Mouth breathing reduces O2 absorption and causes excessive offloading of CO2, disrupting the CO2-O2 dissociation effect and reducing oxygen bioavailability.

“if i'm breathing through my mouth i'm not absorbing as much o2 into the system so i can't diffuse as much right and i offload that o2 but i also am offloading excessive carbon dioxide when there is no metabolic demand so if my energy if my if i'm not working out push-ups moving around more the metabolic demand isn't requiring me to open my mouth i'm now offloading more carbon dioxide which ruins principally speaking the dissociative effect that happens between carbon dioxide and oxygen meaning bioavailability of oxygen to the process you just described in the electron transport chain”