Osmotic relationships

Many cestodes appear to behave like osmometers, and when placed in an environment with a different osmotic pressure can only adjust their internal osmotic pressure by varying their body volume. Moniezia, Hymenolepis, Schistocephalus and Callibothrium change their weight almost arithmetically in response to a change in external osmotic pressure (796). 

The question of the osmotic relationship between a cestode and its environment is complicated by the fact that some substances can pass through membranes by means other than diffusion (p. 42). Hence the actual osmotic pressure of a solution as measured by physico-chemical means may not be as significant to a worm as the actual content to which the tegument of the worm is permeable - using the word in its widest sense as indicated above. The reason for this is that substances in the medium which (theoretically) contribute to the total osmotic pressure of the medium do not actually exert osmotic pressure across the tegument of the worm, which separates the worm from its environment. 

differences in permeability of solute can create curiously anomalous situations. In the case of the avian cestode, Tetrabothrius erostris, both sucrose at 0.192 m (d = 0.36 °C) and NaCl at 0.140 M (zl = 0.56 deg. C) appear to be isosmotic this result can be interpreted as showing that this species is less permeable to sucrose than to Na+ and Cl . 

It is interesting to note that, in the case of cestodes of elasmobranchs, urea -which is well known to contribute substantially to the osmotic pressure of blood in many species of elasmobranchs - occurs in quantity in some cestodes of these fishes. The tetraphyllidean Calliobothrium verticillatum contains urea concentrations of 1.03( 0.46)% of the wet weight, the equivalent of 3.7% dry weight (796). When worms were incubated in solutions lacking urea, the urea in the worm tissues rapidly disappeared, but this did not occur if urea was present in the external medium. Similar results were obtained with the tetraphyllideans Phyllobothrium foliatum and Inermiphyllidium pulvinatum. Later studies with 14C-labelled urea on the kinetics of urea entry into C. verticillatum showed that equilibrium between urea in worm tissues and in the external medium was reached in 60-90 min. 

It is surprising to find that Lacistorhynchus tenuis, from the same host as C. verticillatum, does not behave osmotically in a similar manner. The amount of free urea in this species is low, and later work showed that urea was metabolised by this cestode (Chapter 6). 

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The most commonly used generic term for a dissolved substance is solute, and this is the term that we will employ in most contexts, for both large and small compounds, that is, for macromolecules and micromolecules. A closely related term, cosolvent, is often used by physical chemists when the issue in question involves a dissolved substance that either stabilizes or destabilizes the structures of macromolecules. For instance, cosolvent is often used in literature on the effects of solutes on protein stability. A more restrictive and specific term that will be employed when we discuss the osmotic relationships of organisms is osmolyte. 

Physiologists studying osmotic relationships of organisms, however, are often concerned with the total concentration of all dissolved substances, not just the concentrations of specific solutes. For expressing the total number of osmotically active particles in a solution, the concept of osmolality is commonly employed to refer to the osmotic pressure characteristic of a solution. One osmole is defined as the osmotic pressure of a 1.0 molal solution of an ideal solute. Because conditions of ideality do not pertain to the case of biological fluids, it is not possible to extrapolate precisely from chemical determinations of moles of solute per kilogram (or liter) of fluid to the osmolality of that fluid. Rather, this value must be determined empirically. 

We provide this somewhat lengthy treatment of terminology related to solute concentrations not to overwhelm the reader with nit-picky definitions, but to provide important conceptual tools for understanding the diverse osmotic relationships that form the focus of this chapter. It should be apparent that all of these terms... 

In the maintenance of normal osmotic relationships between the blood and interstitial fluid. 

Albumin is an ellipsoidal molecule 150 A long the ratio of the axes is 4 1 (cf. Fig. 17). The chief functions of albumin are the regulation of osmotic relationships in blood and the provision of a reserve of protein for the organism. In addition, albumin easily binds reversibly to various substances, especially negatively charged ones, so that it also plays the role of a carrier in other words, it has a transport function. 

We shall be interested in determining the effect of electrolytes of low molecular weight on the osmotic properties of these polymer solutions. To further simplify the discussion, we shall not attempt to formulate the relationships of this section in general terms for electrolytes of different charge types-2 l, 2 2, 3 1, 3 2, and so on-but shall consider the added electrolyte to be of the 1 1 type. We also assume that these electrolytes have no effect on the state of charge of the polymer itself that is, for a polymer such as, say, poly (vinyl pyridine) in aqueous HCl or NaOH, the state of charge would depend on the pH through the water equilibrium and the reaction... 

As in osmotic pressure experiments, polymer concentations are usually expressed in mass volume units rather than in the volume fraction units indicated by the Einstein equation. For dilute solutions, however, Eq. (8.100) shows that

partial molar volume of the polymer in solution, and M is the molecular weight of the polymer. Substituting this relationship for (pin Eq. (9.9)gives... 

Our strategy in proceeding, therefore, is to write separate expressions for the forces cited in items (1) and (2), and then set them equal to each other as required by item (3). Since we have discussed osmotic effects in Chap. 8 and elastic forces in Chap. 3, we shall invoke certain concepts and relationships from these chapters in this discussion. In this derivation we continue to omit numerical coefficients and some of the less pertinent parameters (although we retain Vj for the sake of Problem 5 at the end of the chapter), and focus attention on the relationship between a, M, and the interaction parameter x-... 

When the superfluid component flows through a capillary connecting two reservoirs, the concentration of the superfluid component in the source reservoir decreases, and that in the receiving reservoir increases. When both reservoirs are thermally isolated, the temperature of the source reservoir increases and that of the receiving reservoir decreases. This behavior is consistent with the postulated relationship between superfluid component concentration and temperature. The converse effect, which maybe thought of as the osmotic pressure of the superfluid component, also exists. If a reservoir of helium II held at constant temperature is coimected by a fine capillary to another reservoir held at a higher temperature, the helium II flows from the cooler reservoir to the warmer one. A popular demonstration of this effect is the fountain experiment (55). 

The osmotic coefficient 4> and activity coefficient are related in a simple manner through the Gibbs-Duhem equation. We can find the relationship by writing this equation in a form that relates a and 2-... 

Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

Colligative properties measure average relative molar masses, M, and in the case of osmotic pressure, II, the important relationship is ... 

Plant cells selected for tolerance to stress show varied responses to the imposed osmotic gradients. In adapted cells, tolerance to salinity or to water stress was not found to increase proportionately with increases in turgor (Handa et al., 1983 Binzel et al., 1985). It was suggested from these observations and from studies by Heyser Nabors (1981) that no relationship existed between turgor and growth and that stress adaptation may alter the relationship between turgor and cell expansion (see also Chapter 6). 

In the osmotic pressure method, the activity of the solvent in the dilute solution is restored to that of the pure solvent (i.e., unity) by applying a pressure m on the solution. According to a well-known thermodynamic relationship, the change in activity with pressure is given by... 

Calculated from intrinsic viscosities using an empirical relationship based on osmotic measurements. 

But for any arbitrary polymer concentration C2 there will be another osmotic pressure due to the previously considered polymer-solvent interaction and to the associated elastic reaction of the network. Recalling the general relationship tz= — (mi —mi)/vi, we may calculate oTo from Eq. (38). At equilibrium the total osmotic pressure arising from the effects of all solutes must be zero, i.e., — JCq. Hence from Eqs. 

The body s normal daily sodium requirement is 1.0 to 1.5 mEq/kg (80 to 130 mEq, which is 80 to 130 mmol) to maintain a normal serum sodium concentration of 136 to 145 mEq/L (136 to 145 mmol/L).15 Sodium is the predominant cation of the ECF and largely determines ECF volume. Sodium is also the primary factor in establishing the osmotic pressure relationship between the ICF and ECF. All body fluids are in osmotic equilibrium and changes in serum sodium concentration are associated with shifts of water into and out of body fluid compartments. When sodium is added to the intravascular fluid compartment, fluid is pulled intravascularly from the interstitial fluid and the ICF until osmotic balance is restored. As such, a patient s measured sodium level should not be viewed as an index of sodium need because this parameter reflects the balance between total body sodium content and TBW. Disturbances in the sodium level most often represent disturbances of TBW. Sodium imbalances cannot be properly assessed without first assessing the body fluid status. 

For a solution of a single electrolyte, the relationship between the mean activity coefficient and the osmotic coefficient is given by the equation... 

Here Jv is the volumetric flow rate of fluid per unit surface area (the volume flux), and Js is the mass flux for a dissolved solute of interest. The driving forces for mass transfer are expressed in terms of the pressure gradient (AP) and the osmotic pressure gradient (All). The osmotic pressure (n) is related to the concentration of dissolved solutes (c) for dilute ideal solutions, this relationship is given by... 

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