2011B13 Outline the physiological role of cerebrospinal fluid, including a description of its production and fate.

Location

·   150mL total

·   50% in the ventricles

·   50% in the subarachnoid space of cerebral cortex and spinal cord

Production

·   By choroid plexus of first, second, third ventricles and ependymal cells

·   500mL/day, constant production if cerebral perfusion pressure >70mmHg

·   Complete turnover every 24 hours

Circulation

·   Due to pulsatile flow in choroidal arteries

·   Lateral -> third -> fourth

·   Fourth -> around cerebral hemispheres via foramen of Magendi and foramina of Luschka

·   Fourth -> down central canal of spinal cord

Reabsorption

·   90% into arachnoid villi superior sagittal sinus

·   10% Into spinal arachnoid villi -> epidural venous sinuses

·   Minimal below ICP 7mmHg. Linear increase from ICP 7-23mmHg.

Graph

Note: X axis unit mmCSF not mmHg

Note: Y axis zero point not at the X axis

(Ganong’s Review of Medical Physiology)

Physical protection

·   Cushion surrounding the brain

·   Low specific gravity due to low protein content (1.005 cf. 1.03 Plasma)

·   Reduces effective weight of brain from 1.4kg -> 40g

·   Hence ↓inertia, ↓risk of injury if sudden change of direction

ICP buffer

·   ↑ICP -> spinal displacement of CSF

Electrolyte homeostasis

·   BBB and B-CSF-B are impermeable to most ions

·   Active transport maintains concentrations.

·   Stable ionic environment essential for brain electrical activity

Acid-base homeostasis

·   BBB and B-CSF-B are impermeable to H⁺ and OH^-

·   Stable pH important for brain activity e.g. during intense exercise with lactic acidosis

Control of breathing

·   CO₂ can cross BBB, H+ cannot

·   CO₂ plasma -> CSF -> brain ECF

·   CO₂ + H₂O <-> H⁺ + HCO₃^-

Nutrient and waste transport

·   Active transport between blood <-> CSF <-> brain ECF

Plasma

CSF

Appearance

Yellowish

Clear, colourless

SG

Whole blood 1.06

Plasma 1.03

1.005

Osmolality

289

289

Na⁺

140

140

K⁺

4

2.4

Cl^-

100

114

HCO₃^-

24

24

Mg²⁺

1

Higher

Ca²⁺

2.4

1.2 (50%)

Glucose

5

3 (60%)

Protein

70g/L

0.2g/L

pH

7.4

7.32

pCO₂

40

50