Surprisingly, we don’t expel all the carbon dioxide (CO2) when we exhale—we retain a residue required to produce a chemical called H+ to kick oxygen (O2) out of the red blood cells into the tissue where it’s actually used in the mitochondria to produce energy. When we need immediate physical action for “flight or fight” from a predator, the body is designed to breathe faster and deeper to get more O2 while still balancing CO2.
The problem is that we breathe too fast when we’re under emotional stress without the required increase in physical activity, so we end up with too much O2 in our bloodstream and not enough CO2 to create H+ to get all that O2 into the tissues.