Circulation of blood
Blood circulation begins in the tissues, where the metabolism takes place through the walls of the capillaries (blood and lymph). The capillaries constitute the main part of the microcirculatory bed, in the colum microcirculation of blood and lymph occurs.
Lymphatic capillaries and interstitial spaces also belong to the microcirculatory bed. Microcirculation is the movement of blood and lymph in the microscopic part of the vascular bed.
Microcirculatory bed, according to V. V. Kupriyanov, includes 5 links:
1) arterioles as the most distal parts of the arterial system,
2) precapillaries, or precapillary arterioles, which are intermediate between arterioles and true capillaries;
4) postcapillaries, or postcapillary venules,
5) venules, which are the roots of the venous system.
All these links are equipped with mechanisms that ensure the permeability of the vascular wall and the regulation of blood flow at the microscopic level. The microcirculation of blood is regulated by the work of the arterial muscles and arteriol, as well as the special muscle sphincters, the existence of which was predicted by I. Sechenov and called them “cranes”. Such sphincters are in pre- and postcapillaries. Some vessels of the microvasculature (arterioles) perform predominantly distribution function, and the rest (precapillaries, capillaries, postcapillaries and venules) predominantly trophic (exchange).
At each given moment only a part of the capillaries (open capillaries) functions, and the other remains in reserve (closed capillaries).
In addition to these vessels, the Soviet anatomists proved that they belong to the microcirculatory channel of arterio-venous anastomoses, which are present in all organs and represent the paths of the shortened flow of arterial blood into the venous channel, bypassing the capillaries. These anastomoses are divided into true anastomoses, or shunts (with locking devices capable of blocking the flow of blood, and without them), and inter-arterioles, or half-shunts. Due to the presence of arteriovenous anastomoses, the terminal blood flow is divided into two ways of blood flow: 1) transcapillary, which serves for metabolism, and 2) extracapillary juxtacillary (from lat. Juxta – near, beside) flow of blood necessary for regulation of hemodynamic balance; the latter is due to the presence of direct connections (shunts) between the arteries and veins (arteriovenous anastomoses) and arterioles and venules (arteriolenovenular anastomoses).
Due to the extracapillary blood flow, if necessary, the capillary bed is unloaded and the blood transport in the organ or a given area of the body is accelerated. It is like a special form of a roundabout, collateral, blood circulation.