Kapillairen

Kapillairen, or capillaries, are the smallest blood vessels in the circulatory system, forming the connection between arterioles and venules. They consist of a single layer of endothelial cells supported by a basement membrane, and in many tissues they enclose pericytes that provide stability and regulate blood flow.

There are three main structural classes: continuous, fenestrated, and sinusoidal capillaries. Continuous capillaries have an intact endothelial lining with tight junctions and are found in muscle, lung, and many organ systems including the central nervous system. Fenestrated capillaries have small pores (fenestrae) in the endothelium and are common in kidneys, endocrine glands, and the intestinal mucosa, allowing higher permeability. Sinusoidal capillaries have larger gaps and a discontinuous basement membrane, seen in liver, bone marrow, and spleen, enabling the passage of larger molecules and cells.

Functionally, capillaries are the primary site of exchange between blood and tissues. Gases (oxygen and carbon dioxide), nutrients, hormones, and waste products diffuse through capillary walls, aided by transcellular transport and bulk flow. Red blood cells typically pass through capillaries in a single file due to their narrow diameter, about 5–10 micrometers. Capillary density correlates with tissue metabolic activity and is especially high in organs such as the brain, heart, and kidneys.

Regulation and physiology: Blood flow through capillary networks is controlled by arterioles and pre-capillary sphincters; local factors such as nitric oxide, carbon dioxide, pH, and inflammatory mediators modulate permeability and perfusion. Fluid exchange is governed by Starling forces, with lymphatic drainage returning excess interstitial fluid.

Clinical relevance: Capillary function affects edema, inflammation, and microvascular disease. Capillaries also support angiogenesis during wound healing and tumor growth, making capillary health essential for tissue viability.