General Description of the Heart

The salient anatomical features of the heart may be reviewed. The heart is a hollow muscular organ, subdivided internally into four chambers: the right and left auricles (or atria) and the right and left ventricles. The auricles are thin-walled collecting chambers. They have little contractile power and serve primarily by storing the blood brought to them by the veins during the contraction, or systole, of the ventricles and then passing this blood on to the ventricles during their period of relaxation, or diastole. The ventricles, on the other hand, are thick-walled muscular chambers which exert considerable force during contraction. This contractile force imparts the necessary kinetic energy to the blood to maintain its circulation.

From a functional standpoint, the heart may be divided into the "right heart" (right auricle and ventricle) and the "left heart" (left auricle and ventricle). The right heart receives venous blood from all the systemic veins of the body and pumps it through the pulmonary arteries into the lungs, where oxygen is absorbed and carbon dioxide eliminated. The oxygenated blood is returned through the pulmonary veins to the left heart which in turn pumps it through the aorta into the systemic arteries of the body. The orifices between the auricles and their corresponding ventricles, and the exit of the pulmonary artery and the aorta from their respective ventricles are guarded by valves which permit the flow of blood in one direction only.

The heart muscle itself is not nourished by the blood contained within its chambers, but is supplied by the coronary arteries which leave the aorta just beyond the aortic valves.

The stimulus which causes contraction of the heart muscle at each beat arises within the heart itself, in a specialized muscle mass known as the sine-auricular (S-A) node or "pacemaker." The impulse is conducted to all parts of the heart muscle by way of a specialized conducting system, the auriculo-ventricular (A-V) node and bundle. The rate of beating of the heart is regulated by two sets of nerves, the vagus nerves which slow the rate and the accelerator nerves which increase the rate.

The heart is enclosed in a fibrous sac called the pericardium. A thin film of fluid (the pericardial fluid) separates the heart from the pericardium and minimizes the friction which otherwise would occur during contraction and relaxation of the heart. The pericardium probably serves a protective function in preventing dangerous overdistention of the heart. It is uncertain to what extent the pericardium limits the stroke volume (amount of blood pumped by the heart at each beat) during exercise. While the pericardium is relatively nonelastic and hence resists attempts at sudden stretching, it can be stretched very slowly to permit the normal increase in heart size often seen in athletes or the pathological dilatation of the heart which occurs in certain types of heart disease.