Varicose veins are enlarged veins that often appear in the legs. Normal veins have a series of valves, which prevent the pooling of blood in the lower leg. Each normal valve allows blood to move up in the vein, but blocks it from moving back down. If a single valve fails, the valve below has to hold up a longer column of blood, making the lower valve prone to fail as well. This process can go on over years, eventually causing most of the valves to fail. Eventually, this results in a vein that may be stretched to several times its normal size.