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Describe the formation, circulation and functions of cerebrospinal fluid (2/2008 80%)
Describe the formation, flow and absorption of cerebrospinal fluid. (1/2007 29%)
CSF ( cerebral spinal fluid) is a specialised extracellular fluid in the ventricles and subarachnoid space.
Formation
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CSF is formed in the brain and circulates through subarachnoid space and ventricular system
(2 lateral ventricles, 3rd ventricle, aqueduct of Sylvius, 4th ventricle and central canal of spinal
cord)
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~150ml of CSF in the system
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500ml/day of CSF produced
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70% produced by choroid plexuses ( rich network of blood vessels covered by ependymal
cells projecting into the ventricles)
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Found in the:
 Temporal horns of the lateral ventricles
 Posterior portion of the third ventricle
 Roof of the fourth ventricle
 Capillary endothelium of choroid plexus is fenestrated plasma in the
capillaries is filtered across the endothelium forming a protein-rich fluid in
the choroid plexus stroma
 Secretes Na+ by active transport, which pulls Cl- and water, so forming the
majority of the fluid
 Glucose enters by facilitated transport which becomes saturated when
plasma glucose >15-20mmol/L
 Potassium and Bicarbonate are moved out of the CSF back into the vascular
space
 HCO3- and H+ are formed within the epithelial cell by the interaction of CO2
and H2O catalysed by CA
30% from endothelial cells lining the brain capillaries
Composition
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Mg 1.12mmol/L; CSF/plasma ratio =1.4 >plsama
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Cl 124mmol/L, CSF/plasma ratio =1.14 >plasma
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K 2.9mml/L, ratio 0.67 < plasma
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Ca 1.15mmol/L ratio 0.7 < plasma
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Glucose 3.7mmol/L ratio 0.82 < plasma
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Protein 0.18g/L, ratio = 0.002 < plasma
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Na 140mmo/L ~= plasma
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pH 7.32
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Protein concentrations ~0.5% of respective plasma protein concentration. Lowest in
ventricles (26mg/100mL), intermediate in cistern magna (32mg/100mL), highest in lumbar
sac (42mg/100mL). Due to decrease clearance of protein by dural sinuses in the lumbar sac
Circulation
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Circulates through ventricular system and subarachnoid space from formation sites to
absorption sites
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CSF hydrostatic pressure = 5-15mmHg
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Ciliary movements of the ependymal cells propel CSF. Resp oscillations and arterial pulsation
of cerebral arteries also provide momentum for movment
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CSF formed in ventricles, move towards 4th ventricle and foramina of Luschka and magendie
then into the cistern magna
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From cistern Magana passes superiorly into SA sapce around cerebellar hemisphere;
caudally into spinal SA space and cephalad to basilar cisterns
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From basilar cisterns CSF flows through prechiasmatic cistern and Sylvaina fissure to lateral
and frontal cortical regions + via a second route to the medial and posterior cerebral cortex
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Lateral ventricles  foramen of Monro  3rd ventricle  4th ventricle  foramen of
Luschka/magendie  cistern magna  superior to SA space around cerebellum; caudaully
to spinal SA space; cephald to basilar cisterns  via prechiasmatic cistern/ Sylvania fissure
 lateral & frontal cortical region & medial & post cerebral cortex
Reabsorption
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Reabsorbed into venous blood via intracranial arachnoid villi (90%) within the dural walls of
the sagittal and sigmoid sinuses + spinal arachnoid villi (10%) within the dural walls of the
dural sinusoids on dorsal nerve roots
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When CSF pressure – venous sinuses pressure >1.5mmHg  CSF flows into cerebral venous
sinuses (mean CSF pressure =15cmH2O = 11.25mmHg; superior sagittal sinus = 9cmH2O =
6.75mmHg)
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CSF transported across endothelium of arachonoid villi by pinocytosis+ etracellular route by
opening of intercellular spaces.
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CSF formation and reabsorption usually is in equilibrium
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Rate of formation remains constant until ICP high enough to reduce CPP to <70mmHg 
decrease production
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CSF reabsorption rate increases with CSF pressure.
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At ICP 7mmHg – 22.5mmHg CSf reabsorption increases in liner fashion
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If ICP <7mmHg minimal reabsorption occurs
Functions:
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Cushioning effect with buoyancy
o
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Brain and CSF has same low specific gravity (1.007), therefore brain is suspended,
floating with zero buoyancy within the CSF
When the head moves, the brain floats and moves along with the skull, prevents
deformation and damage by acceleration or deceleration forces associated with
head movements
o
Effective weight of brain is reduced from 1400gm to 47gm
Maintenance of ionic environment
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Neuronal tissues are highly sensitive to ion changes
o
CSF provides a constant chemical environment for neuronal activity
o
Ca2+, K+ Mg2+ and HCO3- are transferred into CSF by active pumps
o
H+ and Cl- are transferred by secondary mechanism
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Removal of toxic substances from brain
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Acid-base regulation
o
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Acid base status of CSF influences respiratory control
Nutritional and intracerebral transport
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Sugars, basic and neutral amino acids are transported by active transport between
blood and ECF compartment of the brain
o
O2 and CO2 diffuse freely across the BBB
Neuropeptides secreted into the CSF are carried from one region to another