Flow of Cerebrospinal Fluid

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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. 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 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. The transformed ISF is pumped into the subarachnoid space as CSF across the pial-glial membranes. CSF passes through the ventricles and into the subarachnoid space through the foramina of Magendie and Luschka(see also p. 16). A third source of water for CSF is provided by the complete oxidation of glucose by brain parenchymal cells. The subarachnoid space is a cavity between the arachnoid membrane and pia mater surrounding the brain and spinal cord. 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 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.

CSF flows into the ventricles and through the subarachnoid space, driven by two forces. 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. CSF is propelled through the choroid plexus into the ventricles with a driving pressure of about 15 cm of water. CSF exits the sub-arachnoid space back into the venous system through the arachnoid villi by means of a hydrostatic pressure-dependent mechanism.

The flow of CSF must be regulated to prevent the build-up of excess pressure on brain tissue. 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. ISF constitutes about 15-20% of the total tissue weight, and is the principal source of extracellular fluid in the brain. Cerebral capillary walls are less permeable than those of peripheral capillaries, and the flow across into the CSF is balanced by the return of CSF to the plasma compartment. CSF flows through the ventricles and the subarachnoid space at the rate of about 0.3 ml per minute. Neurons and glia contribute to the maintenance of normal hydrostatic pressures through the activity of their membrane ion transporters (see next spread). CSF flow is turned over about three times in 24 hours. CSF provides not only a buffering and cushioning system for brain, but also carries many substances such as trophic factors and nutrients.

venous sinu:

cerebral hemisphere lateral ventricle.

third ventricle.

venous sinu: cerebellu fourth ventricle.

foramen of. Magendie spinal cord bathed in CSF

lumbar, cistern interventricular foramen choroid plexus venous sinu:

cerebral hemisphere lateral ventricle.

third ventricle.

venous sinu: cerebellu fourth ventricle.

foramen of. Magendie spinal cord bathed in CSF

lumbar, cistern

Netcerebral

interventricular foramen choroid plexus choroid plexus ubarachnoid space choroid plexus ubarachnoid space cerebral capillary epithelium

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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