The venous system of the lower limbs
The veins carry the blood back to the heart and lungs. Its blue color is due to the low oxygen content. When standing, nearly half of the venous blood is in our legs. The blood transit from the legs to the heart has to overcome gravity. This requires the action of a pump. The pump function is mainly achieved by the foot and ankle and calf movement. During each step the muscles compress the veins located inside the calf muscle thereby allowing the venous blood to rush back to the heart.
The venous blood return
In healthy individuals, venous blood flows mainly from the superficial veins (located right under the skin) to the deep veins, through the muscles. Once in the deep veins, the blood moves up to the heart and the lungs to be re-oxygenated.
In order to direct the blood flow from the foot to the upper leg, and to prevent it from falling back, the leg veins have thin valves that work as a lock system. In a normal vein, one-way valves are located every 2-5 cm to aid in the proximal flow toward the heart. When calf muscles relax, the valves close to prevent blood from flowing backward into the lower part of the veins.
Thus, they play a major role in the blood return. These valves are fragile and can be easily damaged. Defective valves are mainly responsible for the development of venous disease.