The purpose of the precipillary sphincter is to adjust the flow of blood into the capillaries. These smooth muscles contract and act as a valve to redirect blood to the area where the body needs it most. They encircle each capillary branch at its exit from the arteriole, according to Reference.com.
As blood flows through the human body, it moves through a branched system of arteries to the capillaries, where it delivers oxygen and nutrients while picking up waste materials. Deoxygenated blood then returns, through the branched system of veins to the heart. Because the body has a limited blood supply and its tissues differ in their energy and oxygen requirements according to their activity at the time, "the precapillary sphincters reduce blood flow to inactive tissues and allow free flow into active tissues," according to Innerbody.com. This mechanism allows blood flow to increase to the digestive system after a meal, aiding in digestion as well as nutrient absorption.
According to About.com, blood is constantly supplied to all parts of the body. The precapillary sphincters only close off certain capillary beds at any time. When a particular capillary bed is closed, the blood flows freely from the arteriole to venule using the thoroughfare channel.