Definition
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. The wall of blood vessels is formed by three different layers. On the outside is the so-called tunica externa (adventitia), in the middle is the tunica media (media) and right inside is the tunica interna (intima).
In the veins, the intima forms wrinkles in the interior of the vessel at regular intervals. These resulting flaps usually consist of two, sometimes three crescent-shaped sails. The free edge of these sails is always turned towards the heart.
The veins transport the oxygen-poor blood back from the body to the heart, arteries carry oxygen-rich blood to the periphery. In the arteries, the blood pressure is still very high due to the directly upstream heart. In addition, these vessels in the media have a pronounced muscle layer and can thus actively contract to transport the blood further.
However, 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 further. This is done by several mechanisms, the most important of which is the so-called muscle pump (when the muscles become tense, the veins are compressed and the blood is practically squeezed out). But in order for the blood to really 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 meets it. If the muscle is tensed again, the blood is transported further towards the heart through the overlying venous valve and so on. The section between two venous valves is called the valvular sinus.
In this area, the wall of the veins is more elastic than in the area where the valve is attached. If these areas are increasingly filled with blood, the so-called varicose veins develop: Sacking between the individual venous valves, which typically occurs in the area of the lower leg and becomes visible under the skin. If, due to a pathological process, the venous valves can no longer close properly and the veins therefore dilate secondarily, fill with blood to an increased extent and the blood flow slows down, this is known as chronic venous insufficiency (CVI).
The valves are the stronger and more numerous the more the blood has to be transported against gravity and the more the valves have to “endure”. In the veins of the legs, especially in the lower legs, there are many 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 sinus, the two large vena cava and the umbilical vein. In humans, valves that function according to the same principle are still present in the vessels of the lymphatic system.
