The skin is the body's largest organ, accounting for approximately 15% of the body mass of the average adult. The skeletal muscles are the largest collection of common structures, totaling 40% of the body mass. The skin and the skeletal muscles are each significantly affected by blood flow during exercise.
The skin is composed of two defined parts, the epidermis and the dermis. The epidermis is the outer covering of the body, acting as both a shield against foreign objects that might enter the various bodily systems, as well as insulation and support for the internal organs and tissues. The dermis is located below the epidermis, and contains the capillaries, the blood vessels that provide the tissue with its necessary nutrients, the subcutaneous glands that release oils and perspiration from the skin, and the roots for human hair. During exercise, the dermis is immediately affected by changes in blood flow.
Skeletal muscles are the source of power for all skeletal movement by the body through their contraction, which is stimulated through a complex interplay of nerve impulses. Skeletal muscles also assist in the support of the entire bodily structure. The individual muscles are each composed of a series of fibers, arranged into bundles of various sizes. The muscle fibers contain cells where the energy-to-power movement is produced. These cells are supplied with oxygen and nutrients, such as glucose or fatty acids, through the capillaries that extend directly into the muscle. The blood also removes the waste products that occur through energy production in the muscle cells. Each muscle fiber is encircled with three or four capillaries. When the body is performing with efficiency, oxygen and nutrients are carried to the muscle cells by the blood at the same rate with which waste carbon dioxide and other metabolites are removed.
An increase in blood flow is described as active hyperemia. The effect of exercise on the flow of blood within the body is progressive. To accommodate the demands for oxygen, the heart will begin to beat faster and to pump more powerfully. An increased heart rate will stimulate increased blood pressure within the cardiovascular system as well as increased blood volume to counter the demands of exercise. Depending on external factors such as temperature and other environmental conditions, the thermoregulatory system of the body will seek to achieve a balance between the maintenance of the body's core temperature and the release of perspiration to cool the body. These functions cause blood to be directed toward the surface of the skin. The warm blood from the internal areas is cooled through this exchange; perspiration causes a reduction in blood volume over time unless the fluids are replaced. In warm weather conditions, as much as 30% of the cardiac output goes to direct the flow of blood to the skin for cooling; the evaporation of perspiration will act to reduce body temperature. The control of the capillaries that expand and contract in the process of increasing and decreasing blood flow to the skin is determined through the nerve structures connected to the autonomic nervous system, the specialized regulation by the brain of a number of essential body functions.
In cold conditions [when the body is exposed to temperatures that fall below 40°F (4°C), the autonomic nervous system will seek to maintain the warmth of the vital internal organs, and blood will correspondingly be directed away from the extremities and the skin surface to the internal organs. In such circumstances, the skin temperature will fall and the skin, both the epidermis and the underlying dermis, becomes vulnerable to freezing, leading to the injury known as frostbite.
When the body is at rest, approximately 20% of the cardiac output is directed to the maintenance of blood flow to the skeletal muscles. The rate of skeletal muscle blood flow in the body's resting state is 3 ml/minute per 100 mg of muscle mass. Very shortly after the start of exercise, the blood flow rate to the muscle will increase by as much as 20 times the resting rate. In sports such as swimming, cross-country skiing, or running, where the entire body and almost all skeletal muscles are working in some capacity, the cardiac output directed to the skeletal muscles approaches 80%. There is a rough correlation between blood flow increases and the increase in the amount of oxygen consumed during exercise.
The increased blood flow is directed through a cardiovascular device known as the skeletal muscle pump. The veins that direct the spent arterial blood back to the heart and lungs are constructed with a one-directional valve that promotes venous return, permitting the body to recycle and recharge the blood more quickly.