Another consequence of thrombophlebitis, especially in the deep veins of the legs, is damage of the valves in the affected veins. These valves prevent backward (downward) flow of blood in the veins when you stand up. Because veins do not have muscle in their walls to help “pump” the blood back to the heart, they are affected by gravity and the gentle squeezing provided by the surrounding skeletal muscle. To improve the flow of blood back to the heart, veins have valves. These valves work like safety cogs on mountain trains, which prevent them from rolling backward if they lose power while climbing up a steep mountain slope. In much the same way, the valves in the veins prevent the blood from “flowing back” as it is gradually pushed uphill toward the heart.When the valves in the veins do not work properly, several problems can occur. The pooling of blood can lead to ballooning of the vein, resulting in varicose veins. In some cases the pooling gets so bad that the leg swells. This condition is commonly referred to as venous insufficiency. With chronic swelling and the associated increase in pressure on the skin, discoloration called stasis pigmentation can develop in some people, and in severe cases, actual skin ulceration can develop.*210\252\8*