Mannitol is an osmotic drug, which reaches the filtrate in Bowman's capsule, but hardly undergo reabsorption. It increases the osmotic pressure in the tubular fluid. As a result less water is reabsorbed and the Na+concentration in the tubular fluid is lower. Actually osmotic diuretics increase the urinary excretion of almost all electrolytes. Osmotic diuretics increase the extracellular volume, decrease blood viscosity and inhibit renin release.
Osmotic diuretics are used to treat acute renal failure and edema in the brain and eye. Because osmotic diuretics increase extracellular volume, pulmonary edema (especially in heart failure patients) and hyponatremia (especially in renal failure patients) can occur. On the other hand, the loss of water in excess of electrolytes raises the risk for hypernatremia and dehydration.
What is true about mannitol?
Extra info: Mannitol is used in case of acute renal insufficiency with oliguria. It increases osmolality of the extracellular spaces, which results in water flow from the intracellular towards the extracellular space.