Exercise

The circulatory system supplies oxygen and other vital substances to tissues of the body. Obviously, the demands on this supply line increase sharply during exercise. Human energy is customarily expressed in terms of oxygen consumption; oxygen consumption is to the body what gasoline consumption is to the automobile. Oxygen consumption is at its minimum during complete rest (resting oxygen consumption ) and measures in an average adult between

200 and 250 cc of oxygen per minute. This level (1 MET ) covers the essential metabolic needs of the body (basal metabolism ). The level of oxygen consumption rises steeply with activity since the oxygen cost of human effort is quite high. For example, walking at a moderate pace increases oxygen consumption to about three times its basal level. Strenuous exercise, such as climbing stairs briskly or running, may increase the basal demands for oxygen by as much as five to eight times. Maximum possible effort for a healthy individual occurs at the cost of 10 to 15 times the basal oxygen consumption; for a trained athlete it may reach 20 times the resting level. This high need for oxygen is one of the principal limiting factors of human exercise. The limitation depends on the possible top performance of the two principal systems involved in the process: respiration, supplying enough oxygen to the air spaces of the lungs; and circulation, delivering enough oxygen to the working tissues.

The burden of delivering 10 to 20 times the basal amount of oxygen to the tissues is considerable, and it is supportable only because of work-saving adaptive mechanisms. These mechanisms apply primarily to the circulatory apparatus, since the respiration is usually capable of increasing its work in proportion to high demands. The circulatory system is so designed that delivery of oxygen can increase much more than the work of the heart and circulation can. This is made possible through three adaptive mechanisms. First, in periods of high demand the circulatory system makes full use of the available oxygen supply. Normally during rest only a small part of the available oxygen is consumed by the tissues. Blood returning to the right side of the heart is, at rest, still 75 percent saturated with oxygen, indicating that only one-fourth of the available supply has been utilized. Better utilization of oxygen is an effective work-saving device for the heart. For example, if a certain form of exercise demands four times the resting amount of oxygen, the tissue can easily draw twice as much oxygen as during rest (reducing the oxygen saturation of the returning venous blood from 75 percent to 50 percent), in which case the volume of blood circulating through the tissues (cardiac output) has only to double instead of quadruple to meet the full demand. Second, blood can be redistributed (by way of regional flow regulation, as discussed earlier) so that the working muscles or organs get a greater share of the flow than do less important regions. Thus during exercise the digestive tract, the kidneys,

the brain, the skin, and other nonparticipating organs are perfused with less blood than during rest in order to supply the heart and the working muscles with more oxygen. Third, working muscles can perform temporarily with a smaller supply of oxygen than needed to meet actual energy demands. This "oxygen debt" is repaid immediately after exercise ceases. This mechanism is particularly important for short-term, high-intensity forms of exercise.

These adaptive mechanisms are essential, since the heart has a rather limited capacity for increasing its work. It is estimated that in a healthy individual, cardiac output can only increase to four or at most five times its resting level (from a normal of 5 liters per minute to 20 or 25 liters per minute). Thus, as a general rule, at peak cardiac performance the potential oxygen supply to the tissues rises fourfold; the tissues can extract up to four times as much oxygen from the blood during exercise than at rest; therefore top muscular performance is about 16 times the resting level when expressed in terms of oxygen consumption.

The following analogy may help illustrate the adaptive circulatory process during exercise. Let us imagine a large industrial plant with ample supplies of raw material but no storage facilities for its principal fuel, coal, which has to be brought in. The normal daily manufacturing activities of the plant require a quantity of coal equivalent to five carloads; however, since the shortest train consists of 20 cars, only one-quarter of each car is unloaded, thereby supplying the needs. At times the plant is called on to increase temporarily the manufacture of one of its principal products. The increased fuel demands are met in part by bringing in longer trains, and in part by unloading more coal from each car. The plant can also, to conserve fuel, slow down or eliminate the manufacture of some less essential product. If one assumes that the rail-loading facilities at the other end of the communication line limit coal delivery to 80 carloads a day, the maximum manufacturing capabilities of the plant would amount to 16 times its ordinary level, if all 80 carloads were brought in and all the coal in them unloaded.

Physiologists and clinicians often find it necessary to calculate the work of the heart, at rest and during exercise, in terms of work delivered (external work ) rather than the actual energy utilized by the heart. External work is expressed by the following formula: work equals blood pressure times cardiac output, or

W = P × F . The increase in work during exercise is directly related to the increase in cardiac output, since blood pressure shows little change, its level being well regulated by the previously described barostatic mechanism.

In spite of the efficient work-saving devices for the heart and the circulation operating in the healthy individual, exercise imposes a heavy strain on the circulatory system. Obviously, under conditions of less than perfect health, certain functions of the circulation begin to lag, and the efficiency of the circulatory adjustment may suffer. Exercise, which may reveal faulty circulatory function long before it becomes evident under resting conditions, thus provides one of the fundamental tests of the circulatory apparatus in the study of cardiac disease.