What is the structure in veins that prevent blood backtracking?

The structures in veins that prevent blood from backtracking are called valves. Veins are blood vessels that carry blood back to the heart. They have one-way valves that ensure that blood only flows in one direction - towards the heart. The valves are made of thin flaps of tissue that are attached to the inner wall of the vein. When blood flows through the vein, the valves open to allow the blood to pass through. When the blood pressure drops, the valves close to prevent the blood from flowing back.