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.
- Definition of Osmotic Pressure
- Significance of Osmotic Pressure in the Absorption of Food
- Factors Regulating Osmotic Pressure
- Osmotic Pressure in Cells
- Physiological Importance of Osmotic Pressure
- Factors Regulating the Osmotic Pressure and the Volume of the Extracellular Water
- ADH and Osmotic Pressure
- Vant Hoff’s Laws of Osmotic Pressure
- Calculations Involving Osmotic Pressure
- Distribution of a Solute between Two Immiscible Solvents
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.
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.