The deep veins lie within and between the muscles of the calf and thigh. All the muscles are surrounded by a firm leathery layer of "facia", so as they contract and relax blood is forced up the deep veins. Normally, the valves in the veins make sure that the blood flows only upwards in the deep and superficial veins, and inwards through the perforating veins.

If the valves stop working, then the muscle pump cannot do its job properly. Damaged valves in the deep veins mean that blood is not pumped upwards, and this can be particularly harmful if the veins at knee level are affected. Failure of valves in the perforating veins allows blood to be pushed out under high pressure into the superficial veins, which can lead to varicose veins.