Humans spend a long time standing up and in this position the heart is about a metre or so higher than the veins in the lower leg. This makes it very difficult to get blood back to the heart from the legs. There are two main mechanisms which normally help the blood back up to the heart. First is the muscle pump in the calf. As the muscles contract they squeeze the blood out of the deep veins lying between them. Some of this blood will tend to be squeezed up and some down. This is where the second mechanism comes in ---- the valves in the veins which, when functioning, allow blood to flow upwards only. Consequently in healthy legs when the muscles contract they squeeze blood out of the deep veins and as the valves stop it going down it all goes up in the direction of the heart. On relaxation of the calf muscles the pressure in the deep veins lowers and blood flows from the superficial into the deep system. There are also valves which stop blood flowing the wrong way from deep to superficial. Blood flow into the legs is nicely balanced by blood being pumped out and pressure in the veins is kept low.