From Big Medical Encyclopedia

OSMOTIC PRESSURE — pressure upon the solution separated from pure solvent by a semipermeable membrane at Krom stops osmosis, i.e. transition of molecules of solvent to solution through the semipermeable membrane dividing them or transition of molecules of solvent through a semipermeable membrane from the solution which was less concentrated to the solution which was more concentrated. Semipermeable membranes represent natural or artificial films, permeable only for molecules of solvent (e.g., waters) and not permeable for molecules of solute. Osmosis and O. of play a large role in maintenance of concentration of the substances dissolved in liquids of an organism at the certain, physiologically necessary level, and, therefore, in distribution of water between fabrics and cells. During the studying of the isolated cells and fabrics important that artificial culture medium was an izotonichna to natural medium. At introduction to different organism of liquids the smallest disturbances cause solutions with O. of, equal O. of of liquids of an organism.

Fig. Scheme of the elementary osmometer: 1 — the external vessel filled with pure water; 2 — the internal not burned clay vessel in which time the mahogany brown forming a semipermeable membrane is besieged is filled with the studied solution; h — height of a fluid column which hydrostatic pressure is equal to pure excessive pressure on solution, i.e. to the osmotic pressure of solution.

O.'s measurement (osmometriya) finds broad application for definition a pier. the weight (weight) of biologically active high-molecular substances, such as proteins, carbohydrates, nucleinic to - you, etc. Measurement of size O. of is performed by means of the devices called by osmometers (fig). The number of the water molecules facing from water the semipermeable membrane formed by mahogany brown are more than number of the water molecules facing this membrane from solution since concentration of water molecules in solution is lower, than in pure water. It is resulted by osmosis and there is an excess hydrostatic pressure on solution, under action to-rogo the speed of transition of water molecules through a membrane in pure water increases. If excessive pressure on solution reaches size, equal O. of of solution, then the number of the water molecules passing through a membrane in both directions becomes identical, osmosis stops, and between the solution and solvent which are on both sides of a semipermeable membrane osmotic balance is established. Thus, osmotic pressure arises only in that case when solution and solvent are separated from each other by a semipermeable membrane.

It is the simplest to lake of of the isolated cells or fabrics to measure by method of a plasmolysis. For this purpose the studied objects place in solutions with different concentration of some substance, in relation to Krom the cellular membrane is impenetrable. Solutions with O. of higher, than O. of contents of cells (hypertensive solutions), cause wrinkling of cells — a plasmolysis owing to transition of water from a cell in solution. Solutions with O. of lower, than O. of contents of cells (hypotonic solutions), cause increase in volume of cells as a result of transition of water from solution in a cell. Solutions with O. of, equal O. of contents of cells (isotonic solutions), do not cause change of volume of cells. Knowing concentration of such solution, find its O. of; the size O. of and contents of cells will be same. Membrane potentials can be the important factor defining passing of water through a cellular membrane, especially in an initial stage of process, to-rye cause electroosmotic movement of water through a cell membrane, so-called abnormal osmosis (see. Elektroosmos ). In similar cases O.'s measurement by by method of a plasmolysis is inexact.

O.'s definition by of the solutions containing low-molecular substances for to-rykh difficult to prepare an impenetrable membrane, make indirect methods, usually by means of measurement of fall of temperature of freezing of solution (see. Kriometriya ).

Ja. Vant Hoff showed that O. of of the diluted solutions of nonelectrolytes submits to the laws established for pressure gases (see), it can also be calculated on the equation similar to Klapeyron's equation — Mendeleyeva for gases:

π\• v = n • RT, (1)

where π — the osmotic pressure, v — the volume of solution in l, n — number of moths of the dissolved substance-not-electrolyte, T — temperature on an absolute scale, R — a constant, a numerical value a cut is same, as well as for gases (R for gases is equal to 82,05*10 - 3 l atm / grad-mol).

The given equation is mathematical expression of the law of Vant Hoff: The lake of of the diluted solution is equal to pressure, a cut would produce solute, being in gaseous state and occupying the volume equal to the volume of solution at the same temperature. Having entered into the equation molar concentration — with = n\v we will receive π = c*RT.

The lake of of solution of electrolyte is more than O. of of solution of a nonelectrolyte of the same molar concentration. It is explained by dissociation of molecules of electrolyte in solution on ions owing to what concentration of kinetic active particles increases, the cut is defined the size O. of.

The number i showing in how many time of O. of (de) of solution of electrolyte is more than O. of (l) of solution of a nonelectrolyte of the same molar concentration, call an isotinic coefficient of Vant Hoff:

i = π e

Numerical size i depends by nature electrolyte and its concentration in solution. For weak electrolytes size i can be calculated on a formula:

i = a * (N — 1) + 1

where and — extent of dissociation of electrolyte, and N — the number of ions, on to-rye breaks up one molecule of electrolyte. For the diluted solutions of strong electrolytes i equal N can accept.

Follows from told that O. of of solution of electrolyte can be found on the equation:

π e = i • with • RT,

where with — molar concentration.

If solution, except low-molecular solutes, contains high-molecular substances (colloids), then O. of caused by high-molecular substances call, according to the offer H. Schade, oncotic, or colloid osmotic pressure.

The general O. to of a blood plasma of the person normal is equal to 7,6 atm, the oncotic pressure caused generally by proteins of plasma makes only 0,03 — 0,04 atm. Oncotic pressure, despite small size in comparison with the general O. of a blood plasma, plays a large role in distribution of water between blood and body tissues.

Many biopolymers, napr, proteins, nucleinic to - you, etc., being polyelectrolytes, at dissociation in solution form multiply charged ions (polyions) big a pier. the weight (weight), for to-rykh a membrane of the osmometer it is impenetrable, and the usual ions of the small sizes passing through a semipermeable membrane. If the solution filling the osmometer contains polyelectrolyte, then the low-molecular ions diffusing through a membrane unevenly are distributed on both sides of a membrane (see. Membrane equilibrium ). Observed at the same time excess hydrostatic pressure will be equal in the osmometer πБ = πБ + π1 — π2 where πБ — O. of caused by biopolymer, and π1 and π2 — O. of of the low-molecular electrolyte which is in an osmotic cell and in external solution respectively. At O.'s measurement by of solutions of biopolymers it is necessary to consider a possibility of uneven distribution of low-molecular electrolytes on both sides of a semipermeable membrane of the osmometer or to take measurements at sufficient excess of the low-molecular electrolyte which is specially entered into solution of biopolymer. In this case low-molecular electrolyte is distributed on both sides of a semipermeable membrane almost evenly, at the same time = π1 = π2 and πБ = πН.


Set of the mechanisms providing O.'s maintenance by in fluid mediums of an organism at the level, optimum for a metabolism, is called osmoregulation. Obtaining information from receptor zones on O.'s change by of blood, c. the N of page turns on a number of the mechanisms returning system in a state, optimum for an organism. Inclusion happens in two ways: nervous and humoral. The deviation of size O. from optimum level is caught in an organism osmoreceptors (see), among to-rykh the leading place occupy the central osmoreceptors located in supraoptic and paraventrikulyarny kernels hypothalamus (see).

Cells of a supraoptic kernel of a hypothalamus are capable to cosecrete antidiuretic hormone (ADG), on axons of these cells it moves to a neurohypophysis where there is its accumulation and removal in the general blood stream (see. Vasopressin ). ADG influences a reabsorption of water in distal departments of nephron and is capable to cause narrowing of a gleam of vessels. The afferent signals regulating allocation of ADG come to a hypothalamus from volume receptors (volyumoretseptor) of the left auricle, from receptors of an aortic arch, from osmoreceptors of an internal carotid artery, from bar of receptors and chemoceptors of a carotid sine. O.'s increase of of extracellular liquid causes increase of secretion of ADG both at the expense of the most osmotic pressure, and due to reduction of volume of extracellular liquid at dehydration of an organism. Thus, allocation of ADG is influenced by two alarm systems: the alarm system from osmoreceptors and the alarm system from baroreceptors and volyumoretseptor. However the leading link in regulation of secretion of ADG nevertheless is O. of of a blood plasma operating on osmoreceptors of a hypothalamus.

A special role in maintenance fiziol. sizes O. of belongs to ions sodium (see). Dehydration arises in connection with change of maintenance of ions of Na + . At dehydration because of change of maintenance of ions of Na + reduction of volume of an arterial blood and intercellular liquid is registered volyumoretseptor, impulses from to-rykh on nerve pathways reach departments of c. N of the page regulating allocation of one of mineralokortikoidny hormones — Aldosteronum (see), to-ry raises a reabsorption of sodium. The central regulation of secretion of Aldosteronum is carried out by the hypothalamus producing an adrenocorticotropin-rileasing-factor (AKTG-rileasing-faktor), to-ry regulates secretion of the adrenocorticotropic hormone (AKTG) formed by a front share of a hypophysis (see. Adrenocorticotropic hormone ). There is an opinion that along with influence of AKTG on secretion of Aldosteronum, there is a special center of regulation of secretion of Aldosteronum located on average a brain. Exactly here the afferent impulsation also arrives at reduction of volume of intercellular liquid as a result of change of maintenance of ions of sodium. Cells of the center of regulation of secretion of Aldosteronum on average a brain are capable to neurosecretion — the formed hormone comes to an epiphysis where collects and from there is emitted in blood. This hormone received the name of an adrenoglomerulotropin (AGTG).

Allocation of ADG and Aldosteronum can be regulated and angiotensin (see), it is obvious in the way of its action on special receptors of hypothalamic neurons. Renin-angiotenzinnaya the system of kidneys can act as the volyumoretseptorny zone reacting to change of a renal blood-groove.

Influence normalization of changed O. also urination (see. Diuresis ), transcapillary exchange of liquid and ions (see. Water salt metabolism ), sweating (see), release of liquid through lungs (with expired air in days 350 — 400 ml of water are lost) and release of liquid through went. - kish. a path (100 — 200 ml of water are lost with a stake).

Ability to O.'s normalization by also blood has. It can carry out a role of the osmotic buffer at various shifts as towards an osmotic hypertension, and hypotonia. Apparently, this function of blood is connected, first, with redistribution of ions between plasma and erythrocytes and, secondly, with ability of proteins of a blood plasma to connect or give ions.

At reduction of water resources of an organism or disturbance of a normal ratio between water and mineral salts (hl. obr. sodium chloride) arises thirst (see), satisfaction the cut promotes maintenance fiziol.

level of a water balance and electrolytic balance in an organism (see. Homeostasis ).

Bibliography: Bladergryon N. V. Physical chemistry in medicine and biology, the lane with it is mute., page 102, etc., M., 1951; Wagner R. G. Determination of osmotic pressure, in book: Fizich. methods of organic chemistry, under the editorship of A. Vaysberger, the lane with English, t. 1, page 270, M., 1950, bibliogr.; Ginetsinsky A. G. Physiological mechanisms of water-salt balance, M. — JI., 1963; Gubanov N. I. and Utepbergenov A. A. Medical biophysics, page 149, M., 1978; H and - t about the h and Yu. V. Ionoreguliruyushchaya's N a funytion of a kidney, D., 1976; With and t p and e-in and X. K. Extrarenal mechanisms of osmoregulation, Alma-Ata, 1971, bibliogr.; Williams V. and Williams of X. Physical chemistry for biologists, the lane with English, page 146, M., 1976; Physiology of a kidney, under the editorship of Yu. V. Natochin, JI., 1972; Andersson B. Regulation of water intake, Physiol. Rev., v. 58, p. 582, 1978, bibliogr.

V. P. Mishin; S. A. Osipovsky (physical.).