The smallest blood vessels.

Capillaries are the smallest blood vessels that form networks throughout the tissues of the body. It is through the capillaries that oxygen, nutrients and waste are exchanged between blood and tissues. The capillary networks are the final destination of arterial blood from the heart and the starting point for the flow of venous blood back to the heart. Between the smallest arteries, or arterioles, and the capillaries are intermediate vessels called precapillaries, which, unlike capillaries, have muscle fibers that allow them to contract. Thus, the precapillaries can control the emptying and filling of the capillaries.