THE DETERMINATION of serum and urine osmolality is a technically simple procedure, and if properly applied, is of considerable use in the clinical management of water and electrolyte disturbances. It is the purpose of this paper to review briefly the concept, physiology, and clinical application of osmolality.
Osmotic Pressure and Osmolality
According to Raoult's law, when a solute is added to a solvent, the chemical potential of the solvent molecules is lowered in proportion to the mole fraction of the solute particles present. This is manifested as lowering of the vapor pressure and freezing point and raising of the osmotic pressure and boiling point of the solvent. The phenomenon of osmotic pressure is one of the colligative properties of a solution which depend upon the number of particles dissolved in the solution but not on their mass, charge, or shape.If the two arms of a U-shaped tube are