That would be the concentration gradient of hydrogen ions(protons). See the attached pic:)
Quite simple really.... The CNS actually doesn't receive as much nutrients or key components from the blood... The blood actually dumps it into the Cerebrospinal fluid. The Cerebrospinal fluid therefore is directly in contact with the brain and if you want to find an infection you take from a source as close to thing as you can.
If you look at the pH scale hydrogen are going to be on your acidic side and your oh is going to be on your base side