capillariescontributes

Capillaries contribute to the body's microcirculation by serving as the principal site of exchange between the blood and tissues. They form an extensive network that connects arterioles to venules and enable the delivery of oxygen and nutrients while removing carbon dioxide and metabolic wastes. Capillaries vary in permeability and structure: continuous capillaries have uninterrupted endothelium and are common in muscle and skin; fenestrated capillaries have pores that accelerate exchange in organs such as the kidneys and small intestine; sinusoidal capillaries have large gaps and a discontinuous basement membrane, allowing movement of cells and large proteins in the liver, spleen, and bone marrow. Exchange across capillary walls occurs mainly by diffusion, with some vesicular transport, and is influenced by the endothelial glycocalyx and the basement membrane.

The Starling forces, including capillary hydrostatic pressure and plasma oncotic pressure, regulate net fluid exchange. Most filtration occurs at the arterial end of a capillary and most reabsorption at the venous end, with the lymphatic system returning excess interstitial fluid to the circulation. Capillary blood flow is dynamically regulated through local autoregulation and precapillary sphincters and can recruit additional capillaries in active tissues.

Pericytes contribute to capillary stability and blood flow regulation. Capillary density can increase with exercise or tissue growth and can expand through angiogenesis in response to hypoxia, mediated by growth factors such as VEGF.

Dysfunction of capillaries underlies many diseases, including diabetic microangiopathy, inflammatory edema, sepsis, and tumor angiogenesis.