(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 at-
tached 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
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. "Function-
al" 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,