Your body experiences many changes during exercise. Some changes are immediate and occur while you are active. Others are cumulative, with long-lasting effects extending over months, years, and decades. Such responses represent attempts at bodily adaptation to the stress of muscular work. Physiology is the study of body function. It is an assessment of bodily reactions under various situations such as rest, exercise, altitude, starvation, advancing age, and many more. It also is a comparison of the degree and efficiency of function of the organs under these circumstances.

In terms of fitness, the physiology of the resting organism is of little interest. Of great importance, however, is an understanding of physiological responses during and after exercise. Improving quality and quantity of performance, increasing functional reserve, and shortening recovery times can be measured and are the significant parameters of the physiology of exercise. With specific stimuli, specific organs are called upon to do the work. After eating, the digestive tract is busy. While studying, the brain is on the alert.

During exercise the muscles are active. The corollary implication is that other organs are relatively quiescent. Blood is diverted from the abdominal organs, where it is not needed, to muscles, where the demand is greater. Thus the kidneys close up shop almost completely.

Muscles are composed of long, narrow cells which can shorten (contract) or lengthen (relax). The number of muscle cells per person remains constant. The width or bulk of these cells, however, is determined by the amount of work they do. The muscles of sedentary people shrink down and become skinny from lack of work. Active muscles widen or hypertrophy because of continuing exercise inducing greater strength and endurance. The muscles of a physically active man are bigger due to hypertrophy, not to an increase in the number of cells. Muscular strength, stamina, and efficiency are modified by fitness. Moreover, blood supply improves.

A limiting factor in muscular work is blood supply he ability of the vascular system to deliver oxygen and nutrient and to remove un-desired and toxic waste products. In animals, at least, there is good evidence to show that training increases perfusion of muscles. Not only is the number of capillaries bathing a muscle important, but also the distance from the wall of the capillary to the muscle cell—the distance which oxygen must travel to be used. The nearer the capillary to the muscle cell, the greater is the efficiency of oxygen utilization. Gnat wing capillaries are many times nearer the wing muscle cells than are human capillaries and muscle cells. And the gnat muscles are accordingly more efficient.

Strength and stamina activities imply "steady state" exercise; that is to say, where sufficient oxygen is supplied to the muscles minute by minute. This is called "aerobic metabolism." If the muscles are exerted faster than oxygen can be supplied, this is maximal work and can be maintained for only a few minutes. An "oxygen debt" is incurred during this brief period of so-called "anaerobic metabolism." When exercise ceases, oxygen consumption continues at a high level until "the oxygen debt" has been paid up. This explains why people breathe hard for a few minutes after exercising.

In the trained individual oxygen consumption is increased, sub maximal and maximal work can be continued longer before fatigue occurs, performance is better, and speed is greater.