Fig. 2. Inferior vena cava flow (_PC) and intra-abdominal pressure (IAP) vary during each breathing cycle. (A) Increasing inspired tidal volumes (open diamonds) correlate with decreasing _PC (r²=–0.71, P<0.0001). Expiration (filled diamonds) causes an increase in _PC but the magnitude of the increase does not correlate with increasing tidal volumes (r²=–0.15, P=0.33). (B) The sum of the inspiratory and expiratory alterations results in no net change in total _PC (open squares) during the breath cycle at low tidal volumes and a net decrease at high tidal volumes. As a result, increasing tidal volumes are correlated with a net decrease in total _PC (r²=–0.57, P<0.0001). IAP (filled squares) is positively correlated with increments in tidal volume (r²=0.72, P<0.0001). Data presented are 50 consecutive breaths from the recovery period post-exercise in one representative alligator (mass 3.55 kg). _PC was calculated as the change relative to the preceding non-ventilatory period. 1 mmHg=133.3 Pa.