capillary

Capillary is a term used for several related concepts in biology and physics. In physiology, a capillary is the smallest blood vessel, connecting arterioles and venules. Capillaries have diameters of about 5–10 micrometers and consist of a single layer of endothelial cells supported by a basement membrane; they usually lack smooth muscle, though pericytes may accompany the endothelium. This thin wall facilitates exchange: nutrients, gases, and wastes diffuse between blood and surrounding tissue. Transport can also occur by vesicular transcytosis and, in some tissues, by filtration and reabsorption linked to hydrostatic and osmotic pressures.

Capillaries are classified by permeability: continuous capillaries have uninterrupted endothelium (found in muscle, skin, lungs); fenestrated capillaries have small pores and are common in the kidney and endocrine glands; discontinuous (sinusoidal) capillaries have larger openings and are found in liver, spleen, and bone marrow.

Capillary action refers to the movement of liquids through narrow spaces due to adhesion to surfaces and cohesion within the liquid; this causes capillary rise in tubes and pores. The height of rise is approximated by Jurin’s law: h = 2γ cosθ /(ρ g r), where γ is surface tension, θ is contact angle, ρ is liquid density, g is gravity, and r is tube radius. Capillary action is important in plants and soils and is exploited in laboratory techniques such as capillary tubes for blood collection and in microfluidics and chromatography.

In summary, capillaries play a central role in microcirculation and exchange, while capillary action describes a separate physical process that also has wide scientific and practical applications.