The Hearth General Features of the Circulation

The tissues of the body require, for their normal functioning, a reasonable degree of constancy with respect to certain factors; among these factors are: temperature, acidity, food supply, and oxygen. The primary function of the circulation of the blood is to ensure the preservation of this constant internal environment by transporting oxygen, food materials and hormones to the tissue cells and by removing the waste products of activity. The interchange of materials between the blood and the tissues occurs in the thin-walled capillaries. The rest of the circulatory system, including the heart, exists solely for the purpose of maintaining the capillary exchange. The heart is a muscular pump which imparts sufficient kinetic energy to the blood to move it through the capillaries. The arteries conduct the blood from the heart to the capillaries and the veins conduct the blood from the capillaries back to the heart again. As will be seen in later chapters, the arteries and veins are not simply passive conducting tubes, but are also responsible, through alterations in their diameters, for the proper distribution of the blood to various organs and tissues in accordance with their metabolic requirements.

From the standpoint of the physiology of exercise, the heart is primarily a respiratory organ. In the periods of rest between bouts of exercise, the muscles are able to store sufficient food materials to initiate exercise and to maintain it until reserves can be mobilized. There is, however, no mechanism for the storage of oxygen in the tissues. Any increase in the oxygen requirement must be satisfied by a corresponding increase in the transport of oxygen to the tissues. This is accomplished in two ways: (1) by diverting blood to the contracting muscles from less active regions and (2) by increasing the volume of blood pumped by the heart per minute. Not only is the flow of blood to the muscles increased when they become active, but in addition a larger volume of oxygen is removed from each volume of blood. Of these adaptive mechanisms, the increased pumping action of the heart is the most important in terms of the share it contributes to the adjustment of oxygen supply to the requirements of the contracting muscles. It is also the most vulnerable link in the chain; the upper limit to the volume of oxygen which can be delivered to active muscles is almost invariably set by the capacity of the heart to increase its output. For these reasons the heart occupies the key position in any scheme of the physiological adjustments in exercise.