Precapillary sphinctersa. serve as valves in veins. b. are found in the tunica media. c. carry blood under high pressure. d. control the blood flow into capillary beds. e. are voluntary structures.

D. Control the blood flow into the capillary bed

Explanation:

The degree of filling of the capillary bed and level of arterial pressure within the vascular system is determined mainly by the degree of tonus (firmness) in the smooth muscle of the arterial wall. Arterioles have relatively narrow lumina, thick muscular wall and precapillary sphincter.