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The Journal of Experimental Biology 205, 2725-2736 (2002)
© 2002 The Company of Biologists Limited

Oxygen transfer during aerobic exercise in a varanid lizard Varanus mertensi is limited by the circulation

Peter Frappell^1,*, Tim Schultz² and Keith Christian²

¹ Department of Zoology, La Trobe University, Melbourne, Victoria, 3086, Australia
² School of Biological Sciences, Northern Territory University, Darwin, NT 0909, Australia

^* Author for correspondence (e-mail: p.frappell{at}latrobe.edu.au)

Accepted 30 May 2002

Oxygen transfer during sustained maximal exercise while locomoting on a treadmill at 0.33 m s^-1 was examined in a varanid lizard Varanus mertensi at 35 °C. The rate of oxygen consumption (_O2) increased with locomotion from 3.49±0.75 (mean ± S.D.) to 14.0±4.0 ml O₂ kg^-1 min^-1. Ventilation (_E) increased, aided by increases in both tidal volume and frequency, in direct proportion to _O2. The air convection requirement (_E/_O2=27) was therefore maintained, together with arterial Pa_CO2 and Pa_O2. The alveolar—arterial P_O2 difference (PA_O2—Pa_O2) also remained unchanged during exercise from its value at rest, which was approximately 20 mmHg. Pulmonary diffusion for carbon monoxide (0.116±0.027 ml kg^-1 min^-1 mmHg^-1) was double the value previously reported in V. exanthematicus and remained unchanged with exercise. Furthermore, exercise was associated with an increase in the arterial—venous O₂ content difference (Ca_O2—Cv_O2), which was assisted by a marked Bohr shift in the hemoglobin saturation curve and further unloading of venous O₂. During exercise the increase in cardiac output (_tot) did not match the increase in _O2, such that the blood convection requirement (_tot/_O2) decreased from the pre-exercise value of approximately 35 to 16 during exercise. Together, the results suggest that ventilation and O₂ transfer across the lung are adequate to meet the aerobic needs of V. mertensi during exercise, but the decrease in the blood convection requirement in the presence of a large arterial—venous O₂ content difference suggests that a limit in the transport of O₂ is imposed by the circulation.

Key words: exercise, ventilation, oxygen consumption, oxygen transport, blood gas, breathing pattern, reptile, Varanus mertensi, lung diffusion, P₅₀, oxygen affinity

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