Solutions mannitol/water, osmolality

Osmolality determinations are usually carried out using a cryoscopic osmometer, which is calibrated with deionized water and solutions of sodium chloride of known concentration. Using this technique, the sodium chloride equivalents and freezing point depressions for more than 500 substances have been determined and reported in a series of papers by Hammerlund and co-workers (e.g., see Hammerlund 1981). Figure 6.18 shows the osmolality of mannitol-water solutions. 

Because water is reabsorbed in direct proportion to salt reabsorption in the proximal tubule, luminal fluid osmolality remains nearly constant along its length, and an impermeant solute like inulin rises in concentration as water is reabsorbed. If large amounts of an impermeant solute such as mannitol (an osmotic diuretic) are present in the tubular fluid, water reabsorption causes the concentration of the solute and osmolality of tubular fluid to rise, eventually preventing further water reabsorption. 

Another use for mannitol and urea is in the treatment of dialysis disequilibrium syndrome. Too rapid removal of solutes from the extracellular fluid by hemo- or peritoneal dialysis reduces the osmolality of the extracellular fluid. Consequently, water moves from the extracellular compartment into the intracellular compartment, causing hypotension and CNS symptoms (i.e., headache, nausea, muscle cramps, restlessness, CNS depression, and convulsions). Osmotic diuretics increase the osmolality of the extracellular fluid compartment and thereby shift water back into the extracellular compartment. 

A. Mannitol is an osmotically active solute diuretic. Mannitol inhibits water reabsorption at the loop of Henie and the proximal tubule. The increase in urine output is usually accompanied by an increase in solute excretion. In addition, mannitol transiently increases serum osmolality and decreases cerebrospinal fluid pressure by creating an osmotic gradient between brain tissue and the vascular compartment. Water moves across this gradient into the blood vessels, lowering the cerebrospinal fluid pressure and decreasing intracranial pressure. 

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