Microcapillaries

Microcapillaries are the smallest vessels in the microcirculation, formed by the terminal portions of capillary networks that connect arterioles to venules. Diameters typically range from about 3 to 8 micrometers, often allowing red blood cells to pass in a single file. They are the principal sites of exchange between blood and surrounding tissues.

Structurally, microcapillaries consist of a single layer of endothelial cells atop a basement membrane. Depending on tissue, the endothelium can be continuous, fenestrated, or, in rare cases, discontinuous. Pericytes may accompany the endothelium, contributing to stability and regulation of capillary tone. The basement membrane and pericyte coverage vary by tissue.

Functionally, their primary role is exchange of gases, nutrients and waste. Oxygen and carbon dioxide diffuse across the endothelium; glucose, amino acids, and electrolytes travel via diffusion or transcytosis. Capillary walls also regulate fluid exchange and maintain selective permeability to protect tissue homeostasis.

Flow and caliber are regulated by local signals from the endothelium and surrounding tissue, including nitric oxide and endothelin, and by pericyte contraction. Capillary density and length vary with tissue metabolism, with higher density in brain, skeletal muscle, and retina compared with less active tissues.

Pathology can affect microcapillaries in ways that impair exchange. Microvascular dysfunction occurs in diabetes, hypertension, sepsis, and ischemia, leading to impaired perfusion, increased permeability, capillary leakage, or dropout. Obstruction by microthrombi or inflammatory cells can also reduce exchange efficiency and tissue oxygenation.

Assessment techniques include capillaroscopy, intravital or confocal microscopy, and advanced imaging to evaluate capillary density, flow, and permeability; electron microscopy provides ultrastructural detail of endothelial junctions and basement membrane. The term "microcapillary" emphasizes the smallest exchanges within the broader microcirculation.