(biology) Narrowest blood vessel in vertebrates, 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. Capillary walls are extremely thin, consisting of a single layer of cells, and so nutrients, dissolved gases, and waste products can easily pass through them. This makes the capillaries the main area of exchange between the fluid (lymph) bathing body tissues and the blood.
1. A tube of small internal diameter; holds liquid by capillary action; SYN. capillary tube, capillary tubing.
2. Any of the minute blood vessels connecting arterioles with venules.

ETYM Latin capillaris, from capillus hair. Related to Capillaire.
1. Long and slender with a very small internal diameter; SYN. hairlike.
2. Relating to or involving capillarity.
3. Pertaining to or like a hair.

In physics, a very narrow, thick-walled tube, usually made of glass, such as in a thermometer. Properties of fluids, such as surface tension and viscosity, can be studied using capillary tubes.