Osmotic Pressure: Definition, Significance and Factors | Biophysics

In this article we will discuss about:- 1. Definition of Osmotic Pressure 2. Significance of Osmotic Pressure in the Absorption of Food 3. Factors Regulating 4. Pressure in Cells 5. Physiological Importance 6. Factors Regulating the Volume of the Extracellular Water 7. ADH 8. Vant Hoff’s Laws 9. Calculations Involving 10. Distribution of a Solute between Two Immiscible Solvents.

Contents:

1. Definition of Osmotic Pressure
2. Significance of Osmotic Pressure in the Absorption of Food
3. Factors Regulating Osmotic Pressure
4. Osmotic Pressure in Cells
5. Physiological Importance of Osmotic Pressure
6. Factors Regulating the Osmotic Pressure and the Volume of the Extracellular Water
7. ADH and Osmotic Pressure
8. Vant Hoff’s Laws of Osmotic Pressure
9. Calculations Involving Osmotic Pressure
10. Distribution of a Solute between Two Immiscible Solvents

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1. Definition of Osmotic Pressure:

Osmotic pressure can be defined as the excess pressure which must be applied to a solution to prevent the flow of solvent of low osmotic pressure when they are separated by a perfectly semi-permeable membrane.

2. Significance of Osmotic Pressure in the Absorption of Food:

i. Osmotic pressure is only permanent if the membrane is truly semipermeable, i.e. if it stops all solute molecules and only passes the solvent molecules.

ii. In case of dialysis, if the collodion or cel­lophane bag is filled with a solution of a dye with small molecules and placed in contact with water, water will pass into the dye, but, at the same time, water plus dye will pass out from the bag.

As water molecules are smaller than the dye mol­ecules, water will pass into the bag more quickly than water plus dye will leave it. Therefore, an osmotic pressure will be de­veloped, but it will only be small and tran­sient because the membrane is permeable to both water and dye.

But, if the bag is filled with a dye of large molecules, the dye cannot pass out into the water but water will pass into the dye—causing a permanent pressure difference.

If the bag is surrounded with a solution of NaCl of greater osmotic pressure than the dye, water will at first pass out of the bag faster but, at the same time, NaCl will pass into the bag. The final equilibrium will be at­tained when the osmotic pressure of the salt solution outside the bag equals the osmotic pressure of salt inside, so that the ultimate permanent osmotic pressure dif­ference will be that of the dye alone.

3. Factors Regulating Osmotic Pressure:

i. Osmotic pressure depends on the number of solute molecules but not on the size of the molecules.

Example: A solution of urea (Mol. wt. 60) of 60 g. per litre has the same osmotic pres­sure as a solution of cane sugar (mol. wt. 342) of 342 g. per litre; because these two solutions contain the same number of mol­ecules per litre.

ii. In case of salts which ionize, it is the number of ions plus molecules which count, so that fully ionised NaCl has twice the osmotic pressure it would have as judged by the number of molecules.

iii. Other substances, such as soaps, form mo­lecular aggregates, so that their solutions have lower osmotic pressure.

iv. The osmotic pressure increases with the rise in temperature.

4. Osmotic Pressure in Cells:

i. If the cell is kept in a hypotonic solution, the cell wall and the vacuolar membrane both will allow water to pass into it and will set up an excess pressure in the inte­rior of the cell causing the cytoplasm to be forced tightly against the cell wall. In normal health, this condition is known as “turgor” and the cell is said to be turgid.

ii. If the cell is immersed in a concentrated solution (high osmotic pressure), water will pass out of the interior of the cell. The cytoplasm will then shrink and detach it­self from the cell wall. This phenomenon is said to be “plasmolysis”. Iso-osmotics: Solutions with the same pres­sure are termed iso-osmotics.

Isotonic solution:

A pair of solutions which produce no flow through a semi­permeable membrane are said to be isot­onic solutions.

5. Physiological Importance of Osmotic Pressure:

i. Absorption from gastro-intestinal tract, as also fluid interchange in various compart­ments of the body follow the principles of osmosis.

ii. The osmotic pressure of plasma proteins regulates water to flow from the protein- free intestinal fluid into the blood vessels.

iii. Living red cells, if suspended in 0.92% NaCl solution, neither gain nor lose wa­ter. Briefly speaking, intracellular fluid of red cells is isotonic with the red cell mem­brane in 0.92% NaCl solution.

6. Factors Regulating the Osmotic Pressure and the Volume of the Extracellular Water:

In the maintenance of life, the body has homeostatic or regulatory mechanisms which help to maintain the osmotic pressure and the volume of the extra­cellular water within physiological limits. The os­motic pressure of the extracellular water is control­led by antidiuretic hormone ADH and the volume of the extracellular water is controlled by aldoster­one, the adrenocortical hormone.

7. ADH and Osmotic Pressure:

ADH, the antidiuretic hormone, is an octapeptide. It regulates the osmotic pressure of the extracellu­lar water and of the cells by regulating the reten­tion or excretion of water by the kidneys. The posterior pituitary gland is stimulated to secrete ADH, when the osmotic pressure of the ex­tracellular water becomes relatively greater than the osmotic pressure of the cells.

ADH liberation is stopped, when the osmotic pressure of the extracel­lular water is relatively less than that of the cells. The site of action of ADH is probably the distal tubules and collecting ducts of the kidneys. ADH increases the permeability of the structures to wa­ter.

When this happens; an increased amount of water is reabsorbed resulting in the volume of urine decrease. ADH secretion is not affected when the total osmotic pressure of the body changes. If urea is infused, it does not cause an antidiuretic effect even though it increases the osmotic pressure of the ex­tracellular water.

Because, it is able to penetrate the cells freely. Hence it also raises the osmotic pressures of the cells to the same degree as the ex­tracellular water.

If, for any reason, the osmotic pres­sure of the cells in relation to the osmotic pressure of the extracellular water changes, ADH secretion either decreases or increases.

If a large amount of hypertonic glucose or sucrose is infused into the body, it raises the osmotic pressure of the extracel­lular water.

The glucose or sucrose is unable to pen­etrate cells freely. Therefore, the osmotic pressure of the extracellular water rises above that of the cells. As a result, ADH secretion occurs and water excretion by the kidneys decreases.

If a person drinks plain water, the immediate effect of the imbibed water is to dilute the extracel­lular water.

This causes the osmotic pressure of the extracellular water to become lower than that of the cells. Therefore, ADH secretion stops and water diuresis takes place.

ADH stimulation occurs in various ways, e.g., it occurs as the result of fear, pain or in acute infec­tions such as pneumonia. The mechanism by which ADH secretion occurs in these conditions is un­known.

Recently it has been found that the liver acts as an osmoreceptor to regulate the intake of water from the gastro-intestinal tract.

When water is im­bibed, it is absorbed into the splanchnic circula­tion and is brought to the liver by way of the portal vein.

This causes the osmolality of the portal vein to decrease very rapidly. The hypothalamus then stimulates the kidneys to excrete water.

8. Vant Hoff’s Laws of Osmotic Pressure:

This is stated as:

1. The osmotic pressure of a solution varies directly with the concentration of the sol­ute in the solution and is equal to the pres­sure the solute would exert if it would be a gas in the volume occupied by the solu­tion, if the volume of the solute molecules relative to volume of solvent be negligi­ble.

2. The osmotic pressure of a solution varies directly with absolute temperature in the same way as the pressure of a gas varies when its volume is kept constant.

These laws of osmotic pressure have been thor­oughly verified by accurate observations. As with gases, the laws of osmotic pressure hold closely only for dilute solutions. Proper correction must be made for concentrated solution.

9. Calculations Involving Osmotic Pressure:

Osmotic pressure is due to the difference in activ­ity of pure solvent molecules and of solvent mol­ecules associated with solute molecules. It is pro­portional to the number of and independent of the kind of dissolved particles present.

The general equation for gases applies to os­motic pressure:

where π = the osmotic pressure in atmosphere, n = the number of moles of solutes, R = the molar gas constant (0.082 litre-atmosphere/mol/Å), T = the absolute temperature, V = the volume in litres.

Since n = g/M, where g = grams of solute and

M = the molecular weight of the solute, the equa­tion may be written

πV = g/M RT, or, π = CRT

where C = the molal concentration of the solution and corrects for the volume occupied by the solute molecules.

10. Distribution of a Solute between Two Immiscible Solvents:

When a water solution of succinic acid is shaken with ether, the molecules of acid distribute them­selves in such a way that the ratio of acid mol­ecules dissolved in water to those dissolved in ether is constant, regardless of the total amount of acid dissolved. The ratio of concentration of acid in water C₁ to concentration of acid in ether C₂ is relatively constant

C₁/C₂ = K

The principle of the distribution of solutes between immiscible solvents is of much importance in the body. In general, a substance that is more soluble in organic solvents than in water is also more soluble in lipids. Consequently, drugs and other molecules that are more soluble in organic solvents than in water will tend to concentrate in the tissues and fluids of the body that contain more lipid material.

On the other hand, molecules that are highly soluble in water and slightly soluble in lipids will be present in greater concentration in the body fluids and tissues that contain little lipid or fat-like material.

For example, when ether is used as an anaesthetic, the concentration in the brain and nervous tissue (rich in lipids) will be much greater than the concentration in the blood and tis­sues, which are much poorer in lipid material. Theo­retically, the ether will be distributed in the body according to its distribution coefficients for the various fluids and tissues that serve as solvents for it.