Veins are blood vessels that carry blood back from the leg towards the heart. There are two sets within the leg. A deep set near the bones and within the muscles and a superficial set under the skin. Most of the blood flowing back towards the heart is carried in the deep set and very little blood flows in the superficial veins.
When standing, blood must flow uphill. As muscles in the legs contract, they squeeze the deep veins and push blood up the leg. As the muscles relax, blood is prevented from flowing back down the leg by the action of valves. These are delicate folds of the inside lining of the veins. The combined action of the muscles and the valves keeps blood flowing up the leg. Problems with these valves can lead to superficial venous reflux and varicose veins.
Superficial venous reflux
The superficial veins join the deep veins at various points along the leg. Blood is prevented from flowing out of the deep veins and into the superficial veins by the action of the valves. In a condition called Superficial Venous Reflux, the valves at these junctions and within the superficial veins are not functioning properly. In Superficial Venous Reflux, blood flows down the superficial veins in the wrong direction, back down towards the feet. This back pressure damages valves further down the leg and stretches the veins under the skin. Varicose veins then develop.