A precapillary sphincter is a band of smooth muscle that adjusts blood flow into capillaries mainly in the mesentericmicrocirculation. At the point where each of the capillaries originates from an arteriole, a smooth muscle fiber encircles the capillary. This is called the precapillary sphincter. The sphincter can open and close the entrance to the capillary, by which contraction causes blood flow in a capillary to change as vasomotion occurs. The entire capillary bed may be bypassed by blood flow through arteriovenous anastomoses or through preferential flow through metarterioles. If the sphincter is damaged or cannot contract, blood can flow into the capillary bed at high pressures. When capillary pressures are high (as per gravity, etc.), fluid passes out of the capillaries into the interstitial space, and edema or fluid swelling is the result.
Precapillary sphincters and metarterioles were discovered in the mesenteric circulation in the 1950s. Medical and physiological textbooks, such as Guyton, Boron and Fulton, etc. were quick to claim the existence of said sphincters and metarterioles all over the body, despite lack of evidence. At least since 1976 there has been considerable debate about the existence of precapillary sphincters and metarterioles. As of 2013[update] it is held that they are unique to the mesenteric circulation and some researchers have suggested the term precapillary resistance instead.