Permeability

PERMEABILITY, the ability of a partition or membrane to allow dissolved substances to pass through. If a membrane allows certain substances to pass while holding back others, it is called semipermeable. Usually, semipermeable membranes let the solvent (e.g., water) and some dissolved substances pass; the fewer substances a membrane passes besides the solvent, the more perfect its semipermeability is considered. Finally, impermeable membranes hold back even the solvent itself. Numerous studies show that the surface of protoplasm represents a semipermeable membrane. The penetration into the cell and the biological action of any chemical substances—both ordinary constituents of the solutions bathing the cell and various medicinal substances—depend on the properties of this membrane and the nature of its permeability. In the case of multicellular organisms, the covering layer of epithelial tissue plays the same role of a semipermeable partition for the penetration of dissolved substances from the outside as the surface layer of an individual cell. The study of the permeability of the living cell therefore constitutes a necessary basis for developing one of the most important problems of biology—the problem of the relationship between the organism and the environment. Along with living cell membranes, the permeability of non-living and in particular artificial membranes, which have found important applications in technology (for dialysis, ultrafiltration, etc.), is also of considerable interest. The study of these simpler membranes provides essential material for building a general theory of permeability, although the results obtained from them cannot be directly transferred to cellular permeability, which is a strictly vital property of the cell, depending on its functional state and sharply disrupted upon death. Permeability of non-living membranes. Membranes possessing permeability for crystalloids, but more or less fully holding back colloids, are especially widespread. Such properties are possessed, for example, by various dead tissues or membranes prepared from them: the wall of the swim bladder or urinary bladder, parchment membranes, etc. Similar membranes can be prepared artificially from various colloidal substances, such as gelatin or collodion, and their porosity varies depending on the preparation method (see Dialysis). The use of a series of membranes of successively varying porosity even makes it possible to fractionate a mixture of colloids, filtering off the colloid with larger particles from the more finely dispersed one passing through the membrane (see Ultrafiltration). It is significantly more difficult to construct an artificial semipermeable membrane that also holds back crystalloids. The most perfect in this respect is undoubtedly the copper ferrocyanide precipitation membrane, which is impermeable to very many crystalloids (see Osmotic pressure). In recent years, dried collodion films have also been studied in great detail. While ordinary collodion membranes pass not only crystalloids but—depending on the preparation method—also the most finely dispersed colloids, upon complete drying their permeability decreases to a huge extent and acquires a sharply pronounced selective character. As thorough studies by Collander showed, the penetration of non-electrolytes through a dried collodion film is determined mainly by their molecular volume: only substances whose molecules do not exceed a certain limiting value pass—the smaller, the denser the collodion film is. Thus, in this case, the theory first advanced by M. Traube is justified, according to which a semipermeable membrane represents a "molecular sieve" that mechanically holds back larger molecules. The precipitation membrane mentioned above behaves in a similar way. The penetration of electrolytes through the same dried collodion film reveals completely different dependencies. If a solution of any electrolyte, e.g., potassium chloride, is separated by such a membrane from pure water, the electrolyte does not penetrate into it. A more precise study shows, however, that in reality the membrane is impermeable not to both ions of the electrolyte, but only to the anion; cations are held back together with anions only due to the impossibility of any significant spatial separation from each other of ions of different signs. This is easy to verify by replacing the water in the external liquid with a sodium chloride solution. Then potassium ions quickly appear in the external liquid, exchanging for a corresponding amount of sodium ions. Analogous experiments on the chlorine ion show that for it the membrane is impermeable under all conditions. Michaelis substantiated the theory of selective ionic permeability of a membrane containing microscopic pores approaching molecular dimensions in a series of studies. At the boundary of two contacting phases, a more or less significant potential difference usually arises. On the surface of a solid body immersed in water, an electric double layer is formed (see); an excess of ions of one sign predominates in the thinnest aqueous layer tightly bound to the solid body and imparting a corresponding electrical charge to it, while an equal excess of ions of the opposite sign forms the outer, mobile sheath of the double layer. Dissolved substances can pass through the latter only when the pore sizes do not exceed the thickness of the double layer. However, only ions having a charge of the opposite sign compared to the pore walls can freely enter the outer sheath of the double layer; for ions of the same sign, passage will be difficult or even completely impossible. According to Michaelis, the selective ionic permeability of fine-pored membranes is based on this. Its existence (and the degree of selectivity) can be verified by separating two solutions of the same electrolyte at different concentrations with such a membrane and measuring the magnitude of the membrane potential difference (see) between them. Due to the fact that the collodion surface has a negative boundary potential, a membrane prepared from it exhibits selective permeability for cations. Similarly, as Mond and Hoffmann showed, a positively charged membrane possesses selective permeability for anions. The permeability of uniform, homogeneous membranes, e.g., a non-aqueous film separating two aqueous phases, is based on a completely different principle. Here there can be no question of penetration through any pores, and consequently of dependence on pore size and electrical forces acting on their surface, on the one hand, and on the size or electrical properties of the passing particles, on the other. The condition for passing through the membrane is dissolution in the substance of the membrane itself. Thus, for example, only substances soluble in oil can pass through an oil layer. Permeability of the cell wall. Research methods. The study of the permeability of the living cell required the development of special methods, each of which has a limited scope of application and more or less significant drawbacks. The most direct and precise method would be the direct chemical determination of substances penetrating into the cell. Unfortunately, only very few plant cells are large enough and contain enough cell sap for chemical analysis. Such cells, as for example in the alga Valonia, therefore represent an exceptionally convenient object that has provided valuable material for studying permeability. In other cases, the amount of substance penetrating into the cell can be estimated, at least approximately, only under the condition that it gives some visible reactions in the cell under the microscope. This takes place, for example, when the cell contains an indicator by the color change of which one can judge the amount of acid or alkali that has penetrated the cell. Some cells contain reaction-sensitive pigments that can be used as natural indicators. For the same purpose, certain artificial indicators that easily penetrate the cell (e.g., neutral red) are also used. Tannic acid contained in the plant cell sap, which is an excellent indicator for the presence of alkaloids, can play a similar role. It forms insoluble precipitates with them, making it possible to detect the penetration of minute traces of alkaloid bases into the cell. For the purpose of the direct observation of permeability, many researchers have studied the penetration of various dyes into the cell. Here, the presence or absence of coloration was used to judge whether a given dye penetrates the cell, and the intensity of coloration was used to judge the degree of permeability and the rate of penetration. This very widespread method is, however, highly unreliable, since coloration depends on more than just permeability. The cell acquires noticeable coloration only when the dye, having penetrated the cell, accumulates in it in a significantly higher concentration than in the surrounding solution. Therefore, changes in stainability depend less on permeability than on changes in the conditions of binding and accumulation of the dye inside the cell. Among indirect methods of studying permeability, osmotic methods played the most important role.

In its relation to surrounding solutions, a cell behaves to a certain extent like an osmotic cell, absorbing water from hypotonic solutions and losing it in hypertonic ones; in the former its volume increases, in the latter it decreases. However, the cell volume changes in strict accordance with the osmotic pressure of the surrounding solution only if its surface is impermeable to the dissolved substance. By the degree of deviation from this simple relationship, one can judge the degree of permeability. Thus, for example, under the action of a substance capable of gradually penetrating the cell, plasmolysis (see) will require a higher osmotic pressure of the solution than in the case of a substance for which the cell surface is completely impermeable. The higher the permeability, the greater this discrepancy. Substances that pass into the cell as freely as water do not cause plasmolysis at any concentration. As the plasmolyzing substance penetrates into the cell and its concentration equalizes, plasmolysis disappears, giving way to deplasmolysis. The latter occurs the faster, the higher the permeability, thus providing another method for its characterization. In conclusion, we should dwell on methods that study tissue permeability rather than cell permeability proper. They are based on using not the surface of an individual cell as a membrane, but an entire lamellar tissue, which makes it possible to move from micromethods to macromethods. As such a tissue, the lamellar alga Laminaria, frog skin, and the intestinal wall have been used. Tissue permeability can be judged by directly measuring the amount of substances passing through it or by determining its electrical conductivity; the latter value characterizes exclusively ionic permeability. Results obtained on whole tissue cannot always be directly translated to cellular permeability. Certain features of tissue permeability will be discussed below. At present, an enormous amount of experimental material has accumulated on the questions of cellular permeability, which, unfortunately, is of very unequal value. As a result of a insufficiently critical attitude towards the applied methods, very many studies represent ballast that cannot be used in establishing the laws of permeability. First of all, it should be noted that instead of the rate of penetration (characterized by the amount of substance penetrating per unit of time), the distribution of a given substance between the cell and the solution was often investigated in essence. Meanwhile, the concentration in which this or that substance can accumulate in the cell does not give a correct idea of the rate of its penetration. An even more serious source of errors is the insufficient consideration of the basic fact that cell permeability is sharply disrupted when it is damaged, whereas the permeability of a living, undamaged cell is precisely what is subject to investigation. Meanwhile, many of the studied substances themselves more or less strongly damage the cell and disrupt its normal semipermeability. First they damage the cell and only after that gain access into it. This relates in particular to very many observations on the permeability of cells for acids and alkalis. Most researchers did not distinguish their truly vital penetration from passage into a cell previously damaged by an abnormal reaction of the solution. Of course, such "secondary penetration" has nothing in common with normal cellular permeability, which is the subject of study. To avoid this error, there is only one method, unfortunately far from always applied; it is necessary at the end of the experiment to check whether the cell has retained its viability. For a plant cell, the criterion for this can be the deplasmolysis of a (previously plasmolyzed) cell in a hypotonic solution; for muscle tissue, the preservation (or restoration) of normal excitability; for an egg, the capacity for development, etc. For the same purpose, one can also check whether the cell wall still retains one of the substances for which it is normally impermeable. Rules of cellular permeability. The permeability of cells to dissolved substances was first systematically studied by Overton at the end of the 19th century. The regularities established on the basis of the huge number of experiments performed by him turned out to be in main features identical for all cells studied by him, plant and animal. They therefore represent general rules characterizing the permeability of any cell wall, although individual types of cells may differ in specific features. Subsequent authors, who significantly developed and supplemented the doctrine of cellular permeability, at the same time confirmed the main regularities established by Overton. Overton's studies showed that there is a close relationship between the chemical structure of various compounds and their ability to penetrate into a living cell. The relationships existing between them receive a very general and clear formulation if one uses the basic concepts of the modern theory of molecular structure to express them. All chemical compounds can be divided into polar and nonpolar (see Molecule, Dipoles). Examples of completely nonpolar compounds are various hydrocarbons. Polar compounds are generally more soluble in water (which is itself polar) than in such organic nonpolar solvents as benzene, ether, chloroform. Not only the molecule as a whole, but even its individual radical can represent a "polar group," characterized by a more or less uneven distribution of electrical charges between the atoms constituting it. In organic compounds, the most widely distributed polar groups are the carboxyl (-COOH), hydroxyl (-OH), and amino group (-NH₂); the first is distinguished by a particularly large polarity. Since polar groups have an affinity for water, with an increase in the number of polar groups in the molecule, solubility in water increases and solubility in non-aqueous solvents decreases. Turning now to the rules of permeability, it must first be said that typical nonpolar compounds, like hydrocarbons, penetrate the cell very quickly. Overton showed this for a wide variety of hydrocarbons—saturated, unsaturated, cyclic, such as methane, pentane, acetylene, benzene, etc. Conditions change somewhat when any polar group, such as -COOH, -OH, -NH₂, is introduced into the molecule. The ability to penetrate the cell depends mainly on the ratio of the polar and nonpolar parts of the molecule: it increases with an increase in the latter. Indeed, a comparison of monohydric alcohols, fatty acids, etc., shows that in homologous series, permeability increases as one passes to higher members of the series, as the carbon chain lengthens, despite the fact that the overall dimensions of the molecule increase at the same time. Conversely, an increase in the number of polar groups in the molecule decreases permeability. In general, if a molecule contains only one polar group, and the rest of it has the character of a hydrocarbon, the properties of the latter predominate, and the molecule easily penetrates the cell (as, for example, monohydric alcohols, corresponding aldehydes, ketones, etc.). With a larger number of polar groups, the ratios change. A good illustration of such relationships can be served by various alcohols. Monohydric alcohols penetrate the cell extremely quickly, dihydric ones somewhat slower; this is easily verified by comparing, for example, the passage of ethyl alcohol (C₂H₅OH) and ethylene glycol [C₂H₄(OH)₂]. The trihydric alcohol glycerin [C₃H₅(OH)₃] penetrates much more slowly, which can even temporarily cause plasmolysis, gradually disappearing as it penetrates. The penetration of the tetrahydric alcohol erythritol [C₄H₆(OH)₄] proceeds even more slowly, although it can still be proved by deplasmolysis. With respect to the hexahydric alcohol mannitol [C₆H₈(OH)₆], this method turns out to be already unproductive. The cell wall is equally impermeable to sugars: the introduction of an aldehyde or ketone group into the alcohol molecule does not make it more capable of penetrating the cell. Similar relationships are observed in organic acids. A comparison of fatty acids with the corresponding hydroxy acids shows that in this case too, the introduction of a hydroxyl group hinders penetration. An increase in the number of carboxyl groups has an even more unfavorable effect: permeability for dibasic acids is significantly lower than for the corresponding monobasic ones. The addition of amino groups acts in a similar way. For amino acids, as well as for sugars, Overton was not able to notice any penetration of them through the cell wall. Such complete impermeability of the cell to hexoses and amino acids, which undoubtedly must penetrate into it as basic nutrients, is biologically completely incomprehensible. At present, it is still difficult to say what explains this contradiction—the insufficient sensitivity of the applied methodology or the fact that under physiological conditions the cell exhibits a different permeability in relation to these substances necessary for it than under experimental conditions.

While the introduction of a polar group decreases permeability, the substitution of hydrogen by some hydrocarbon group (-CH₃, -C₂H₅, -C₆H₅, etc.) increases it. This effect is manifested especially sharply if hydrogen entering into the composition of some polar group is subjected to such substitution (by methyl, ethyl, phenol, etc.). In this regard, a comparison of glycerin or urea with their mono-, di-, and triethyl derivatives is particularly revealing. The rate of penetration rapidly increases with an increase in the number of substituting ethyl groups: while it is very small for urea itself, triethylurea penetrates the cell almost instantaneously. Thus, summarizing the observations on various organic substances, it can be said that they fully confirm the proposition stated above: a nonpolar (or homeopolar) structure of the molecule favors penetration into the cell, while a heteropolar structure hinders it. Polarity is most sharply expressed in strong electrolytes, which include mineral salts, strong inorganic acids, and alkalis. Therefore, it can be expected that they are also incapable of rapidly passing into a living cell. Cell permeability for acids and alkalis was studied using natural indicators or indicators artificially introduced into the cell. Among various acids, weak acids such as salicylic, valeric, and benzoic acids possess the greatest ability to penetrate the cell and acidify its contents. To correctly imagine how much faster weak acids penetrate the cell compared to stronger acids, one must take into account that even to obtain the same acidification, the former would have to reach a significantly higher concentration in the cell. Even easier than the mentioned acids, almost instantaneously, such weak and at the same time volatile acids as carbon dioxide and hydrogen sulfide penetrate the cell. Conversely, strong mineral acids (hydrochloric, nitric, sulfuric)—in other words, free hydrogen ions—pass extremely poorly into the living cell. If many authors obtained relatively high permeability values for them, this depended, apparently, on "secondary penetration" into a previously damaged cell. From this, of course, it does not follow that the cell is completely impermeable to hydrogen ions, but at that low concentration of strong acids that is still harmless to the cell, this penetration cannot be detected. Obviously, in the case of organic acids as well, it is not free hydrogen ions that penetrate the cell, but only neutral molecules of the acid, which then undergo dissociation inside the cell. Similar relationships have been established for alkalis. Strong caustic alkalis penetrate the cell only after they destroy or damage its surface, whereas for weak organic bases a very significant permeability is observed. It is especially great for ammonia, which in this respect presents a complete analogy to the exceptional position that carbon dioxide occupies among acids. Thus, living cells are very little permeable to strong, practically completely dissociated acids and alkalis, and pass many weak, poorly dissociated acids and alkalis much more easily. In other words, they are impermeable or very little permeable to free hydrogen and hydroxyl ions, but easily pass the undissociated molecules of many acids and alkalis. Hydrogen and hydroxyl ions, incapable of directly penetrating the cell in a free state, enter it in a masked form, in combination with a corresponding acid anion or with a cation of some base. Therefore, there is no direct correspondence between the concentration of hydrogen and hydroxyl ions in the external solution and the pH of the cell contents. Intracellular reaction is determined to a significantly greater extent by the permeability conditions for acidic and alkaline substances present in the external solution than by its pH. A less acidic solution can sometimes exert a stronger acidifying effect on the cell; the same applies to the alkalinizing effect of alkaline solutions. In the limiting case, as Jacobs showed, such a paradoxical effect as the acidification of a cell by an alkaline solution and alkalinization by an acidic one is even possible. Thus, for example, in a mixture of soda and carbon dioxide having an alkaline reaction—at a sufficient concentration of carbon dioxide, due to its free penetration into the cell, acidification of the latter occurs. Similarly, the cell contents can be alkalinized by a mixture of ammonia and ammonium chloride, which has a slightly acidic reaction due to the hydrolytic dissociation of the latter. As a consequence of the described influence of permeability, the physiological action of acids and alkalis cannot be unambiguously determined by their reaction and therefore often completely does not correspond to their chemical activity. With regard to permeability, hydrogen and hydroxyl ions do not constitute an exception; access to the cell for other ions is also extremely difficult. This relates in particular to anions of organic acids. The cell membrane is just as little permeable to them as it is to hydrogen ions. This is easy to verify by comparing the penetration of organic acids and their salts. In contrast to weak organic acids, which are very little dissociated, the salts that they form with alkali metals are dissociated almost completely. In the first case, neutral molecules of the acid are formed, in the second—free anions; the former pass easily into the cell, the latter—do not. This explains the characteristic influence that the pH of the solution exerts on the penetration of weak acids into the cell, for example, acid dyes; it is suppressed by an alkaline reaction and sharply accelerated upon acidification. In an alkaline medium, an acid dye gives a strongly dissociated and therefore poorly penetrating salt; in an acidic solution, it forms undissociated molecules, for which the cell is sufficiently permeable. Similar relationships take place for alkalis. They explain the dependence of the toxicity of alkaloid bases on the reaction of the solution, described as early as Overton. In his experiments, fish and tadpoles survived for some time in a 0.1% solution of strychnine nitrate, but quickly died when a small amount of soda was added to this solution. The fact is that free alkaloid bases (strychnine, cocaine, caffeine, etc.) are negligibly little dissociated and, in the form of molecules of undissociated alkali, easily penetrate the cell. Conversely, the cell membrane is impermeable to the alkaloid cation. Therefore, salts of alkaloids with strong acids, strongly dissociated electrolytically and containing a large amount of free alkaloid cations, are distinguished by very low toxicity. The latter depends exclusively on the hydrolytic dissociation occurring in an aqueous solution, due to which a small amount of undissociated molecules of the free alkaloid base is formed in the solution. Their content increases with an increase in the alkalinity of the solution, and along with this, the toxicity of the latter also increases. Conversely, acidification suppresses hydrolytic dissociation and correspondingly neutralizes the solution of the alkaloid salt. Turning from acids and alkalis to the ions of neutral salts, it is necessary to state an extremely small permeability for them as well. Overton actually believed that mineral salts do not penetrate the cell at all. More careful observations by Osterhout showed, however, that pure solutions of alkali metal salts (NaCl, KCl) penetrate the cell with a noticeable speed. True, even in these experiments, as will be shown below, the transfer of a cell from a normal salt medium into a pure solution of a single salt preliminarily created an abnormal increase in permeability in the cell, which is of decisive significance for the outcome of the experiment. It is therefore more correct to experiment on cells located in their normal environment, to which the studied ion has been added in the smallest possible "concentration harmless to the cell." For this purpose, for example, salts of cesium and rubidium are used—ions close in their chemical nature to potassium and permitting at the same time sensitive spectroscopic determination. For the same purpose, the penetration of bromine or other monovalent ions and their exchange for chlorine ions contained in the cell are studied. When investigating ionic permeability, great care is required, since permeability increases sharply with any (even reversible) damage to the cell. However, even a completely undamaged, normal cell exhibits some permeability for ions—for various ions (and in different cells) it is very dissimilar. An example is the marine alga Valonia, which contains a sufficient amount of cell sap for chemical analysis. Its surface is impermeable to the alkali and alkaline-earth cations contained in it (Na, K, Ca, Mg), the content of which in the cell does not depend at all on their concentration in seawater. It is just as impermeable to the sulfate ion. However, other anions (having a smaller ionic radius) freely penetrate through the surface of the protoplast. Thus, in the presence of bromine ions in the external solution, they penetrate inside, displacing a corresponding amount of chlorine, so that the total amount of halogens in the cell sap remains unchanged. Similar phenomena of selective permeability for anions are exhibited by erythrocytes.

It has long been known that when carbon dioxide is passed through blood, its titratable alkalinity increases, while the chlorine content decreases at the same time (whereas the distribution of Na and K remains unchanged). Koeppe pointed out that this is explained by the permeability of erythrocytes to both the bicarbonate ion and the chloride ion. In erythrocytes, a larger amount of alkaline cations than in plasma is held by protein anions. Displacing the latter, carbon dioxide forms bicarbonates with the alkaline cations. Thus, an excess of bicarbonate ions (HCO₃') is produced in erythrocytes, part of which passes into the blood plasma, displacing an equivalent amount of Cl^- ions from it in return. The exchange character of the described redistribution of anions is best illustrated by the circumstance that erythrocytes suspended in an isotonic solution of a nonelectrolyte do not release alkali into the solution at all when carbon dioxide is passed through. On the contrary, after passing CO₂ through a suspension of erythrocytes in an isotonic solution of Na₂SO₄ or NaNO₃, the appearance of titratable alkali (HCO₃' ion) is observed in the solution, while an equivalent amount of sulfate or nitrate ions penetrates into the cell. However, it is not even necessary to pass carbon dioxide to be convinced of the selective permeability of erythrocytes for anions. As Rohonyi showed, in an isotonic solution of calcium nitrite, NO₂' ions penetrate into erythrocytes, displacing Cl^- ions from them, which pass into the external solution and, by such washing, can be almost completely extracted from the cells; the calcium concentration in the solution remains unchanged in this process. Thus, while retaining cations, the erythrocyte membrane at the same time freely passes various crystalloid anions. However, the relationships found in erythrocytes cannot be generalized: in other cases, inverse relationships are observed—the cell membrane turns out to be less permeable to anions than to cations. Selective ionic permeability is most frequently manifested in the fact that some cells are permeable to a greater or lesser degree only to certain cations, others only to some anions. In conclusion, it is necessary to consider the penetration of water itself, which serves as a natural solvent for all substances studied. Osmotic phenomena and the ability of the cell to change its volume in solutions of altered osmotic pressure provide clear proof of their permeability to water. However, the rate of penetration or exit of water for different cells is very nonuniform; it is far from always as great as is commonly thought. During the plasmolysis of plant cells, equilibrium is often achieved only after an hour or even later; in many animal egg cells, the exchange of water occurs even much more slowly. Taking into account the small size of the cell, such a rate must be considered very moderate. Thus, water does not occupy any exceptional place in terms of the rate of penetration into the cell. Permeability to water, in many cases very large, turns out to be extremely limited in others. Gases penetrate into the cell with a very high rate. The exceptionally high permeability of the cell to such volatile substances as carbon dioxide, ammonia, and hydrogen sulfide was already noted above. However, there are extremely few measurements allowing one to judge even the relative rate of passage of various gases. Krogh measured the rate of diffusion of several gases through a thin tissue plate. The results obtained by him turn out to be very close to the relative rates of diffusion of the same gases through a layer of water. Carbon dioxide passes with the greatest rate in both cases, oxygen passes significantly more slowly, yet exhibiting a somewhat greater rate than nitrogen. Functional changes in permeability. It has repeatedly been pointed out that any damage to the cell is accompanied by a sharp disruption of its normal semipermeability. The extreme ease of the disruption of permeability is a serious source of errors that distorted the results of many experimental studies in this field, but at the same time, such a close connection between the physiological state of the cell and its permeability makes measurements of the latter a valuable and promising method for studying cellular pathology. Using electrical conductivity as a convenient measure of cell permeability, Osterhout was able to directly monitor changes in a plant cell upon its damage, death, or recovery and to construct quantitative curves of the course of these processes. However, changes in permeability occur not only upon the death of a cell. Under the influence of various experimental treatments, permeability can change reversibly, returning to its initial value upon the completion of the experiment. Among such influences, changes in the ionic composition of the surrounding solution must be placed in the first place. As is known, transferring a cell from its natural environment containing an equilibrated mixture of salts to a pure solution of a single salt causes cell damage and, consequently, an irreversible increase in its permeability (see Ion antagonism). However, dissimilar salts exert different effects in this case. Alkali metal salts increase permeability from the very beginning. For some time, this increase in permeability is reversible: the cell, transferred back into the equilibrated solution, remains alive and restores normal semipermeability. A longer stay in a pure solution of NaCl or another similar salt causes cell death, accompanied by a further increase in permeability. On the contrary, calcium and other alkaline earth cations initially decrease permeability. It increases again, and already irreversibly, only as a result of cell damage upon their longer action. If salts of alkali and alkaline earth cations are in a mixture, forming an equilibrated solution, small changes in their relative concentration, insufficient to damage the cell, can exert a significant effect on its permeability, "tuning" it to a higher or lower level. In a usual equilibrated mixture, an increase in the content of calcium ions (or other alkaline earth cations) decreases permeability, while the predominance of alkali cations increases it. These observations were made on ionic permeability. Electrolytes affect the permeability of the cell to water in a similar way: calcium and magnesium salts decrease it, whereas sodium and potassium salts act in the opposite direction. The phenomenon of so-called salt glucosuria is probably explained by the described effect of salts on cell permeability. It often appears after the infusion of a physiological NaCl solution as a result of the loosening of renal cells caused by the latter, which acquire increased permeability to the glucose contained in the blood. The addition of a small amount of calcium chloride to the sodium salt solution, as might be expected, quickly eliminates glucosuria. Cell permeability changes not only depending on the ionic composition of the surrounding solution. Of particularly great interest are functional changes in permeability closely associated with physiological processes occurring in the cell, in particular with excitation processes. Lillie studied larvae of the marine polychaete worm Arenicola containing in their cells a soluble yellow pigment to which the cell membrane is impermeable at rest. A pure NaCl solution isotonic with seawater causes a general tonic contraction accompanied by such a significant increase in permeability that the pigment freely exits outward. The addition of calcium or magnesium prevents both muscle contraction and the increase in permeability, which thus prove to be closely related to each other. On the basis of his experiments, Lillie first expressed the idea that upon cell excitation, its permeability to dissolved substances increases. True, such an interpretation of these experiments seems controversial in many respects. In particular, the observed changes in permeability may depend on the direct action of salts rather than on the excitation phenomena caused by them. But in other cases, such an explanation is excluded. A reversible increase in permeability was discovered upon electrical and even upon normal nervous stimulation of various tissues—muscles, glands, skin. Other physicochemical changes observed in the excited tissue, in particular the electrical phenomena occurring in it (see Animal electricity), are associated with this increase in permeability. It is characteristic that electrical phenomena on the cell surface turn out to be similar upon excitation and upon damage. In both cases, they depend on a sharp increase in cell permeability, which loses its selective ionic character to a significant degree. The only difference is that in one case (upon excitation) this increase in permeability is reversible, while in the second (upon damage) it is irreversible. Fertilization, which excites the developmental processes of the egg cell, should also be attributed to irritation phenomena in the broad sense. Accordingly, a number of observations indicate an increase in the ionic permeability of the fertilized egg compared to the unfertilized one. Similar phenomena are observed in plant cells, and one of the most important agents affecting their permeability is light. According to Lepeshkin and Tröndle, illumination in plants usually increases the permeability of cell walls.

Ultraviolet rays prove to be most effective in this regard. However, in some cases, the very change in the brightness of illumination (even a transition from light to darkness) can serve as a stimulus increasing permeability. It also increases under other types of stimulation. According to Pfeffer, changes in cellular permeability condition the turgor movements of sensitive plants, for example, the mimosa. The cells of the joints of sensitive plants possess, similar to the contractile elements of the animal organism, the ability to temporarily increase their permeability in response to external stimuli (primarily mechanical). The increase in permeability caused by mechanical stimulation leads to the exit of a portion of the dissolved substances from the cell and, as a consequence, to a drop in cellular turgor. In the so-called joints of sensitive plants, which serve as their motor organs, the cellulosic cell walls, which usually harden in adult cells, retain their elasticity. Therefore, when the turgor drops due to external stimulation, the cell contracts, and the plant organs, no longer held by the elastic resistance of the joint, drop downward. As soon as the joint cells regain their normal semipermeability, the osmotically active products formed within them in the process of metabolism once again increase turgor, and the plant parts straighten out, becoming once again susceptible to subsequent stimuli. Thus, apparently, a reversible increase in permeability invariably occurs upon the excitation of any cell, both animal and plant. In the course of cellular processes, permeability does not remain strictly constant, but undergoes greater or lesser significant functional changes. If there is a general connection between excitation and an increase in permeability, then on the other hand, it is natural to expect that narcotic agents, which make the living cell temporarily unresponsive to stimulation, produce the opposite effect, causing a temporary decrease in permeability. This has indeed been established by a number of researchers. Thus, for example, Lillie found that various narcotizing substances (alcohol, ether, chloroform), while paralyzing the excitatory action of pure sodium chloride solutions on Arenicola larvae, simultaneously decrease their permeability (stopping the outflow of pigment under the influence of NaCl). Osterhout studied this influence particularly thoroughly and precisely on the thalli of the alga Laminaria, and later on frog skin. Their electrical conductivity decreases in a 1% ether solution, and this change—as is characteristic of narcosis—is completely reversible; permeability returns to normal after the alga is transferred to clean sea water. In a 3% ether solution, the alga soon dies: the temporary decrease in ionic permeability, the reversible phase of narcosis, is followed by a sharp and irreversible increase as a result of cell death. Similarly, according to Winterstein's data, narcosis retards the passage of salts through a layer of frog muscle cells. Both these and many other observations show that narcosis lowers the permeability of the cell membrane. In establishing such a general proposition, it is necessary to guard against one gross error which has unfortunately become widespread in studies of functional changes in permeability. The fact is that cell permeability for different substances does not change in parallel: while increasing for some substances, it may remain unchanged (and in some cases even decrease) in relation to others. Thus, for example, cell permeability for water and salts changes incomparably more strongly than permeability for basic dyes that vitally stain the cell, or for narcotics. Therefore, results obtained on any single substance cannot always be generalized and do not yet give the right to speak of a corresponding change in permeability for all soluble substances in general. Theories of cell permeability. The enormous number of observations, partially examined here, compels us to attribute properties to the protoplasm surface different from those of the cell contents—the properties of a special semipermeable membrane. Numerous attempts to explain otherwise the characteristic features of the penetration of dissolved substances into the cell and their distribution between the cell and the external solution have proved fruitless. To clarify the nature of the semipermeable cell membrane, it was first of all necessary to establish by what physicochemical properties substances that pass rapidly through the cell membrane differ from those that are completely devoid of this ability or possess it only to a limited extent. Overton, who was the first to systematically investigate cell permeability, drew attention to the fact that substances soluble in fats or fat-like, "lipoid" substances freely penetrate the cell. Cells behave as if they were surrounded by a thin fat-like, lipoid membrane, dissolution in which is a necessary condition for penetration inside. The lipoid membrane theory developed by Overton remained the prevailing theory of cell permeability for a long time. Given the presence of a continuous fat film on the surface of the protoplasm, a substance can penetrate inside only after dissolving in it. The only question is to what extent cell permeability for various substances actually corresponds to their relative solubility in fats. The latter can be expressed by the so-called partition coefficient—the ratio of their concentration in a fatty solvent and in an aqueous solution in contact with it. Using olive oil as a fatty solvent, Overton found for many substances a complete parallelism between the rate of their penetration into the cell and the oil/water partition coefficient. Most of the rules established above for cell permeability are fully applicable to the latter; the more strongly polar the compound is, the lower its solubility in fats in general. However, a detailed study of a large number of various substances—primarily vital dyes—revealed a whole series of exceptions and contradictions. Thus, for example, methylene blue and many other dyes penetrate the cell and vitally stain it without being soluble in olive oil and other neutral fats. Such observations led Overton to suggest that the cell membrane contains not true neutral fats, but compounds close to them in their physical properties (solubility in alcohol, ether, benzene, and other organic liquids) which received the general name of lipoids. Lipoids, the main representatives of which are lecithin and cholesterol, are contained in every cell. Individual differences in the permeability of different cells can be explained by the unequal composition of lipoids taking part in the construction of the cell membrane, in particular by the unequal ratio of lecithin and cholesterol in it. An exact study of the distribution of dissolved substances between lipoids and water presents significant difficulties. It cannot be studied directly by shaking both solvents together (in one of which the given substance has been previously dissolved), as is done in the case of water and liquid vegetable oil. Cholesterol is a solid body; lecithin swells upon contact with water, changes its physical properties, etc. Therefore, they have to be preliminarily dissolved in chloroform or some other organic liquid that is as indifferent as possible, i.e., not chemically binding and not dissolving the substance under investigation; then the distribution of the dissolved substance between water and such a chloroform-lipoid phase is investigated. The absorption by such a mixture of substances insoluble in pure chloroform is attributed to the lipoid contained in it. However, later studies by Loewe revealed the incorrectness and unsuitability of such a methodology: a colloidal solution of lipoid in chloroform can absorb substances insoluble either in the lipoid or in chloroform, but adsorbed on the surface of the lipoid particles. By citing the fact that the protoplasm surface contains not neutral fats, but lipoids, Overton eliminated the discrepancy observed between the rate of penetration of various substances into the cell and the magnitude of their oil/water partition coefficient. However, along with considerable flexibility, the theory in this form acquired a high degree of uncertainty, hindering its precise experimental verification. Nirenstein attempted to introduce some modification into it. Using the paramecium as an object, he, like Overton, found that in many cases the penetration of dyes into a living cell does not correspond at all to their absorption by pure olive oil. But it was sufficient to add a little oleic acid to the latter to achieve complete parallelism between the partition coefficient and the ability to stain the paramecium for all investigated basic dyes. A similar result for acidic dyes could be achieved by adding an oil-soluble organic base—diamylamine. The mixture of olive oil, oleic acid, and diamylamine in its relation to various dyes represents an exact likeness of the paramecium. However, this model does not have universal significance and cannot provide a general scheme of permeability.

However, if the idea of the dissimilar lipoid composition of the membranes of different cells makes it possible to explain the individual differences observed between them, the lipoid theory faces incomparably more serious difficulties which cannot be overcome in this way. Completely inexplicable remains, first of all, the easy penetration of certain substances that are not at all soluble in lipoids. This includes water itself, which passes more or less quickly into any living cell. True, lecithin swollen in water becomes permeable to water, but at the same time it loses the lipoid semi-permeability characteristic of it in the dry state. Equally incomprehensible is the penetration into cells of other substances insoluble in lipoids. Furthermore, Overton's theory completely fails to explain the reversible functional changes in permeability that accompany very many cellular processes. Obviously, it cannot lay claim to being an exhaustive theory of cellular permeability and requires a number of corrections and additions. As one of such corrections, mention should be made of the emulsion theory of Clowes, which enjoys great popularity in America. According to Clowes, lipoids are not located in the cell membrane in a continuous layer, but form an emulsion. This emulsion is in an unstable state, making the easy reversal of its phases possible; from an emulsion of lipoid in water, under the influence of various influences, it can transform into an emulsion of the reverse type, water in oil. The emulsion film is permeable exclusively to substances soluble in its continuous phase; an emulsion of water in lipoid passes only substances soluble in lipoids, whereas the reverse emulsion (lipoid in water) is permeable to water and substances dissolved in it. Under the influence of external conditions (ionic composition of the solution, narcotics, etc.), the type of lipoid emulsion changes, and along with it the nature of cellular permeability. This very visual concept, upon closer examination, turns out to be untenable, however. The cell membrane has not an emulsion structure, but a significantly more finely dispersed, probably colloidal structure. The possibility of the reversal of its phases is in no way proven, and the similarity of its behavior to an emulsion represents only an external analogy. The actual functional changes in permeability are far from being as profound as one would expect in the case of an actual phase reversal. Another modification of the lipoid theory was put forward by Nathansohn, who suggested that the cell membrane has a mosaic structure formed by a combination of lipoid and protein particles. The protein sections must pass water and certain substances dissolved in it, whereas the lipoid inclusions cause typical lipoid permeability. This concept, extremely schematic in its initial form, concerning the presence of two pathways for the penetration of dissolved substances into the cell, subsequently received very clear and concrete development. The lipoid theory of permeability proceeds from the idea that the condition for penetration through the cell membrane is dissolution in the substance itself of which it is built. The whole problem reduces to determining the chemical nature of the membrane, which serves as a solvent for the passing substances. A completely different path is opened by the concept of heterogeneous membranes that pass dissolved substances depending on their finely porous structure, of membranes acting, in the figurative expression of M. Traube, as a "molecular sieve." Such a concept for a long time seemed completely inapplicable to cellular permeability. A sufficient refutation of it was considered to be the circumstance that the possibility of penetration into the cell is not only not limited by certain molecular sizes, but in homologous series penetration is even accelerated with the elongation of the carbon chain. However, the picture changes dramatically if one turns specifically to substances insoluble in lipoids. Substances having sufficiently small molecules pass freely into the cell, completely independently of whether they are soluble in lipoids or not. This explains the unhindered penetration into any cell of water, gases and volatile substances (NH₃, CO₂), as well as any dissolved substances with sufficiently small molecules. With an increase in molecule size beyond a known limit, access to the cell turns out to be closed for all substances except those for which significant lipoid solubility (associated with a sufficiently homeopolar structure) provides another pathway of penetration. The most vivid proof of the finely porous structure of the cell membrane is provided by its selective ionic permeability. As is known, the size of an ion, its "ionic radius," depends on its electrical charge. Due to the latter, hydration occurs in an aqueous solution, and the ion is surrounded by a shell of oriented water molecules attracted by it. Therefore, its true dimensions turn out to be many times larger than those indicated by its chemical formula. As a result of the decay of a neutral molecule into ions, each of them (along with its water shell) usually turns out to be larger than the initial molecule. It is precisely the large size of ions that explains the low permeability of many membranes to them. On the contrary, the selective permeability of the membrane for ions of one sign, as Michaelis showed on collodion membranes, depends on the electrical charge of the walls of its pores. Positively charged membranes are selectively permeable to anions, negatively charged ones to cations. If the pores are small enough, the membrane is completely impermeable to identically charged ions. With some loosening of the colloidal structure and a general increase in permeability, the difference becomes only quantitative: the membrane turns out to be less permeable to identically charged ions than to ions of the opposite sign. The larger the pore size becomes, the more these differences are smoothed out. Thus, it can be considered established that according to the method of their penetration into the cell, all dissolved substances are divided into two large groups. To one of them belong substances whose penetration to one degree or another depends on the magnitude of their molecular volume. As the latter increases, the penetration rate drops rapidly; it becomes equal to zero when the molecular volume reaches a certain value characteristic of each membrane. For charged particles, as already indicated, a complicating circumstance is the action of electrical forces, which make the pores unequally permeable to ions of different sign. Such relationships present indisputable proof that for a given group of substances permeability is determined by the filtration of the dissolved substance through the smallest pores of the membrane. The second group consists of substances soluble in lipoids. Their penetration is not affected in the least by the magnitude of molecular volume. No matter how much the size of the molecules increases, the rate of penetration even increases, provided only that with this increase in the molecule its polarity is weakened and thereby lipoid solubility is enhanced. Such behavior fundamentally contradicts the concept of passage through any pores. It can be explained only by solubility in the lipoids of the cell membrane, and the latter may have an unequal composition in different cells, differing in the ratio of various lipoid fractions (in particular lecithin and cholesterol). The collapse of the lipoid theory was caused by the fact that too much was demanded of it—a complete explanation of all phenomena of permeability. For the given vast group of substances, however, it cannot be replaced by any other concept and fully retains its significance. Thus, the participation of lipoids in the construction of the cell membrane appears unquestionable. It also follows from the Gibbs principle, with which it would be difficult to reconcile the absence of lipoids on the surface of the protoplasm given their constant presence in the cell contents. However, the now firmly established ability of many substances to penetrate by filtration through the smallest pores of the membrane proves that lipoids do not form a continuous phase in the membrane, but are in a dispersed state. This disperse system can by no means be considered a lipoid emulsion. Only in the colloidal state, in a gel, with the closest approach of the colloidal micelles, does the pore size turn out to be small enough to give the effect of selective permeability. Like all aqueous phases of the cell, the intermicellar fluid located between the lipoid micelles must contain a large amount of proteins. Through these water-protein pores, in the spaces between the lipoid particles, water itself and sufficiently small molecules of other substances insoluble in lipoids penetrate into the cell. Ions penetrate by the same path, and the electrical charge of the protein pore walls causes selective permeability for ions of the opposite sign. Thus, for dissolved substances, there are two different pathways of penetration into the cell: micellar and intermicellar, dissolution in the lipoid micelles of the membrane or filtration (complicated by electrical interactions) between them.

This duality of the principles of penetration corresponds to a profound difference in the response to various influences. Cell permeability for substances soluble in lipoids is characterized by great constancy. Conversely, permeability for other substances (in particular for ions) undergoes significant fluctuations as a result of both external influences and internal functional changes. This difference is entirely understandable: the solubility of some substances depends very little on the presence of others, whereas the most diverse conditions affect filtration. Thus, a change in the pH of a solution can cause a recharging of the protein walls of the pores and thereby a sharp change in the character of ionic permeability. Much more frequently, changes occur in the size of the protein pores, and any loosening of the membrane, any swelling of its protein phase must lead to a general increase in filtration permeability. Consequently, cell permeability for substances insoluble in lipoids depends on all agents that alter the swelling of the protein gel. Thus, for example, it is lowered by calcium and other polyvalent cations that suppress the swelling of negatively charged colloids. In this case, salts can alter the degree of permeability as evenly and gradually as they alter the degree of swelling. But they never cause that profound change in the character of permeability that would correspond to Claus's hypothesized "phase reversal." The most diverse vital processes are associated with changes in the state of cellular colloids. Hence the functional changes in permeability. As a general rule, excitation is accompanied by a loosening of the protein phase of the membrane and an increase in its permeability; narcosis exerts the opposite effect. Just as with the action of electrolytes, only the permeability for substances insoluble in lipoids undergoes functional changes, first of all ionic permeability. Permeability of tissues. In the bodies of multicellular animals, many tissues play the role of membranes regulating through their permeability the intake of dissolved substances and their exchange in the organism. Such are the epithelial tissues of the outer integuments, the endothelium of blood vessels, the intestinal wall, and others. At first glance, it would appear that the permeability of tissue membranes should not differ in any way from the permeability of the cells composing them. However, in reality, there are very substantial differences between them. These depend primarily on the intercellular substances connecting the cells to each other and cementing them into a continuous tissue. Dissolved substances, in their passage through a tissue membrane, obtain the ability to penetrate through these colloidal intercellular junctions, bypassing the protoplasm itself with its semipermeable cell membrane. The tissue membrane then acquires the properties of a colloidal filter of greater or lesser density, impermeable only to colloidal substances. Such membranes are widespread in the organism and play a major role in its water metabolism. To them belongs, for example, the endothelial wall of capillaries. Passing through it, the blood is freed from the greater part of the colloidal substances contained in it and yields lymph, which thus represents an ultrafiltrate of blood. Along with changes in the colloid-osmotic pressure of the blood and with the production of osmotically active products as a result of organ activity, changes in the permeability of the endothelial ultrafiltrate represent one of the essential factors influencing lymph formation (see). The porosity of the blood vessel endothelium undergoes particularly significant changes under pathological conditions. The permeability of vascular walls increases sharply during inflammation (see). This increase in permeability, as in the case of artificial ultrafiltrates, can be determined by the size of the colloidal particles passed by the membrane. The latter increases in the sequence: albumin, globulins, fibrinogen. Only albumins can pass through normal endothelium. Inflammatory exudates usually contain, besides albumin, significant amounts of globulins, and in many cases also fibrinogen. The passage of the latter indicates a strong loosening of the vascular ultrafiltrate and a significant increase in its pores. Calcium ions can compact the endothelial ultrafiltrate, just as they compact many other colloidal membranes. According to the observations of Chiari and Januschke, the abundant introduction of calcium prevents the formation of exudates caused by poisoning with iodine compounds and other substances; the bond between the endothelial cells, loosened by them, is cemented by calcium. In a similar manner, through ultrafiltration, other tissue fluids are formed from the blood, for example, cerebrospinal fluid. The tissue filter serves in this case as well as a barrier regulating through its permeability the penetration of dissolved substances into the cerebrospinal fluid. Apparently, even during urine formation, the first stage is the filtration of blood through the renal glomeruli, and only then does the resulting primary ultrafiltrate undergo more profound changes in its chemical composition (see Diuresis). In all the examples considered, the diffusing substance moves along the line of least resistance through the intermediate intercellular junctions. As a consequence of this, the role of the cells is obscured, and the tissue approaches in its behavior non-living colloidal membranes. In other cases, however, tissue membranes exhibit very characteristic activity associated with their vital activity, and the passage of dissolved substances through cell layers acquires interesting features compared to their penetration inside the cell. Such a characteristic feature, observed during the penetration of dissolved substances through certain tissue membranes, is their one-way permeability. A necessary condition for diffusion or osmosis through a conventional semipermeable membrane is a difference in the composition or concentration of the solutions separated by it. The movement of substances ceases when identical solutions are present on both sides of the membrane. However, in a living organism, the movement of fluid through cell layers is frequently one-way. Especially in phenomena of resorption and secretion, the movement of fluid can occur not in the direction of the osmotic gradient. It is well known, for example, that even a hypertonic solution present in the intestinal lumen is absorbed by its wall. If one cuts a living cell wall and stretches it as a membrane across a vessel containing Ringer's solution, the equilibrium on both sides of the membrane will be disturbed. The solution will move through the intestinal wall in a definite direction (from its inner, mucosal surface to the outer, serous one) as if the membrane were permeable to it only in this direction. Such one-way permeability is preserved only in living tissue; it disappears upon death and also during anesthesia. One-way permeability to water and dissolved substances was studied in detail by Wertheimer. As a membrane, he used the skin of a frog, easily separated from its hind limbs. The upper part of it, corresponding to the thighs, tied off at the level of the knee joint, formed two skin bags. One of them preserved its normal position, the other was turned inside out (pigmented side outward). Wertheimer's experiments showed that water passes from the outside inward faster than in the opposite direction (the skin surface is called outer or inner here according to its normal position on the animal's body). Various dissolved substances behave quite differently. Peptones, polypeptides, and amino acids easily penetrate inward, but do not pass from the inside outward. Sugars pass more easily in the opposite direction (from inside outward), but their behavior can change under the influence of other substances present in the solution. Similar differences are exhibited by various dyes. Some of them pass predominantly in one direction, others in the opposite direction. Thus, for example, with respect to methylene blue, the skin possesses one-way permeability from the inside outward, whereas eosin, Bismarck brown, or Bordeaux pass more easily from the outside inward. Particularly graphic results are given by experiments on a mixture of two dyes belonging to both of these groups (e.g., methylene blue and eosin). Using the selective permeability of the skin for one dye in one direction and for the other in the opposite direction, they can be partially separated, isolating each of them in a more or less pure form. The direction of this selective permeability does not remain absolutely constant. It can be altered with the help of certain influences, of which the most important is a change in the pH of the solution, as well as the concentration of other ions. Thus, methylene blue behaves in the manner described above in a neutral and alkaline medium, but in an acidic reaction, it moves in the opposite direction. The reaction similarly influences other dissolved substances. Thus, in many cases, changes in pH can alter the direction of one-way permeability. The phenomenon of one-way permeability is of very great importance for understanding the processes of secretion and resorption occurring in the organism. However, the theory of this phenomenon is still very little developed. As a general proposition, one can only state that a necessary condition for one-way permeability is asymmetry in the structure of the membrane itself.

Such asymmetry may, for example, be expressed in the opposite electrical charge of both membrane surfaces. The passage of dissolved substances through the membrane then has the character of electrical transfer. Indeed, according to Keller, positively charged dyes penetrate the frog's skin predominantly in the direction from inside to outside, whereas a negative charge of the dye favors penetration in the reverse direction. At present, it is still difficult to say to what extent such an explanation can have general significance. Of course, for the prolonged maintenance of such differences between both membrane surfaces, a continuous expenditure of energy is required, at the expense of which the work of transferring water or dissolved substances against the direction of the concentration gradient is accomplished. Its source is the chemical processes occurring in the living cell. Therefore, when cell vitality is suppressed, unidirectional permeability disappears. The most diverse processes of movement and distribution of water and dissolved substances in the organism, their penetration into the cell, and their physiological action ultimately depend on cellular permeability, the study of which thus represents one of the most important problems of biology. D. Rubinstein. Medical significance of permeability. A whole range of questions of both theoretical and practical medicine is closely connected with the problem of permeability. The following three main groups of questions deserve to be noted: a) the explanation of the pathogenesis of a number of pathological processes using concepts of altered permeability, b) attempts to influence the course of pathological processes by means of artificial changes in permeability, c) taking into account the features of permeability of various parts of the organism in their normal and pathological state when introducing medicinal substances into the organism. In this case, the medical significance lies not so much in the permeability of individual cells, but rather of cell complexes, the so-called biological membranes (walls) separating from each other parts of the organism that differ in their physiological and physicochemical features. Among walls of this kind, the vascular wall should be put in first place in terms of its significance, changes in the permeability of which are observed in a number of pathological processes. a) In venous stasis, an increase in the permeability of vascular walls (predominantly capillaries, as well as small veins) is found, which owes its origin, apart from mechanical stretching of the wall, also to the disruption of gas exchange and nutrition of endothelial cells. The consequence of this increase in permeability is edema and possibly diapedesis of erythrocytes (see Edema, Diapedesis). The connection between the increase in vascular wall permeability and edema is expressed in the fact that the damaged vascular wall becomes permeable to blood proteins, in connection with which there is a decrease in the oncotic pressure of blood and a decrease in the reabsorption of water from tissues into the blood (Schade, Krogh). The connection between the increase in vascular wall permeability and diapedesis has not been clarified more closely. Apparently, the matter boils down to a change in the permeability of the intercellular substance between endothelial cells. There are opinions linking the increase in vascular permeability during venous stasis with a shift of the tissue reaction to the acidic side (see Inflammation, morphology and pathological physiology of inflammation). The increase in the permeability of vascular walls during venous stasis may also have some positive significance in the sense that thanks to the exit into the tissues of such a strongly buffered fluid as blood, the neutralization and dilution of underoxidized products formed in the tissues during venous stasis is achieved (Okunev). b) In inflammation, a very significant increase in vascular walls permeability occurs in exactly the same way. The phenomena of exudation and emigration of leukocytes characteristic of the inflammatory process are closely connected with the aforementioned increase in vascular permeability (see Inflammation). The increase in the permeability of vascular walls leads to the formation of exudate for the same reasons for which it leads to edema: blood proteins pass through the vascular wall, the colloid-osmotic pressure of the latter drops, the reabsorption of water by blood vessels weakens, and the escaped water is retained in the tissues thanks to the oncotic pressure in the tissues increased at the expense of blood proteins (Schade, Habler). The increase in vascular permeability in inflammation is significantly higher than in venous stasis, which, among other things, is evident from the significantly greater exit of protein from the blood into tissues in inflammation than in venous stasis. The increase in vascular wall permeability in inflammation not only for water, but also for colloids is confirmed by numerous experimental data [enhanced absorption from the inflammatory focus of colloidal dyes, their enhanced deposition at the site of inflammation; Goldmann, Kuznetsovsky, Okunev, Malkin]. The circumstance that in the later periods of the inflammatory process the increase in vascular permeability is replaced by a decrease appears to be very important. Associated with this decrease in permeability is the cessation of exudate formation and the return of the colloid-osmotic pressure of the blood to normal figures (Habler). The cause of the initial increase in vascular permeability in inflammation is the toxic effect on the vascular wall of both the substances that caused the inflammation and the products of tissue breakdown, including H-ions. The cause of the decrease in vascular permeability in the later periods of the inflammatory process has not been precisely elucidated. According to some views, there occurs a "clogging" of the vascular wall by proteins passing through it and settling on it (Habler), while others reduce it to processes of multiple thrombus formation in small vessels. The role of changes in vascular wall permeability in inflammation for emigration has not been sufficiently elucidated. There is an opinion that the escape of the leukocyte occurs through a previously damaged ("softened") intercellular substance (Abramson). The significance of changes in vascular permeability in inflammation is very great. The phenomena of absorption from the inflammatory focus are connected with changes in vascular permeability (Okunev), and consequently also the question of general phenomena in inflammation. It should be noted that increased and decreased vascular permeability can be observed simultaneously in one and the same inflammatory focus, with increased permeability being observed in the more peripheral parts of the inflammatory focus, and decreased permeability in the more central ones. c) In addition to venous stasis and inflammation, an increase in the permeability of vascular walls is also observed in cases of so-called toxic and nervous edema. In the first case, the change in permeability is caused by intoxication (edema in anaphylaxis can also be referred here), in the second case, we are talking about insufficiently investigated nervous influences on vascular permeability. The next natural membrane following the vascular wall, the permeability of which has important medical significance, is the wall of the gastrointestinal tract. The immensely important questions of absorption from the stomach and intestine are connected with the permeability of this membrane. Changes in the permeability of the stomach and intestine wall lead to significant disruptions in absorption processes. It has been established that under the influence of harmful agents (toxins, poisonous substances), the unidirectional permeability typical of the gastrointestinal tract wall is disrupted, and the wall begins to behave like a simple dialyzing membrane. At the same time, the normal impermeability of the intestinal wall to colloids is disrupted, and the latter begin to penetrate from the lumen of the intestine into the blood. In this respect, experiments on absorption from the intestine upon damage to its mucosa by colloidal dyes are very indicative (Okunev). The described phenomena of changes in the permeability of the gastrointestinal tract wall upon damage make it possible to explain the absorption of toxic substances from the intestine, which occurs in intestinal diseases. Of huge practical significance is the normal impermeability of the intestinal wall to microbes, which under pathological conditions can be replaced by permeability (for example, in strangulated hernias, intestinal obstruction). An increase in the permeability of the gastrointestinal tract wall can also be caused by surface-active substances, as well as by a state of excitation. Despite their apparent importance, both of these recent effects on the permeability of the gastrointestinal tract wall have been studied very little (Fürth, Lasch). Finally, mention should be made of the higher permeability of the gastrointestinal tract wall in young animals, especially in relation to colloids (Möllendorff). In the third place in terms of its medical significance should be placed the permeability of the blood-brain barrier. As numerous studies show, changes in the permeability of this barrier are observed not only in inflammatory diseases of the meninges, but also in a number of mental and nervous diseases (Walter, Hauptmann). Thus, in progressive paralysis, tabes, senile psychoses, myelitis, and delirium tremens, an increase in the permeability of the blood-brain barrier is observed, while in schizophrenia some authors note a decrease in permeability. The degree of disruption of the blood-brain barrier permeability is stronger the more severe the disease. Depending on the nature of the course of the disease, the permeability of the blood-brain barrier can change. Thus, in acute alcohol psychoses it is increased, in chronic alcoholism it is decreased. Disruptions in the blood-brain barrier permeability in either direction can be considered equally harmful.

The harmful consequences of a decrease in the permeability of the blood-brain barrier are clear from the fact that this obviously also reduces the entry of nutrients from the blood into the cerebrospinal fluid. In the case of an increase in permeability, one must reckon with a disruption of the protective function of the blood-brain barrier, and with an enrichment of the cerebrospinal fluid with protein, which alters its physicochemical properties. In addition to the aforementioned psychiatric and nervous diseases, an increase in the permeability of the blood-brain barrier is also noted during pregnancy and menstruation (hormonal action), at a young age, in uremia, anaphylactic shock, and under the influence of certain toxic substances. The permeability of the blood-brain barrier plays a very important role when attempting to affect the central nervous system by means of medicinal substances introduced through the blood. In order to increase the permeability of the blood-brain barrier and thereby provide the medicinal substance with access to the central nervous system, warming of the animal is used. There is an opinion that the favorable effect of malaria in the treatment of progressive paralysis is based precisely on this kind of increase in the permeability of the blood-brain barrier caused by high temperature. These views on influencing the permeability of the blood-brain barrier are, however, not shared by all authors (Stern). In view of the great practical significance of the permeability of the blood-brain barrier, a number of methods have been developed for its determination. Of no small medical significance is the permeability of the renal filter (glomerular vessels, epithelium of the convoluted tubules). An increase in the permeability of the renal filter (primarily the walls of the glomerular vessels) leads to the excretion of urine with protein. In this regard, great importance is attached to the shift in the reaction of the renal tissue to the acidic side, which facilitates the swelling of protoplasm and thereby promotes the "loosening" of cell complexes. Experimental material is based on observations of an increase in the permeability of the renal filter in acidosis caused by poisoning with CO₂ and uranium (Eppinger, MacNider). The increased permeability of the diseased kidney with respect to colloids is well demonstrated by experiments on its excretion of colloidal dyes in significant quantities, while a healthy kidney allows only traces of these dyes to pass (Seyderhelm and Lampe). Undoubtedly, besides the influence of acidity, it is also necessary to take into account a number of other factors contributing to the increase in the permeability of the renal filter (toxic effects on the glomerular vessels and on the epithelium of the convoluted tubules). A change in the permeability of the renal filter theoretically should lead to a clear disruption of the excretory activity of the kidneys, the accumulation in the body of substances subject to excretion, and so forth. However, the connection between these latter phenomena and the change in renal permeability has not yet been sufficiently clarified. This particularly concerns issues related to a possible decrease in the permeability of the renal filter, the perversion of normal permeability, and so on. The medical significance of skin permeability under normal and pathological conditions stems both from the protective function of the skin (impermeability of normal skin to microbes; see Infection) and from the fact that the integument serves as a site for the introduction of many medicinal substances (see Skin, Absorption). Research in this latter direction has yielded a number of indications that agree well with the so-called lipoid theory of permeability (see above). An increase in skin permeability, apart from mechanical damage, is also caused by ultraviolet rays and potassium ions. Conversely, X-rays and calcium ions lower skin permeability (Gans and Schlossmann, Ebbecke). Skin permeability to ions is well proven by the so-called psycho-galvanic phenomenon of Tarkhanov, where a close connection between the state of excitation and increased permeability to ions is clearly outlined (Tarkhanov, Gildemeister, Schwarz). The medical significance of the permeability of the endothelium of serous cavities is acquired mainly in connection with the possibility of the absorption of toxins from these cavities and the penetration of microbes from there. At the present time, the possibility of the absorption of colloids from serous cavities has been proven (Okunev), but the mechanism of this phenomenon cannot be considered completely elucidated, since it is unknown how the penetration of colloids from serous cavities into the blood occurs, whether by passing through the endothelium of the cavities and vessel walls or through the lymphatic pathways. There are indications in favor of the fact that during inflammations of the serous cavities, an enhanced penetration of colloidal substances into the blood occurs in the initial stage, and a delayed penetration in the later stages (Okunev). Changes in the permeability of the barrier between the blood and the aqueous humor of the eye are observed during inflammatory processes and injuries in the region of the anterior chamber of the eye. Usually, an increase in permeability takes place, leading to the appearance of protein in the fluid of the anterior chamber of the eye. Repeated punctures lead to the same result. Attempts to influence permeability by artificial means are contained in a whole series of therapeutic measures. This includes the use of calcium as an anti-inflammatory agent (lowering the permeability of the vascular wall), the use of astringents (lowering the permeability of mucous membranes), elevated temperature (local arterial hyperemia accompanied by an increase in vascular permeability), irritating substances (the same effect as in the previous case), and the like. It must be borne in mind that in all the enumerated cases, the change in permeability represents only a part of the complex phenomena arising as a result of the use of the named therapeutic measures.