See figure 9-4 for an illustration depicting cardiovascular circulatory
patterns.
a. General. The human cardiovascular circulatory system is
described as a closed, two-cycle system.
(1) It is closed because at no place is the blood as whole blood ever
outside the system.
(2) It is two-cycle because the blood passes through the heart twice with
each complete circuit of the body. In the pulmonary cycle, the blood passes from the right
heart, through the lungs, and to the left heart. In the systemic cycle, the blood passes
from the left heart, through the body in general, and returns to the right heart.
(3) It is common for an area of the body to be supplied by more than one
blood vessel so that if one is damaged, the others will continue the supply. This is known
as collateral circulation. However, there are situations, such as in the heart and the
brain, where a single artery supplies a specific part of a structure. Such an artery is
called an end artery. When an end artery is damaged, that area supplied by it will usually
die, as in the case of the coronary artery (para 9-7c) above or in the case of a
"stroke" in the brain.
b. Pulmonary Cycle. The pulmonary cycle begins in the right
ventricle of the heart. Contraction of the right ventricular wall applies pressure to the
blood. This forces the tricuspid valve closed and the closed valve prevents blood from
going back into the right atrium. The pressure forces blood past the semilunar valve into
the pulmonary trunk. Upon relaxation of the right ventricle, back pressure of the blood in
the pulmonary trunk closes the pulmonary semilunar valve. The blood then passes into the
lungs through the pulmonary arterial system. Gases are exchanged between the alveoli of
the lungs and the blood in the capillaries next to the alveoli. This blood, now saturated
with oxygen, is collected by the pulmonary veins and carried to the left atrium of the
heart. This completes the pulmonary cycle.
c. Systemic Cycle.
(1) Left ventricle of the heart. The oxygen-saturated blood is moved from
the left atrium into the left ventricle. When the left ventricular wall contracts, the
pressure closes the mitral valve, which prevents blood from returning to the left atrium.
The contraction of the left ventricular wall therefore forces the blood through the aortic
semilunar valve into the aortic arch. Upon relaxation of the left ventricular wall, the
back pressure of the aortic arch forces the aortic semilunar valve closed.
(2) Arterial distributions. The blood then passes through the various
arteries to the tissues of the body. See figure 9-5 for an illustration of the main
arteries of the human body.
(a) The carotid arteries supply the head. The neck and upper members are
supplied by the subclavian arteries.
(b) The aortic arch continues as a large single vessel known as the aorta
passing down through the trunk of the body in front of the vertebral column. It gives off
branches to the trunk wall and to the contents of the trunk.
(c) At the lower end of the trunk, the aorta divides into right and left
iliac arteries, supplying the pelvic region and lower members.
(3) Capillary beds of the body tissues. In the capillary beds of the
tissues of the body, materials (such as food, oxygen, and waste products) are exchanged
between the blood and the cells of the body.
(4) Venous tributaries. See figure 9-6 for an illustration of the main
veins of the human body.
(a) The blood from the capillaries among the tissues is collected by a
venous system parallel to the arteries. This system of deep veins returns the blood back
to the right atrium of the heart.
(b) In the subcutaneous layer, immediately beneath the skin, is a network
of superficial veins draining the skin areas. These superficial veins collect and then
join the deep veins in the axillae (armpits) and the inguinal region (groin).
(c) The superior vena cava collects the blood from the head, neck, and
upper members. The inferior vena cava collects the blood from the rest of the body. As the
final major veins, the venae cavae empty the returned blood into the right atrium of
heart.
(d) The veins are generally supplied with valves to assist in making the
blood flow toward the heart. It is of some interest to note that the veins from the head
do not contain valves.
(e) From that portion of the gut where materials are absorbed through the
walls into the capillaries, the blood receives a great variety of substances. While most
of these substances are useful, some may be harmful to the body. The blood carrying these
substances is carried directly to the liver by the hepatic portal venous system. This
blood is specially treated and conditioned in the liver before it is returned to the
general circulation by way of the hepatic veins.
Primary Content Providers: The U. S. Army, The
U.S. Navy
Ancillary Content and Online Version: David L. Heiserman
Publisher: SweetHaven Publishing Services
