blood. Now, we wish to discuss the supply of nutrient blood to the heart. This blood
nourishes the tissues of the heart. The nutrient blood supplies oxygen and food
materials to the tissues of the heart and removes waste materials. This nutrient blood is
supplied to the walls of the heart by the right and left coronary arteries.
(1) The openings leading into the coronary arteries are located in the base
of the ascending aorta, just above (behind the cusps of) the semilunar valve (aortic
valve). When this valve is open, its cusps cover the openings of the coronary arteries.
When the valve is closed, the backpressure of the blood in the aorta fills the coronary
arteries with blood. The coronary arteries then distribute the blood to all of the tissues
of the relaxed heart.
(2) Many of the branches of the coronary arteries are of the end artery type.
This means that such a branch is the sole supply of nutrient blood to a specific area of
the heart. If the branch should be closed for any reason, the tissue in that area will die
for lack of oxygen and nourishment.
c. Cardiac Veins and Coronary Sinus. The blood from the tissues of the
heart is collected by the cardiac veins. These veins empty into the coronary sinus, a
vessel, which in turn empties into the right atrium.
d. Thebesian Veins. The thebesian veins are many minute sinuses found in
the myocardium of the ventricles. They extend from the lumen into the myocardium of
10-29. HEART SOUNDS
When the valves of the heart close, they produce audible sounds. First, the
closing of the AV valves produces a noticeable "LUB." When the semilunar valves
subsequently close, another sound "DUB" is produced to complete the cycle. These
are referred to as the heart sounds--"LUB DUB, LUB DUB," etc.
10-30. ELECTROCARDIOGRAM (EKG)
Since the myocardial tissue is living material, its activity produces electrical
impulses. With an electrocardiogram, the pattern of these electrical impulses can be
10-31. THE PERICARDIUM
a. General. The heart is an active organ of the human body. Its pumping
action, which begins in the very early embryo, continues without stopping until death.
During each cycle of its activity, the heart changes in shape and size and tends also to
rotate. (The number of cycles per minute is called heart rate.) To reduce the amount of
friction resulting from this activity, the heart is includes within a serous sac, called the
pericardium, or pericardial sac.