Interestingly, the discomfort you feel when you hold your breath for too long is caused not by the depletion of oxygen but by a build-up of carbon dioxide. That’s why the first thing you do when you stop holding your breath is blow out. You would think that the most urgent need would be to get fresh air in rather than stale air out, but no. The body so abhors CO2 that you must expel it before gulping in replenishment.
