Which chemical does blood-brain and CSF-brain barrier control with easy entrance?

The blood-brain barrier and the cerebrospinal fluid (CSF) brain barrier, provides the means for maintaining the stable chemical environment of the brain. Only water, Carbon dioxide, and oxygen enter the brain with ease. The purpose of the blood-brain barrier is to protect against circulating toxins or pathogens that could cause brain infections, while at the same allowing the chemicals and vital nutrients to reach the brain.

Oxygen

Explanation:

There are few chemicals that have easy access to the brain through the two barriers, the blood-brain barrier and the cerebrospinal fluid (CSF) - brain barrier, and they are only water, carbon dioxide, and oxygen.