Organ Notes

Anatomy of the liver:
The liver is located in the upper right-hand portion of the abdominal
cavity, beneath the diaphragm and on top of the stomach, right
kidney, and intestines. The liver, a dark reddish-brown organ that
weighs about 3 pounds, has multiple functions.
There are two distinct sources that supply blood to the liver:
Oxygenated blood flows in from the hepatic artery.
Nutrient-rich blood flows in from the hepatic portal vein.
The liver holds about 13 percent of the body's blood supply at any
given moment.
The liver consists of two main lobes, both of which are made up of
thousands of lobules. These lobules are connected to small ducts
that connect with larger ducts to ultimately form the hepatic duct. The
hepatic duct transports bile produced by the liver cells to the
gallbladder and duodenum (the first part of the small intestine).
What are the functions of the liver?
The liver regulates most chemical levels in the blood and excretes a
product called bile, which helps to break down fats, preparing them
for further digestion and absorption. All of the blood leaving the
stomach and intestines passes through the liver. The liver processes
this blood and breaks down the nutrients and drugs in the blood into
forms that are easier to use for the rest of the body. More than 500
vital functions have been identified with the liver. Some of the more
well-known functions include the following:
 production of bile, which helps carry away waste and break
down fats in the small intestine during digestion
 production of certain proteins for blood plasma
 production of cholesterol and special proteins to help carry fats
through the body
 conversion of excess glucose into glycogen for storage (This
glycogen can later be converted back to glucose for energy.)
 regulation of blood levels of amino acids, which form the
building blocks of proteins
 processing of hemoglobin for use of its iron content (The liver
stores iron.)
 conversion of poisonous ammonia to urea (Urea is one of the
end products of protein metabolism that is excreted in the
 clearing the blood of drugs and other poisonous substances
 regulating blood clotting
 resisting infections by producing immune factors and removing
bacteria from the bloodstream
When the liver has broken down harmful substances, they are
excreted into the bile or blood. Bile by-products enter the intestine
and ultimately leave the body in the feces. Blood by-products are
filtered out by the kidneys and leave the body in the form of urine.
The kidneys are a pair of vital organs that perform many functions to
keep the blood clean and chemically balanced. Understanding how
the kidneys work can help a person keep them healthy.
What do the kidneys do?
The kidneys are bean-shaped organs, each about the size of a fist.
They are located near the middle of the back, just below the rib cage,
one on each side of the spine. The kidneys are sophisticated
reprocessing machines. Every day, a person's kidneys process about
200 quarts of blood to sift out about 2 quarts of waste products and
extra water. The wastes and extra water become urine, which flows
to the bladder through tubes called ureters. The bladder stores urine
until releasing it through urination.
Wastes in the blood come from the normal breakdown of active
tissues, such as muscles, and from food. The body uses food for
energy and self-repairs. After the body has taken what it needs from
food, wastes are sent to the blood. If the kidneys did not remove
them, these wastes would build up in the blood and damage the
The actual removal of wastes occurs in tiny units inside the kidneys
called nephrons. Each kidney has about a million nephrons. In the
nephron, a glomerulus-which is a tiny blood vessel, or capillaryintertwines with a tiny urine-collecting tube called a tubule. The
glomerulus acts as a filtering unit, or sieve, and keeps normal
proteins and cells in the bloodstream, allowing extra fluid and wastes
to pass through. A complicated chemical exchange takes place, as
waste materials and water leave the blood and enter the urinary
At first, the tubules receive a combination of waste materials and
chemicals the body can still use. The kidneys measure out chemicals
like sodium, phosphorus, and potassium and release them back to
the blood to return to the body. In this way, the kidneys regulate the
body's level of these substances. The right balance is necessary for
In addition to removing wastes, the kidneys release three important
 erythropoietin, or EPO, which stimulates the bone marrow to
make red blood cells
 renin, which regulates blood pressure
 calcitriol, the active form of vitamin D, which helps maintain
calcium for bones and for normal chemical balance in the body
What is renal function?
The word "renal" refers to the kidneys. The terms "renal function" and
"kidney function" mean the same thing. Health professionals use the
term "renal function" to talk about how efficiently the kidneys filter
blood. People with two healthy kidneys have 100 percent of their
kidney function. Small or mild declines in kidney function-as much as
30 to 40 percent-would rarely be noticeable.
Some people are born with only one kidney but can still lead normal,
healthy lives. Every year, thousands of people donate one of their
kidneys for transplantation to a family member or friend. For many
people with reduced kidney function, a kidney disease is also present
and will get worse. Serious health problems occur when people have
less than 25 percent of their kidney function. When kidney function
drops below 10 to 15 percent, a person needs some form of renal
replacement therapy—either blood-cleansing treatments called
dialysis or a kidney transplant-to sustain life.
The heart is an amazing organ. It continuously pumps oxygen and
nutrient-rich blood throughout the body to sustain life. This fist-sized
powerhouse beats (expands and contracts) 100,000 times per day,
pumping five or six quarts of blood each minute, or about 2,000
gallons per day.
How Does Blood Travel Through the Heart?
As the heart beats, it pumps blood through a system of blood vessels,
called the circulatory system. The vessels are elastic, muscular tubes
that carry blood to every part of the body.
Blood is essential. In addition to carrying fresh oxygen from the lungs
and nutrients to the body's tissues, it also takes the body's waste
products, including carbon dioxide, away from the tissues. This is
necessary to sustain life and promote the health of all parts of the
There are three main types of blood vessels:
Arteries. They begin with the aorta, the large artery leaving the
heart. Arteries carry oxygen-rich blood away from the heart to
all of the body's tissues. They branch several times, becoming
smaller and smaller as they carry blood further from the heart
and into organs.
Capillaries. These are small, thin blood vessels that connect the
arteries and the veins. Their thin walls allow oxygen, nutrients,
carbon dioxide, and other waste products to pass to and from
our organ's cells.
Veins. These are blood vessels that take blood back to the heart;
this blood has lower oxygen content and is rich in waste
products that are to be excreted or removed from the body.
Veins become larger and larger as they get closer to the heart.
The superior vena cava is the large vein that brings blood from
the head and arms to the heart, and the inferior vena cava
brings blood from the abdomen and legs into the heart.
This vast system of blood vessels -- arteries, veins, and capillaries -is over 60,000 miles long. That's long enough to go around the world
more than twice!
Blood flows continuously through your body's blood vessels. Your
heart is the pump that makes it all possible.
Where Is Your Heart and What Does It Look Like?
The heart is located under the rib cage, slightly to the left of your
breastbone (sternum) and between your lungs.
Looking at the outside of the heart, you can see that the heart is
made of muscle. The strong muscular walls contract (squeeze),
pumping blood to the rest of the body. On the surface of the heart,
there are coronary arteries, which supply oxygen-rich blood to the
heart muscle itself. The major blood vessels that enter the heart are
the superior vena cava, the inferior vena cava, and the pulmonary
veins.The pulmonary artery and the aorta exit the heart and carry
oxygen-rich blood to the rest of the body.
Where Is Your Heart and What Does It Look Like? continued...
On the inside, the heart is a four-chambered, hollow organ. It is
divided into the left and right side by a muscular wall called the
septum. The right and left sides of the heart are further divided into
two top chambers called the atria, which receive blood from the veins,
and two bottom chambers called ventricles, which pump blood into
the arteries.
The atria and ventricles work together, contracting and relaxing to
pump blood out of the heart. As blood leaves each chamber of the
heart, it passes through a valve. There are four heart valves within
the heart: Mitral valve, Tricuspid valve, Aortic valve, Pulmonic valve
The tricuspid and mitral valves lie between the atria and ventricles.
The aortic and pulmonic valves lie between the ventricles and the
major blood vessels leaving the heart.
The heart valves work the same way as one-way valves in the
plumbing of your home. They prevent blood from flowing in the wrong
Each valve has a set of flaps, called leaflets or cusps. The mitral
valve has two leaflets; the others have three. The leaflets are
attached to and supported by a ring of tough, fibrous tissue called the
annulus. The annulus helps to maintain the proper shape of the
The leaflets of the mitral and tricuspid valves are also supported by
tough, fibrous strings called chordae tendineae. These are similar to
the strings supporting a parachute. They extend from the valve
leaflets to small muscles, called papillary muscles, which are part of
the inside walls of the ventricles.
How Does Blood Flow Through the Heart?
The right and left sides of the heart work together. The pattern
described below is repeated over and over, causing blood to flow
continuously to the heart, lungs, and body.
Right Side of the Heart
Blood enters the heart through two large veins, the inferior and
superior vena cava, emptying oxygen-poor blood from the body
into the right atrium of the heart.
As the atrium contracts, blood flows from your right atrium into your
right ventricle through the open tricuspid valve.
When the ventricle is full, the tricuspid valve shuts. This prevents
blood from flowing backward into the atria while the ventricle
As the ventricle contracts, blood leaves the heart through the
pulmonic valve, into the pulmonary artery and to the lungs
where it is oxygenated.
Left Side of the Heart
The pulmonary vein empties oxygen-rich blood from the lungs into
the left atrium of the heart.
As the atrium contracts, blood flows from your left atrium into your
left ventricle through the open mitral valve.
When the ventricle is full, the mitral valve shuts. This prevents blood
from flowing backward into the atrium while the ventricle
As the ventricle contracts, blood leaves the heart through the aortic
valve, into the aorta and to the body.
How Does Blood Flow Through Your Lungs?
Once blood travels through the pulmonic valve, it enters your lungs.
This is called the pulmonary circulation. From your pulmonic valve,
blood travels to the pulmonary artery to tiny capillary vessels in the
Here, oxygen travels from the tiny air sacs in the lungs, through the
walls of the capillaries, into the blood. At the same time, carbon
dioxide, a waste product of metabolism, passes from the blood into
the air sacs. Carbon dioxide leaves the body when you exhale. Once
the blood is purified and oxygenated, it travels back to the left atrium
through the pulmonary veins.
What Are the Coronary Arteries of the Heart?
Like all organs, your heart is made of tissue that requires a supply of
oxygen and nutrients. Although its chambers are full of blood, the
heart receives no nourishment from this blood. The heart receives its
own supply of blood from a network of arteries, called the coronary
Two major coronary arteries branch off from the aorta near the point
where the aorta and the left ventricle meet:
Right coronary artery supplies the right atrium and right ventricle
with blood. It branches into the posterior descending artery,
which supplies the bottom portion of the left ventricle and back
of the septum with blood.
Left main coronary artery branches into the circumflex artery and
the left anterior descending artery. The circumflex artery
supplies blood to the left atrium, side and back of the left
ventricle, and the left anterior descending artery supplies the
front and bottom of the left ventricle and the front of the septum
with blood.
These arteries and their branches supply all parts of the heart muscle
with blood.
Coronary artery disease occurs when plaque builds up in the
coronary arteries and prevents the heart from getting the enriched
blood it needs. If this happens, a network of tiny blood vessels in the
heart that aren't usually open called collateral vessels may enlarge
and become active. This allows blood to flow around the blocked
artery to the heart muscle, protecting the heart tissue from injury.
How Does the Heart Beat? The atria and ventricles work together,
alternately contracting and relaxing to pump blood through your heart.
The electrical system of the heart is the power source that makes this
possible. Your heartbeat is triggered by electrical impulses that travel
down a special pathway through the heart.
The impulse starts in a small bundle of specialized cells called the
SA node (sinoatrial node), located in the right atrium. This node
is known as the heart's natural pacemaker. The electrical
activity spreads through the walls of the atria and causes them
to contract.
A cluster of cells in the center of the heart between the atria and
ventricles, the AV node (atrioventricular node) is like a gate that
slows the electrical signal before it enters the ventricles. This
delay gives the atria time to contract before the ventricles do.
The His-Purkinje network is a pathway of fibers that sends the
impulse to the muscular walls of the ventricles, causing them to
At rest, a normal heart beats around 50 to 99 times a minute.
Exercise, emotions, fever, and some medications can cause your
heart to beat faster, sometimes to well over 100 beats per minute.
It's important to understand the complexity of the human brain. The
human brain weighs only three pounds but is estimated to have about
100 billion cells. It is hard to get a handle on a number that large (or
connections that small). Let's try to get an understanding of this
complexity by comparing it with something humans have created--the
entire phone system for the planet. If we took all the phones in the
world and all the wires (there are over four billion people on the
planet), the number of connections and the trillions of messages per
day would NOT equal the complexity or activity of a single human
brain. Now let's take a "small problem"--break every phone in
Michigan and cut every wire in the state. How long would it take for
the entire state (about 15 million people) to get phone service back?
A week, a month, or several years? If you guessed several years, you
are now beginning to see the complexity of recovering from a head
injury. In the example I used, Michigan residents would be without
phone service while the rest of the world had phone service that
worked fine. This is also true with people who have a head injury.
Some parts of the brain will work fine while others are in need of
repair or are slowly being reconnected.
Let's start looking at the building blocks of the brain. As previously
stated, the brain consists of about 100 billion cells. Most of these cells
are called neurons. A neuron is basically an on/off switch just like the
one you use to control the lights in your home. It is either in a resting
state (off) or it is shooting an electrical impulse down a wire (on). It
has a cell body, a long little wire (the "wire" is called an axon), and at
the very end it has a little part that shoots out a chemical. This
chemical goes across a gap (synapse) where it triggers another
neuron to send a message. There are a lot of these neurons sending
messages down a wire (axon). By the way, each of these billions of
axons is generating a small amount of electrical charge; this total
power has been estimated to equal a 60 watt bulb. Doctors have
learned that measuring this electrical activity can tell how the brain is
working. A device that measures electrical activity in the brain is
called an EEG (electroencephalograph).
Each of the billions of neurons "spit out" chemicals that trigger other
neurons. Different neurons use different types of chemicals. These
chemicals are called "transmitters" and are given names like
epinephrine, norepinephrine, or dopamine. Pretty simple, right? Well,
no. Even in the simplified model that I'm presenting, it gets more
Is the brain like a big phone system (because it has a lot of
connections) or is it one big computer with ON or OFF states (like the
zeros and ones in a computer)? Neither of the above is correct.
Let's look at the brain using a different model. Let's look at the brain
as an orchestra. In an orchestra, you have different musical sections.
There is a percussion section, a string section, a woodwind section,
and so on. Each has its own job to do and must work closely with the
other sections. When playing music, each section waits for the
conductor. The conductor raises a baton and all the members of the
orchestra begin playing at the same time playing on the same note. If
the drum section hasn't been practicing, they don't play as well as the
rest of the orchestra. The overall sound of the music seems "off" or
plays poorly at certain times. This is a better model of how the brain
works. We used to think of the brain as a big computer, but it's really
like millions of little computers all working together.
How does information come into the brain? A lot of information comes
in through the spinal cord at the base of the brain. Think of a spinal
cord as a thick phone cable with thousands of phone lines. If you cut
that spinal cord, you won't be able to move or feel anything in your
body. Information goes OUT from the brain to make body parts (arms
and legs) do their job. There is also a great deal of INCOMING
information (hot, cold, pain, joint sensation, etc.). Vision and hearing
do not go through the spinal cord but go directly into the brain. That’s
why people can be completely paralyzed (unable to move their arms
and legs) but still see and hear with no problems.
Information enters from the spinal cord and comes up the middle of
the brain. It branches out like a tree and goes to the surface of the
brain. The surface of the brain is gray due to the color of the cell
bodies (that's why it's called the gray matter). The wires or axons
have a coating on them that's colored white (called white matter).
We have two eyes, two hands, and two legs, so why not two brains?
The brain is divided in half, a right and left hemisphere. The right
hemisphere does a different job than the left. The right hemisphere
deals more with visual activities and plays a role in putting things
together. For example, it takes visual information, puts it together,
and says "I recognize that--that's a chair," or "that's a car" or "that's a
house." It organizes or groups information together. The left
hemisphere tends to be the more analytical part; it analyzes
information collected by the right. It takes information from the right
hemisphere and applies language to it. The right hemisphere "sees" a
house, but the left hemisphere says, "Oh yeah, I know whose house
that is--it's Uncle Bob's house."
So what happens if one side of the brain is injured? People who have
an injury to the right side of the brain "don't put things together" and
fail to process important information. As a result, they often develop a
"denial syndrome" and say "there's nothing wrong with me." For
example, I treated a person with an injury to the right side of the
brain--specifically, the back part of the right brain that deals with
visual information--and he lost half of his vision. Because the right
side of the brain was injured, it failed to "collect" information, so the
brain did not realize that something was missing. Essentially, this
person was blind on one side but did not know it. What was scary
was that this person had driven his car to my office. After seeing the
results of the tests that I gave him, I asked, "Do you have a lot of
dents on the left side of your car?" He was amazed that I magically
knew this without seeing his car. Unfortunately, I had to ask him not
to drive until his problems got better. But you can see how the right
side puts things together.
The left side of the brain deals more with language and helps to
analyze information given to the brain. If you injure the left side of the
brain, you're aware that things aren't working (the right hemisphere is
doing its job) but are unable to solve complex problems or do a
complex activity. People with left hemisphere injuries tend to be more
depressed, have more organizational problems, and have problems
using language.
Information from our eyes goes to areas at the very back of the brain.
We've all seen cartoons where the rabbit gets hit on the head and the
rabbit sees stars. This can actually happen in human beings (trust
me, not a good thing to do at home!). If you take a hard enough blow
to the back of the head, this brain area bangs against back of your
skull. This stimulates it and you can see stars and flashing lights.
Remember those two hemispheres? Each hemisphere processes half
the visual information. Visual information that we see on the left gets
processed by the right hemisphere. Information on the right gets
processed by the left hemisphere. Remember, wires that bring in
information to the brain are "crossed"--visual information from the left
goes to the right brain.
The area of the brain that controls movement is in a very narrow strip
that goes from near the top of the head right down along where your
ear is located. It's called the motor strip. If I injure that area, I'll have
problems controlling half of my body. If I have a stroke in the left
hemisphere of my brain, the right side of the body will stop working. If
I have an injury to my right hemisphere in this area, the left side of my
body stops working (remember, we have two brains). This is why one
half of the face may droop when a person has had a stroke.
In the general population, 95 percent of people are right-handed,
which means that the left hemisphere is the dominant hemisphere.
(For you left-handers, the right hemisphere is dominant.) With righthanded people, the ability to understand and express language is in
this left temporal lobe. If I were to take a metal probe, and charge it
with just a bit of electricity, and put it on the "primary" area of my left
temporal lobe, I might say "hey, I hear a tone." If I move this probe to
a more complex area of the temporal lobe, I might hear a word being
said. If I move the electrical probe to an even more complex area, I
might hear the voice of somebody I recognize; "I hear Uncle Bob's
voice." We have simple areas of the temporal lobe that deal with
basic sounds and other areas of the temporal lobe that look at more
complex hearing information.
The right temporal lobe also deals with hearing. However, its job is to
process musical information or help in the identification of noises. If
this area is damaged, we might not be able to appreciate music or be
able to sing. Because we tend to think and express in terms of
language, the left temporal lobe is more critical for day-to-day
The vision areas and the hearing areas of the brain have a boundary
area where they interact. This is the area of the brain that does
reading. We take the visual images and convert them into sounds. So
if you injure this area (or it doesn't develop when you are very young),
you get something called dyslexia. People who have dyslexia have
problems that may include seeing letters backwards or have
problems understanding what written words mean.
If something lands on my left hand, this information will be transmitted
to the right side of my brain. It goes to the area of the brain next to
the area that deals with movement. The tactile area of the brain deals
with physical sensation. Movement and feeling are closely related, so
it makes sense that they are next to each other in the brain. Because
movement and tactile areas are located close to each other, it is not
uncommon for people with a brain injuries to lose both movement and
feeling in parts of their body. Remember--tactile information from the
left side of the body goes to the right brain, just like movement and
FRONTAL LOBES--Planning, Organizing, Controlling
The biggest and most advanced part of the brain is the frontal lobe.
(It's called the frontal lobe because it's in the front part of brain.) One
job of the frontal lobe is planning. You have probably heard of "frontal
lobotomies." At the turn of the century, this surgery was done on
people who were very violent or who were in a psychiatric hospital
because they were very agitated. Doctors used surgery to damage
this area of the brain. Following this surgery, people became very
passive and less violent. At first, scientists saw this as a great thing.
Neurosurgery could stop behavioral problems such as violence. The
problem was that the patients stopped doing a lot of other things.
They didn't take care of themselves and they stopped many activities
of daily living. They basically sat there. In head injury, individuals with
frontal lobe impairment seem to lack motivation and have difficulty
doing any task that requires multiple steps (e.g., fixing a car or
planning a meal). They have problems with planning.
The frontal lobe is also involved in organizing. For a lot of activities,
we need to do step A, then step B, then step C. We have to do things
in order. That's what the frontal lobes help us do. When the frontal
lobe is injured, there is a breakdown in the ability to sequence and
organize. A common example is people who cook and leave out a
step in the sequence. They forget to add an important ingredient or
they don't turn the stove off. I've met a lot of patients who've burned
or melted a lot of pans.
Additionally, the frontal lobes also play a very important role in
controlling emotions. Deep in the middle of the brain are sections that
control emotions. They're very primitive emotions that deal with
hunger, aggression, and sexual drive. These areas send messages
to other parts of the brain to DO SOMETHING. If you're mad, hit
something or someone. If you're hungry, grab something and eat it.
The frontal lobes "manage" emotions. In general, the frontal lobe has
a NO or STOP function. If your emotions tell you to punch your boss,
it's the frontal lobes that say "STOP or you are going to lose your
job." People have often said to me "a little thing will set me off and I'm
really mad." The frontal lobes failed to stop or turn off the emotional
On the other hand, we have talked about how the frontal lobes plan
activities. The frontal lobes may fail to plan for some types of
emotion. For example, sexual interest involves some level of planning
or preparation. Without this planning, there is a lack of sexual
interest. A lack of planning can also affect the expression of anger.
I've had some family members say "You know, the head injury
actually improved him, he's not such a hot-head anymore." If you
listen very carefully, you're also going to hear "he's not as motivated
anymore." Remember, the frontal lobe plans activities as well as
controls emotions.
The male reproductive system starts with the testes. The testes
consist of two circular glands that reside in a skin sack called the
scrotum. The testes produce two major products of the reproductive
system called sperm and testosterone. The sperm are the male
gamete, that combine with the female ovum during fertilization to
produce a fetus. The testes also produce testosterone, which is an
important hormone responsible for producing male sex
Duct System
Once sperm has been produced, the testes discharge the sperm cells
into a series of ducts. The first duct is the epididymis. This directs the
sperm from the testis into the vas deferens. From the vas deferens,
the sperm travels to the seminal vesicles.
The seminal vesicles are two glands located directly below the
bladder. The seminal vesicles discharge a sticky and thick fluid that
makes up the first part of semen. As the sperm continues to travel,
the prostate secretes additional alkaline solution that makes up a
large portion of the semen. This alkaline solution helps protect the
sperm from the acidity in the man's urethra, and eventually the
woman's, during intercourse.
Sperm exits the body through the penis. When a man becomes
sexually excited, blood rushes to the large spaces in the penis called
the corpora cavernosa. This causes the penis to become hard and
rigid enough to enter the female reproductive tract through the
vagina. Once the man has reached a certain point of sexual
stimulation, ejaculation occurs. During ejaculation, the sperm-filled
semen is ejected out of the urethra. If ejaculation occurs during
sexual intercourse, the semen travels into the female reproductive
tract where it can contribute to conception.
Female Reproductive System
The female reproductive system is designed to carry out several
functions. It produces the female egg cells necessary for
reproduction, called the ova or oocytes. The system is designed to
transport the ova to the site of fertilization. Conception, the
fertilization of an egg by a sperm, normally occurs in the fallopian
tubes. The next step for the fertilized egg is to implant into the walls
of the uterus, beginning the initial stages of pregnancy. If fertilization
and/or implantation does not take place, the system is designed to
menstruate (the monthly shedding of the uterine lining). In addition,
the female reproductive system produces female sex hormones that
maintain the reproductive cycle.
What Parts Make up the Female Anatomy?
The female reproductive anatomy includes parts inside and outside
the body.
The function of the external female reproductive structures (the
genitals) is twofold: To enable sperm to enter the body and to protect
the internal genital organs from infectious organisms. The main
external structures of the female reproductive system include:
Labia majora: The labia majora enclose and protect the other
external reproductive organs. Literally translated as "large lips,"
the labia majora are relatively large and fleshy, and are
comparable to the scrotum in males. The labia majora contain
sweat and oil-secreting glands. After puberty, the labia majora
are covered with hair.
Labia minora: Literally translated as "small lips," the labia minora
can be very small or up to 2 inches wide. They lie just inside the
labia majora, and surround the openings to the vagina (the
canal that joins the lower part of the uterus to the outside of the
body) and urethra (the tube that carries urine from the bladder
to the outside of the body).
Bartholin's glands: These glands are located beside the vaginal
opening and produce a fluid (mucus) secretion.
Clitoris: The two labia minora meet at the clitoris, a small, sensitive
protrusion that is comparable to the penis in males. The clitoris
is covered by a fold of skin, called the prepuce, which is similar
to the foreskin at the end of the penis. Like the penis, the clitoris
is very sensitive to stimulation and can become erect.
The internal reproductive organs in the female include:
Vagina: The vagina is a canal that joins the cervix (the lower part of
uterus) to the outside of the body. It also is known as the birth
Uterus (womb): The uterus is a hollow, pear-shaped organ that is
the home to a developing fetus. The uterus is divided into two
parts: the cervix, which is the lower part that opens into the
vagina, and the main body of the uterus, called the corpus. The
corpus can easily expand to hold a developing baby. A channel
through the cervix allows sperm to enter and menstrual blood to
Ovaries: The ovaries are small, oval-shaped glands that are
located on either side of the uterus. The ovaries produce eggs
and hormones.
Fallopian tubes: These are narrow tubes that are attached to the
upper part of the uterus and serve as tunnels for the ova (egg cells)
to travel from the ovaries to the uterus. Conception, the fertilization of
an egg by a sperm, normally occurs in the fallopian tubes. The
fertilized egg then moves to the uterus, where it implants into the
lining of the uterine wall.
What Happens During the Menstrual Cycle?
Females of reproductive age experience cycles of hormonal activity
that repeat at about one-month intervals. With every cycle, a
woman's body prepares for a potential pregnancy, whether or not that
is the woman's intention. The term menstruation refers to the periodic
shedding of the uterine lining. (Menstru means "monthly"; hence the
term menstrual cycle.)
The average menstrual cycle takes about 28 days and occurs in
phases: the follicular phase, the ovulatory phase (ovulation), and the
luteal phase.
There are four major hormones (chemicals that stimulate or regulate
the activity of cells or organs) involved in the menstrual cycle: folliclestimulating hormone, luteinizing hormone, estrogen, and
Follicular Phase of the Menstrual Cycle
This phase starts on the first day of your period. During the follicular
phase of the menstrual cycle, the following events occur:
Two hormones, follicle stimulating hormone (FSH) and luteinizing
hormone (LH) are released from the brain and travel in the
blood to the ovaries.
The hormones stimulate the growth of about 15 to 20 eggs in the
ovaries each in its own "shell," called a follicle.
These hormones (FSH and LH) also trigger an increase in the
production of the female hormone estrogen.
As estrogen levels rise, like a switch, it turns off the production of
follicle-stimulating hormone. This careful balance of hormones
allows the body to limit the number of follicles that mature.
As the follicular phase progresses, one follicle in one ovary
becomes dominant and continues to mature. This dominant
follicle suppresses all of the other follicles in the group. As a
result, they stop growing and die. The dominant follicle
continues to produce estrogen.
Ovulatory Phase of the Menstrual Cycle
The ovulatory phase, or ovulation, starts about 14 days after the
follicular phase started. The ovulatory phase is the midpoint of the
menstrual cycle, with the next menstrual period starting about two
weeks later. During this phase, the following events occur:
The rise in estrogen from the dominant follicle triggers a surge in
the amount of luteinizing hormone that is produced by the brain.
This causes the dominant follicle to release its egg from the ovary.
As the egg is released (a process called ovulation) it is captured by
finger-like projections on the end of the fallopian tubes
(fimbriae). The fimbriae sweep the egg into the tube.
Also during this phase, there is an increase in the amount and
thickness of mucous produced by the cervix (lower part of the
uterus). If a woman were to have intercourse during this time,
the thick mucus captures the man's sperm, nourishes it, and
helps it to move towards the egg for fertilization.
Luteal Phase of the Menstrual Cycle
The luteal phase of the menstrual cycle begins right after ovulation
and involves the following processes:
Once it releases its egg, the empty follicle develops into a new
structure called the corpus luteum.
The corpus luteum secretes the hormone progesterone.
Progesterone prepares the uterus for a fertilized egg to implant.
If intercourse has taken place and a man's sperm has fertilized the
egg (a process called conception), the fertilized egg (embryo)
will travel through the fallopian tube to implant in the uterus.
The woman is now considered pregnant.
If the egg is not fertilized, it passes through the uterus. Not needed to
support a pregnancy, the lining of the uterus breaks down and sheds,
and the next menstrual period begins.
How Many Eggs Does a Woman Have?
The vast majority of the eggs within the ovaries steadily die, until they
are depleted at menopause. At birth, there are approximately 1
million eggs; and by the time of puberty, only about 300,000 remain.
Of these, 300 to 400 will be ovulated during a woman's reproductive
lifetime. The eggs continue to degenerate during pregnancy, with the
use of birth control pills, and in the presence or absence of regular
menstrual cycles.