Osmotic diuresis.

Osmotic diuresis occurs, if nonreabsorbed solutes such as mannitol impair the reabsorption of water. The reduced reabsorption of volume affects in turn the reabsorption and excretion of solutes. Thus, mannitol leads to modest impairment of proximal tubular reabsorption not only of water, but as well of electrolytes (Na, Cl, K, Pi, Ca, but not Mg), urea, and uric acid. Infusion of hypertonic mannitol increases renal blood flow and the glomerular filtration rate of superficial nephrons. The increased perfusion of medulla leads to wash out of medullary hypertonicity. The decline of medullary osmolarity leads to a marked impairment of water reabsorption in descending limbs and possibly to moderate impairment of NaCl, Ca, and Mg reabsorption in the ascending limbs of Henle's loop. In the collecting duct, inhibition is marked of water and urea reabsorption and modest of NaCl reabsorption. A number of open questions remain, such as the mechanisms underlying decrease of renal vascular resistance, increased proximal tubular reabsorption of magnesium, or impaired NaCl reabsorption in thick ascending limbs.