Overview Defintion measure of ability to dilute urine equal to the volume of plasma cleared of pure water per unit time Equation free water clearance (CH2O) = urine flow rate (V) - water occupied with solute (Cosm) Cosm = UosmV/Posm ADH with ADH CH2O < 0 retention of free water without ADH CH2O > 0 excretion of free water Loop diuretics produce isotonic urine CH2O = 0