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First published online August 18, 2005
Journal of Experimental Biology 208, 3331-3339 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01758

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Terrestrial locomotion does not constrain venous return in the American alligator, Alligator mississippiensis

Suzanne L. Munns^*, Lynn K. Hartzler, Albert F. Bennett and James W. Hicks

Department of Ecology and Evolutionary Biology, University of California, 321 Steinhaus Hall, Irvine, CA 92697, USA

^* Author for correspondence at present address: School of Veterinary and Biomedical Sciences, James Cook University, Townsville, QLD 4811, Australia (e-mail: suzy.munns{at}jcu.edu.au)

Accepted 22 June 2005

The effects of treadmill exercise on components of the cardiovascular (heart rate, mean arterial blood pressure, central venous pressure, venous return) and respiratory (minute ventilation, tidal volume, breathing frequency, rate of oxygen consumption, rate of carbon dioxide production) systems and on intra-abdominal pressure were measured in the American alligator, Alligator mississippiensis, at 30°C. Alligators show speed-dependent increases in tidal volume and minute ventilation, demonstrating that the inhibition of ventilation during locomotion that is present in some varanid and iguanid lizards was not present in alligators. Exercise significantly increases intra-abdominal pressure; however, concomitant elevations in central venous pressure acted to increase the transmural pressure of the post caval vein and thus increased venous return. Therefore, despite elevated intra-abdominal pressure, venous return was not limited during exercise in alligators, as was the case in Varanus exanthematicus and Iguana iguana. Respiratory cycle variations in intra-abdominal pressure, central venous pressure and venous return indicate that, at high tidal volumes, inspiration causes a net reduction in venous return during active ventilation and thus may act to limit venous return during exercise. These results suggest that, while tonically elevated intra-abdominal pressure induced by exercise does not inhibit venous return, phasic fluctuations during each breath cycle may contribute to venous flow limitation during exercise.

Key words: exercise, hemodynamics, intra-abdominal pressure, locomotion, oxygen consumption, reptile, venous return, ventilation, central venous pressure, heart rate

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