I can tell you for sure that when your oxygen coverts to CO2 that does not make it out when we exhale, can put you to sleep, and kill you. When I was younger, I could hold my breath underwater for a little over 5 minute.  I use to win money, doing this.  Won $100 once from Caruth Bird of Dallas, Tx fame, and another $100 from Robert Conrad (James West).  I made the mistake of doing this a couple time with lot of alcohol in my system, and ended up in ICU twice.    Ah, the ignorance of youth.