In the first place, it is the most important stimulus to the circulation of the blood and the lymph. The movement of the muscles and other organs experts a massaging action on these two streams in the capillaries and lymph spaces; and the increased respiration experts a suction upon the venous blood into the great veins near the heart by the same bellows-like action that draws air into the lungs. This increased breathing is also important to the lungs themselves in filling air spaces, which under normal conditions may not be used, and particularly in inflating the top or apical regions of the lungs.

The increased bodily activity increases oxidation in the body. The extent of this action is shown by the fact that the output of carbon dioxide is increased from three to ten times by muscular activity. Exercise increases the demand of oxygen in the tissues, and thereby stimulates the formation of red corpuscles in red bone marrow and leads to an increase in haemoglobin. The skin is cleared by the generous excretion of the sweat glands and the consequent softening of the horny layers.

The most generally obvious aspect of exercise is the building up of the skeletal muscular system, which constitutes about one half of the bulk and the weight of the body. The need of reserve strength is especially important in the case of heart, which, like the skeletal muscles is build up and strengthens by proper physical exercise.