Small versions of arteries that can dilate to supply active organs.

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

Answer: (A)

The ability to regulate blood flow to active organs relies on the small arteries, known as arterioles, which can dilate to increase perfusion. These vessels are the tiny branches of arteries just before the capillaries and have smooth muscle in their walls. When organs become more active, chemical signals cause this smooth muscle to relax, widening the vessel (vasodilation) and lowering resistance. That drop in resistance lets more blood reach the tissues to meet the higher metabolic demand. Arteries are larger and do not adjust flow to individual tissues as directly, capillaries are where exchange occurs, and veins return blood to the heart. So arterioles are the vessels that dilate to supply active organs.