Which structure prevents backflow of blood in veins?

Veins contain specialized structures known as valves that play a crucial role in maintaining the unidirectional flow of blood towards the heart. These valves are designed to close and prevent the backflow of blood, which is particularly important in the context of the lower extremities where blood must travel against the force of gravity.

As blood is pushed through the veins during contraction of nearby skeletal muscles, valves open to allow blood to move forward. When the muscles relax or if blood tries to flow backward, the valves close, effectively stopping any reverse flow. This mechanism ensures that blood continuously returns to the heart, maintaining proper circulation.

The other structure mentioned, muscles, do assist in moving blood through veins by compressing them during physical activity, but they do not specifically prevent backflow. Capillaries are small blood vessels where the exchange of substances occurs and do not contain valves. Arteries are responsible for carrying blood away from the heart and have a different structure that does not involve valves for backflow prevention. Hence, the defining feature of veins that facilitates the prevention of backflow is indeed the presence of valves.