But for any nutrients to enter and leave the CSF, and the brain itself for the matter, they have to go through the tightly regulated by the blood-brain barrier.
This is constantly replenished, with around 500 ml of new CSF produced everyday and the excess, or 500 minus 150 mL or 350 mL, is absorbed into the blood.
The CSF fills the space that surrounds the brain and wastes from inside their brain make their way out to the CSF which gets dumped along with the waste into the blood.
For example, it could be a congenital defect like spina bifida, or an acquired one like a skull fracture, where there might be CSF leaking through the sinuses.