During exercise, pulmonary gas exchange in reptiles was predicted to differ from that in mammals because of their less complex lung structure, which might reduce ventilation-perfusion heterogeneity (V/QL) at the expense of pulmonary diffusion limitation. To investigate this, the multiple inert gas elimination technique was used in six Varanus exanthematicus at rest and during maximal exercise. Trace amounts of six inert gases were infused into the external jugular vein and blood samples were collected from the pulmonary artery and the left atrium. Mixed expired gas samples and ventilatory and metabolic data were acquired. Indices of V/QL heterogeneity, calculated using a 50-compartment model, were low at rest (log standard deviation of perfusion distribution, logSDQ = 0.39) and increased significantly with exercise (logSDQ = 0.78). Oxygen diffusion limitation was apparent during exercise and was comparable to reported mammalian values. A molecular-mass-dependent limitation, suggesting limited intrapulmonary gas mixing, was evident only at rest. An increase in left atrial PO2 from 82mmHg at rest to 96 mmHg during exercise was associated with a corresponding decrease in PCO2. These data indicate adequacy of pulmonary ventilation and gas exchange for metabolic demands in exercising varanid lizards and suggest that less complex lung structures are not necessarily linked to increased pulmonary diffusion limitation.

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