Cerebrospinal fluid
To keep waste products from building up in the CSF, the choroid plexuses make new CSF about four times a day.  The old CSF drains out into the bloodstream, bringing waste products and toxins with it.  In this way, the CSF rinses things that could hurt the brain out into the bloodstream.  The bloodstream can then carry these chemicals to organs that can get rid of them, like the kidneys and the lungs.  For example, the bloodstream carries carbon dioxide to the lungs, where it can be breathed out.