capillaries

Capillaries are the smallest blood vessels in the circulatory system, forming the microcirculation between arterioles and venules. Their primary function is to enable the exchange of gases, nutrients, and wastes between blood and surrounding tissues. Capillaries have a very small diameter, about 5 to 10 micrometers, just large enough for a red blood cell to pass through. They consist of a single layer of endothelial cells and a surrounding basement membrane, with occasional supportive pericytes.

There are three main structural types: continuous capillaries, fenestrated capillaries, and discontinuous (sinusoidal) capillaries. Continuous capillaries have intact endothelium with tight junctions and are common in muscle, skin, lungs, and the brain, where the endothelial barrier is relatively restrictive. Fenestrated capillaries have pores (fenestrae) that permit higher permeability and are found in the kidneys, small intestine, and endocrine glands. Discontinuous capillaries, or sinusoids, have large gaps and a discontinuous basement membrane, allowing passage of larger molecules; they occur in the liver, spleen, and bone marrow.

Exchange across capillaries occurs mainly by diffusion of gases and small solutes, aided by water and small molecule pores, and sometimes by vesicular transport. The rate of flow through capillary beds is regulated by precapillary sphincters and local signals, coordinating tissue perfusion with metabolic demand. Some tissues have high capillary density, while others are sparsely vascularized, reflecting functional needs and metabolic rate.

Clinical relevance: capillary permeability and density can change with inflammation, injury, and disease. Diabetic microangiopathy, aging, and ischemia can impair capillary function, while angiogenesis accompanies wound healing and tumor growth. Capillaries play a key role in maintaining fluid balance through Starling forces, affecting edema and tissue hydration.