Which vessel type is most directly involved in the exchange of gases and nutrients with tissues?

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

Answer: (B)

Gas and nutrient exchange with tissues happens most efficiently where blood is in close contact with cells and can diffuse across a very thin wall. Capillaries are the smallest vessels with walls that are basically a single layer of endothelial cells, forming a huge, winding network. This setup creates a large surface area and very slow blood flow, so oxygen and nutrients can diffuse into surrounding tissue and carbon dioxide and wastes can diffuse back into the blood over a tiny distance. Arterioles and venules feed into and drain from these capillary beds, but their thicker walls and smaller role in diffusion limit direct exchange compared to capillaries. Different capillary types adjust permeability for various tissues, but the essential point remains: capillaries are the primary site of gas and nutrient exchange.