What are the functions of cerebrospinal fluid?

Cerebrospinal fluid (CSF) is the transcellular fluid located within the cerebral ventricles and subarachnoid space that bathes the brain and spinal cord. CSF has a number of functions:

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  Buoyancy and cushioning. The adult brain weighs around 1400 g. However, when suspended in CSF, the brain has an effective weight of less than 50 g. Sudden head movement produces potentially damaging acceleration and deceleration forces – the lower effective weight of the brain reduces its inertia, protecting it from damage. CSF also cushions the brain, protecting it from damage, especially from the ridged skull base.

  
  
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  Maintenance of a constant ionic environment. Neurons are highly sensitive to changes in their external environment; maintaining a constant ionic and osmotic environment is essential for normal neuronal activity.

  
  
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  Buffering changes in intracranial pressure (ICP). Displacement of CSF from the cranium is an important, but limited, compensatory mechanism that occurs following an increase in ICP (see Chapter 49).

  
  
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  Control of respiration. As a small, lipid-soluble molecule, CO₂ can freely diffuse from the blood to the CSF. The CSF has a much lower protein concentration than plasma and therefore has a reduced buffering capacity, making the CSF pH very sensitive to changes in blood PCO₂. The central chemoreceptors detect changes in CSF pH, causing the respiratory centre to make corresponding adjustments in V̇_E (see Chapter 22).

  
  
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  Glymphatic system. Outside the central nervous system (CNS), the lymphatic system is responsible for removing extracellular proteins, excess fluid and some metabolic waste products. The brain also produces waste products that need to be cleared, but it lacks a lymphatic system. Instead, the glymphatic system allows CSF to circulate in paravascular channels between the blood vessels and the astrocyte foot processes, where it collects and removes waste products. The name ‘glymphatic’ comes from astrocytes (a type of glial cell) performing the role of the lymphatic system in the CNS.

Where is CSF produced?

CSF is produced by the choroid plexus, located in the ventricles of the brain: the two lateral ventricles, third ventricle and fourth ventricle. CSF is produced by a combination of filtration and active secretion of water and solutes at a rate of 0.3 mL/min, equivalent to 500 mL/day. The choroid plexus is formed by modified ependymal cells, ciliated cells that line the surface of the ventricles of the brain and the central canal of the spinal cord. Ciliary action propels CSF through the ventricles:

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  From the lateral ventricles, CSF flows through the two foramina of Monro into the third ventricle, located between the right and left thalamic nuclei.

  
  
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  CSF travels through the aqueduct of Sylvius, located within the midbrain, to the fourth ventricle, located within the pons.

  
  
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  From the fourth ventricle, CSF flows into the subarachnoid space via the two lateral foramina of Luschka and the midline foramen of Magendie.¹ Most of the CSF flows around the cerebral hemispheres, whilst the remainder flows around the spinal cord.

Overall, the total volume of CSF is 100–150 mL, around half of which is located within the ventricular system and half is located within the subarachnoid space.