Osmotic Pressure

osmotic pressure

[äz′mäd·ik ′presh·ər] (physical chemistry) The applied pressure required to prevent the flow of a solvent across a membrane which offers no obstruction to passage of the solvent, but does not allow passage of the solute, and which separates a solution from the pure solvent. The applied pressure required to prevent passage of a solvent across a membrane which separates solutions of different concentration, and which allows passage of the solute, but may also allow limited passage of the solvent. Also known as osmotic gradient.

Osmotic Pressure

the force that tends to reduce the concentration of a solution that is in contact with a pure solvent by the reciprocal diffusion of molecules in the solute and solvent. If the solution is separated from the pure solvent by a semipermeable membrane, diffusion is possible in only one direction: the solvent is osmotically drawn across the membrane into the solution. In this case the osmotic pressure may be measured directly as the excess pressure that exists on the side of the solution at osmotic equilibrium.

Osmotic pressure results from a reduction in the chemical potential of a solvent in the presence of a solute. The tendency of a system to have equal chemical potentials over its entire volume and to reach a state of lowest free energy gives rise to the osmotic diffusion of matter. In ideal and dilute solutions, the osmotic pressure is independent of the nature of the solvent and solutes. At constant temperature it is determined only by the number of kinetically active particles—ions, molecules, associated species, and colloidal particles—in a unit volume of the solution.

The first measurements of osmotic pressure were made in 1877 by W. Pfeffer, who studied aqueous solutions of cane sugar. In 1887 his data were used by J. H. van’t Hoff to establish the dependence of osmotic pressure on the concentration of the solute; van’t Hoff’s expression for this dependence is identical in form to the Boyle-Mariotte law for ideal gases. The osmotic pressure (π) was found to be equal to the pressure that would be imparted by the solute, if, at a given temperature, the solute were an ideal gas and occupied a volume equal to that of the solution. For very dilute solutions of nondissociating compounds, osmotic pressure is described with sufficient accuracy by the equation πV = nRT, where n is the number of moles of solute, V is the volume of the solution, R is the universal gas constant, and T is the absolute temperature. When the solute dissociates into ions, a factor i—the van’t Hoff coefficient—is introduced into the right-hand side of the equation; i is greater than 1 with dissociation and less than 1 with association of the solute. The osmotic pressure of a real solution (π ′) always exceeds that of an ideal solution (π″), and the ration π′/π″, which is called the osmotic coefficient g, increases with increasing concentration.

Solutions with the same osmotic pressure are called isotonic, or isosmotic, solutions. Various blood substitutes and physiological solutions are isotonic relative to the internal fluids of organisms. A solution with a higher osmotic pressure relative to another solution is hypertonic, while a solution with a lower osmotic pressure is hypotonic.

Osmotic pressure is measured with an osmometer. A distinction is made between static and dynamic methods of measurement. By the static method, the excess hydrostatic pressure is represented by the height H that a column of liquid in an osmometer tube reaches at osmotic equilibrium with the equal external pressures p_A and p_B acting on chambers A and B (Figure 1). The dynamic method entails measurement of v—the rates of ascent and descent of the solvent within the osmotic cell—at several values of excess pressure Δ p, which is the difference P_A—p_B; subsequently the results are extrapolated to v = 0 at Δp = π.

Figure 1. Schematic diagram of an osmometer: (A) solution chamber; (6) solvent chamber; (M) semipermeable membrane; (p_A) and (p_B) external pressure on chambers A and B, respectively. At osmotic equilibrium, H is the height of the columns of liquid that counterbalances the osmotic pressure when P_A = p_b; when p_A ≠ p_B, that is, when π = p_A—p_B, the height is b.

Many osmometers are able to measure by both methods. One of the major difficulties in the measurement of osmotic pressure is the selection of a semipermeable membrane. Porous ceramic or glass partitions or films made of cellophane or of natural or synthetic polymers are usually used. Osmometry is the study of the techniques used in measuring osmotic pressure.

Osmometry is used primarily to determine the molecular weight (M) of polymers. M is calculated using the formula

where c is the concentration of the polymer by weight, and A is a coefficient that depends on the structure of the macromolecule.

Osmotic pressure can reach considerable values. For example, a 4 percent solution of sugar at room temperature has an osmotic pressure of about 0.3 meganewtons per square meter (MN/m²); a 53 percent solution, about 10 MN/m². The osmotic pressure of sea water is about 0.27 MN/m².

L. A. SHITS

The osmotic pressure in biological fluids and in the cells of animals, plants, and microorganisms depends on the concentration of solutes in the liquid mediums. The salt composition of biological fluids and cells—which is characteristic for each type of organism—is maintained by active transport of ions and by the selective permeability of biological membranes to various salts. Osmotic pressure is kept at a relatively constant level by water-salt metabolism, that is, by the uptake, distribution, use, and elimination of water and salts. Internal osmotic pressure is greater than external osmotic pressure in hyperosmotic organisms, while the reverse is true of hypoosmotic organisms; the internal and external osmotic pressures of poikilosmotic organisms are equal.

Ions are actively absorbed and retained by hyperosmotic organisms, while water passively crosses the biological membranes according to the osmotic gradient. Hyperosmotic regulation is characteristic of all plants, freshwater organisms, and marine chondrichthians, including sharks and members of the suborder Batoidei. Organisms with hypoosmotic regulation are adapted to actively eliminate salts. Teleosts eliminate the most common marine ions—sodium and chlorine—through gills, while birds and marine reptiles—such as marine snakes and turtles—eliminate these ions through salt glands that are located in the head region. In these organisms, magnesium, sulfate, and phosphate ions are eliminated through the kidneys.

Osmotic pressure in hyperosmotic and hypoosmotic organisms may be created either by ions that are predominant in the environment or by products of metabolism. For example, in sharks and members of the suborder Batoidei, 60 percent of the osmotic pressure is maintained by urea and trimethylammonia; in blood plasma of mammals, mainly by sodium and chlorine ions; and in insect larvae, by various low-molecular-weight metabolites. In poikilosmotic organisms, including unicellular marine organisms, echinoderms, cephalopods, and members of the subclass Myxini, the osmotic pressure is determined by and is equal to the osmotic pressure of the environment; other than certain intracellular processes, osmoregulatory mechanisms do not exist in these animals.

The range of mean osmotic pressures in the cells of organisms that are incapable of osmotic homeostasis is rather broad and depends on the type and age of the organism, the type of cells, and the osmotic pressure of the environment. Under optimal conditions, the osmotic pressure in the cytoplasm of cells of surface organs varies from 2 to 16 atmospheres (atm) in swamp plants and from 8 to 40 atm in plains plants. The osmotic pressure in a plant may differ sharply from cell to cell. For example, in mangroves the osmotic pressure in the cytoplasm is about 60 atm, while in the xylem channels it does not exceed 1–2 atm.

Among homoiosmotic organisms, which are capable of maintaining a relatively constant osmotic pressure, the average value and range of osmotic pressures differ, for example, from 3.6 to 4.8 atm in earthworms, from 6.0 to 6.6 atm in freshwater fish, from 7.8 to 8.5 atm in marine teleosts, from 22.3 to 23.2 atm in elasmobranchs, and from 6.6 to 8.0 atm in mammals.

Except for such glandular fluids as saliva, sweat, and urine, the osmotic pressure of most mammalian bodily fluids is equal to that of the blood. In animal cells, the osmotic pressure that is contributed by high-molecular-weight compounds, including proteins and polysaccharides, is insignificant, although these substances are metabolically important in that they create the oncotic pressure.

IU. V. NATOCHIN and V. V. KABANOV

REFERENCES

Moelwyn-Hughes, E. A. Fizicheskaia khimiia, vols. 1–2. Moscow, 1962. (Translated from English.)
Kurs fizicheskoi khimii, vols. 1–2. Edited by la. I. Gerasimov. Moscow-Leningrad, 1963–66.
Pasynskii, A. G. Kolloidnaia khimiia, 3rd ed. Moscow, 1968.
Prosser, C. L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)
Griffin, D. and A. Novick. Zhivoi organizm. 1973. (Translated from English.)
Nobel, P. Fiziologiia rastitel’noi kletki (fiziko-khimicheskii podkhod). Moscow, 1973. (Translated from English.)

osmotic pressure

pressure

(P) [presh´ur] force per unit area.arterial pressure (arterial blood pressure) blood pressure (def. 2).atmospheric pressure the pressure exerted by the atmosphere, usually considered as the downward pressure of air onto a unit of area of the earth's surface; the unit of pressure at sea level is one atmosphere. Pressure decreases with increasing altitude.barometric pressure atmospheric p.blood pressure 1. see blood pressure.2. pressure of blood on walls of any blood vessel.capillary pressure the blood pressure in the capillaries.central venous pressure see central venous pressure.cerebral perfusion pressure the mean arterial pressure minus the intracranial pressure; a measure of the adequacy of cerebral blood flow.cerebrospinal pressure the pressure of the cerebrospinal fluid, normally 100 to 150 mm Hg.continuous positive airway pressure see continuous positive airway pressure.filling pressure see pressure" >mean circulatory filling pressure.high blood pressure hypertension.intracranial pressure see intracranial pressure.intraocular pressure the pressure exerted against the outer coats by the contents of the eyeball.intrapleural pressure (intrathoracic pressure) pleural pressure.intrinsic positive end-expiratory pressure elevated pressure" >positive end-expiratory pressure and dynamic pulmonary hyperinflation caused by insufficient expiratory time or a limitation on expiratory flow. It cannot be routinely measured by a ventilator's pressure monitoring system but is measurable only using an expiratory hold maneuver done by the clinician. Its presence increases the work needed to trigger the ventilator, causes errors in the calculation of pulmonary compliance, may cause hemodynamic compromise, and complicates interpretation of hemodynamic measurements. Called also auto-PEEP and intrinsic PEEP.maximal expiratory pressure maximum expiratory pressure.maximal inspiratory pressure the pressure during inhalation against a completely occluded airway; used to evaluate inspiratory respiratory muscle strength and readiness for weaning from mechanical ventilation. A maximum inspiratory pressure above −25 cm H₂O is associated with successful weaning.maximum expiratory pressure (MEP) a measure of the strength of respiratory muscles, obtained by having the patient exhale as strongly as possible against a mouthpiece; the maximum value is near capacity" >total lung capacity.maximum inspiratory pressure (MIP) the inspiratory pressure generated against a completely occluded airway; used to evaluate inspiratory respiratory muscle strength and readiness for weaning from mechanical ventilation. A maximum inspiratory pressure above −25 cm H₂O is associated with successful weaning.mean airway pressure the average pressure generated during the respiratory cycle.mean circulatory filling pressure a measure of the average (arterial and venous) pressure necessary to cause filling of the circulation with blood; it varies with blood volume and is directly proportional to the rate of venous return and thus to cardiac output.negative pressure pressure less than that of the atmosphere.oncotic pressure the osmotic pressure of a colloid in solution.osmotic pressure the pressure required to stop osmosis through a semipermeable membrane between a solution and pure solvent; it is proportional to the osmolality of the solution. Symbol π.partial pressure the pressure exerted by each of the constituents of a mixture of gases.peak pressure in mechanical ventilation, the highest pressure that occurs during inhalation.plateau pressure in mechanical ventilation, the pressure measured at the proximal airway during an end-inspiratory pause; a reflection of alveolar pressure.pleural pressure the pressure between the visceral pleura and the thoracic pleura in the pleural cavity. Called also intrapleural or intrathoracic pressure.positive pressure pressure greater than that of the atmosphere.positive end-expiratory pressure (PEEP) a method of control mode ventilation in which positive pressure is maintained during expiration to increase the volume of gas remaining in the lungs at the end of expiration, thus reducing the shunting of blood through the lungs and improving gas exchange. A PEEP higher than the critical closing pressure prevents alveolar collapse and can markedly improve the arterial Po₂ in patients with a lowered functional residual capacity, as in acute respiratory failure.

Effects of the application of positive end-expiratory pressure (PEEP) on the alveoli. A, Atelectatic alveoli before PEEP application. B, Optimal PEEP application has reinflated alveoli to normal volume. C, Excessive PEEP application overdistends the alveoli and compresses adjacent pulmonary capillaries, creating dead space with its attendant hypercapnia. From Pierce, 1995.pulmonary artery wedge pressure (PAWP) (pulmonary capillary wedge pressure (PCWP)) intravascular pressure, reflecting the left ventricular end diastolic pressure, measured by a swan-ganz catheter wedged into a small pulmonary artery to block the flow from behind.pulse pressure the difference between the systolic and diastolic pressures. If the systolic pressure is 120 mm Hg and the diastolic pressure is 80 mm Hg, the pulse pressure is 40 mm Hg; the normal pulse pressure is between 30 and 40 mm Hg.urethral pressure the pressure inwards exerted by the walls of the urethra, which must be counteracted in order for urine to flow through; see also profile" >urethral pressure profile.venous pressure the blood pressure in the veins; see also central venous pressure" >central venous pressure.water vapor pressure the tension exerted by water vapor molecules, 47 mm Hg at normal body temperature.wedge pressure blood pressure measured by a small catheter wedged into a vessel, occluding it; see also pulmonary capillary wedge pressure and wedged hepatic vein pressure.wedged hepatic vein pressure the venous pressure measured with a catheter wedged into the hepatic vein. The difference between wedged and free hepatic vein pressures is used to locate the site of obstruction in portal hypertension; it is elevated in that due to cirrhosis, but low in cardiac ascites or portal vein thrombosis.

os·mot·ic pres·sure (OP, Π),

the pressure that must be applied to a solution to prevent the passage into it of solvent when solution and pure solvent are separated by a membrane permeable only to the solvent (sometimes less correctly viewed as the force with which the solution attracts solvent through the semipermeable membrane).

os·mot·ic pres·sure

(Π) (oz-mot'ik presh'ŭr) The pressure that must be applied to a solution to prevent the passage into it of solvent when solution and pure solvent are separated by a membrane permeable only to the solvent.

osmotic pressure

see OSMOTIC POTENTIAL.

Osmotic pressure

Pressure that occurs when two solutions of differing concentrations are separated by a semipermeable membrane, such as a cellular wall, and the lower concentration solute is drawn across the membrane into the higher concentration solute (osmosis).Mentioned in: Electrolyte Disorders, Thoracentesis

os·mot·ic pres·sure

(Π) (os-mot'ik presh'ŭr) The pressure that must be applied to a solution to prevent the passage into it of solvent when solution and pure solvent are separated by a membrane permeable only to the solvent.

osmotic pressure

osmotic pressure

osmotic pressure

osmot′ic pres′sure

osmotic pressure

Osmotic Pressure

osmotic pressure

Osmotic Pressure

REFERENCES

osmotic pressure

pressure

os·mot·ic pres·sure (OP, Π),

os·mot·ic pres·sure

osmotic pressure

Osmotic pressure

os·mot·ic pres·sure

osmotic pressure

Words related to osmotic pressure

noun (physical chemistry) the pressure exerted by a solution necessary to prevent osmosis into that solution when it is separated from the pure solvent by a semipermeable membrane

Related Words