The amphibian ancestor had two gas exchangers, one working in water and one in air. It also ventilated the aerial exchanger in an intermittent fashion. The functional repercussions of thie ancestry can be seen in all amphibians and reptiles.
During periods of apnoea, which almost all amphibians and reptiles show in their breathing patterns, there are variable fluctuations in lung and arterial PO2 and PCO2. Arterial PCO2 and pH show only small fluctuations during apnoea induced by voluntary dives, and blood lactate does not build up. More considerable acidoses and lactate accumulations occur in forced dives which the animals can readily survive, even when the dives are prolonged. Anaerobic metabolism plays an important role in their capacity to survive prolonged apnoea.
Amphibians ventilate their lungs with a buccal pump whose pattern of activity differs from species to species. Short periods of apnoea characterize the more terrestrial forms. Long dives punctuated by bursts of lung ventilation, or irregular bouts of breathing with no distinctive pattern occur in more aquatic forms. Similar differences in pattern can be seen in terrestrial and aquatic reptiles. The most striking characteristic of ventilation is its extreme variability, even in a single individual.
Responses occur to decreasing oxygen tensions and increasing carbon dioxide tensions in inspired air, alveolar air, and arterial blood, the animals almost always increasing lung ventilation and decreasing the duration of apnoea. However, no simple relationship can be seen between the tensions of respiratory gases in lungs or arterial blood and the beginning or end of an apnoeic period.
Periods of apnoea are accompanied by vasoconstriction in the lung vasculature. This has the effect of decreasing blood flow to the lung and increasing right to left shunt in the incompletely divided heart.