CSF is a clear, plasma-like fluid that circulates around the outside of the brain, in cavities within the brain (ventricles), and in the space surrounding the spinal cord.
Most CSF originates in the choroid plexus. The choroid plexus is composed of a mass of tiny blood vessels that are located in the third lateral and fourth ventricles. The remaining CSF, about 30%, is formed in other sites such as the subarachnoid space and the ependymal lining of the ventricles.
The volume of CSF in an adult is approximately 90-150 mL of CSF. Babies up to about four weeks of age have an average volume of 10 - 60 mL of CSF.