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The Circulatory System

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The Circulatory System

Post  Joe_Morningstar on Wed Feb 10, 2010 6:08 am

The circulatory system in anatomy and physiology is the course taken by
the blood through the arteries, capillaries, and veins and back to the heart. In
humans and the higher vertebrates, the heart is made up of four chambers the
right and left auricles, or atria, and the right and left ventricles. The right
side of the heart pumps oxygen-poor blood from the cells of the body back to the
lungs for new oxygen; the left side of the heart receives blood rich in oxygen
from the lungs and pumps it through the arteries to the various parts of the
body. Circulation begins early in fetal life. It is estimated that a given
portion of the blood completes its course of circulation in approximately 30
seconds.

Pulmonary circulation is where the blood from the entire body is
transported to the right auricle through two large veins. The superior vena cava
and the inferior vena cava. When the right auricle contracts, it forces the
blood through an opening into the right ventricle. Contraction of this ventricle
drives the blood to the lungs. Blood is prevented from returning into the
auricle by the tricuspid valve, which completely closes during contraction of
the ventricle. In its passage through the lungs, the blood is oxygenated, that
is, then it is brought back to the heart by the four pulmonary veins, which
enter the left auricle. When this chamber contracts, blood is forced into the
left ventricle and then by ventricular contraction into the aorta. The bicuspid,
or mitral, valve prevents the blood from flowing back into the auricle, and the
semilunar valves at the beginning of the aorta stop it from flowing back into
the ventricle. Similar valves are present in the pulmonary artery.

The aorta divides into a number of main branches, which in turn divide
into smaller ones until the entire body is supplied by an elaborately branching
series of blood vessels. The smallest arteries divide into a fine network of
still more minute vessels, the capillaries, which have extremely thin walls;
thus, the blood is enabled to come into close relation with the fluids and
tissues of the body. In the capillaries, the blood performs three functions then
it releases its oxygen to the tissues, it furnishes to the body cells the
nutrients and other essential substances that it carries, and it takes up waste
products from the tissues. The capillaries then unite to form small veins. The
veins, in turn, unite with each other to form larger veins until the blood is
finally collected into the superior and inferior venae cavae from which it goes
to the heart, completing the circuit.

In addition to the pulmonary and systemic circulations described above,
a subsidiary to the venous system exists, known as portal circulation. A certain
amount of blood from the intestine is collected into the portal vein and carried
to the liver. There it enters into the open spaces called sinusoids, where it
comes into direct contact with the liver cells. In the liver important changes
occur in the blood, which is carrying the products of the digestion of food
recently absorbed through the intestinal capillaries. The blood is collected a
second time into veins, where it again joins the general circulation through the
right auricle. In its passage through other organs, the blood is further
modified.

Coronary circulation is the means by which the heart tissues themselves
are supplied with nutrients and oxygen and are freed of wastes. Just beyond the
semilunar valves, two coronary arteries branch from the aorta. These then break
up into an elaborate capillary network in the heart muscle and valve tissue.
Blood from the coronary capillary circulation enters several small veins, which
then enter directly into the right auricle without first passing into the vena
cava.

The action of the heart consists of successive alternate contraction
and relaxation of the muscular walls of the auricles and ventricles. During the
period of relaxation, the blood flows from the veins into the two auricles,
gradually distending them. At the end of this period, the auricles are
completely dilated then their muscular walls contract, forcing almost the entire
contents through the auriculoventricular openings into the ventricles. This
action is sudden and occurs almost simultaneously in both auricles. The mass of
blood in the veins makes it impossible for any blood to flow backward. The force
of blood flowing into the ventricles is not powerful enough to open the
semilunar valves, but it distends the ventricles, which are still in a condition
of relaxation. The tricuspid and mitral valves open with the blood current and
close readily at the beginning of ventricular contraction.

The ventricular systole immediately follows the auricular systole. The
ventricular contraction is slower, but far more forcible then the ventricular
chambers are virtually emptied at each systole. The apex of the heart is thrown
forward and upward with a slight rotary motion then this impulse, called the
apex beat, can be detected between the fifth and sixth ribs. The heart is
entirely at rest for a short time after the ventricular systole occurs. The
entire cycle can be divided into three periods then in the first, the auricles
contract and in the second, the ventricles contract; in the third, both the
auricles and the ventricles remain at rest. In humans, with a normal heart rate
of approximately 72 heartbeats per minute, the cardiac cycle has a duration of
about 0.8 second. Auricular systole requires about 0.1 second; ventricular
systole occupies approximately 0.3 second. Thus, the heart is completely at rest
for about 0.4 second, or during perhaps half of each cardiac cycle.

With every beat, the heart emits two sounds, which are followed by a short
pause. The first sound, coinciding with the ventricular systole, is dull and
protracted. The second sound, made by the sudden closure of the semilunar valves,
is shorter and much sharper. Diseases of the heart valves may change these
sounds, and many factors, including exercise, cause wide variations in the
heartbeat, even in healthy people. The normal heart rate of animals varies
widely from species to species. At one extreme, the heart of a hibernating
mammal may beat only a few times a minute; at the other, the hummingbird has a
heart rate of 2000 heartbeats per minute.

When it enters the arteries at the moment of ventricular contraction,
the blood stretches the walls of the arteries. During diastole, the distended
arteries return to their normal diameter, in part because of the elasticity of
connective tissue and in part because of the contraction of muscles in the
arterial walls. This return to normal is important in maintaining a continuous
flow of blood through the capillaries during the period while the heart is at
rest. The expansion and contraction of the arterial walls that can be felt in
all the arteries near the surface of the skin is called the pulse.

The rate and strength of the heartbeat are controlled by nerves through
a series of reflexes that speed it up or slow it down. The impulse to
contraction, however, is not dependent on external nervous stimuli, but arises
in the heart muscle itself. A small bit of specialized tissue called the
sinoauricular node, embedded in the wall of the right auricle, is responsible
for initiating the heartbeat. The contraction then spreads over the auricles in
the septum between the auricles, it excites another node called the
auriculoventricular node. The auriculoventricular bundle conducts the impulse
from this node to the muscles of the ventricles, and in this way contraction and
relaxation of the heart are coordinated. Each phase of the cardiac cycle is
associated with the production of an electrical potential that can be recorded
by electrical instruments to produce a reading known as an electrocardiogram.

Circulation of the blood in superficial capillaries can be observed
under the microscope. The red blood cells can be seen moving along rapidly in
the middle of the blood current, while the white cells advance more slowly along
the walls of the capillaries. The capillaries present a far larger surface with
which the blood comes in contact than do other blood vessels end because they
consequently offer the greatest resistance to the progress of the blood, they
have a great influence on the circulation. Capillaries expand when temperature
rises and help to cool the blood then they contract in cold and help preserve
internal heat.

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Joe_Morningstar

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