Final answer:
Mannitol disrupts tight junctions, which makes the blood-brain barrier more permeable to medications that need to reach the brain's extra-cellular fluid. The blood-brain barrier's selective permeability normally prevents many substances, including drugs, from entering the CNS.
Step-by-step explanation:
Mannitol is a drug that temporarily disrupts tight junctions. This drug is often added to medications that need to get into the extra-cellular fluid of the brain because it will make the capillaries of the blood-brain barrier more permeable to larger molecules. The blood-brain barrier (BBB) is a physiologically significant barrier that controls the transfer of substances from the bloodstream to the central nervous system (CNS). It consists of capillary endothelial cells with tight junctions that prevent the movement of nearly all substances.
Because of the BBB's permeability control, many substances, including certain pharmaceuticals, are inhibited from entering the brain, which is a challenge for drug delivery. The assistance of mannitol enables these drugs to pass through the BBB by affecting the permeability of the brain's capillaries, thereby allowing the treatment of CNS conditions more effectively.