osmolalno

Osmolalno, or osmolalnost, is a measure of the concentration of osmotically active particles in a solution, expressed as osmoles of solute per kilogram of solvent (Osm/kg, commonly shown as mOsm/kg in medical contexts). It reflects the number of dissolved particles rather than their chemical identity and is a key concept in physiology and clinical chemistry because it governs water movement across semipermeable membranes.

In human physiology, osmolalnost is tightly regulated. Normal plasma osmolality in healthy adults is roughly 275–295 mOsm/kg. The main contributors are sodium and its accompanying anions, glucose, and urea. Potassium contributes to osmolality as well, but to a smaller extent. Osmolalnost can be estimated from routine solute concentrations, and it is distinct from osmolarnost (osmolarity), which is per liter of solution rather than per kilogram of solvent.

Clinical use and measurement. Osmolalnost can be measured directly by freezing point depression or by vapor pressure osmometers. It can also be estimated with the commonly used formula: estimated osmolality ≈ 2 × [Na+] + [glucose]/18 + [BUN]/2.8, with concentrations in mg/dL. The osmolality gap—the difference between measured osmolality and calculated osmolality—helps detect unmeasured solutes such as etanol or toxic alcohols.

Applications and relevance. Osmolalnost is used to assess hydration status, diagnose and monitor disorders of water balance (like hyponatremia or hypernatremia), and evaluate fluid therapy. Abnormal osmolality can indicate imbalances in sodium, glucose, or urea and may prompt further testing for toxins or additional solutes.