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the concentration of a solution in terms of osmoles of solutes per kilogram of solvent.
serum osmolality 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 means a higher than usual amount of water in relation to the amount of particles dissolved in it, and accompanies overhydration, or edema. An increased serum osmolality indicates deficient fluid volume. 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 osmolar 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. The normal value for serum osmolality is 270–300 mOsm/kg water.
urine osmolality 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. The normal value is 500 to 800 mOsm/L.
The osmotic concentration of the urine.
See also: osmolality