Microcirculatory bed

The arterial link of the vascular system ends with the vessels of the microcirculatory bed. In each organ, according to its structure and functions, the vessels of the microcirculatory bed may have features of structure and microtopography. The beginning of the microcirculatory bed is an arteriole with a diameter of about 30-50 μm, in the walls of which there is one layer of spirally oriented myocytes - smooth muscle cells. Precapillaries (arterial capillaries) extend from the arterioles. In the walls of their initial sections there are one or two smooth myocytes that form precapillary sphincters that regulate blood flow from the arteriole towards the capillaries.

Precapillaries continue into capillaries, the walls of which do not contain smooth muscle cells. The walls of true capillaries are formed by a single layer of endothelial cells, a basement membrane, and pericytes (pericapillary cells). The endothelial layer lying on the basement membrane is 0.2-2.0 μm thick. Adjacent endothelial cells are connected to each other by desmosomes and nexuses. There are 3-15 nm wide gaps between endothelial cells. These gaps facilitate the passage of various substances through the capillary walls. The basement membrane is formed by intertwined connective tissue fibers and amorphous substance. Pericytes (pericapillary cells, Rouget cells) are located within the basement membrane or outside it. Long and numerous processes of these cells pass through the basement membrane and contact each endothelial cell. Each pericyte is connected to the ending of a sympathetic neuron, capable of transmitting a nerve impulse.

The diameter of blood capillaries is 3-11 µm. The thinnest capillaries (3-7 µm) are located in the muscles, thicker ones (up to 11 µm) - in the skin, mucous membranes. In the liver, endocrine glands, hematopoiesis organs and the immune system, capillaries have a diameter of up to 25-30 µm, which is why they are called sinusoids. Capillaries are exchange vessels, through their walls nutrients pass from the blood to the tissues, and in the opposite direction (from the tissue to the blood) - metabolic products. The transport of substances through the capillary walls is carried out both by diffusion and by filtration.

The total number of capillaries in the human body is approximately 40 billion, with a total cross-sectional area of 11,000 cm (1.1 m ² ). For comparison, the cross-sectional area of the aorta is 2.8 cm ² with a diameter of 2.5 cm. Postcapillaries (postcapillary venules) are formed from capillaries; their diameter is 8-30 μm. The walls of postcapillaries are similar in structure to the walls of capillaries, but postcapillaries have a wider lumen than capillaries and a greater number of pericytes in their walls. Postcapillaries form venules with a diameter of 30-50 μm, which are the initial link in the venous system. In the walls of larger venules, whose diameter is 50-100 μm, there are single smooth muscle cells (myocytes). Venules do not have an elastic membrane.

The microcirculatory bed also includes arterial vessels that directly connect the arteriole and venule - arteriovenous anastomoses. Smooth myocytes are present in the walls of these anastomoses. When the myocytes relax, the arteriovenous anastomoses open (expand) and blood from the arterioles is directed directly to the venules, bypassing the capillaries.

[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ]