Through the action of its very muscular walls, the heart produces the primary motive force to drive the blood through the arterial system. In humans, the heart is located just above the diaphragm, in the middle of the thorax, and extending slightly to the left. It is said that the heart of an average individual is about the size of that individual's clenched fist.
a. General Construction of the Human Heart. See figure 9-2 for an illustration of the human heart.
(1) Chambers. The heart is divided into four cavities known as the chambers. The upper two chambers are known as the atria, right and left. Each atrium has an ear-like projection known as an auricle. The lower two chambers are known as ventricles, right and left. Between the two atria is a common wall known as the interatrial septum. Between the two ventricles is a common wall known as the interventricular septum.
ATRIUM = hall
AURICLE = ear-like flap
VENTER = belly
SEPTUM = fence
(2) Wall layers. The walls of the chambers are in three general layers. Lining the cavity of each chamber is a smooth epithelium known as the endocardium. (Endocarditis is an inflammation of the endocardium.) The middle layer is made up of cardiac muscle tissue and is known as the myocardium. The outer layer of the heart is another epithelium known as the epicardium.
B. INTERIOR VIEW Figure 9-2. The human heart.
B. INTERIOR VIEW Figure 9-2. The human heart.
(3) Relationship of wall thickness to required pressure levels. A cross-section of the chambers shows that the atrial walls are relatively thin. The right ventricular wall is much thicker. The left ventricular wall is three to five times thicker than that of the right. These differences in wall thickness reflect the amount of muscle tissue needed to produce the amount of pressure required of each chamber.
(a) Between the atrium and ventricle of each side is the atrioventricular (A-V) valve. Each A-V valve prevents the blood from going back into the atrium from the ventricle of the same side. The right A-V valve is known as the tricuspid valve. The left A-V valve is known as the mitral valve. ("Might is never right.") The leaflets (flaps) of the A-V valves are prevented from being pushed back into the atria by fibrous cords. These fibrous cords are attached to the underside (the ventricular side) of the leaflets and are called chordae tendineae. At their other ends, the chordae tendineae are attached to the inner walls of the ventricles by papillary muscles.
(b) A major artery leads away from each ventricle--the pulmonary trunk from the right ventricle and the aortic arch from the left ventricle. A semilunar valve is found at the base of each of the pulmonary trunk and the aortic arch. These semilunar valves prevent blood from flowing back into the ventricles. The pulmonary (semilunar) valve and the aortic (semilunar) valve are each made up of three semilunar ("pocket-like") cusps.
b. Control of the Heart Beat. The heart is under several different control systems--extrinsic nervous control, intrinsic nervous control, and humoral control.
(1) Extrinsic nervous control. Extrinsic nervous control is control from outside of the heart. Extrinsic control is exerted by nerves of the autonomic nervous system. The sympathetic cardiac nerves accelerate (speed up) the heart. The vagus parasympathetic nerve decelerates (slows down) the heart.
(2) Intrinsic "nervous" control. Intrinsic "nervous" control is control built within the heart. The intrinsic "nervous" system consists of the sinoatrial (S-A) node (often referred to as the "pacemaker"), the atrioventricular (A-V) node, and the septal bundles. The septal bundles spread through the walls of the ventricles, just beneath the endocardium. This combination of nodes and bundles initiates the heart beat automatically and transmits the impulse through the atria and the ventricles.
(3) Humoral control. In addition to the "nervous" control of heart action, it appears that there are substances in the blood itself which have varying effects on the functioning of the heart. Although these substances are not as yet well understood, they appear to have some importance. The transplanted heart seems to depend to a degree on this control mechanism, since much of its "nervous controls" have been lost for the initial period in the recipient's body.
c. Coronary Arteries and Cardiac Veins. We may say that the heart deals with two different kinds of blood flow--"functional" blood and "nutritive" blood. "Functional" blood is the blood that the heart works on or pushes with its motive force. However, the walls of the heart require nutrition that they cannot get directly from the blood within the chambers. "Nutritive" blood is supplied to these walls by the coronary arteries, right and left. The coronary arteries arise from the base of the aortic arch and are distributed over the surface of the heart. This blood is collected by the cardiac veins and empties into the right atrium of the heart. Should a coronary artery, or one of its branches, become closed for whatever reason, that part of the heart wall formerly supplied nutrient blood by the closed vessel will very likely die.
d. Pericardial Sac. The average heart contracts in what is known as a heart beat, about 70-80 times a minute. To reduce the frictional forces that would be applied to its moving surfaces, the heart is enclosed in a special serous sac known as the pericardium ("around the heart").
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