Capillary

Any of the vessels that carry blood between the smallest arteries, or arterioles, and the smallest veins, or venules (see circulatory system). Capillaries form a fine network throughout the body’s organs and tissues. Their thin walls are permeable and allow blood and cells to exchange constituents such as oxygen, glucose, carbon dioxide, and water (see respiration). Capillaries open and close to blood flow according to the requirements of different organs. The opening and closing of skin capillaries helps to regulate temperature. A direct blow to the body may rupture the thin capillary walls, causing bleeding under the surface of the skin, which in turn causes swelling and bruising. Increasing age, high doses of corticosteroid drugs, and scurvy (vitamin C deficiency) make capillaries more fragile; a tendency to purpura (small areas of bleeding under the skin) may develop.