How do osmotic diuretics, like mannitol, work in the renal system?

Osmotic diuretics, such as mannitol, function primarily by increasing the osmolarity of blood and renal tubules. When mannitol is administered, it is freely filtered at the glomerulus but is not reabsorbed in the renal tubules. This leads to an increase in osmotic pressure within the renal tubules, which draws water into the tubules and prevents its reabsorption. As a result, there is an increase in urine output. The elevated osmolarity in both the blood and renal tubular fluid effectively causes fluid to be retained within the tubules rather than being absorbed back into the body, increasing urine production.

This mechanism is particularly beneficial in situations where rapid diuresis is necessary or when reducing intracranial pressure, as mannitol can help to reduce cerebral edema by drawing excess fluid out of the brain and into the vascular space.