Editing Tetrapod (section)

==== Carbon dioxide metabolism ====
Although air-breathing fish can absorb oxygen through their lungs, the lungs tend to be ineffective for discharging carbon dioxide. In tetrapods, the ability of lungs to discharge CO<sub>2</sub> came about gradually, and was not fully attained until the evolution of amniotes. The same limitation applies to gut air breathing (GUT), i.e., breathing with the lining of the digestive tract.<ref name="Nelson2014">{{cite journal|last1=Nelson|first1=J. A.|title=Breaking wind to survive: fishes that breathe air with their gut|journal=Journal of Fish Biology|volume=84|issue=3|date=March 2014|pages=554–576|issn=0022-1112|doi=10.1111/jfb.12323|pmid=24502287|bibcode=2014JFBio..84..554N }}</ref> Tetrapod skin would have been effective for both absorbing oxygen and discharging CO<sub>2</sub>, but only up to a point. For this reason, early tetrapods may have experienced chronic [[hypercapnia]] (high levels of blood CO<sub>2</sub>). This is not uncommon in fish that inhabit waters high in CO<sub>2</sub>.<ref>{{harvnb|Clack|2012|p=235}}</ref>
According to one hypothesis, the "sculpted" or "ornamented" dermal skull roof bones found in early tetrapods may have been related to a mechanism for relieving [[respiratory acidosis]] (acidic blood caused by excess CO<sub>2</sub>) through compensatory [[metabolic alkalosis]].<ref name="JanisDevlin2012">{{cite journal|last1=Janis|first1=C. M.|last2=Devlin|first2=K.|last3=Warren|first3=D. E.|last4=Witzmann|first4=F.|title=Dermal bone in early tetrapods: a palaeophysiological hypothesis of adaptation for terrestrial acidosis|journal=Proceedings of the Royal Society B: Biological Sciences|volume=279|issue=1740|date=August 2012|pages=3035–3040|issn=0962-8452|doi=10.1098/rspb.2012.0558|pmid=22535781|pmc=3385491}}</ref>