Blood vessel, veins, systemic circulation
A vein is a blood vessel that contains blood that flows to the heart. In the great systemic circulation, oxygen-poor blood always flows through the veins, whereas in the pulmonary circulation oxygen-rich blood always flows from the lungs to the heart. Veins have a different structure and functions than arteries.
Among the most important veins of the human body are the inferior and superior vena cava ( inferior and superior vena cava), which conduct all of the body's venous blood to the heart. They are the largest veins of the body.
Parallel to this drainage system there is the Azygos or Hemiazygossystem. These two veins continue to run parallel to the inferior and superior vena cava, thus providing a second drainage pathway for venous blood so that bottlenecks can be bypassed. The naming of the veins is almost always like the associated arteries. Exceptions are, for example, the large vena saphena magna, a superficial vein in the legs, or the internal and external jugular vein (internal and external jugular vein ), which carry the venous blood from the head and neck region back into the superior vena cava.
Considering the microscopic ( histological ) construction of the veins, it is found that this 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 equally sized arteries. This can be explained by the fact that in the venous system of the body a much lower blood pressure prevails, so that fewer muscle cells in the vessel wall are needed to counter the high internal pressure.
There are also local differences in veins. Thus one finds in the leg veins a thicker muscle layer in the vessel wall than in the arm veins, since in the legs a higher water pressure ( hydrostatic pressure ) prevails. This is because there is more blood above the legs than above the arms and so the weight of the overlying blood is higher for the leg veins than for the arm veins.
The outer layer ( tunica adventitia ) of the veins is the thickest layer and often strongly networked with the neighboring tissue. This is done by connective tissue trains, which radiate into the surrounding tissue and fix the vein so. Furthermore, the vein is kept open and does not collapse (collapse) when the internal pressure decreases. This ensures that, even with low blood pressure and in blood-poor regions of the body, the blood can always flow back to the heart and is not blocked by closed veins.
In contrast to the arteries, there is a low pressure in the veins. Thus, the blood from parts of the body lying below the heart plane can not be pumped back so easily against gravity to the heart. In order to facilitate this venous return, venous valves can be found in all major veins below the heart plane. Venous valves are folds of the innermost layer ( tunica intima, endothelial layer ), which are additionally supported by collagen fiber tissue. Through the venous valves, a reversal of the flow direction of the blood can be prevented because venous valves pass the blood only in one direction, namely back to the heart. If blood flows in the opposite direction, there is no blood flow (standstill), so the venous valves inflate like small valve leaflets, lie close together and close the vein.
By contracting muscles, venous blood can be pumped into the next venous valve floor. That's because many veins are overgrown with muscle. When the muscle tightens, contracts and thickens, it tenses the sheath of the muscle (fascia) that surrounds the muscle and fuses with the veins. This pressure is applied to the blood-filled vein and since the venous valves allow only one direction of blood flow, the blood flows back further to the heart.