Osmotic Pressure

OSMOTIC PRESSURE, pressure produced by molecules of a dissolved substance on the semipermeable walls of a vessel. Theory of O.P. If pure water and any solution are separated by a partition that retains dissolved molecules but allows water to pass, the latter begins to pass through such a semipermeable membrane. If the solution is in a closed vessel, the entry of water into it (endosmosis) creates increased pressure in such a vessel, which stops further penetration of water. This hydrostatic pressure, balancing the tendency of water to penetrate into the solution, is called the osmotic pressure of the solution. It can be measured with a manometer connected to the inner vessel (see figure). The corresponding apparatus is called an osmometer. As a semipermeable membrane for his osmometer, Dutrochet used various animal membranes. However, these membranes were also permeable to dissolved substances. Significantly more perfect artificial semipermeable membranes that retain most dissolved substances were constructed by M. Traube. He noticed that when two substances that form an insoluble precipitate upon interaction come into contact, the latter in many cases is located at the boundary between them in the form of an extremely thin boundary film. Of various precipitate membranes, the film of ferrocyanide copper [Cu₂Fe(CN)₆], formed when potassium ferrocyanide comes into contact with

any copper salt. It holds back most of the dissolved salts, while water passes through it at a very significant speed. To give the membrane sufficient strength and stability, Pfeffer used vessels from unburned clay (usually used for galvanic elements). Acting on them from opposite sides with solutions of copper sulfate and potassium ferrocyanide, he achieved precipitation in the thickness of the porous wall of the clay vessel or at its inner surface. Pfeffer's measurements showed that there is a direct proportionality between the concentration of the dissolved substance and the O. d. it produces. For example, in a 1% solution of cane sugar, the O.d. was found to be equal to 53.5 cm of mercury, in a 6% solution it was approximately 6 times higher (307.5 cm). A remarkable analogy between the O.d. of solutions and gas pressure was established by van't Hoff; it formed the basis of the solution theory developed by him. The O.d. of solutions can be calculated by the equation of state of an ideal gas: pv=RT, where R is the gas constant equal to 0.0821 l/atm. However, for all electrolytes, the theoretical value is greater than the experimental one. Their ratio (P_meas: P_theor) is called the "isotonic coefficient" (i). This discrepancy between measurement results and theoretical calculation was resolved in the theory of electrolytic dissociation (see Electrolytic dissociation). The work required to concentrate a solution during endosmotic water absorption is the same as for the corresponding compression of a gas. In both cases, the magnitude of this work depends on the ratio of pressure at the beginning and end of the process. If p₁ denotes the O.d. of the initial solution, p₂ of the final one, then the osmotic work is equal to: A = RT ln p₂/p₁. O.d. is produced only by those molecules for which the membrane, permeable to the solvent, is impermeable. Molecules freely passing through the membrane must reach approximately equal concentrations on both sides of it, as a result of which their O.d. becomes zero. Due to this, membranes impermeable only to any particular group of dissolved substances allow measuring their partial O.d. For example, colloidal membranes (e.g., collodion film), freely passing all crystalloids but holding back most colloids, allow measuring the O.d. of the latter, which is very small. Therefore, the O.d. of colloids (oncotic pressure) can be accurately measured only if the O.d. of the crystalloids contained in the same solution is eliminated by the method described. At the same time, colloidal O.d. is extremely variable, extremely dependent on the electrolytes contained in the solution; The latter can first change the degree of dispersion of colloids, and thereby the number of osmotically active particles. Even stronger influence is exerted by electrolytes due to the membrane equilibrium Donnan that occurs between them and colloids (see Donnan equilibrium). In a living organism, there are two types of semipermeable membranes. The cell wall, which holds back most dissolved substances, can serve as a representative of one type; therefore, a living cell—both plant and animal—represents a true osmotic cell for them. Completely different properties are possessed by partitions such as the endothelial wall of blood capillaries, separating blood from tissue lymph, or the wall of renal glomeruli, through which urine is filtered. Freely passing the crystalloids dissolved in water, they hold back only colloidal substances. Methods for measuring O.d. The osmometer is the most accurate instrument for measuring the O.d. of a solution. However, it allows accurate measurement of the O.d. only for those dissolved substances for which its membrane is impermeable. This extremely limits the scope of application of the osmometer. In reality, there are no completely semipermeable membranes, i.e., membranes that would freely pass water while holding back all substances dissolved in it. Therefore, the osmometer cannot give the complete O.d. of a solution of any substances, and to determine it, one has to resort to other, indirect methods of measurement. These indirect methods are based on the dependence existing between the concentration of the solution and the pressure (or "tension") of its saturated vapor. A solution has a lower vapor tension than the pure solvent. As van't Hoff theoretically showed, this lowering of vapor tension, produced by the dissolved substance, is proportional to its O.d. The lowering of vapor tension can therefore be used to measure O.d. If in a closed space there are two aqueous solutions, then in the one that has a higher vapor tension, water evaporates, condensing in the second solution. This is the basis of the Barger method, later modified by Rast. It has the essential advantage for biological purposes that the measurement is carried out at ordinary temperatures and very small amounts of liquid are required. In a series of capillary tubes, a drop of the liquid under investigation is introduced together with a drop of a second solution of known and sequentially changing concentration. Air here plays the role of a semipermeable partition, holding back the dissolved substances but passing water (in a vapor state) from a hypotonic to a hypertonic solution. The absence of change in the volume of the drops (established under a microscope) serves as an indicator of the isotonicity of the liquid under investigation with the solution used for comparison. The ebullioscopic method, based on the increase in boiling temperature proportional to the lowering of the vapor tension of the solution, is not suitable for biological purposes, since the fluids in the body contain proteins and other colloids that coagulate and change during boiling. The cryoscopic method—determination of O.d. by the freezing point of the solution (see Cryoscopy)—is used not only for studying various biological fluids (blood, urine), but even for determining the O.d. prevailing inside living cells and tissues. For this purpose, the depression (lowering of the freezing point) of the juice obtained from various tissues by grinding, crushing, and pressing under a Buchner press is measured. To avoid postmortem chemical changes, which easily occur with such treatment, the tissue under investigation is first subjected to rapid heating to immediately destroy tissue enzymes and stop their action, or the enzymatic activity is stopped by rapid cooling—immersion of the tissue in liquid air. To avoid possible errors associated with the preparation of tissue juice, experiments were made on cryoscopy of whole tissues and organs, the freezing temperature of which was measured using a thermoelectric needle. Such measurements turn out to be no more reliable because living tissues usually freeze with very strong supercooling, which, as Bachmetiev's experiments showed, can reach 10° (only after the release of the first ice crystals does the temperature jump, approaching the freezing point of the solution). Under such conditions, the determination of the true freezing point becomes extremely inaccurate. Due to the aforementioned shortcomings of physical methods, biological methods acquire great importance in measuring the O.d. of cell contents. They are based on the fact that the cell wall represents a very perfect semipermeable membrane, thanks to which the living cell itself can be used as a "microosmometer". With the help of the plasmolysis method first developed by de Vries (see) and its various modifications, numerous measurements of the O.d. of plant cells have been made. In the case of an animal cell, the difference in O.d. between the surrounding solution and the cell manifests itself in its absorption or release of water, and consequently in a change in its volume, which can be directly measured. In the case of a suspension of isolated cells (e.g., red blood cells), it is more convenient to measure the total volume of the entire cell mass using a hematocrit (see). The cell contents are isotonic with a solution in which the cell volume remains unchanged. To measure the partial O.d. of colloids, the liquid under investigation is placed in an osmometer with a colloidal wall, and instead of water, the same liquid, previously freed from colloidal substances by ultrafiltration (see), is poured into the outer vessel. O.d. of blood and tissues. The contents of plant cells usually have a very significant O.d.; its values average 5-20 atmospheres. Such pressures allow the plant to more widely spread its light-requiring leaves and shoots than would be possible with supporting, skeletal structures. Below 4 atmospheres, the pressure rarely falls; more often it can have a larger value. For example, in the cells of stem nodes of cereals, the pressure reaches 50 atmospheres; the highest pressures—up to 100 atmospheres—were found in some desert plants, which are forced to particularly tenaciously hold on to the scarce water available to them.

Such hypertonicity of the plant cell in relation to the external environment has very significant importance for it, causing the necessary tension for growth processes in the plant cell, its turgor. If by increasing the concentration of the external solution the excess of internal O. d. over the external one is eliminated, the cell contracts and its further growth ceases. In the animal organism, internal fluids acquire very great importance - blood, cavity fluids, tissue lymph. They directly bathe the tissue cells, constituting their 'internal environment' - in contrast to the external environment surrounding the entire organism from the outside. At the beginning of the evolutionary ladder, in marine invertebrates, this internal environment does not yet differ from the external environment in either salt composition or O. d. - the sea water, which in the ocean and in open seas contains on average 3.5% salts, corresponding to a depression of 2.3° and an O. d. of approximately 28 atmospheres. Approximately the same O. d. is possessed by the fluid of the body cavity and the blood of invertebrate animals living in such water - coelenterates, worms, crustaceans, echinoderms, mollusks. In those cases where the surrounding sea water, desalinated by river waters, has a lower O. d., the latter is equally reduced in the internal environment. Similar changes in internal fluids can be caused experimentally by subjecting the animal to the action of an artificially concentrated or diluted solution. A significant complication in the character of the internal environment is observed in the lower group of marine vertebrates - in sharks. Their blood contains approximately half as much salt as the surrounding sea water, but a large amount (up to 2-3%) of urea, which participates in balancing the external O. d. Only in the next higher group of animals - in ganoid (sturgeon) and bony fishes - does the blood become independent of the external environment not only in composition but also in O. d. In a number of bony fishes, the latter was found to be equal to 9-13 atmospheres, whereas in the surrounding sea water it was 28 atmospheres. This independence of the internal environment from external osmotic conditions explains the ability of many bony fishes to periodically move from the sea to fresh waters of rivers for the purpose of spawning. In fresh waters, the O. d. and salt concentration apparently lie below the minimum which is necessary for the internal environment of the animal organism. Therefore, in fresh waters even invertebrate animals have a different (higher) O. d. in their internal fluid than in the external environment. It usually does not fall below 3-4 atmospheres (whereas the O. d. of fresh water is measured in tenths of an atmosphere). In vertebrate animals (fish, amphibians), it stabilizes at approximately the same level, invariably remaining lower than in corresponding marine organisms. The greatest stability is characteristic of the O. d. of the blood of higher terrestrial animals (as well as marine mammals, e.g. the whale). At the body temperature of a warm-blooded animal, it averages 8 atmospheres, i.e. lies between the figures established, on the one hand, for marine, and on the other hand, for freshwater vertebrates. Thus, one can trace the successive stages in the development of the properties of the internal environment, stages corresponding to the general course of organic evolution. From passive subordination to external conditions, from the identity of the properties of the internal and external environment, evolution first leads (in sharks) to a certain isolation of the internal environment in terms of salt composition, then to independence in the magnitude of O. d. The transition of marine organisms to fresh waters or to terrestrial existence accelerates this process. The biological significance of this phenomenon is clear: in the organism, with the help of special osmoregulatory organs, O. d. is maintained at a constant level and thus becomes a physiological constant. O. d. is no less a universal biological factor than temperature, and osmoregulation develops in the course of evolution much earlier than thermoregulation. The establishment of constant osmotic conditions in the internal environment of the organism, in the fluid directly bathing the tissue elements, has very significant importance for the animal cell. In contrast to the plant cell with its hypertonicity creating normal turgor tension, the contents of the animal cell (deprived in most cases of an elastic shell) usually differ little in their O. d. from the solution bathing it, and any sharp osmotic difference between them is quickly equalized by the movement of water or of dissolved substances. However, complete isotony with the surrounding environment is observed only in the resting cell. For active, working animal tissue, a somewhat elevated O. d. and turgor tension are also apparently characteristic. Although the applied methods in most cases do not give sufficiently reliable numerical results, they nevertheless reveal interesting dependencies between O. d. and the physiological state of the organs studied. In Sabbatani's experiments, the greatest depression was found in the liver and kidneys - organs which, due to their participation in continuous metabolic processes and secretion, perform particularly intensive work. During digestion, the depression in these organs increases even more: in the livers of dogs killed during active digestion, the depression was 1.00-1.20°, in fasting animals - 0.94°. An increase in osmotic concentration always occurs in active tissue; it is the result of the intensifying dissimilation during work, leading to the formation of smaller osmotically active molecules. In the working muscle, an increase in O. d. was discovered by Buglia's experiments. The muscle cells, the contents of which become hypertonic in relation to the blood, osmotically absorb water, swell and stretch their shell: hence the increase in volume and turgescence of the working muscle. A similar phenomenon was observed by Bottazzi and Enriquez in the salivary glands (octopus). The death of an organ causes in it, due to autolytic processes, the same increase in O. d. as work. Liacre's experiments clearly established a very significant increase in O. d. in liver tissue subjected to autolysis under aseptic conditions. In the living organism, local tissue disintegration is accompanied by a similar change, although not as strong as in the isolated organ. In the focus of an inflammatory process, Schade could confirm noticeable hypertonicity; the greatest depression is observed in the center of inflammation, where it usually equals 0.6-0.8°, in some cases rising even to 1.4°, which corresponds to pressures of 8-11, and in extreme cases up to 19 atmospheres. Osmotic absorption of water probably plays a significant role in the formation of inflammatory swellings. Thus, only in resting, inactive tissue can complete isotony between the cell contents and the fluid bathing it prevail. In the living organism, processes of metabolism, oxidation, dissimilation continuously disrupt osmotic equilibrium and create a difference in concentration between cells and tissue fluids, lymph, blood. In the active state, during the work of an organ (muscle, gland), the osmotic gradient increases; in the resting state, it returns to the original norm. The concentration successively decreases from tissue cells to arterial blood; in lymph and in venous blood it has intermediate values. Osmoregulation. Thus, osmotically active products continuously enter the bloodstream from the tissues. Besides these endogenous disturbances, the O. d. of blood can be subjected to even more drastic changes from the outside - from the external surfaces (in aquatic organisms) or from the intestine, which rapidly absorbs both water and dissolved substances entering it. Therefore, it is necessary to determine by what processes, with the help of what osmoregulatory processes in the higher animal organism, excess water and dissolved substances are removed and the constancy of O. d. of blood is maintained. Part of the water is continuously removed by the lungs, but the respiratory center is not excited by changes in the osmotic state of the blood and does not regulate the evaporation of water. Another path is represented by the sweat glands. The depression of sweat varies from 0.08 to 0.70°. Usually it is strongly hypotonic in comparison with the blood. Its amount varies within unusually wide limits - from a few cm³ to several liters per day. Thus, in the form of sweat, large amounts of water and dissolved substances can be excreted from the organism, but the hypotonia of the blood, the lowering of its normal O. d., is not a specific stimulus for sweating, and the secretion of sweat from the skin surface in humans, the evaporation of large amounts of water from the tongue in dogs, serve primarily as a means of thermoregulation and only indirectly participate in maintaining the osmotic properties of the blood. The kidneys play the main role in the latter. The kidneys excrete on average 1-2 liters of urine per day, the depression of which usually fluctuates between 1.3-2.2°.

Thus, normally urine possesses a significantly higher O. d. than blood, and the formation of urine requires the kidneys to perform very considerable osmotic^v work, which can be calculated knowing the concentration of osmotically active substances in urine and in blood. However, the removal of water and osmotically active substances from the body by the methods described here requires a certain amount of time, more or less prolonged. They do not allow one to understand the remarkable speed with which any more or less significant disturbance of the normal O. d. of blood is equalized. The speed of regulation is clearly shown by the experiments of Hamburger. In one of them he injected intravenously into a horse 7 liters of a 5% solution of Na₂SO₄. According to calculation, this amount was sufficient to double the O. d. of the blood. Meanwhile, only 10 minutes after the injection, the increase in pressure was only 5-6%, and after 1-2 hours it was completely smoothed out. In another experiment, when infusing 5 liters of the same solution_ch, after only 30 minutes the O. d. returned to normal. Equilibrium was restored by regulating the O. d. before the foreign substances (sulfates) introduced into the blood were removed. The excretion of the latter by the kidneys occurred much more slowly. Obviously, first the blood returns to a constant O. d. and only then is its normal chemical composition restored. Such rapid restoration of normal blood O. d. occurs mainly by the redistribution of water and dissolved substances between the blood and other fluids and tissues of the body, primarily between the blood and lymph. According to Schade, connective tissue (especially of subcutaneous fat and muscles) is the main depot of water in the body. By binding it or again releasing it, the intercellular substance of connective tissue participates in the phenomena of osmoregulation. From the extracellular fluids, osmotic disturbances are transmitted further by cells, which are bathed by these fluids and concentrate hypotonic solutions in contact with them and dilute hypertonic ones. Thanks to the huge surface of contact of circulating fluids with living cells, the process of bringing fluids to isotony with the cell contents occurs very quickly. Every sharp osmotic disturbance, spreading outwards, loses in intensity and can then be finally eliminated without harm to the body by the action of physiol. regulators that remove excess water and osmotically active substances from the body. O. d. of blood colloids. In the phenomena of movement and distribution of water in the body, and consequently in the phenomena of osmoregulation, blood colloids play a large role. Their O. d. was first measured by Starling, using ultrafiltrate of serum as the external fluid of the osmometer. According to Starling's measurements, the colloid-osmotic pressure of the blood averages 30-40 mm of mercury. Later measurements by Schade and Claussen gave for it an even lower figure—about 25 mm. Compared to the total O. d. of blood, which exceeds 7 atmospheres, this value (constituting about ^x/a% of the O. d. of blood) seems completely negligible. At first glance, it seems quite natural to think that the physiol. role played by the O. d. of colloids in the body must be equally negligible. However, this assumption is not justified in reality. In the body there are widely distributed partitions which, like the membranes used in ultrafiltration, are impermeable to colloids and freely allow crystalloids to pass through. Through such partitions, water and dissolved crystalloids are osmotically absorbed until the hydrostatic pressure inside equals the O. d.; with a greater increase in hydrostatic pressure, ultrafiltration occurs, and the colloids are again separated from the salt solution. The small magnitude of the O. d. of blood colloids acquires special significance due to the fact that in the body hydrostatic pressures of the same order are usually observed, and the circulatory system allows their magnitude to be quickly and precisely regulated. A small change in one or the other pressure—osmotic or hydrostatic—is sufficient to disturb the equilibrium and produce a flow of fluid in one direction or the other. The interplay of these two forces forms the basis of the functioning of a number of physiol. apparatuses regulating the distribution and movement of water in the body. They play a particularly essential role in the mechanism of lymph formation and urine secretion (see Diuresis), as well as in the pathogenesis of edema (see Edema). Since lymph contains less protein than blood, it is necessary to overcome the O. d. of blood proteins during its formation. In an even purer form, such ultrafiltration of a crystalloid solution from blood occurs in the renal glomeruli during urine formation. Only secondarily, during the further movement of urine through the renal tubules, does the subsequent processing of the initial ultrafiltrate and the change in its crystalloid composition occur. Therefore, the O. d. of blood colloids is one of the decisive factors in the water balance of the body. For a long time, this value was not given proper attention in attempts to replace blood with artificial physiological solutions. For the first time, Starling clearly put forward such a significance of colloid-osmotic blood pressure, and Bayliss proposed adding colloids in a concentration with an osmotic concentration equal to that of blood to crystalloid physiological solutions (see).