Blood is pumped from the heart to the legs through the arteries. The arteries divide into fine capillaries where oxygen and nutrients are delivered to the cells. The blood in the capillaries then flows into the venous system to be returned to the heart. The contraction of the muscles in the legs helps this flow. When we walk the calf and thigh muscles squeeze the deep veins and force the blood upwards.
There are two sets of veins at work in the legs; the deep system within the muscles and the superficial system just beneath the skin. The superficial system has two major veins; the great saphenous vein, which runs along the inner side of the thigh and joins the deep veins at the groin, and the short saphenous vein, which runs along the back of the leg and joins the deep veins behind the knee. Numerous smaller channels called perforators also connect the superficial system to the deep system. Both systems are equipped with one-way valves that only allow the blood to flow from the superficial system to the deep system and upward towards the heart.