The blood-brain barrier is formed by endothelial cells of the capillary wall, astrocyte end-feet ensheathing the capillary, and pericytes embedded in the capillary basement membrane.
This system allows the passage of some molecules by passive diffusion, as well as the selective transport of various nutrients, ions, organic anions, and macromolecules such as glucose, water and amino acids that are crucial to neural function.
The blood-brain barrier restricts the passage of pathogens, the diffusion of solutes in the blood, and large or hydrophilic molecules into the cerebrospinal fluid, while allowing the diffusion of hydrophobic molecules (O2, CO2, hormones) and small polar molecules. Cells of the barrier actively transport metabolic products such as glucose across the barrier using specific transport proteins.
These capillaries are relatively impermeable because (1) their endothelial cells lack fenestrations and are joined at their borders by tight junctions and (2) they are surrounded by the cytoplasmic processes of neuroglia called astrocytes. These features contribute to a structural and functional barrier that protects CNS neurons from many extraneous influences and prevents certain antibiotics and chemotherapeutic agents from reaching the CNS.