<P> CSF is produced by the choroid plexus in two steps . Firstly, a filtered form of plasma moves from fenestrated capillaries in the choroid plexus into an interstitial space, with movement guided by a difference in pressure between the blood in the capillaries and the interstitial fluid . This fluid then needs to pass through the epithelium cells lining the choroid plexus into the ventricles, an active process requiring the transport of sodium, potassium and chloride that draws water into CSF by creating osmotic pressure . Unlike blood passing from the capillaries into the choroid plexus, the epithelial cells lining the choroid plexus contain tight junctions between cells, which act to prevent most substances flowing freely into CSF . </P> <P> Water and carbon dioxide from the interstitial fluid diffuse into the epithelial cells . Within these cells, carbonic anhydrase converts the substances into bicarbonate and hydrogen ions . These are exchanged for sodium and chloride on the cell surface facing the interstitium . Sodium, chloride, bicarbonate and potassium are then actively secreted into the ventricular lumen . This creates osmotic pressure and draws water into CSF, facilitated by aquaporins . Chloride, with a negative charge, moves with the positively charged sodium, to maintain electroneutrality . Potassium and bicarbonate are also transported out of CSF . As a result, CSF contains a higher concentration of sodium and chloride than blood plasma, but less potassium, calcium and glucose and protein . Choroid plexuses also secrete growth factors, vitamins B1, 12 C, folate, beta - 2 microglobulin, arginine vasopressin and nitrous oxide into CSF . A Na - K - Cl cotransporter and Na / K ATPase found on the surface of the choroid endothelium, appears to play a role in regulating CSF secretion and composition . </P> <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>

Csf that is around the brain is found in the