ABSTRACT
Th e lungfi sh (Dipnoi) forms a probable sister group to the land vertebrates (Tetrapoda) and the two ramifi cations share many features of pulmonary function. Th e African lungfi sh, Protopterus (4 species) and the South American lungfi sh (Lepidosiren paradoxa) (1 species) have well developed lungs combined with rudimentary gills. Diff erently, the Australian lungfi sh (Neoceratodus) predominantly depends on gill ventilation, while its lung structure is more simple. Like the land vertebrates, Lepidosiren and Protopterus possess central cerebral CO2 and H+ receptors that monitor acid-base status by adjustments of pulmonary ventilation, while less information is available for Neoceratodus. In land vertebrates, the central cerebral receptors account for 70 to 80% of the response to hypercarbia, while the peripheral blood screening CO2/pH-receptors account for 20 to 30%. In Lepidosiren, the peripheral CO2/H
+-receptors accounted for 20% of the hypercarbia-induced ventilatory responses, which suggests a very early origin of the central and peripheral CO2/H
receptors. In tetrapods, the specifi c O2-stimulus is O2 partial pressure rather than O2-content, which also applies to Lepidosiren. When exposed to higher temperatures, lungfi sh and amphibians become more dependent on the lung for CO2-elimination. Th is increases intrapulmonary and arterial PCO2 which, in turn, accounts for a negative ΔpHa/Δt as expected for ectothermic vertebrates. Recent publications emphasize a very similar respiratory control in lungfi sh and the amphibians, which raises a number of questions about a common ancestor.
