Capillaries - Circulation

We know that most tissues have such an extensive network of capillaries that any single cell is hardly 2-3 cells away from any capillary. The small terminal arteries subdivide to form arterioles which divide to form metarterioles and then capillaries, which rejoin to form the venules and veins.

The smooth muscles of the arterioles become discontinuous in the metarterioles and end in a muscle ring the precapillary sphincter that controls the blood supply to each capillary bed. Through the precapillary sphincters, the capillary bed can be bypassed altogether and blood can be diverted to areas of greater demand. All capillaries of an animal have the potential to hold 14% of the total blood volume. However, only 30% to 50% of all capillaries are open at a time and thus only 5-7% of the total volume is contained in them.