Regulation of Heartbeat

The heart muscle pumps the blood through the body by means of rhythmical contraction (systole) and dilations (diastole). On the heart system there are 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, in the sinoatrial node (SA node), or pacemaker. This node lies just in front of the opening of the superior vena cava. It consists of heart cells that emit regular impulses. This electrical stimulus becomes propagated 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, which proceeds for about a centimeter and then divides into a left and aright bundle branch. The two bundle branches lie a long the two sides of the heart’s septum and then proceeds toward the apex. 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 muscles cells and conduct the stimuli more rapidly. The AV node conduct the stimulus relatively slowly, however. 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 sends nerve branches to the heart and the determines the resultant heart rate. In adults a rest this is between 60 and 74 beats a minute. In infants and young children it may be between 100 and 120 beats a minute. Tension, exertion, or ever may cause the rate of a healthy heart to vary between 55 and 200 beats a minute.

The output of the heart is expressed as the amount of blood pumped out of the heart each minute: the heart minute-volume (HMV). This is the product of the heart rate and the stroke volume (SV), The amount of blood pumped out of the heart at each contraction.