A. Description, Purpose, and Composition
CSF is a clear, colorless fluid that fills subarachnoid space, cisterns, and ventricles. It's pH is slightly lower than the pH of blood (7.33 vs. 7.36-7.44). It contains very few white blood cells (less than 3 lymphocytes/cc). CSF also has lower glucose and much lower protein concentration, but a higher NaCl concentration than plasma. The purpose of CSF is to preserve homeostasis in the central nervous system. It also cushions the brain; providing buoyancy, protection from impact, and weight reduction. Waste products produced by the brain are collected and removed through the CSF which also provides a source of distribution for neuroactive substances.
a. Site of production
1) Choroid plexus (70%) - extensive capillary network supplies cells of choroid; CSF from choroid plexus secreted into ventricles
2) Other (30%) - metabolic water production (water as a product of various chemical reactions)
1. At any given time, total volume of CFS is about 100-150 mls in ventricles, cisterns, and subarachnoid space
2. Turnover rate is high, total volume produced daily averages 500 mls (~21 ml/hr)
3. Blood-CSF barrier (BBB) - substances in blood cannot access CSF. Tight junctions between capillary endothelial cells prevent substances (e.g. macromolecules) from passively diffusing from blood to CSF.