Distinguish between arteries, veins, and capillaries in terms of structure and function.

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Understanding how structure maps to function in blood vessels helps explain why arteries, veins, and capillaries differ. Arteries carry blood away from the heart under high pressure, so they have thick, muscular walls with elastic tissue to withstand and regulate that pressure and to push blood forward. Veins return blood to the heart at lower pressure; they have thinner walls, a larger lumen, and valves to prevent backflow as blood travels through the venous system. Capillaries are the sites of exchange; their walls are extremely thin—often just a single cell layer—so gases, nutrients, and wastes can diffuse between blood and surrounding tissues quickly. This combination of features—arteries with thick, muscular walls; veins with valves to support one-way flow; and capillaries with thin walls for exchange—best reflects how structure supports function.