Severe head injury can result in swelling of the brain. This is a very serious condition because the brain is entirely contained within the bony skull-it has nowhere to go when it swells so the intracranial pressure increases. Mannitol is a simple sugar that can be administered in  intravenous fluids to reduce intracranial pressure. Using your knowledge of osmolarity, try to describe how mannitol exerts its effect.

When administered intravenously the mannitol becomes a new solute in the plasma. Because mannitol is a sugar and is therefore water soluble it can't across the cell membrane and remains in the blood. Because the mannitol is a new socket and is therefore increased the number of solutes in the blood, it attracts water from the intracellular fluid into the blood plasma. The water then travels to the kidneys where it gets excreted.  The mannitol causes the cells in the brain to dehydrate mildly. In effect, the mannitol is causing the cells of the brain to dehydrate slightly. Cell volume depends in part on the volume of the intracellular fluid, so a decreased intracellular fluid volume decreases cell size. This reduces the overall swelling of the brain.