Tight junctions between endothelial cells, supported by astrocyte foot processes
Expansion
Structure
- Tight junctions between capillary endothelial cells, with no fenestrations
- Thick basement membrane
- Astrocyte foot processes inducing and maintaining the barrier
- Pericytes
- Efflux pumps such as P-glycoprotein actively removing drugs
Crosses freely: oxygen, carbon dioxide, water, and lipid-soluble drugs such as anaesthetics and many psychotropics. Glucose and amino acids need specific transporters.
Excluded: large or charged molecules, most antibiotics, catecholamines, and protein.
Circumventricular organs lack the barrier, because they must sample blood: the area postrema (chemoreceptor trigger zone), organum vasculosum (osmoreceptors), subfornical organ, median eminence and posterior pituitary.
Clinical consequences: levodopa crosses but dopamine does not, which is why the precursor is given with a peripheral decarboxylase inhibitor; meningitis disrupts the barrier, which is why penicillin becomes effective and why steroids are given to limit inflammation; and the barrier is immature in neonates, allowing kernicterus.