Blood flow through tissues is matched with the metabolic needs of the tissues. During exercise, blood flow through tissues is changed dramatically. Its rate of flow through exercising skeletal muscles can be 15 to 20 times greater than through resting muscles. The increased blood flow is the product of local, nervous, and hormonal regulatory mechanisms. When skeletal muscle is resting, only 20% to 25% of the capillaries are open, whereas during exercise 100% of the capillaries are open.

Low oxygen tensions resulting from greatly increased muscular activity or the release of vasodilator substances such as lactic acid, carbon dioxide, and potassium ions causes dilation of precapillary sphincters. Increased sympathetic stimulation and epinephrine released from the adrenal medulla cause some vasoconstriction in the blood vessels of the skin and viscera and some vasodilation of blood vessels in skeletal muscles. Consequently, resistance to blood flow in skeletal muscle decreases, and resistance to blood flow in the skin and viscera increases somewhat. Therefore blood is shunted from the viscera and the skin through the vessels in skeletal muscles.

The movement of skeletal muscles that compresses veins in a cyclic fashion and the constriction of veins greatly increase the venous return to the heart. The resulting increase in the preload and increased sympathetic stimulation of heart result in elevated heart rate and stroke volume, which increases the cardiac output. As a consequence, the blood pressure usually increases by 20 to 60 mm Hg, which helps sustain the increased blood flow through skeletal muscle blood vessels.

In response to sympathetic stimulation, some decrease in the blood flow through the skin can occur at the beginning of exercise. However, as the body temperature increases in response to the increased muscular activity, temperature receptors in the hypothalamus are stimulated. As a result, action potentials in sympathetic nerve fibers causing vasoconstriction decrease, resulting in vasodilation of blood vessels in the skin. As a consequence, the skin turns a red or pinkish color, and a great deal of excess heat is lost as blood flows through the diilated blood vessels.

The overall effect of exercise on circulation is to greatly increase the blood flow through exercising muscles and to keep blood flow through other organs at a low value, which is just adequate to supply their metabolic needs.