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When we breathe we breathe in air (EAN21). When you breathe out, theres still most of the oxygen left in that air. We are simply unable to utilize the ammount of oxygen present in air and even less so the oxygen in richer mixes.
We do however fill our lungs when we breathe and as we cant absorb more oxygen from nitrox than we do for air, we have to breathe at the same rate wether we have air or EAN in the tank.
Also, what trigger us to breathe is not lack of O2, but the need to expell the CO2 that build up.
Exactly the question I asked on Marine Science and Physiology thread. Obviously, O2 delivery can be increased with increased FiO2 or mechanical ventilation would be ineffective. I still do not know what drives SAC, O2 requirement or CO2 elimination.
Best, Craig