Cerebrospinal fluid
[KB-fluid:3.6;WG21:p614-620]
A type of transcellular fluid
Function
- Act as cushion
--> Protection from injury
- Water bath effect
--> Brain effectively weights only 50g
NB:
- No lymphatic channels in brain
- CSF fills the role of returning interstitial fluid and protein back into circulation
Location
In the ventricles and subarachnoid space, around the brain and spinal cord
Within the meninges
Quantity
Total volume: 150mLs
Daily production: 550 mLs/day
--> turnover of 3-4 times/day
Circulation
- Formed by
* Choroid plexus (50-70%)
* Ependyma of the walls of the ventricles (30-50%)
- CSF flows through the foramens of Magendie & Luschka
--> Into the subarachnoid space of the brain and spinal cord
- Absorption by
* Arachnoid villi (90%)
* Directly into cerebral venules (10%)
Relationship with ICP
- Normal ICP: 5 - 15 mmHg
- Rate of CSF formation
--> Constant, not affected by ICP
- Absorption of CSF
--> Increases linearly as ICP increases above 68 mmCSF
- At a pressure about 11 cmH2O (or 112mmCSF, 8mmHg)
--> Rate of secretion = absorption
NB:
- 1 mmHg = 1.36 cmH2O
- 1 cmH2O = 0.735mmHg
Composition of CSF
- Identical to brain ECF
- Different from plasma
Difference between CSF and plasma
[WG21:p616]
In CSF
- pCO2 is higher (50mmHg)
--> Lower CSF pH (7.33)
- Protein content low
* 0.2g/L (compare to 60g/L in plasma)
--> Low buffering capacity
- Glucose concentration is lower
* 64% of plasma level
- Cholesterol content is very low
* 0.1% of plasma level
- Chloride concentration is slightly higher
* 113 meq/kg H2O
- Potassium concentration is lower
* 2.9 meq/kg H2O or 60% of plasma level
- Bicarbonate level is about the same
- [Ca2+] is lower
- [Mg2+] is higher
NB:
- Na+ is actively transported into CSF at the apical membrane of ependymal cells
- Ependymal cells also contain carbonic anhydrase and produce HCO3- into CSF to provide buffering