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

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 degrees C. The rate of oxygen consumption (V*O2) increased with locomotion from 3.49+/-0.75 (mean +/- S.D.) to 14.0+/-4.0 ml O2 kg(-1) min(-1). Ventilation (V*E) increased, aided by increases in both tidal volume and frequency, in direct proportion to (V*O2). The air convection requirement (V*E/V*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 (CaO2-CvO2), 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 (Q*tot) did not match the increase in V*O2, such that the blood convection requirement (Q*tot/V*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.