Fig. 8

Flow diagram of routes taken by large hydrophilic solutes emerging from the parenchyma by extravascular routes. The eventual destination is blood but there are multiple routes that a solute can follow. The initial extravascular pathways be they periarterial, perivenous, subependymal or white matter tracts may lead to CSF. In addition, particularly the intramural perivascular pathways may lead to lymphatics. Solutes that reach CSF at the parenchymal surface in anterior and/or ventral portions of the brain may be taken via the olfactory nerve to the cribriform plate and thus to lymphatics. Alternatively the solutes may be mixed into CSF that at some point traverses the cisterna magna, perhaps on route to spinal sites of elimination. A significant portion of the CSF may also be directed to lymph. It cannot be assumed that solutes reaching the cervical lymph nodes have not first mixed with CSF