Varicose veins affecting a man's legs. Varicose veins are veins that have become distended, lengthened and tortuous. They form due to defective valves in the veins of the legs. As blood is circulated around the legs, valves prevent it from sinking to the feet under gravity. If the valves stop working, blood can pool downwards, giving the external appearance seen here. The superficial veins (saphenous veins) of the legs are most commonly affected. Treatment includes elastic support, stripping (avulsion, surgical removal of the veins) or sclerotherapy, where the vein is injected with an irritant which eventually blocks it, re-routing the blood.