What structural feature in veins helps maintain one-way flow and assist venous return?

• A. Valves preventing backflow
• B. Thick tunica media
• C. High-resistance arterioles
• D. Abundant elastic arteries

Answer: (A)

Valves are the key feature in veins that promotes one-way flow toward the heart. These leaflets act as cusps that open to permit blood to move forward when veins are squeezed by surrounding muscles or when pressure from blood flowing toward the heart rises, and then close to prevent backward flow when pressure would push blood back toward the feet. This valve mechanism is especially important in the legs, where gravity opposes return; together with the skeletal muscle pump and the respiratory pump, valves keep venous return efficient despite low venous pressure.

In contrast, the other options describe properties typical of the arterial system or of veins only in a general sense. Thick tunica media is characteristic of arteries that handle higher pressure, high-resistance arterioles are part of the arterial microcirculation, and abundant elastic arteries are large arteries near the heart. These features do not provide the unidirectional, low-pressure return mechanism that valves provide in veins.