Oxygen, carbon dioxide, nutrients, and wastes are exchanged between the blood and the body tissues via capillaries. Capillaries are part of a large branching network of circulatory vessels and are the final destination of arterial blood flowing from the heart. Correspondingly, the capillaries are also the starting point for flow of venous blood returning to the heart. Located between the arterioles and the capillaries are precapillaries, transitional vessels possessing muscle fibers that enable them to contract. Therefore, the precapillaries are able to dictate the draining and filling of the capillary beds.