Osmolality and Catheterization

Osmotic activity is the process when water in one solution, with lower concentration of e.g. salt ions or sugar molecules, moves through a semipermeable barrier in the direction of a higher concentration to equalize concentrations. The osmotic activity could both be measured in milliosmoles/kilogram (mOsm/kg) solvent, i.e. osmolality, and calculated as milliosmoles/liter (mOsm/L) solution, i.e. osmolarity. 2,3 In human serum osmolarity is approximately equivalent to osmolality given the density of ≈1 kg/L of the fluid. 2,3