Regulate blood flow by ensuring that it moves in one direction. They open to allow blood to pass through, and then close to prevent back flow.

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

Answer: (C)

Valves regulate blood flow by ensuring blood moves in one direction. They open to let blood pass and close to prevent backflow, a mechanism seen in the heart (the atrioventricular and semilunar valves) and in many veins. When forward pressure pushes blood through, the valve leaflets separate; when pressure would push blood backward, the leaflets come together to block the path. This unidirectional control is essential for efficient circulation and to prevent pooling or backflow. Other vessels like veins, arteries, and capillaries carry blood or exchange substances, but they don’t actively regulate direction through opening and closing like valves do.