Osmotic pressure The hydrostatic

Osmotic pressure The hydrostatic pressure produced on the surface of a semipermeable membrane by osmosis. 

Practical Aspects of Osmometry In static osmometers, the heights of liquid in capillary tubes attached to the solvent and solution compartments (Fig. 4.3) are measured. At equilibrium, the hydrostatic pressure corresponding to the difference in liquid heights is the osmotic pressure. The main disadvantage of this static procedure is the length of time required for attainment of equilibrium. 

From our effort and flow variable perspective, there are two effort variables that act in concert, either adding or subtracting. These are concentration (or osmotic) pressure and hydrostatic pressure (see Section 2.9). Either one can canse the flow of water through the semipermeable membrane (Figure 2.8.4). 

When hydrostatic pressure in the right environment (initial solute richer environment) equals the osmotic pressure %, the net solvent flux ends. According to nonequiUbrium... 

Ccmments A pressure of lo bar corresponds to the hydrostatic pressure of approximately lo m of water and is produced by less than i% by mol of solute The calculation demonstrates that it takes a very small mole fraction of solute to produce a fairly large osmotic pressure. The result may seem counterintuitive, but it is just another manifestation of the fact the pressure has a small effect on liquid properties it takes a fairly large osmotic pressure to change the fugacity of the mixed solvent enough to make it equal to that of the pure solvent. The mathematical explanation is that the molar volume of liquids, which appears in the denominator of eq. is veiy small... 

Figure 4.4.16. Principle scheme of a membrane osmometer 1 - solvent, 2 - polymer, 7C - osmotic pressure. Ah - hydrostatic height difference, - ordinary pressure or measuring pressure, Vj- partial molar volume of the solvent in the polymer solution.

Osmosis It is the movement of a solvent through a membrane that is impermeable to a solute, from the more dilute to the more concentrated solution. Water passes through the membrane in both directions, but it flows more rapidly in the direction of the salt solution resulting in a difference in hydrostatic pressure. The tendency of the solvent to flow can be stopped by applying pressure to the salt solution. The excess pressure that must be applied to the solution to produce equilibrium is known as osmotic pressure. The pressure exerted in excess of the osmotic pressure reverses the flow of the solvent and is called reverse osmosis. ... 

The force that pulls the solvent through the membrane toward the more concentrated solution is called osmotic pressure. It can be exactly balanced by applying hydrostatic pressure to the membrane. Consequently, when n, osmotic pressure=p, hydrostatic pressure, fluid will not move. 

Ultrafiltration in the glomeruli, too, depends on the reversal of osmotic dilution. The hydrostatic pressure of the capillary system actually pushes fluids into Bowman s capsule. With respect to proteins, a concentration increase is achieved since proteins are not able to pass through. 

The simplest osmotic dosage form, ALZA Corporation s OROS elementary osmotic pump (Fig. 7), combines the dmg and sometimes an osmotic agent in a monolithic core and deflvers the dmg in solution (102). The mass dehvery rate with time dm df) of the dmg solution is described by equation 4, where is the hydrauHc permeabiUty of the membrane, a is the membrane reflection coefficient, Atz is the osmotic pressure gradient, APis the hydrostatic back pressure, A is the area of the membrane, C is the dissolved concentration of the dmg, and b is the membrane thickness. 

Plastic membrane This is done by the use of a water permeable plastic membrane held deep enough under the sea so that the hydrostatic pressure is greater than the osmotic pressure of the seawater. The water distills out of the solution through the membrane and is pumped to the surface. Large areas of the membranes, mechanically supported to withstand the very high pressures are essential to make the process perform rapidly for the most economical production. 

In arterioles, the hydrostatic pressure is about 37 mm Hg, with an interstitial (tissue) pressure of 1 mm Hg opposing it. The osmotic pressure (oncotic pressure) exerted by the plasma proteins is approximately 25 mm Hg. Thus, a net outward force of about 11 mm Hg drives fluid out into the interstitial spaces. In venules, the hydrostatic pressure is about 17 mm Hg, with the oncotic and interstitial pressures as described above thus, a net force of about 9 mm Hg attracts water back into the circulation. The above pressures are often referred to as the Starling forces. If the concentration of plasma proteins is markedly diminished (eg, due to severe protein malnutrition), fluid is not attracted back into the intravascular compartment and accumulates in the extravascular tissue spaces, a condition known as edema. Edema has many causes protein deficiency is one of them. 

The figures in Table XXVII show that a 0.001°C depression in the melting temperature corresponds approximately to a 10-cm. change in hydrostatic pressure head in the osmotic pressure. With appropriate pains, osmotic pressures may be measured within 0.01 cm. of liquid... 

Understanding the effects of colloid administration on circulating blood volume necessitates a review of those physiologic forces that determine fluid movement between capillaries and the interstitial space throughout the circulation (Fig. 10—5).4 Relative hydrostatic pressure between the capillary lumen and the interstitial space is one of the major determinants of net fluid flow into or out of the circulation. The other major determinant is the relative colloid osmotic pressure between the two spaces. Administration of exogenous colloids results in an increase in the intravascular colloid osmotic pressure. In the case of isosomotic colloids (5% albumin, 6% hetastarch, and dextran products), initial expansion of the intravascular space is essentially that of the volume of colloid administered. In the case of hyperoncotic solutions such as 25% albumin, fluid is pulled from the interstitial space into the vasculature... 

Fig. 6.4 Electroosmotic pressure. Hydrostatic pressure difference Ap compensates the osmotic pressure difference between the compartments 1 and 1 and prevents the solvent from flowing through the membrane 2...

Possible driving forces for solute flux can be enumerated as a linear combination of gradient contributions [Eq. (20)] to solute potential across the membrane barrier (see Part I of this volume). These transbarrier gradients include chemical potential (concentration gradient-driven diffusion), hydrostatic potential (pressure gradient-driven convection), electrical potential (ion gradient-driven cotransport), osmotic potential (osmotic pressure-driven convection), and chemical potential modified by chemical or biochemical reaction. 

For osmotic drug delivery systems, Eq. (2) is of critical importance. This equation demonstrates that the quantity of water that can pass a semipermeable film is directly proportional to the pressure differential across the film as measured by the difference between the hydrostatic and osmotic pressures. Osmotic delivery systems are generally composed of a solid core formulation coated with a semipermeable film. Included in the core formulation is a quantity of material capable of generating an osmotic pressure differential across the film. When placed in an aqueous environment, water is transported across the film. This transported water in turn builds up a hydrostatic pressure within the device which leads to expulsion of the core material through a suitably placed exit port. 

The third mechanism of capillary exchange is bulk flow. In this case, water and dissolved solutes move across capillaries due to hydrostatic pressure and osmotic pressure. When the balance of these two forces causes fluid to move out of the capillary, it is referred to as filtration. When these forces cause fluid to move into the capillary, it is referred to as reabsorption. 

Figure 15.7 Starling principle a summary of forces determining the bulk flow of fluid across the wall of a capillary. Hydrostatic forces include capillary pressure (Pc) and interstitial fluid pressure (PJ. Capillary pressure pushes fluid out of the capillary. Interstitial fluid pressure is negative and acts as a suction pulling fluid out of the capillary. Osmotic forces include plasma colloid osmotic pressure (np) and interstitial fluid colloid osmotic pressure (n,). These forces are caused by proteins that pull fluid toward them. The sum of these four forces results in net filtration of fluid at the arteriolar end of the capillary (where Pc is high) and net reabsorption of fluid at the venular end of the capillary (where Pc is low).

Although the interstitial fluid hydrostatic pressure is "negative," it causes fluid to be pulled out of the capillary, so this pressure is "added" to the other outward forces. The only force pulling fluid into the capillary is the plasma colloid osmotic pressure ... 

Increase hydrostatic pressure results in an increase in the solvent activity on the solution side until the applied pressure becomes equal to the Osmotic pressure when equilibrium is attained. 

If the osmotic pressure be a function of the hydrostatic pressure of the solution p and of the numerical concentration we have the mathematical identity... 

In order for solvent and solution to be in equilibrium in an apparatus such as that shown in Figure 3.2, the solution side must be at a higher pressure than the solvent side. This excess pressure is what is known as the osmotic pressure of the solution. If no external pressure difference is imposed, solvent will diffuse across the membrane until an equilibrium hydrostatic pressure head has developed on the solution side. In practice, to prevent too much dilution of the solution as a result of the solvent flow into it, the column in which the pressure head develops is generally of a very narrow diameter. We return to the details of osmotic pressure experiments in the next section. First, however, the theoretical connection between this pressure and the concentration of the solution must be established. 

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