Synonyms
blood vessel, veins, body circulation
A vein is a blood vessel containing blood that flows towards the heart. In the major circulation of the body, blood that is always low in oxygen flows through the veins, whereas in the pulmonary circulation, blood that is always rich in oxygen flows from the lungs to the heart. Compared to arteries, veins have a different structure and functions.
Important veins in the body
Among the most important veins in the human body are the inferior and superior vena cava (inferior and superior veins), which conduct all the venous blood in the body to the heart. They are the largest veins in the body. Parallel to this drainage system there is also the azygos or hemiazygos system.
These two veins run parallel to the inferior and superior vena cava further down the back, thus providing a second drainage route for venous blood, so that constrictions can be bypassed. The veins are almost always named in the same way as the corresponding arteries. Exceptions are, for example, the great rose vein (Vena saphena magna), a superficial vein in the legs, or the internal and external jugular veins (Vena jugularis interna and externa), which lead the venous blood from the head and neck region back into the upper vena cava.
Special features in the construction
Looking at the microscopic (histological) structure of the veins, it can be seen that it corresponds to that of the artery of the muscular type. However, the individual layers of the vein are thinner and looser and contain more connective tissue than arteries of the same size. This can be explained by the fact that the body’s venous system has a much lower blood pressure, so that fewer muscle cells are needed in the vascular wall to counteract the high internal pressure.
Furthermore, there are also local differences in veins. In leg veins, for example, there is a thicker muscle layer in the vascular wall than in the arm veins, since there is a higher water pressure (hydrostatic pressure) in the legs. This is due to the fact that there is more blood above the legs than above the arms, so the weight of the blood above is higher for the leg veins than for the arm veins.
The outer layer (tunica adventitia) of the veins is the thickest layer and is often highly interconnected with the adjacent tissue. This is done by means of connective tissue trains that radiate into the surrounding tissue and thus fix the vein. In addition, the vein is kept open in this way and does not collapse when the internal pressure decreases. This ensures that even with low blood pressure and in anemic regions of the body, the blood can always flow back to the heart and is not blocked by closed veins.
