How do the three layers of a vein wall differ from those of an artery?

Both arteries and veins are composed of three layers—the tunica intima, tunica media, and tunica adventitia—but their structural composition differs significantly to accommodate different blood pressures.123

• Tunica Intima: In arteries, this layer includes an elastic membrane lining that is absent in veins. Many veins, particularly in the limbs, contain valves formed by folds in the tunica intima to prevent the backflow of blood, a feature not found in arteries.123
• Tunica Media: This middle layer is much thicker in arteries, consisting of substantial smooth muscle and elastic fibers to withstand high pressure. In veins, the tunica media is thinner, containing less muscle and elastic tissue but proportionately more collagen fibers.132
• Tunica Adventitia: This outermost layer is the thickest layer of the vein and is composed chiefly of connective tissue. While it serves as a supportive element in both vessels, the arterial tunica adventitia is generally designed to protect against overexpansion under high pressure.213

Overall, veins have thinner, more compliant walls and larger lumens than arteries, allowing them to function as blood reservoirs.23