This is achieved through the regulation of water flow across the blood-brain barrier, both at the level of the ISF and the level of glial and neuronal and intracellular fluid. The flow of CSF must be regulated to prevent the build-up of excess pressure on brain tissue. CSF exits the sub-arachnoid space back into the venous system through the arachnoid villi by means of a hydrostatic pressure-dependent mechanism. CSF is propelled through the choroid plexus into the ventricles with a driving pressure of about 15 cm of water. There is (i) the gradient set up between the point of secretion of CSF into the ventricles from the choroid plexus to the point where CSF drains through the arachnoid villi into the venous sinuses, and (ii) the mechanical propulsion provided by the pulsing of the cerebral arteries in the subarachnoid space and the movement of the trabecular tethers. The trabeculae are delicate columns of tissue bridges tethered to the arachnoid mater, which strengthen the subarachnoid space and they provide stable support for the brain within its fluid cushion of CSF.ĬSF flows into the ventricles and through the subarachnoid space, driven by two forces. CSF flows through the cisternae and trabeculae of the sub-arachnoid space, which, when filled with CSF, provides buoyancy for the brain, effectively reducing its weight from around 1400 g to about 45 g when it is suspended in CSF. The subarachnoid space is a cavity between the arachnoid membrane and pia mater surrounding the brain and spinal cord. A third source of water for CSF is provided by the complete oxidation of glucose by brain parenchymal cells. CSF passes through the ventricles and into the subarachnoid space through the foramina of Magendie and Luschka(see also p. The transformed ISF is pumped into the subarachnoid space as CSF across the pial-glial membranes. The choroid plexus supplies at least 75% of the CSF, which is also derived from the interstitial fluid (ISF), which is produced by the endothelial cells of the blood-brain barrier in the choroid plexus. Average blood flow through the cerebral circulation is about 0.5 ml/min/g of brain tissue, and flow to the choroid plexus is about ten times higher. The cerebrospinal fluid (CSF) is an ultra-filtrate of plasma actively secreted into the cerebral ventricles by the choroid plexus, a highly vascularized and perfused lining of the ventricles.
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