Osmotic diuretics are small molecular weight substances that are filtered by the glomerulus but not reabsorbed by the renal tubule, and thus increase the osmolarity of the tubular fluid. Consequently they prevent the reabsorption of water (and also, by more complex mechanisms, of sodium) principally in the proximal convoluted tubule and probably also the loop of Henle. The result is that urine volume increases according to the load of osmotic diuretic.
Mannitol, a polyhydric alcohol (mol. wt. 452), is most commonly used; it is given i.v. In addition to its effect on the kidney, mannitol encourages the movement of water from inside cells to the extracellular fluid, which is thus transiently expanded before diuresis occurs. These properties define its uses, which are for rapid reduction of intracraninal or intraocular pressure, and to maintain urine flow to prevent renal tubular necrosis. Because it increases circulatory volume, mannitol is contraindicated in congestive cardiac failure and pulmonary oedema.
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