Osmotic diuresis, solutions Mannitol

The answer is b. (Hardman, pp 695-697.) A significant increase in the amount of any osmotically active solute in voided urine is usually accompanied by an increase in urine volume Osmotic diuretics affect diuresis through this principle. The osmotic diuretics (such as mannitol) are nonelectrolytes that are freely filtered at the glomerulus, undergo limited re absorption by the renal tubules, retain water in the renal tubule, and promote an osmotic diuresis, generally without significant Na excretion. Ln addition, these diuretics resist alteration by metabolic processes. 

Mannitol Stimulation of osmotic diuresis is possible using mannitol (10-20% solution). (128) Mannitol is neither metabolized in the body nor reabsorbed by the tubules and is excreted almost totally through the kidney. Renal circulation and renal filtration are raised, and by reducing tubular absorption (= osmotic diuresis), water excretion is increased ( diuresis starter ). The saluretic effect is, however, relatively small. In the case of restricted renal function, application of mannitol is contraindicated. If necessary, the mannitol test (i.v. injection of 75 ml of a 20% solution) can be carried out beforehand. With enhanced diuresis of > 40 ml/hr, the kidneys still function adequately, so that it is possible to stimulate osmotic diuresis by means of a mannitol infusion. 

Hyponatremia occurs with an increased amount of other solutes in the ECF, causing an extracellular shift of water or intracellular shift of Na. to maintain osmotic balance between the ECF and ICF compartments. The most common cause of this type of hyponatremia is severe hyperglycemia. As a general rule, the Na" decreases —1.6 mmol/L for every 100 mg/dL increase of glucose above 100 mg/dL. The clinical use of mannitol for osmotic diuresis can have a similar effect. 

Anesthetized rats were prepared for micropuncture as previously reported (2). Osmotic diuresis was induced by infusion of a 5 % mannitol solution in isotonic saline to permit rapid serial collections of urine necessary to determine fractional recoveries of the isotopes. Prior to abdominal surgery some rats were slowly infused i.v. with benzbromarone or benziodarone at a dose of 10 mg/kg/BW. 

Osmotic diuretics such as mannitol act on the proximal tubule and, in particular, the descending limb of the Loop of Henle — portions of the tubule permeable to water. These drugs are freely filtered at the glomerulus, but not reabsorbed therefore, the drug remains in the tubular filtrate, increasing the osmolarity of this fluid. This increase in osmolarity keeps the water within the tubule, causing water diuresis. Because they primarily affect water and not sodium, the net effect is a reduction in total body water content more than cation content. Osmotic diuretics are poorly absorbed and must be administered intravenously. These drugs may be used to treat patients in acute renal failure and with dialysis disequilibrium syndrome. The latter disorder is caused by the excessively rapid removal of solutes from the extracellular fluid by hemodialysis. 

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