What percentage of the oxygen that we breathe is used by the brain?
The brain consumes 25% of the total required oxygen for a normal functioning of the body.
Each time you breathe in your body goes through cellular respiration, which takes glucose (C6H12O6) and oxygen (O2) and turns it into water (H2O) Carbon Dioxide (CO2) and Energy (ATP), the Carbon Dioxide is breathed out in place of the used up oxygen. When you breathe in then all that oxygen is translated into carbon dioxide, as previously stated, which is breathed out.
When you respire you "Breathe" and when you intake oxygen (o2) it goes into your lungs where it is bound to iron (Fe) in the blood so that it can travel to the brain. Oxygen is carried by iron to the brain via the circulatory system, which is the bloodstreams, heart and veins. Once the oxygen has been used it is bound with the carbon atom then exhaled as carbon dioxide (CO2).