Re: How does the choroid plexus produce CSF?

CSF is a filtrate of the blood produced by the choroid plexuses.  These 
plexuses consist of a layer of ependymal ("lying upon") cells that are 
covered by the capillary-rich pia mater.  Capillary blood is ultrafiltered 
through the pia mater capillaries and the ependymal cells.  About 0.5 
liters of CSF are produced a day, all from (hydrostatic pressure-powered) 
filtration through this "ultrafilter." 

CSF eventually percolates through the subarachnoid space through the 
lateral and median apertures in the fourth ventricle.  From there it 
enters the dural sinuses, thuse, the blood, by passing through structures 
called "arachnoid villi."