the movement of blood through the smallest blood vessels, or capillaries, providing for the exchange of substances between the blood and tissues.

Capillary circulation is made possible by the difference between the hydrostatic pressures of the venous and arterial ends of capillaries. The pressure in the arterial end is 30–35 mm Hg, or 8–10 mm higher than the osmotic pressure of blood plasma. As a result of this difference in pressures, water and many of the substances dissolved in it (except macromolecular proteins) pass from the blood plasma into the interstitial fluid, transporting vitally needed substances to the tissues. As the blood flows through a capillary, hydrostatic pressure drops in the venous end to 12–17 mm Hg, or approximately 10 mm below the osmotic pressure of the blood. As a result, water and the substances dissolved in it pass from the interstitial fluid into the plasma, helping thereby to remove metabolic products from the tissues. The rate of capillary circulation corresponds to the rate of metabolism. For example, in a body at rest, there are 30–50 functioning capillaries per sq mm of cross section of skeletal muscle; during intensive muscular activity the number may increase 50–100 times.