Venous valves (Valvulae) are structures in the veins that perform a valve-like function and thus prevent the blood from flowing back in the wrong direction.
Function of the venous valves
The wall of blood vessels is made up of three different layers. On the outside is the so-called tunica externa (adventitia), in the middle the tunica media (media) and on the inside the tunica interna (intima). In the case of the veins, the intima forms folds into the interior of the vessel at regular intervals. These flaps that are created in this way usually consist of two, sometimes three crescent-shaped sails. The free edge of these sails is always turned towards the heart.
The Veins transport that low oxygen blood back from the body to the heart, Arteries to lead oxygenated Blood in the periphery. In the arteries is that Blood pressure Due to the directly upstream heart, these vessels also have a pronounced muscle layer in the media and can thus actively contract in order to transport the blood further. Since the blood pressure in the veins is very low and their muscles are also very weak, these vessels have to find another way to transport the blood.
This happens through several mechanisms, the most important of which is the so-called muscle pump (when the muscles are tensed, the veins are compressed and the blood practically squeezed out). In order for the blood to actually flow towards the heart, there are the venous valves. These close the vein in the direction opposite to the regular flow as soon as the blood hits them. If the muscle is tensed again, the blood is transported through the overlying venous valve towards the heart and so on. The section between two venous valves is called the valvular sinus. In this area the wall of the veins is more flexible than in the area of the valve attachment.
If these areas are increasingly filled with blood, the so-called varicose veins develop: bulges between the individual venous valves that typically arise in the area of the lower legs and become visible under the skin. If the venous valves can no longer close properly due to a pathological process and the veins therefore widen secondarily, fill more with blood and the blood flow slows down, this is called chronic venous insufficiency (CVI).
The valves are stronger and more numerous, the more the blood has to be transported against gravity and the more the valves have to "withstand". In the veins of the legs, especially the lower legs, there are many venous valves, but fewer in the veins of the upper half of the body. In a few veins there are even no valves at all, including the pulmonary veins, the cerebral sinuses, the two large vena cavae and the umbilical vein.
Valves that work according to the same principle still exist in humans in the vessels of the lymphatic system.