Cerebrospinal Fluid (CSF): Production, Circulation, and Absorption
1. Introduction
Cerebrospinal fluid (CSF) is a clear, colorless fluid that surrounds the brain and spinal cord, providing mechanical protection, metabolic support, and waste removal.
• Total CSF Volume: 150 mL
• Daily CSF Production: ~500 mL/day
• CSF Turnover Rate: Replaced ~3-4 times per day
- CSF Pressure: 5-15 mmHg (similar to ICP)
2. CSF Production
A. Site of Production
• Choroid Plexus (70%) – Located in lateral, third, and fourth ventricles
• Ependymal Cells & Interstitial Fluid (30%) – Contribute minor amounts
B. Mechanism of Production
CSF is formed by active transport and ultrafiltration of plasma through the choroid plexus epithelium, which has tight junctions forming the blood-CSF barrier.
• Active transport: Na⁺, Cl⁻, HCO₃⁻ move into CSF → water follows passively
• Ultrafiltration: Small molecules and ions passively diffuse
C. Composition of CSF (Compared to Plasma)
|
Component |
CSF Value |
Plasma Value |
Remarks |
|
pH |
7.33 |
7.40 |
Slightly more acidic |
|
Glucose |
50-80 mg/dL |
80-120 mg/dL |
~2/3 of plasma glucose |
|
Protein |
15-45 mg/dL |
6-8 g/dL |
Much lower than plasma |
|
Sodium (Na⁺) |
138-150 mEq/L |
135-145 mEq/L |
Similar to plasma |
|
Potassium (K⁺) |
2.5-3.0 mEq/L |
3.5-5.0 mEq/L |
Lower than plasma |
|
Chloride (Cl⁻) |
120-130 mEq/L |
100-110 mEq/L |
Higher than plasma |
3. CSF Circulation
1. Lateral ventricles (Choroid plexus)
↓ Through Foramen of Monro
2. Third ventricle
↓ Through Cerebral aqueduct (Aqueduct of Sylvius)
3. Fourth ventricle
• Foramen of Luschka (Lateral) → Subarachnoid space
• Foramen of Magendie (Midline) → Cisterna magna
4. Subarachnoid space (Around brain and spinal cord)
5. Absorption into venous system via arachnoid granulations
4. CSF Absorption
A. Sites of Absorption
• Arachnoid granulations (Main site) – Located in superior sagittal sinus
• Spinal arachnoid villi – Minor role in absorption
B. Mechanism of Absorption
• Bulk flow across arachnoid villi into venous sinuses
• CSF is absorbed passively when CSF pressure > venous pressure (>5 mmHg)
• If CSF absorption is impaired, it leads to hydrocephalus
5. Clinical Correlations
A. CSF Disorders
|
Condition |
Cause |
CSF Changes |
|
Hydrocephalus |
Blocked CSF flow or impaired absorption |
Increased CSF volume → ICP rise |
|
Meningitis |
Infection (bacterial, viral, fungal) |
↑ WBCs, ↑ protein, ↓ glucose |
|
Subarachnoid Hemorrhage (SAH) |
Ruptured aneurysm |
Xanthochromia, ↑ RBCs |
|
CSF Leak (Post LP, Skull Fracture) |
Dural tear |
Low-pressure headache |
B. Lumbar Puncture (LP) & CSF Analysis
• Site: L3-L4 or L4-L5 (below spinal cord termination)
• Indications: Infection, SAH, idiopathic intracranial hypertension
• Contraindications: Raised ICP (risk of herniation), coagulopathy