Precapillaire

Precapillaire (precapillary) is a term used in physiology to describe the vascular segment that regulates the entrance of blood into capillary networks. It commonly refers to the distal arterioles and metarterioles that feed capillaries and, in some tissue beds, to discrete precapillary sphincters—rings of smooth muscle at the capillary entrance.

Functionally, precapillaries regulate microvascular perfusion by adjusting vascular resistance and flow distribution according to tissue metabolic needs. When arteriolar tone increases, blood flow into certain capillaries is reduced or redirected; when tone decreases, perfusion increases. This modulation ensures delivery of oxygen and nutrients while minimizing waste accumulation.

Mechanisms include smooth muscle contraction of arterioles, regulatory signals from pericytes, endothelial-derived factors (such as nitric oxide and endothelin), and local metabolic cues like oxygen tension, carbon dioxide, pH, and adenosine. The concept of a true, anatomically discrete precapillary sphincter is debated; in many beds flow control is more diffusely distributed, with metarterioles and arteriolar smooth muscle and pericytes contributing to regulation.

Clinical relevance: microvascular dysfunction involving precapillary resistance can accompany shock, diabetes, and inflammatory states, contributing to impaired tissue perfusion despite normal macrocirculatory parameters. Variations across tissues mean the degree of precapillary control differs between organs such as skeletal muscle, the gut, and the brain; cerebral perfusion, for example, is tightly regulated by autoregulatory mechanisms and the blood-brain barrier rather than a simple precapillary sphincter model.