Circulation
The body contains about 5 liters of blood. Assuming a heart rate of 4-5 liters per minute, a circulation through the large and small circulatory system takes about a minute. The blood circulation of the individual organs depends strongly on the current work.
After a meal, 1/3 of all blood flows through the gastrointestinal tract and only a small part through the muscles of the musculoskeletal system. During physical exertion, the blood circulation in the muscles can increase 20 times and the blood flow through the digestive organs decreases. Various mechanisms are used to control the blood flow.
- Baroreceptor reflexIn the wall of the carotid arteries (Arteria carotis communis) are pressure sensors that measure the current blood pressure. If the blood pressure rises, a throttling signal is sent to the heart; if the blood pressure falls, the heart’s performance is increased.
- AutoregulationThe kidney depends on a constant blood flow with relatively stable pressures. If the pressure in the renal artery is too high, the muscles of the vessel wall contract – it contracts.
As a result, the blood supply to the kidney decreases and with it the pressure.
- Local-chemical The blood circulation in the brain and muscles is regulated by substances that provide indirect information about the activity of the cells. Substances that are released during work (hydrogen and potassium) increase the blood circulation by relaxing the vascular muscles; if their concentration falls below normal, the blood circulation is reduced.
- NervalThe vessels are supplied (with a few exceptions: erectile tissue, salivary glands) only by sympathetic nerve fibers. Depending on the protein equipment (receptors) of the muscle cells, they react either by constricting or expanding the vessel.
- HormonalNumerous hormones and other messenger substances (e.g. adrenalin, histamine, caffeine, etc.)
influence the tension of the muscles. The effects also depend on the protein content of the cell wall.
Wall structure of the vesselsDirectly adjacent to the blood are the cells of the vessel wall (endothelium). They are very smooth and thus reduce the probability of blood clots (thrombosis) forming.
Together with the underlying musculature, they are fused via connective tissue. All vessels (except the capillaries) contain muscles (smooth muscles) in their wall. This allows them to change the diameter of the vessels and thus control the blood flow to the downstream tissues.
Various stimuli (hormones, metabolic products, nerves, automatisms) can increase or decrease the tension of the muscles. Depending on the effect, this is referred to as vasodilatation or vasoconstriction. The main artery (aorta) and the initial sections of the major arteries have a special feature in their wall structure, which is that they contain a particularly large number of elastic fibers.
As a result, they function like an air vessel: in the so-called systole, when blood is expelled from the heart, they are stretched and blood is stored temporarily, so to speak. When blood no longer flows from the heart during diastole, the elastic fibers return to their original state and release the stored blood. By emptying its reservoir, the blood is kept in motion and the heart is relieved.
This mechanism is also known from everyday life: It is easier to push an already rolling car than to push a stationary one. With age, the elasticity of the blood vessels naturally decreases, so that the heart is no longer relieved or its work is made more difficult. The situation is made worse when the arteries become even stiffer due to calcification.
