How does the circulatory system work?
Arteries carry blood away from the heart. They make sure that oxygen and nutrients reach the organs and tissues. The veins then collect the oxygen-depleted “used” blood in their capillary layer, such as in the hands and feet.
What is the job of veins?
The main task of the veins is to carry blood back to the heart. We can distinguish between two different types of veins in the legs: surface veins and deep veins. Surface veins run just under the skin and in many people are easily visible. They carry the blood to the large deep veins that run between the muscles. Most of the blood (90 %) is carried by the deep veins away from the legs towards the heart.
How is a vein constructed?
In comparison to arteries, veins have a far thinner muscle layer and are far more elastic. This means that large quantities of blood can collect in the veins. To prevent blood from fl owing backwards, veins differ from arteries in that they are equipped with valves.
These valves in the veins guide blood up from the feet in the direction of the heart. They are particularly numerous in those veins in which the blood fl ows against gravity, for example, in the legs. Here the valves in the veins function as check valves. As long as the blood fl ows in the direction of the heart, the valves open. If gravity pulls the blood towards the feet, then the valves close.
How is blood pumped “upwards”?
The most important mechanism involved in transporting blood from the feet back to the heart is the so-called “calfmuscle pump.” When the legs move, in particular when we walk, the calf muscles are exerted. This causes squeezing of the deep leg veins running between the calf muscles. The valves in the upstream veins open, and blood contained in the veins is pushed “upwards” against gravity.
When the calf muscles relax, pressure on the veins is reduced and they enlarge. The fl ow of blood to the heart slows down, and the valves in the veins close so that the blood does not fl ow back in the wrong direction. At the same time, the emptied veins fi ll up again with “fresh” blood from the surface venous system.