ABSTRACT
The permeability of the adult blood-brain barrier resembles that of the cell membrane. Lipophilic molecules pass the barrier relatively freely and their delivery to neural tissue is determined primarily by blood flow. Hydrophilic molecules, however, (with important exceptions) enter neural tissue extremely slowly since they cannot pass through the lipid cell membrane, and since the diffusion pores that are present in other vascular beds are extremely rare in the blood-brain barrier. Accordingly, the blood-brain barrier has evolved a number of specific transport mechanisms that deliver needed nutrients to the brain, removed unwanted molecules, and protect the brain from circulating toxins.
