For the patient with head injury and increased Intracranial Pressure (ICP), an osmotic diuretic, such as mannitol, is very helpful. An osmotic diuretic works by creating an osmotic gradient in the blood by the presence of the additional osmolyte (mannitol). In other words, the osmolyte stays in the bloodstream and creates a greater osmotic force to pull water out of tissues and third spaces (interstitium of cerebrum). This aids in alleviating the addition ICP by fluid removal.
The extra fluid that was "sucked" into the bloodstream via osmosis, can then travel to the kidneys and be filtered out into the urine (diuresis). This explains its two purposes as an "osmotic" and a "diuretic."
