kapiláry

Kapiláry are the smallest blood vessels in the circulatory system, forming a network that links arterioles to venules and enabling microcirculation. With diameters around 5–10 micrometers, capillaries allow red blood cells to pass in single file, maximizing surface area for exchange of gases, nutrients, and wastes between blood and tissues.

Structure: The capillary wall is mainly a single layer of endothelial cells on a basement membrane. Pericytes, when present, regulate blood flow and stability. Capillaries are classified as continuous, fenestrated, or sinusoidal. Continuous capillaries have uninterrupted endothelium and are common in muscle and brain; fenestrated capillaries have pores in the endothelium and are found in kidneys and endocrine glands; sinusoidal capillaries have large gaps and are typical of liver, bone marrow, and spleen.

Function: Exchange occurs primarily by diffusion, supported by capillary permeability and, in some types, transcytosis. Oxygen and carbon dioxide, glucose, amino acids, and other solutes move between blood and interstitial fluid. Water movement is governed by hydrostatic and oncotic pressures. Local metabolic activity and inflammatory signals regulate flow, with nitric oxide playing a key role.

Physiology and health: Capillary density and perfusion adapt to tissue demand such as exercise. Structural changes or impaired regulation—capillary rarefaction, leakage, or endothelial dysfunction—are linked to diseases including hypertension and diabetes, and to tissue inflammation.

Development: Capillaries form through vasculogenesis and angiogenesis during development and repair, driven by endothelial progenitors and growth factors.