The normal function of leg veins - both the deep veins in the leg and the superficial veins - is to carry blood back to the heart. During walking, for instance, the calf muscle acts as a pump, contracting veins and forcing blood back to the heart.
To prevent blood from flowing in the wrong direction, veins have numerous valves. If the valves fail (a cause of venous reflux), blood flows back into superficial veins and back down the leg. This results in veins enlarging and becoming varicose. The process is like blowing air into a balloon without letting the air flow out again- the balloon swells.
To succeed, treatment must stop this reverse flow at the highest site or sites of valve failure. In the legs, veins close to the surface of the skin drain into larger veins, such as the saphenous vein, which run up to the groin. Damaged valves in the saphenous vein are often the cause of reversed blood flow back down into the surface veins.