Which structure contains valves to prevent blood from flowing in the wrong direction?

• A. Arteries
• B. Veins
• C. Capillaries
• D. Atria

Answer: (B)

Valves that prevent backflow are most characteristic of veins. As blood returns to the heart, especially from the limbs, pressure can drop and gravity can pull blood downward. One-way valves in veins open to let blood move toward the heart and close when it might try to flow backward, keeping the circulation moving in the right direction. Arteries carry blood away from the heart under higher pressure and don’t have these valves along their length. Capillaries are tiny exchange vessels with no room for valves. The heart’s atrial chambers themselves don’t contain valves that regulate flow within the atria; valves are at the junctions between chambers and vessels. So the structure that contains valves to prevent backward flow is the veins.