What is the mechanism by which mannitol exerts its effect in reducing cerebral edema?

Mannitol primarily reduces cerebral edema through its ability to alter serum osmolality. As an osmotic diuretic, mannitol increases the osmolarity of the blood and extracellular fluid, which draws water out of the cerebral tissue. This shift in fluid helps to reduce the volume of edema in the brain. The hyperosmotic state created by mannitol leads to osmotic diuresis, where water is excreted in the urine, further contributing to the reduction in total body water and thereby lowering intracranial pressure.

The other options do not accurately describe the primary mechanism by which mannitol acts. While increasing blood viscosity or promoting vasodilation could influence cerebral perfusion dynamics, these mechanisms do not specifically relate to the reduction of cerebral edema. Decreasing urine output contradicts the effect of mannitol, as it actually promotes diuresis, leading to increased urine output rather than a decrease.