The choroid plexus is a plexus of cells that produces the cerebrospinal fluid in the ventricles of the brain. The choroid plexus consists of modified ependymal cells. CSF is recycled (flushed) 4 times per day in order to clean out metabolites and toxins like beta amyloid. Hence the choroid plexus must produce about 500 milliliters of CSF daily (or 21 mL per hour).