<P> Orešković and Klarica hypothesise that CSF is not primarily produced by the choroid plexus, but is being permanently produced inside the entire CSF system, as a consequence of water filtration through the capillary walls into the interstitial fluid of the surrounding brain tissue, regulated by AQP - 4 . </P> <P> There are circadian variations in CSF secretion, with the mechanisms not fully understood, but potentially relating to differences in the activation of the autonomic nervous system over the course of the day . </P> <P> CSF returns to the vascular system by entering the dural venous sinuses via arachnoid granulations . These are outpouchings of the arachnoid mater into the venous sinuses around the brain, with valves to ensure one - way drainage . This occurs because of a pressure difference between the arachnoid mater and venous sinuses . CSF has also been seen to drain into lymphatic vessels, particularly those surrounding the nose via drainage along the olfactory nerve through the cribriform plate . The pathway and extent are currently not known, but may involve CSF flow along some cranial nerves and be more prominent in the neonate . CSF turns over at a rate of three to four times a day . CSF has also been seen to be reabsorbed through the sheathes of cranial and spinal nerve sheathes, and through the ependyma . </P> <P> The composition and rate of CSF generation are influenced by hormones and the content and pressure of blood and CSF . For example, when CSF pressure is higher, there is less of a pressure difference between the capillary blood in choroid plexuses and CSF, decreasing the rate at which fluids move into the choroid plexus and CSF generation . The autonomic nervous system influences choroid plexus CSF secretion, with activation of the sympathetic nervous system increasing secretion and the parasympathetic nervous system decreasing it . Changes in the pH of the blood can affect the activity of carbonic anhydrase, and some drugs (such as frusemide, acting on the Na - K - Cl cotransporter) have the potential to impact membrane channels . </P>

The csf returns to the bloodstream by way of