During exercise, which statement explains the increased blood flow to muscles?

During exercise, which statement explains the increased blood flow to muscles?

• A. Arterioles in muscles constrict to reduce flow
• B. Venous return decreases
• C. Capillary density increases rapidly
• D. Arterioles dilate in muscles to increase blood flow

Answer: (D)

During exercise, working muscles demand more oxygen and nutrients, so blood flow to those muscles must increase. The key mechanism is arteriolar dilation in the active muscles. Local metabolic signals produced by exercising muscle—such as low oxygen, high carbon dioxide, increased hydrogen ion concentration, adenosine, and nitric oxide—cause the smooth muscle in small arteries to relax. This vasodilation lowers resistance to blood flow, allowing a larger volume of blood to reach the muscle’s capillaries, which enhances oxygen delivery and waste removal. Constricting arterioles would do the opposite and reduce flow, while a decrease in venous return doesn’t explain why more blood reaches the muscles during activity. Capillary density can adapt with training over longer periods, but it doesn’t change rapidly during a single bout of exercise; the immediate increase in flow comes from dilation and the recruitment of capillaries within the active muscle.

During exercise, working muscles demand more oxygen and nutrients, so blood flow to those muscles must increase. The key mechanism is arteriolar dilation in the active muscles. Local metabolic signals produced by exercising muscle—such as low oxygen, high carbon dioxide, increased hydrogen ion concentration, adenosine, and nitric oxide—cause the smooth muscle in small arteries to relax. This vasodilation lowers resistance to blood flow, allowing a larger volume of blood to reach the muscle’s capillaries, which enhances oxygen delivery and waste removal.

Constricting arterioles would do the opposite and reduce flow, while a decrease in venous return doesn’t explain why more blood reaches the muscles during activity. Capillary density can adapt with training over longer periods, but it doesn’t change rapidly during a single bout of exercise; the immediate increase in flow comes from dilation and the recruitment of capillaries within the active muscle.