Buccal jet streaming and dead space determination in the South American lungfish, Lepidosiren paradoxa

The "jet stream" model predicts an expired flow within the dorsal part of the buccal cavity with small air mixing during buccal pump ventilation, and has been suggested for some anuran amphibians but no other species of air breathing animal using a buccal force pump has been investigated. The presence of a two-stroke buccal pump in lungfish, i.e. expiration followed by inspiration, was described previously, but no quantitative data are available for the dead-space of their respiratory system and neither a detailed description of airflow throughout a breathing cycle. The present study aimed to assess the degree of mixing of fresh air and expired gas during the breathing cycle of Lepidosiren paradoxa and to verify the possible presence of a jet stream during expiration in this species. To do so, simultaneous measurements of buccal pressure and ventilatory airflows were carried out. Buccal and lung gases (PCO₂ and PO₂) were also measured. The effective ventilation was calculated and the dead space estimated using Bohr equations. The results confirmed that the two-stroke buccal pump is present in lungfish, as it is in anuran amphibians. The present approaches were coherent with a small dead space, with a very small buccal-lung PCO₂ difference. In the South American lungfish the dead space (V_D) as a percentage of tidal volume (V_T) (V_D / V_T) ranged from 4.1 to 12.5%. Our data support the presence of a jet stream and indicate a small degree of air mixing in the buccal cavity. Comparisons with the literature indicate that these data are similar to previous data reported for the toad Rhinella schneideri.