Blood returning to the heart must travel upward against gravity, especially from the legs. Veins accomplish this by using one way valves placed along their inner walls. These valves are thin flaps of tissue that open when blood moves toward the heart and close when blood tries to flow backward. When muscles around a vein contract, they squeeze the vein and push blood upward. As soon as the muscle relaxes, the valves close to prevent the blood from slipping downward. This step by step movement cr