Blood Vessels

Anastomoses

Smaller arteries often unite to form a branching network before entering capillaries. These networks are called anastomoses and can occur between branches of equal or unequal size. Anastomoses allow an alternative source of blood from two or more arteries to enter a capillary bed so that if a single artery is occluded by pressure, disease, or damage, the blood supply is not interrupted.

Vascular Shunts

In many areas, blood may be routed directly from arterioles to veins without passing through the capillary bed. Arterioles have precapillary sphincters that can narrow or even close down the vessel. At times of low demand by the tissue supplied by the capillary bed in question, most of the precapillary sphincters will be closed so that only a maintenance supply of blood passes through the capillary bed. When demand is high, most sphincters will be opened.