ABSTRACT
Ventilation is a critically important process in providing O2 to the respiratory surfaces and removing CO2 from them. When either environmental gas composition or tissue demands change, then adjusting ventilation through rate and amplitude modifications is the most direct response to ensure respiratory gas exchange. In amphibious vertebrates breathing both air and water and using a suite of respirator structures (which can include skin, external gills, internal gills, lungs, gas bladders and intestines), the process of ventilatory adjustment can be complex indeed. The present review examines the morphology, physiology and evolutionary biology of ventilatory responses to altered O2 and CO2 levels in amphibians and air-breathing fishes. Additionally, the vital role in modulating ventilatory responses of both centrally and peripherally located chemoreceptors and mechanoreceptors, as investigated by in vitro and in vivo methods, is examined. Finally, this analysis concludes by posing an extensive list of areas in lower vertebrate respiratory control deserving future investigation.
