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.

Explanation:

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.  

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-