Precapillary sphincters: (a) Precapillary sphincters are rings of smooth muscle that regulate the flow of blood through capillaries; they help control the blood flow to where it is needed. (b) Valves in the veins prevent blood from moving backward.

What is the action of precapillary sphincters?

Therefore, precapillary sphincters represent a mechanism to equalize pressure and RBC flux between the capillary networks that branch off from the upper, middle, and lower parts of the PA. Simultaneously, sphincters protect downstream capillaries and brain tissue against adverse blood pressure.

What causes precapillary sphincters to relax?

The smooth muscle of the metarterioles and the precapillary sphincters contracts and relaxes regularly causing intermittent flow in the capillaries: this is known as vasomotion. A local drop in pO2 is the most important factor causing relaxation of the precapillary sphincters.