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

Highlights

• Buccal pump ventilation was investigated and dead space calculated in lungfish.

• L. paradoxa is able to renew almost all of its pulmonary air in one breathing cycle.

• Lungfish do possess two-strike buccal pump for pulmonary ventilation.

• The results suggest the presence of a Jet Stream in the lungfish.

• A low VD/VT ratio (~4–12%) suggests a small air mixing within the buccal cavity.

Abstract

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.

Introduction

The six species of extant Dipnoi (Neoceratodus forsteri; Lepidosiren paradoxa; Protopterus aethiopicus, P. amphibius, P. annectens, P. dolloi) possess lungs as their principal air-breathing organs (Graham, 1997). The morphological characteristics of dipnoan lungs have been studied for centuries (Owen, 1841; Quekett, 1844; Hyrtl, 1843; Günther, 1871; Parker, 1892; Poll, 1962; Grigg, 1965; Klika and Lelek, 1967; Hughes and Weibel, 1976; Maina and Maloiy, 1985; de Moraes et al., 2005). Several aspects of lung ventilation, gas exchange, blood gas chemistry, or chemoreception have also been of interest for comparative physiologists, especially over the last 50 years (e.g. Johansen and Lenfant, 1967; Johansen and Lenfant, 1968; Lenfant et al., 1970; McMahon, 1970; Fritsche et al., 1993; Amin-Naves et al., 2004; DeLaney et al., 1983; Sanchez and Glass, 2001; Glass, 2010; Sanchez and Glass, 2001; Sanchez et al., 2001b; Bassi et al., 2005; Bassi et al., 2010; Sanchez et al., 2005; Amin-Naves et al., 2007a, Amin-Naves et al., 2007b; da Silva et al., 2008, da Silva et al., 2017; Zena et al., 2017). Such attention given to morphology and physiology of lungfish can be explained by their phylogenetic position close to the origin of the Sarcopterygii (Brinkmann et al., 2004; Yokobori et al., 1994; Zardoya et al., 1998) and their ability to exchange gases bimodally with water (across gills and/or integument) and air (using lungs), representing the remaining living species of a lineage of vertebrates that evolved air breathing in the mid-Devonian (~380 Ma; Clement and Long, 2010).

One aspect of lungfish respiration, however, has not received much attention up to now: the breathing mechanism (Bishop and Foxon, 1968; McMahon, 1969; Brainerd, 1994). While studies of the buccal pump mechanism have been carried out on L. paradoxa (Bishop and Foxon, 1968; Brainerd, 1994) and P. aethiopicus (McMahon, 1969; Brainerd et al., 1993), only superficial descriptions of breathing behavior are available for N. forsteri (Dean, 1906; Grigg, 1965). It seems clear, however, that all lungfish use a two-stroke buccal pump, composed of one expiration and at least one inspiration, to renew the air within the lungs (Brainerd, 1994). The events during one breathing cycle can be described as follows (Brainerd and Ferry-Graham, 2006): 1) Air breathing starts with the animal putting its head out of the water and the mouth being opened; 2) The buccal cavity expands to draw in oxygen-rich air; 3) As the buccal cavity expands, the glottis opens and the lungs empty through elastic recoil of body wall and lungs; 4) Once the buccal cavity has been fully expanded, the mouth closes and the air trapped within the buccal cavity is pushed into the lungs. Since fresh air and gas expired from the lungs enter the buccal cavity during buccal expansion, some degree of mixing of oxygen-rich and oxygen-poor gas occurs during the first phase of the two-stroke buccal pump, reducing the amount of oxygen entering the lungs. McMahon (1969) estimates the gas mixing to be 20–40% of expired air being returned to the lungs in P. aethiopicus.

A two-stroke buccal pump ventilation has also been identified in anuran amphibians. De Jongh and Gans (1969) and Gans et al. (1969) performed classical experiments using the bullfrog (Lithobates catesbeianus) to describe the complex functioning of the buccal pump. De Jongh and Gans (1969) identified three different types of cyclical phenomena associated to buccal movements. 1) Oscillations of the buccal floor to ventilate the buccal cavity through the open nares. 2) Ventilatory cycles to renew the air within the lungs through coordinated opening and closing of nares and glottis. 3) Inspiratory movements with pulmonary ventilation interspaced by non-ventilatory periods. One fundamental proposal of de Jongh and Gans (1969) was the presence of an expiratory airflow, known as jet stream, postulated to result in minimal mixing of oxygen rich air in the buccal cavity with the oxygen poor air leaving the lungs during expiration, and passing rapidly along the dorsal part of the buccal cavity. For Lithobates pipiens a similar ventilatory pattern as in the bullfrog has been described, but no evidence for the presence of an expiratory jet stream has been found (Vitalis and Shelton, 1990), since the air being pumped into the lungs during inspiration represents a mixture of air, containing 30–50% of air from the previous expiration (West and Jones, 1975). Brett and Shelton (1979), studying Xenopus laevis, found in this species a ventilatory cycle starting with an expiration that is followed by an inspiration, thereby suggesting no mixing of fresh and used air within the buccal cavity. Fernandes et al. (2005), investigating the ventilatory pattern and jet stream hypothesis in the toad Rhinella schneideri, found a buccal cavity dead-space representing 30–40% of tidal volume, suggesting some degree of inspiratory and expiratory flow separation, with limited mixing within the buccal cavity.

Regarding the ventilatory cycle of L. paradoxa, it has to be assumed that this species possibly expires almost all of the air contained within the lungs at the beginning of a ventilatory cycle. This was demonstrated by Bishop and Foxon (1968) using X-ray imaging of a breathing cycle, where the lungs were no longer visible during expiration due to a near-complete loss of air, also has been suggested by da Silva et al. (2017), based on measurements of expiratory tidal volume. Since pulmonary dead space seems to be very small, a significant amount of gas mixing may only be possible during the expansion phase of the two-stroke buccal pump, when air is being expired from the lungs and fresh air is being drawn into the buccal cavity. To determine the degree of mixing of fresh air and expired gas during the breathing cycle of L. paradoxa and to verify the possible presence of a jet stream during expiration in this species, we performed measurements of buccal pressure, buccal and pulmonary PCO₂ and PO₂, pulmonary ventilation and calculated dead space and effective ventilation by using the Bohr dead space equation (see Eq. 3).