Milliosmol

Milliosmol, often written milliosmole, is a unit of osmotic concentration equal to one thousandth of an osmole. The symbol most commonly used is mOsm (or mOsmol). In practice, it expresses the amount of osmotically active particles in a solution and is commonly reported as milliosmoles per liter (mOsm/L) or, less frequently, per kilogram (mOsm/kg) depending on whether osmolarity or osmolality is being described.

Osmolarity and osmolality are related concepts. Osmolarity is the concentration of osmotically active particles per liter of solution, while osmolality is per kilogram of solvent. In dilute aqueous solutions at body temperature, osmolarity approximates osmolality, but the two terms are distinct and the units differ (L vs kg). Osmols account for all solute particles that contribute to osmotic pressure, including electrolytes that dissociate into multiple particles.

Measurement of milliosmols in fluids can be done with osmometry. Freezing-point depression osmometers and vapor-pressure osmometers are common methods. Clinically, osmolality or osmolarity of body fluids is used to assess hydration, electrolyte balance, and renal function. The osmolar gap, calculated from measured osmolality minus the estimated osmolality based on major solutes, helps detect unmeasured solutes such as toxic alcohols.

Notes: milliosmol is not an SI unit, but mOsm/mOsm/L remain standard in medical and physiological contexts. The unit provides a convenient scale for describing osmotically active particle concentrations in biological systems.