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
(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 pulmo-
nary 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 con-
traction 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
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