between the esophagus and the wind pipe (trachea) where there shouldn't be one. In Esophageal atresia
the esophagus of a newborn does not connect to the stomach but comes to a dead end right before the
stomach. Both conditions require corrective surgery and are usually detected right after the baby is
born. In some cases, it can be detected before the baby is born.


Esophagitis
Esophagitis is inflammation of the esophagus and is a non-congenital condition. Esophagitis can


be caused by certain medications or by infections. It can also be caused by gastroesophageal reflux
disease (gerd), a condition where the esophageal sphincter allows the acidic contents of the stomach to
move back up into the esophagus. Gastroesophageal reflux disease can be treated with medications, but
it can also be corrected by changing what you eat.


Conditions Affecting the Stomach and Intestines
Everybody has experienced constipation or diarrhea in their lifetime. With constipation, the


contents of the large intestines don't move along fast enough and waste material stays in the large
intestines so long. All water is extracted out of the waste and it becomes hard. With diarrhea you get
the exact opposite reaction. Waste moves along too fast and the large intestines can't absorb the water
before the waste is pushed through. Common flora bacteria assists in the prevention of many serious


Wikibooks | 229




Chapter 12
problems. Here are some more examples of common stomach and intestinal disorders:


Appendicitis
Appendicitis is the inflammation of the appendix, the finger-like pouch that extends from the


cecum. The most common symptoms are abdominal pain, loss of appetite, fever, and vomiting. Kids
and teenagers are the most common victims of appendicitis and must be corrected by surgery. While
mild cases may resolve without treatment, most require removal of the inflamed appendix, either by
laparotomy or laparoscopy. Untreated, mortality is high, mainly due to peritonitis and shock.


Celiac Disease
Celiac disease is a disorder in which a person's digestive system is damaged by the response of the


immune system to a protein called gluten, which is found in rye, wheat, and barley, and also in foods
like breakfast cereal and pizza crust. People that have celiac disease experience abdominal pain,
diarrhea, bloating, exhaustion, and depression when they eat foods with gluten in them. They also have
difficulty digesting their food. Celiac disease runs in families and becomes active after some sort of
stress, like viral infections or surgery. The symptoms can be managed by following a gluten free diet.
Doctors can diagnose this condition by taking a full medical history or with a blood test.


Diverticulitis
Diverticulitis is a common disease of the bowel, in particular the large intestine. Diverticulitis


develops from diverticulosis, which involves the formation of pouches (diverticula) on the outside of
the colon. Diverticulitis results if one of these diverticula becomes inflamed. In complicated
diverticulitis, bacteria may subsequently infect the outside of the colon if an inflamed diverticula bursts
open. If the infection spreads to the lining of the abdominal cavity (peritoneum), this can cause a
potentially fatal peritonitis. Sometimes inflamed diverticula can cause narrowing of the bowel, leading
to an obstruction. Also, the affected part of the colon could adhere to the bladder or other organ in the
pelvic cavity, causing a fistula, or abnormal communication between the colon and an adjacent organ.


Gastritis and Peptic ulcers
Usually the stomach and the duodenum are resistant to irritation because of the strong acids


produced by the stomach. But sometimes a bacteria called Helicobacter pylori or the chronic use of
drugs or certain medications, weakens the mucous layer that coats the stomach and the duodenum,
allowing acid to get through the sensitive lining beneath. This can cause irritation and inflammation of
the lining of the stomach, which is called gastritis, or cause peptic ulcers, which are holes or sores that
form in the lining of the stomach and duodenum and cause pain and bleeding. Medications are the best
way to treat this condition.


Gastrointestinal Infections
Gastrointestinal infections can be caused by bacteria such as Campylobacter, Salmonella, E. coli,


or Shigella. They can also be caused by viruses or by intestinal parasites like amebiasis and Giardiasis.
The most common symptoms of gastrointestinal infections Abdominal pain and cramps, Diarrhea, and
vomiting. These conditions usually go away on there own and don't need medical attention.


Inflammatory Bowel Disease


230 | Human Physiology




The Gastrointestinal System


Inflammatory bowel disease is the chronic inflammation of the intestines, which usually affect
older kids, teens and adults. There are two major types, ulcerative colitis and Crohn's disease and
indeterminate colitis, which occurs in 10-15% of patients. Ulcerative colitis usually affects just the
rectum and small intestine, while Crohn's disease can affect the whole gastrointestinal tract from mouth
to anus along with some other parts of the body. Patients with these diseases also suffer from
extraintestinal symptoms including joint pain and red eye, which can signal a flare of the disease. These
diseases are treated with medications and if necessary, Intravenous or IV feeding, or in the more
serious cases, surgery to remove the damaged areas of the intestines.


Polyp
A polyp is an abnormal growth of tissue (tumor) projecting from a mucous membrane. If it is


attached to the surface by a narrow elongated stalk it is said to be pedunculated. If no stalk is present it
is said to be sessile. Polyps are commonly found in the colon, stomach, nose, urinary bladder and
uterus. They may also occur elsewhere in the body where mucous membranes exist like the cervix and
small intestine.


Disorders of the Pancreas, Liver, and Gallbladder
Disorders of the pancreas, liver, and gallbladder affect the ability to produce enzymes and acids


that aid in digestion. examples of these disorders are.
Cystic Fibrosis
Cystic fibrosis is a chronic, inherited illness where the production of abnormally thick mucous


blocks the duct or passageways in the pancreas and prevents the digestive fluids from entering the
intestines, making it difficult for the person with the disorder to digest protein and fats which cause
important nutrients to pass through without being digested. People with this disorder take supplements
and digestive enzymes to help manage their digestive problems.


Hepatitis
Hepatitis is a viral condition that inflames a person's liver which can cause it to lose it's ability to


function. Viral hepatitis, like hepatitis A, B, and C, is extremely contagious. Hepatitis A, which is a
mild form of hepatitis, can be treated at home, but more serious cases that involve liver damage, might
require hospitalization.


Cholecystitis
Acute or chronic inflammation if the gallbladder causes abdominal pain. 90% of cases of acute


cholecystitis are caused by the presence of gallstones. The actual inflammation is due to secondary
infection with bacteria of an obstructed gallbladder, with the obstruction caused by the gallstones.
Gallbladder conditions are very rare in kids and teenagers but can occur when the kid or teenager has
sickle cell anemia or in kids being treated with long term medications.


Cholestasis
Cholestasis is the blockage in the supply of bile into the digestive tract. It can be "intrahepatic" (the


Wikibooks | 231




Chapter 12
obstruction is in the liver) or "extrahepatic" (outside the liver). It can lead to jaundice, and is identified
by the presence of elevated bilirubin level that is mainly conjugated.


Biliary colic
This is when a gallstone blocks either the common bile duct or the duct leading into it from the


gallbladder. This condition causes severe pain in the right upper abdomen and sometimes through to
the upper back. It is described by many doctors as the most severe pain in existence, between childbirth
and a heart attack. Other symptoms are nausea and vomiting and diarrhea, bleeding caused by continual
vomiting, and dehydration caused by the nausea and diarrhea. Another more serious complication is
total blockage of the bile duct which leads to jaundice, which if it is not corrected naturally or by
surgical procedure can be fatal as it causes liver damage. The only long term solution is the removal of
the gallbladder.


Gastrointestinal Dysfunctions
As we age, the amount of digestive enzymes produced by the body drops way down. This leads to


decreased and slower digestion, slower absorption of nutrients and increased accumulation of fecal
mater in the intestinal tract. Undigested food material and metabolic waste can also build up due to
slow elimination, starting of a series of health problems.


When digestion slows, it turns the intestines into a toxic environment. Helpful organisms cannot
live in toxic environments. When the beneficial organisms die they are replaced by harmful organisms,
such as yeasts and parasites, the most common being Candida albicans. This leads to changes in the
intestinal wall which produces leaky gut syndrome which allows many toxic chemicals to be introduced
into the blood stream. As a result the entire toxic load of the body is increased, which causes a bigger
burden on the liver, kidneys and other body organs. When this happens the organs that are normally
used for eliminating waste and supplying nutrients the GI tract becomes into a large dump for waste.
This problem is made worse by the use of junk food, prescriptions, over the counter medications,
antibiotics and a diet that is too low in fiber.


Most people never even think about their GI tract. We are all concerned about what the outside of
our body look like, but we completely ignore the inside. Because our bodies a very resilient.
deterioration of the digestive system can go on for years with no symptoms or side-effect. When
symptoms finally do appear they are usually very non-specific, they include: decreased energy,
headaches, diarrhea, constipation, heartburn, and acid reflux. Over the years these symptoms become
more serious, they include: asthma, food allergies, arthritis, and cancer.


Poor digestion, poor absorption, and bacterial imbalance can be traced to a lot of chronic
conditions. Every organ in the body receives nutrients for the GI tract. I if the GI tract is
malfunctioning then the whole body suffers.


It is possible to return good health to your GI tract by improving digestion, consuming the right
amount of fiber, cutting out junk food and refined sugars.


You can improve the function of the intestines by taking fiber supplements and vitamins
(especially B12 and vitamin K). Some doctors suggest herbal or vitamin enema's to cleanse and relieve
constipation and to help stimulate peristaltic movement which will help to move the bowels.


232 | Human Physiology




The Gastrointestinal System


Irritable Bowel Syndrome
Irritable Bowel Syndrome (IBS) is a disorder with symptoms that are most commonly bloating,


abdominal pain, cramping, constipation, and diarrhea. IBS causes a lot of pain and discomfort. It does
not cause permanent damage to the intestines and does not lead to serious diseases such as cancer. Most
of the people affected with IBS can control their symptoms with stress management, diet, and
prescription medication. For others IBS can be debilitating, they may be unable to go to work, travel,
attend social events or leave home for even short periods of time.


About 20 percent of the adult population has some symptoms of IBS, making it one of the most
common intestinal disorders diagnosed by physicians. It is more common in men than women and in
about 50 percent of people affected it starts at about age 35.


Researchers have not found out what exactly causes IBS. One idea is that people with IBS have a
large intestine (colon) that is sensitive to certain foods and stress. The immune system may also be
involved. It has also been reported that serotonin is linked with normal GI functioning. 95 percent of
the body's serotonin is located in the GI tract (the other 5 percent is in the brain). People with IBS have
diminished receptor activity, causing abnormal levels of serotonin in the GI tract. Because of this IBS
patients experience problems with bowel movement, mobility, and sensation having more sensitive
pain receptors in their GI tract. Many IBS patients suffer from depression and anxiety which can make
symptoms worse.


There is no cure for IBS, but medications are an important part of relieving symptoms. Fiber
supplements or laxatives are helpful for constipation. Anti diuretics such as Imodium can help with
diarrhea. An antispasmodic is commonly prescribed for colon muscle spasms. Antidepressants and pain
medication are also commonly prescribed. [12]


Gastrointestinal Stromal Tumor
Gastrointestinal Stromal Tumors or GIST is an uncommon type of cancer in the GI tract


(esophagus, stomach, small intestine, and colon). These types of cancers begin in the connective tissue
like fat, muscles, nerves , cartilage, etc.


GIST originates in the stroma cells. Stroma cells are strung along the GI tract and are part of the
system that helps the body to know when to move food through the digestive system. Over half of
Gist's occur in the stomach. Most cases occur in people between the ages of forty and eighty, but can
also show up in a person of any age.


All GIST's of any size or location have the ability to spread. Even if a GIST is removed, it can
reappear in the same area, or may even spread outside of the GI tract.


In the early stages, GIST is hard to diagnose because in the early stages symptoms cannot be
recognized. In the later stages a person can have vague abdominal pain, vomiting, abdominal bleeding
that shows up in stool or vomit, low blood counts causing anemia, and having an early feeling of being
full causing a decrease in appetite.


GIST is now recognized as an aggressive cancer that is able to spread to other parts of the body.
People who have been diagnosed with GIST should get treatment as soon as possible.


Wikibooks | 233




Chapter 12
Food Allergies
Food allergies occur when the immune system thinks that a certain protein in any kind of food is a


foreign object and will try to fight against it.
Only about eight percent of children and two percent of adults actually have a food allergy. A


person can be allergic to any kind of food, but the most common food allergies are from nuts, cow's
milk, eggs, soy, fish, and shellfish. Most people who have a food allergy are allergic to less than four
different foods.


The most common signs of food allergies are hives, swelling, itchy skin, itchiness, tingling or
swelling in the mouth, coughing, trouble breathing, diarrhea, and vomiting. The two most common
chronic illness that are associated with food allergies are eczema and asthma.


Food allergies can be fatal if it causes the reaction called anaphylaxis. This reaction makes it hard
for the person to breathe. This can be treated by an epinephrine injection.


GERD, Heartburn, Acid Reflux
GERD, or Gastroesophageal Reflux Disease occurs when the lower esophageal sphincter is not


able to close properly. When this happens, contents from the stomach called reflux leak back into the
esophagus and the stomach.


When the stomach refluxes, stomach acid touches the lining of the esophagus and causes it to have
a burning feeling in the throat or the chest. This is what heartburn is. When you taste the fluid in the
back of your throat, it is called acid indigestion. It is common for a person to get occasional heartburn,
but when it occurs more than twice a week it can be considered as GERD. GERD can occur in people
of all ages including infants.


Some symptoms of GERD include having a pain in your chest, hoarseness, having trouble
swallowing, or having the feeling of food being stuck in your throat. The main symptoms are having
persistent heartburn and acid regurgitation. GERD can also cause bad breath and a dry cough.


No one knows why people get GERD. Some things that could contribute to GERD are alcohol use,
pregnancy, being overweight and smoking. Certain foods night also contribute like citrus fruits,
caffeine, spicy, fatty, and dried foods, and also mint flavorings.


Over-the-counter antacids or medications that help stop acid production and help the muscles
empty the stomach are commonly used to treat GERD.


Constipation
Not everyone is on the same schedule for having a bowel movement. Depending on the person, a


"normal" schedule can range anywhere from three times a day to three times a week. If you start having
bowel movements less than your own personal schedule, then you might be getting the signs of
constipation.


Constipation is when you have trouble having bowel movements. The stool is very hard making it
hard to pass and causing a person to strain. You may even feel like you have to have a bowel


234 | Human Physiology




The Gastrointestinal System


movement even after you have already had one.
When you digest food, the waste products go through your intestines by the muscles contracting.


When in the large intestine, most of the water and salt from the waste products are reabsorbed because
they are needed by the body for our everyday functions. You can become constipated if too much water
is absorbed, or if waste products move too slowly.


Not getting enough fluids, a low fiber diet, age, not being physically active, depression, stress and
pregnancy can all be causes of constipation. Medications and narcotics can also cause a person to get
constipated. Chronic constipation may be a symptom of a liver problem such as a urea cycle disorder.


The best way for a person to treat constipation is to make sure that you are getting enough fluids as
well as fiber in your diet. By doing this, the bulk of your stool is increased and also makes the stool
softer so that it can move through your intestines more easily. Being more active and increasing your
daily exercise also helps keep you regulated.


Hemorrhoids Hemorrhoids (also known as haemorrhoids, emerods, or piles) are varicosities or
swelling and inflammation of veins in the rectum and anus.


Two of the most common types of hemorrhoids are external and internal hemorrhoids.
• External hemorrhoids are those that occur outside of the anal verge (the distal end of the


anal canal). They are sometimes painful, and can be accompanied by swelling and irritation.
Itching, although often thought to be a symptom from external hemorrhoids, is more commonly
due to skin irritation.


• If the vein ruptures and a blood clot develops, the hemorrhoid becomes a
thrombosed hemorrhoid.


• Internal hemorrhoids are those that occur inside the rectum. As this area lacks pain
sensory receptor|receptors, internal hemorrhoids are usually not painful and most people are not
aware that they have them. Internal hemorrhoids, however, may bleed when irritated.


• Untreated internal hemorrhoids can lead to two severe forms of hemorrhoids: prolapsed and
strangulated hemorrhoids.


• Prolapsed hemorrhoids are internal hemorrhoids that are so distended that they are
pushed outside of the anus.


• If the anal sphincter muscle goes into spasm and traps a prolapsed hemorrhoid outside of
the anal opening, the supply of blood is cut off, and the hemorrhoid becomes a strangulated
hemorrhoid.


Bleeding in the Gastrointestinal tract
Bleeding in the gastrointestinal tract doesn't always mean you have a disease, it's usually a


symptom of a digestive problem. The cause of the bleeding may not be that serious, it could be
something that can be cured or controlled such as hemorrhoids. However, locating the source of the
bleeding is very important. The gastrointestinal tract contains many important organs like the
esophagus, stomach, small intestine, large intestine or colon, rectum, and anus. Bleeding can come
from one or more of these area from a small ulcer in the stomach, or a large surface like the


Wikibooks | 235




Chapter 12
inflammation of the colon. Sometimes a person doesn't even know they are bleeding. When this
happens, it is called hidden, or occult bleeding. Simple tests can detect hidden blood in the stool.


What Causes Bleeding in the Digestive Tract
Esophageal bleeding may be caused by Mallory-Weiss syndrome which is a tear in the esophagus.


Mallory-Weiss syndrome is usually caused by excessive vomiting or may be caused by childbirth, a
hiatal hernia, or increased pressure in the abdomen caused by coughing. Various medications can cause
stomach ulcers or inflammations. Medications containing aspirin or alcohol, and various other
medications(mainly those used for arthritis) are some examples of these.


Benign tumors or cancer of the stomach may also cause bleeding. These disorders don't usually
produce massive bleeding. The most common source of bleeding usually occurs from ulcers in the
duodenum. Researchers believe that these ulcers are caused by excessive stomach acid and a bacteria
called Helicobacter Pylori.


In the lower digestive tract, the most common source of bleeding is in the large intestine, and the
rectum. Hemorrhoids are the most common cause of bleeding in the digestive tract. Hemorrhoids are
enlarged veins in the anal area which produces bright red blood that you see in the toilet or on the toilet
paper.


How do you Recognize Bleeding in the Digestive Tract
The signs of bleeding in the digestive tract vary depending on the site and severity of the bleeding.


If the blood is coming from the rectum, it would be bright red blood. If it was coming from higher up in
the colon or from the small intestine, the blood would be darker. When the blood is coming from the
stomach, esophagus, or the duodenum, the stool would be black and tarry.


If the bleeding is hidden, or occult, a person may not notice changes in the stool color. If extensive
bleeding occurs, a person may feel dizzy, faint, weak, short of breath, have diarrhea or cramp
abdominal pain. Shock can also occur along with rapid pulse, drop in blood pressure, and difficulty
urinating. Fatigue, lethargy, and pallor from anemia will settle in if the bleeding is slow. Anemia is
when the bloods iron-rich substance, hemoglobin, is diminished.


Common Causes of Bleeding in the Digestive Tract
• Hemorrhoids
• Gastritis (inflammation)
• Inflammation (ulceratice colitis)
• Colo rectal Polyps
• Colo rectal Cancer
• Duodenal Ulcer
• Enlarged Veins
• Esophagitis (inflammation of the esophagus)
• Mallory-Weiss Syndrome
• Ulcers


Iron and beets can also turn the blood red or black giving a false indication of blood in the stool.


236 | Human Physiology




The Gastrointestinal System


How Bleeding in the Digestive Tract is Diagnosed
To diagnose bleeding in the digestive tract the bleeding must be located and a complete history and


physical are very important. Here are some of the procedures that diagnose the cause of bleeding.
Endoscopy
An endoscopy is a common diagnostic technique that allows direct viewing of the bleeding site.


Since the endoscope can detect lesions and confirm the absence or presence of bleeding, doctors often
use this method to diagnose acute bleeding, the endoscope can also be used to treat the cause of
bleeding as well.


The endoscope is a flexible instrument that can be inserted through the mouth or rectum. The
instrument allows the doctors to see inside the esophagus, stomach,
duodenum(esophagoduodenoscopy), sigmoid colon(sigmoidoscopy), and rectum(rectoscopy, to collect
small samples of tissues, take pictures, and stop the bleeding. There is a new procedure out using a long
endoscope that can be inserted during surgery to locate a source of bleeding in the small intestine.


Capsule Endoscopy
Capsule endoscopy helps doctors to see and examine the lining of the middle part of the


gastrointestinal tract, which includes the three parts of the small intestine (duodenum, jejunum, ileum).
The capsule is a small pill sized video camera called an endoscope. It has its own lens and light that
transfers the images to a monitor so the doctor can view them outside of the body. This process is also
referred to as small bowel endoscopy, capsule endoscopy, or wireless endoscopy.


The most common reason for doing a capsule endoscopy is to look for the causes of bleeding that
is coming from the small intestine. It is also able to help detect ulcers, tumors, and Crohn's disease.


Angiography
Angiography is a technique that uses dye to highlight blood vessels. This procedure is used when


the patient is bleeding badly enough that it allows the dye to leak out of the blood vessels and identifies
the bleeding site. In some situations, Angiography allows the patient to have medication injections that
may stop the bleeding.


Radionuclide Scanning
Radionuclide scanning is a non-invasive screening technique used for locating sites of acute


bleeding, especially in the lower GI tract. This procedure injects small amounts of radioactive material
that either attach to the persons red blood cells or are suspended in the blood. Special pictures are taken
that allows doctors to see the blood escaping. Barium x-rays, angiography, and radionuclide scans can
be used to locate sites of chronic occult bleeding.


How to Recognize Blood in the Stool and Vomit
• Bright red blood coating the stool
• Dark blood mixed with the stool
• Black or tarry stool


Wikibooks | 237




Chapter 12
• Bright red blood in the vomit
• Grainy appearance in vomit


Symptoms of Acute Bleeding
• Weakness
• Shortness of breath
• Dizziness
• Cramp abdominal pain
• Feeling light headed
• Diarrhea


Symptoms of Chronic Bleeding
• Fatigue
• Shortness of breath
• Lethargy
• Pallor


Colonoscopy
A colonoscopy is a test to look at the inside of your colon. Everyone should have a colonoscopy by


the time they are 50 to check for diseases of the colon. Colonoscopy is best known for its use in early
detection of colorectal cancer, the second leading cause of cancer deaths in the United States. Colon
cancer develops from growths like polyps within the intestinal wall. These growths often take 5-10
years to develop usually without symptoms. You are at a higher risk to have this disease if you have a
close relative who has had it. If you are going to develop a polyp, you will probably do so after age 50.
So the American College of Gastroenterology (the digestive specialists) recommends screening
examinations every 5 years for early detection and removal of these cancer-causing growths after that
age. Don't make excuses! It's not so bad and it may save your life!


Case Study
Bob had a history of chronic pain in his intestinal area. He wasn't so sure what it was. The doctor


suspected what it was and gave Bob antibiotics. It helped. It so happened that whenever Bob ate
popcorn or nuts he would get this pain. Sometimes it would just go away... other times he had to go on
antibiotics. The doctor ordered some tests. Bob would have to stay away from nuts, popcorn, tomatoes,
strawberries, and anything else with seeds or hard parts. Seems something in his bowels couldn't
tolerate those foods. Bob ate a pretty healthy diet so he couldn't understand what was happening. A few
years later, Bob had another series of painful episodes. The pain was so great Bob could hardly stand
let alone go to work. This time the doctor did more tests and found out that his lower intestine was
almost blocked. Surgery was ordered. What did Bob have?


238 | Human Physiology




The Gastrointestinal System


Glossary
Amebiasis


An inflammation if the intestines caused by infestation with Entameba histolytica (a type of
ameba) and characterized by frequent loose stools flecked with blood and mucus


Amylase
An enzyme produces in the pancreas and salivary glands that help in the digestions of starches.


Bile
A bitter, alkaline, brownish-yellow or greenish-yellow fluid that is secreted by the liver, stored in
the gallbladder, and discharged into the duodenum and aids in the emulsification, digestion, and
absorption of fats. Also called gall.


Biotin
Biotin is used in cell growth, the production of fatty acids, metabolism of fats, and amino acids. It
plays a role in the Krebs Cycle. Biotin is also helpful in maintaining a steady blood sugar level. It
is often recommended for strengthening hair and nails.


B12
A vitamin important for the normal formation of red blood cells and the health of the nerve
tissues. Undetected and untreated B12 deficiency can lead to anemia and permanent nerve and
brain damage


Candida Albicans
Found in animals and in man. Has been isolated from the skin and mucosa of man, but has also
been recovered from leaves, flowers, water, and soil. Reported to be allergenic. A common cause
of superficial infection, oral and vaginal infection, sepsis, and disseminated disease. Cells from
the organism are usually not airborne and are considered to be normal component of the flora of
the mouth and other mucous membranes on the body.


Chemical digestion
Is a chemical breakdown of food when being in the mouth (oral cavity). Is the digestive
secretions of saliva that moistens food and introduces gastric juices and enzymes that are
produced in the stimulation to certain macronutrients, such as, carbohydrates. In this, the mouth
saliva carries an enzyme called amylase for breaking down carbohydrates.


Cholecystokinin (CCK)
Cholecystokinin (also called pancreozymin), this is a hormone in the small intestinal cells
(intestinal mucosa) that is produced in response to food. This hormone regulates the release of
secretions of many organs that aid digestion, such as, bicarbonate from the pancreas to reduce the
acidity of digestive juices like the chyme that enters the small intestine form the stomach that
contains hydrochloric acid (HCL).


Chylomicrons
The lipoproteins first formed after absorption of lipids form food.


Chyme
The thick semi fluid mass of partly digested food that is passed from the stomach to the


Wikibooks | 239




Chapter 12
duodenum.


Crohn's Disease
Described as skip lessions in the large and small bowel it is a malabsorption disorder that can
affect the gastrointestinal tract for the mouth to the anus.


Deamination
When an amino acid group breaks off an amino acid that makes a molecule of ammonia and keto
acid.


Emulsifier
A mixture of two immiscible (unblendable) substances.


Gastrin
The stomach mucosa secretes a hormone gastrin that increases the release of gastric juices.


GI tract
Gastrointestinal Tract, The tube that extends from the mouth to the anus in which the movement
of muscles and release of hormones and enzymes digest food.


Hydrochloric
The chemical substance hydrochloric acid is the water-based solution of hydrogen chloride (HCI)
gas. It is a strong acid, the major component of stomach acid and of wide industrial use.


Lactobacillus Acidophilus
Important resident inhabitant of the human small and large intestines, mouth, and vagina.
Secretes natural antibiotic substances which strengthen the body against various disease-causing
microbes


Leaky gut syndrome
Abnormal level of intestinal permeability


Lingual lipase
An enzyme produced only in infancy to aid digestion of long-chain fatty acids.


Lipase
An enzyme produced by microorganisms that split the fat molecules into fatty acids which create
flavor


Mechanical digestion
The crushing of the teeth and rhythms made by the movement of the tongue, the teeth aid in
tearing and pulverizing food, while the tongue helps with peristalsis (movement), of food down
the esophagus.


Micelles
A product of lipids and bile assist in lipid absorption.


Microvilli
On the villi in the small intestine is mivrovilli, these projections called brush border microvilli


240 | Human Physiology




The Gastrointestinal System


secrete specific enzymes for disaccharide hydrolysis, these further aid the absorption of the
carbohydrate by yielding a monosaccharide that then can go through portal circulation to liver
circulation to be further processed into immediate use for energy or glycogen storage.


Peristalsis
The wavelike muscular contractions of the intestine or other tubular structure that propel the
contents onward by alternate contraction and relaxation.


Proliferation
The process of reproduction or division of cells


Proteases
Protein enzyme


Rennin
Only produced during infancy and is a gastric protease and functions with calcium to clot with
milk proteins casein, to slow the movement of milk so that digestion is prolonged.


Serotonin
chemical messenger in the brain that affects emotions, behavior, and thought


Synthesize
To create something, such as chemicals in the body, from simpler, raw materials


Ulcerative Colitis
Villi


A minute projection arising from a mucous membrane, especially one of the vascular projections
of the small intestine.


Vitamin K
A substance that promotes the clotting of blood


Case Study Answer Bob has diverticulitis. The doctor was afraid that if he had another bad
infection that scar tissue would eventually block his colon completely and burst, which would
necessitate a colostomy. Bob ended up having to have surgery to remove the damaged part of his colon.
The doctor removed almost 18 inches of Bob's large intestine. Bob is doing fine now and most
importantly, he can now eat his favorite food - nuts! Note: Sometimes a diet rich in fiber can help you
avoid this dread problem. Sometimes, like in Bob's case, the predisposition to have this problem runs in
the family. All of his siblings and his father suffered from this same ailment. Stress is another factor
that can exacerbate this disease. So.. don't worry, be happy and eat fiber!


Review Questions
1. This is released in the duodenum in response to acidic chyme


A) Cholecystokinin


Wikibooks | 241




Chapter 12
B) Gastrin
C) Secretin
D) Peptide


2. In the GI tract, this layer is responsible for absorption and secretions
A) Mucosa
B) Sub mucosa
C) Muscularis
D) Serosa


3. This digestive enzyme is produced in the salivary glands and the pancreas
A) Maltase
B) Amylase
C) Pepsin
D) Nuclease
E) Lipase


4. This keeps the chyme in the stomach until it reaches the right consistency to pass into the small
intestine


A) Esophageal sphincter
B) Intrinsic sphincter
C) Cardiac sphincter
D) pyloric sphincter


5. The site where most of the chemical and mechanical digestion is carried out
A) Pylorus
B) Fundus
C) Stomach
D) Large intestine
E) Small intestine


6. Parietal cells secret
A) Serotonin
B) Mucus
C) Pepsinogen
D) Hydrochloric Acid
E) Gastrin


7. The cells at the base of fundic or oxyntic glands
A) Chief cells
B) G cells
C) Argentaffin cells
D) Goblet cells


242 | Human Physiology




The Gastrointestinal System


E) Parietal cells
8. The movement and the flow of chemicals into the stomach is controlled by


A) Nervous system
B) Pancreas
C) Various digestive system hormones
D) Liver
E) Both the nervous system and various digestive system hormones


9. The function of the Ileum is
A) Absorb nutrients
B) Absorb vitamin B12 and bile salts
C) To introduce bile and pancreatic juices
D) Absorb alcohol and aspirin


10. The liver does this
A) Glycogen storage
B) Plasma protein synthesis
C) Bile production
D) Drug detoxification
E) All of the above


References
1. Chen Ts, Chen PS. Intestinal autointoxication: A gastrointestinal leitmotive: Journal


Clinical Gastroenterology
2. Ernst E. Colonic irrigation and the theory of autointoxication: A triumph of ignorance over


science. Journal of Gastroenterology
3. Alvarez WC. Origin of the so-called auto-intoxication symptoms.
4. Donaldson AN. Relation of constipation to intestinal intoxication.
5. Kenney JJ. Fit for Life: Some notes on the book and its roots. Nutrition Forum
6. Use of enemas is limited. FDA consumer
7. Amebiasis associated with colonic irrigation - Colorado. Morbidity and Mortality Weekly


Report
8. Istre GR and others. An outbreak of amebiasis spread by colonic irrigation at a chiropractic


clinic
9. Benjamin R and others. The case against colonic irrigation
10.Eisele JW, Reay DT. Deaths related to coffee enemas
11.Jarvis WT. Colonic Irrigation. National Council Against Health Fraud.
12.National Digestive Disease Information Clearinghouse (NDDIC)


live version • discussion • edit lesson • comment • report an error


Wikibooks | 243




Chapter 13


13 NUTRITION
live version • discussion • edit lesson • comment • report an error


The Community and Nutrition Programs
onnections between nutrition and health have probably been understood, at least to some degree,
among all people of all places and times. For example, around 400 BC Hippocrates said, "Let food


be your medicine and medicine be your food." Understanding the physiological needs of our cells helps
us understand why it is that food has such an impact on overall health. In this chapter we introduce
nutrition by examining how cells use different nutrients and then discuss disease conditions that are
tied to nutritional problems.


C


Nutrition and Health in the Community
The nutritional status of people in our communities is a concern not only for quality of life, but


also for economics (treating illness costs far more than preventing it). Various public health agencies
are striving to prevent nutritional deficiencies and improve overall health. In the U.S., the government
supplies a variety of resources such as state assistance, WIC (Women Infant and Child), and so forth. In
addition, there have been many government agencies and voluntary health and scientific associations,
such as the American Heart Association, that focus on life style and dietary factors that prevent chronic
and life-threatening diseases. The U.S. Department of Agriculture (USDA) and the U.S. Department of
Health and Human Services (USDHHS) developed dietary guidelines in 1977 that were compiled and
displayed as the food guide pyramid. The food guide pyramid was revised as "My Pyramid," but this
new chart is confusing to most people. Harvard School of Public Health developed an alternative
healthy eating pyramid (shown at left) based on long-term nutritional studies. This pyramid differs
from the old USDA pyramid in several key aspects: for example, exercise is at the bottom to remind us
of its important role in our health. Also, not all carbohydrates are at the bottom (white bread, white rice,
and potatoes are now at the top with sugars), and not all oils are at the top (plant oils are at the bottom).
Other resource, such as the Recommended Daily Allowance (RDA) have helped people become more
aware of nutritional needs, yet obesity and chronic health problems continue to rise.


Nutritional Requirements
Our bodies have certain nutritional needs and if they are not met will cause catabolism of its own


fats, carbohydrates and proteins. Molecules are continuously broken down, so we must replace them.
Food molecules, essential fatty acids and essential amino acids are particularly important in replacing
these molecules. Vitamins (Vital Emines) and minerals are not used as energy, but are essential in
enzyme reactions. Living tissue is kept alive by using the expenditure of ATP, found in the break down
of food. Foods energy value is measured in kilocalories. 1 kilocalorie is equal to 1000 calories.


Carbohydrates
Macronutrient
244 | Human Physiology




Nutrition


An energy-yeilding nutrient. Macronutrients are those nutrients that together provide the vast
majority of metabolic energy to an organism. The three main macronutrients are carbohydrates,
proteins, and fat.


Micronutrients
Microminerals or trace elements, are dietary minerals needed by the human body in very small
quantities (generally less than 100mg/day) as opposed to macrominerals which are required in
larger quantities.


Functions
Glucose it is the most easily used by the body. It is a simple carbohydrate that circulates in the
blood and is the main source of energy for the muscles, central nervous system, and is the ONLY
source of energy for the brain.


Carbohydrates are made of organic compounds carbon, hydrogen, and oxygen.
There are three sizes of carbohydrate and they are distinguished by a classification of two that is,


Simple carbohydrates (mono saccharides and disaccharides) and complex carbohydrates
(polysaccharides). Polysaccharides are the most abundant carbohydrate in the body along with
glycogen.


The break down of polysaccharides goes as follows: Polysaccharides are digested into
monosaccchorides including glucose which goes into the intestinal epithelium and into the
bloodstream. The molecules of glucose are taken by glucose transporters and delivered into the cells of
the body. While glucose is in the cells it can be oxidized for energy or provide substrates to other
metabolic reactions or of course into glycogen for storage.


A. Monosaccharides = Single carbohydrate unit, such as, Glucose, Fructose, and Galactose.
B. Disaccharides = Two single carbohydrates bound together these are Sucrose, Maltose, and
Lactose.
C. Polysaccharides = Have many units of monosaccharides joined together such as, Starch and
Fiber.


Proteins
Functions
Protein forms hormones, enzymes, antibodies; it is part of fluid and electrolyte regulation, the
buffering effect for pH, and transporter of nutrients. A good example of a protein is the oxygen
carrying hemoglobin found in red blood cells.


Proteins are made of carbon, hydrogen, oxygen, and nitrogen, an inorganic molecule, the thing
that clearly distinguishes them form the other macronutrients.


A. Amino acids are the building blocks of proteins.
Wikibooks | 245




Chapter 13
B. Polypeptide a group of amino acids bonded together 1000 or more.
The body requires amino acids to produce new body protein (protein retention) and to replace


damaged proteins (maintenance) that are lost in the urine.
Proteins are relatively large molecules made of amino acids joined together in chains by peptide


bonds. Amino acids are the basic structural building units of proteins. They form short polymer chains
called peptides or longer poly-peptides which in turn formstructures called proteins. The process of
protein synthesis is controlled by an mRNA template. In this process tRNA transfers amino acids to the
mRNA to form protein chains.


There are twenty standard amino acids used by cells in making proteins. Vertebrates, including
humans, are able to synthesize 11 of these amino acids from other molecules. The remaining nine
amino acids cannot be synthesized by our cells, and are termed "'essential amino acids'". These
essential amino acids must be obtained from foods.


The 9 Essential Amino Acids have the following names: Histidine, Isoleucine, Leucine, Lysine,
Methionine, Phenylalanine, Threonine, Tryptophan, Valine


You can remember these with this saying “Hey It's Like Lovely Material; Please Touch The
Velvet”.


The 11 Non-essential Amino Acids are as follows:
Alanine, Arginine, Aspartic acid, Cysteine, Cystine, Glutamic acid, Glutamine, Glycine, Proline,


Serine, Tryosine
How about this memory device, "Almost Always Aunt Cindy Can Get Great Gum Popping Sounds


Together"


The 20 Amino Acids and What They Do!
Amino Acid Abbrev. Remarks


Alanine A Ala
Very abundant, very versatile. More stiff than glycine, but small enough to
pose only small steric limits for the protein conformation. It behaves fairly
neutrally, can be located in both hydrophilic regions on the protein outside
and the hydrophobic areas inside.


Cysteine C Cys


The sulfur atom binds readily to heavy metal ions. Under oxidizing
conditions, two cysteines can join together in a disulfide bond to form the
amino acid cystine. When cystines are part of a protein, insulin for example,
this stabilises tertiary structure and makes the protein more resistant to
denaturation; disulphide bridges are therefore common in proteins that have
to function in harsh environments including digestive enzymes (e.g., pepsin
and chymotrypsin) and structural proteins (e.g., keratin). Disulphides are
also found in peptides too small to hold a stable shape on their own (eg.
insulin).


246 | Human Physiology




Nutrition


Aspartic acid D Asp
Behaves similarly to glutamic acid. Carries a hydrophilic acidic group with
strong negative charge. Usually is located on the outer surface of the protein,
making it water-soluble. Binds to positively-charged molecules and ions,
often used in enzymes to fix the metal ion. When located inside of the
protein, aspartate and glutamate are usually paired with arginine and lysine.


Glutamate E Glu Behaves similar to aspartic acid. Has longer, slightly more flexible side chain.


Phenylalanine F Phe
Essential for humans. Phenylalanine, tyrosine, and tryptophan contain large
rigid aromatic group on the side chain. These are the biggest amino acids.
Like isoleucine, leucine and valine, these are hydrophobic and tend to orient
towards the interior of the folded protein molecule.


Glycine G Gly
Because of the two hydrogen atoms at the α carbon, glycine is not optically
active. It is the smallest amino acid, rotates easily, adds flexibility to the
protein chain. It is able to fit into the tightest spaces, e.g., the triple helix of
collagen. As too much flexibility is usually not desired, as a structural
component it is less common than alanine.


Histidine H His


In even slightly acidic conditions protonation of the nitrogen occurs,
changing the properties of histidine and the polypeptide as a whole. It is used
by many proteins as a regulatory mechanism, changing the conformation and
behavior of the polypeptide in acidic regions such as the late endosome or
lysosome, enforcing conformation change in enzymes. However only a few
histidines are needed for this, so it is comparatively scarce.


Isoleucine I Ile
Essential for humans. Isoleucine, leucine and valine have large aliphatic
hydrophobic side chains. Their molecules are rigid, and their mutual
hydrophobic interactions are important for the correct folding of proteins, as
these chains tend to be located inside of the protein molecule.


Lysine K Lys


Essential for humans. Behaves similarly to arginine. Contains a long flexible
side-chain with a positively-charged end. The flexibility of the chain makes
lysine and arginine suitable for binding to molecules with many negative
charges on their surfaces. E.g., DNA-binding proteins have their active
regions rich with arginine and lysine. The strong charge makes these two
amino acids prone to be located on the outer hydrophilic surfaces of the
proteins; when they are found inside, they are usually paired with a
corresponding negatively-charged amino acid, e.g., aspartate or glutamate.


Leucine L Leu Essential for humans. Behaves similar to isoleucine and valine. See isoleucine.


Methionine M Met
Essential for humans. Always the first amino acid to be incorporated into a
protein; sometimes removed after translation. Like cysteine, contains sulfur,
but with a methyl group instead of hydrogen. This methyl group can be
activated, and is used in many reactions where a new carbon atom is being
added to another molecule.


Asparagine N Asn Similar to aspartic acid. Asn contains an amide group where Asp has a carboxyl.
Proline P Pro Contains an unusual ring to the N-end amine group, which forces the CO-


NH amide sequence into a fixed conformation. Can disrupt protein folding
structures like α helix or β sheet, forcing the desired kink in the protein
chain. Common in collagen, where it often undergoes a posttranslational


Wikibooks | 247




Chapter 13
modification to hydroxyproline. Uncommon elsewhere.


Glutamine Q Gln Similar to glutamic acid. Gln contains an amide group where Glu has a carboxyl. Used in proteins and as a storage for ammonia.
Arginine R Arg Functionally similar to lysine.


Serine S Ser
Serine and threonine have a short group ended with a hydroxyl group. Its
hydrogen is easy to remove, so serine and threonine often act as hydrogen
donors in enzymes. Both are very hydrophilic, therefore the outer regions of
soluble proteins tend to be rich with them.


Threonine T Thr Essential for humans. Behaves similarly to serine.
Valine V Val Essential for humans. Behaves similarly to isoleucine and leucine. See isoleucine.


Tryptophan W Trp Essential for humans. Behaves similarly to phenylalanine and tyrosine (see phenylalanine). Precursor of serotonin.
Tyrosine Y Tyr Behaves similarly to phenylalanine and tryptophan (see phenylalanine). Precursor of melanin, epinephrine, and thyroid hormones.


Dietary proteins fall into two categories: complete proteins and incomplete proteins. Complete proteins
include ample amounts of all essential amino acids. What I can eat that will include these great
complete proteins include meat, fish, poultry, cheese, eggs, and milk. Incomplete proteins contain some
but not all of the essential amino acids required by the human body. Examples of incomplete proteins
include legumes, rice, and leafy green vegetables. Someone who chooses a vegan lifestyle must be
careful to combine various plant proteins to obtain all the essential amino acids on a daily basis, but it
can be accomplished.


Ingested proteins are broken down into amino acids during digestion. They are then absorbed by
the villi of the small intestine and enter the blood stream. Our cells use these amino acids to assemble
new proteins that are used as enzymes, cell receptors, hormones, and structural features. Each protein
has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene
encoding that protein (see Genetics and Inheritance). If we are deficient in even a single amino acid
then our cells cannot make the proteins they require.


Lipids
Macronutrient


Provide 9 Kcalories per gram; it is an energy-yielding nutrient
Functions are stored energy (adipose tissue), organ protection, temperature regulator, insulation


such as myelin that covers nerve cells, lipid membrane around cells, and emulsifiers to keep fats
dispersed in body fluids.


Lipids are made of organic molecules carbon, hydrogen, and oxygen. Fats consist of glycerol fatty
acids joind by an ester bond.


• A. Triglycerides composed of three fatty acids and one glycerol molecule.
• B. Saturated fatty acid fatty acid with carbon chains fully saturated with hydrogen.
• C. Monounsaturated fatty acid fatty acid that has a carbon chain with one unsaturated


248 | Human Physiology




Nutrition


double bond.
• D. Polyunsaturated fatty acid a fatty acid that has two or more double bonds on the carbon


chain.
Essential fatty acids part of the polyunsaturated fatty acids


• E. Linoleic acid and essential polyunsaturated fatty acid, it first double bond is a t the 6th
carbon this is why it can be called Omega 6.


• F. Linolenic acid an essential polyunsaturated fatty acid, it first double bond is at the 3rd
carbon this is why it can be called Omega 3, and is the main member of the omega-3 family.


• G. Eicosapentaenoic acid (EPA) , is derived form linolecin acid and is the main fatty acid
found in fish, also called omega 3.


• H. Docosahexaenoic acid (DHE), is an omega 3 fatty acid is synthesized in body from
alph-linolenic acid and is present in fish. DHA is present in retina and brain.


Nonessential
• I. Sterols serve a vital function in the body and are produced by the body and are not


essential nutrients, this structure of a lipid is cholesterol. This is a waxy substance that doesn't
look like a triglyceride it doesn't have a glycerol backbone or fatty acids but because it is
impermeable in water it is a lipid.


• J. CIS- Trans Fatty acids hydrogenation makes monounsaturated and polyunsaturated fatty
acids go from a state of their original form that is cis to a trans form. Addition of hydrogens
ions will cause vegetable oil to harden. Additionally, they may stimulate cholesterol synthesis,
and are potentially carcinogenic.


Absorption process of triglycerides. This is the fat that your body deals with most of the time.
They are absorbed with the transport of chylomincres into the lymphatic system which in turn will pour
into the blood stream at the thoracic duct. Once it enters the blood stream the chylomicrons take the
triglycerides into the cells. The triglycerides that are on the outer part of the chylomicrons are broken
down by lipoprotein lipase. Lipoprotein lipase can be found on the walls of capillaries. It is this enzyme
that will break it into fatty acids and monoglycerides. The fatty acids are taken by the body's cells while
the monoglycerides are taken to the liver to be processed.


More Info on Lipids:
• 1. Lipids are structural components found in every cell of the human body. That is they


form the lipid bilayer found in individual cells. They also serve as the myelin sheath found in
neurons.


• 2. Lipids provide us with energy, most of that energy is in the form of triacylglycerols.
• 3. Both lipids and lipid derivatives serve as vitamins and hormones.
• 4. Lipophilic bile acids aid in lipid solubility


Vitamins and Minerals
We all need micronutrients in small quantities to sustain health. Micronutrients include dietary


minerals and vitamins. While all minerals and vitamins can be obtained through food, many people do
not consume enough to meet their micronutrient needs and instead may take a supplement.


Wikibooks | 249




Chapter 13
Microminerals or trace elements include at least iron, cobalt, chromium, copper, iodine,


manganese, selenium, zinc, and molybdenum. They are dietary minerals needed by the human body in
very small quantities (generally less than 100mg/day) as opposed to macrominerals which are required
in larger quantities. (Note that the use of the term "mineral" here is distinct from the usage in the
geological sciences.)


Vitamins
Vitamins are organic compounds that are essential for our body to function properly. Most


vitamins are obtained from what you consume, because the body is unable to manufacture most of the
essential vitamins that you need to survive. Here are types of vitamins and their roles:


Vitamin Food Sources Functions Problems When Deficient
Problems With
Taking Too


Much


A (retinol)


Ingested in a precursor
form. Found in animal
sources such as milk
and eggs. Also found
carrots and spinach
(contain pro vitamin A
carotenoids).


Vitamin A is a fat-soluble
vitamin. It helps cells
differentiate, also lowering your
risk of getting cancer. Vitamin
A helps to keep vision healthy.
It is required during pregnancy.
Vitamin A also influences the
function and development of
sperm, ovaries and placenta and
is a vital component of the
reproductive process.


Night blindness,
impaired growth
of bones and
teeth


Headache,
dizziness,
nausea, hair
loss, abnormal
development of
fetus


B1
(thiamine)


Found in wheat germ,
whole wheat, peas,
beans, enriched flour,
fish, peanuts and
meats.


Vitamin B1 is a water-soluble
vitamin that the body requires to
break down carbohydrates, fat
and protein. The body needs
vitamin b in order to make
adenine triphosphate (ATP).
Vitamin B1 is also essential for
the proper functioning of nerve
cells.


Beriberi,
muscular
weakness,
enlarged heart


Can interfere
with the
absorption of
other vitamins


B2
(riboflavin)


Found in milk cheese,
leafy green vegetables,
liver, soybeans yeast
and almonds.
Exposure to light
destroys riboflavin.


Vitamin B2 is a water-soluble
vitamin that helps the body
process amino acids and fats.
Activated vitamin B6 and folic
acid helps convert
carbohydrates to adenosine
triphosphate (ATP). Sometimes
vitamin B2 can act as an
antioxidant.


Dermatitis,
blurred vision,
growth failure


Unknown


B3 (niacin) Found in beets,
brewer's yeast, beef
liver, beef kidney,


Vitamin B3 is required for cell
respiration and helps release the
energy in carbohydrates, fats,


Pellagra,
diarrhea, mental
disorders


High blood
sugar and uric
acid,


250 | Human Physiology




Nutrition


pork, turkey, chicken,
veal, fish, salmon,
swordfish, tuna,
sunflower seeds, and
peanuts.


and proteins. It helps with
proper circulation and healthy
skin, functioning of the nervous
system, and normal secretion of
bile and stomach fluids. It is
used in the synthesis of sex
hormones, treating
schizophrenia and other mental
illnesses, and as a memory-
enhancer.


vasodilation


C (ascorbic
acid)


Found citrus fruits
such as oranges,
grapefruit and lemon.


Vitamin C is an essential water-
soluble vitamin. It is needed to
make collagen. Vitamin C also
aids in the formation of liver
bile which helps to detoxify
alcohol and other substances.
Evidence indicates that vitamin
C levels in the eye decrease with
age and this may be a cause of
cataracts. Vitamin C has been
reported to reduce activity of the
enzyme, aldose reductase,
which helps protect people with
diabetes. It may also protect the
body against accumulation or
retention of the toxic mineral,
lead.


Scurvy, delayed
wound healing,
infections


Gout, kidney
stones,
diarrhea,
decreased
copper


D
Produced by the
human body during
exposure to the
ultraviolet rays of the
sun.


Vitamin D is a fat-soluble
vitamin that helps maintain
blood levels of calcium.
Vitamin D is necessary for
healthy bones and teeth.
Vitamin D plays a role in
immunity and blood cell
formation and also helps cells
differentiate this lowers your
chance of getting cancer.


Lack of Vitamin
D results in
rickets for
children and
osteomalacia for
adults.


Calcification of
soft tissue,
diarrhea,
possible renal
damage


E


Found in vegetable
oils, nuts, and green
leafy vegetables.
Fortified cereals are
also an important
source of vitamin E in
the United States.


Vitamin E is an antioxidant that
protects cell membranes and
other fat-soluble parts of the
body, such as LDL cholesterol
(the “bad” cholesterol), from
damage.


Unknown
Diarrhea,
nausea,
headaches,
fatigue, muscle
weakness


K Found in kale, collard
greens, spinach,
mustard greens, turnip
greens and Brussels


Vitamin K by helping transport
Ca, vitamin K is necessary for
proper bone growth and blood
coagulation.


Easy bruising
and bleeding


Can interfere
with
anticoagulant
medication


Wikibooks | 251




Chapter 13
sprouts. Also found
vegetable oils such as
soybean, canola,
cottonseed, and olive.
Additionally, the
normal flora of the
large intestine produce
vitamin K, which our
body is able to absorb
and use


Folic acid


Found in many
vegetables including,
broccoli, peas,
asparagus, spinach,
green leafy types.
Also found in fresh
fruit, liver and yeast.


Coenzyme needed for
production of hemoglobin and
formation of DNA.


Megaloblastic
anemia, spina
bifida


May mask B12
deficiency


B12
Found in meat, fish,
eggs and milk but not
in vegetables.


Vitamin B12 is needed to make
red blood cells. Supplements
can help some types of anemia.


Pernicious
anemia Unknown


B6
(pyridoxine
)


Found in cereals,
yeast, liver, and fish.


Vitamin B6 is a coenzyme in
amino acid synthesis.


Rare to be
deficient,
convulsions,
vomiting,
seborrhea,
muscular
weakness


Insomnia,
neuropathy


Fat soluble vitamins A, D, E, K
With fat soluble vitamins you need the presence of fat in your diet to absorb them, this is because


the bile will not be secreted to help with emulsification and therefore the fat vitamins will not be
broken down for absorption. Fat soluble vitamins are stored in organs such as the liver, spleen, and
other fatty tissues in the body. Because of this excessive amounts of fat-soluble vitamins can
accumulate in the body resulting in toxicity, but this rarely comes form excessive dietary intake but
rather form improper use of vitamin supplements.


Minerals
Minerals are atoms of certain chemical elements that are essential for body processes. Minerals are


inorganic, meaning that they are not man-made. They are either produced by our body, or we obtain
them by eating certain foods that contain them. They are ions found in blood plasma and cell
cytoplasm, such as sodium, potassium, and chloride. In addition, minerals represent much of the
chemical composition of bones (calcium, phosphorus, oxygen). They also contribute to nerve and
muscle activity (sodium, potassium, calcium). Minerals serve several many other functions as well.
There are 21 minerals considered essential for our bodies. Nine of the essential minerals in the body
account for less than .01% of your body weight. Because of the small amount of these minerals that our
body needs, we call them trace minerals. The 12 most important minerals and their functions are listed


252 | Human Physiology




Nutrition


below:
Mineral Source Use in the body


Calcium
(Ca)


Calcium can be found in dairy
products, dark green vegetables
and legumes.


It contributes to bone and teeth formation. In addition,
calcium also contributes to nerve and muscle action, and
blood clotting.


Chloride (Cl) Chloride is mainly found in table salt.
It plays a role in the acid-base balance, stomach acid
formation, and body water balance.


Copper (Cu) Copper can be found in seafood, nuts, and legumes.
It participates in the synthesis of hemoglobin and
melanin.


Flourine (F)
Flourine is evident in
flouridated water, tea, and
seafood.


It accounts for the maintenance of teeth, and perhaps the
maintenance of bone as well.


Iodine (I)
Iodine is a component in
iodized salt, marine fish and
shellfish.


Although a very small amount is needed for our body,
according to some, iodine still plays a role in our body's
function. It can also be found in seaweed. It is needed
for the thyroid hormone.


Iron (Fe)
Iron can be found in green
leafy vegetables, whole grains
foods, legumes, meats, and
eggs.


It is needed for composition of hemoblogin, myoblobin,
and certain enzymes.


Magnesium
(Mg)


Magnesium is found in whole
grains foods, and in green leafy
vegetables.


It is the coenzyme found in several enzymes.


Phophorus
(P)


Phosphorus can be found in
meat, poultry, and whole grain
foods.


It serves as components of bones, teeth, phospholipids,
ATP, and nucleic acids.


Potassium
(K)


Potassium is widespread in the
diet, especially in meats and
grains.


It deals with muscle and nerve function, and also is a
major component of intracellular fluid.


Sodium (Na)
Sodium is found in table salt, is
a major component of water
and also widespread in the diet.


It participates in the functioning of muscles and nerves.


Sulfur (S) Sulfur is found in meat and diary products. It is a component of many proteins.


Zinc (Zn) Zinc is found in whole grain foods, meats, and seafood. It is a component of many enzymes.


Nutritional Disorders
Body Mass Index became popular during the early 1980s as obesity started to become a discernible


issue in prosperous Western society. BMI provided a simple numeric measure of a person's "fatness" or
"thinness", allowing health professionals to discuss the problems of over- and under-weight more
objectively with their patients. However, BMI has become controversial because many people,
including physicians, have come to rely on its apparent numerical "authority" for medical diagnosis –
but that has never been the BMI's purpose. It is meant to be used as a simple means of classifying


Wikibooks | 253




Chapter 13
sedentary (physically inactive) individuals with an average body composition.[1] For these individuals,
the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI
lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is
overweight; a BMI below 15 may indicate the person has an eating disorder; a number above 30
suggests the person is obese (over 40, morbidly obese).


In physiology the term “weight” is used interchangeably with “mass”. For a given body shape and
given density, the BMI will be proportional to weight e.g. if all body weight increase by 50%, the BMI
increases by 50%.


BMI is defined as the individual's body weight divided by the square of their height. The formulas
universally used in medicine produce a unit of measure that is not dimensionless; it has units of kg/m2.
Body mass index may be accurately calculated using any of the formulas below.
SI units US units UK mixed units


BMI Weight Status
Below 18.5 Underweight
18.5 - 24.9 Normal
25.0 - 29.9 Overweight
30.0 and Above Obese


The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of
American men and 49% of women have BMIs over 25. Extreme obesity — a BMI of 40 or more —
was found in 2% of the men and 4% of the women. There are differing opinions on the threshold for
being underweight in females, doctors quote anything from 18.5 to 20 as being the lowest weight, the
most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the
health risks involved, with a BMI <17.5 being one of the criteria for the diagnosis of anorexia nervosa.


Anorexia nervosa: is a psychiatric diagnosis that describes an eating disorder characterized by
low body weight and body image distortion with an obsessive fear of gaining weight. Individuals with
anorexia often control body weight by voluntary starvation, purging, vomiting, excessive exercise, or
other weight control measures, such as diet pills or diuretic drugs. It primarily affects young adolescent
girls in the Western world and has one of the highest mortality rates of any psychiatric condition, with
approximately 10% of people diagnosed with the condition eventually dying due to related factors.[1]
Anorexia nervosa is a complex condition, involving psychological, neurobiological, and sociological
components.[2]


Bulimia nervosa: commonly known as bulimia, is generally considered a psychological condition
in which the subject engages in recurrent binge eating followed by an intentional purging. This purging
is done in order to compensate for the excessive intake of the food and to prevent weight gain. Purging
typically takes the form of vomiting; inappropriate use of laxatives, enemas, diuretics or other
medication; and excessive physical exercise.


254 | Human Physiology




Nutrition


Metabolism
Absorptive and post absorptive stage of metabolism
The body has two phases to it metabolic cycle. The first is known as the absorptive stage. This


stage happens 3-4 hours after a typical meal. during this phase nutrients are absorbed by the body. In
other words this is the stage where energy is stored into macromolecules. During the post-absorptive
stage the nutrients are not being absorbed instead this is the stage where it is being mobilized.


Insulin
The changes in the body that occur between the absorptive an post-absorptive state are triggered by


the changes in the plasma concentration of insulin. Insulin encourages the synthesis of energy storage
molecules. When plasma glucose levels in the bloodstream increase during the absorptive stage insulin
is secreted from the pancreas. When the plasma glucose levels decrease that begins the post-absorptive
phase. Insulin acts on several different tissues in the body and influences almost every major aspect of
energy metabolism. Insulin supports and promotes all aspects of the absorptive phase by helping store
energy in all tissues. It also inhibits the reactions of the post-absorptive phase. Insulin also affects the
transport of nutrients across the membrane of ALL body cells except for those located in the liver and
CNS. Insulin also has a part in growth where it needs to be present in the blood stream in order for the
hormones to effect normally.


Epinephrine and sympathetic nervous activity on metabolism
The sympathetic system and epinephrine suppress insulin and stimulate glycogen secretion. This


effects the post absorptive phase by making metabolic adjustments. During the post absorptive phase
plasma glucose levels decrease and cause an increase of glycogen secretion. It also acts directly on
glucose receptors in the CNS. This causes a rise in epinephrine secretion by the adrenal medulla. The
rise in epinephrine creates a cascade event where the body sends signals to all the tissues (except
skeletal muscles)to switch to the post absorptive phase.


Case Study
Diabetes


With insulin having such an effect throughout the body you can understand why a lack of proper
insulin control can effect the body. With insulin deficient people they have a uptake of glucose into
muscle and fat tissues and an increase of releasing glucose into the bloodstream. With the uptake of
glucose in the muscles it causes them to use their alternate energy. This will produce muscle wasting,
weakness and weight loss. The can be tested as hyperglycemia in the blood. These effects are caused
by abnormally high plasma glucose levels and although hyperglycemia has a suppressive effect on
glucagon secretion, glucagon secretion is often elevated in people with diabetes because the glucose
permeability of alpha cells in the pancreas is insulin dependent. Diabetes is the seventh leading cause
of death in the United States. Their are two different types of diabetes, Type I and Type II. Type I also
known as insulin dependent diabetes Mellitus (IDDM) occurs mostly in children as it is a result of of
the destruction of B cells within the body. Treatment for this type of diabetes is insulin injection


Wikibooks | 255




Chapter 13
therapy and if left untreated can cause death by ketoacidosis or dehydration shock. Type II also known
as non-insulin dependent diabetes ( NIDDM) appears usually after the age of 40 and accounts for the
majority of diabetes cases. The cause of NIDDM is usually a reduction in target cell responsiveness to
insulin.


Calories, Exercise, and Weight
Energy Balance and Body Weight


Energy is measured in units called calories. A calorie is the amount of energy that is needed to
raise the temperature of 1 gram of water by one degree Celsius. Because a calorie is such a small
amount, scientists use a larger unit to measure intake, called a kilocalorie. A kilocalorie is also referred
to often as a capital "C" Calorie, and is equal to 1000 calories. When we "count" calories, we are
actually counting the big Calories.


The old saying, "you are what you eat" is very much true. According to scientists, the average
adult consumes 900,000 calories per year. Most people tend to take in more calories than their body
needs. An intake of 12 extra calories a day, or around 5% excess in calories, yields an annual increase
of 12 extra pounds of body weight. The more developed countries tend to consume more calories than
others because of the increasing availability and dieting habits of eating refined foods with little
nutrition in them and lots of saturated fat. In our society, there is a huge emphasis put on a person's
image and how thin they are, and less emphasis put on what's most important--the nutrition our body
receives. While our body do need calories every day to keep us going, we need to watch the amount of
calories we consume in order to maintain good health and proper body weight.


Our Caloric intake is linked directly to our health status. Being overweight is generally defined as
being 15-20% above ideal body weight, while obesity is defined as being more than 20% above it.
People who weigh 10% less than ideal are considered underweight. This is less common in the more
developed countries. In less developed countries such as South Africa, being underweight is quite
common because they lack the nutrition to maintain good health.


How do we gain weight? When we consume more calories than our body can burn in a day, the
excess energy is stored in specialized cells as fat. It is also important to know that the three classes of
nutrients have different Caloric contents. Carbohydrates and proteins contain only four Calories per
gram, while fat contains about nine. Because of this, it is essential that we watch our amount of fat
intake. If we continuously feed our body more calories than is needed, our body will produce more fat
cells, to story the excess energy. This contributes to gaining weight.


It is more difficult for chronically overweight persons to lose weight than normal-weight persons.
This is because they are constantly fighting the body's own weight-control system, which responds as if
the excess weight were normal. Our body is capable of measuring how much we intake, and
maintaining our weight. When an overweight person goes on a diet, and consumes less calories, their
body will respond as if they are starving, and try to save energy where it can to make up for the
decrease in received calories.


Maintaining a healthy body weight


256 | Human Physiology




Nutrition


To maintain a stable body weight, our consumption of calories needs to be equal to the amount of
calories we use in a day. You can determine your daily energy needs by determining your basal
metabolic rate (BMR). Your BMR is the energy your body needs to perform essential activities. Some
examples of essential activities are breathing, and maintaining organ function. Your metabolic rate can
be influenced by your age, gender, muscular activity, body surface area and enviromental tempature.


Physical Activity: An efficient way to use calories
Although the BMR stays about the same, we can dramatically change the amount of calories we


burn in a day by participating in physical activity. It is important to note that heavier people do more
work per hour than normal-weight people, for the same level of activity. We must spend about 3,500
Calories to lose one pound of fat. The best approach to weight loss, recommended by nutritionists, is to
reduce the Caloric intake by a small amount each day while gradually increasing your amount of
physical activity.


BMR: Determining how many calories we need
There are several factors that influence the BMR. Each person's body has different needs. BMR


needs vary with gender and body composition. Muscle tissue consumes more energy than fat tissue.
Typically, males need more calories than females, because they generally have more muscle tissue.
Males use up calories faster than women. BMR also varies with your age as well. As we age, our body
needs less and less calories. In addition, some health conditions can contribute to our needed calories.
Health conditions such as fever, infections, and hyperthyroidism are examples of health conditions that
lower your BMR. Our stress level effects our needed calorie intake as well. So does our increase or
decrease in consumption, and our rate of metabolism, which varies with individual genetics.


Calculating Your BMR
Here are the steps to determining your BMR, or, the amount of energy your body needs to perform


essential activities:
1. First calculate your weight into kilograms. This is obtained by dividing the number of


pounds by 2.2.
2. For Males: multiply your weight in kilograms by 1.0. For Females: multiply your weight in


kilograms by 0.9.
3. This number approximates the number of Calories you consumer per hour. Now multiply


this number by 24 to estimate how many Calories you need per day to support basic metabolic
functions.
4. The end result is your personal basal metabolic rate!


Glossary
Amino acids


The building blocks of protein in the body. There are nine essential amino acids that are not
manufactured by the body and must come from the diet.


Anabolism
Refers the cumulative metabolic intracellular, molecular processes by which every cell repairs


Wikibooks | 257




Chapter 13
itself and grows.(synthesizing).


Anorexia
A common eating disorder characterized by an abnormal loss of the appetite for food


antioxidants
Compounds that protect against cell damage inflicted by molecules called oxygen-free radicals,
which are a major cause of disease and aging.


Bulimia Nervosa
Eating disorder characterized by binge eating followed by an intentional purging.


Catabolism
The opposite of Anabolism. The metabolic process that breaks down molecules into smaller
units. It is made up of degradative chemical reactions in the living cell.


Cirrhosis of the liver
An irreversible advanced scarring of the liver as a result of chronic inflammation of the liver. Can
be caused by alcoholism or obesity.


Complete Proteins
Proteins that contain ample amounts of all of the essential amino acids


Deamination
When an amino acid group breaks off an amino acid that makes a molecule of ammonia and keto
acid.


Diverticulosis
A diet low in dietary fiber increases the risk, this is the pouches called diverticula formation on
the outer portion of the large intestine.


Gastric Bypass Surgery
An operation where a small gastric pouch is created and the remainder of the stomach bypassed


Incomplete Proteins
Proteins that contain some but not all of all of the essential amino acids required by the body


Ipecac
A drug used to induce vomiting


Kwashiorkor
A childhood form of malnutrition caused by general lack of protein or deficiency in one or more
amino acids. Appearance of a person with this is a swollen belly due to inadequate production of
albumin, which causes the blood to have a lower osmotic pressure, resulting in more fluids
escaping from the plasma.


Marasmus
malnutrition cause by a lack of kcalorie intake. Appearance of a person with this is a skeletal one.


258 | Human Physiology




Nutrition


Malnutrition
An imbalanced nutrient and or energy intake.


Obesity
A condition in which the natural energy reserve in fatty tissue increased to a point where it is
thought to be a risk factor for certain health conditions or increased mortality


Peptide
Two or more amino acids linked together by a bond called a peptide bond.


Polypeptide
A string of amino acids linked together by peptide bonds. A protein is an example of a
polypeptide.


Starvation
A severe reduction in vitamin, nutrient, and energy intake, and is the most extreme form of
malnutrition


Review Questions
1. Nonessential amino acids


A) are stored in the body
B) are only needed occasionally
C) can be produced in the body
D) can be taken in supplements


2. Micronutrients include
A) minerals and vitamins
B) lipids and fatty acids
C) amino acids and proteins
D) vitamins and minerals


3. The body requires amino acids to
A) produce new red blood cells
B) produce new protein
C) replace damaged red blood cells
D) replace damaged protein
E) A and C
F) B and D


4. The function of lipids
A) store energy
B) organ protection
C) temperature regulator


Wikibooks | 259




Chapter 13
D) emulsifiers
E) all of the above


5. This vitamin is a vital component of the reproductive process and lowers the risk of getting cancer
A) B12
B) Folic Acid
C) Niacin
D) Thiamine
E) Retinol


6. This vitamin is needed to make red blood cells
A) B1
B) B2
C) B6
D) B12


7. This participates in the synthesis of hemoglobin and melanin
A) Copper
B) Chloride
C) Calcium
D) Iron
E) Iodine


8. I go to visit my grandmother and see that she has multiple bruises- from this I may assume that
A) she has a vitamin A deficiency
B) she is old and just clumsy
C) she has a vitamin K deficiency
D) she has scurvy
E) she has rickets


9. As a pirate I may get scurvy because
A) I am not getting enough vegetables on the ship
B) I am not getting enough fruit on the ship
C) I am eating too much fish on the ship
D) I am getting too much sun on the ship
E) I am drinking too much rum on the ship


10. I am taking anticoagulant medication and it doesn’t seem to be working, this could be because
A) I have too much vitamin A
B) I have too much B12
C) I have too much sodium
D) I have too much vitamin E
E) I have too much vitamin K


260 | Human Physiology




Nutrition


References
• Van De Graaff (2002) Human Anatomy 6th ed. McGraw-Hill Higher Education
• Windmaier, P.W. Raff, H. Strang, T.S. (2004) Vander, Sherman, & Luciano's Human


Physiology, the Mechanisms of Body Function 9th ed. Mcgraw-Hill
• Starr & McMillan (2001) Human Biology 6th ed. Thomson-Brooks/cole.
• Spurlock, Morgan (2004) Super Size Me Hart Sharp Video


live version • discussion • edit lesson • comment • report an error


Wikibooks | 261




Chapter 14


14 THE ENDOCRINE SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction To The Endocrine System
he endocrine system is a control system of ductless glands that secrete chemical messengers called
hormones that circulate within the body via the bloodstream to affect distant cells within specific


organs. Endocrine glands secrete their products immediately into the blood or interstitial fluid, without
storage of the chemical. Hormones act as "messengers," and are carried by the bloodstream to different
cells in the body, which interpret these messages and act on them.


T
It seems like a far fetched notion or idea that a small chemical can enter the bloodstream and cause


an action at a distant location in the body. Yet this occurs in our bodies everyday of our lives. The
ability to maintain homeostasis and respond to stimuli is largely due to hormones secreted within the
body. Without hormones, you could not grow, maintain a constant temperature, produce offspring, or
perform the basic actions and functions that are essential for life.


The endocrine system provides an electrochemical connection from the hypothalamus of the brain
to all the organs that control the body metabolism, growth and development, and reproduction.


There are two types of hormones secreted in the endocrine system: (1) steroidal and (2)
nonsteroidal, or protein based hormones.


The endocrine system regulates its hormones through negative feedback, except in very specific
cases like childbirth. Increases in hormone activity decrease the production of that hormone. The
immune system and other factors contribute as control factors also, altogether maintaining constant
levels of hormones.


Types of Glands
Exocrine Glands are those which release their cellular secretions through a duct which empties to


the outside or into the lumen (empty internal space) of an organ. These include certain sweat glands,
salivary and pancreatic glands, and mammary glands. They are not considered a part of the endocrine
system.


Endocrine Glands are those glands which have no duct and release their secretions directly into
the intercellular fluid or into the blood. The collection of endocrine glands make up the endocrine
system.


The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroids,
adrenal (cortex and medulla), pancreas and gonads.
The pituitary gland is attached to the hypothalamus of the lower forebrain.
The thyroid gland consists of two lateral masses, connected by a crossbridge, that are attached to


262 | Human Physiology




The Endocrine System


the trachea. They are slightly inferior to the larynx.
The parathyroids are four masses of tissue, two embedded posteriorly in each lateral mass of the
thyroid gland.
One adrenal gland is located on top of each kidney. The cortex is the outer layer of the adrenal
gland. The medulla is the inner core.
The pancreas is along the lower curvature of the stomach, close to where it meets the first region
of the small intestine, the duodenum.
The gonads are found in the pelvic cavity.


Hormones and Types
A hormone is a type of chemical signal. They are a means of communication between cells.
The endocrine system produces hormones that are instrumental in maintaining homeostasis and


regulating reproduction and development. A hormone is a chemical messenger produced by a cell that
effects specific change in the cellular activity of other cells (target cells). Unlike exocrine glands
(which produce substances such as saliva, milk, stomach acid, and digestive enzymes), endocrine
glands do not secrete substances into ducts (tubes). Instead, endocrine glands secrete their hormones
directly into the surrounding extracellular space. The hormones then diffuse into nearby capillaries and
are transported throughout the body in the blood.


The endocrine and nervous systems often work toward the same goal. Both influence other cells
with chemicals (hormones and neurotransmitters). However, they attain their goals differently.
Neurotransmitters act immediately (within milliseconds) on adjacent muscle, gland, or other nervous
cells, and their effect is short-lived. In contrast, hormones take longer to produce their intended effect
(seconds to days), may affect any cell, nearby or distant, and produce effects that last as long as they
remain in the blood, which could be up to several hours.


In the following table there are the major hormones, their target and their function once in the
target cell.


Endocrine
Gland Hormone Released


Chemical
Class


Target
Tissue/Organ


Major Function of
Hormone


Hypothalamus Hypothalamic releasing and inhibiting hormones Peptide
Anterior
pituitary


Regulate anterior pituitary
hormone


Posterior
Pituitary Antidiuretic (ADH) Peptide Kidneys


Stimulates water
reabsorption by kidneys


Posterior
Pituitary Oxytocin Peptide


Uterus,
mammary
glands


Stimulates uterine muscle
contractions and release of
milk by mammary glands


Anterior
Pituitary


Thyroid stimulating
(TSH) Glycoprotein Thyroid Stimulates thyroid
Adrenocorticotropic Peptide Adrenal cortex Stimulates adrenal cortex


Wikibooks | 263




Chapter 14
(ACTH)
Gonadotropic (FSH,
LH) Glycoprotein Gonads


Egg and sperm production,
sex hormone production


Prolactin (PRL) Protein Mammary glands Milk production


Growth (GH) Protein Soft tissue, bones
Cell division, protein
synthesis and bone growth


Thyroid Thyroxine (T4) and Triiodothyronie (T3)
Iodinated
amino acid All tissue


Increase metabolic rate,
regulates growth and
development


Calcitonin Peptide Bones, kidneys and intestine
Lowers blood calcium
level


Parathyroids Parathyroid (PTH) Peptide Bones, kidneys and intestine Raises blood calcium level


Adrenal Cortex Glucocorticoids (cortisol) Steroid All tissue
Raise blood gluclose level,
stimulates breakdown of
protein


Mineralocoticoids
(aldosterone) Steroid Kidneys


Reabsorb sodium and
excrete potassium


Sex Hormones Steroid
Gonads, skin,
muscles and
bones


Stimulates reproductive
organs and brings on sex
characteristics


Adrenal
Medulla


Epinephrine and
norepinephrine


Modified
amino acid


Cardiac and
other muscles


Released in emergency
situations, raises blood
glucose level, “fight or
flight” response


Pancreas Insulin Protein Liver, muscles, adipose tissue
Lowers blood glucose
levels, promotes formation
of glycogen


Glucagon Protein Liver, muscles, adipose tissue Raises blood glucose levels


Testes Androgens (testosterone) Steroid
Gonads, skin,
muscles and
bone


Stimulates male sex
characteristics


Ovaries Estrogen and progesterone Steroid
Gonads, skin,
muscles and
bones


Stimulates female sex
characteristics


Thymus Thymosins Peptide T lymphocytes
Stimulates production and
maturation of T
lymphocytes


Pineal Gland Melatonin Modified amino acid Brain
Controls circadian and
circannual rhythms,
possibly involved in
maturation of sexual
organs


Hormones can be chemically classified into four groups:


264 | Human Physiology




The Endocrine System


1. Amino acid-derived: Hormones that are modified amino acids.
2. Polypeptide and proteins: Hormones that are chains of amino acids of less than or more


than about 100 amino acids, respectively. Some protein hormones are actually glycoproteins,
containing glucose or other carbohydrate groups.
3. Steroids: Hormones that are lipids that are synthesized from cholesterol. Steroids are


characterized by four interlocking carbohydrate rings.
4. Eicosanoids: Are lipids that are synthesized from the fatty acid chains of phospholipids


found in plasma membrane.
Hormones circulating in the blood diffuse into the interstitial fluids surrounding the cell. Cells with


specific receptors for a hormone respond with an action that is appropriate for the cell. Because of the
specificity of hormone and target cell, the effects produced by a single hormone may vary among
different kinds of target cells.


Hormones activate target cells by one of two methods, depending upon the chemical nature of the
hormone.


• Lipid-soluble hormones (steroid hormones and hormones of the thyroid gland) diffuse
through the cell membranes of target cells. The lipid-soluble hormone then binds to a receptor
protein that, in turn, activates a DNA segment that turns on specific genes. The proteins
produced as result of the transcription of the genes and subsequent translation of mRNA act as
enzymes that regulate specific physiological cell activity.


• Water-soluble hormones (polypeptide, protein, and most amino acid hormones) bind to a
receptor protein on the plasma membrane of the cell. The receptor protein, in turn,stimulates the
production of one of the following second messengers:


Cyclic AMP (cAMP) is produced when the receptor protein activates another membrane-bound
protein called a G protein. The G protein activates adenylate cyclase, the enzyme that catalyzes the
production of cAMP from ATP. Cyclic AMP then triggers an enzyme that generates specific cellular
changes.


Inositol triphosphate (IP3) is produced from membrane phospholipids. IP3, in turn, triggers the
release of CA2+ from the endoplasmic reticulum, which then activates enzymes that generate cellular
changes.


Endocrine glands release hormones in response to one or more of the following stimuli:
1. Hormones from other endocrine glands.
2. Chemical characteristics of the blood (other than hormones).
3. Neural stimulation.


Most hormone production is managed by a negative feedback system. The nervous system and
certain endocrine tissues monitor various internal conditions of the body. If action is required to
maintain homeostasis, hormones are released, either directly by an endocrine gland or indirectly
through the action of the hypothalamus of the brain, which stimulates other endocrine glands to release
hormones. The hormones activate target cells, which initiate physiological changes that adjust the body
conditions. When normal conditions have been recovered, the corrective action - the production of
hormones - is discontinued. Thus, in negative feedback, when the original (abnormal) condition has


Wikibooks | 265




Chapter 14
been repaired, or negated, corrective actions decrease or discontinue. For example, the amount of
glucose in the blood controls the secretion of insulin and glucagons via negative feedback.


The production of some hormones is controlled by positive feedback. In such a system, hormones
cause a condition to intensify, rather than decrease. As the condition intensifies, hormone production
increases. Such positive feedback is uncommon, but does occur during childbirth, where hormone
levels build with increasingly intense labor contractions. Also in lactation, hormone levels increase in
response to nursing, which causes milk production to increase. The hormone produced by the
hypothalamas causing the milk let down and uterine contraction is oxytocin.


Endocrine Glands
Pituitary gland


The hypothalamus makes up the lower region of the diencephalons and lies just above the brain
stem. The pituitary gland (hypophysis) is attached to the bottom of the hypothalamus by a slender stalk
called the infundibulum. The pituitary gland consists of two major regions, the anterior pituitary gland
(anterior lobe or adenohypophysis) and the posterior pituitary gland (posterior lobe or
neurohypophysis). The hypothalamas also controls the glandular secretion of the pituitary gland.


The hypothalamus oversees many internal body conditions. It receives nervous stimuli from
receptors throughout the body and monitors chemical and physical characteristics of the blood,
including temperature, blood pressure, and nutrient, hormone, and water content. When deviations
from homeostasis occur or when certain developmental changes are required, the hypothalamus
stimulates cellular activity in various parts of the body by directing the release of hormones from the
anterior and posterior pituitary glands. The hypothalamus communicates directives to these glands by
one of the following two pathways:


The Pituitary gland is found in the inferior part of the brain and is connected by the Pituitary Stalk.
It can be referred to as the master gland because it is the main place for everything that happens within
the endocrine system. It is divided into two sections: the anterior lobe (adenohypophysis) and the
posterior lobe (neurohypophysis). The Posterior pituitary is involved in sending hormones that control
all other hormones of the body.


Posterior pituitary
Communication between the hypothalamus and the posterior pituitary occurs through


neurosecretory cells that span the short distance between the hypothalamus and the posterior pituitary.
Hormones produced by the cell bodies of the neurosecretory cells are packaged in vesicles and
transported through the axon and stored in the axon terminals that lie in the posterior pituitary. When
the neurosecretory cells are stimulated, the action potential generated triggers the release of the stored
hormones from the axon terminals to a capillary network within the posterior pituitary. Two hormones,
oxytocin and antidiuretic hormone (ADH), are produced and released this way. If one's not able to
produce ADH it can cause Diabetes Insipidus which means a person is producing large amounts of
urine, resulting in loss of ions from the blood.


266 | Human Physiology




The Endocrine System


The posterior lobe is composed of neural tissue [neural ectoderm] and is derived from
hypothalamus. Its function is to store oxytocin and Antidieuretic hormone. When hypothalamic
neurons fire: hormones release into the capillaries of the lobe.


The posterior pituitary is, in effect, a projection of the hypothalamus. It does not produce its own
hormones, but only stores and releases the hormones oxytocin and antidiuretic hormone (ADH - also
known as vasopressin).


Anterior pituitary
Communication between the hypothalamus and the anterior pituitary occurs through chemicals


(releasing hormones and inhibiting hormones) that are produced by the hypothalamus and delivered to
the anterior pituitary through blood vessels. The releasing and inhibitating hormones are produced by
specialized neurons of the hypothalamus called neurosecretory cells. The hormones are released into a
capillary network or primary plexus, and transported through veins or hypophyseal portal veins, to a
second capillary network or secondary plexus that supplies the anterior pituitary. The hormones then
diffuse from the secondary plexus into the anterior pituitary, where they initiate the production of
specific hormones by the anterior pituitary. Many of the hormones produced by the anterior pituitary
are tropic hormones or tropins, which are hormones that stimulate other endocrine glands to secrete
their hormones


The anterior lobe is derived from oral ectoderm, composed of glandular epithelium it
communicates with the hypothalamus via a network of capillaries.


The anterior pituitary lobe receives releasing hormones from the hypothalamus via a portal vein
system known as the hypothalamic-hypophyseal portal system.


The anterior pituitary secretes:
• growth hormone
• prolactin
• follicle-stimulating hormone
• luteinizing hormone
• thyroid-stimulating hormone
• adrenocorticotropic hormone
• endorphins
• and other hormones


It does this in response to a variety of chemical signals from the hypothalamus, which travel to the
anterior lobe by way of a special capillary system from the hypothalamus, down the median eminence,
to the anterior lobe. These include:


• TRH (thyrotropin-releasing hormone)
• CRH (corticotropin-releasing hormone)
• DA (dopamine, "prolactin inhibiting factor"/PIF)
• GnRH (gonadotropin-releasing hormone)
• GHRH (growth hormone releasing hormone)


Wikibooks | 267




Chapter 14
These hormones from the hypothalamus cause release of the respective hormone from the


pituitary. The control of release of hormones from the pituitary happens when there is negative
feedback from the gland on which they act. Meaning that when the hormones increase on the effected
gland the pituitary will stop sending hormones to them.


Also, the heart, gastrointestinal tract, the placenta, the kidneys and the skin, whose major function
is not the secretion of hormones, nonetheless, contain some specialized cells that produce hormones.


In addition, all cells, except red blood cells secrete a class of hormones called eicosanoids. These
hormones are paracrines, or local hormones, that primarily affect neighboring cells. Two groups of
eicosanoids, the prostaglandins (PGs) and the leukotrienes (LTs), have a wide range of varying effects
that depend upon the nature of the target cell. Eicosanoid activity, for example, may impact blood
pressure, blood clotting, immune and inflammatory responses, reproductive processes, and the
contraction of smooth muscles.


Antagonistic Hormones
Maintaining homeostasis often requires conditions to be limited to a narrow range. When


conditions exceed the upper limit of homeostasis, specific action, usually the production of a hormone,
is triggered. When conditions return to normal, hormone production is discontinued. If conditions
exceed the lower limits of homeostasis, a different action, usually the production of a second hormone,
is triggered. Hormones that act to return body conditions to within acceptable limits from opposite
extremes are called antagonistic hormones. The two glands that are the most responsible for
homeostasis is the thyroid and the parathyroid.


The regulation of blood glucose concentration (through negative feedback) illustrates how the
endocrine system maintains homeostasis by the action of antagonistic hormones. Bundles of cells in the
pancreas called the islets of Langerhans contain two kinds of cells, alpha cells and beta cells. These
cells control blood glucose concentration by producing the antagonistic hormones insulin and
glucagon.


Beta cells secrete insulin. When the concentration of blood glucose rises such in after eating, beta
cells secret insulin into the blood. Insulin stimulates the liver and most other body cells to absorb
glucose. Liver and muscle cells convert glucose to glycogen, for short term storage, and adipose cells
convert glucose to fat. In response, glucose concentration decreases in the blood, and insulin secretion
discontinues through negative feedback from declining levels of glucose.


Alpha cells secrete glucagon. When the concentration of blood glucose drops such as during
exercise, alpha cells secrete glucagon into the blood. Glucagon stimulates the liver to release glucose.
The glucose in the liver originates from the breakdown of glycogen and the conversion of amino acids
and fatty acids into glucose. When blood glucose levels return to normal, glucagon secretion
discontinues through negative feedback.


Another example of antagonistic hormones occurs in the maintenance of Ca2+ concentration in the
blood. Parathyroid hormone (PTH) from the parathyroid glands increases Ca2+ in the blood by
increasing Ca2+ absorption in the intestines and reabsorption in the kidneys and stimulating Ca2+
release from bones. Calcitonin (CT) produces the opposite effect by inhibiting the breakdown of bone
matrix and decreasing the release of calcium in the blood.


268 | Human Physiology




The Endocrine System


Thyroid gland
The Thyroid gland is one of the largest endocrine glands in the body. It is positioned on the neck


just below the Larynx and has two lobes with one on either side of the trachea. It is involved in the
production of the hormones T3 (triiodothyronine) and T4 (thyroxine). These hormones increase the
metabolic activity of the body‘s cells. The thyroid also produces and releases the hormone calcitonin
(thyrocalcitonin) which contributes to the regulation of blood calcium levels. Thyrocalcitonin or
calcitonin decreases the concentration of calcium in the blood. Most of the calcium removed from the
blood is stored in the bones.


The thyroid hormone consists of two components, thyroxine and iodine. This hormone increases
the metabolism of most body cells. A deficiency of iodine in the diet leads to the enlargement of the
thyroid gland, known as a simple goiter. Hypothyroidism during early development leads to cretinism.
In adults, it produces myxedema, characterized by obesity and lethargy. Hyperthyroidism leads to a
condition known as exophthalmic goiter, characterized by weight loss as well as hyperactive and
irritable behavior.


The thyroid gland is a two-lobed gland that manifests a remarkably powerful active transport
mechanism for uptaking iodide ions from the blood. As blood flows through the gland, iodide is
converted to an active form of iodine. This iodine combines with an amino acid called tyrosine. Two
molecules of iodinated tyrosine then combine to form thryroxine. Following its formation, the
thyroxine becomes bound to a polysaccharide-protein material called thyroglobulin. The normal
thyroid gland may store several weeks supply of thyroxine in this bound form. An enzymatic splitting
of the thyroxine from the thyroglobulin occurs when a specific hormone is released into the blood. This
hormone, produced by the pituitary gland, is known as thyroid-stimulating hormone (TSH). TSH
stimulates certain major rate-limiting steps in thyroxine secretion, and thereby alters its rate of release.
A variety of bodily defects, either dietary, hereditary, or disease induced, may decrease the amount of
thyroxine released into the blood. The most popular of these defects is one that results from dietary
iodine deficiency. The thyroid gland enlarges, in the continued presence of TSH from the pituitary, to
form a goiter. This a futile attempt to synthesize thyroid hormones, for iodine levels that are too low.
Normally, thyroid hormones act via a negative feedback loop on the pituitary to decrease stimulation of
the thyroid. In goiter, the feedback loop cannot be in operation - hence continual stimulation of the
thyroid and the inevitable protuberandce on the neck. Formerly, the principal source of iodine came
from seafood. As a result, goiter was prevalent amongst inland areas far removed from the sea. Today,
the incidence of goiter has been drastically reduced by adding iodine to table salt.


Thyroxine serves to stimulate oxidative metabolism in cells; it increases the oxygen consumption
and heat production of most body tissues, a notable exception being the brain. Thyroxine is also
necessary for normal growth, the most likely explanation being that thyroxine promotes the effects of
growth hormone on protein synthesis. The absence of thyroxine significantly reduces the ability of
growth hormone to stimulate amino acid uptake and RNA synthesis. Thyroxine also plays a crucial role
in the closely related area of organ development, particularly that of the central nervous system.


If there is an insufficient amount of thyroxine, a condition referred to as hypothyroidism results.
Symptoms of hypothyroidism stem from the fact that there is a reduction in the rate of oxidative
energy-releasing reactions within the body cells. Usually the patient shows puffy skin, sluggishness,
and lowered vitality. Other symptoms of hypothyroidism include weight gain, decreased libido,
inability to tolerate cold, muscle pain and spasm, insomnia and brittle dry hair. Hypothyroidism in
children, a condition known as cretinism, can result in mental retardation, dwarfism, and permanent


Wikibooks | 269




Chapter 14
sexual immaturity. Sometimes the thyroid gland produces too much thyroxine, a condition known as
hyperthyroidism. This condition produces symptoms such as an abnormally high body temperature,
profuse sweating, high blood pressure, loss of weight, irritability, and muscular pain and weakness. It
also causes the characteristic symptom of the eyeballs protruding from the skull called exopthalmia.
This is surprising because it is not a symptom usually related to a fast metabolism. Hyperthyroidism
has been treated by partial removal or by partial radiation destruction of the gland. More recently,
several drugs that inhibit thyroid activity have been discovered, and their use is replacing the former
surgical procedures. Unfortunately thyroid conditions require lifetime treatment and because of the
body's need for a sensitive balance of thyroid hormone both supplementing and suppressing thyroid
function can take months or even years to regulate.


T3 and T4 Function within the body
The Production of T3 and T4 are regulated by thyroid stimulating hormone (TSH), released by the


pituitary gland. TSH Production is increased when T3 and T4 levels are too low. The thyroid hormones
are released throughout the body to direct the bodies metabolism. They stimulate all cells within the
body to work at a better metabolic rate. Without these hormones the bodies cells would not be able to
regulate the speed at which they performed chemical actions. Their release will be increased under
certain situations such as cold temperatures when a higher metabolism is needed to generate heat.
When children are born with thyroid hormone deficiency they have problems with physical growth and
developmental problems. Brain development can also be severely impaired


The significance of iodine
Thyroid hormone cannot be produced without an abundant source of iodine. The iodine


concentration within the body, although significant, can be as little as 1/25th the concentration within
the thyroid itself. When the thyroid is low on iodine the body will try harder to produce T3 and T4
which will often result in a swelling of the thyroid gland, resulting in a goiter.


Calcitonin
Calcitonin is a 32 amino acid polypeptide hormone. It is an additional hormone produced by the


thyroid, and contributes to the regulation of blood calcium levels. Thyroid cells produce calcitonin in
response to high calcium levels in the blood. This hormone will stimulate movement of calcium into
the bone structure. It can also be used therapeutically for the treatment of hypercalcemia or
osteoporosis. Without this hormone calcium will stay within the blood instead of moving into bones to
keep them strong and growing. Its importance in humans has not been as well established as its
importance in other animals.


Parathyroid gland
There are four parathyroid glands. They are small, light-colored lumps that stick out from from


the surface of the thyroid gland. All four glands are located on the thyroid gland. They are butterfly-
shaped and located inside the neck, more specifically on both sides of the windpipe. One of the
parathyroid glands most important functions is to regulate the bodies calcium and phosphorus levels.
Another function of the parathyroid glands is to secrete parathyroid hormone, which causes the release


270 | Human Physiology




The Endocrine System


of the calcium present in bone to extracellular fluid. PTH does this by activating the production of
osteoblasts, special cells of the body involved in the production of bone and slowing down osteoclasts,
other specialized cells involved in the removal of bone.


There are two major types of cells that make up parathyroid tissue:
• One of the major cells is called oxyphil cells. Their function is basically unknown.
• The second type are called chief cells. Chief cells produce parathyroid hormone.


The structure of a parathyroid gland is very different from that of a thyroid gland. The chief cells
that produce parathyroid hormone are arranged in tightly-packed nests around small blood vessels,
quite unlike the thyroid cells that produce thyroid hormones, which are arranged in spheres called the
thyroid follicles.


PTH or Parathyroid Hormone is secreted from these four glands. It is released directly into the
bloodstream and travels to its target cells which are often quite aways away. It then binds to a structure
called a receptor, that is found either inside or on the surface of the target cells.


Receptors bind a specific hormone and the result is a specific physiologic response, meaning a
normal response of the body. The activity of all the hormones or growth factors secreted by endocrine
glands and circulating in blood is controlled by the exocrine system of the body.


PTH finds its major target cells in bone, kidneys, and the gastrointestinal system.
Calcitonin, a hormone produced by the thyroid gland that also regulates ECF calcium levels and


serves to counteract the calcium-producing effects of PTH.
The adult body contains as much as 1 kg of calcium. Most of this calcium is found in bone and


teeth.
The four parathyroids secrete the parathyroid hormone (PTH). It opposes the effect of


thyrocalcitonin. It does this by removing calcium from its storage sites in bones, releasing it into the
bloodstream. It also signals the kidneys to reabsorb more of this mineral, transporting it into the blood.
It also signals the small intestine to absorb more of this mineral, transporting it from the diet into the
blood.


Calcium is important for steps of body metabolism. Blood cannot clot without sufficient calcium.
Skeletal muscles require this mineral in order to contract. A deficiency of PTH can lead to tetany,
muscle weakness due to lack of available calcium in the blood.


The parathyroids were long thought to be part of the thyroid or to be functionally associated with
it. We now know that their close proximity to the thyroid is misleading: both developmentally and
functionally, they are totally distinct from the thyroid.


The parathyroid hormone, called parathormone, regulates the calcium-phosphate balance between
the blood and other tissues. Production of this hormone is directly controlled by the calcium
concentration of the extracellular fluid bathing the cells of these glands. Parathormne exerts at least the
following four effects: (1)it increases gastrointestinal absorption of calcium by stimulating the active
transport system and moves calcium from the gut lumen into the blood; (2) it increases the movement


Wikibooks | 271




Chapter 14
of calcium and phosphate from bone into extracellular fluid. This is accomplished by stimulating
osteoclasts to break down bone structure, thus liberating calcium phosphate into the blood. In this way,
the store of calcium contained in bone is tapped; (3) it increases reabsorption of calcium by the renal
tubules, thereby decreasing urinary calcium excretion; (4) it reduces the reabsorption of phosphate by
the renal tubules.


The first three effects result in a higher extracellular calcium concentration. The adaptive value of
the fourth is to prevent the formation of kidney stones.


If parathyroids are removed accidentally during surgery on the thyroind, there would be a rise in
the phosphate concentration in the blood. There would also be a drop in the calcium concentration as
more calcium is excreted by the kidneys and intestines, and more incorporated into the bone. This can
produce serious disturbances, particularly in the muscles and nerves, which use calcium ions for normal
functioning. Overactivity of the parathyroids, which can result from a tumor on the glands, produces a
weakening of the bones. This is a condition that makes them much more vulnerable to fracturing
because of excessive withdrawal of calcium from the bones.


=Adrenal glands
Adrenal glands are a pair of ductless glands located above the kidneys. Through hormonal


secretions, the adrenal glands regulate many essential functions in the body, including biochemical
balances that influence athletic training and general stress response. The glucocorticoids include
corticosterone, cortisone, and hydrocortisone or cortisol. These hormones serve to stimulate the
conversion of amino acids into carbohydrates which is a process known as gluconeogenesis, and the
formation of glycogen by the liver. They also stimulate the formation of reserve glycogen in the tissues,
such as in the muscles. The glucocorticoids also participate in lipid and protein metabolism. The cortex
of the adrenal gland is known to produce over 20 hormones, but their study can be simplified by
classifying them into three categoreies: glucocorticoids, mineralcorticoids, and sex hormones.


They are triangular-shaped glands located on top of the kidneys. They produce hormones such as
estrogen, progesterone, steroids, cortisol, and cortisone, and chemicals such as adrenalin (epinephrine),
norepinephrine, and dopamine. When the glands produce more or less hormones than required by the
body, disease conditions may occur.


The adrenal cortex secretes at least two families of hormones, the glucocorticoids and mineral
corticoids. The adrenal medulla secretes the hormones epinephrine (adrenalin) and norepinephrine
(noradrenalin).


Adrenal Cortex: The hormons made by the Adrenal Cortex supply long-term responses to stress.
The two major hormons produced are the Mineral Corticoids and the Glucocorticoids. The Mineral
Corticoids regulate the salt and water balance, leading to the increas of blood volume and blood
pressure. The Glucocorticoids are monitoring the ACTH, inturn regulating carbohydrates, protiens,
and fat metabilisum. Resulting into an increase in blood gluclose. Glucocorticoids also reduce the
bodys inflammitory responce.


Cortisol is one of the most active glucocorticoids. It usually reduces the effects of inflammation or
swelling throughout the body. It also stimulates the production f glucose from fats and proteins, which
is a process referred to as gluconeogenesis.


272 | Human Physiology




The Endocrine System


Aldosterone is one example of a mineralcorticoid. It signals the tubules in the kidney nephrons to
reabsorb sodium while secreting or eliminating potassium. If sodium levels are low in the blood, the
kidney secretes more renin, which is an enzyme that stimulates the formation of angiotensin from a
molecule made from the liver. Angiotensin stimulates aldosterone secretion. As a result, more sodium
is reabsorbed as it enters the blood.


Aldosterone, the major moineralcorticoid, stimulates the cells of the distal convoluted tubules of
the kidneys to decrease reabsorption of potassium and increase reabsorption of sodium. This in turn
leads to an increased reabsorption of chloride and water. These hormones, together with such hormones
as insulin and glucagon, are important regulators of the ionic environment of the internal fluid.


The renin-angiotensin-aldosterone mechanism can raise blood pressure if it tends to drop. It does
this in two ways. Angiotensin is a vasoconstrictor, decreasing the diameter of blood vessels. As vessels
constrict, blood pressure increases. In addition, as sodium is reabsorbed, the blood passing through the
kidney becomes more hypertonic. Water follows the sodium into the hypertonic blood by osmosis. This
increases the amount of volume in the blood and also increases the blood pressure.


Adrebal Medulla The hypothalamas starts nerve impulses that travel the path from the
bloodstream, spinal cord, sympathetic nerve fibers to the Adrenal Medulla, which then releases
hormones. The effects of these hormones provide a short-term response to stress


Excessive secretion of the glucocorticoids causes Cushing's syndrome, characterized by muscle
atrophy or degeneration and hypertension or high blood pressure. Under secretion of these substances
produces Addison's disease, characterized by low blood pressure and stress.


Epinephrine and norepinephrine produce the "fight or flight" response, similar to the effect from
the sympathetic nervous system. Therefore, they increase heart rate, breathing rate, blood flow to most
skeletal muscles, and the concentration of glucose in the blood. They decrease blood flow to the
digestive organs and diminish most digestive processes.


The adrenal sex hormones consist mainly of male sex hormones (androgens) and lesser amounts of
female sex hormones (estrogens and progesterone). Normally, the sex hormones released from the
adrenal cortex are insignificant due to the low concentration of secretion. However, in cases of excess
secretion, masculinizing or feminizing effects appear. The most common syndrome of this sort is
virilism of the female.


Should there be an insufficient supply of cortical hormones, a condition known as Addison's
disease would result. This disease is characterized by an excessive excretion of sodium ions, and hence
water, due to lack of mineralcorticoids. Accompanying this is a decreased blood glucose level due to a
deficient supply of glucocorticoids. The effect of a decreased androgen supply cannot be observed
immediately. Injections of adrenal cortical hormones promptly relieve these symptoms.


Hormonal production in the adrenal cortex is directly controlled by the anterior pituitary hormone
called adrenocorticotropic hormone (ACTH).


The two adrenal glands lie very close to the kidneys. Each adrenal gland is actually a double gland,
composed of an inner core like medulla and an outer cortex. Each of these is functionally unrelated.


The adrenal medulla secretes two hormone, adrenalin or epinephrine and noradrenalin or


Wikibooks | 273




Chapter 14
norepinephrine, whose functions are very similar but not identical. The adrenal medulla is derived
embriogically from neural tissue. It has been likened to an overgrown sympathetic ganglion whose cell
bodies do not send out nerve fibers, but release their active substances directly into the blood, thereby
fulfilling the criteria for an endocrine gland. In controlling epinephrine secretion, the adrenal medulla
behaves just like any sympathetic ganglion, and is dependent upon stimulation by sympathetic
preganglionic fibers.


Epinephrine promotes several responses, all of which are helpful in coping with emergencies: the
blood pressure rises, the heart rate increases, the glucose content of the blood rises because of glycogen
breakdown, the spleen contracts and squeezes out a reserve supply of blood, the clotting time
decreases, the pupils dilate, the blood flow to skeletal muscles increase, the blood supply to intestinal
smooth muscle decreases and hairs become erect. These adrenal functions, which mobilize the
resources of the body in emergencies, have been called the fight-or-flight response. Norepinephrine
stimulates reactions similar to those produced by epinephrine, but is less effective in conversion of
glycogen to glucose.


The significance of the adrenal medulla may seem questionable since the complete removal of the
gland causes few noticeable changes; humans can still exhibit the flight-or-fight response. This occurs
because the sympathetic nervous system complements the adrenal medulla in stimulating the fight-or-
flight response, and the absence of the hormonal control will be compensated for by the nervous
system.


Pancreas
The pancreas is very important organ in the digestion system and the circulatory system because it


helps to maintain our blood sugar levels. The pancreas is considered to be part of the gastrointestinal
system. It produces digestive enzymes to be released into the small intestine to aid in reducing food
particles to basic elements that can be absorbed by the intestine and used by the body. It has another
very different function in that it forms insulin, glucagon and other hormones to be sent into the
bloodstream to regulate blood sugar levels and other activities throughout the body.


It has a pear-shape to it and is approximately 6 inches long. It is located in the middle and back
portion of the abdomen. The pancreas is connected to the first part of the small intestine, the
duodenum, and lies behind the stomach. The pancreas is made up of glandular tissue: any substance
secreted by the cells of the pancreas will be secreted outside of the organ.


The digestive juices produced by the pancreas are secreted into the duodenum via a Y-shaped duct,
at the point where the common bile duct from the liver and the pancreatic duct join just before entering
the duodenum. The digestive enzymes carried into the duodenum are representative of the exocrine
function of the pancreas, in which specific substances are made to be passed directly into another
organ.


The pancreas is unusual among the body's glands in that it
also has a very important endocrine function. Small groups of
special cells called islet cells throughout the organ make the
hormones of insulin and glucagon. These, of course, are hormones that are critical in regulating blood
sugar levels. These hormones are secreted directly into the bloodstream to affect organs all over the
body.


274 | Human Physiology


Note:
The pancreas is both an exocrine and


an endocrine organ.




The Endocrine System


Insulin acts to lower blood sugar levels by allowing the
sugar to flow into cells. Glucagon acts to raise blood sugar
levels by causing glucose to be released into the circulation


from its storage sites. Insulin and glucagon act in an opposite but balanced fashion to keep blood sugar
levels stable.


A healthy working pancreas in the human body is important for maintaining good health by
preventing malnutrition, and maintaining normal levels of blood sugar. The digestive tract needs the
help of the enzymes produced by the pancreas to reduce food particles to their simplest elements, or the
nutrients cannot be absorbed. Carbohydrates must be broken down into individual sugar molecules.
Proteins must be reduced to simple amino acids. Fats must be broken down into fatty acids. The
pancreatic enzymes are important in all these transformations. The basic particles can then easily be
transported into the cells that line the intestine, and from there they can be further altered and
transported to different tissues in the body as fuel sources and construction materials. Similarly, the
body cannot maintain normal blood sugar levels without the balanced action of insulin and glucagon.


The pancreas contains exocrine and endocrine cells. Groups of endocrine cells, the islets of
Langerhans, secrete two hormones. The beta cells secrete insulin; the alpha cells secrete glucagon.
The level of sugar in the blood depends on the opposing action of these two hormones.


Insulin decreases the concentration of glucose in the blood. Most of the glucose enters the cells of
the liver and skeletal muscles. In these cells, this monosaccharide is converted to the polysaccharide
glycogen. Therefore, insulin promotes glycogenesis (glycogen formation), stimulating the breakdown
of glycogen into glucose for release into the blood..


Insulin deficiency leads to the development of diabetes mellitus, specifically type I, juvenile
diabetes. As the pancreas does not produce sufficient insulin, it is treated by insulin injections. In type
II or maturity onset diabetes, the pancreas does produce enough insulin, but the target cells do not
respond to it.


As already stated, the pancreas is a mixed gland having both endocrine and exocrine functions.
The exocrine portion secretes digestive enzymes into the duodenum via the pancreatic duct. The
endocrine portion secretes two hormones, insulin and glucagon, into the blood.


Insulin is a hormone that acts directly or indirectly on most tissues of the body, with the exception
of the brain. The most important action of insulin is the stimulation of the uptake of glucose by many
tissues, particularly the liver, muscle and fat. The uptake of glucose by the cells decreases blood
glucose and increases the availability of glucose for the cellular reactions in which glucose participates.
Thus, glucose oxidation, fat synthesis, and glycogen synthesis are all accentuated by an uptake of
glucose. It is important to note that insulin does not alter glucose uptake by the brain, nor does it
influence the active transport of glucose across the renal tubules and gastrointestinal epithelium.


As stated, insulin stimulates glycogen synthesis. In addition, it also increases the activity of the
enzyme that catalyzes the rate-limiting step in glycogen synthesis. Insulin also increases triglyceride
levels by inhibiting triglyceride breakdown, and by stimulating production of triglyceride through fatty
acid and glycerophosphate synthesis. The net protein synthesis is also increased by insulin, which
stimulates the active membrane transport of amino acids, particularly into muscle cells. Insulin also has
effects on other liver enzymes, but the precise mechanisms by which insulin induces these changes are
not well understood.


Wikibooks | 275


No organ except the pancreas
makes significant amounts of insulin


or glucagon.




Chapter 14
Insulin is secreted by beta cells, which are located in the part of the pancreas known as the islets of


Langerhans. These groups of cells, which are located randomly throughout the pancreas, also consist of
other secretory cells called alpha cells. It is these alpha cells. It is these alpha cells that secrete
glucagon. Glucagon is a hormone that has the following major effects: it increases hepatic synthesis of
glucose from pyruvate, lactate, glycerol, and amino acids (a process called gluconeogenesis, which also
raises the plasma glucose level); and it increases the breakdown of adipose tissue triglyceride, thereby
raising the plasma levels of fatty acids and glycerol. The glucagon secreting alpha cells in the pancreas,
like the beta cells, respond to changes in the concentration of glucose in the blood flowing through the
pancreas; no other nerves or hormones are involved.


It should be noted that glucagon has the opposite effects of insulin. Glucagon elevates the plasma
glucose, whereas insulin stimulates its uptake and thereby reduces plasma glucose levels; glucagon
elevates fatty acid concentrations, whereas insulin converts fatty acids and glycerol into triglycerides,
thereby inhibiting triblyceride breakdown.


The alpha and beta cells of the pancreas make up a push-pull system for regulating the plasma
glucose level.


Sex organs
The Sex organs (Gonads) are the testes in the male, and the ovaries in the female. Both of these


organs produce and secrete hormones that are balanced by the hypothalamus and pituitary glands.
The main hormones from the reproductive organs are:
Testosterone is more prominent in males. It belongs to the family of androgens, which are steroid


hormones producing masculinizing effects. Testosterone stimulates the development and functioning of
the primary sex organs. It also stimulates the development and maintenance of secondary male
characteristics, such as hair growth on the face and the deep pitch of the voice.


Estrogen In females, this hormone stimulates the development of the uterus and vagina. It is also
responsible for the development and maintenance of secondary female characteristics, such as fat
distribution throughout the body and the width of the pelvis.


Male
The testes produce androgens (i.e., "testosterone"). Testosterone is classified as a steroid and is


responsible for many of the physical characteristics in males like.
• Broad shoulders
• Muscular body
• Hair


Testosterone increases protein production. Hormones that build up protein are called anabolic
steroids. Anabolic steroids are available commercially and are being used by athletes because they
help improve their physical ability, however, they do have major side effects such as:


276 | Human Physiology




The Endocrine System


• Liver and kidney disorders
• Hypertension (high blood pressure)
• Decreased sperm count and impotency
• Aggressive behavior ("roid mania)
• Balding
• Acne


Female
The ovaries produce estrogen and progesterone. Estrogen increases at the time of puberty and


causes the growth of the uterus and vagina. Without estrogen egg maturation would not occur. Estrogen
is also responsible for secondary sex characteristics such as female body hair and fat distribution.
Estrogen and Progesterone are responsible for the development of the breast and for the uterine cycle.
Progesterone is a female hormone secreted by the corpus luteum after ovulation during the second half
of the menstrual cycle. It prepares the lining of the uterus for implantation of a fertilized egg and allows
for complete shedding of the endometrium at the time of menstruation. In the event of pregnancy, the
progesterone level remains stable beginning a week or so after conception.


Pineal gland
The pineal gland (also called the pineal body or epiphysis) is a small endocrine gland in the brain.


It is located near the center of the brain, between the two hemispheres, tucked in a groove where the
two rounded thalamic bodies join.


The pineal gland is a reddish-gray body about the size of a pea (8 mm in humans) located just
rostro-dorsal to the superior colliculus and behind and beneath the stria medullaris, between the
laterally positioned thalamic bodies. It is part of the epithalamus.


The pineal gland is a midline structure, and is often seen in plain skull X-rays, as it is often
calcified. The main hormone produced and secreted by the pineal gland is melatonin. Secretion is
highest at night and between the ages of 0-5.


Glossary
Adrenal Gland: endocrine gland that is located on top of each kidney
Amino Acid-derived: hormones that are modified amino acids
Antagonistic Hormones: hormones that act to return body conditions to within acceptable limits


from opposite extremes
Calcitonin: hormone produced by the thyroid; contributes to the regulation of blood calcium


levels
Eicosanoids: lipids that are synthesized from the fatty acid chains of phospholipids found in


plasma membrane


Wikibooks | 277




Chapter 14
Endocrine Glands: glands that have no duct and release their secretions directly into the


intercellular fluid or into the blood
Endocrine System: a control system of ductless glands that secrete chemical messengers called


hormones
Estrogen: hormone in females; stimulates the development of the uterus and vagina
Exocrine Glands: glands that release their cellular secretions through a duct which empties to the


outside or into the lumen (empty internal space) of an organ
Hormone: a specific chemical substance produced by certain cells that control, or help to control,


cellular processes elsewhere in an organism
Insulin: hormone that acts to lower blood sugar levels by allowing the sugar to flow into cells
Iodine: chemical in the body; Thyroid hormone can not be produced with out it
Lipid-soluble Hormones: diffuse through the cell membranes of target cells
Parathyroid: four masses of tissue, two embedded posterior in each lateral mass of the thyroid


gland
Pancreas: organ involved with the digestion system and the circulatory system; helps to maintain


blood sugar levels
Pineal Gland: small endocrine gland in the brain located near the center of the brain, between the


two hemispheres, tucked in a groove where the two rounded thalamic bodies join
Pituitary Gland: endocrine gland that is attached to the hypothalamus of the lower forebrain
Polypeptide and Proteins: hormones that are chains of amino acids of less than or more than


about 100 amino acids
Steroids: hormones that are lipids that are synthesized from cholesterol; characterized by four


interlocking carbohydrate rings
Testosterone: hormone more prominent in males; belongs to the family of androgens, which are


steroid hormones producing masculinizing effects
Thyroid Gland: endocrine gland that consists of two lateral masses that are attached to the trachea
Thyroxine: serves to stimulate oxidative metabolism in cells; increases the oxygen consumption


and heat production of most body tissues
Water-soluble Hormones: bind to a receptor protein on the plasma membrane of the cell


278 | Human Physiology




The Endocrine System


Review Questions
1. My child just fell and was hurt, the anxious feeling that I feel is caused by


A) glucagon
B) insulin
C) epinephrine
D) adrenocorticotropic
E) None of these


2. All of Bob’s life he has had to take insulin shots, this is caused because
A) his beta cells don’t function correctly
B) his alpha cells don’t function correctly
C) his DA hormone isn’t functioning correctly
D) his GHRH hormone isn’t functioning correctly


3. The reason iodine is in salt is
A) to prevent diabetes
B) to prevent simple goiters
C) to prevent Addison’s disease
D) to prevent Cushing's syndrome


4. All hormones react to a negative feedback except
A) progesterone
B) estrogen
C) prolactin
D) oxytocin
E) none of these


5. If I have a high blood calcium level it may be due to
A) calcitonin
B) parathyroid
C) glucocorticoids
D) glucagon


6. Hormones that are lipids that are synthesized from cholesterol
A) protein
B) amino acid-derived
C) polypeptide
D) steroids
E) eicosanoids


7. This type of hormone must bind to a receptor protein on the plasma membrane of the cell


Wikibooks | 279




Chapter 14
A) water soluble
B) lipid soluble
C) steroid
D) polypeptide
E) a and d
F) b and c


8. Endocrine glands release hormones in response to
A) Hormones from other endocrine glands
B) Chemical characteristics of the blood
C) Neural stimulation
D) All of the above


9. The anterior pituitary secretes
A) oxytocin
B) endorphins
C) ADH
D) TRH


10. Chief cells produce
A) epinephrine
B) glucagon
C) insulin
D) mineralocoticoids
E) parathyroid hormone


live version • discussion • edit lesson • comment • report an error


280 | Human Physiology




The Male Reproductive System


15 THE MALE REPRODUCTIVE SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction
n simple terms, reproduction is the process by which organisms create descendants. This miracle is a
characteristic that all living things have in common and sets them apart from nonliving things. But


even though the reproductive system is essential to keeping a species alive, it is not essential to keeping
an individual alive.
I


In human reproduction, two kinds of sex cells or gametes are involved. Sperm, the male gamete,
and an egg or ovum, the female gamete must meet in the female reproductive system to create a new
individual. For reproduction to occur, both the female and male reproductive systems are essential.


While both the female and male reproductive systems are involved with producing, nourishing and
transporting either the egg or sperm, they are different in shape and structure. The male has
reproductive organs, or genitals, that are both inside and outside the pelvis, while the female has
reproductive organs entirely within the pelvis.


The male reproductive system consists of the testes and a series of ducts and glands. Sperm are
produced in the testes and are transported through the reproductive ducts. These ducts include the
epididymis, ductus deferens, ejaculatory duct and urethra. The reproductive glands produce secretions
that become part of semen, the fluid that is ejaculated from the urethra. These glands include the
seminal vesicles, prostate gland, and bulbourethral glands.


Structure
Testes


The testes (singular, testis) are located in the scrotum (a sac of skin between the upper thighs). In
the male fetus, the testes develop near the kidneys, then descend into the scrotum just before birth.
Each testis is about 1 1/2 inches long by 1 inch wide. Testosterone is produced in the testes which
stimulates the production of sperm as well as give secondary sex characteristics beginning at puberty.


Scrotum
The two testicles are each held in a fleshy sac called the scrotum. The major function of the scrotal


sac is to keep the testes cooler than thirty-seven degrees Celsius (ninety-eight point six degrees
Fahrenheit). The external appearance of the scrotum vaires at different times in the same individual
depending upon temperature and the subsequent contraction or relaxation of two muscles. These two
muscles contract involuntarily when it is cold to move the testes closer to the heat of the body in the
pelvic region. This causes the scrotum to appear tightly wrinkled. On the contrary, they relax in warm
temperatures causing the testes to lower and the scrotum to become flaccid. The temperature of the
testes is maintained at about thirty-five degrees Celsius (ninety-five degrees Fahrenheit) below normal


Wikibooks | 281




Chapter 15
body temperature. Temperature has to be lower than normal in order for spermatogenis (sperm
production) to take place.


A male can become sterile when testes have been exposed too often to high temperatures, such as
when frequently in a hot tub.


The two muscles that regulate the temperature of the testes are the dartos and cremaster muscles:
• Dartos Muscle


The dartos muscle is a layer of smooth muscle fibers in the subcutaneous tissue of the scrotum
(surrounding the scrotum). This muscle is responsible for wrinkling up the scrotum, in conditions of
cold weather, in order to maintain the correct temperature for spermatogenisis.


• Cremaster Muscle
The cremaster muscle is a thin strand of skeletal muscle associated with the testes and spermatic


cord. This muscle is a continuation of the internal oblique muscle of the abdominal wall, from which it
is derived. It is responsible for raising or lowering the testes to keep them at the correct temperature.
Because it is skeletal muscle, it can also be contracted voluntarily. Some males, such as athletes, have
the ability to consciously raise their scrotum up, to protect themselves against injury, while playing
sports. However, not all males have this ability.


Seminiferous Tubules
Each testis contains over 100 yards of tightly packed seminiferous tubules. Around 90% of the


weight of each testes consists of seminiferous tubules. The seminiferous tubules are the functional units
of the testis, where spermatogenisis takes place. Once the sperm are produced, they moved from the
seminiferous tubules into the rete testis for further maturation.


Interstitial Cells (Cells of Leydig)
In between the seminiferous tubules within the testes, are instititial cells, or, Cells of Leydig. They


are responsible for secreting the male sex hormones (i.e., testosterone).
Sertoli Cells
A Sertoli cell (a kind of sustentacular cell) is a 'nurse' cell of the testes which is part of a


seminiferous tubule.
It is activated by follicle-stimulating hormone, and has FSH-receptor on its membranes.
Its main function is to nurture the developing sperm cells through the stages of spermatogenesis.


Because of this, it has also been called the "mother cell." It provides both secretory and structural
support.


The junctions of Sertoli cells form the blood-testis barrier, a structure that partitions the interstitial
blood compartment of the testis from the adluminal compartment of the seminiferous tubules. Sertoli
cells control the entry and exit of nutrients, hormones and other chemicals into the tubules of the testis


282 | Human Physiology




The Male Reproductive System


as well as make the adluminal compartment an immune-privileged site.
During the time of sperm maturation, large sertoli cells nourish the immature sperm and filter out


harmful things before they reach the sperm. In these ways, they assist the sperm to mature. They are
support cells that form a blood barrier with the blood. They filter what comes to sperm cells.


The cell is also responsible for establishing and maintaining the spermatogonial stem cell niche,
which ensures the renewal of stem cells and the differentiation of spermatogonia into mature germ cells
that progress stepwise through the long process of spermatogenesis, ending in the release of
spermatozoa.


Other functions During the Maturation phase of spermiogenesis, the Sertoli cells consume the
unneeded portions of the spermatazoa.


Efferent ductules
The sperm are transported out of the testis and into the epididymis through a series of efferent


ductules.
Blood Supply
The testes receive blood through the testicular arteries (gonadal artery). Venous blood is drained


by the testicular veins. The right testicular vein drains directly into the inferior vena cava. The left
testicular vein drains into the left renal vein.


Epididymis
The seminiferous tubules join together to become the epididymis. The epididymis is a tube that is


about 20 feet long that is coiled on the posterior surface of each testis. Within the epididymis the sperm
complete their maturation and their flagella become functional. This is also a site to store sperm until
the next ejaculation. Smooth muscle in the wall of the epididymis propels the sperm into the ductus
deferens.


Ductus Deferens
The ductus (vas) deferens, also called sperm duct, or, spermatic deferens, extends from the


epididymis in the scrotum on its own side into the abdominal cavity through the inguinal canal. The
inguinal canal is an opening in the abdominal wall for the spermatic cord (a connective tissue sheath
that contains the ductus deferens, testicular blood vessels, and nerves. The smooth muscle layer of the
ductus deferens contracts in waves of peristalsis during ejaculation.


Seminal Vesicles
The pair of seminal vesicles are posterior to the urinary bladder. They secrete fructose to provide


an energy source for sperm and alkalinity to enhance sperm mobility. The duct of each seminal vesicle
joins the ductus deferens on that side to form the ejaculatory duct.


Wikibooks | 283




Chapter 15


Ejaculatory Ducts
There are two ejaculatory ducts. Each receives sperm from the ductus deferens and the secretions


of the seminal vesicle on its own side. Both ejaculatory ducts empty into the single urethra.


Prostate Gland
The prostate gland is a muscular gland that surrounds the first inch of the urethra as it emerges


from the bladder. The smooth muscle of the prostate gland contracts during ejaculation to contribute to
the expulsion of semen from the urethra.


Bulbourethral Glands
The bulbourethral glands also called Cowper's glands are located below the prostate gland and


empty into the urethra. The alkalinity of seminal fluid helps neutralize the acidic vaginal pH and
permits sperm mobility in what might otherwise be an unfavorable environment.


Penis
The penis is an external genital organ. The distal end of the penis is called the glans penis and is


covered with a fold of skin called the prepuce or foreskin. Within the penis are masses of erectile
tissue. Each consists of a framework of smooth muscle and connective tissue that contains blood
sinuses, which are large, irregular vascular channels.


Urethra
The urethra, which is the last part of the urinary tract, traverses the corpus spongiosum and its


opening, known as the meatus, lies on the tip of the glans penis. It is both a passage for urine and for
the ejaculation of semen.


Overview of Male Reproductive System Structure and Function
STRUCTURE LOCATION & DESCRIPTION FUNCTION


Bulbourethral
glands (2)


Pea sized organs posterior to the
prostate on either side of the
urethra.


Secretion of gelatinous seminal fluid called pre-
ejaculate. This fluid helps to lubricate the
urethra for spermatozoa to pass through, and to
help flush out any residual urine or foreign
matter. (< 1% of semen)


Cells of Leydig
(Interstitial cells
of Leydig)


Adjacent to the seminiferous
tubules in the testicle.


Responsible for production of testosterone.
Closely related to nerves.


Cremaster muscle Covers the testes. Raises and lowers scrotum to help regulate
temperature and promote spermatogenesis.


284 | Human Physiology




The Male Reproductive System


Voluntary and involuntary contraction.


Dartos muscle
Layer of smooth muscular fiber
outside the external spermatic
fascia but below the skin


Contraction by wrinkling to decrease surface
area available for heat loss to testicles, or
expansion to increase surface area available to
promote heat loss; also helps raise and lower
scrotum to help regulate temperature


Efferent ductules Part of the testes and connect the rete testis with the epididymis Ducts for sperm to get to epididymis


Ejaculatory ducts
(2)


Begins at the vas deferens, passes
through the prostate, and empties
into the urethra at the Colliculus
seminalis.


Causes reflex for ejaculation. During
ejaculation, semen passes through the ducts and
exits the body via the penis.


Epididymis
Tightly coiled duct lying just
outside each testis connecting
efferent ducts to vas deferens.


Storage and maturation of sperm.


Penis
Three columns of erectile tissue:
two corpora cavernosa and one
corpus spongiosum. Urethra passes
through penis.


Male reproductive organ and also male organ of
urination.


Prostate gland
Surrounds the urethra just below
the urinary bladder and can be felt
during a rectal exam.


Stores and secretes a clear, slightly alkaline
fluid constituting up to one-third of the volume
of semen. Raise vaginal pH.(25-30% of semen)


Scrotum Pouch of skin and muscle that holds testicles.
Regulates temperature at slightly below body
temperature.


Semen
Usually white but can be yellow,
gray or pink (blood stained). After
ejaculation, semen first goes
through a clotting process and then
becomes more liquid.


Components are sperm, and "seminal plasma".
Seminal plasma is produced by contributions
from the seminal vesicle, prostate, and
bulbourethral glands.


Seminal vesicles
(2)


Convoluted structure attached to
vas deferens near the base of the
urinary bladder.


About 65-75% of the seminal fluid in humans
originates from the seminal vesicles. Contain
proteins, enzymes, fructose, mucus, vitamin C,
flavins, phosphorylcholine and prostaglandins.
High fructose concentrations provide nutrient
energy for the spermatozoa as they travel
through the female reproductive system.


Seminiferous
tubules (2)


Long coiled structure contained in
the chambers of the testis; joins
with vas deferens.


Meiosis takes place here, creation of gametes
(sperm).


Sertoli cells


Junctions of the Sertoli cells form
the blood-testis barrier, a structure
that partitions the interstitial blood
compartment of the testis from the
adluminal compartment of the
seminiferous tubules.


Cells responsible for nurturing and
development of sperm cells , provides both
secretory and structural support; activated by
FSH. Also called "mother cells" or "nurse
cells".


Testes Inside scrotum, outside of body. Gonads that produce sperm and male sex
hormones.Production of testosterone by cells of


Wikibooks | 285




Chapter 15
Leydig in the testicles.


Testicular arteries
(Gonadal arteries)


Branch of the abdominal aorta. It is
a paired artery. Each passes
obliquely downward and laterally
behind the peritoneum.


Supplies blood to the testes.


Urethra Connects bladder to outside body, about 8 inches long.
Tubular structure that receives urine from
bladder and carries it to outside of the body.
Also passage for sperm.


Vas deferens
Muscular tubes connecting the left
and right epididymis to the
ejaculatory ducts to move sperm.
Each tube is about 30 cm long.


During ejaculation the smooth muscle in the
vas deferens wall contracts, propelling sperm
forward. Sperm are transferred from the vas
deferens into the urethra, collecting fluids from
accessory sex glands en route


Composition of human semen
The components of semen come from two sources: sperm, and "seminal plasma". Seminal plasma,


in turn, is produced by contributions from the seminal vesicle, prostate, and bulbourethral glands.
Seminal plasma of humans contains a complex range of organic and inorganic constituents.
The seminal plasma provides a nutritive and protective medium for the spermatozoa during their


journey through the female reproductive tract. The normal environment of the vagina is a hostile one
for sperm cells, as it is very acidic (from the native microflora producing lactic acid), viscous, and
patrolled by immune cells. The components in the seminal plasma attempt to compensate for this
hostile environment. Basic amines such as putrescine, spermine, spermidine and cadaverine are
responsible for the smell and flavor of semen. These alkaline bases counteract the acidic environment
of the vaginal canal, and protect DNA inside the sperm from acidic denaturation.


The components and contributions of semen are as follows:


GLAND APPROXIMATE % DESCRIPTION


testes 2-5%
Approximately 200- to 500-million spermatozoa (also called sperm
or spermatozoans), produced in the testes, are released per
ejaculation


seminal
vesicle 65-75%


amino acids, citrate, enzymes, flavins, fructose (the main energy
source of sperm cells, which rely entirely on sugars from the
seminal plasma for energy), phosphorylcholine, prostaglandins
(involved in suppressing an immune response by the female
against the foreign semen), proteins, vitamin C


prostate 25-30%
acid phosphatase, citric acid, fibrinolysin, prostate specific antigen,
proteolytic enzymes, zinc (serves to help to stabilize the DNA-
containing chromatin in the sperm cells. A zinc deficiency may
result in lowered fertility because of increased sperm fragility.
Zinc deficiency can also adversely affect spermatogenesis.)


bulbourethral
glands


< 1% galactose, mucus (serve to increase the mobility of sperm cells in
the vagina and cervix by creating a less viscous channel for the


286 | Human Physiology




The Male Reproductive System


sperm cells to swim through, and preventing their diffusion out of
the semen. Contributes to the cohesive jelly-like texture of
semen.), pre-ejaculate, sialic acid


A 1992 World Health Organization report described normal human semen as having a volume of 2
ml or greater, pH of 7.2 to 8.0, sperm concentration of 20x106 spermatozoa/ml or more, sperm count of
40x106 spermatozoa per ejaculate or more and motility of 50% or more with forward progression
(categories a and b) of 25% or more with rapid progression (category a) within 60 minutes of
ejaculation.[2]


Functions
Hormone Regulation


Hormones which control reproduction in males are:
Gonadotropin-Releasing Hormone (GnRH):


• The hypothalamus secretes this hormone into the pituitary gland in the brain.
• There are two gonadotropic hormones, FSH and LH.


Luteinizing Hormone (LH):
• The pituitary gland secretes this hormone after receiving a GnRH signal from the


hypothalamus.
• LH stimulates Leydig cells, in the testes, telling them to produce testosterone.


Follicle-Stimulating Hormone (FSH):
• The pituitary gland also secretes this hormone.
• Testosterone helps FSH run through the bloodstream to make Sertoli cells, located in the


seminiferous tubules of the testes, to make immature sperm to mature sperm.
Testosterone:


• Also know as "the male hormone" and "androgen".
• Testosterone is vital for the production of sperm.


Erection
The erection of the penis is its enlarged and firm state. It depends on a complex interaction of


psychological, neural, vascular and endocrine factors. The term is also applied to the process that leads
to this state.


A penile erection occurs when two tubular structures that run the length of the penis, the corpora
cavernosa, become engorged with venous blood. This is a result of parasympathetic nerve induced


Wikibooks | 287




Chapter 15
vasodilation. This may result from any of various physiological stimuli. The corpus spongiosum is a
single tubular structure located just below the corpora cavernosa, which contains the urethra, through
which urine and semen pass during urination and ejaculation, respectively. This may also become
slightly engorged with blood, but less so than the corpora cavernosa.


Penile erection usually results from sexual stimulation and/or arousal, but can also occur by such
causes as a full urinary bladder or spontaneously during the course of a day or at night, often during
erotic or wet dreams. An erection results in swelling and enlargement of the penis. Erection enables
sexual intercourse and other sexual activities (sexual functions), though it is not essential for all sexual
activities.


Ejaculation
Emission is the term used when sperm moves into the urethra. Ejaculation is the term used when


sperm is forced out of the urethra and penis. These are both stimulated by sympathetic nerves.


Sperm Production
A spermatozoon or spermatozoan (pl. spermatozoa), from the ancient Greek σπέρμα (seed) and


ζ ον (living being) and more commonly known as a sperm cell, is the haploid cell that is the maleῷ
gamete.


Spermatagonia divides several times during the process of sperm development. The entire process
of sperm formation and maturation takes about 9-10 weeks. The separate divisions that take place and
what happens in each are as follows:


• First division: The first division is done by mitosis, and ensures a constant supply of
spermatocytes, each with the diploid number of chromosomes.


• Second division: Spermatocytes then undergo a series of two cell divisions during meiosis
to become secondary spermatocytes.


• Third division: Secondary Spermatocytes finally become spermatids. Spermatids, which
are haploid cells, mature slowly to become the male gametes, or sperm.


The sperm is the main reproductive cell in males. The sperms differ in that each carry a set of
chromosomes dividing each into either a male, or female sperm. The females differ in that they carry a
XX gene, while the male sperm carry a XY gene. The female sperm also differ phenotypically in that
they have a larger head in comparison to the male sperms. This contributes to the male sperm being
lighter, and therefore faster and stronger swimmers than their female counterparts (although
statistically there is still a 50% chance of an either XY or XX embryo forming).


Spermatozoan stream lines are straight and parallel. The tail flagellates, which we now know
propels the sperm cell (at about 1-3 mm/minute in humans) by rotating like a propeller, in a circular
motion, not side to side like a whip. The cell is characterized by a minimum of cytoplasm. During
fertilization, the sperm's mitochondria gets destroyed by the egg cell, and this means only the mother is
able to provide the baby's mitochondria and mitochondrial DNA, which has an important application in


288 | Human Physiology




The Male Reproductive System


tracing maternal ancestry. However it has been recently discovered that mitochondrial DNA can be
recombinant.


Spermatozoa are produced in the seminiferous tubules of the testes in a process called
spermatogenesis. Round cells called spermatogonia divide and differentiate eventually to become
spermatozoa. During copulation the vagina is inseminated, the spermatozoa move through chemotaxis
(see glossary) to the ovum inside a Fallopian tube or the uterus.


Sperm Pathway
Spermatogenesis takes place inside a male’s testes, specifically in the walls of the seminiferous


tubules. The epididymis is a tortuously coiled structure topping the testis, it receives immature sperm
from the testis and stores it for several days. When ejaculation occurs, sperm is forcefully expelled
from the tail of the epididymis into the ductus deferens. Sperm travels through the ductus deferens and
up the spermatic cord into the pelvic cavity, over the ureter to the prostate behind the bladder. Here, the
vas deferens joins with the seminal vesicle to form the ejaculatory duct, which passes through the
prostate and empties into the urethra. Upon the sperm's exit from the testes, into the vas deferens,
muscular movements take over. When ejaculation occurs, rhythmic muscle movements of peristalsis
propel the sperm forward. This continues throughout the remainder of the sperm's journey through the
male reproductive system.


Sperm cells become even more active when they begin to interact with the fertilizing layer of an
egg cell. They swim faster and their tail movements become more forceful and erratic. This behavior is
called "hyper activation."


A recent discovery links hyper activation to a sudden influx of calcium ions into the tails. The
whip-like tail (flagellum) of the sperm is studded with ion channels formed by proteins called CatSper.
These channels are selective, allowing only calcium ion to pass. The opening of CatSper channels is
responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to form
deeper bends, propelling the sperm more forcefully through the viscous environment.


The sperm use their tails to push themselves into the epididymis, where they complete their
development. It takes sperm about 4 to 6 weeks to travel through the epididymis. The sperm then move
to the vas deferens, or sperm duct. The seminal vesicles and prostate gland produce a whitish fluid
called seminal fluid, which mixes with sperm to form semen when a male is sexually stimulated.


The penis, which usually hangs limp, becomes hard when a male is sexually excited. Tissues in the
penis fill with blood and it becomes stiff and erect (an erection). The rigidity of the erect penis makes it
easier to insert into the female's vagina during sexual intercourse, and the extended length allows it to
reach deeper into the female's oviduct, the passage from the ovaries to the outside of the body (allowing
a shorter travel distance for the spermatozoa).


When the erect penis is stimulated to orgasm, muscles around the reproductive organs contract and
force the semen through the duct system and urethra. Semen is pushed out of the male's body through
his urethra - ejaculation. The speed of the semen is about 70 mph when ejaculation comes and it can
contain 100 to 600 million sperm cells. When the male ejaculates during intercourse, semen is
deposited into the fornix at the base of the female's vagina, near the cervix. From the fornix, the sperm
make their way up through the cervix and move through the uterus with help from uterine contractions.


Wikibooks | 289




Chapter 15
Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg


from fertilization. The first barrier to sperm is made up of so-called cumulus cells embedded in a gel-
like substance made primarily of hyaluronic acid. The cumulus cells develop in the ovary with the egg
and support it as it grows.


The second barrier coating the oocyte is a thick shell formed by glycoproteins called the zona
pellucida. One of the proteins that make up the zona pellucida binds to a partner molecule on the
sperm. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of
different species from fusing. There is some evidence that this binding is what triggers the acrosome to
release the enzymes that allow the sperm to fuse with the egg.


When a sperm cell reaches the egg the acrosome releases its enzymes. These enzymes weaken the
shell, allowing the sperm cell to penetrate it and reach the plasma membrane of the egg. Part of the
sperm's cell membrane then fuses with the egg cell's membrane, and the sperm cell sinks into the egg
(at which point the sperm tail falls off).


Upon penetration, the egg cell membrane undergoes a change and becomes impenetrable,
preventing further fertilization.


The binding of the sperm to an ovum is called a zygote. A zygote is a single cell, with a complete
set of chromosomes, that normally develops into an embryo.


Puberty
In addition to producing sperm, the male reproductive system also produces sex hormones, which


help a boy develop into a sexually mature man during puberty. When a baby boy is born, he has all the
parts of his reproductive system in place, but it isn't until puberty that his reproductive organs mature
and become fully functional. As an newborn FSH and LH levels are high and afrter a few weeks levels
drop to extremly low. When puberty begins, usually between the ages of 10 and 14, the pituitary gland
- which is located in the brain - secretes hormones that stimulate the testicles to produce testosterone.
The production of testosterone brings about many physical changes. Although the timing of these
changes is different for each individual male, the stages of puberty generally follow a set sequence.


• First stage: the scrotum and testes grow larger, the apocrine glands develop (see
explanation of apocrine glands in glossary).


• Second stage: the penis becomes longer, and the seminal vesicles and prostate gland grow.
Hair begins to grow in the pubic region. Reproductive capacity has usually developed by this
stage.


• Third stage: hair begins to appear on the face and underarms. During this time, a male's
voice also deepens. Fertility continues to increase.


Testicular size, function, and fertility
In boys, testicular enlargement is the first physical manifestation of puberty (and is termed


gonadarche). Testes in prepubertal boys change little in size from about 1 year of age to the onset of


290 | Human Physiology




The Male Reproductive System


puberty, averaging about 2–3 cc in volume and about 1.5-2 cm in length. Testicular size continues to
increase throughout puberty, reaching maximal adult size about 6 years later. While 18-20 cc is
reportedly an average adult size, there is wide variation in the normal population.


The testes have two primary functions: to produce hormones and to produce sperm. The Leydig
cells produce testosterone (as described below), which in turn produces most of the changes of male
puberty. However, most of the increasing bulk of testicular tissue is spermatogenic tissue (primarily
Sertoli and interstitial cells). The development of sperm production and fertility in males is not as well
researched. Sperm can be detected in the morning urine of most boys after the first year of pubertal
changes (and occasionally earlier).


Genitalia
A boy's penis grows little from the fourth year of life until puberty. Average prepubertal penile


length is 4 cm. The prepubertal genitalia are described as stage 1. Within months after growth of the
testes begins, rising levels of testosterone promote growth of the penis and scrotum. This earliest
discernible beginning of pubertal growth of the genitalia is referred to as stage 2. The penis continues
to grow until about 18 years of age, reaching an average adult size of about 10-16 cm.


Although erections and orgasm can occur in prepubertal boys, they become much more common
during puberty, accompanied by development of libido (sexual desire). Ejaculation becomes possible
early in puberty; prior to this boys may experience dry orgasms. Emission of seminal fluid may occur
due to masturbation or spontaneously during sleep (commonly termed a wet dream, and more clinically
called a nocturnal emission). The ability to ejaculate is a fairly early event in puberty compared to the
other characteristics, and can occur even before reproductive capacity itself. In parallel to the
irregularity of the first few periods of a girl, for the first one or two years after a boy's first ejaculation,
his seminal fluid may contain few active sperm.


If the foreskin of a boy does not become retractable during childhood, it normally begins to retract
during puberty. This occurs as a result of the increased production of testosterone and other hormones
in the body.


Genital Erection
The penis contains two chambers called the corpora cavernosa, which run the length of the organ.


A spongy tissue, full of muscle, veins, arteries, etc. fills these chambers. The corpora cavernosa are
surrounded by a membrane, called the tunica albuginea.


Erection begins with sensory or mental stimulation, or both. Impulses from the brain and local
nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the
spaces. The blood creates pressure in the corpora cavernosa, making the penis expand. The tunica
albuginea helps trap the blood in the corpora cavernosa, thereby sustaining erection. When muscles in
the penis contract to stop the inflow of blood and open outflow channels, erection is reversed.


Wikibooks | 291




Chapter 15


Pubic hair in boys
Pubic hair often appears on a boy shortly after the genitalia begin to grow. As in girls, the first


appearance of pubic hair is termed pubarche and the pubic hairs are usually first visible at the dorsal
(abdominal) base of the penis. The first few hairs are described as stage 2. Stage 3 is usually reached
within another 6 to 12 months, when the hairs are too numerous to count. By stage 4, the pubic hairs
densely fill the "pubic triangle." Stage 5 refers to spread of pubic hair to the inner thighs and upward
towards the umbilicus as part of the developing abdominal hair.


Body and facial hair in boys
In the months and years following the appearance of pubic hair, other areas of skin which respond


to androgens (see glossary) develop heavier hair (androgenic hair) in roughly the following sequence:
underarm (axillary) hair, perianal hair, upper lip hair, sideburn (preauricular) hair, periareolar hair, and
the rest of the beard area. Arm, leg, chest, abdominal, and back hair become heavier more gradually.
There is a large range in amount of body hair among adult men, and significant differences in timing
and quantity of hair growth among different ethnic groups.


Voice change
Under the influence of androgens, the voice box, or larynx, grows in both genders. This growth is


far more prominent in boys, causing the male voice to drop, rather abruptly, usually about one octave,
because the larger vocal folds have a lower fundamental frequency. Occasionally, this is accompanied
by cracking and breaking sounds in the early stages. Most of the voice change happens during stage 4
of male puberty around the time of peak growth. However, it usually precedes the development of
significant facial hair by several months to years. The time it takes for the voice to drop varies
according to how far it has to go (tenor, baritone, or bass).


Height growth in boys
Compared to girls' early growth spurt, growth accelerates more slowly in boys and lasts longer,


resulting in a taller adult stature among males than females (on average about 10 cm or 4 inches). The
difference is attributed to the much greater potency of estradiol compared to testosterone in promoting
bone growth, maturation, and epiphyseal closure. In boys, growth begins to accelerate about 9 months
after the first signs of testicular enlargement and the peak year of the growth spurt occurs about 2 years
after the onset of puberty, reaching a peak velocity of about 8.5-12 cm or 3.5-5 inches per year. The
feet and hands experience their growth spurt first, followed by the limbs, and finally ending in the
trunk. Epiphyseal closure and adult height are reached more slowly, at an average age of about 17.5
years. As in girls, this last growth primarily involves the spine rather than the limbs.


Male musculature and body shape
By the end of puberty, adult men have heavier bones and nearly twice as much skeletal muscle.


Some of the bone growth (e.g., shoulder width and jaw) is disproportionately greater, resulting in
noticeably different male and female skeletal shapes. The average adult male has about 150% of the


292 | Human Physiology




The Male Reproductive System


lean body mass of an average female, and about 50% of the body fat.
This muscle develops mainly during the later stages of puberty, and muscle growth can continue


even after a male is biologically adult. The peak of the so-called "strength spurt," the rate of muscle
growth, is attained about one year after a male experiences his peak growth rate.


Body odor, skin changes, acne
Rising levels of androgens can change the fatty acid composition of perspiration, resulting in a


more "adult" body odor. As in girls, another androgen effect is increased secretion of oil (sebum) from
the skin and the resultant variable amounts of acne.


Sexual Homology
In short, this is a known list of sex organs that evolve from the same tissue in a human life.


Indifferent Male Female
Gonad Testis Ovary
Mullerian duct Appendix testis Fallopian tubes
Mullerian duct Prostatic utricle Uterus, proximal vagina
Wolffian duct Rete testis Rete ovarii
Mesonephric
tubules Efferent ducts Epoophoron
Wolffian duct Epididymis Gartner's duct
Wolffian duct Vas deferens
Wolffian duct Seminal vesicle
Wolffian duct Prostate Skene's glands
Urogenital sinus Bladder, urethra Bladder, urethra, distal vagina
Urogenital sinus Bulbourethral gland Bartholin's gland
Genital swelling Scrotum Labia majora
Urogenital folds Distal urethra Labia minora
Genital tubercle Penis Clitoris
Prepuce Foreskin Clitoral hood


Bulb of penis Vestibular bulbs
Glans penis Clitoral glans
Crus of penis Clitoral crura


Wikibooks | 293




Chapter 15


Aging
For most men, testosterone secretion continues throughout life, as does sperm production, though


both diminish with advancing age. Probably the most common reproductive problem for older men is
prostatic hypertropy, enlargement of the prostate gland. This causes the urethra to compress and
urination becomes difficult. Residual urine in the bladder increases the chance of urinary tract
infections. Prostate hypertrophy is usually benign, but cancer of the prostate is one of the more
common cancers in elderly men. A TURP is commonly used to correct this problem if the symptoms
do not improve in response to home treatment and medication.


Erectile dysfunction (ED) is another common problem seen in aging males. In older men, ED
usually has a physical cause, such as disease, injury, or side effects of drugs. Any disorder that impairs
blood flow in the penis or causes injury to the nerves has the potential to cause ED. Although it is not
an inevitable part of aging, incidences increases with age: About 5 percent of 40-year-old men and
between 15 and 25 percent of 65-year-old men experience ED. As discouraging as Erectile dysfunction
may be, it is treatable at any age, and awareness of this fact has been growing. More men have been
seeking help and returning to normal sexual activity because of improved, successful treatments for
ED.


Things That Can Go Wrong With the Male Reproductive
System


Boys may sometimes experience reproductive system problems. Below are some examples of
disorders that affect the male reproductive system (Disorders of the Scrotum, Testicles, or Epididymis).
Conditions affecting the scrotal contents may involve the testicles, epididymis, or the scrotum itself.


• Testicular trauma. Even a mild injury to the testicles can cause severe pain, bruising, or
swelling. Most testicular injuries occur when the testicles are struck, hit, kicked, or crushed,
usually during sports or due to other trauma. Testicular torsion, when 1 of the testicles twists
around, cutting off the blood supply, is also a problem that some teen males experience -
although it's not common. Surgery is needed to untwist the cord and save the testicle.


• Varicocele. This is a varicose vein (an abnormally swollen vein) in the network of veins
that run from the testicles. Varicoceles commonly develop while a boy is going through
puberty. A varicocele is usually not harmful, although in some people it may damage the
testicle or decrease sperm production, so it helps for you to take your child to see his doctor if
he is concerned about changes in his testicles.


• Testicular cancer. This is one of the most common cancers in men younger than 40. It
occurs when cells in the testicle divide abnormally and form a tumor. Testicular cancer can
spread to other parts of the body, but if it's detected early, the cure rate is excellent. Teen boys
should be encouraged to learn to perform testicular self-examinations.


• Epididymitis is inflammation of the epididymis, the coiled tubes that connect the testes
with the vas deferens. It is usually caused by an infection, such as the sexually transmitted
disease chlamydia, and results in pain and swelling next to 1 of the testicles.


294 | Human Physiology




The Male Reproductive System


• Hydrocele. A hydrocele occurs when fluid collects in the membranes surrounding the
testes. Hydroceles may cause swelling of the testicle but are generally painless. In some cases,
surgery may be needed to correct the condition.


• Inguinal hernia. When a portion of the intestines pushes through an abnormal opening or
weakening of the abdominal wall and into the groin or scrotum, it is known as an inguinal
hernia. The hernia may look like a bulge or swelling in the groin area. It can be corrected with
surgery.


Disorders of Penis
Disorders of the Penis Disorders affecting the penis include the following:


• Inflammation of the penis. Symptoms of penile inflammation include redness, itching,
swelling, and pain. Balanitis occurs when the glans (the head of the penis) becomes inflamed.
Posthitis is foreskin inflammation, which is usually due to a yeast or bacterial infection.


• Hypospadias. This is a disorder in which the urethra opens on the underside of the penis,
not at the tip.


• Phimosis. This is a tightness of the foreskin of the penis and is common in newborns and
young children. It usually resolves itself without treatment. If it interferes with urination,
circumcision (removal of the foreskin) may be recommended.


• Paraphimosis. This may develop when a boy's uncircumcised penis is retracted but doesn't
return to the unretracted position. As a result, blood flow to the penis may be impaired, and your
child may experience pain and swelling. A doctor may try to use lubricant to make a small
incision so the foreskin can be pulled forward. If that doesn't work, circumcision may be
recommended.


• Ambiguous genitalia. This occurs when a child is born with genitals that aren't clearly
male or female. In most boys born with this disorder, the penis may be very small or
nonexistent, but testicular tissue is present. In a small number of cases, the child may have both
testicular and ovarian tissue.


• Micro penis. This is a disorder in which the penis, although normally formed, is well
below the average size, as determined by standard measurements.


• Sexually transmitted diseases. Sexually transmitted diseases (STDs) that can affect boys
include human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS),
human papillomavirus (HPV, or genital warts), syphilis, chlamydia, gonorrhea, genital herpes,
and hepatitis B. They are spread from one person to another mainly through sexual intercourse.


• Erectile dysfunction. E.D. is the inability to get or keep an erection firm enough for sexual
intercourse. This can also called impotence. The word "impotence" may also be used to describe
other problems that can interfere with sexual intercourse and reproduction, such as problems
with ejaculation or orgasm and lack of sexual desire. Using the term erectile dysfunction
clarifies that those other problems are not involved.


Wikibooks | 295




Chapter 15


Contraceptive for Men
Vasectomy: In the procedure the vas deferens of each testes is cut and tied off to prevent the


passage of sperm. Sperm is still produced and stored in crypt sites causing inflammation. Because of
this inflammatory response the immune system acts on them destroying them and then having
antisperm antibodies. This causes a lower possibility if the vasectomy is reversed to becoming fertile
again.


Condoms: A device, usually made of latex, or more recently polyurethane, that is used during
sexual intercourse. It is put on a man's penis and physically blocks ejaculated semen from entering the
body of a sexual partner. Condoms are used to prevent pregnancy, transmission of sexually transmitted
diseases (STDs - such as gonorrhea, syphilis, and HIV), or both.


Review Questions
1. This is needed to make immature sperm mature


A) FHS
B) LH
C) FSH
D) HL


2. These become engorged with blood in an erection
A) corpora cavernosa
B) fiberous envelope
C) septum pectiniforme
D) integument
E) dorsal veins


3. The difference between male and female sperm
A) female sperm have a larger head
B) male sperm are lighter
C) female sperm are faster
D) male sperm are weaker
E) A and B
F) C and D


4. The entire process of sperm formation takes about
A) 5-6 weeks
B) 7-8 weeks
C) 3-4 weeks
D) 9-10 weeks


5. Hyper Activation occurs when


296 | Human Physiology




The Male Reproductive System


A) the sperm are introduced into the urethra
B) the sperm are ejaculated into the vaginal canal
C) the sperm begin to interact with the fertilizing layer of an egg cell
D) the sperm reach the cervix


6. It takes sperm ___________ weeks to travel through the epididymis
A) 6-8
B) 1-3
C) 2-4
D) 4-6


7. While singing in the choir, Ben suddenly notices his voice is constantly cracking. This is caused by
A) androgens
B) LH
C) FSH
D) Ben’s inability to sing


8. In sexual homology, the glans penis in the male is equal to _____________ in the female
A) clitoral hood
B) clitoris
C) clitoral glans
D) clitoral crura


9. In sexual homology, the ___________ in the male is equal to the fallopian tubes in the female
A) testis
B) appendix testis
C) vas deferens
D) seminal vesicle
E) efferent ducts


10. Joe has a bulge in the groin area that seems to get worse when he lifts things. This most likely is
A) epididymitis
B) testicular cancer
C) varicocele
D) hydrocele
E) inguinal hernia


Glossary
Androgen: The generic term for any natural or synthetic compound, usually a steroid hormone,


that stimulates or controls the development and maintenance of masculine characteristics in vertebrates
by binding to androgen receptors. This includes the activity of the accessory male sex organs and
development of male secondary sex characteristics. They are also the precursor of all estrogens, the


Wikibooks | 297




Chapter 15
female sex hormones. The primary and most well-known androgen is testosterone.


Apocrine Glands: Apocrine sweat glands develop during the early to mid puberty ages
approximately around the age of 15 and release more than normal amounts of sweat for approximately
a month and subsequently regulate and release normal amounts of sweat after a certain period of time.
They are located wherever there is body hair. These glands produce sweat that contains fatty materials.
Mainly present in the armpits and around the genital area, their activity is the main cause of sweat odor,
due to the bacteria that break down the organic compounds in the sweat.


Bulbourethral Glands: male accessory sex glands that secrete mucus for lubrication
Chemotaxis: Chemotaxis is a kind of taxis, in which bodily cells, bacteria, and other single-cell or


multicellular organisms direct their movements according to certain chemicals in their environment.
This is important for bacteria to find food (for example, glucose) by swimming towards the highest
concentration of food molecules, or to flee from poisons (for example, phenol). In multicellular
organisms, chemotaxis is critical to development as well as normal function. In addition, it has been
recognized that mechanisms that allow chemotaxis in animals can be subverted during cancer
metastasis.


Corpora Cavernosa: one of a pair of a sponge-like regions of erectile tissue which contain most
of the blood in the male penis during erection


Ductus Deferens: epididymal ducts from each testis converge to form a large, thick walled,
muscular duct


Ejaculatory Ducts: two ducts, receive sperm from the ductus deferensand secretions from the
seminal vesicle; the ducts then empty into the urethra


Epididymis: comma shaped and loosely attached to the rear surface of each testis
Erectile Tissue: smooth muscle and connective tissue inside the penis that contain blood sinuses;


large, irregular vascular channels
Erection: the penis at it’s enlarged and firm state; occurs when the corpora cavernosa become


engorged with venous blood
Flagellum: the whip-like tail of a sperm, propels the sperm towards the egg in hopes of achieving


fertilization
Follicle-Stimulating Hormone (FSH): hormone that stimulates production of sertoli cells, to


make immature sperm to mature sperm
Glans Penis: distal end of the penis, covered with the foreskin
Gonadotropin-Releasing Hormone (GnRH): hormone secreted by the hypothalamus into the


pituitary gland; two types, FSH and LH
Libido: In its common usage, it means sexual desire; however, more technical definitions, such as


those found in the work of Carl Jung, are more general, referring to libido as the free creative—or


298 | Human Physiology




The Male Reproductive System


psychic—energy an individual has to put toward personal development, or individuation.
Luteinizing Hormone (LH): hormone that stimulates Leydig cells in the testes to produce


testosterone
Oviduct: the passage in females from the ovaries to the outside of the body.
Penis: external genital organ of the male
Prostate Gland: male accessory sex gland that secretes an alkaline fluid, which neutralizes acidic


vaginal secretions
Puberty: the period of maturation and arousal of the dormant and nonfunctional reproductive


system; usually occurs in males between the ages of 10 and 15
Scrotum: skin covered sac that houses the male testicals; keeps the testicals away form the body


so that it can stay a few degrees cooler than the body, for better sperm production
Seminal Vesical: male accessory sex glands that supply fructose to ejaculated sperm and secrete


prostaglandins
Seminiferous Tubules: highly coiled tubules within the testes that produce spermatozoa
Sertoli Cell: A Sertoli cell (a kind of sustentacular cell) is a 'nurse' cell of the testes which is part


of a seminiferous tubule.
It is activated by follicle-stimulating hormone, and has FSH-receptor on its membranes.
Its main function is to nurture the developing sperm cells through the stages of spermatogenesis.


Because of this, it has also been called the "mother cell." It provides both secretory and structural
support.


Sexual Homology: sex organs that evolve from the same tissues in both male and females
Sperm: main reproductive cell in males
Spermatogenesis: sperm production
Testes: located in the scrotum, produces testosterone which stimulates production of sperm
Testosterone: male sex hormone secreted by the leydig cells of the testes, vital for the production


of sperm
TURP: transurethral resection of the prostate. During TURP, an instrument is inserted up the


urethra to remove the section of the prostate that is blocking urine flow. This is most commonly caused
by benign prostatic hyperplasia (BPH). A TURP usually requires hospitalization and is done using a
general or spinal anesthetic. It is now the most common surgery used to remove part of an enlarged
prostate.


Wikibooks | 299




Chapter 15
Urethra: the last part of the urinary tract; in males, it is the passage for both urine and sperm
Varicocele: varicose vein of the testicles, sometimes a cause of male infertility
Vasectomy: most common sterilization procedure in males; small segment of each ductus deferens


is surgically removed after it passes from the testis


Summary
Both male and female reproductive systems may seem somewhat isolated from other body systems


in that their purpose is to create new life and not just to maintain existing life. There are however
significant relationships between the reproductive system and other body systems. All systems relate in
one way or another to help our bodies maintain homeostasis.


References
• "Essentials of Anatomy and Physiology" by Valerie C. Scanlon and Tina Sanders
• "Web MD": http://www.webmd.com
• Wikibook: Sexual Health


live version • discussion • edit lesson • comment • report an error


300 | Human Physiology




The Female Reproductive System


16 THE FEMALE REPRODUCTIVE SYSTEM
live version • discussion • edit lesson • comment • report an error


Introduction
ll living things reproduce. This is something that sets the living apart from non-living. Even
though the reproductive system is essential to keeping a species alive, it is not essential to keeping


an individual alive. This chapter describes the different parts of the female reproductive system: the
organs involved in the process of reproduction, hormones that regulate a woman's body, the menstrual
cycle, ovulation and pregnancy, the female's role in genetic division, birth control, sexually transmitted
diseases and other diseases and disorders.


A


Reproduction
Reproduction can be defined as the process by which an organism continues its species. In the


human reproductive process, two kinds of sex cells (gametes), are involved: the male gamete (sperm),
and the female gamete (egg or ovum). These two gametes meet within the female's uterine tubes
located one on each side of the upper pubic cavity,and begin to create a new individual. The female
needs a male to fertilize her egg; she then carries offspring through pregnancy and childbirth.


Similarities between male and female reproductive systems
The reproductive systems of the male and female have some basic similarities and some


specialized differences. They are the same in that most of the reproductive organs of both sexes
develop from similar embryonic tissue, meaning they are homologous. Both systems have gonads that
produce gametes (sperm and egg or ovum) and sex organs. And both systems experience further
development of the reproductive organs, which mature and become functional during puberty as a
result of the gonads secreting sex hormones.


In short, this is a known list of sex organs that evolve from the same tissue in a human life.


Indifferent Male Female
Gonad Testis Ovary
Mullerian duct Appendix testis Fallopian tubes
Mullerian duct Prostatic utricle Uterus, proximal vagina
Wolffian duct Rete testis Rete ovarii
Mesonephric
tubules Efferent ducts Epoophoron
Wolffian duct Epididymis Gartner's duct
Wolffian duct Vas deferens
Wolffian duct Seminal vesicle


Wikibooks | 301




Chapter 16
Wolffian duct Prostate Skene's glands
Urogenital sinus Bladder, urethra Bladder, urethra, distal vagina
Urogenital sinus Bulbourethral gland Bartholin's gland
Genital swelling Scrotum Labia majora
Urogenital folds Distal urethra Labia minora
Genital tubercle Penis Clitoris
Prepuce Foreskin Clitoral hood


Bulb of penis Vestibular bulbs
Glans penis Clitoral glans
Crus of penis Clitoral crura


Differences between male and female reproductive systems
The differences between the female and male reproductive systems are based on the functions of


each individual's role in the reproduction cycle. A male who is healthy, and sexually mature,
continuously produces sperm. A woman's ova (eggs) are completely formed during fetal development.


At about 5 months gestation, the ovaries contain approximately six to seven million oogonia,
which then go through meiosis, and are matured into oocytes.


The ovaries of a newborn baby girl contain about two million oocytes. This number declines to
300,000 to 400,000 by the time puberty is reached. On average, 400 oocytes are ovulated during a
woman's reproductive lifetime.


When a young woman reaches puberty around age 12 or 13, an ova (one of the matured oocytes) is
discharged from alternating ovaries every 28 days. This continues until the woman reaches menopause,
usually around the age of 50 years. Occytes are present at birth, and ages as a woman ages. This may
be one reason older women are more likely to have a hard time becoming pregnant or more likely to
have children with genetic defects.
Female Reproductive System


• Produces eggs (ova)
• Secretes sex hormones
• Receives the male spermatazoa during sexual intercourse
• Protects and nourishes the fertilized egg until it is fully developed
• Delivers fetus through birth canal
• Provides nourishment to the baby through milk secreted by mammary glands in the breast


External Genitals
Vulva


302 | Human Physiology




The Female Reproductive System


The external female genitals are collectively referred to as The Vulva. This consists of the labia
majora and labia minora (while these names translate as "large" and "small" lips, often the "minora"
can be larger, and protrude outside the "majora"), mons pubis, clitoris, opening of the urethra (meatus),
vaginal vestibule, vestibular bulbs, vestibular glands.


The term "vagina" is often improperly used as a generic term to refer to the vulva or female
genitals, even though - strictly speaking - the vagina is a specific internal structure and the vulva is the
exterior genitalia only. Calling the vulva the vagina is akin to calling the mouth the throat.


Mons Veneris
The mons veneris, Latin for "mound of Venus" (Roman Goddess of love) is the soft mound at the


front of the vulva (fatty tissue covering the pubic bone). It is also referred to as the mons pubis. The
mons veneris is sexually sensitive in some women and protects the pubic bone and vulva from the
impact of sexual intercourse. After puberty it is covered with pubic hair, usually in a triangular shape.
Heredity can play a role in the amount of pubic hair an individual grows.


Labia Majora
The labia majora are the outer "lips" of the vulva. They are pads of loose connective and adipose


tissue, as well as some smooth muscle. The labia majora wrap around the vulva from the mons pubis to
the perineum. The labia majora generally hides, partially or entirely, the other parts of the vulva. There
is also a longitudinal separation called the pudendal cleft. These labia are usually covered with pubic
hair. The color of the outside skin of the labia majora is usually close to the overall color of the
individual, although there may be some variation. The inside skin is usually pink to light brown. They
contain numerous sweat and oil glands. It has been suggested that the scent from these oils are sexually
arousing.


Labia Minora
Medial to the labia majora are the labia minora. The labia minora are the inner lips of the vulva.


They are thin stretches of tissue within the labia majora that fold and protect the vagina, urethra, and
clitoris. The appearance of labia minora can vary widely, from tiny lips that hide between the labia
majora to large lips that protrude. There is no pubic hair on the labia minora, but there are sebaceous
glands. The two smaller lips of the labia minora come together longitudinally to form the prepuce, a
fold that covers part of the clitoris. The labia minora protect the vaginal and urethral openings. Both the
inner and outer labia are quite sensitive to touch and pressure.


Clitoris
The clitoris, visible as the small white oval between the top of the labia minora and the clitoral


hood, is a small body of spongy tissue that functions solely for sexual pleasure. Only the tip or glans of
the clitoris shows externally, but the organ itself is elongated and branched into two forks, the crura,
which extend downward along the rim of the vaginal opening toward the perineum. Thus the clitoris is
much larger than most people think it is, about 4" long on average.


The clitoral glans or external tip of the clitoris is protected by the prepuce, or clitoral hood, a
covering of tissue similar to the foreskin of the male penis. However, unlike the penis, the clitoris does
not contain any part of the urethra.


Wikibooks | 303




Chapter 16
During sexual excitement, the clitoris erects and extends, the hood retracts, making the clitoral


glans more accessible. The size of the clitoris is variable between women. On some, the clitoral glans is
very small; on others, it is large and the hood does not completely cover it.


Urethra
The opening to the urethra is just below the clitoris. Although it is not related to sex or


reproduction, it is included in the vulva. The urethra is actually used for the passage of urine. The
urethra is connected to the bladder. In females the urethra is 1.5 inches long, compared to males whose
urethra is 8 inches long. Because the urethra is so close to the anus, women should always wipe
themselves from front to back to avoid infecting the vagina and urethra with bacteria. This location
issue is the reason for bladder infections being more common among females.


Hymen
The hymen is a thin fold of mucous membrane that separates the lumen of the vagina from the


urethral sinus. Sometimes it may partially cover the vaginal orifice. The hymen is usually perforated
during later fetal development.


Because of the belief that first vaginal penetration would usually tear this membrane and cause
bleeding, its "intactness" has been considered a guarantor of virginity. However, the hymen is a poor
indicator of whether a woman has actually engaged in sexual intercourse because a normal hymen does
not completely block the vaginal opening. The normal hymen is never actually "intact" since there is
always an opening in it. Furthermore, there is not always bleeding at first vaginal penetration. The
blood that is sometimes, but not always, observed after first penetration can be due to tearing of the
hymen, but it can also be from injury to nearby tissues.


A tear to the hymen, medically referred to as a "transection," can be seen in a small percentage of
women or girls after first penetration. A transection is caused by penetrating trauma. Masturbation and
tampon insertion can, but generally are not forceful enough to cause penetrating trauma to the hymen.
Therefore, the appearance of the hymen is not a reliable indicator of virginity or chastity.


Perineum
The perineum is the short stretch of skin starting at the bottom of the vulva and extending to the


anus. It is a diamond shaped area between the symphysis pubis and the coccyx. This area forms the
floor of the pelvis and contains the external sex organs and the anal opening. It can be further divided
into the urogenital triangle in front and the anal triangle in back.


The perineum in some women may tear during the birth of an infant and this is apparently natural.
Some physicians however, may cut the perineum preemptively on the grounds that the "tearing" may
be more harmful than a precise cut by a scalpel. If a physician decides the cut is necessary, they will
perform it. The cut is called an episiotomy.


304 | Human Physiology




The Female Reproductive System


Internal Genitals
Vagina


The vagina is a muscular, hollow tube that extends from the vaginal opening to the cervix of the
uterus. It is situated between the urinary bladder and the rectum. It is about three to five inches long in
a grown woman. The muscular wall allows the vagina to expand and contract. The muscular walls are
lined with mucous membranes, which keep it protected and moist. A thin sheet of tissue with one or
more holes in it, called the hymen, partially covers the opening of the vagina. The vagina receives
sperm during sexual intercourse from the penis. The sperm that survive the acidic condition of the
vagina continue on through to the fallopian tubes where fertilization may occur.


The vagina is made up of three layers, an inner mucosal layer, a middle muscularis layer, and an
outer fibrous layer. The inner layer is made of vaginal rugae that stretch and allow penetration to occur.
These also help with stimulation of the penis. The middle layer has glands that secrete an acidic mucus
(pH of around 4.0.) that keeps bacterial growth down. The outer muscular layer is especially important
with delivery of a fetus and placenta.
Purposes of the Vagina


• Receives a males erect penis and semen during sexual intercourse.
• Pathway through a womans body for the baby to take during childbirth.
• Provides the route for the menstrual blood (menses) from the uterus, to leave the body.
• May hold forms of birth control, such as a diaphragm, FemCap, Nuva Ring, or female


condom.


Wikibooks | 305




Chapter 16


Cervix
The cervix (from Latin "neck") is the lower, narrow portion


of the uterus where it joins with the top end of the vagina.
Where they join together forms an almost 90 degree curve. It is
cylindrical or conical in shape and protrudes through the upper
anterior vaginal wall. Approximately half its length is visible
with appropriate medical equipment; the remainder lies above
the vagina beyond view. It is occasionally called "cervix uteri",
or "neck of the uterus".


During menstruation, the cervix stretches open slightly to
allow the endometrium to be shed. This stretching is believed to
be part of the cramping pain that many women experience.
Evidence for this is given by the fact that some women's cramps
subside or disappear after their first vaginal birth because the
cervical opening has widened.


The portion projecting into the vagina is referred to as the
portio vaginalis or ectocervix. On average, the ectocervix is three cm long and two and a half cm wide.
It has a convex, elliptical surface and is divided into anterior and posterior lips. The ectocervix's
opening is called the external os. The size and shape of the external os and the ectocervix varies widely
with age, hormonal state, and whether the woman has had a vaginal birth. In women who have not had
a vaginal birth the external os appears as a small, circular opening. In women who have had a vaginal
birth, the ectocervix appears bulkier and the external os appears wider, more slit-like and gaping.


The passageway between the external os and the uterine cavity is referred to as the endocervical
canal. It varies widely in length and width, along with the cervix overall. Flattened anterior to
posterior, the endocervical canal measures seven to eight mm at its widest in reproductive-aged
women. The endocervical canal terminates at the internal os which is the opening of the cervix inside
the uterine cavity.


During childbirth, contractions of the uterus will dilate the cervix up to 10 cm in diameter to allow
the child to pass through. During orgasm, the cervix convulses and the external os dilates.


Uterus
The uterus is shaped like an upside-down pear, with a thick lining and muscular walls. Located


near the floor of the pelvic cavity, it is hollow to allow a blastocyte, or fertilized egg, to implant and
grow. It also allows for the inner lining of the uterus to build up until a fertilized egg is implanted, or it
is sloughed off during menses.


The uterus contains some of the strongest muscles in the female body. These muscles are able to
expand and contract to accommodate a growing fetus and then help push the baby out during labor.
These muscles also contract rhythmically during an orgasm in a wave like action. It is thought that this
is to help push or guide the sperm up the uterus to the fallopian tubes where fertilization may be
possible.


306 | Human Physiology


Clinical Application:
Pelvic inflammatory disease (PID) is a
widespread infection that originates in
the vagina and uterus and spreads to


the uterine tubes, ovaries, and
ultimately the pelvic peritoneum. This
condition, which occurs in about 10%


of women is usually caused by
chlamydial or gonorrheal infection,
other bacteria infecting the vagina
may be involved as well. Signs and
symptoms include tenderness of the
lower abdomen, fever, and a vaginal
discharge. Even a single episode of
PID can cause infertility, due to


scarring that blocks the uterine tubes.
Therefore, patients are immediately
given broad-spectrum antibiotics
whenever PID is suspected.




The Female Reproductive System


The uterus is only about three inches long and two inches wide, but during pregnancy it changes
rapidly and dramatically. The top rim of the uterus is called the fundus and is a landmark for many
doctors to track the progress of a pregnancy. The uterine cavity refers to the fundus of the uterus and
the body of the uterus.


Helping support the uterus are ligaments that attach from the body of the uterus to the pelvic wall
and abdominal wall. During pregnancy the ligaments prolapse due to the growing uterus, but retract
after childbirth. In some cases after menopause, they may lose elasticity and uterine prolapse may
occur. This can be fixed with surgery.


Some problems of the uterus include uterine fibroids, pelvic pain (including endometriosis,
adenomyosis), pelvic relaxation (or prolapse), heavy or abnormal menstrual bleeding, and cancer. It is
only after all alternative options have been considered that surgery is recommended in these cases. This
surgery is called hysterectomy. Hysterectomy is the removal of the uterus, and may include the
removal of one or both of the ovaries. Once performed it is irreversible. After a hysterectomy, many
women begin a form of alternate hormone therapy due to the lack of ovaries and hormone production.


Fallopian Tubes
At the upper corners of the uterus are the fallopian tubes. There are two fallopian tubes, also


called the uterine tubes or the oviducts. Each fallopian tube attaches to a side of the uterus and connects
to an ovary. They are positioned between the ligaments that support the uterus. The fallopian tubes are
about four inches long and about as wide as a piece of spaghetti. Within each tube is a tiny passageway
no wider than a sewing needle. At the other end of each fallopian tube is a fringed area that looks like a
funnel. This fringed area, called the infundibulum, lies close to the ovary, but is not attached. The
ovaries alternately release an egg. When an ovary does ovulate, or release an egg, it is swept into the
lumen of the fallopian tube by the frimbriae.


Once the egg is in the fallopian tube, tiny hairs in the tube's lining help push it down the narrow
passageway toward the uterus. The oocyte, or developing egg cell, takes four to five days to travel
down the length of the fallopian tube. If enough sperm are ejaculated during sexual intercourse and
there is an oocyte in the fallopian tube, fertilization will occur. After fertilization occurs, the zygote, or
fertilized egg, will continue down to the uterus and implant itself in the uterine wall where it will grow
and develop.


If a zygote doesn't move down to the uterus and implants itself in the fallopian tube, it is called a
ectopic or tubal pregnancy. If this occurs, the pregnancy will need to be terminated to prevent
permanent damage to the fallopian tube, possible hemorrhage and possible death of the mother.


Mammary glands
Mammary glands are the organs that produce milk for the sustenance of a baby. These exocrine


glands are enlarged and modified sweat glands.


Wikibooks | 307




Chapter 16
Structure


The basic components of the mammary gland are the alveoli (hollow cavities, a few millimetres
large) lined with milk-secreting epithelial cells and surrounded by myoepithelial cells. These alveoli
join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into
openings in the nipple. The myoepithelial cells can contract, similar to muscle cells, and thereby push
the milk from the alveoli through the lactiferous ducts towards the nipple, where it collects in
widenings (sinuses) of the ducts. A suckling baby essentially squeezes the milk out of these sinuses.


The development of mammary glands is controlled by hormones. The mammary glands exist in
both sexes, but they are rudimentary until puberty when - in response to ovarian hormones - they begin
to develop in the female. Estrogen promotes formation, while testosterone inhibits it.


At the time of birth, the baby has lactiferous ducts but no alveoli. Little branching occurs before
puberty when ovarian estrogens stimulate branching differentiation of the ducts into spherical masses
of cells that will become alveoli. True secretory alveoli only develop in pregnancy, where rising levels
of estrogen and progesterone cause further branching and differentiation of the duct cells, together with
an increase in adipose tissue and a richer blood flow.


Colostrum is secreted in late pregnancy and for the first few days after giving birth. True milk
secretion (lactation) begins a few days later due to a reduction in circulating progesterone and the
presence of the hormone prolactin. The suckling of the baby causes the release of the hormone
oxytocin which stimulates contraction of the myoepithelial cells.


The cells of mammary glands can easily be induced to grow and multiply by hormones. If this
growth runs out of control, cancer results. Almost all instances of breast cancer originate in the lobules
or ducts of the mammary glands.
STRUCTURE LOCATION & DESCRIPTION FUNCTION


Breasts
Upper chest one on each side
containing alveolar cells (milk
production), myoepithelial cells
(contract to expel milk), and duct
walls (help with extraction of milk).


Lactation milk/nutrition for newborn.


Cervix The lower narrower portion of the uterus.


During childbirth, contractions of the uterus
will dilate the cervix up to 10 cm in diameter
to allow the child to pass through. During
orgasm, the cervix convulses and the external
os dilates


Clitoris Small erectile organ directly in front of the vestibule. Sexual excitation, engorged with blood.


Fallopian tubes Extending upper part of the uterus on either side.
Egg transportation from ovary to uterus
(fertilization usually takes place here).


Hymen Thin membrane that partially covers the vagina in young females.
Labia majora Outer skin folds that surround the entrance to the vagina. Lubrication during mating.
Labia minora Inner skin folds that surround the Lubrication during mating.


308 | Human Physiology




The Female Reproductive System


entrance to the vagina.
Mons Mound of skin and underlying fatty tissue, central in lower pelvic region
Ovaries (female
gonads)


Pelvic region on either side of the
uterus.


Provides an environment for maturation of
oocyte. Synthesizes and secretes sex hormones
(estrogen and progesterone).


Perineum
Short stretch of skin starting at the
bottom of the vulva and extending to
the anus.


Urethra Pelvic cavity above bladder, tilted. Passage of urine.
Uterus Center of pelvic cavity. To house and nourish developing human.


Vagina
Canal about 10-8 cm long going
from the cervix to the outside of the
body.


Receives penis during mating. Pathway
through a womans body for the baby to take
during childbirth. Provides the route for the
menstrual blood (menses) from the uterus, to
leave the body. May hold forms of birth
control, such as an IUD, diaphragm, neva ring,
or female condom


Vulva
Surround entrance to the
reproductive tract.(encompasses all
external genitalia)


Endometrium The innermost layer of uterine wall. Contains glands that secrete fluids that bathe the utrine lining.
Myometrium Smooth muscle in uterine wall. Contracts to help expel the baby.


The Female Reproductive Cycle
Towards the end of puberty, girls begin to release eggs as part of a monthly period called the


female reproductive cycle, or menstrual cycle (menstrual referring to "monthly"). Approximately
every 28 days, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes. Unless the
egg is fertilized by a sperm while in the fallopian in the two to three days following ovulation, the egg
dries up and leaves the body about two weeks later through the vagina. This process is called
menstruation. Blood and tissues from the inner lining of the uterus (the endometrium) combine to form
the menstrual flow, which generally lasts from four to seven days. The first period is called menarche.
During menstruation arteries that supply the lining of the uterus constrict and capillaries weaken. Blood
spilling from the damaged vessels detaches layers of the lining, not all at once but in random patches.
Endometrium mucus and blood descending from the uterus, through the liquid creates the menstruation
flow.


The reproductive cycle can be divided into an ovarian cycle and a uterine cycle (compare ovarian
histology and uterine histology in the diagram on the right). During the uterine cycle, the endometrial
lining of the uterus builds up under the influence of increasing levels of estrogen (labeled as estradiol in
the image). Follicles develop, and within a few days one matures into an ovum, or egg. The ovary then
releases this egg, at the time of ovulation. After ovulation the uterine lining enters a secretory phase, or
the ovarian cycle, in preparation for implantation, under the influence of progesterone. Progesterone is
produced by the corpus luteum (the follicle after ovulation) and enriches the uterus with a thick lining
of blood vessels and capillaries so that it can sustain the growing fetus. If fertilization and implantation


Wikibooks | 309




Chapter 16
occur, the embryo produces Human Chorionic Gonadotropin (HCG), which maintains the corpus
luteum and causes it to continue producing progesterone until the placenta can take over production of
progesterone. Hence, progesterone is "pro gestational" and maintains the uterine lining during all of
pregnancy. If fertilization and implantation do not occur the corpus luteum degenerates into a corpus
albicans, and progesterone levels fall. This fall in progesterone levels cause the endometrium lining to
break down and sluff off through the vagina. This is called menstruation, which marks the low point for
estrogen activity and is the starting point of a new cycle.


Common usage refers to menstruation and menses as a period. This bleeding serves as a sign that a
woman has not become pregnant. However, this cannot be taken as certainty, as sometimes there is
some bleeding in early pregnancy. During the reproductive years, failure to menstruate may provide the
first indication to a woman that she may have become pregnant.


Menstruation forms a normal part of a natural cyclic process occurring in healthy women between
puberty and the end of the reproductive years. The onset of menstruation, known as menarche, occurs
at an average age of 12, but is normal anywhere between 8 and 16. Factors such as heredity, diet, and
overall health can accelerate or delay the onset of menarche.


Signs of ovulation
The female body produces outward signs that can be easily recognized at the time of ovulation.


The two main signs are thinning of the cervical mucus and a slight change in body temperature.
Thinning of the Cervical Mucus
After menstruation and right before ovulation, a woman will experience an increase of cervical


mucus. At first, it will be thick and yellowish in color and will not be very plentiful. Leading up to
ovulation, it will become thinner and clearer. On or around the day of ovulation, the cervical mucus
will be very thin, clear and stretchy. It can be compared to the consistency of egg whites.


Temperature Change
A woman can also tell the time of ovulation by taking her basal body temperature daily. This is a


temperature taken with a very sensitive thermometer first thing in the morning before the woman gets
out of bed. The temperature is then tracked to show changes. In the uterine cycle, a normal temperature
will be around 97.0 – 98.0. The day of ovulation the temperature spikes down, usually into the 96.0 –
97.0 range and then the next morning it will spike up to normal of around 98.6 and stay in that range
until menstruation begins.


Both of these methods are used for conception and contraception. They are more efficient in
conception due to the fact that sperm can live for two to three days inside of the fallopian tubes. A
woman could be off by a couple of days in her calculations and still become pregnant.


Menopause is the physiological cessation of menstrual cycles associated with advancing age.
Menopause is sometimes referred to as "the change of life" or climacteric. Menopause occurs as the
ovaries stop producing estrogen, causing the reproductive system to gradually shut down. As the body
adapts to the changing levels of natural hormones, vasomotor symptoms such as hot flashes and
palpitations, psychological symptoms such as increased depression, anxiety, irritability, mood swings
and lack of concentration, and atrophic symptoms such as vaginal dryness and urgency of urination


310 | Human Physiology




The Female Reproductive System


appear. Together with these symptoms, the woman may also have increasingly scanty and erratic
menstrual periods.


Technically, menopause refers to the cessation of menses; the gradual process through which this
occurs, which typically takes a year but may last as little as six months or more than five years, is
known as climacteric. A natural or physiological menopause is that which occurs as a part of a
woman's normal aging process. However, menopause can be surgically induced by such procedures as
hysterectomy.


The average onset of menopause is 50.5 years, but some women enter menopause at a younger
age, especially if they have suffered from cancer or another serious illness and undergone
chemotherapy. Premature menopause is defined as menopause occurring before the age of 40, and
occurs in 1% of women. Other causes of premature menopause include autoimmune disorders, thyroid
disease, and diabetes mellitus.


Premature menopause is diagnosed by measuring the levels of follicle stimulating hormone (FSH)
and luteinizing hormone (LH). The levels of these hormones will be higher if menopause has occurred.
Rates of premature menopause have been found to be significantly higher in both fraternal and
identical twins; approximately 5% of twins reach menopause before the age of 40. The reasons for this
are not completely understood. Post-menopausal women are at increased risk of osteoporosis.


Perimenopause refers to the time preceding menopause, during which the production of hormones
such as estrogen and progesterone diminish and become more irregular. During this period fertility
diminishes. Menopause is arbitrarily defined as a minimum of twelve months without menstruation.
Perimenopause can begin as early as age 35, although it usually begins much later. It can last for a few
months or for several years. The duration of perimenopause cannot be predicted in advance.


Premenstrual Syndrome (PMS) It is common for women to experience some discomfort in the
days leading up to their periods. PMS usually is at its worst the seven days before a period starts and
can continue through the end of the period. PMS includes both physical and emotional symptoms:
acne, bloating, fatigue, backaches, sore breasts, headaches, constipation, diarrhea, food cravings,
depression, irritability, difficulty concentrating or handling stress.


Ovarian and Uterine Cycles in the Nonpregnant Woman
Ovarian Cycle Events Uterine Cycle Events


Follicular phase -
Days 1-13 FSH secretion begins.


Menstruation - Days
2-5 Endometrium breaks down.


Follicle maturation
occurs.


Proliferative phase -
Days 6-13 Endometrium rebuilds.


Estrogen secretion is
prominent.


Ovulation - Day
14* LH spike occurs.
Luteal phase - Days
15-28 LH secretion continues.


Secretory phase - Days
15-28


Endometrial thickens, and
glands are secretory.


Wikibooks | 311




Chapter 16
Corpus luteum forms.
Progesterone secretion is
prominent.


(*)Assuming a 28 day cycle.
There are two phases of the ovarian cycle the follicular phase and the luteal phase. In the follicular


phase about 10-25 follicles are taken from preantral or early antrial follicles to develop further. Seven
days later the dominant follicle is selected to develop to full maturity. This is the pre-cursor for
ovulation. Follicles themselves secrete FSH and estrogen, and these two hormones stimulate follicular
growth and development. Ovulation marks the beginning of the luteal phase. This is started by the wall
of the Graffian follicle to rupture and cause a flow of antral fluid that will carry the oocyte to the
ovary's surface. The ruptured follicle is then turned into a gland (corpus luteum). Which secretes
estrogens and progesterone. This is all triggered by and abrupt change in plasma LH levels. After
ovulation the released oocyte enters the uterine tube, where it will be either fertilized or discarded.


The uterine cycle operates in sync with the ovarian cycle and is divided into three phases. The first
phase in the menstrual phase. It is named the menstrual phase because in corresponds with the shedding
the the uterine lining or more commonly called menstruation. The corpus luteum degenerates causing
plasma estrogen and progesterone levels to decrease and in turn causes menstruation. Blood vessels in
the outer most layer of the endometrium constrict and decrease blood flow to the tissues killing these
tissues. After the tissues die they start to separate from the underlying endometrail tissues. Eventually
the dead tissue is shed. This shedding of the tissues ruptures blood vessels and causes bleeding. Now
we have the proliferative phase. During this phase the uterus renews itself and prepares for pregnancy.
The endomitrial tissue that is left after menstruation begins to grow. The endometrial glands grow and
enlarge causing more blood vessels. The cervical canal has glands that secrete a thin mucous that helps
deposited sperm. Estrogen promotes uterine changes in this phase. The last phase is the secretory
phase. This is where the endometrium is transformed to make it the best environment for implantation
and subsequent housing and nourishment of the developing embryo. By doing this the endometrium
will do things like have an enriched blood supply, begin to secrete fluids rich in glycogen, and even
form a plug at the end of the cervical canal so that microorganisms can not enter. These changes in the
uterus are caused by progesterone, due to the corpus luteum. At the end of the secretory phase the
corpus luteum degenerates, and progesterone levels fall. This will trigger menstruation.


Sexual Reproduction
Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the


offspring. In sexual reproduction, genes from two individuals are combined in random ways with each
new generation. Sex hormones released into the body by the endocrine system signal the body when it
is time to start puberty. The female and male reproductive systems are the only systems so vastly
different that each sex has their own different organs. All other systems have "unisex" organs.


Reproduction is characterized by two processes. The first, meiosis, involves the halving of the 46
of chromosomes. The second process, fertilization, leads the fusion of two gametes and the restoration
of the original number of chromosomes: 23 chromosomes from the paternal side and 23 from the
maternal side. During meiosis, the chromosomes of each pair usually cross over to achieve genetic
recombination.


Sexual reproduction cannot happen without the sexual organs called gonads. Both sexes have


312 | Human Physiology




The Female Reproductive System


gonads: in females, the gonads are the ovaries. The female gonads produce female gametes (eggs); the
male gonads produce male gametes (sperm). After an egg is fertilized by the sperm, the fertilized egg is
called the zygote.


The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then
implants itself in the wall of the uterus, where it begins the processes of embryogenesis and
morphogenesis. The womens body carries out this process of reproduction for 40 weeks, until delivery
of the fetus from the uterus through the vagina or birth canal. Even after birth, the female continues
with the reproduction process by supplying the milk to nourish the infant.


Infertility
Infertility is the inability to naturally conceive a child or the inability to carry a pregnancy to term.


There are many reasons why a couple may not be able to conceive without medical assistance.
Infertility affects approximately 15% of couples. Roughly 40% of cases involve a male contribution or
factor, 40% involve a female factor, and the remaining 20% involve both sexes. Healthy couples in
their mid-20s having regular sex have a one-in-four chance of getting pregnant in any given month.
This is called "Fecundity".


Primary vs. secondary
According to the American Society for Reproductive Medicine, infertility affects about 6.1 million


people in the United States, equivalent to 10% of the reproductive age population. Female infertility
accounts for one third of infertility cases, male infertility for another third, combined male and female
infertility for another 15%, and the remainder of cases are "unexplained.


"Secondary infertility" is difficulty conceiving after already having conceived and carried a normal
pregnancy. Apart from various medical conditions (e.g. hormonal), this may come as a result of age
and stress felt to provide a sibling for their first child. Technically, secondary infertility is not present if
there has been a change of partners.


Factors of Infertility
Factors relating to female infertility are:


• General factors
• Diabetes mellitus,thyroid disorders,adrenal disease


• Significant liver,kidney disease
• Psychological factors
• Hypothalamic-pituitary factors:


• Kallmann syndrome
• Hypothalamic dysfunction
• Hyperprolactinemia
• Hypopituitarism
• Ovarian factors


• Polycystic ovary syndrome
Wikibooks | 313




Chapter 16
• Anovulation
• Diminished ovarian reserve
• Luteal dysfunction
• Premature menopause
• Gonadal dysgenesis (Turner syndrome)
• Ovarian neoplasm
• Tubal/peritoneal factors


• Endometriosis
• Pelvic adhesions
• Pelvic inflammatory disease(PID, usually due to chlamydia)
• Tubal occlusion
• Uterine factors


• Uterine malformations
• Uterine fibroids (leiomyoma)
• Asherman's Syndrome
• Cervical factors


• Cervical stenosis
• Antisperm antibodies
• Insufficient cervical mucus (for the travel and survival of sperm)
• Vaginal factors


• Vaginismus
• Vaginal obstruction
• Genetic factors


• Various intersexuality|intersexed conditions, such as androgen insensitivity
syndrome


Combined Infertility
In some cases, both the man and woman may be infertile or sub-fertile, and the couple's infertility


arises from the combination of these factors. In other cases, the cause is suspected to be immunological
or genetic; it may be that each partner is independently fertile but the couple cannot conceive together
without assistance.


Unexplained Infertility
In about 15% of cases of infertility, investigation will show no abnormalities. In these cases


abnormalities are likely to be present but not detected by current methods. Possible problems could be
that the egg is not released at the optimum time for fertilization, that it may not enter the fallopian tube,
sperm may not be able to reach the egg, fertilization may fail to occur, transport of the zygote may be
disturbed, or implantation fails. It is increasingly recognized that egg quality is of critical importance.


Diagnosis of Infertility
Diagnosis of infertility begins with a medical history and physical exam. The healthcare provider


may order tests, including the following:


314 | Human Physiology




The Female Reproductive System


• an endometrial biopsy, which tests the lining of the uterus
• hormone testing, to measure levels of female hormones
• laparoscopy, which allows the provider to see the pelvic organs
• ovulation testing, which detects the release of an egg from the ovary
• Pap smear, to check for signs of infection
• pelvic exam, to look for abnormalities or infection
• a postcoital test, which is done after sex to check for problems with secretions
• special X-ray tests


Treatment
• Fertility medication which stimulates the ovaries to "ripen" and release eggs (e.g.


Clomifene|clomifene citrate, which stimulates ovulation)
• Surgery to restore potency of obstructed fallopian tubes (tuboplasty)
• Donor insemination which involves the woman being artificially inseminated or artificially


inseminated with donor sperm.
• In vitro fertilization (IVF) in which eggs are removed from the woman, fertilized and then


placed in the woman's uterus, bypassing the fallopian tubes. Variations on IVF include:
• Use of donor eggs and/or sperm in IVF. This happens when a couple's eggs and/or


sperm are unusable, or to avoid passing on a genetic disease.
• Intracytoplasmic sperm injection (ICSI) in which a single sperm is injected directly into an


egg; the fertilized egg is then placed in the woman's uterus as in IVF.
• Zygote intrafallopian transfer(ZIFT) in which eggs are removed from the woman, fertilized


and then placed in the woman's fallopian tubes rather than the uterus.
• Gamete intrafallopian transfer(GIFT) in which eggs are removed from the woman, and


placed in one of the fallopian tubes, along with the man's sperm. This allows fertilization to take
place inside the woman's body.


• Other assisted reproductive technology (ART):
• Assisted hatching


• Fertility preservation
• Freezing (cryopreservation) of sperm, eggs, & reproductive tissue
• Frozen embryo transfer (FET)
• Alternative and complimentary treatments


• Acupuncture Recent controlled trials published in Fertility and Sterility have shown
acupuncture to increase the success rate of IVF by as much as 60%. Acupuncture was
also reported to be effective in the treatment of female anovular infertility, World Health
Organization, Acupuncture: Review and Analysis of Reports on Controlled Trials
(2002).


• Diet and supplements
• Healthy lifestyle


Types of Birth Control
Birth control is a regimen of one or more actions, devices, or medications followed in order to


deliberately prevent or reduce the likelihood of a woman becoming pregnant. Methods and intentions
typically termed birth control may be considered a pivotal ingredient to family planning. Mechanisms
which are intended to reduce the likelihood of the fertilization of an ovum by a sperm may more


Wikibooks | 315




Chapter 16
specifically be referred to as contraception. Contraception differs from abortion in that the former
prevents fertilization, while the latter terminates an already established pregnancy. Methods of birth
control which may prevent the implantation of an embryo if fertilization occurs are medically
considered to be contraception. It is advised to talk with a doctor before choosing a contraceptive. If
you have genetics problems or blood conditions, such as factor V leiden, certain contraceptives can be
deadly.


Type Procedure Method Effectiveness Risks
Abstinence Refrain from sexual intercourse No sperm in vagina 100% None


Rhythm Method


Intercourse is avoided for
about an 8-day span every
month in middle of her
cycle, from about five days
before ovulation to three
days after ovulation.


fertilization is only
possible during 8-
day span in middle
of menstrual cycle


70-80% None


Withdrawal
The man withdraws his
penis from the vagina at just
the right moment before
ejaculation.


sperm are unable to
enter vagina if male
penis is removed at
the right time


70-80% None


Tubal Ligation
(Vasectomy) Oviducts are cut and tied No eggs in oviduct Almost 99%


About 75%
Irreversible


Hormonal IUD
(intrauterine
device)


Flexible, plastic coil inserted
by physician


Releases small
amounts of estrogen.
In most cases, stops
egg from developing
and being released


About 99%
May cause
infections,
uterine
perforation


Oral
Contraceptive


Hormone medication taken
daily


Stops release of FSH
and LH More than 90%


Blood clots,
especially in
smokers


Contraceptive
Implants


Tubes of progesterone
implanted under the skin


Stops release of FSH
and LH More than 90% None known


Contraceptive
Injections Injections of hormones


Stops release of FSH
and LH About 99%


Possible
osteoporosis


Diaphragm
Latex cup inserted into
vagina to cover cervix
before intercourse


Blocks entrance of
sperm into uterus


With
spermicide,
about 90%


Latex or
spermicide
allergy


Cervical Cap Latex cup held by suction over cervix
Delivers spermicide
near cervix Almost 85%


UTI, latex or
spermicide
allergy


Female Condom Polyurethane liner fitted inside vagina
Blocks entrance of
sperm into uterus
and prevents STD’s


Almost 85% None


Male Condom
soft sheath, made of latex or
animal membrane, encloses
penis, trapping ejaculated
sperm


Blocks entrance of
sperm into vagina
and prevents STD's


90% None


316 | Human Physiology




The Female Reproductive System


Jellies, Cream,
Foams


Spermicidal products
inserted before intercourse


Kills large number of
sperm About 75%


UTI, allergy to
spermicides


Natural Family
Planning


Keep record of ovulation
using various methods


Avoid sexual
intercourse near
ovulation


About 70% None known


Douche Vagina cleansed after intercourse Washes out sperm Less than 70% None known


Plan B Pill Pill taken after intercourse
Prevents release of
egg, fertilization of
egg, or egg from
attaching to uterus


About 89% Same as oral contraceptive


Sexually Transmitted Diseases
Sexually transmitted diseases (STDs) are diseases or infections that have a significant probability


of transmission between humans by means of sexual contact: vaginal intercourse, oral sex, and/or anal
sex. Many STDs are (more easily) transmitted through the mucous membranes of the penis, vulva, and
(less often) the mouth. The visible membrane covering the head of the penis is a mucous membrane,
though, for those who are circumcised it is usually dry and produces no mucus (similar to the lips of the
mouth). Mucous membranes differ from skin in that they allow certain pathogens (viruses or bacteria)
into the body (more easily).


The probability of transmitting infections through sex is far greater than by more casual means of
transmission, such as non-sexual contact—touching, sharing cutlery, and shaking hands. Although
mucous membranes exist in the mouth as well as in the genitals, many STDs are more likely to be
transmitted through oral sex than through deep kissing. Many infections that are easily transmitted
from the mouth to the genitals or from the genitals to the mouth, are much harder to transmit from one
mouth to another. With HIV, genital fluids happen to contain a great deal more of the pathogen than
saliva. Some infections labeled as STDs can be transmitted by direct skin contact. Herpes simplex and
HPV are both examples. Depending on the STD, a person who has has the disease but has no
symptoms may or may not be able to spread the infection. For example, a person is much more likely
to spread herpes infection when blisters are present than when they are absent. However, a person can
spread HIV infection at any time, even if he/she has not developed symptoms of AIDS.


All sexual behaviors that involve contact with the bodily fluids of another person should be
considered to hold some risk of transmission of sexually transmitted diseases. Most attention has
focused on controlling HIV, which causes AIDS, but each STD presents a different situation.


As may be noted from the name, sexually transmitted diseases are transmitted from one person to
another by certain sexual activities rather than being actually caused by those sexual activities.
Bacteria, fungi, protozoa or viruses are still the causative agents. It is not possible to catch any sexually
transmitted disease from a sexual activity with a person who is not carrying a disease; conversely, a
person who has an STD received it from contact (sexual or otherwise) with someone who is infected.


Although the likelihood of transmitting diseases by sexual activities varies a great deal, in general,
all sexual activities between two (or more) people should be considered as being a two-way route for
the transmission of STDs (i.e. "giving" or "receiving" are both risky).


Wikibooks | 317




Chapter 16


Prevention of Sexually Transmitted Diseases
Although healthcare professionals suggest that safer sex, such as the use of condoms, as the most


reliable way of decreasing the risk of contracting sexually transmitted diseases during sexual activity,
safer sex should by no means be considered an absolute safeguard. Abstinence is the best control of and
protection from the transmission of sexually transmitted infections. The transfer of and exposure to
bodily fluids, such as blood transfusions and other blood products, sharing injection needles, needle-
stick injuries (when medical staff are inadvertently jabbed or pricked with needles during medical
procedures), sharing tattoo needles, and childbirth are all avenues of transmission. These means put
certain groups, such as doctors, haemophiliacs and drug users, particularly at risk.


Human Papillomavirus (HPV)
There are over 100 types of this virus which is often asymptomatic. Nearly 3 out of 4 Americans


between ages 15 and 49 have been infected. It can be contracted through one partner and remain
dormant allowing it to be transmitted to another. Some types can cause cervical cancer.


Genital HPV infection is a sexually transmitted disease that is caused by human papillomavirus.
Human papillomavirus is the name of a group of viruses that includes more than 100 different strains.
More than 30 of these are sexually transmitted and they can infect the genital area of men and women.
Approximately 20 million people are currently infected with HPV and at least 50% of sexually active
men and women will acquire HPV at some point in their lives. By age 50 at least 80% of women will
have acquired HPV and about 6.2 million Americans get a new HPV infection each year. Most people
who have HPV don't know that they are infected. The virus lives in the skin or mucous membranes and
usually causes no symptoms. Commonly some people get visible genital warts or have pre-cancerous
changes in the cervix, vulva, anus, or penis. Very rarely, HPV results in anal or genital cancers. Genital
warts usually appear soft, moist, pink, or flesh colored swellings. They can be raised, flat, single, or
multiple, small or large and sometimes cauliflower shaped. Warts may not appear for weeks or months
or not at all and the only way to diagnose them is by visible inspection. Most women are diagnosed
with HPV on the basis of abnormal pap tests and there are no tests available for men. There is no cure
for HPV. The surest way to eliminate risk for HPV is to refrain from any genital contact with another
individual. For those who choose to be sexually active, a long term monogamous relationship with an
uninfected partner is the strategy most likely to prevent future HPV infections.The next best way to
help reduce risk is using a condom but the effectiveness is unknown.


What is the connection between HPV and cervical cancer? All types of HPV cause mild pap
test abnormalities which do not have serious consequences. Approximately 10 of the 30 identified HPV
types can lead to development of cervical cancer. Research as shown that for most women, 90%
cervical HPV infection becomes undetectable within two years. Although only a small proportion of
women have persistent infection, persistent infection with the high risk types of HPV is the main risk
factor for cervical cancer.


A pap test can detect pre-cancerous and cancerous cells on the cervix. Regular pap testing and
careful medical follow up, with treatment if necessary, can help ensure that pre-cancerous changes in
the cervix caused by HPV infection do not develop into life threatening cervical cancer. The pap test
used in the U.S. cervical cancer screening programs is responsible for greatly reducing deaths from
cervical cancer.


318 | Human Physiology




The Female Reproductive System


Diseases and Disorders of the Female Reproductive
System


Women are commonly dealing with many different diseases and disorders that pertain to the
reproductive system. Here are some of the most common:


1. Vulvovaginitis (pronounced:vul-vo-vah-juh-ni-tus) is an inflammation of the vulva and
vagina. It may be caused by irritating substances such as laundry soap, bubble baths or poor
hygiene such as wiping from back to front. Symptoms include redness and itching in these areas
and sometimes vaginal discharge. It can also be caused by an overgrowth of candida, a fungus
normally present in the vagina.
2. Nonmenstrual vaginal bleeding is most commonly due to the presence of a foreign body


in the vagina. It may also be due to urethral prolapse, a condition in which the mucous
membranes of the urethra protrude into the vagina and forms a tiny, donut shaped mass of tissue
that bleeds easily. It can also be due to a straddle injury or vaginal trauma from sexual abuse.
3. Ectopic Pregnancy occurs when a fertilized egg or zygote doesn't travel into the uterus,


but instead grows rapidly in the fallopian tube. Women with this condition can develop severe
abdominal pain and should see a doctor because surgery may be necessary.
4. Ovarian tumors,although rare, can occur. Women with ovarian tumors may have


abdominal pain and masses that can be felt in the abdomen. Surgery may be needed to remove
the tumor.
5. Ovarian cysts are noncancerous sacs filled with fluid or semi-solid material. Although they


are common and generally harmless, they can become a problem if they grow very large. Large
cysts may push on surrounding organs, causing abdominal pain. In most cases, cysts will pass or
disappear on their own and treatment is not necessary. If the cysts are painful and occur
frequently, a doctor may prescribe birth control pills to alter their growth and occurrences.
Surgery is also an option if they need to be removed.
6. Polycystic ovary syndrome is a hormone disorder in which too many hormones are


produced by the ovaries. This condition causes the ovaries to become enlarged and develop
many fluid filled sacs or cysts. It often first appears during the teen years. Depending on the
type and the severity of the condition, it may be treated with drugs to regulate hormone balance
and menstruation.
7. Trichomonas vaginalis inflammatory condition of the vagina usually a bacterial infection


also called vaginosis.
8. Dysmenorrhea is painful periods.
9. Menorrhagia is when a women has very heavy periods with excess bleeding.
10.Oligomenorrhea is when a woman misses or has infrequent periods, even though she has


been menstruating for a while and is not pregnant.
11.Amenorrhea is when a girl has not started her period by the time she is 16 years old or 3


years after puberty has started, has not developed signs of puberty by 14, or has had normal
periods but has stopped menstruating for some reasons other than pregnancy.
12.Toxic shock syndrome is caused by toxins released into the body during a type of


bacterial infection that is more likely to develop if a tampon is left in too long. It can produce
high fever, diarrhea, vomiting, and shock.
13.Candidasis symptoms of yeast infections include itching, burning and discharge. Yeast


organisms are always present in all people, but are usually prevented from "overgrowth"
(uncontrolled multiplication resulting in symptoms) by naturally occurring microorganisms.


Wikibooks | 319




Chapter 16
At least three quarters of all women will experience candidiasis at some point in their lives. The


Candida albicans organism is found in the vaginas of almost all women and normally causes no
problems. However, when it gets out of balance with the other "normal flora," such as lactobacilli
(which can also be harmed by using douches), an overgrowth of yeast can result in noticeable
symptoms. Pregnancy, the use of oral contraceptives, engaging in vaginal sex after anal sex in an
unhygienic manner, and using lubricants containing glycerin have been found to be causally related to
yeast infections. Diabetes mellitus and the use of antibiotics are also linked to an increased incidence of
yeast infections. Candidiasis can be sexually transmitted between partners. Diet has been found to be
the cause in some animals. Hormone Replacement Therapy and Infertility Treatment may be factors.


There are also cancer's of the female reproductive system, such as:
1. Cervical cancer
2. Ovarian cancer
3. Uterine cancer
4. Breast cancer


Endometriosis
Endometriosis is the most common gynecological diseases, affecting more than 5.5 million women


in North America alone! The two most common symptoms are pain and infertility. In this disease a
specialized type of tissue that normally lines the inside of the uterus,(the endometrium) becomes
implanted outside the uterus, most commonly on the fallopian tubes, ovaries, or the tissue lining the
pelvis. During the menstrual cycle, hormones signal the lining of the uterus to thicken to prepare for
possible pregnancy. If a pregnancy doesn't occur, the hormone levels decrease, causing the thickened
lining to shed.


When endometrial tissue is located in other parts it continues to act in it's normal way: It thickens,
breaks down and bleeds each month as the hormone levels rise and fall. However, because there's
nowhere for the blood from this mislocated tissue to exit the body, it becomes trapped and surrounding
tissue becomes irritated. Trapped blood may lead to growth of cysts. Cysts in turn may form scar tissue
and adhesions. This causes pain in the area of the misplaced tissue, usually the pelvis. Endometriosis
can cause fertility problems. In fact, scars and adhesions on the ovaries or fallopian tubes can prevent
pregnancy. Endometriosis can be mild, moderate or severe and tends to get worse over time without
treatment. The most common symptoms are:


1. Painful periods Pelvic pain and severe cramping, intense back pain and abdominal pain.
2. Pain at other times Women may experience pelvic pain during ovulation, sharp deep pain


in pelvis during intercourse, or pain during bowel movements or urination.
3. Excessive bleeding Heavy periods or bleeding between periods.
4. Infertility Approximately 30-40% of women


The cause of endometriosis remains mysterious. Scientists are studying the roles that hormones
and the immune system play in this condition. One theory holds that menstrual blood containing
endometrial cells flows back through the fallopian tubes, takes root and grows. Another hypothesis
proposes that the bloodstream carries endometrial cells to other sites in the body. Still another theory
speculates that a predisposition toward endometriosis may be carried in the genes of certain families.


Other researchers believe that certain cells present within the abdomen in some women retain their


320 | Human Physiology




The Female Reproductive System


ability to specialize into endometrial cells. These same cells were responsible for the growth of the
woman's reproductive organs when she was an embryo. It is believed that genetic or environmental
influences in later life allow these cells to give rise to endometrial tissue outside the uterus.


Experts estimate that up to one in ten American women of childbearing age have endometriosis.
There is some thinking that previous damage to cells that line the pelvis can lead to endometriosis.
There are several ways to diagnose endometriosis:


1. Pelvic exam
2. Ultrasound
3. Laparoscopy Usually used, most correct diagnosis
4. Blood test


Enometriosis can be treated with:
1. Pain medication
2. Hormone therapy


1. Oral contraceptives
3. Gonadotropin-releasing hormone(Gn-Rh)agonists and antagonists
4. Danazol(Danocrine)
5. Medroxyprogesterone(Depo-Provera)
6. Conservative surgery which removes endometrial growths.
7. Hysterectomy


Check Your Understanding
1. In homology, the ___________ in the female is equal to the penis in the male


A) labia majora
B) clitoral hood
C) clitoris
D) labia minora
E) none of the above


2. This contains some of the strongest muscles in the human body
A) uteras
B) clitoris
C) cervix
D) labia majora


3. This protects the vaginal and urethral openings
A) labia majora
B) labia minora
C) clitoris
D) urethra


Wikibooks | 321




Chapter 16
4. Sally has noticed that her cervical mucus has changed and now resembles egg whites- from this Sally
could assume


A) her period will begin soon
B) nothing, this is a normal occurance
C) she has a yeast infection
D) she is ovulating


5. Debbie recently went to the OBGYN and was diagnosed with PCOD (polycystic ovary syndrome)
because of this she has


A) nothing, its normal in women
B) antisperm antibodies
C) an overproduction of LH
D) leaking of milk from her mammary glands
E) problems becoming pregnant


6. Angie went to the doctor because she has had pain in her leg recently- this could be caused by
A) ovulation pain
B) her period that will be starting tomorrow
C) premenstrual syndrome
D) a blood clot resulting from her birth control pill


7. Sue recently started her period and has noticed that they are very heavy and painful, and that they are
inconsistent in their timing. One explanation could be


A) endometriosis
B) ovarian cancer
C) candidasis
D) toxic shock syndrome
E) amenorrhea


8. Mary is getting married and is not ready to become a mother- she chooses this birth control because
of its high effectiveness


A) natural family planning
B) a diaphragm
C) contraceptive injections
D) a spermicide foam


9. The release of LH in woman causes
A) menstration
B) ovulation
C) increase of endometrial lining
D) decrease of endometrial lining
E) nothing LH only does something in the male reproductive system


322 | Human Physiology




The Female Reproductive System


10. When the ovaries stop producing estrogen, this occurs
A) ovulation
B) implantation
C) premenstrual syndrome
D) menopause


Glossary
Adhesions: Abnormal tissue that binds organs together
Alveoli: Basic components of the mammary glands; lined with milk-secreting epithelial cells
Birth Control: regimen of one or more actions, devices, or medications followed in order to


deliberately prevent or reduce the likelihood of a woman becoming pregnant
Cervical Mucus: Mucus secreted by the cervix, near ovulation it helps to lower the acidity of the


vagina
Cervix: Lower, narrow portion of the uterus where it joins with the top of the vagina
Clitoris: Small body of spongy tissue that functions solely for sexual pleasure
Chromosomes: Structures in the nucleus that contain the genes for genetic expression
Ectocervix: Portion of the cervix projecting into vagina
Endocervical Canal: Passageway between the external os and the uterine cavity
Endometrium: The inner lining of the uterus
Fallopian Tubes: Located at the upper end of the vagina, passage way for the egg from the ovary
Factor V Leiden: This is the name given to a variant of human factor V that causes a


hypercoagulability disorder. In this disorder the Leiden variant of factor V, cannot be inactivated by
activated protein C. Factor V Leiden is the most common hereditary hypercoagulability disorder
amongst Eurasians. It is named after the city Leiden (The Netherlands), where it was first identified in
1994 by Prof R. Bertina et al.


Gamete: A haploid sex cell; either an egg cell or a sperm cell
Gene: That portion of the DNA of a chromosome containing the information needed to synthesize


a particular protein molecule
Gonad: A reproductive organ, testis or ovary that produces gametes and sex hormones
Hormone: A chemical substance produced in an endocrine gland and secreted into the


bloodstream that acts on target cells to produce a specific effect
Wikibooks | 323




Chapter 16
Hymen: Thin fold of mucous membrane that separates the lumen of the vagina from the urethral


sinus
Infertility: Inability to naturally conceive a child or the inability to carry a pregnancy to term
Labia Majora: Outer "lips" of the vulva, made of loose connective tissue and adipose tissue with


some smooth muscle
Labia Minora: Inner lips of the vulva, folds and protects the vagina, urethra and clitoris
Mammary Glands: Organs that produce milk for the sustenance of a baby
Meiosis: A specialized type of cell division by which gametes, or haploid sex cells, are formed
Menarche: The first menstrual discharge; occurs normally between the ages of 9 and 17
Menopause: The period marked by the cessation of menstrual periods in the human female
Menstrual Cycle: The rhythmic female reproductive cycle characterized by physical changes in


the uterine lining
Menstruation: The discharge of blood and tissue from the uterus at the end of menstrual cycle
Mittelschmerz: Pain near the lower abdomen site at the time of ovulation; German for ovulation


pain
Mons Veneris: soft mound at the front of the vulva (fatty tissue covering the pubic bone)
Ovarian Cycle: Last phase of the reproductive cycle; if no implantation occurs, causes the


breakdown of the endometrial lining and causes menstruation
Ovulation: The rupture of an ovarian follicle with the release of an ovum
Perineum: External region between the scrotum and the anus in a male or between the vulva and


anus in a female
Premenstrual Syndrome (PMS): Time leading up to menstruation; includes both physical and


emotional symptoms: acne, bloating, fatigue, backaches, sore breasts, headaches, constipation,
diarrhea, food cravings, depression, irritability, difficulty concentrating or handling stress


Puberty: The period of development in which the reproductive organs become functional and the
secondary sex characteristics are expressed


Reproduction: Process by which an organism continues its species
Sexually transmitted diseases (STDs): diseases or infections that have a significant probability of


transmission between humans by means of sexual contact
Urethra: Located below the clitoris, used for the passage of urine


324 | Human Physiology




The Female Reproductive System


Uterine Cycle: First part of the reproductive cycle; the time when the endrometrial lining builds
up and follicles develop


Uterus: Major reproductive organ, receives fertilized eggs which become implanted in the lining,
the lining (endometrium) provides nourishment to developing fetus; contains some of the strongest
muscles in the female body and is able to stretch during fetus development


Vagina: Muscular, hollow tube that extends from the vaginal opening to the cervix
Vulva: External female genitals, includes labia majora, labia minora, mons pubis, clitoris, meatus,


vaginal vestibule, vestibule bulbs and vestibular glands


References
• Essentials of Anatomy and Physiology. Fourth Edition. Valerie C. Scanlon and Tina


Sanders.
• Human Anatomy. Sixth Edition. Van De Graaff.
• Wikibook: Sexual Health
• http://www.fda.gov/cder/drug/infopage/planBQandAhtm
• http://www.goplanb.com/Forconsumers
• American Social Health Association;ashastd.org
• http://www.cdc.gov
• http://www.mayoclinic.com


live version • discussion • edit lesson • comment • report an error


Wikibooks | 325




Chapter 17


17 PREGNANCY AND BIRTH
live version • discussion • edit lesson • comment • report an error


Introduction
n this chapter we will discuss the topics covering pregnancy, from conception to birth. The chapter
will cover fertilization, implantation of the zygote, to becoming a fetus, the three trimesters, and the


progressive development of the fetus through the weeks of pregnancy. It will cover the topic of birth
and different birthing methods.
I
Fertilization


Fertilization is the joining of a sperm and an egg. A sperm is a male gamete that is released into the
vagina of a female during intercourse. In order for fertilization to occur there must be a mature ovum
present. Every month one of the ovaries releases an egg which will meet one of the A 4 million sperm
the male ejaculates into the vagina. The sperm swim through the cervix and into the uterus which lead
to the fallopian tubes. This is where fertilization is most likely to take place. The high amount of sperm
in the ejaculate is needed because only around 100 survive to enter reach the fertilization site. In order
to penetrate the egg the sperm must first break through 2 barriers surrounding the ovum. The
acromsome of sperm comes in contact with the corona radiata and releases digestive enzymes that
break down a gelatinous layer around the egg called, the zona pellucida. Once a sperm reaches the
plasma membrane of the egg it sets of a reaction that spreads across the membrane of the egg
preventing other sperm from breaking through the egg membrane. Once the sperm reaches the inside of
the egg it sheds its tail and the two nuclei fuse and now the 23 chromosomes from the egg and the 23
chromosomes of the sperm join and they become a zygote. Chromosomes contain all the information
needed to determine the genetic structure of the new baby. Normally all human beings have two
chromosomes that determine sex: A combination of X and Y makes a male or a combination of X and
X makes a female. All ovum have X sex chromosomes where as sperm have both X or Y sex
chromosomes. Therefore, the male gametes determine the sex of the baby.


Pre-embryonic Period
After fertilization, the zygote begins a process of dividing by mitosis in a process called cleavage.


It divides until it reaches 16 cells. It is now referred to as a morula. As the morula floats freely within
the uterus, it starts to bring nutrients into the cells. The morula fills with fluid and the cells inside start
to form two separate groups. At this stage it is now a blastocyte. The inner layer of cells is called the
embryoblast, and will become the fetus. The outer layer is called a trophoblast which will develop into
part of the placenta. At this point the zona pellucida is disintegrating. The trophoblast contains
specialized cells that become extensions, like fingers, that grow into the endometrium once in contact
with the well thickened endometrium.


326 | Human Physiology




Pregnancy and Birth


Implantation
The blastocyst preserves itself by secreting a hormone that indirectly stops menstruation. The


trophoblast cells secrete hCG hormones that help maintain the corpus luteum that would normally
regress. In turn, the corpus luteum continues to secrete progesterone, which maintains the endometrium
of the uterus in the secretory phase. This helps the blastocyst to continue to grow and stay embedded
within the endometrium. The fetal life support system and the placenta begin to form, and eventually
the placenta will take over the job of producing progesterone.


• Gastrulation and Formation
The embryoblast within the blastocyst forms 3 primary germs layers: ectoderm, mesoderm, and


endoderm.


Ectoderm
This forms the nervous tissue and the epithelium covering the outer body surface. Epidermis of


skin, including hair and nails, glands of skin, linings of oral cavity, nasal cavity, anal canal, vagina,
brain, spinal cord, sensory organs, lens of eye and epithelium of conjunctiva (a membrane that covers
the sclera and lines the inside of the eyelids), pituitary gland, adrenal medulla, and enamel of teeth.


Mesoderm
This forms all of the muscle tissue and the connective tissue of the body, as well as the kidneys


and the epithelium of the serous membranes and blood vessels. All muscle tissue (skeletal, smooth,
cardiac), all connective tissue (fibrous connective tissue, bone, blood, cartilage), dentin of teeth,
adrenal cortex, kidneys and ureters, internal reproductive viscera, epithelium lining vessels, joint
cavities, and the serous body cavities.


Endoderm
Forms the lining epithelium and glands of the visceral body systems. Lining epithelium and glands


of digestive, respiratory, and parts of urogenital systems, thyroid and parathyroid glands, and thymus.


Formation of Placenta & Amniotic Sac
The endometrium makes changes. Cellular growth and accumulation of glycogen occur causing


fetal tissue and maternal tissue to come together. This formation makes the functional unit called the
placenta. The placenta envelops the entire fetus. It provides protection from harmful substances. The
amniotic sac contains amniotic fluid. Amniotic fluid is the watery liquid surrounding and cushioning a
growing fetus within the amnion. It allows the fetus to move freely without the walls of the uterus
being too tight against its body. Buoyancy is also provided here for comfort.


Wikibooks | 327




Chapter 17


Amniotic Fluid
The amnion grows and begins to fill, mainly with water, around two weeks after fertilization. After


a further 10 weeks the liquid contains proteins, carbohydrates, lipids and phospholipids, urea and
electrolytes, all which aid in the growth of the fetus. In the late stages of gestation much of the amniotic
fluid consists of fetal urine.


The forewaters are released when the amnion ruptures, commonly known as when a woman's
"waters break" or "spontaneous rupture of membranes" (SROM). The majority of the hindwaters
remain inside the womb until the baby is born.


Endocrine Function of the Placenta
There are pituitary like hormones and steroid hormones secreted from the placenta. The pituitary


like hormones are hCG and hCS. hCG is similar to LH and helps maintain the mothers corpus luteum.
hCS is like prolactin and growth hormone and help aid in increasing fat breakdown that spares the use
of glucose from the mothers tissues. This effect leaves more glucose available to the placenta and the
fetus for necessary growth. The steroid hormone are progesterone and estrogen. Progesterone helps
maintain the endrometrium and supports the growth of mammary glands. Estrogen also helps maintain
the endrometrium and growth of mammary glands. It also inhibits prolactin secretion.


Developing Baby
The womb is expanding, the baby is growing and taking all the nourishment from the mother.


What once started as a microscopic two-celled egg, will be formed into a baby in just 12 weeks. The
baby develops from conception to term, in a month-to-month progress.


Overview of Developmental Milestones
WEEK CHANGES IN MOTHER DEVELOPMENT OF BABY


Pre-embryonic Development
1 week Ovulation Occurs Fertilization occurs, cell division begins and continues, chorion appears


Embryonic Development


2 weeks
Symptoms of early pregnancy
(nausea, breast swelling and
tenderness, fatigue); blood pregnancy
tests may show positive


Implantation occurs; amnion and yolk sac appear;
embryo has tissue; placenta begins to form


3 weeks
First period missed; urine pregnancy
test may show positive; early
pregnancy symptoms continue


Nervous system begins to develop; allantois and
blood vessels are present and placenta is well formed


4 weeks
Limb buds form; heart is beating; nervous system
further develops; embryo has tail; other systems are
forming


328 | Human Physiology




Pregnancy and Birth


5 weeks Uterus is the size of a hen's egg; mother may need to urinate frequently
Embryo is curved, head is large, limb buds are
showing division, nose, ears and eyes are noticeable


6 weeks Uterus is the size of an orange Fingers and toes are present and skeleton is cartilaginous


8 weeks Uterus can be felt above the pubic bone
Fetus begins to look human; limbs are developing
and major organs forming; facial features are
becoming refined
Fetal Development


12
weeks Uterus is the size of a grapefruit


Head grows faster than the rest of the body; facial
features are apparent, but there is no layer of fat yet
and the skin is translucent; gender can be
distinguished via ultrasound; fingernails appear


16
weeks Fetal movement can be felt


Fine hair (lanugo) grows over the body; fetus
resembles a tiny human being; skeleton is visible


20-22
weeks


Uterus reaches up to the level of
umbilicus and pregnancy is obvious


Vernix caseosa, the protective fatty coating, begins to
be deposited; heartbeat can be heard


24
weeks


Doctor can tell where baby's head,
back and limbs are; breasts have
enlarged and nipples and areola are
darker, colostrum is produced


Fully formed but still thin; much larger and very
active, all major organs are working, the lungs and
digestive system need more time to develop; body is
covered in fine hair called lanugo


32
weeks


Uterus reaches halfway between
umbilicus and rib cage


Most babies are in a head down position in the
womb; head is more in proportion to the body; eyes
are open; babies born at this stage have a good chance
of living


36
weeks


Weight gain is averaging about a
pound a week; standing ans walking
are becoming very difficult because
the center of gravity is thrown
forward


Body hair begins to disappear, fat is being deposited


40
weeks


Uterus is up to the rib cage, causing
shortness of breath and heartburn;
sleeping is very difficult


Not much room to move in the womb; fully mature,
baby moves less, and the surrounding fluid reduces
and the womb expands its limits


Embryonic Development at Specific Stages
First trimester


An embryo this tiny shows very distinct anatomic features, including tail, limb buds, heart (which
actually protrudes from the chest), eye cups, cornea/lens, brain, and prominent segmentation into
somites. The gestational sac is surrounded by a myriad of chorionic villi resembling elongate party
balloons. This embryo is about five weeks old (or seven weeks in the biologically misleading but
eminently practical dating system used in obstetrics).


Wikibooks | 329




Chapter 17
4 Weeks


• There are only the beginnings of facial features. All the major organs are starting to form.
Gill-like folds that develop into facial features, beginnings of the spinal cord, skin is translucent,
and rudimentary (basic; minimal) heart develops.


6 Weeks
• The length from crown to rump is about the size of a finger tip, ¾ ". The beginnings of all


the major organs will have formed.
• The embryo floats in a fluid filled bubble that will develop into the amniotic sac. The sac is


covered by a protective layer of cells, called chorion. The yolk sac supplies the embryo with all
its nutrients until the placenta is fully developed and takes over at around the twelfth week.
During the first 12 weeks, the embryo will develop features and major organs of a human being.
The embryo is susceptible to harmful environmental influences. This is a vital time for the
embryo to develop healthily; taking supplements of folic acid, avoiding certain foods, and
eliminating alcohol, cigarettes, and any unnecessary drugs or medicines.


9 Weeks
• The length from crown to rump approximately 1 1/4". The facial features are becoming


more distinct, and the “tail” has disappeared. The muscles are also developing. Eyes are formed
but eyelids are still closed over them. Arms now bend at the elbow and rudimentary hands and
fingers develop. Knees will have formed and developing feet with distinct toes.


• Heart- is now a four-chambered and fully formed organ; it beats about 180 times per
minute.


• Brain and nervous system- is four times the size it was at 6 weeks. Special glial cells are
being formed within the neural tube; they allow nerve cells to be joined so that messages can be
transmitted from the brain to the body.


• Digestive system- the mouth, intestine, and stomach are developing very rapidly, but do not
function yet.


• The fetal life-support system- the placental tissue that initially surrounds the fetus and the
amniotic sac is becoming concentrated in one circular area on the womb wall to form the
placenta.


12 Weeks
• At twelve weeks the fetus looks like a tiny human. It is about 2 1/2" long and weighs 1/2


oz. Arms and legs are now beginning to move. Skin is red and translucent. Fingers and toes are
more defined, and nails are starting to grow.


• Heart is complete and working, pumping blood to all parts of the body. Digestive system
has formed and is linked to the mouth and intestines. Sexual organs have formed inside the
body, but cannot yet establish the sex of the baby.


330 | Human Physiology




Pregnancy and Birth


Second Trimester


20 Weeks
• By 20 weeks the fetus will be about 6 1/3" long and weighs 12 oz. Movements are for more


coordinated. The sexual organs are well developed and are usually visible on ultra sound.
• The fetus is growing very quickly. At this stage, the mother should feel the movements of


the fetus. Movements are more noticeable as the fetus's leg bones achieve their final relative
proportions in a process called quickening. Quickening is the process of muscles contracting
that cause movement at the fetus's sinovial joints. The joint movement enhances the nutrition of
the articular cartilage and prevents the fusion of connective tissues within the joint. It also
promotes bone hardening.


• From now on, the fully developed placenta will provide all the fetus' needs until birth;
oxygen, nutrients and protective antibodies.


Third Trimester


29 Weeks
• By 29 weeks the baby is about 10" long and weighs about 2 lbs. 7 oz.
• The brain grows much larger, and fatty protective sheath covers the nerve fibers; this


important development allows brain impulses to travels faster, enhancing the ability to learn.
The lungs have developed most of their airways and air sacs. The placenta is quite selective in
what it allows to pass from the mother to the baby's blood, stopping some harmful substances,
such as certain drugs, from crossing over.


40 Weeks
• The baby is now ready to be born. When the head of the baby moves down from high in the


mother's abdomen and settles deeper into her pelvis in preparation for birth, it is called
engagement. This can happen any time between 36 weeks and labor.


• In the last four weeks of pregnancy the baby puts on a lot of weight and develops a thick
layer of fat. All organs are completely formed and functioning.


Umbilical Cord
This is the life support for a growing embryo. The umbilical cord stretches between the placenta


and the fetus. This cord contains the umbilical arteries and vein. The umbilical cord forms by week 5 of
conception. The average cord is close to 22 inches long and may have the appearance of a coil. The
umbilical cord is very rich in stem cells and is often used for parents who choose to store their stem
cells in a blood bank or donate it to a blood bank. These stem cells can be used to treat over 45
disorders and is an alternative from extracting the stem cells from a donor.


• Umbilical Arteries


Wikibooks | 331




Chapter 17
The exchange of gases, nutrients and oxygen takes place between the maternal blood and fetal


blood. There are 2 main arteries.
• Umbilical Vein


Vein that carries nutrients and oxygen away from the placenta to the growing fetus. It also carries
rich blood. There is only 1 main vein.


• Fetus doesn't use its lungs for gas exchange.


Umbilical Abnormalities
• Single Umbilical Artery


One artery instead of two will result in chromosomal abnormalities. Some of these defects include
poor fetal growth, preterm delivery, and still births. This can be detected by a routine ultrasound. If an
ultrasound is done and no other complications or abnormalities are detected, the baby will usually be
born healthy.


• Umbilical Prolapse
This condition usually happens when a cord is too long. The baby may be born prematurely or will


be breech.
• Umbilical Nuchal Loops


This condition happens when the umbilical cord is wrapped around the baby's head at least one or
more times. This can be detected when a baby is in stress or by a simple ultrasound. In most cases the
mother will have a cesarean delivery. In other cases the cord may be wrapped around the hands or feet.


• Vasa Previa
This occurs in one in every 3,000 births, which can become life threatening for the unborn baby.


This complication happens when the umbilical cord inserts abnormally in the fetal membranes of the
placenta, which appears abnormally shaped or positioned. Major risks include unprotected fetal blood
vessels cross the cervix, oftentimes rupturing the membranes. Also, lack of blood pressure due from
pressure, causes the loss of oxygen to the baby. Women who will be at risk for this would be those who
already have experienced placenta previa or have used in vitro fertilization.


• Umbilical Cord Knots
About 1% of babies are born with one or more knots in their umbilical cord. Some knots happen


during labor; others happen from moving around in the womb. Most knots occur when the umbilical
cord is too long. In some cases the knots can become tight, cutting off the oxygen supply to the baby.
Cord knots result in miscarriages and stillbirth in 5% and 10% of most cases. Most will require a
cesarean delivery.


• Umbilical Clotting


332 | Human Physiology




Pregnancy and Birth


This is more common with genetic defects, such as Factor V Leiden. This complication will
prevent blood flow to and from the baby and many times will cause the placenta to also clot and die. If
this is not caught early enough, the baby will die of starvation in the womb. A simple ultrasound can
determine if there are problems with the blood flow.


Pregnancy from the mother's perspective
The first sign you may be pregnant is missing your period. This is because the blastocyte that is in


your uterus has special cells that release the hormone hCG (beta-human chorion gonadotrophin). HCG
is the hormone used in a home pregnancy test to determine if a woman is pregnant. If it is positive, the
woman should follow up with a visit to the doctor in which a blood sample will be taken for
confirmation.


Pregnancy is the carrying of one or more embryos, or fetuses, by female mammals including
humans, inside their bodies. In a pregnancy, there can be multiple gestations (for example, in the case
of twins, or triplets). Human pregnancy is the most studied of all mammalian pregnancies.


Human pregnancy lasts approximately 9 months between the time of the last menstrual cycle and
childbirth (38 weeks from fertilization). The medical term for a pregnant woman is genetalian, just as
the medical term for the potential baby is embryo (early weeks) and then fetus (until birth). A woman
who is pregnant for the first time is known as a primigravida or gravida 1: a woman who has never
been pregnant is known as a gravida 0; similarly, the terms para 0, para 1 and so on are used for the
number of times a woman has given birth.


In many societies' medical and legal definitions, human pregnancy is somewhat arbitrarily divided
into three trimester periods, as a means to simplify reference to the different stages of fetal
development. The first trimester period carries the highest risk of miscarriage (spontaneous death of
embryo or fetus). During the second trimester the development of the fetus can start to be monitored
and diagnosed. The third trimester marks the beginning of viability, which means the fetus might
survive if an early birth occurs.


Changing Body
As soon as a woman becomes pregnant, her body begins to change so that it can support both


herself and the unborn baby. All of the body functions start to work much harder. The heart has to
pump more blood around the body, in particular to the womb, placenta, and the fetus. As well as
physical demands, pregnancy also causes a range of emotional reactions.


• The first trimester, the first twelve weeks, little is visible.
• The second trimester, 13-27 Weeks, the waistline is rapidly growing, the abdomen becomes


noticeably pregnant.
• The third trimester, 28-40 weeks, the body expands rapidly and the womb enlarges and


presses against the diaphragm.


Wikibooks | 333




Chapter 17
First Trimester


In the early weeks the mother is likely to be more tired. Most expectant mothers are still in shock!
As your uterus begins to grow your "bump" begins to be more noticeable. This is a good time to start
looking into options on birthing and doctors.


• Physical feelings: tiredness, nausea, constipation, frequent urination, food cravings, change
in size of breasts, fainting or dizziness, bloated stomach, and high emotions.


Second Trimester
The mother will probably be feeling full of energy and excitement.


• Physical feelings: More energy, constipation, heartburn, and indigestion. The breasts
continue to grow, as does an increase in appetite. There is mild swelling in the feet,
ankles, hands, and face. There is also more baby movement. There may be emotional ups
and downs in the feeling of pregnancy, and short-term memory may be poor.


• The hormones estrogen, progesterone, human placental lactogen, oxytocin, and prolactin
prepare the body for feeding the baby, and cause the breasts to enlarge, becoming painful
and tender.


• The fetus, placenta, and amniotic fluid account for just over a third of the weight gain
during pregnancy. The remaining weight comes from increased blood volume, fluid
retention, and extra body fat. The suggested weight gain in most pregnancies is between
25-35 lbs.


Third Trimester
Physical feelings


Shortness of breath, tiredness, difficulty in moving and sleeping, and frequent urination. The
emotional mood swings ease off, but the mother begins to feel less enthusiastic about being
pregnant. She may become impatient and restless and just wants for the birth to be over.


• The body is changing to cope with the ever increasing size of the womb. The baby grows
and pushes out the lower back of the mother. The breathing rate of the baby is growing
very quickly. At this stage, the mother should feel the movements of the fetus. Other signs
may be the nipples secreting colostrum, Braxton-Hicks' contractions may begin, and
blood flow to the womb has increased tenfold since conception.


Prenatal Care
Once the female confirms her pregnancy, she will need to find out her physical condition and what


to expect in the coming months. Prenatal tests, prenatal care, and what type of birthing methods, are
many options that she will need to decide what is best for her.


• Prenatal Visits
The main purpose of the visit is to troubleshoot for potential problems. A series of tests will be


334 | Human Physiology




Pregnancy and Birth


most critical in the first weeks such as: mother's history, urine tests, weighing in, blood tests, physical
examination, blood pressure, prenatal monitoring, and ultrasound scans.


Labor and Birth
Labor is defined as contractions and cervical change, contractions alone are not labor.


• Pre-Labor Signs: as your body is preparing for labor, there are a few things that should be
expected to happen within four to six weeks of labor.
1. Pressure on the pelvic area
2. Occasional brownish discharge
3. Energy level is noticeably increasing or decreasing
4. Loss of the mucus plug (does not always exist)/increasing discharge
5. Braxton Hicks contractions (painless contraction of the uterus)
6. Movement of the baby into the pelvis
• False Labor Signs: there are a few signs that indicate false labor.
1. Timing of the contractions are irregular and do not become more frequent
2. Contractions stop during rest, stopping what the mother is doing, walking, or changing


position
3. Inconsistent in strength (strong one minute then weak the next)
4. Location of pain is in the front only
• True Labor
1. Pain in the lower back, radiating towards the front abdomen, possibly also the legs
2. Contractions increase in strength and are closer together; coming now on a regular basis, 30


to 70 seconds apart
3. The mucous plug is detached, showing bloody discharge
4. The water breaks (usually this does not break until the doctor does it), when this happens,


contractions become much stronger
5. Some women have the sudden need to go to the bathroom, diarrhea is common
6. Contractions continue despite movement
7. The cervix is thinning and dilating


When the contractions of labor begin, the walls of the uterus start to contract. They are stimulated
by the release of the pituitary hormone oxytocin. The contractions cause the cervix to widen and begin
to open. As labor progresses the amniotic sac can rupture causing a slow or a fast gush of fluids. Labor
usually begins within a 24 hour period after the amniotic sac has ruptured. As contractions become
closer and stronger the cervix will gradually start to dilate. The first stage of labor is broken into three
parts:


• Latent Phase First is the latent phase of labor, when the cervix dilates from 1-4
centimeters, this can be the longest and most exhausting part for the mother.


• Active Phase The cervix dilates on average 1 cm per hour in the active phase of labor


Wikibooks | 335




Chapter 17
dilating from 4-8 centimeters. If am epidural is requested it is usually given in this phase.


• Transition In this phase the mother becomes "complete" meaning to dilation to
approximately 10 cm and full effacement. The baby begins to move through the cervix into the
birth canal.


At this point the labor enters the second stage, or the delivery of the baby. The mother begins
pushing to aid in the birth of the baby, this part of labor can last minutes, or even hours. A fetus usually
delivered head first. 'Crowning' is the term used when the fetus' head can be seen between the mothers
labia as it emerges. At this point if necessary the birth attendant may perform an episiotomy, which is a
small surgical incision in the peritoneum. This procedure is usually done to avoid severe tearing of the
mother, or to aid in the speed of the delivery.


The third stage of labor is the delivery of the afterbirth (placenta).
Oxytocin continues to be released to shrink the size of the uterus and aid in the limiting of blood


loss from the site of the placenta. As the uterus shrinks the attachment site blood vessels, some of
which can be as large as an adult finger, shrink also. The average blood loss in a routine vaginal
delivery is 400-500 cc.


There are times when a mother may need outside aid in the delivery of the baby, some of these
methods include:


• Forceps, an instrument used to cradle the fetus' head and manipulate the head under the
pubic bone to more easily pass through the birth canal.


• Vacuum Extraction, a suction cup is applied to the baby's head, and a plunger is used to
suck any air from between the suction cup and the head to create a good seal. The babies head is
then manipulated through the birth canal. This usually leaves a baby's head bruised, but the
mark fades within weeks after birth.


• Cesarean section, or C-section, is the delivery of a baby through a surgical abdominal
incision (Abdominal delivery - Abdominal birth - Cesarean section). A C-section delivery is
performed when a vaginal birth is not possible or is not safe for the mother or child. Surgery is
usually done while the woman is awake but anesthetized from the chest to the legs by epidural
or spinal anesthesia. An incision is made across the abdomen just above the pubic area. The
uterus is opened, and often brought through the incision after delivery for better visualization.
The amniotic fluid is drained, and the baby is delivered. The baby's mouth and nose are cleared
of fluids, and the umbilical cord is clamped and cut. After delivery a nursery nurse or
pediatrician check the make sure that the baby is breathing and responding. Due to a variety of
medical and social factors, C-sections have become fairly common; around 25% of births are
performed by C-section. C-sections carry some risks to mother and baby. Compared to a vaginal
birth, the risks to mother include increased risk of death, surgical injury, infection, postpartum
depression, and hemorrhage, although these are rare. Babies born by c-section are more likely to
be admitted to the ICU for breathing problems. Mothers are advised to carefully weigh the risks
of C-section versus vaginal birth.


336 | Human Physiology




Pregnancy and Birth


Delivery Options
Hospital Births


The chances of having natural, uncomplicated birth are optimized by carefully selecting your
obstetrician and hospital. Doctors who work with midwives have lower cesarean section rates
because midwives handle less complicated pregnancies. Delivering babies by abdominal surgery
has been steadily rising in America over the past two decades, so that now 22-30% of births in
American hospitals are cesarean section. The U.S., despite having the most advanced technology
and highly trained medical personnel, ranks 23rd in infant mortality and 18th in perinatal
mortality.
Medical interventions such as epidural anesthesia, pitocin augmentation of labor, vacuum
extraction of fetus, episiotomy and separation of newborn and mother are common in American
hospitals. There are circumstances where medical procedures such as these are necessary, but
many parents and professionals now question the routine use of such interventions. In some
cases, the routine use of these procedures have lead to further complications. For example, the
epidural anesthetic, while providing pain relief, has shown to increase the operative vaginal
delivery rate (i.e. forceps and vacuum extraction rates slightly) especially in first time mothers.
Epidurals have not been shown to increase the cesarean section rate in recent well documented
studies.


Freestanding Birth Centers & Water Birth
"Freestanding" Birth Centers are not inside of or affiliated with a hospital. They are run by
collaboration of midwives or physicians. This is an alternative choice for the woman who does
not wish to birth in a hospital environment yet is not comfortable giving birth at home. Birth
centers do not provide any additional measure of safety than most planned home births with
qualified midwives; they may provide the expectant couple with the physiological comfort
necessary to enable the mother to relax.
Out of hospital birth centers are designed for women having low-risk pregnancies who want
drug-free birth with minimal intervention in a home-like environment. Family members may
participate in the birth. C-sections rates are lower than most hospitals because the pregnancies are
low risk. Freestanding Birth Centers are an alternative choice for a woman who has had a
previous cesarean and wishes to maximize her chances of a vaginal delivery. However, vaginal
birth attempts after a prior cesarean section have a 1-2% risk of uterine rupture. Heath insurance
may cover costs. Many birth centers offer birthing tubs where one can give birth in water.


Homebirth
Birth at home provides parents with intimacy, privacy, comfort and family-centered experience.
Childbirth at home may be a safe option for healthy women having normal pregnancies. It is for
those who have a very strong desire for natural childbirth and who are willing to take high degree
of responsibility for their health care and baby's birth. At home, the parents and midwife are in
control of the birthing environment, and strict time perimeters for length of labor are not
imposed, or routine medical interventions such as IVs done. However, the World Health
Organization (WHO) states that "giving birth in a health facility (not necessarily a hospital) with
professional staff is safer by far than doing so at home." (The World Health Report 2005). Also,
the American College of Obstetricians and Gynecologists (ACOG) opposes out of hospital births.
In choosing the comfort of home parents are also choosing to be further away from lifesaving
measures should complications arise.


Wikibooks | 337




Chapter 17
Homebirth midwives provide complete prenatal care including monthly visits, laboratory tests,
screening for infections. They provide nutritional counseling and support for psycho-social
issues. There is a chance that a rare, but critical emergency might occur during the birth where
hospital services may not be able to be obtained quick enough. Again, the WHO states that "it is
just before, during, and in the very first hours and days after birth that life is most at risk," (The
World Health Report 2005) and that "many of the complications that result in maternal deaths
and many that contribute to perinatal deaths are unpredictable, and their onset can be both sudden
and severe." (WHO B