capillars

Capillars is a term sometimes encountered in historical or regional contexts as an alternative spelling for capillaries, the smallest blood vessels of the circulatory system. They form the microvasculature between arterioles and venules and have luminal diameters of about 5 to 10 micrometers. The walls consist of a single layer of endothelial cells supported by a basement membrane, and in many tissues they contain pericytes that help regulate blood flow and vascular stability. The endothelial lining is often covered by a glycocalyx that participates in barrier function and mechanotransduction.

Capillaries are classified by permeability into continuous, fenestrated, and discontinuous (sinusoidal) types. Continuous capillaries have an uninterrupted endothelium and are common in muscle and connective tissue; fenestrated capillaries possess pores and are found in organs with active exchange, such as the kidneys and small intestine; discontinuous capillaries have larger gaps and occur in the liver, spleen, and bone marrow, enabling transport of larger molecules.

Microcirculation in capillaries enables exchange between blood and tissues through diffusion, transcytosis, and filtration. Gases like oxygen and carbon dioxide diffuse across the endothelial layer; nutrients, hormones, and waste products are exchanged with interstitial fluid. Exchange rates depend on blood flow, capillary density, and the balance of hydrostatic and oncotic pressures across the capillary wall.

Capillaries form during angiogenesis and can adapt to tissue needs. Capillary dysfunction or loss of density is associated with diseases such as hypertension, diabetes, and ischemia, contributing to edema and impaired tissue perfusion.