Veins and their function
Against gravity – back to the heart
The heart serves as a pump for the transport of blood in the arteries and transports in the case of a human adult 7,000 litres of blood per day in the circulatory system. This volume of blood must also be carried back to the heart by the veins and on account of man's upright posture an additional pump mechanism is needed to carry blood against gravity towards the heart.
This requires a complicated mechanism: The „muscle-vein pump“ of the leg muscles, also called the“calf muscle pump“. It fulfils the most important function in transporting back blood, as the veins in the leg cover the longest distance to the heart.
How the muscle-vein pump functions:
The leg’s movement activates the calf muscles. The muscle belly thickens and presses together the deep veins lying between the muscles.
The veins narrow, blood has less space to diffuse and flows faster against gravity in the direction of the heart. So-called venous valves support the blood in flowing back from the legs. Valves located at opposite sites act as non-return valves. If the blood flows towards the heart, the valves lie back against the vascular wall and allow the blood to flow unhindered in the direction of the heart.
However, if the blood flows back, the venous valves shut, obstruct the passage and prevent the blood from flowing “back”. Should the muscles relax, this results in suction which refills the empty veins with venous blood. The number of venous valves in the different veins varies from 2 to 20 valves per vein.