CSF formation: CSF is formed in the choroid plexus of the cerebral ventricles (70%) and across the pial and ependymal surfaces (30%) at a rate of 0.4 mL/minute. The total volume of CSF is between 100 and 150 mL.
CSF is an ultrafiltrate of plasma whose final composition is modified by active (mainly Na) and passive (most importantly glucose) transport of ions and other metabolites (Table-1). Proteins and other hydrophilic molecules are poorly permeable and excluded from the CSF. The capillary endothelium in the choroid plexus is freely permeable to substances. The epithelial cells of the choroid plexus contain tight junctions and are the site of the blood-CSF barrier.
CSF formation is reduced by (a) decreased choroidal blood flow and capillary hydrostatic pressure, (b) hypothermia, (c) increased serum osmolarity, and (d) increased ICP.
CSF flows through the ventricles and out to the subarachnoid space of the brain and the spinal cord. It is reabsorbed into venous blood via the arachnoid villi. If reabsorption is impeded, CSF builds up and ICP increases.

 

Table-1. Composition of cerebrospinal fluid and serum in humans
Component	CSF	Serum
Sodium (mEq/L)	141	140
Potassium (mEq/L)	2.9	4.6
Calcium (mEq/L)	2.5	5.0
Magnesium (mEq/L)	2.4	1.7
Chloride (mEq/L)	124	101
Bicarbonate (mEq/L)	21	23
Glucose (mg/100 mL)	61	92
Protein (mg/100 mL)	28	7000
pH	7.31	7.41
Osmolality (mosmol/kg H2O)	289	289

Blood-brain barrier: The BBB isolates the brain from substances in the plasma and is important for normal brain function.
The site of the BBB is the capillary endothelial cells. They are connected to each other via tight junctions that exclude the passage of substances between them. Any substance that crosses into the brain from the blood must cross the capillary epithelial cell. The tight junctions and therefore the BBB are not present in the choroid plexus and certain other small areas of the brain (e.g., restricted areas of the hypothalamus).
Water, gases, and lipophilic substances are freely permeable to the BBB. Proteins and polar (hydrophilic) substances are poorly permeable to the BBB and cross this barrier only if there is a specific transport system for them. Glucose, ions, and certain amino acids are transported across the BBB. Because glucose is passively transported to and metabolized in the CNS, its concentration in the brain is usually 60% of its plasma level. This transport is saturable, so that large changes in glucose concentrations require time to equilibrate (Table-1)