Cerebrospinal fluid

CSF is produced at ≈500 mL/day, mainly by the choroid plexus (≈80%) and partly as interstitial fluid crossing the ependyma. Total volume is only ≈150 mL, ≈25 mL in the ventricles and ≈125 mL in the subarachnoid space, so it is replaced 3–4 times a day. It circulates in one direction, propelled by continuous secretion, arterial pulsations and ependymal cilia:
1. lateral ventricles;
2. interventricular foramina;
3. third ventricle;
4. cerebral aqueduct;
5. fourth ventricle;
6. median and lateral apertures;
7. subarachnoid space (cisterns, over the convexities and around the spinal cord);
8. arachnoid granulations, into the dural venous sinuses, chiefly the superior sagittal sinus.
A smaller proportion drains along cranial and spinal nerve sheaths and through the cribriform plate to lymphatics, and through recently described dural (meningeal) lymphatic vessels.

CSF is not a simple filtrate of plasma. Compared with plasma it has:
• very little protein (≈0.15–0.45 g/L, versus ≈70 g/L);
• about 60% of the plasma glucose;
• higher Cl⁻ and Mg²⁺;
• lower K⁺, Ca²⁺ and HCO₃⁻;
• a slightly lower pH (≈7.33);
• almost no cells (≤5 lymphocytes/µL, no red cells).
Because CSF is poorly buffered, its H⁺ concentration follows arterial PCO₂ closely. This is how central chemoreceptors in the medulla drive breathing in response to CO₂.