Which vessels are the site of nutrient and gas exchange with tissues?

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

Answer: (A)

Capillaries are where nutrient and gas exchange with tissues occurs because their walls are only a single layer of endothelial cells, creating the tiny, highly permeable interface between blood and tissue cells. This thin barrier minimizes diffusion distance, and the extensive capillary network provides a large surface area so oxygen and nutrients can move from blood into interstitial fluid and into cells, while carbon dioxide and wastes move in the opposite direction. Blood flow slows as it passes through capillaries, giving more time for diffusion and exchange. Arteries and veins transport blood but have much thicker, muscular walls and are focused on bulk transport and pressure; venules are small veins that carry blood back toward the heart and also aren’t the primary sites of exchange.