Question 15

College Answer

This question was generally well answered. Better answers noted production including an amount, site and mechanism. Similarly, absorption included the site, the rate and factors which affect the rate. The electrolyte and pH and how they compare to extracellular fluid should have been included in the section on composition.

Discussion

• Production:  
  • 400-600ml/day, by highly regulated ultrafiltration and active secretion
  • Ultrafiltrate of plasma is formed by the fenestrated choroidal capillaries
  • It collects in the choroid interstitial space
  • Ions are actively transported out of this ultrafiltrate, and into the CSF:
    • Sodium is actively secreted from the apical membrane (Na/K/ATPase)
    • Carbonic anhydrase also provides H⁺ to power Na/H⁺ exchange at the basal membrane
    • The sodium gradient created and maintained in this way is then used to co-transport HCO₃- and CL^-  into CSF
  • This creates an osmotic gradient which pulls water across the membrane through aquaporin channels
  • This process is disconnected from ultrafiltration, i.e. CSF production is constant and is not pressure-dependent (though at low CPP, <55 mmHg, CSF production decreases)
• Absorption:
  • Absorbed at 25ml/hr (CSF secretion rate = reabsorption rate) into the dural venous blood, mainly from arachnoid granulations
  • Driving force is hydrostatic gradient between CSF and venous blood
  • Thus, CSF reabsorption stops when CSF pressure is less than ~ 7 cm H₂O
• Composition:
  • CSF contains minimal protein (~0.2g/L)
  • CSF sodium potassium and calcium are slightly lower than plasma
  • CSF chloride CO2 and bicarbonate are higher than plasma
  • CSF glucose is about 2/3rds of the plasma value
• Function:
  • Barrier function (the blood-CSF barrier)
  • Chemical stability and waste removal
  • Buoyancy and mechanical cushioning of the CNS
  • Hydraulic pressure buffering of ICP with arterial pulse and respiration

Sakka, Laurent, Guillaume Coll, and Jean Chazal. "Anatomy and physiology of cerebrospinal fluid." European annals of otorhinolaryngology, head and neck diseases 128.6 (2011): 309-316.

Segal, Malcolm B. "The choroid plexuses and the barriers between the blood and the cerebrospinal fluid." Cellular and molecular neurobiology 20.2 (2000): 183-196.

Brown, P. D., et al. "Molecular mechanisms of cerebrospinal fluid production." Neuroscience 129.4 (2004): 955-968.