Microcirculatory bed
Last reviewed: 23.04.2024
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The arterial link of the vascular system is terminated by the vessels of the microcirculatory bed. In each organ according to its structure and functions, the vessels of the microcirculatory bed can have structural features and microtopography. The beginning of the microcirculatory bed is an arteriol with a diameter of about 30-50 μm, in the walls of which there is one layer of spirally oriented myocytes - smooth muscle cells. From the arterioles precapillaries leave (arterial capillaries). In the walls of their initial sections, there are one or two smooth myocytes that form precapillary sphincters that regulate blood flow from the arteriolus towards the capillaries.
Precapillaries continue into the capillaries, the walls of which do not have smooth muscle cells. The walls of the true capillaries are formed by a single layer of endotheliocytes, the basal membrane and pericycapes (pericapillary cells). The endothelial layer lying on the basal membrane has a thickness of 0.2-2.0 μm. Neighboring endotheliocytes are connected to each other with the help of desmosomes and nexus. There are gaps 3-15 nm wide between the endotheliocytes. These slots facilitate the passage through the walls of capillaries of various substances. Basal membrane is formed by interwoven connective tissue fibers and amorphous substance. In the thickness of the basal membrane or outside it are pericytes (pericapillary cells, Ruge cells). The long and numerous processes of these cells pass through the basal membrane and contact each endotheliocyte. For each pericyte the end of the sympathetic neuron, capable of transmitting a nerve impulse, is suitable.
The diameter of the blood capillaries is 3-11 μm. The thinnest capillaries (3-7 μm) are located in the muscles, thicker (up to 11 μm) - in the skin, mucous membranes. In the liver, endocrine glands, organs of blood and immune system, capillaries have a diameter of up to 25-30 microns, in connection with which they are called sinusoids. Capillaries are exchange vessels, nutrients pass through their walls from blood to tissues, and in the opposite direction (from tissue to blood) are metabolic products. Transport of substances through the walls of capillaries is carried out both by diffusion and by filtration.
The total number of capillaries in the human body is about 40 billion, the total area of their cross-section reaches 11,000 cm (1,1 m 2 ). For comparison, the aortic cross-sectional area is 2.8 cm 2 with its diameter of 2.5 cm. Post capillaries (postcapillary venules) with a diameter of 8-30 μm are formed from the capillaries. The walls of post-capillaries are similar in structure to capillary walls, but postcapillaries have a wider lumen than capillaries, and a greater number of pericytes in their walls. From post-capillaries, venules 30-50 μm in diameter are formed, which are the initial link of the venous system. In the walls of larger venules, whose diameter is 50-100 microns, there are single smooth muscle cells (myocytes). The elastic membrane of venules is absent.
The microcirculatory channel also includes arterial vessels that connect the arterioles and venules directly - arteriolovenous anastomoses. Smooth myocytes are present in the walls of these anastomoses. When the myocytes relax, the arteriolovenous anastomoses unfold (expand) and the blood from the arterioles is directed directly to the venules, bypassing the capillaries.
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