How does Mannitol primarily exert its action?

Mannitol primarily exerts its action by increasing plasma osmolarity, which draws water out of the eye. This mechanism is particularly important in the treatment of conditions such as increased intraocular pressure or cerebral edema. Mannitol is an osmotic diuretic that works by creating an osmotic gradient in the renal tubules, leading to the retention of water in the urine. When administered intravenously, mannitol increases the osmotic pressure in the blood, which then draws fluid from the surrounding tissues, including those in the eye, thereby reducing swelling and lowering intraocular pressure effectively. This action is beneficial for patients with conditions like glaucoma or when managing acute situations that require rapid fluid withdrawal.

The other options do not accurately describe the primary action of mannitol. While it does not primarily increase sodium reabsorption, decrease blood pressure through vasodilation, or inhibit glucose absorption in the kidneys, its primary role as an osmotic agent is critical for its therapeutic effects.