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Cardiovascular System – Overview
The heart is a hollow pear-shaped muscular organ placed between the lungs in the middle of the
chest that pumps blood through the body, supplying cells with oxygen and nutrients. It is
attached to the breastbone by special connective tissues called ligaments. The apex (blunt point
of the lower edge of the heart) lies on the diaphragm, pointing toward the left. The apex pulses
with every beat of the heart. This is what you feel when you hold your hand to your heart. The
heart has four cavities: a small upper cavity (atrium) and a large lower cavity (ventricle) on each
side. The adult human heart is approximately the size of a fist. In an average adult, it is about
five inches long and three and a half inches across at its broadest part, and it weighs less than a
pound.
The small vessels which collect blood from the front of the right ventricle of the heart are known
as the anterior cardiac veins. They are usually three or four in number and deliver the blood to
the right atrium of the heart.
The left atrium is a small upper cavity of the heart. Oxygen rich blood returns from the lungs
through the four pulmonary veins into the smooth chamber of the left atrium. The chamber is
constructed of two overlapping layers of muscle: a superficial layer and an inner layer, composed
of many small bundles. The wall of the chamber is slightly thicker and more powerful than the
right atrium. As the heart contracts (ventricular systole), blood flows into the ascending aorta
through the aortic arch. As the heart relaxes (ventricle diastole), the blood flows through the
mitral valve to the left ventricle.
The right atrium is a small upper cavity of the heart that holds about three-and-a-half
tablespoons of blood. It serves as the receiving chamber for all the venous blood (short of
oxygen and laden with carbon dioxide) returning through the superior and inferior vena cava,
and from many minute blood vessels that drain blood from the walls of the chamber itself. The
right atrium is slightly larger than the left atrium, which is slightly more powerful. The walls of
the right atrium are less than an eighth of an inch thick. Two layers of muscle form the wall.
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The superficial layer spans both atria, and the inner layer, composed of many small bundles,
arches over the atrial cavity at right angles to the superficial layer. As the heart contracts
(ventricular systole), the blood is pushed through the pulmonary valve into pulmonary
circulation. As the heart relaxes (ventricular diastole), the blood exits the right atrium through
the tricuspid valve to the right ventricle. In the upper part of the right atrium there is a small
patch of special heart tissue called the sinus node or the sinoatrial node. It is the hearts
pacemaker. It triggers the heartbeat and establishes its pace.
The body's largest blood vessel, the aorta, arches out of the heart and down toward the lower
body. It has a diameter of about one inch and blood rushes through it at a speed of about eight
inches per second. The aorta is divided into several parts: the ascending aorta, the arch of the
aorta, and the thoracic and abdominal portions of the descending aorta.
The pulmonary artery carries blood from the right side of the heart to the lungs. It is the only
artery that carries dark, low-oxygen blood. The artery is short and wide (about 2 inches in length
and 1 1/5 inches in diameter). It arises from the base of the right ventricle and extends upward
and branches into two arteries of nearly equal size. The right pulmonary artery is longer and
larger and runs horizontally outward to the base of the right lung where it divides in two
branches for the two lobes. The left pulmonary artery is shorter and somewhat smaller. It runs
horizontally to the base of the left lobe where it divides in two branches for the two lobes.
The superior vena cava returns the blood from the head, arms and upper body to the right
atrium of the heart. The inferior vena cava returns blood from the lower body and legs.
The left ventricle is the large lower cavity of the heart. Its walls are three times thicker than
those of the right ventricle, making it the most powerful chamber in the heart. Along with the
three spiral layers shared by the right ventricle (the superficial spiral layer, the deep sinospiral
layer, and the superficial bulbo layer), a fourth layer of muscle, the deep bulbo spiral layer,
winds from the aortic and mitral valve rings around the left ventricle and back, making the
ventricle stronger to withstand higher blood pressure. When the left ventricle contracts, the
blood is pushed through the aortic semilunar valve and into the aorta.
The right ventricle is a large lower cavity of the heart. The walls of the ventricle are a quarter
of an inch thick and are composed of three spiraling layers of muscle: the superficial layer, the
deep sinospiral layer, and the bulbo spiral layer. All three bands of muscle anchor firmly onto
the skeleton of the heart. Toward the top of the chamber, near the pulmonary artery, the inner
surface is smooth. Throughout the rest of the chamber, small bundles of muscle arise from the
walls, stretch and then merge with other bundles. They weave an intricate web of tissue across
the ventricle. When the right ventricle contracts, blood rushes into the pulmonary artery and into
the lungs. The right ventricle holds slightly more than a quarter of a cup of blood.
In order for the body to stay alive, each of its cells must receive a continuous supply of food and
oxygen. At the same time, carbon dioxide and other materials produced by the cells must be
picked up for removal from the body. This process is continually maintained by the body's
circulatory system. The primary circulatory system consists of the heart and blood vessels,
which together maintain a continuous flow of blood through the body delivering oxygen and
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nutrients to and removing carbon dioxide and waste products from peripheral tissues. A
subsystem of the circulatory system, the lymphatic system, collects interstitial fluid and returns it
to the blood. The heart pumps oxygen-rich blood from the lungs to all parts of the body through
a network of arteries, and smaller branches called arterioles. Blood returns to the heart via small
venules, which lead to the larger veins. Arterioles and venules are linked even smaller vessels
called metarterioles. Capillaries, blood vessels a single cell thick, branch off from the
metarterioles and then rejoin them. The network of tiny capillaries is where the exchange of
oxygen and carbon dioxide between blood and body cells takes place. The average adult has
over 60,000 miles of blood vessels in their body.
About ten billion capillaries lace all body tissues, bringing blood within reach of every cell.
They are the smallest blood vessels, microscopic in size, and contain less than five percent of the
total circulating blood volume at any one time. Capilaries branch off from the metarterioles
which connect arterioles with venules. The capillaries have thin walls, only one cell thick, across
which oxygen and metabolic exchanges take place. As blood flows through the capillaries in the
lungs, it changes from venous blood to arterial blood by unloading carbon dioxide and picking
up oxygen. Its color changes in the process from a deep crimson to a bright scarlet. As blood
flows through tissue capillaries, it changes back from arterial blood to venous blood. The
oxygen leaves the blood to enter cells, and the carbon dioxide leaves the cells and enters the
blood.
The brain is supplied by a dense network of blood vessels. It requires a constant supply of
oxygen and rapid removal of wastes. Brain cells suffer permanent damage if their blood supply
stops for more than two minutes. Once brain cells die, they are not replaced. The consequence
of ruptured or blocked blood vessels in the brain can be a stroke. The seriousness of paralysis or
loss of faculties following a stroke depends on how much brain tissue has been starved of oxygen
and nutrients.
The pulmonary arterioles and venules wind along the bronchioles, then branch out to form dense
webs of capillaries that surround each alveolus. Some 300 million alveoli fill the two lungs.
Enveloped in a thin film of water, the alveoli and their capillaries make up the respiratory
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membrane. The tiny capillaries of the lungs hold just three ounces of blood, which is spread
thinly, making it possible for oxygen and carbon dioxide to be exchanged in just a quarter of a
second. As blood flows through the lung capillaries, carbon dioxide leaves it and oxygen enters
it. Venous blood enters lung capillaries (these are the only capillaries where this occurs).
Arterial blood enters all other capillaries). The hemoglobin in the red blood cells of venous
blood is combined with carbon dioxide rather than oxygen, and is called carbaminohemoglobin.
As the blood flows through the lung capillaries the carbaminohemoglobin is broken down into
carbon dioxide and hemoglobin. The hemoglobin picks up oxygen molecules, while the carbon
dioxide molecules move out of the blood and are expelled from the lungs. This process changes
the venous blood to arterial blood, which is then circulated through the body.
One quarter of the total blood output from
the heart comes to the kidneys along the
renal arteries. The incoming artery divides
up repeatedly inside the kidney, eventually
forming arterioles, each one of which
leads to the Bowman's capsule. The tiny,
thin walled capillaries branch from the
arterioles and form a compact ball known
as the glomerules. These capillaries then
reunite to form the arterioles which leaves
the capsule. The capillary walls of the
kidney consist of many minute pores,
which are so small that they act, in effect,
like a filter, allowing water, glucose, salts,
amino acids, vitamins, and urea from the
blood to pass through into a tubule, but
preventing the passage of blood cells and
proteins. Every minute 125 ccs (1/4 pint)
of fluid is filtered through the capillaries.
This fluid contains about one kilo (2 1/2
pounds) of sodium chloride, .45 kilo (1
pound) of sodium bicarbonate, and .15
kilo (1/3 pound) of glucose. The body
then reabsorbs about 99% of the fluid back into the blood stream. The kidney filters and
reabsorbs the body's blood supply nearly fifteen times every day, which results in a remarkably
thorough cleaning operation.
The liver receives 28% of the total cardiac output of blood to the body. The two lobes of the
liver are supplied with oxygen-rich blood from the hepatic artery and deoxygenated blood, rich
in newly digested nutrients, from the portal vein. These two large blood vessels divide and
subdivide into a network of thousands of tiny capillaries weaving their way between the millions
of specialized cells of the liver. These cells are arranged in functional groups called lobules. All
the products of digestion and oxygen are brought to each liver cell by the network of capillaries,
providing them with the raw materials and fuel to fulfil their biochemical role.
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The carbohydrates and food broken down in the small intestine is absorbed by the finger-like
structures called villi. It is then passed into the tiny capillaries of the intestine, which run into
small blood vessels draining each villus. The small vessels run into larger ones and then
eventually into the hepatic portal vein which leads to the liver. The next steps of the breakdown
process continues in the liver. The nutrients are then delivered to other cells in the body. A
collection of capillaries also supplies the muscular walls of the large intestine with oxygen and
nutrients, and also removes water, vitamins, and minerals taken into the liver.
Blood vessels are tubes that carry blood to and from all parts of the body. The human circulatory
system is made up of three types of blood vessels: arteries, veins, and capillaries. An artery is a
large blood vessel with muscular tissue in the wall that carries blood away from the heart to the
organs and tissues of the body. Arterioles are small branches of the artery that are about 0.2mm
in diameter. The veins and smaller venules are vessels that carry blood from the organs and
tissues toward the heart. Veins have small valves that open to let blood through, and close to
prevent it from flowing back. Capillaries are microscopic in size. They link the arterioles and
venules through a network of metarterioles. At any one time the veins contain about 75% of the
body's blood. About 20% of the body's blood is in the arteries and only about 5% is in the
capallaries.
Healthy blood vessels have smooth, flexible walls. The
arteries transport blood under high pressure and,
therefore, have walls that are much more elastic than the
veins of the body. The flow of blood in the arteries
pulsates. It constantly increases and decreases as a
result of the heart pumping a new volume of blood into
the arteries 70 times per minute. This effect is what
causes the pulse you can feel by placing your fingertips
over the arteries in your wrist and neck. As the artery
receives each new volume of blood, the pressure is
highest. This is called systolic pressure. When the
hearts ventricles are relaxed, the pressure is at its lowest.
This is called diastolic pressure. Because the pressure in
the veins is very low, their walls are thinner than those
of the arteries. Veins are capable of contracting and
expanding according to the needs of the body. They can
act as a reservoir because their walls are 6 to 10 times as
expandable as those of arteries. When you need a blood
sample drawn for most diagnostic tests, it is taken from a vein.
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