Gas exchange, hypercarbia and acid-base balance, paleoecology, and the evolutionary transition from water-breathing to air-breathing among vertebrates

The evolutionary transition from water-breathing to air-breathing among vertebrates was accompanied by a shift in acid-base status from a low-PCO2, low [HCO3−] acid-base status to a high-PCO2, high-[HCO3−] acid-base status. It has been hypothesized that this change occurred concomitant with the reduced ventilation rate associated with breathing O2-rich (relative to water) air, and that the integument was used in the initial stages of the transition as a primary site for CO2 elimination. Decoupling of O2 and CO2 exchange thereby avoided a respiratory acidosis that would be associated with the relative hypoventilation expected of protoamphibians while they were air-breathing. However, at least some primitive amphibians appear to have had integuments and body sizes that would make it unlikely that the skin could be a primary site for either O2 uptake or CO2 elimination, which casts doubt on any general theory that requires the skin to play a major role in acid-base balance in early amphibians. In this review, I survey the responses of lower vertebrates to hypercarbia and the role of their integument in gas exchange, and suggest that protoamphibians may have inhabited aquatic environments that were hypercarbic, and conclude that they therefore already may have had an acid-base status similar to terrestrial forms (i.e., high-PCO2, high [HCO3−]) before they emerged onto land, which would obviate the need for a gas-permeable integument.