Most of our blood volume is carried in the veins (64%). Veins can expand to hold large amounts of blood. Veins are blood vessels that carry blood from the body back to the heart. Blood return from the legs occurs mainly through the deep veins. Within the veins, especially those of the legs are valves. Venous valves are bicuspid (two) flap like structures made of elastic tissue. The valves function to keep blood moving in one direction.
The flow of blood in the venous system is complex for several reasons: the low pressure within the veins, flow rates that vary from high (during muscle contraction) to almost no flow during quiet standing or sitting positions, the effects of gravity, the collapsible nature of the venous wall, the presence of valves, and the large volume of blood carried in the veins.
Once the blood has passed from the arteries through the capillaries, it is flowing at a slower rate because little pressure remains to move the blood along. Blood flow in the veins below the heart is helped back up to the heart by the muscle pump. The walls of the veins are thin and somewhat floppy. To compensate for this many veins are located in the muscles. Movement of the leg squeezes the veins, which pushes the blood toward the heart. When the muscles contract the blood within the veins is squeezed up the vein and the valves open. When the muscle is at rest, the valves close helping to prevent the backward flow of blood. This is referred to as the muscle pump.
Healthy legs have veins with smooth, elastic walls that are perfectly designed to adapt to the changes in pressure within a vein. Veins have valves that keep blood moving in one direction: back toward the heart. As the leg muscles are activated, the venous valves open to allow one-way flow in the direction of the heart. When the muscles relax, the valves close to stop any back-flow.
But if the walls of a vein have been damaged by varicosis or thrombosis, the vein may dilate and the valves fail to close properly. When valves fail to work properly, blood flows backward into the veins. This results in blood pooling, putting pressure on lower leg veins, which may cause even more valves to fail over time. So when the body is upright, the blood being transported back to the heart may stagnate in the legs. The pressure in the superficial veins directly under the skin rises and the veins become swollen. Tired, aching legs are the mostcommon early symptoms – particularly after prolonged standing. Later, fluid may collect in the feet and ankles causing them to swell. The skin above the ankles may become thin and discoloured or even break to form a venous stasis ulcer.
In the adult population of western countries it is estimated that 40–50% of men and 50–55% of women have some form of venous disorder of the lower limb* – with all the pain and discomfort that it involves. Venous disorders range from varicose veins to deep vein thrombosis and postthrombotic syndrome, a particular serious sequel to such conditions.