Expansion
Tight junctions, basement membrane and astrocyte foot processes
Expansion
Three layers
- Continuous capillary endothelium with tight junctions, and no fenestrations
- A continuous basement membrane
- Astrocyte foot processes ensheathing the capillary, plus pericytes
What crosses: small lipid soluble molecules (oxygen, carbon dioxide, anaesthetics, ethanol) freely; glucose and amino acids by specific transporters (GLUT1); and almost nothing else. P-glycoprotein actively pumps many drugs back out.
Circumventricular organs lack the barrier, so they can sample the blood:
- Area postrema - the chemoreceptor trigger zone, which is why chemotherapy and opioids cause vomiting
- Posterior pituitary, median eminence, subfornical organ, pineal, OVLT
Clinical consequences
- Antibiotic penetration is poor, which is why meningitis needs high dose agents that cross, such as ceftriaxone, and why the barrier becomes more permeable when inflamed, improving penetration early in treatment
- Levodopa crosses but dopamine does not, which is why levodopa is given with a peripheral decarboxylase inhibitor
- Domperidone does not cross, which is why it is the preferred antiemetic in Parkinson’s disease
- Vasogenic oedema results from barrier breakdown around tumours and abscesses, and responds to dexamethasone, unlike the cytotoxic oedema of stroke