What is the flap that opens and closes to allow blood into veins travel in Only One Direction?

The flap that opens and closes to allow blood into veins travel in only one direction is called a semilunar valve (also known as a venous valve). These valves are one-way valves that prevent backflow of blood in veins. They are located at regular intervals along the length of the veins, especially in the limbs and organs where there is a risk of pooling or obstruction to blood flow.