Abstract
The migration of substances between blood circulation and central nervous system (CNS) is regulated by the blood-brain barrier (BBB). Small lipophilic molecules such as carbon dioxide, oxygen or ethanol can pass the BBB by passive, transcellular diffusion, while the paracellular transport of hydrophilic substances is restricted by intercellular tight-junctions. Due to accessory transport systems the BBB is able to regulate specifically the permeation of substances (e.g. nutrients) [1].