4,5 As this equation shows, plasma sodium concentration is the major determinant of plasma osmolality. Plasma osmolality (mOsm/kg) in healthy animals can be calculated by the equation shown in Box 50-1. Because they are the most plentiful, these molecules are the main determinants of plasma osmolality in healthy dogs and cats. 3 The most abundant osmoles in the extracellular fluid are sodium (and the accompanying anions chloride and bicarbonate), glucose, and urea. Every molecule dissolved in the total body water contributes to osmolality, regardless of size, weight, charge, or composition. In physiologic systems there is no appreciable difference between osmolality and osmolarity, so the term osmolality will be used for the rest of this discussion for simplicity. Osmolarity is the concentration of osmoles in a volume of solvent and in biologic systems is expressed as mOsm/L of water. In biologic systems, osmolality is expressed as mOsm/kg of water and can be measured using an osmometer. Osmolality is the concentration of osmoles in a mass of solvent. Burkitt Creedon DVM, DACVECC, in Small Animal Critical Care Medicine (Second Edition), 2015 Osmolality and Osmotic PressureĪn osmole is 1 mole of any fully dissociated substance dissolved in water. Because of hypotonic-induced hemolysis, parenterally administered fluids should ideally be isotonic or hypertonic. Erythrocytes are resistant to increases in plasma osmolarity, whereas they are susceptible to mild decreases in osmolarity this is the basis of the red blood cell fragility test in which red blood cell suspensions are placed in solutions of decreasing osmolarity. Using this categorization, it is readily apparent that some routinely used crystalloid solutions are hypotonic in particular, lactated Ringer's solution (275 mOsm/L) is mildly hypotonic and 5% dextrose (250 mOsm/L) is moderately hypotonic, although, as glucose is metabolized, 5% dextrose becomes an increasingly hypotonic solution.
The normal plasma osmolality in large animals is approximately 285 mOsm/kg, and plasma osmolality is aggressively defended by increasing water intake (osmolality >285 mOsm/kg) or promoting free water excretion (osmolality 312 mOsm/L), or hypotonic (<300 mOsm/L). Osmolality is the number of dissolved particles per kilogram of solution and is expressed as mOsm/kg of solution. Complete understanding of the tonicity concept requires differentiation of two terms, osmolality and osmolarity. The tonicity of the solution is an important clinical issue. In Veterinary Medicine (Eleventh Edition), 2017 Isotonic, Hypertonic, and Hypotonic Crystalloid Solutions
Table 1-1 shows the relationships among molecular weight, equivalence, and osmoles for a number of physiologically significant solutes.
Because of the dilute nature of physiologic solutions and because water is the solvent, osmolalities are expressed as milliosmoles per kilogram of water (mOsm/kg H 2O). For this reason, osmolality is the preferred term for biologic systems and is used throughout this and subsequent chapters. In contrast, osmolality, which is based on the mass of the solvent, is temperature independent. Measurements of osmolarity are temperature dependent because the volume of solvent varies with temperature (i.e., the volume is larger at higher temperatures).
For dilute solutions, the difference between osmolarity and osmolality is insignificant. Osmolarity refers to the number of solute particles per 1 L of solvent, whereas osmolality is the number of solute particles in 1 kg of solvent. Osmolarity and osmolality are frequently confused and incorrectly interchanged. Stanton PhD, in Renal Physiology (Fifth Edition), 2013 Osmolarity and Osmolality