capillary (biology)

In biology, narrowest blood vessel in vertebrates measuring 0.008–0.02 mm in diameter, barely wider than a red blood cell. Capillaries are distributed as beds, complex networks connecting arteries and veins. The function of capillaries is to exchange materials with their surroundings. For this reason, capillary walls are extremely thin, consisting of a single layer of cells through which nutrients, dissolved gases, and waste products can easily pass. This makes the capillaries the main area of exchange between the fluid (lymph) bathing body tissues and the blood. They provide a large surface area in order to maximize diffusion.

All body cells lie close to capillaries so they can receive the food and oxygen they require from the capillaries. Networks of capillaries are especially extensive in certain parts of the body. For example, in the lungs they surround the air sacs (alveoli), taking up oxygen and releasing carbon dioxide. They are also found in the lining of the gut – in little finger-like projections called villi – where they absorb the products of the digestion of food.