How normal veins function
Veins carry blood to the heart. The veins in the leg have a particular problem in getting the blood back to the heart, because they have to carry the blood uphill, against the force of gravity. But the body has a mechanism to deal with the situation, consisting of the following two elements.

• Some of the leg veins are deep in the muscle. These are called the deep veins. The contractions of the leg muscles during walking squeeze these veins, forcing blood along; this is called the muscle pump.
• To prevent the blood going backwards and away from the heart, veins have one-way valves.

Not all the veins in the leg lie in the muscle like the deep veins; some are nearer the surface of the leg, in the skin and fatty tissue outside the muscle. These are called superficial veins. There are connections between the deep veins and those outside the muscle. Each connection has a valve, which ensures that blood flows from the vein outside the muscle into the deep vein, and not back the other way.