A
precapillary sphincter is a band of
smooth muscle that adjusts
blood flow into
capillaries mainly in the
mesenteric microcirculation. 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.