Definition of the barrier The interstitial fluid (rSF) of the brain is separated from the blood by the blood-brain barrier (BBB). This barrier must not be thought of as a single entity or as an absolute restriction to all molecules, but as a multiple structure located at several sites within the brain. The first of these interfaces is located at the endothelium of the brain capillaries. Secondly there is a potential site for interchange on the outer linings of the brain between the dura and the arachnoid membranes. Thirdly, there are the choroid plexuses and the cerebrospinal fluid (CSF)...