Metarteriole

A metarteriole is a short vessel in the microcirculation that connects a terminal arteriole to a capillary bed. It typically arises from an arteriole and ends at the postcapillary side of the capillary network. In many anatomical descriptions, the metarteriole serves as the link between arterial and capillary segments of a microvascular unit.

Structure and components include a small diameter, lined endothelium, and a surrounding layer of smooth muscle that may form brief rings at the entrance to capillaries, sometimes referred to as precapillary sphincters. Pericytes may be present along the metarteriole. A thoroughfare channel can run through the metarteriole and proximal capillary segment, providing a relatively direct path from the arteriole toward the venule.

Function and regulation involve controlling blood flow into the capillary bed. The smooth muscle rings modulate entry into capillaries in response to local metabolic signals such as oxygen, carbon dioxide, pH, and temperature, as well as neural or hormonal input. When precapillary sphincters constrict, flow into the capillary network is reduced and blood may pass preferentially through the thoroughfare channel; when they relax, perfusion of capillaries increases. Thus the metarteriole contributes to the distribution of blood flow and exchange within tissues.

Clinical and physiological relevance centers on tissue perfusion. Variations in metarteriolar tone or density can influence microvascular exchange and perfusion, with implications for conditions such as shock, inflammation, and edema. The exact organization of metarterioles and precapillary sphincters can vary among tissues, and some descriptions classify the metarteriole as part of terminal arterioles rather than as a distinct structure.