[3] Osmotic diuretics work by expanding extracellular fluid and plasma volume, therefore increasing blood flow to the kidney.
Osmotic diuretics increase plasma volume, but because they do not cross the blood-brain barrier, this does not affect the nervous system.
Osmotic diuretics have their major effect in the proximal convoluted tubule and the descending limb of the Loop of Henle.
The presence of a nonreabsorbable solute such as mannitol prevents the normal absorption of water by interposing a countervailing osmotic force.
The increase in urine flow rate decreases the contact time between fluid and the tubular epithelium, thus reducing sodium as well as water reabsorption.
The second effect occurs through creation of an osmotic gradient across the blood-brain barrier, leading to the movement of water from the parenchyma to the intravascular space.