In this article we will discuss about some physico-chemical principles which plays a great role in physiology with the help of suitable diagrams. Also learn the physiological importance of each physico-chemical principles.
Filtration is the process by which un-dissolved particles are separated from a liquid through a membrane as a result of a mechanical force (filtering force). It is done through a porous substance, such as a piece of linen or filter paper. This filtering force is either gravity or hydrostatic pressure positive or negative.
Whenever there is a difference of hydrostatic pressure between the two sides of a membrane, filtration will occur.
The important examples are:
(a) Absorption from the small intestine, and
(b) Passage of water, salts, foodstuffs, etc., from the blood stream to the tissue fluid—hydrostatic pressure in the capillaries being higher than in the latter.
Molecules of a substance are continuously in motion. This motion is least in the solids, maximum in the gases and intermediate in the liquids. When two such substances, (for instance—a solid and a liquid, or two miscible liquids, or a liquid and a gas or two gases) are kept in contact, the molecules of the two substances will pass into each other, until a uniform admixture is obtained.
If a layer of water be carefully added upon a layer of concentrated sugar solution, the two liquids will gradually run into each other and ultimately the sugar concentration will be same everywhere. This spontaneous admixture of the molecules of the two substances in contact (due to inherent molecular movement) is called diffusion (Fig. 3.1).
Anything that alters the molecular movement also alters the rate of diffusion proportionally, viz., heat increases and cold depresses. The diffusibility of different substances is not same. Other factors remaining constant, it depends upon the weight and size of the molecules.
When two substances, as mentioned above, come in contact, directly or through a permeable membrane, diffusion will take place, when the molecular concentration of a substance in solution is higher in one part of a liquid than in another and when any absolute barrier does not intervene (Fig. 3.2).
The diffusion of water through a semipermeable membrane is called osmosis. If a layer of water be separated from a sugar solution by a semipermeable membrane (which will allow only the water molecules to pass but not the sugar molecules) it will be seen that the sugar solution gradually increases in volume for some time and then there will be no further change (Fig. 3.3). What happens is that, the sugar molecules being impermeable, more water molecules will pass from the water layer into the sugar solution than will pass from the latter to the former (Fig. 3.3 A & B).
Due to this, the volume and therefore the level of sugar solution will rise. This will raise the hydrostatic pressure of the sugar solution and this increased pressure will force more and more water molecules to pass out of the sugar solution. Thus a time will come when the movement of water molecules on either side will be same, so that no further alteration of volume will take place.
At this stage (Fig. 3.3 C), the hydrostatic pressure of sugar solution S exactly neutralises the attractive force of the solution for water molecules. This attractive force is the osmotic pressure. The amount of pressure which has to be applied on solution S to prevent the movement of water molecules is defined as the osmotic pressure of the particular concentration of the substance.
If the semipermeable membrane is placed between two sugar solutions of different strengths, water molecules will continuously pass from one side to the other till the concentrations on two sides are equal. Stronger solution will always draw more water from the weaker solution. Therefore, the force under which a solvent moves from a solution of lower solute concentration to a solution of higher solute concentration when a selectively permeable membrane separates these solutions is called osmotic (oncotic) pressure (Fig. 3.3).
In true sense, it is the pressure which must be put upon a solution to keep it in equilibrium with the pure solvent when the two are separated by a semipermeable membrane. For this reason osmotic pressure does not depend upon the size of the molecules but upon the total number of discrete particles per unit volume. If the solute be ionisable, the osmotic pressure (O.P.) will be proportionally more.
If more pressure (weight) is applied on piston (Fig. 3.4) water will be forced out from the stronger solution S to the weaker solution W. This passage of water against concentration gradient is known as ultrafiltration. If two solutions, separated by a membrane, have the same O.P., they are called isotonic.
One having lesser O.P. is called hypotonic, while that having higher O.P., hypertonic. 0.9 percent sodium chloride solution is isotonic with blood plasma, commonly known as normal saline or physiological saline. A 5 percent solution of glucose has also similar osmotic pressure.
Either of them can be injected intravenously in human patient if properly sterilised. Two solutions having same number of particles per unit volume are called isosmotic. But since, their permeability through a membrane may vary, they are not necessarily isotonic.
Osmotic pressure plays a great role in physiology.
Ultrafiltration is a kind of filtration through a jelly filter or any ultrafilter which serves to separate colloid solutions from crystalloids and to separate particles of different size in a colloid mixture. Due to the opposite of osmosis ultrafiltration results from the exertion of a pressure on a solution. This pressure forces the solution through a membrane impermeable to one or more of the solutes.
The blood plasma is placed in a vessel of which one end is a collodion membrane. Now, if a pressure is exerted on the blood plasma an ultra filtrate will be separated. This separation will take out from the solution all the constituents of the plasma except the protein which has been contained in the gross plasma.
This occurrence results in due to small pores of the collodion membrane. The extracellular fluid (ECF) is also an ultra filtrate into the plasma through the porous capillary membrane. If the counter pressure is not exerted on the plasma, the ECF shall pass back into the plasma.
Dialysis is a process by which the more diffusible materials can be separated from non-diffusible material. In Fig. 3.6 A, water solution of egg albumin and sugar has taken in the upper smaller container whose open bottom is covered with a semipermeable membrane. The semipermeable membrane has got selective permeability to water and sugar molecules but not to macromolecules—the egg albumin.
This container is suspended (partially) in the water of a large container. Due to selective permeability, the sugar molecules will ultimately go into the water leaving behind only the albumin and little water. As the albumin is impermeable to this membrane, this will rebound from the membrane during the process of dialysis (Fig. 3.6).
Whenever the bigger particles are held back and only the smaller particles are allowed to pass through a membrane— depending upon osmotic pressure (i.e., diffusion), dialysis comes into action.
(a) During absorption from the intestine bigger food particles are held back,
(b) In the capillary area the bigger albumin, globulin, etc., particles are not ordinarily allowed to pass into the tissue fluid. It is to be noted that diffusion, osmosis and dialysis are the manifestations of the same principle (inherent molecular movement) and usually go on simultaneously.
6. Surface Tension:
Surface tension is the manifestation of attracting forces in between atoms or molecules. As elsewhere, so also in a liquid, the molecules attract each other. Within the depth of the liquid, each molecule is attracted equally from all directions (Fig. 3.7).
Hence, the molecule can move freely in all directions. But a molecule at the liquid-air surface is attracted only by the molecules within the depth of the liquid and there are relatively few molecules in the gas above the water surface to exert any upward force. Consequently, it tends to be pulled inwards and its freedom of movement is restricted.
Thus at the surface of a liquid a layer is formed in which the molecules are arranged more densely. For the same reason the surface of the liquid tries to pull itself together and shrinks in order to occupy the least possible area. This energy with which the surface molecules closely adhere together is called surface tension.
Solutes alter surface tension. Inorganic salts generally rise while organic substances reduce surface tension of water. Of the latter—bile salts, proteins, phospholipids, oils, soaps, etc., are important examples (Hay’s sulphur test, for the detection of bile salts in urine, depends upon this principle.)
(a) The globular shape of an oil drop in water, of the fat particles in milk, etc., is due to surface tension.
(b) Bile salts reduce the surface tension of fat converting it into an emulsion in the intestine. This helps digestion and absorption of fat.
(c) The formation of cell membrane is at least partly due to surface tension of the cell cytoplasm. There are innumerable such examples.
Adsorption is a peculiar form of combination in which substances adhere together on their surfaces. It is a sort of union by surface contact. It is not a true chemical reaction, because no definite quantitative relation is found.
In physiology adsorption plays a great role.
Some of them are as follows:
(a) Enzyme action—both the enzyme and the substrate are colloidal in nature. By process of adsorption they come in closer contact and the interaction is hastened,
(b) The combination between toxin and antitoxin, by which they neutralise each other, is another adsorption phenomenon,
(c) Various adsorption compounds are formed in the body, such as lecithin with protein (found in the brain) and such others. The blue compound formed by adding iodine with starch is an adsorption compound.
Certain substances have the property of making water-insoluble substance soluble in water. This is called hydrotropy or hydrotropic action. How this action is brought about is not definitely known.
It is said that the hydrotropic substances form loose compounds with the insoluble substances and thereby make them soluble and diffusible through membranes. This view is supported by the fact that a quantitative relation is often found between them. For instance, when glycocholic acid forms hydrotropic compound with oleic acid they have a molecular ratio of 3:1.
Hydrotropic substances of physiological importance are:
i. Bile salts and such other compounds of cholic acid (vide under ‘Bile Salts’). This is possibly the chief member.
ii. Lecithin, soaps of higher fatty acids, phenylacetic acid, benzoic acid, hippuric acid, etc.
iii. Hydrotropic substances of unknown nature are also found in the intestinal juice, in the intestinal mucosa, in the blood plasma and possibly in other tissues and body fluids.
The insoluble substances which are made soluble in this way are—fats, certain phospholipids, sterols— specially cholesterol, insoluble soaps, uric acid and inorganic salts of Ca, Mg, and possibly of Fe, Cu, Mn, etc.
This is of immense physiological value. There are many chemicals in the body which are kept in solution with the help of hydrotropic action.
9. Donnan Equilibrium:
When a saline solution and distilled water are separated by a permeable membrane final equilibrium will be reached when the concentration of salt on both sides will be same. But if there be any non-diffusible ion on one side, a different phenomenon will be seen. Suppose NaP is an ionisable compound of which P is non- diffusible.
If this compound is kept on one side and NaCI on the other, Na and CI ions will freely pass but not P. When the final equilibrium will be reached, it will be seen that the product of Na and CI on one side is equal to the product of the same two ions on the other side.
In the final state Na × CI (left) = Na × Cl (right). It is obvious that total Na of the left side is greater than total Na of the right side and the total chloride of the right side is greater than total chloride of the left side.
Similarly, if NaP be on the left side and H2O on the right side, in the final state the reaction on the right side will be alkaline due to Na and OH ions. If a compound Cl-P be on the left and H2O on the right, in the final state the reaction on the right side will be acid due to H and CI ions.
This type of equilibrium in which the products of the same pair of ions on two sides of the permeable membrane become same, is called Donnan equilibrium. It is obvious that in the final state there will be a great difference in the nature and quantity of diffusible ions on two sides of the membrane which will lead to a difference of electric potential and chemical reaction on the two sides. Since, in our body there are many compounds of the NaP type, Donnan equilibrium is of great physiological importance.
It explains how difference of electric potential can be established on two sides of a membrane, how stomach can secrete a strongly acid juice and pancreas can secrete an alkaline juice. The phenomenon of chloride shift can also be explained from this standpoint.
When sugar, urea, NaCI, etc., be dissolved in water the result is a clear permanent true solution. But if proteins, starch, glycogen, etc., be placed in water, they will make a thick opalescent unstable solution. Graham (1861) called the first type as the crystalloidal solution, because solute particles form crystals and pass through parchment membrane, and called the second group, colloidal solution, for the solute particles in this case show reverse properties.
Modern idea indicates that the difference between true solution and colloidal solution depends on the size of the solute (the dispersed phase) molecules in the solvent (the dispersion medium). If the size is greater than 200 mµ they remain as suspensions and if less than 1 mµ as true solution. The size of the molecules in colloidal solutions varies from 1 mµ to 200 mµ.
Emulsoid and Suspensoid:
Colloids fall into two classes—emulsoid (emulsion) or lyophilic colloids and suspensoid (suspension) or lyophobic colloids. Difference between the two is shown in Table 3.1.
Emulsoids being more stable, can impart their own stability to suspensoids. In other words, when suspensions of solid particles are made in a solvent which is already an emulsion, the resulting suspensoid will be relatively more stable. This is called the protective action of emulsoids. For instance, blood is a suspension of red cells (7.2µ) in plasma, which in itself is an emulsion of proteins in water. This makes blood, a relatively stable suspensoid.
Sol and Gel:
A colloid may remain in two states—as a liquid or as a solid. The former is called sol, the latter, gel. These, two states are reversible. Sol can be transformed into gel by altering temperature, H-ion concentration, salt concentration, concentration of the solute, etc. During this process, a relatively small amount of solute particles runs together and sets into a semisolid mass, entangling in its meshes a fairly large amount of the solvent in the form of isolated droplets.
The explanation is as follows. Being heterogeneous, colloidal solution has two distinct components—called phases. The solvent, being continuous, is called the continuous phase. The solute, being discontinuous or dispersed, is known as dispersed phase. The two phases are interchangeable.
When the solute is more or less solid and the solvent (continuous phase) is liquid—it becomes a sol (if the liquid is water—a hydrosol). If, on the other hand, the solvent (continuous phase) is more or less solid and the solute (dispersed phase) be liquid-it makes a gel (if the latter is water—a hydrogel). For instance, a hot solution of agar is a liquid—a sol. On cooling, it sets into a semisolid jelly—a gel. Milk is a sol; butter is a gel (Fig. 3.8).
Colloids are of immense physiological value.