b. 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.
c. 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").
THE PROPERTY OF INHERENT RHYTHMICITY
a. The heart muscle (myocardium), like other muscles, is dependent upon
electrical energy for its proper contraction. One property of cardiac muscle that cannot
be found in any other muscle is inherent rhythmicity. Inherent rhythmicity is the property
of the cardiac muscle that allows cardiac muscle cells to beat separately without any
stimulation. If a cardiac muscle cell is placed in a saline (salt) bath containing the
required amount of essential electrolytes the muscle cell will contract and relax
rhythmically with no external stimulation. Furthermore, if another cardiac cell is placed
in the same bath it, too, will beat at its own separate rate. It is interesting that when the
two cardiac cells are placed together (in contact) the two cells will begin to beat as a
unit. The property of inherent rhythmicity allows the myocardium to beat together with a
minimal amount of nervous stimulation.
b. Instead of initiating the contractile process, nervous stimulation functions
rather to govern the rate of the heartbeat. The property of inherent rhythmicity appears
to be embryonic in origin. That is, the heart begins beating and systemic circulation
occurs before any nervous tissue is formed.
a. Initiation of the Cardiac Impulse. The initiation of the cardiac impulse begins
in a highly specialized node of nervous tissue known as the sinoatrial node (also known
as the SA node). As the name implies, the sino-atrial node is located in one of the atria-
specifically the right atrium. The SA node initiates the electrical impulse that spreads
out over both the atria causing the atrial muscles to contract. The fact that the SA node
is located within the right atrium explains why the right atrium contracts 0.08 seconds
before the left atrium contracts--although the contraction of the atria can still be
considered to be simultaneous. As the atrial, muscle contracts the impulse travels
through the atrial muscle to the atrioventricular (AV).node