Osmotic diuretics (mannitol) are pharmacologically inert drugs freely filtered in the glomerulus but poorly reabsorbed by the tubules. The mechanism of action resides in the osmotic activity that mannitol produces along the tubular segments, counteracting the normal reabsorption of sodium and water. Three mechanisms are described. First, the effective intraluminal Na + is lower secondary to the increased water in the tubular lumen producing a decrease in the reabsorption force (gradient) for the Na+ ion. Second, there is an increased flux of Na+ from the peritubular (extraluminal) fluid back to the tubular lumen; in summary, extraluminal Na + is greater than luminal Na+. Finally, mannitol increases the intravascular volume, decreases the viscosity of the blood, thus increasing the medullary blood flow, and decreases the medullary solute gradient (washout of the normal medullary hypertonicity), preventing concentration of the urine. Another effect lies in the scavenging property of mannitol for oxygen radicals, which may be a beneficial therapeutic tool in the case of reperfusion injuries.

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