How can the heart be strong enough to pump blood up your legs against gravity?
The heart is not strong enough by itself to get the blood back up the veins in your legs and back to your heart. The human body relies on a second system to finish that task. This system involves small valves throughout the veins and muscle contractions from your skeletal muscles when you walk and move about. The valves close when blood starts to flow in one direction, so that blood in the veins can only flow in the direction back to the heart, which is up the legs. When you squeeze your leg muscles to walk, stand, kick, and move about, the muscles squeeze the veins and force the blood to get moving. Because of the valves, the blood can only move in one direction as it gets squeezed along. So it is a combination of blood pressure from the heart's pumping action, the valves, and muscle movement that gets the blood up the legs against gravity. If the valves malfunction, then the blood falls back down to some extent after every muscle contraction and begins to pool in the veins. This causes the veins to swell with blood, which can be painful and unsightly, and is known as varicose veins
