Which vessel type is primarily responsible for exchange of nutrients and gases between blood and body tissues?

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

Answer: (B)

Capillaries handle exchange because their walls are extremely thin—essentially a single layer of endothelial cells—and they form dense networks right next to tissue cells. This setup creates a large surface area and the short distance needed for diffusion, so oxygen and nutrients move from blood into tissues and carbon dioxide and other wastes move from tissues into blood efficiently as blood flows slowly through these tiny vessels. Arteries have thick, muscular walls built for high-pressure transport away from the heart, not exchange. Veins and venules transport blood back to the heart and have thinner walls, but the exchange of substances occurs mainly in capillaries, not in these larger vessels.