Blood in the legs is returned to the heart through two systems of veins. One set of veins is located deep inside the leg, near the bone. The other system consists of superficial veins near the surface. The systems are linked by veins called perforators. To prevent blood from flowing downward instead of back to the heart, veins have one-way valves that close off in between heartbeats. If something causes these valves to malfunction, whether it's an injury or a hereditary weakness, blood flows backward and the veins swell. A blood clot deep in the veins can also impede blood flow and lead to pooling of blood. The long-term inability of leg veins to return blood to the heart is known as chronic venous insufficiency, or CVI (C-V-I). CVI can cause leg swelling; varicose veins; chronic pain; and skin discoloration, itching, or ulcers near the ankles. Minor cases may be treated with support stockings, which help leg muscles push blood upwards. In advanced cases, surgery is necessary. Past techniques involved tying off the perforator veins, but newer methods may allow doctors to repair the faulty valves or to re-route blood flow by grafting on a healthy length of vein from another area.