The blood vessel that alters the flow of blood to the tissues is the:

• A. Capillaries
• B. Venules
• C. Arterioles
• D. Arteries

Answer: (A)

Flow to tissues is controlled primarily by arterioles, the small arteries that surround each capillary bed. Their smooth muscle can contract or relax, changing the diameter and thus the resistance to blood flow. When arterioles constrict, resistance goes up and less blood reaches the tissue; when they dilate, resistance drops and more blood flows to meet metabolic needs. Capillaries are the sites where exchange occurs, so their role is to receive the appropriate amount of blood delivered by upstream arterioles; the flow through capillaries depends on the tone of those arterioles. Arteries deliver blood at high pressure to the body, and venules drain blood from capillaries, but they don’t actively tune how much blood reaches a tissue in real time.