The circulatory system
These functions are carried out most effectively and efficiently when the involved tissues receive an abundant blood flow. Under normal resting conditions, the kidneys, which account for only 1% of the body’s weight, receive 20% of the cardiac output (CO); the gastrointestinal tract receives approximately 27% of the CO; and the skin receives 6 to 15% of the blood pumped by the heart per minute. Because these tissues receive more blood than they need to support metabolic activity, they can easily tolerate a
sustained decrease in blood flow. During exercise, when the metabolic demand of the working skeletal muscles and the heart increases substantially, blood flow is directed away from the kidneys and organs of the digestive system and toward the skeletal and cardiac muscles.