Ependymal cells make up a specialized epithelial structure called the choroid plexus, which produces CSF at a rate that completely replenishes it about three times per day.CSF circulates through the brain’s interconnected ventricular system to the brainstem, where it passes through small openings out to the subarachnoid space surrounding the brain and spinal cord.CSF ultimately flows to the top of the outer surface of the brain, where most of it enters the bloodstream through one-way valves in large veins.