Smallest blood vessels with walls one cell thick that allow gas exchange.

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

Answer: (B)

Gas exchange happens most efficiently in the capillaries because their walls are only one cell thick, formed by a single layer of endothelium. This ultra-thin barrier provides a short diffusion path so oxygen can move from blood to tissues and carbon dioxide can move from tissues into the blood. Capillaries are the smallest vessels, connecting arterioles to venules, so they are right where cells are located. Arteries and arterioles have thicker, muscular walls to cope with high pressure, and veins have thinner walls with larger lumens and are focused on returning blood to the heart rather than gas exchange.