Реферат: Heart Essay Research Paper HEARTThe human heart

directly beneath the endocardium receive a sufficient amount of oxygen from the blood within the cavities

of the heart.

REGULATION OF THE HEARTBEAT

The heart muscle pumps the blood through the body by means of rhythmical contractions (systole) and

dilations (diastole). The heart’s left and right halves work almost synchronously. When the ventricles

contract (systole), the valves between the atria and the ventricles close, as the result of increasing

pressure, and the valves to the pulmonary artery and the aorta open.

When the ventricles become flaccid during diastole and the pressure decreases, the reverse process takes

place: through the valves between the atria and the ventricles, which are now open again, blood is drawn

from the atria into the ventricles, and the valves to the pulmonary artery and the aorta close.

At the end of diastole the atria also contract and thus help to fill the ventricles. This is followed by

systole. The electrical stimulus that leads to contraction of the heart muscle originates in the heart

itself, that is, in the sinoatrial node (SA node), or pacemaker. This node, which lies just in front of

the opening of the superior vena cava, measures no more than a few millimeters. It consists of heart

cells that emit regular impulses. Because of this spontaneous discharge of the sinus node, the heart

muscle is automated, and a completely isolated heart can contract on its own, as long as its metabolic

processes remain intact. The electrical stimulus from the SA node becomes propagated regularly over the

muscle cells of both atria and reaches the atrioventricular node (AV node), which lies on the border

between the atria and the ventricles. The stimulus continues into the bundle of His. This bundle proceeds

for about a centimeter and then divides into a left and a right!

bundle branch. The two bundle branches lie along the two sides of the heart’s septum and then proceed

toward the apex. The small side branches that come off are the Purkinje fibers, which conduct the

stimulus to the muscle cells of the heart’s ventricles.

The Purkinje fibers differ from the cardiac muscle cells and conduct the stimuli more rapidly. However,

the AV node conducts the stimulus relatively slowly. As a result, the heart chambers contract regularly

and evenly during systole, and ventricular contraction does not coincide with that of the atria; so the

pumping function is well-coordinated. Potentially, the whole conduction system is able to discharge

spontaneously and can take over the function of the SA node. The rate at which the cells of the SA node

discharge under normal circumstances is externally influenced through the autonomic nervous system, which