kapillare

Capillary, in its biological sense, refers to the smallest blood vessels in the circulatory system as well as to the related physical phenomenon of capillary action. The term derives from the Latin capillus, meaning hair, reflecting their slender, hair-like dimensions.

In biology, capillaries form the microcirculation network that connects arterioles and venules. Their walls are typically one cell thick, formed mainly by endothelial cells with a surrounding basal lamina and, in many tissues, pericytes. Diameters range about 5 to 10 micrometers, just wide enough for a red blood cell to pass in single file. Capillaries enable the exchange of gases, nutrients, and waste between blood and surrounding tissues by diffusion across the thin wall, aided by porous or fenestrated endothelium in some vessels. Blood flow through capillary beds is regulated to meet local metabolic demand, and the overall density of capillaries varies by tissue and physiological state.

Capillaries are categorized by wall structure and permeability into continuous, fenestrated, and discontinuous (sinusoidal) types. Continuous capillaries have intact walls and tight junctions, fenestrated capillaries possess small openings, and discontinuous capillaries have larger gaps, allowing greater exchange. They are distributed throughout most tissues, with specialized arrangements in organs such as the brain, kidneys, and liver.

Capillary action, a separate but related concept, describes the ability of liquids to flow in narrow spaces due to cohesion and adhesion, often observed in thin tubes or porous materials. The height of capillary rise depends on surface tension, contact angle, fluid density, tube radius, and gravity, as described by capillary rise equations. This phenomenon underpins many natural and technological processes, including soil moisture movement and ink transport in wicks.