(A) Is actively secreted by choroidal plexus > CSF fills the ventricles and subarachnoid space.> In humans, the volume of CSF is about 150 mL and the rate of CSF production is 550 mL/d.> Thus the CSF turns over about 3.7 times a day.> 50-70% of the CSF is formed in the choroid plexuses and the remainder is formed around blood vessels and along ventricular walls.> The CSF in the ventricles flows through the foramens of Magendie and Luschka to the subarachnoid space and is absorbed through the arachnoid villi into veins, primarily the cerebral venous sinuses.> Villi act as valves which permit bulk flow (direct flow) of CSF into venous blood and these villi is about 500 mL/d, with additional small amounts of CSF being absorbed by diffusion into cerebral blood vessels.> The composition of CSF is essentially the same as that of brain ECF, which in living humans makes up 15% of the brain volume and contains 50-80 mg/dL of glucose.# Functions of CSF:> Buoyancy: The brain weighs about 1400 g in air, but in its "water bath" of CSF it has a net weight of only 50 g.> The buoyancy of the brain in the CSF permits its relatively flimsy attachments to suspend it very effectively.> Protection: When the head receives a blow, the arachnoid slides on the dura & the brain moves, but its motion is gently checked by the CSF cushion and by the arachnoid trabeculae.> CSF pressure ranges from 180-10 mmg Hg); .4.4-7.3 mm Hg in newborns and < 200 mm Hg in children and adults.> Chemical stability drains metabolic wastes and maintains hemostasis.> Prevention of ischaemia is by reducing CSF pressure by decreasing its amount.> CSF pH is 7.30-7.36