MICROCIRCULATION

From Big Medical Encyclopedia

MICROCIRCULATION — the movement of blood in system of microcirculation, the central part a cut are capillaries. To. to. carries out the main function of microcirculator system — transcapillary exchange, i.e. a metabolism between blood and fabrics. Total number capillaries (see) in a big circle of blood circulation makes several billions. By data A. Kroga (1927), in 1 mm 3 skeletal muscle of the person is apprx. 2000 capillaries, a dog — 2630, horses — 1350. By calculations of 1 ml of the blood which is in capillaries of skeletal muscles has the contact surface with an endothelium of capillaries reaching 0,5 m 2 . Such big contact surface of blood with walls of capillaries favors to the metabolism occurring in them, in particular gas exchange between blood and fabrics.

Though density of a capillary network is unusually big, number of perfusing capillaries is wide varies depending on funkts, conditions of fabric or body. Morfol. the analysis of a capillary bed at various animals showed that the structure of a capillary is steady and low-changing. The wall of a capillary does not contain smooth muscle cells that indicates impossibility of active reduction. Intravital researches and results of the electronic and microscopic analysis allow to come to a conclusion that endothelial cells of capillaries, without possessing specific sokratitelny function, in certain conditions are capable to be reduced. Also passive mechanism of change of a gleam of a capillary caused by a difference of hydrostatic pressure in a capillary and surrounding fabric is very probable.

Along with the capillaries performing exchange function (them sometimes call nutritive saved la-rami), G. I. Mchedlishvili (1958), Tsveyfakh (V. of Zweifach, 1961), V. V. Kupriyanov et al. (1975) describe also so-called main capillaries. Morphologically main capillaries are identical usual, however have a large diameter. Speed of a blood-groove in such capillaries is 2 — 3 times higher than the speed of a blood-groove in usual capillaries. Functionally, according to V. V. Kupriyanov et al. (1975), the main capillaries carry out a role of semi-shunts, providing transition of an arterial blood to venous vessels.

Function of capillaries consists in ensuring transcapillary exchange, i.e. in supply of cells of bodies and fabrics with nutritious and plastic substances and removal of products of metabolism. Implementation of this function requires observance of a number of conditions, the most important of which are certain sizes of hydrostatic and oncotic pressure in a capillary (see. Capillary pressure ), speeds of a blood-groove in a capillary, permeability of a wall of a capillary, a certain number of perfusing capillaries per unit of volume of fabric.

Total number of capillaries in various fabrics is unequal. In fabrics with a high level of exchange the number of capillaries on 1 mm2 of cross-section are more, than in fabrics with less intensive exchange. E.g., in a cardiac muscle the number of capillaries on 1 mm2 of section are twice more, than in a skeletal muscle; in gray matter of a brain the capillary network is much more dense, than in white matter.

The metabolism through a capillary wall is carried out by filtering, diffusion, and also microvesicular transport. Filtering happens due to hydrostatic capillary tension. This process provides a water-salt homeostasis of fabrics and specialized forms of transport at lymphization, exudate, etc. The coefficient of capillary filtering is expressed amount of liquid (in mkl), edges it is filtered through a certain area of a vascular wall (in micron 2 ) in unit of time (sec.) with a certain pressure of blood (in cm w.g.). Diffusion (see) provides transfer plastic and nutrients, and also elimination of products of metabolism. At the same time capillary permeability can be determined by the first Fick's law:

— dn/dt = +pS*Δc,

where — dn/dt — diffusion rate, pS — the work of capillary permeability of solution (p) on an effective surface (S) of an endothelium of a capillary, Δc — a difference of concentration on both sides of a capillary wall.

In addition to passive diffusion in capillaries advance of substances against a gradient of concentration — by so-called active transfer of molecules is observed (see. Transport of ions ). Believe that in membranes of cells there are special substances — permeaza, or ionophores (see) which by formation of a complex with this or that substance provide its receipt in a cell.

Electronic microscopic examinations showed that microvesicles (so-called microvesicular transport) take part in transendothelial transfer of substances. The microvesicles which are formed on one surface move to opposite where connect to a cellular cover and are exempted from contents in subendothelial space. At electronic and microscopic studying of ways of an exit of macromolecules and microparticles from a blood-groove process of «loading» of microvesicles at their formation on one cellular surface, movement of vesicles to an opposite surface and release of contents in subendothelial space was tracked. Rate of volume flow of vesicular transport reaches 6 — 10 vesicles/micron 2 endothelium in a second.

The most important indicator of functioning of a microcirculator bed is the speed of a blood-groove in capillaries. Intravital researches showed that at animals the average speed of the movement of erythrocytes in capillaries makes 0,5 — 1 mm/sec., and in capillaries of skin of the person — 0,74 mm/sec. In an experiment it is shown that in pulmonary capillaries of a cat speed can reach 2 mm/sec. The erythrocyte passes through an alveolar capillary 248 microns long for 0,12 sec.; this interval also determines duration of contact of an erythrocyte with an alveolar air. Speed of a blood-groove in capillaries is defined by a pressure gradient in precapillaries and post-capillaries. The gradient in turn depends on the size of the general arterial and venous pressure and peripheric resistance.

The flow of the erythrocytes passing through a capillary widely varies and depending on a functional condition of body can fluctuate from 300 to 1500 erythrocytes a minute.

The size of transcapillary exchange depends, in particular, on number of perfusing capillaries, i.e. such to whom erythrocytes move. Capillaries, free of at present erythrocytes and filled with plasma, received the name of plasmatic. In conditions funkts, rest of body the number of perfusing capillaries makes 30 — 50% of total number of capillaries. During the strengthened work of body plasmatic capillaries are filled with erythrocytes. The terms «perfusing» and «closed» capillaries are very conditional as well as the terms «not perfusing» and «open» capillaries. So, e.g., a capillary, on Krom erythrocytes do not move, is not perfusing in strict sense because on it plasma can move. The closed capillaries, i.e. vessels which gleam is almost completely blocked by the fallen-down walls meet only in parenchymatous bodies (lungs, a spleen, a liver) in connection with elasticity of their stroma. In fabrics with more rigid stroma as showed intravital observations, there are no closed capillaries.

In the conditions of pathology at emergence of units from the stuck together erythrocytes corking separate capillaries the number of plasmatic capillaries and microvessels increases. Process aggregations of erythrocytes (see) it is reversible, and at recovery of hemodynamic parameters units «crash» (disaggregation) to separate erythrocytes.

There is an opinion that the number of perfusing capillaries is defined by work of a precapillary sphincter. However this point of view is not divided by many researchers. The precapillary sphincter is formed by two smooth muscle cells in the place of an otkhozhdeniye of a precapillary from a metarteriole (a precapillary arteriole). The main data on a precapillary sphincter were received during the studying of microvessels of a retrolingvalny membrane of a frog. The motor innervation of a precapillary sphincter, independence of its function of reduction of a metarteriole and high sensitivity to vasoactive substances, mechanical influences and products of fabric metabolism was shown. Assume that smooth muscle cells of a precapillary sphincter have the certain tone causing a condition of a relative konstriktion. During the strengthened work of body the collecting products of metabolism reduce a tone of smooth muscle cells, cause dilatation). Strengthening of a capillary blood-groove (increase in number of active capillaries) arising at the same time provides removal of surplus of metabolites that leads to recovery of a tone of muscle cells and reduction of a blood-groove. Whether at a long konstriktion of a precapillary sphincter in an experiment of an otme-ch strengthening of adsorption (intake of liquid from fabric in capillaries) whereas dominance of long dilatation strengthened filtering (an exit of liquid from capillaries). The question of function of a precapillary sphincter at mammals remains open, however some authors in work of a precapillary sphincter see the only mechanism of regulation To. to. The number of perfusing capillaries is defined by a ratio of arterial and venous pressure at the level of a precapillary sphincter. Aperiodic intermittence of a blood-groove in capillaries can be caused by a blockage of the mouth of a precapillary of leukocytomas which hardly overcomes the narrow mouth of a precapillary. After passing of a leukocyte the blood stream in capillaries is recovered.

Thus, regulation To. to. it is carried out generally by means of humoral mechanisms. At the same time it is necessary to consider that the microcirculator bed of bodies and fabrics is involved in the general system of haemo circulation. Therefore, with the expressed autonomy of a capillary blood-groove the last substantially is caused by the central hemodynamics that is especially accurately shown at falloff of the ABP. Nervous control of function of capillaries (in particular, their permeability) is carried out indirectly — by means of the vasoactive substances emitted, e.g., by mast cells under the influence of neurotransmitters (see. Neurohumoral regulation ).

According to A. L. Chizhevsky's (1959) representations, the erythrocyte in a capillary holds such position, at Krom its side surfaces are located along an axis of a vessel. At the same time rotation of an erythrocyte stops, but there is its deformation. The intravital microscopy allowed to observe deformation of the erythrocyte moving in a capillary and taking the form of a drop, a pear, a hand bell, a horseshoe, the cylinder, etc. The erythrocyte takes such forms in post-capillaries which diameter considerably exceeds its diameter. In capillaries which diameter is close to diameter of erythrocytes the last are located the wide surface across a flow and move almost closely one by one, performing thereby function of peculiar pistons (the piston mechanism of passing of erythrocytes). Speed of the movement of such erythrocytes on precapillaries is much higher, than at deformed. The movement of erythrocytes in a capillary closely one after another provides hydrodynamic stabilization of the situation of an erythrocyte, and also excludes a possibility of its rotation. Such provision of an erythrocyte is most favorable to process of diffusion of oxygen.

Rheological properties of blood also influence on To. to. The flowability of blood depends on degree of its viscosity. Direct dependence between the size of a hematocrit is revealed (i.e. the volume of erythrocytes as a percentage) and viscosity of blood, however even at a hematocrit of 98% blood keeps flowability. At the size of a hematocrit of 20% viscosity of blood is 10 times lower, than at a hematocrit of 90%. In capillaries the size of a hematocrit (Nanosecond) can be calculated by a formula:

where N — number of erythrocytes in a capillary, VR — the average volume of an erythrocyte, D — the average diameter of a capillary, a L — its length. As in capillaries the hematocrit is rather constant, in capillaries with internal to dia. 5 microns and less viscosity of blood already practically do not depend on a hematocrit.

One of the most frequent forms of pathology in system of microcirculation is intravascular aggregation of erythrocytes and other uniform elements of blood. Emergence in blood of a large number of units of various form and size reduces a total surface of erythrocytes, creates conditions for mechanical obstruction of micro vessels and capillaries in which the blood stream stops. The developing hypoxia of fabrics, including a vascular wall, causes increase in its adhesive properties that leads to sticking of the leukocytes reducing a gleam of a microvessel and complicating a blood stream. The stronger aggregation of erythrocytes is expressed, the suspension stability of blood is reduced more sharply that leads to separation of plasma from erythrocytes and emergence of plasmatic capillaries, free of erythrocytes. An essential factor in the mechanism of aggregation of erythrocytes is primary reduction of speed of a blood-groove.

Secondary reduction of a blood-groove at burns, a fatty embolism, toxic hemotransfusionic and cardiogenic shock, fibrinferments, an oliguria, heart operations and vessels, acute arterial insufficiency, a hypothermia, extracorporal blood circulation, at infections and injuries is caused by the aggregation of erythrocytes. Aggregation of erythrocytes depends also on a ratio of concentration highly - and low-molecular proteins of a blood plasma. At increase in concentration of high-molecular proteins (fibrinogen) real premises for aggregation of erythrocytes are created. Aggregation of erythrocytes is the secondary process reflecting reaction of system of blood to damage.

At many patol, processes (an injury, an inflammation, hypostasis) the main link of a pathogeny is increase in permeability of a wall of a capillary (see. Permeability ).

Passing of leukocytes and erythrocytes (emigration) through a capillary wall is the main component of a pathogeny inflammations (see). The method of a submicroscopy studied dynamics in detail emigration (see). Leukocytes get generally through interendothelial connections. The neutrophil allows thin pseudopodiums into the conjunction of endothelial cells, and then, being as if poured in the got part a pseudopodium, passes through a wall of a capillary without destruction of the last. Emigration of lymphocytes follows after pass of leukocytes which, apparently, somehow influence an endothelial cell and facilitate transition of lymphocytes. Lymphocytes pass through an endothelial cell by formation of the big vacuole which is gradually moving ahead from a gleam of a vessel to perivascular space. Emigration of erythrocytes is probably carried out passively, at the expense of pressure of blood against the background of the increasing permeability of walls of capillaries which become passable and for the fibrinogen turning in extravasated space into fibrin.

Change of permeability of capillaries can be caused not only an intravascular factor (delay of a blood-groove, thrombocytopenia, a hypoproteinemia, plasma kinina, effect of toxins, etc.), but also extravasated factors among which an essential role is played by system mast cells (see). Mast cells, being an obligatory component of connecting fabric, contain highly active substances (a histamine, serotonin, heparin, noradrenaline, hyaluronidase, proteolytic enzymes, mucopolysaccharides, etc.). Various physical., chemical, flotogenny and antigenic irritants, a hypoxia and many other factors cause degranulation of mast cells, i.e. their destruction. At degranulation of cells of a granule get to surrounding space where their contents can influence a wall of a capillary, changing its permeability, and also adhesive properties of an endothelium.

See also Blood circulation , Blood circulation regional , Mikrotsirkulyation .


Bibliography: Kupriyanov V. V., Karaganov Ya. I. and Kozlov V. I. Microcirculator bed, M., 1975; Mchedlishvili G. I. Microcirculation, Tbilisi, 1958, bibliogr.; Nesterov A. I. To the doctrine about circulatory capillaries and a kapillyaroskopiya as a method of their studying in normal and pathological conditions, Tomsk, 1929, bibliogr.; Chernukh A. M., Alexandrov P. N. and Alekseev of O. V. Mikrotsirkulyation, M., 1975, bibliogr.; Chizhevsky A. L. Structural analysis of moving blood, M., 1959, bibliogr.; Shushengko K. A. Circulatory capillaries, Novosibirsk, 1975, bibliogr.; By about g h A. Anatomie und Physiologie der Capillaren, B. u. a., 1970, Bibliogr.

A. M. Chernukh, P.N. Alexandrov.

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