FREE ESSAY ON THE HUMAN CIRCULATORY SYSTEM

THE HUMAN CIRCULATORY SYSTEM

The human circulatory is one of, if not the, most important system in the body. "It
consists of such organs as the heart, and lungs" (Dunbar 4). "However every organ and
organ system in the body is nourished and kept alive through the use of the circulatory
system" (2-4). The main organ in the circulatory system is the heart. "Basically, the
heart is a pump that keeps fresh blood coursing through your body, bringing oxygen and
nutrients to all your organs and cells" (2). "A mathematical description of what the
heart accomplishes is astonishing. Your heart keeps approximately ten pints (4.73 liters)
of blood constantly circulating through seventy -five -thousand miles (one -hundred
-thousand kilometers) of blood vessels" (2). Those thousands of miles of blood vessels
reach every organ in your body. "Once they reach the organ they deliver much needed
oxygen and take away carbon dioxide and other waste products that your body cannot use.
This must be done without interruption if you are to remain healthy and alive" (2). 
"Every living cell in your body depends on oxygen for life. Any cell deprived of oxygen
for more than thirty minutes will die. The cells in your brain are even more dependent on
fresh oxygen. If your brain cells are deprived of oxygen for only five seconds it will
cause you to become unconscious. If your brain is deprived of oxygen for more than four
minutes your brain will die" (2).
But where does all of this oxygen come from? , You may ask yourself. The lungs, I
believe, are the second most important organs in the circulatory system. For one reason
they are the only way for oxygen to be received into the body. It is in the lungs that
cells absorb fresh oxygen, and where carbon dioxide is released to be expelled from the
body. The transfer of these two gasses takes place in the alveoli in the lungs. "Alveoli
are 'small cavities' or 'air sacs in the lungs'" (Marieb G-2). "Blood entering the lungs
through the pulmonary circulation is dark colored, low on oxygen, and high in carbon
dioxide. The blood is pumped by the right side of the heart into the pulmonary arteries,
which leads to the lungs. The pulmonary arteries divide into smaller and smaller blood
vessels, which ending with pulmonary capillaries in the walls of the alveoli"(World Book
L 528). These alveoli are the smallest of a series of cavities in the lungs.
"The lungs are designed to receive air, which enters the body through the mouth or the
nose. The air passes through the pharynx and the larynx and enters the airways, beginning
with the trachea and then into the two primary bronchi"(World Book L 527). "After the
primary bronchi enter the lungs they subdivide into smaller and smaller branches, called
secondary and tertiary bronchi. Finally ending in the smallest of the conducting
passageways, the bronchioles. This branching and re-branching is often referred to as the
bronchial tree. The bronchioles lead into the respiratory zone structures, even smaller
conduits, which eventually terminate in alveoli" (Marieb 380). Unlike the heart, which is
made entirely of muscle, the lungs have no muscles in them. "They are forced to expand
and allowed to contract through movements of the ribs and the diaphragm. Like your
heartbeat, the rhythmic motion of your lungs expanding and relaxing is a constant
process. You can however control your breathing to a certain extent. For instance, if you
feel like taking a deep breath you can do so instantly. But you cannot make your heart
beat faster or slower on impulse. 
You have the power to hold your breath, but only for a short time. If you hold your
breath long enough you will fall into unconsciousness and then resume breathing
automatically. The reason behind this is that the muscles that control your breathing are
controlled by your body's nervous system" (Dunbar 5). You see if your brain senses that
it is not getting enough oxygen it will automatically force you to breath so that you can
continue to live.
"William Harvey discovered that blood circulates throughout the body. He could not
explain, however, the motion of the heart that makes circulation possible. As far as he
was concerned, that could be 'comprehended only by God'" (3). However, more than three
-hundred years later we now have a vast knowledge of how the heart works. 
"Each side of the heart performs a different pumping job. The right side takes blood from
the body and pumps it to the lungs. The left side of 
the heart collects blood from the lungs and pumps it to the body. Blood goes through the
right side of the heart, to the lungs, and then to the left side of the heart where it is
once again pumped through the body" (World Book H 139). 
"Blood flows into the right atrium through two large veins, called the superior and the
inferior vena cava's" (139). The superior vena cava carries blood from the head and arms.
The inferior vena cava carries blood from the trunk and legs. 
"Blood from the body fills the right atrium. The right atrium then contracts, squeezing
blood through the tricuspid valve into the ventricle. The tricuspid valve is made of
three little triangular flaps of thin, strong fibrous tissue. The valve allows blood to
flow into the ventricle but does not allow it to flow back out in the same direction. The
ventricle is at first relaxed, but it contracts when it is filled with blood. The
resulting pressure closes the tricuspid valve and opens the semilunar valve, which is
between the ventricle and the pulmonary artery. Blood gushes through the semilunar valve
and into the pulmonary artery, which leads to the lungs. The semilunar valve gets its
name because its three flaps are in the shapes of a half moon" (139). 
"Blood flows from the lungs back into the left side of the heart through the four
pulmonary veins and into the left atrium. The left atrium then, like its neighbor to the
right, then contracts, squeezing blood through the mitral valve into the left ventricle.
The mitral valve is similar to the tricuspid valve except that it has only two flaps. The
left ventricle then contracts sending the blood through another semilunar valve into the
aorta. The aorta with it's numerous branches then sends the blood throughout the body"
(140). 
The heart has two phases, the first is a relaxing/filling phase, and the second is a
contracting/emptying phase. "The relaxing/filling phase is called the diastole phase. The
contracting/pumping phase is called the systole phase" (140).
The blood and blood vessels are also a necessary component of the circulatory system.
"The blood vessels form a complicated system of connecting tubes throughout the body.
There are three major types of blood vessels. Arteries carry blood from the heart. Veins
return blood to the heart. Capillaries are extremely tiny vessels that connect the
arteries and the veins (World Book Ci-Cz 559). "Blood is the life stream of the human
body. This red fluid performs many tasks, and no part of the body can survive without it.
Blood supplies cells with the food and oxygen they need, and removes waste from the cells
and carries them to organs that remove them from the body, or break them down into
harmless substances. In addition the blood fights germs in the body" (World Book B 422).
"Blood has four major parts: plasma, red blood cells, white blood cells, and
platelets"(423).
"Plasma is the liquid part of the blood. This straw colored fluid makes up about fifty
-five to sixty -five percent of the total volume of blood. The red and white blood cells
and the platelets are all suspended in the plasma. Plasma consists mainly of water.
However, it also contains hundreds of other substances, including proteins, digested
food, and waste products. The major proteins in plasma are albumin, fibrinogen, and
globulin. Albumin helps keep plasma in the blood vessels. If the amount of albumin falls,
fluid in the plasma may escape into the surrounding tissues, causing edema. Fibrinogen
plays an important part in clotting. Globulin proteins, especially the gamma globulins,
help fight infection" (423).
"Red blood cells, also called erythrocytes, are the most numerous of the formed elements.
Each micro liter of blood contains from four to six -million red blood cells. Red blood
cells are constantly circulating throughout the bloodstream. The main job of these cells
is to carry oxygen from the lungs to the body tissues, and to carry carbon dioxide from
the body tissues to the lungs. 
A mature erythrocyte has the shape of a donut without the whole. It is a flat disk with
thick, rounded edges and a thin indented center. Red blood cells have no nucleus, they
consist of only three parts: the cell membrane, hemoglobin, and a group of chemicals. The
cell membranes main function is to act as an envelope and hold the cell together. The
hemoglobin enables the red blood cells to carry oxygen. The chemicals in red blood cells
provide energy and help keeps the erythrocytes in good condition. The red blood cells are
produced in bone marrow" (423-423). 
"White blood cells, also called leucocytes, help protect the body against infection. A
micro liter of blood contains four -thousand to ten -thousand leucocytes. These cells
range from seven to twelve microns in diameter, and, unlike red blood cells they have a
nucleus. There are several kinds of white blood cells, including: neutrophils,
lymphocytes, and monocytes. 
Neutrophils are the most common kind of leucocytes, their main function is to fight off
infection. When an infection is detected in the body, the bone marrow releases far more
neutrophils than normal. The development process usually takes about twelve days, and the
neutrophils leave the blood a few hours after being released from the marrow. They then
move through body tissues to an area of bacterial infection. The neutrophils swallow up
and destroy the bacteria.
There are two kinds of lymphocytes, both of which form in the bone marrow. One kind
leaves the marrow in search of the thymus organ, here they become T cells. The other type
of lymphocytes, B cells, release antibodies into the blood when an infection is detected.
The T cells release other substances that fight invaders in the bloodstream.
Monocytes are produced mainly in the bone marrow. Like neutrophils, they carry out
phagocytosis. Monocytes work with neutrophils to destroy bacteria. They play an
especially important role in killing the bacteria that causes tuberculosis" (425).
Platelets are disk like structures that develop from cells found in the marrow. They help
prevent the loss of blood from damaged vessels. If a small blood vessel is cut or broken,
platelets stick to the damaged surface and to one another. As they pile up, they form a
temporary seal over the injury. At the same time, the platelets release substances that
start the process of blood clotting. Platelets are the smallest of the formed elements.
They measure only about two to four microns in diameter. A micro liter of blood contains
one -hundred -fifty -thousand to four -hundred -thousand platelets" (425).
Bibliography
Bibliography
"Blood"
The World Book Encyclopedia: Scott Fetzer Company.
1990.
2: 422-425
Dunbar, Robert E.
The Heart and Circulatory System
New York: Franklin Watts,
1984
Hildebrant, Jack and Modell, Harold I.
"Lung"
The World Book Encyclopedia: Scott Fetzer Company.
1990.
12:527-528
"How the Heart Works"
The World Book Encyclopedia: Scott Fetzer Company.
1990.
9:139-141
Marieb, Elaine N.
Essentials of Human Anatomy and Physiology
New York: The Benjamin/Cummings Publishing Company, Inc.,
1994
"The Circulatory System."
The World Book Encyclopedia: Scott Fetzer Company.
1990.
4:559-560