ABSTRACT
Diuretics act on the kidney to increase urine output and electrolyte excretion. Particularly since the introduction of chlorothiazide and frusemide, they have become very widely used. Many millions of prescriptions are written each year and it is estimated that diuretics are prescribed for one in four medical patients. Clearance is a concept crucial to the evaluation of diuretics: it is defined as the volume of plasma completely cleared of a given substance in unit time. Renal clearance is derived by dividing total urinary excretion by the plasma concentration during the collection interval: it expresses the input-output relationship for any endogenous or exogenous substance and measures renal excretory function. Proper characterisation of a diuretic also requires an understanding of basic renal physiology. Some of the beneficial effects of diuretics in heart failure and hypertension seem to be the result of direct vasodilator activity.
