Definition. A diuretic is any substance which increases urine and solute excretion. This wide definition, however, includes substances not commonly thought of as diuretics, e.g. water. To be therapeutically useful a diuretic should increase the output of sodium as well as of water, since diuretics are normally required to remove oedema fluid, composed of water and solutes, of which sodium is the most important. Diuretics are among the most commonly-used drugs, perhaps because the evolutionary advantages of sodium retention have left an aging population without salt-losing mechanisms of matching efficiency.
Each day the body produces 1801 of glomerular filtrate which is modified in its passage down the renal tubules to appear as 1.51 of urine. Thus a 1% reduction in reabsorption of tubular fluid will more than double urine output. Clearly, drugs that act on the tubule have considerable scope to alter body fluid and electrolyte balance. Most clinically useful diuretics are organic anions, which are transported directly from the blood into tubular fluid. The following brief account of tubular function with particular reference to sodium transport will help to explain where and how diuretic drugs act; it should be read with reference to Figure 26.1.
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