|MadSci Network: Neuroscience|
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."
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