The
blood–brain barrier (
BBB) is a highly selective
permeability barrier that separates the circulating
blood from the brain
extracellular fluid (BECF) in the
central nervous system (CNS). The blood–brain barrier is formed by brain endothelial cells, which are connected by
tight junctions with an extremely high
electrical resistivity of at least 0.1 O·m. The blood–brain barrier allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function. On the other hand, the blood–brain barrier may prevent the entry of lipophilic, potential
neurotoxins by way of an active transport mechanism mediated by P-glycoprotein.
Astrocytes are necessary to create the blood–brain barrier. A small number of regions in the brain, including the
circumventricular organs (CVOs), do not have a blood–brain barrier.