SN4 SHORT NOTE 5M ★ High-yield

Describe the formation, circulation and functions of cerebrospinal fluid. Give its normal values.

Physiology · 1st Professional — Pre-Clinical

Normal Values

Cerebrospinal fluid = a clear, colourless fluid that surrounds and cushions the brain and spinal cord.

Parameter	Normal value
Total volume	150 mL (25 mL in ventricles)
Rate of formation	500–550 mL/day (0.35 mL/min)
Turnover	Replaced 3–4 times a day
Pressure (lateral recumbent)	60–150 mm H2O
Protein	15–45 mg/dL
Glucose	45–80 mg/dL — about two-thirds of blood glucose
Cells	0–5 lymphocytes/µL; no neutrophils or RBC
Chloride	120–130 mEq/L — higher than plasma

Formation and Circulation

Formed by the choroid plexus (70%) by active secretion, and by the ependyma and brain capillaries (30%)
It is an active secretion, not a filtrate — hence its composition differs from plasma

Lateral ventricles → Foramen of Monro → Third ventricle → Aqueduct of Sylvius → Fourth ventricle → Foramina of Luschka and Magendie → Subarachnoid space → Arachnoid villi → dural venous sinuses

Absorption through the arachnoid villi is by bulk flow, and depends on CSF pressure exceeding venous pressure

Functions

Mechanical protection — a water cushion; buoyancy reduces the effective weight of the brain from 1400 g to about 50 g
Maintains a constant intracranial pressure — the Monro–Kellie doctrine: brain + blood + CSF must total a constant volume
Nutrition and removal of metabolites
Chemical stability of the neuronal environment
Central chemoreceptor function — CSF H+ drives respiration
Route for hormones and neuromodulators

CSF in Disease

Condition	Appearance	Cells	Protein	Glucose
Pyogenic meningitis	Turbid	↑↑ Neutrophils	↑↑	↓↓
Tuberculous meningitis	Clear; cobweb coagulum	↑ Lymphocytes	↑↑	↓
Viral meningitis	Clear	↑ Lymphocytes	Normal or ↑	Normal
Subarachnoid haemorrhage	Blood-stained, xanthochromic	RBC	↑	Normal
Guillain–Barré	Clear	Normal	↑↑	Normal

The last pattern is albuminocytological dissociation — high protein with a normal cell count

Applied Aspects

Lumbar puncture is done at L3–L4 or L4–L5, below the termination of the cord at L1–L2
Contraindicated in raised intracranial pressure with papilloedema — risk of coning (tonsillar herniation)
Hydrocephalus — communicating (impaired absorption) or obstructive (block within the ventricular system, commonly at the aqueduct)
Post-LP headache — from continued leak; treated with bed rest, fluids and a blood patch

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