Ventilation in An Aquatic and A Terrestrial Chelonian Reptile

Periods of breath-holding are interrupted by episodes of continuous breathing in the aquatic turtle Pelomedusa subrufa, whereas single breaths and short periods of breath-holding alternate in the terrestrial tortoise Testudo pardalis. This implies that partial pressures of O₂ and CO₂ in expired air are stable in Testudo in contrast to cyclic fluctuations in Pelo-medusa.

In spite of this, air convection requirements (⁠⁠, ml BTPS/ml STPD) are not significantly different for the two species (25·4 in Testudo, 27·3 in Pelomedusa), and differences in weight-specific ventilation between the species can only be ascribed to a difference in mean body weight.

The end-tidal in Pelomedusa (mean 15·2 mmHg) is lower than in Testudo (mean 24·7 mmHg), which reflects aquatic CO₂ elimination in Pelomedusa.

In Testudo, the time course of ventilation correlates with the time course of increase of end-tidal during CO₂ breathing, but no simple relationship is evident between ventilation and blood in Pelomedusa.

In both species tidal volume as well as respiratory frequency increase approximately in proportion to the end-tidal , although the response to 6% CO₂ breathing could be less than expected from extrapolation of the responses to 2 and 4% CO₂. Both species also increase ventilation during hypoxia, but hyperoxia depresses ventilation.