Pulmonary oxygen transport during activity in lizards

Oxygen consumption (MO2), effective alveolar ventilation (Veff), arterial and alveolar PO2 (PaO2, PAO2) and the alveolar-arterial PO2 difference [(A--a)PO2] were determined in the lizards Varanus exanthematicus and Iguana iguana at rest and during treadmill exercise at 35 degrees C. In both species, Veff increased more rapidly than MO2 giving rise to an increased PAO2. In contrast, PaO2 remained unchanged through the highest levels of MO2 attained. As a result, the (A--a)PO2 increased with increasing MO2. We suggest that the observed increase in (A--a)PO2 may be due to a rather low pulmonary oxygen diffusing capacity (DLO2) and limited capacity to increase DLO2 during exercise. Arterial desaturation was prevented by a compensatory hyperventilation, thus enhancing the gradient for alveolar-capillary gas exchange. These results indicate that both lizard species increase pulmonary oxygen transport sufficiently so that it is not a limiting factor to aerobic scope under the conditions of this study.