c. Function. The function of the heart and the blood vessels is to keep the
blood circulating throughout the body. The heart provides the pumping mechanism and
the vessels provide the system through which the blood flows. The human body cannot
live if the heart or the vessels stop functioning.
d. Functional Unit of Closed Tubes. The heart and vessels of the circulatory
system provide an essentially closed unit within the body. The same amount of blood
that is pumped out of the heart must return to the heart. In the arteries, blood flows
away from the heart. The large arteries lead into smaller arteries called arterioles.
From the arterioles blood passes into capillaries, which return blood to the heart by
means of small veins called venules and then larger veins. (One of the lymphatic
system's primary functions is to return proteins and other substances, which have
escaped from the capillaries back into the veins.)
25.
THE HEART
a. Three Layers. The heart (figure 24) has three layers: a thin, outer, serous
covering called the epicardium; a thick, muscular wall, the myocardium; and an inner
lining, the endocardium, which is continuous with the lining (endothelium) of the blood
vessels.
b. Four Chambers. The interior of the heart is divided into halves by a
muscular wall, the septum; and each half is further divided into an upper chamber, the
atrium; and a lower chamber, the ventricle. Consequently, there are four chambers in
the heart; two atria and two ventricles. Each atrium communicates with its
corresponding ventricle (that is, the one on the same side) by means of an opening
called the atrioventricular opening. The muscular walls of the ventricles are much
thicker than those of the atria, and the wall of the left ventricle is thicker than that of the
right ventricle. This difference in structure is due to the fact, that the ventricles, which
eject the blood from the heart, perform more work than the atria, which receive the
blood. The left ventricle pumping blood to the body performs more work than does the
right ventricle, which pumps blood only to the lungs.
c. Valves. The four chambers of the heart are lined with endocardium. At each
of the openings from the chambers, this lining folds on itself and extends into the
opening to form valves. These valves allow the blood to pass from a chamber but
prevent its return. The tricuspid valve lies between the right atrium and ventricle. It has
three flaps, or cusps, from which it derives its name. The bicuspid valve, between the
left atrium and ventricle, is also called the mitral valve. These two valves serve to
prevent the backflow of blood from the ventricles to the atria. At the outlets of the
ventricles are the semilunar valves, which prevent the backflow of blood from the
arteries. The semilunar valve on the right is called the pulmonary valve; the one on the
left is the aortic valve.
MD0851
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