In mannitol therapy, which description correctly explains its action?

• A. It increases renal perfusion
• B. It decreases CSF production
• C. It increases serum osmolarity, drawing water from brain
• D. It inhibits kidney function

Answer: (C)

Mannitol works by creating an osmotic gradient in the bloodstream. It is freely filtered by the kidneys but not reabsorbed, so it stays in the intravascular space and increases the plasma osmolality. This higher osmolarity pulls water out of brain tissue into the blood, reducing brain edema and lowering intracranial pressure. The water then is excreted by the kidneys. This is why the description that best fits its action is that it increases serum osmolarity, drawing water from the brain.