osmolality

(chemistry) The molality of an ideal solution of a nondissociating substance that exerts the same osmotic pressure as the solution being considered.

chemistry For a given solution of a non-dissociating substance, the molality of an ideal solution of it that would exert the same osmotic pressure; this is usually expressed as osmoles of the original.

The ratio of solutes (such as electrolytes) to fluid. It is measured in osmoles per kilogram of fluid. One osmole is equal to the molecular weight of the substance in grams divided by the number of ions or other particles the substance dissociates into when in solution.

The concentration of a solution in terms of osmoles of solutes per kilogram of solvent.

• serum o. — a measure of the number of dissolved particles per unit of water in serum. In a solution, the fewer the particles of solute in proportion to the number of units of water (solvent), the less concentrated the solution. A low serum osmolality would be indicative of a higher than usual amount of water in relation to the amount of particles dissolved in it. It would be expected, then, that a low serum osmolality would accompany overhydration, or edema, and an increased serum osmolality would be present in a state of fluid volume deficit.
— Measurement of the serum osmolality gives information about the hydration status within the cells because of the osmotic equilibrium that is constantly being maintained on either side of the cell membrane (homeostasis). — Water moves freely back and forth across the membrane in response to the osmotic pressure being exerted by the molecules of solute in the intracellular and extracellular fluids. Serum osmolality reflects the status of hydration of the intracellular as well as the extracellular compartments and thus describes total body hydration.
• urine o. — a measure of the number of dissolved particles per unit of water in the urine. A more accurate measure of urine concentration than specific gravity, urine osmolality is useful in diagnosing renal disorders of urinary concentration and dilution and in assessing status of hydration.