Which is the smallest vessel for gas exchange?

• A. Artery
• B. Vein
• C. Capillary
• D. Right Atrium

Answer: (C)

Gas exchange occurs across the walls of capillaries, the smallest blood vessels. Their walls are thin and consists of a single layer of endothelial cells, and they form vast networks that create a large surface area with very short diffusion distances. This setup allows oxygen to diffuse into the blood and carbon dioxide to diffuse out efficiently. Arteries and veins are much larger and have thicker walls designed for transport under pressure, not diffusion. The right atrium is a heart chamber, not a vessel, so it isn’t involved in gas exchange. Thus, capillaries are the smallest vessels where gas exchange happens.