The precapillary sphincter is a band of smooth muscle that adjusts the blood flow into each capillary. Blood flow in a capillary changes as vasomotion occurs. The entire capillary bed may be bypassed by blood flow through arteriovenous anastomoses. A precapillary sphincter encircles each capillary branch at the point where it branches from the arteriole. Contraction of the precapillary sphincter can close the branches off to blood flow. If the sphincter is damaged or can not contract, blood can flow into the capillary bed at high pressures. When capillary pressures are high (and this can be the result of gravity), fluid passes out of the capillaries into the interstitial space, and edema or fluid swelling is the result.

References

• Fundamentals of Anatomy & Physiology: seventh edition, Frederic H. Martini
• Dzulfitree B. Ahmad, CUCMS MBBS 07/08 student