Cerebrospinal Fluid

CEREBROSPINAL FLUID (CSF)

• Clear, colorless, almost protein free filtrate/transduate of blood.

LOCATION:

In brain:

• Around brain  – In subarachnoid space.
• Inside brain – In ventricles.

In Spinal cord:

• Around spinal cord – In subarachnoid space.
• Inside spinal cord – In its central canal.

FORMATION:

• Major source of CSF – 
• Choroidal plexus of all 4 ventricles.
• Mainly two lateral ventricles.
• Other sources – 
• Ependymal cells of ventricles & brain itself, via perivascular spaces.
• Total CSF volume in adult – 125ml – 150 ml.
• Rate of CSF formation – 500-550 ml/day.
• CSF production – 0.3 – 0.35 ml/minute.
• CSF replaced 3-4 times every day.

• Choroid epithelium secretes sodium ions by active transport.
• With help of Na2+– K+– ATPase. 
• Na2+ entry generates an electrical gradient —> Cl– move into CSF passively.
• Movement of sodium & chloride ions raises ventricular fluid osmotic pressure —-> Resulting in passive water flow into ventricles.
• Potassium & bicarbonate ions – Actively transported out of CSF into bloodstream.
• HCO–3 enters CSF – With help of “Carbonic anhydrase” enzyme.
• Glucose enters CSF – Via “Facilitated diffusion” mediated by “GLUT-1”. 

• CSF Osmolarity & Na2+ concentration “Same” as of plasma.
• CSF Glucose & K+ ion concentration is lower than plasma.
• CSF glucose level is 2/3 of plasma glucose.
• CSF H+ ion concentration is slightly higher than plasma.
• Ie., pH is slightly low.
• Due to brain’s glucose utilization.
• CSF Cl– ion concentration is higher than plasma.

CIRCULATION:

From lateral ventricles, CSF flows through interventricular foramina of monro into third ventricle.

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Continues to flow through “Aqueduct of Sylvius” in midbrain into fourth ventricle.

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Finally, flows out of fourth ventricle through a median (magendie) & two lateral (Luschka) formina.

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Ultimately, circulates in subarachnoid space around brain & spinal cord.

• Note: CSF does not pass through Epidural space.

ABSORPTION:

• Absorbed from subarachnoid space by arachnoid villi (arachnoid granulation tissue).
• 80% in cerebral arachnoid villi & 20% in spinal arachnoid villi into Subdural venous sinuses.
• CSF is partly absorbed by lymphatics around cranial nerves I, II, VII, VIII.

Factors influencing absorption:

• Plasma colloidal osmotic pressure & Hydrostatic pressure.

1. Plasma colloidal osmotic pressure –

• 25 mm Hg higher than that of CSF.
• Because CSF does not have plasma proteins.

2. CSF Hydrostatic pressure –

• 0.5-5 mm Hg higher than in subdural venous sinuses.
• Hence, providing an additional force favouring CSF absorption.

CSF PRESSURE:

• Measured by lumbar puncture.
• Obtained from space between Arachnoid & pia mater.

• Pressure recorded – Higher in sitting position.
• One may add 200 mm CSF/water to pressure in sitting posture.
• Normal range during sitting – 265-400 mm CSF/water (20-30 mm Hg).

Pressure regulating factors:

Major regulator:

• “Rate of CSF absorption” by arachnoid villi.
• Approximately 500-550 ml per day.
• CSF absorption directly proportional to pressure. 
• On normal & continuous CSF production, circulation & absorption cycle –
• Regulation of CSF volume & pressure in skull is at constant level.
• At Average CSF pressure –
•  CSF Filteration & absorption equals – Ie., 112 mm H2O.

At Lowered CSF pressure – 

CSF Absorption stops – Ie., Approximately 68 mm H2O

• Variations:
• 1. Defective CSF absorption/drainage → Increases intracranial pressure (CSF pressure in brain).
• 2. Excessive CSF drainage → Decreases CSF volume.
• Causes severe headache.
• Eg: In Continuous leak after lumbar puncture.
• Important metrics to note:
• CSF Specific gravity – 1.007.
• Ie., Density close to brain tissue.

Exam Question 

CEREBROSPINAL FLUID

• CSF major source – Choroidal plexus.
• Other sources of CSF are ependymal cells of the ventricles & brain itself, via perivascular spaces.
• CSF pressure is regulated by “Rate of CSF absorption” by arachnoid villi.
• CSF Total volume in an adult –  125 – 150 ml.
• Rate of CSF formation – 500-550 ml/day.
• CSF production per minute – 0.3 – 0.35 ml/minute.
• Each of CSF constituents is actively transported.
• CSF circualtes from lateral ventricle into 3rd ventricle and ultimately into subarachnoid space.
• CSF is absorbed from subarachnoid space by arachnoid villi (arachnoid granulation tissue) into Subdural venous sinuses.
• CSF is partly absorbed by lymphatics around cranial nerves I, II, VII, VIII.
• CSF does not pass through Epidural space.
• CSF Pressure – Measured through a lumbar puncture.
• CSF is obtained from space between Arachnoid & pia mater.
• About 150 mm of water/CSF in supine (horizontal).
• Range – 65-200 mm water/CSF (5-15 mm Hg).
• CSF pressure is regulated by rate of CSF absorption by arachnoid villi.
• Decreased CSF volume results in severe headache in case of continuous leak after lumbar puncture.
• Process of CSF absorption stops, below pressure of 68 mm H2O.
• Choroid epithelium secretes sodium ions by active transport.
• Osmolarity & sodium ion concentration of CSF – “Same” as that of plasma.
• CSF Glucose & K+ ion concentration is lower than plasma.
• CSF H+ ion concentration is slightly higher than plasma.
• Ie., pH is slightly low.
• CSF Cl- ion concentration is higher than plasma.
• β – 2 transferrin is found only in CSF; Hence, “Specific CSF marker”.
• CSF Specific gravity – 1.007