From Big Medical Encyclopedia

CAPILLARY PRESSURE — hydrostatic pressure of blood in capillaries. Direct measurement of size K. to in capillaries it is shown that in arterial department of a capillary pressure is higher, than in venous. At the expense of this difference in pressure a certain speed of the movement of blood through a capillary is created and the possibility of the transcapillary metabolism which is carried out on the basis of filtering and absorption of liquid through a capillary wall is provided (see. Microcirculation , Mikrotsirkulyation ).

Fluid movement through a capillary wall happens at the expense of a difference of hydrostatic pressure of blood and hydrostatic pressure of surrounding fabric (so-called effective hydrostatic pressure), and also under the influence of a difference of sizes of osmoonkotichesky pressure of blood and intercellular liquid.

During the calculation of filtrational pressure it is necessary to consider that movement of plasma or an intercellular lymph is carried out towards smaller hydrostatic and bigger osmotic pressure. Therefore in a habit view the size of filtrational (Russian Federation) pressure (in mm of mercury.) it can be calculated on a formula:

The Russian Federation = (Pk — Rtk) — (πпл — πтк),

where sizes of Rk and Rtk — sizes of capillary pressure of blood and hydrostatic pressure of an intercellular lymph; πпл and πтк — sizes of osmoonkotichesky pressure of a blood plasma in a capillary and an intercellular lymph.

At positive value of size of the filtrational pressure >(Rf0) there is filtering of liquid from a capillary, and at negative (the Russian Federation< 0) — its absorption. So, e.g., for capillaries of a nail bed of the person the following values were received: pressure in arterial department of a capillary (Rka) of 30 mm of mercury.; pressure in its venous end (Rkv) — 15 mm of mercury.; pressure of intercellular liquid (Rtk) — 8 mm of mercury.; osmoonkotichesky pressure of a blood plasma (πпл) — 25 mm of mercury. and an intercellular lymph (πтк) — 10 mm of mercury.

After a podstavleniye of these values in a formula it turns out for arterial (Rf) and venous (Rfv) departments of a capillary respectively:

Rfa = 7 mm of mercury.

Rfv = — 8 mm of mercury.

Therefore, straining action from capillaries in intercellular liquid was carried out under pressure of 7 mm of mercury., and a reversed current of liquid in a gleam of a capillary — under pressure of 8 mm of mercury.

Normal the size of filtration rate of liquid is almost equal to the speed of its reabsorption. Only small part of intercellular liquid comes, in addition to circulatory capillaries, to a gleam limf, capillaries and in the form of a lymph is returned to a circulatory bed on limf, system (see. Lymphopoiesis ).

Change of one of parameters of balance leads to change of other parameters and, as a result, to recovery of a dynamic equilibrium between intensity of straining actions and absorption. So, increase in the capillary hydrostatic pressure (Rk) leads to strengthening of filtering of water from a capillary therefore increase in hydrostatic pressure (Rtk) and decrease in osmoonkotichesky pressure of an intercellular lymph for the second time develops (πтк). At the same time the osmoonkotichesky pressure of proteins of a blood plasma causing strengthening of absorption of liquid in the venous end of a capillary increases. Thus, strengthening of filtering causes the corresponding increase in absorption of liquid.

In renal tubules, where size K. reaches 70 mm of mercury., as a result of strengthening of straining actions concentration of proteins sharply increases. The size of osmoonkotichesky pressure at the same time can reach 35 mm of mercury. and to promote process of a reabsorption of liquid in circulatory system.

The condition of such dynamic equilibrium between straining actions and absorption is inherent in a healthy organism. In the conditions of pathology this balance is broken and hypostases develop (see. Swelled ). E.g., at a cardiovascular decompensation venous pressure with firmness increases that leads in turn to permanent increase in capillary hydrostatic pressure of a blood plasma. Strengthening of filtering of liquid from a capillary in intercellular space without strengthening of absorption of liquid in venous system results. Similarly, excessive fall of the contents of proteins in plasma (e.g., at hunger, nefroza etc.) leads to falling of osmoonkotichesky pressure of plasma and development of hypooncotic hypostasis.

Regulation To., according to a number of researchers, is carried out not by means of pericytes (Ruzhe's cells) as A. Krog assumed, and by means of a precapillary sphincter of a muscular layer of a metarteriole (a precapillary arteriole). The tone of a precapillary sphincter is under sympathetic and humoral control of intermediate products of a metabolism, vasoactive substances and mediators to which action the sphincter is very sensitive. Role of c. N of page in regulation To. by it is studied not enough.

At reduction of a sphincter (i.e. vasoconstrictions of a metarteriole) decrease To. promotes absorption of an intercellular lymph in a gleam of a capillary and to its outflow in venous system. In these conditions To. can become equal venous. At a vazodilatation of a metarteriole To. increases, becomes above venous pressure and the blood stream in a capillary begins.

In experiments it is shown that the erythrocyte during the passing through a capillary corks its gleam owing to what there is an insignificant decrease To. in a distal piece of a capillary (the erythrocyte at the same time plays a role of the piston). After passing of an erythrocyte To. in this site is recovered. The piston mechanism of passing of erythrocytes promotes straining action through a wall of a capillary. Same convection of intercellular liquid promotes.

In diagnosis measurement To. did not find practical application for. In experiments measurement To. carry out direct or indirect by methods. For its direct measurement under control of specially adapted kapillyaroskop (see. Kapillyaroskopiya ) by means of the micromanipulator enter the glass microcannula connected to the micromanometer into a cavity of a capillary of a nail bed of the person. Also measure To. at animals in acute experiences.

Under control of a microscope determine by the transparent inflated membrane pressing the studied capillary to glass by an indirect way pressure in capillaries of a mesentery, microvessels of an ear of experimental animals. See also Capillaries , Blood pressure .

Bibliography: Hydrodynamics of blood circulation, the lane with in., under the editorship of S. A. Regirer „page 252, M., 1971, bibliogr.; Folkov B. and Neil E. Blood circulation, the lane with English, M., 1976; Chernukh A. M., Alexandrov P. N. and Alekseev of O. V. Mikrotsirkulyation, M., 1975, bibliogr.; Shushengko K. A. Circulatory capillaries, Novosibirsk, 1975, bibliogr.; Handbook of physiology, Sect. 2 — Circulation, ed. by W. F. Hamilton, v. 2, Washington, 1963.

H. K. Saradzhev.