CSF Formation

90% of the CSF is produced by the choroids plexus epithelium which is found in all of the ventricles. The other 10% is felt to come from brain interstitial fluid. CSF is a very dilute fluid with 99% of it being water. The first step in CSF formation is filtration of intravascular fluid across the open junctions of the choroid plexus capillary endothelium. Water, sodium, bicarbonate, and chloride are then moved across the choroid plexus epithelium by a secretory process. This secretion occurs by creating ion gradients. In the epithelial cell, carbonic anhydrase breaks down water and carbon dioxide to form a hydrogen ion and bicarbonate ion. On the blood interfacing side of the cell, the hydrogen ion is exchanged for a sodium ion and bicarbonate ion is exchanged for a chloride ion. These ions then move across the cell and on the CSF side of the cell, the sodium ion is exchanged for a potassium ion and then both chloride and bicarbonate ions are secreted into the CSF. Water follows the ion gradient across the cell and into the CSF.  CSF is formed at the rate of .35 ml/min. The CSF volume for the average adult is 90-150 ml. From the observed rate of formation and the CSF volume, it is estimated that the CSF turns over 4-5 times in a 24 hour period.

Barrier Systems of the Brain

The brain requires a much narrower range of chemical and nutrition fluctuation than what the other organs of the body can tolerate. The barrier systems of the brain help maintain this required privileged homeostasis. These barriers are created by tight junctions that are found at the following locations:
1. Cerebral capillary endothelium- this creates the blood-brain barrier.
2. Choroid plexus epithelium- this creates the blood-CSF barrier.
3. Arachnoid membrane- this creates the CSF-arachnoid barrier.
4. There is a fourth barrier system that consists of restricted specialized ependymal cells that are located in the 3^rd and 4^th ventricles adjacent to the circumventricular organs (CVO). These CVOs are have open junctions in their capillary endothelium so that the brain can sample what is occurring in the systemic circulation and respond with the appropriate control.

Because of the bood-brain and blood-CSF barriers, the interstitial fluid and the CSF are tightly regulated. It is important to note that the ependymal lining of the ventricles has open junctions. This allows for communication between the CSF and contiguous interstitial fluid. Because there are no lymphatics in the brain this communication allows for the CSF to become a sink and aid the process of removing brain metabolites.

Physiologic Function of the Choroid Plexus

As noted above the choroid plexus produces most of the CSF. Because of the tight junctions of the choroid plexus epithelium there is restricted passive diffusion. The concentration of a protein in the CSF is inversely proportional to its molecular size and the protein content of CSF is only 1%. Also lipophilic substances diffuse across the epithelium more readily than hydrophilic substances.

Another important function of the choroid plexus epithelium is facilitated transport of glucose and certain amino acids. This is rapid, energy free transport by a transport protein with a “down hill” gradient, i.e. the concentration in the CSF is less than in the plasma.

Summary of Function of the CSF

The basic function of the CSF can be summarized as three fold:
1. It helps to preserve homeostasis for the brain in a more restricted, narrower range than what the other organs of the body require.
2. It provides a mechanical protection for the brain. The brain essentially floats in the CSF. Because of this, the effective weight of the brain is reduced 30 fold- so the effective weight of a 1500 gram brain suspended in CSF is 50 grams.