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.
n. an extremely narrow blood vessel, approximately 5–20 ?m in diameter. Capillaries form networks in most tissues; they are supplied with blood by arterioles and drained by venules. The vessel wall is only one cell thick, which enables exchange of oxygen, carbon dioxide, water, salts, etc., between the blood and the tissues (see illustrations).
a quickly performed test to assess the adequacy of circulation in an individual with poor cardiac output. An area of skin is pressed firmly by (say) a fingertip until it loses its colour; the number of seconds for the area to return to its original colour indicates capillary refill time. Normal capillary refill takes around 2 seconds. Slow capillary refill may occur globally in an individual with poor circulation or in a small area (e.g. a toe) in which local circulation is compromised (due, for example, to peripheral vascular disease). This test may not be very useful in people with dark skin.... capillary refill time
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