capillary

/kap"euh ler'ee/, adj., n., pl. capillaries.

adj.

1. pertaining to or occurring in or as if in a tube of fine bore.

2. resembling a strand of hair; hairlike.

3. Physics.

a. pertaining to capillarity.

b. of or pertaining to the apparent attraction or repulsion between a liquid and a solid, observed in capillarity.

4. Anat. pertaining to a capillary or capillaries.

n.

5. Anat. one of the minute blood vessels between the terminations of the arteries and the beginnings of the veins.

6. Also called capillary tube. a tube with a small bore.

[1570-80; capill(ar) (obs., < L capillaris pertaining to hair, equiv. to capill(us) hair + -aris -AR¹) + -ARY]

* * *

Any of the minute blood vessels that form networks where the arterial and venous circulation (see artery, vein) meet for exchange of oxygen, nutrients, and wastes with body tissues.

Capillaries are just large enough for red blood cells to pass through in single file. Their thin walls are semipermeable, allowing small molecules to pass through in both directions. The smallest lymphatic vessels and minute bile channels in the liver are also called capillaries.

* * *

▪ anatomy

      in human physiology, any of the minute blood vessels that form networks throughout the bodily tissues; it is through the capillaries that oxygen, nutrients, and wastes are exchanged between the blood and the tissues. The capillary networks are the ultimate destination of arterial blood from the heart and are the starting point for flow of venous blood back to the heart. Between the smallest arteries, or arterioles, and the capillaries are intermediate vessels called precapillaries, or metarterioles, that, unlike the capillaries, have muscle fibres that permit them to contract; thus the precapillaries are able to control the emptying and filling of the capillaries.

      The capillaries are about 8 to 10 microns (a micron is 0.001 millimetre) in diameter, just large enough for red blood cells to pass through them in single file. The single layer of cells that form their walls are endothelial cells, like those that form the smooth channel surface of the larger vessels.

      The networks of capillaries have meshes of varying size. In the lungs and in the choroid—the middle coat of the eyeball—the spaces between capillaries are smaller than the vessels themselves, while in the outer coat of arteries—the tunica adventitia—the intercapillary spaces are about 10 times greater than the diameter of the capillaries. In general, the intercapillary spaces are smaller in growing parts, in the glands, and in mucous membranes; larger in bones and ligaments; and almost absent in tendons.

      The smallest vessels in the lymphatic system are also called capillaries, as are the minute channels for bile in the liver. See also artery; vein.

* * *