Which vessel is the site of gas exchange?

• A. Artery
• B. Vein
• C. Capillary
• D. Aorta

Answer: (C)

Gas exchange happens across the walls of tiny vessels called capillaries, where a very thin barrier sits between the air in the alveoli and the blood. Capillaries have enormous surface area and extremely thin endothelium, so oxygen can diffuse into the blood and carbon dioxide can diffuse out into the alveolar air efficiently. Arteries and veins transport blood but their walls are thicker and their main role is conduction, not diffusion, so they aren’t the sites where gas exchange occurs. The aorta is a major artery and likewise not involved in diffusion of gases. So the best answer is the capillary.