Cerebrospinal fluid, its formation and circulation

Introduction of CSF

  • It is a clear, colorless liquid and an ultra-filtrate of blood.
  • It is present in the sub-arachnoid space of the meninges which provides a special environment in which the brain floats.
  • It cushions the brain from hard blows and sudden movements.
  • The weight of brain floating in CSF is only about 14% of its actual weight.
  • It also maintains chemical environment of the central nervous system other than providing buoyancy to the brain.
  • Also, unwanted substances and excess components are conveyed away from the extracellular fluid and into the venous portion of the blood circulatory system by it.

Formation of CSF

  • It is formed at the choroid plexuses of the lateral, third and fourth ventricles by a combination of diffusion and active transport.
  • Choroid plexus is formed by the ependymal cells that line the ventricles and the pia mater.
  • It is supplied richly by blood vessels and has intricate networks of capillaries.
  • It is considered to be components of blood brain barrier.
  • Blood vessels of choroid plexus are selectively permeable that makes it able to produce CSF.
  • It does not allow blood cells and largest protein molecules to pass into the ventricles.
  • But it permits some traces of proteins, oxygen and carbon dioxide in solution, various minerals like sodium, potassium, calcium, magnesium and chloride ions, glucose and a few white blood cells.
  • The total volume of CSF i.e. about 125ml in adult is formed and renewed about three times a day.

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Circulation of CSF

  • CSF moves from the ventricles inside the brain to the sub-arachnoid space outside the brain normally.
  • It flows slowly from the two lateral ventricles of the brain, where much of the fluid is formed.
  • From there it reaches the third ventricle through the paired inter-ventricular foramina.
  • Finally it reaches the fourth ventricle through the cerebral aqueduct.
  • The fluid leaves the ventricular system through three apertures (two lateral and one median) in the roof of fourth ventricle.
  • The fluid oozes into the sub-arachnoid space (the cisterna magna) behind the medulla oblongata from there.
  • Some CSF slowly makes its way down the spinal cord to the lumbar cisterna.
  • However, most of the fluid circulates slowly towards the top of the brain through the sub-arachnoid space.
  • Arachnoid villi contain pressure sensitive valves. These valves permits a one-way flow of CSF from the sub-arachnoid space to the superior sagittal sinus.
  • This sinus drains venous blood from the brain.
  • The valves open if pressure of the fluid inside the sub-arachnoid space exceeds that of the blood in the sinus.
  • The valves close if the pressures in these two channels are equal or reversed.

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i) https://www.physio-pedia.com/CSF_Cerebrospinal_Fluid

ii) https://teachmephysiology.com/nervous-system/components/cerebrospinal-fluid/

Cerebrospinal fluid, its formation and circulation