Viscosity

VISCOSITY, or internal friction, is the resistance that manifests itself when some particles of a substance move in relation to others. The concept of "internal friction" is applicable to liquid, solid, and gaseous substances alike, whereas the term viscosity is used predominantly in relation to liquids. In a biological respect, of the greatest significance is precisely the viscosity, or internal friction, of liquid and semi-liquid substances, particularly protoplasm. This significance of viscosity lies in the fact that changes in it substantially affect both the speed of purely chemical reactions proceeding within the system and a number of physical phenomena of paramount importance for vital processes in the cell. Thus, with an increase in viscosity, the speed of diffusion decreases, electrical conductivity drops, and Brownian motion (see) slows down up to a complete stop. Furthermore, the mechanical resistance experienced by a liquid when passing through narrow spaces (capillaries, intercellular spaces) changes sharply. The dependence of this resistance on viscosity is often taken as the basis for measuring viscosity; according to the so-called Poiseuille law, this dependence can be expressed by the following formula: where v is the volume of liquid flowing during time t through a capillary with cross-section q and length l, at constant pressure p and liquid viscosity. While the viscosity of dispersions and suspension colloids hardly differs from the viscosity of the liquid part of the suspension, the presence of emulsion (lyophilic) colloids has a sharp effect on viscosity. This is caused by the general rule according to which viscosity is higher the more strongly the suspended particles are hydrated (more precisely, solvated, i.e., the more closely they are bound to the solvent molecules). Therefore, all factors influencing the degree of hydration of a colloid sharply alter its viscosity. From this follows an important consequence: the possibility of judging the degree of swelling or hydration of a colloid from its viscosity. In colloids, and particularly in the constituent parts of the cell, true viscosity often passes, through completely imperceptible gradations, into elasticity, which is already predominantly a property of a solid body. Therefore, one often speaks of elastic, or structural, viscosity. The difference is most clearly seen in an example: with true viscosity, a particle of iron located in a given medium under the influence of an electromagnet will move slowly and, when the current is turned off, will remain in the very place where it was at the last moment. In the presence of elastic forces, however, such a particle can be displaced from its initial position, and when the electromagnet current is turned off, it will return back again. The viscosity of protoplasm fluctuates within extremely wide limits; thus, for instance, Heilbronn found values of 1-2 for the eggs of certain echinoderms (water = 1), and Fetter found 8.726 for Paramecium. It reaches a maximum at 15°. Under the influence of narcotics, the viscosity of protoplasm decreases. To measure the viscosity of protoplasm, one usually uses the measurement of the falling speed of intracellular inclusions (for example, starch grains), and to increase the effect of gravity, centrifugation is resorted to. A more perfect method is the application of micrurgy (see): a microscopic piece of iron is introduced into the cell and, acting upon it with an electromagnet, the current strength necessary for a certain movement of this particle in the cell protoplasm and in pure water is compared. (For the methodology of measuring the viscosity of ordinary liquids, see Viscosimetry.)

Blood viscosity is of primary importance in the physiology and pathology of blood circulation; it is measured by Hess or Determann viscometers (Hess, Determann; see Viscometry). Blood viscosity fluctuates under physiological conditions around 4-5 (assuming the viscosity of water=1), and under pathological conditions even from 1.72 to 22.89 (according to Hess). Blood viscosity changes mainly depending on the quantity and properties of the formed elements, and to a lesser extent on changes in the properties of the plasma (primarily its colloids; salt concentration is of lesser importance). Blood gases also influence blood viscosity: carbon dioxide significantly increases blood viscosity, causing, on the one hand, swelling of erythrocytes and, consequently, an increase in the surface of internal friction, and on the other hand, causing the diffusion of proteins, sugar, etc., from erythrocytes into the plasma. Oxygen lowers blood viscosity. With an increase in temperature, blood viscosity decreases, and vice versa; therefore, hot baths lower blood viscosity. Bile and its derivatives, alcohol, and ether increase blood viscosity; quinine, phosphoric acid, and potassium iodide lower it. In men, blood viscosity is higher than in women: thus, according to Hess, in men it averages 4.74, in women 4.4; according to Determann, 4.79 and 4.57, respectively. In vegetarians, for example, the Japanese, blood viscosity is lower; likewise, in experiments on dogs, a carbohydrate diet lowers, and a meat diet increases, blood viscosity. During the day, only insignificant fluctuations in blood viscosity occur. Prolonged moderate work lowers, while heavy work increases, blood viscosity. For normal hemodynamic conditions, blood viscosity is of some importance as a resistance that the work of the heart must overcome. In cardiac decompensation, with a deterioration of pulmonary ventilation, the content of CO2 in the blood increases, which significantly increases blood viscosity, and thereby, perhaps, the work of the heart as well. Therefore, some authors have proposed systematically determining blood viscosity in cardiac patients for diagnostic and prognostic purposes. Thus, Martinet draws attention to the proportionality of changes in blood pressure under normal conditions depending on blood viscosity, specifically, with an increase in blood viscosity, the amplitude of blood pressure increases, and vice versa. It is necessary, however, to assume that the organism, with normal function of gas motors, easily balances the increase in resistance due to increased viscosity by a corresponding dilation of blood vessels, and vice versa. How easily the influence of increased blood viscosity is neutralized in this way is fully proven by common cases of polycythemia, in which, as a rule, neither an increase in arterial pressure nor cardiac insufficiency is observed. And in polycythemia, there is the strongest increase in blood viscosity that is observed at all. Therefore, in those cases of polycythemia in which hypertension is present (Gaisböck's form), one must assume a different origin for the hypertension. For the treatment of arteriosclerosis, based on the well-known studies of the Romberg school, the property of large doses of potassium iodide to lower blood viscosity is of importance. Inhalation of oxygen acts in the same way in severe cardiac and pulmonary insufficiency. On the other hand, after profuse hemorrhages, with the depletion of the circulatory system, intravenous infusions of colloidal solutions (gum) isoviscous to blood are prescribed, which retain the introduced water longer in the bloodstream and thereby support the fallen blood pressure. Among blood diseases, the maximum figures for blood viscosity are determined in polycythemia; blood viscosity also increases in leukemias, especially myeloid, and less so in lymphatic; a significant increase in the number of platelets acts on blood viscosity in the same way. Fluctuations of leukocytes within the limits of ordinary leukocytosis do not affect blood viscosity. In some anemias (pernicious and others), we have the most severe degrees of decrease in blood viscosity, which partly causes the appearance of a venous hum over the bulbus v. jugularis, appearing whenever blood viscosity falls below 3. The determination of serum viscosity provides, according to Naegeli, an approximate idea of the amount of protein in the serum (the influence of non-protein bodies of the serum on viscosity is so insignificant that it can be neglected). In a normal person, the viscosity of blood serum fluctuates within the range of 1.4 to 1.9, and the viscosity of plasma (obtained by adding hirudin or novirudin to the blood) between 1.7 and 2.2. To determine serum viscosity, a special viscometer (Serumviskosimeter) is required, which allows determinations to be made with an accuracy to the second decimal place. According to Naegeli, with changes in serum viscosity from 1.43 to 1.90, the protein content correspondingly increases from 5 to 9.5%. Globulins influence serum viscosity more sharply than albumins, therefore, with the same protein content (determined refractometrically), but with different ratios of the protein coefficient, viscosity will change regularly. Rohrer determined the value of viscosity for various amounts of albumins and globulins and presented them in the form of a diagram, by which, knowing the refractive index and viscosity of a given serum, one can easily determine the percentage content of protein fractions. In many diseases, blood viscosity changes in parallel with fluctuations in globulins (for example, in tuberculosis - Rhodin and others; in scarlet fever - Petschacher and others). Therefore, the determination of blood viscosity can have prognostic significance in general infections, in particular in pulmonary tuberculosis, since a shift of the protein formula to the left has an unfavorable significance in the course of the process. It is understandable, therefore, that in tuberculosis, blood viscosity changes in general parallel to the erythrocyte sedimentation rate, which also depends on the protein shift. As an indicator of the strength of inflammatory changes, the determination of blood viscosity can also be of importance in surgery; thus, in appendicitis, blood viscosity increases when the peritoneum is involved in the process and, especially, in perforated peritonitis (up to 7.1); in acute abdominal symptoms, high blood viscosity speaks in favor of peritonitis and against, for example, internal hemorrhage, in which blood viscosity is lowered. The Naegeli-Rohrer theory on the dependence of serum viscosity on the protein coefficient is criticized by Wanner and the Ellinger school. By determining the ratio of the found viscosity of the serum under investigation to the normal viscosity of serum with the same protein concentration, one obtains the so-called specific viscosity (Spiro), reduced viscosity (Rusznyak), or the viscosity coefficient of Hellwig and Neuschloss. The latter authors consider a specific viscosity of 0.96 to 1.04 to be normal and believe that more sharp changes in specific viscosity, excluding infection, cardiac decompensation, and renal ailments, can be used for the diagnosis of hyperthyroidism (specific viscosity 1.04). The indicated value of specific viscosity for the diagnosis of thyroid diseases was not confirmed by the works of other authors. Neuschloss and Trelles were also unable to confirm the regularity indicated by Naegeli-Rohrer and believe that specific viscosity changes depending on the degree of hydration of the serum protein bodies. Specific viscosity can fluctuate under the influence of purine diuretics, incretes, and poisons. According to Spiro, the organism normally maintains the colloidal state of serum proteins at a degree of minimal swelling with minimal specific viscosity, and thereby facilitates blood circulation; in all severe general infections, in tissue decay, and cancer, specific viscosity increases, and the optimal conditions for blood circulation are disturbed. Blood viscosity has also been determined in many other individual diseases, but these determinations are few and partly contradictory (for example, in diabetes, spontaneous gangrene) and therefore do not deserve special analysis.