Smallest vessels for nutrient and waste exchange.

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Nutrient and waste exchange happens at the capillaries because they are the smallest blood vessels with extremely thin walls, just a single layer of endothelial cells. This ultra-thin barrier plus their vast networks surrounding every cell provides the ideal setup for diffusion, allowing oxygen and nutrients to move from the blood into tissues and wastes to move from tissues into the blood. Arteries and the aorta are large, thick-walled vessels designed to carry blood away from the heart under high pressure, not to exchange substances. Veins return blood to the heart and have thinner walls than arteries, but they don’t provide the primary site for nutrient and waste exchange. So the vessels specialized for this exchange are the capillaries.