ABSTRACT
The blood–brain barrier governs strictly what is allowed to cross into the brain extracellular fluid from the blood. The physical barrier is provided by brain capillary endothelial cells which are coupled to each other by tight junctions with a very high electrical resistance. The lack of a blood–brain barrier at the posterior pituitary permits oxytocin and vasopressin to be secreted directly into the systemic circulation, and at other sites it allows the brain to monitor the concentrations of water, ions and selected molecules for homeostatic functions. The blood–brain barrier is able to actively exclude a wide range of lipophilic compounds that are potentially neurotoxic, many of which are ingested as part of a natural diet. This is achieved by a transport protein, P-glycoprotein, expressed in high levels in the plasma membrane of endothelial cells. The blood–brain barrier opens in cerebral ischemia causing cytotoxic cerebral edema, which is a medical emergency.
