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Why do veins have thinner walls and larger internal spaces than arteries?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

Veins have thinner walls and larger internal spaces (lumens) than arteries primarily because the blood within them is under substantially lower blood pressure.12

While arteries must withstand the intense, pulsating pressure generated by the heart to transport blood to the body's tissues, veins carry blood back to the heart at a much lower pressure. Consequently, veins do not require the thick, highly elastic, and muscular walls that arteries use to manage high-pressure flow and recoil.132

The thinner, more compliant walls of veins allow them to expand easily, enabling these vessels to serve as reservoirs that hold a large proportion of the body’s blood volume. Because the pressure in veins is so low, they rely on other mechanisms—such as the contraction of surrounding skeletal muscles and a system of internal valves—to ensure blood continues to flow in one direction toward the heart, particularly against the force of gravity.123