Control of arrhythmic breathing in bimodal breathers: Amphibia

Amphibians employ a system of gas exchange whereby various combinations of the lungs, gills, and skin are used to exploit gas exchanges in both air and water (bimodal breathing). Continuous lung ventilation is rarely observed in these animals. Instead, the dominant breath pattern is arrhythmic in nature and is believed to have evolved in response to a periodic need to supplement aquatic gas exchange. Such a need is largely dependent on the activity state of the animal concerned and its capacity for aquatic gas exchange. The overall control system appears to be one that turns lung ventilation on and off by trigger signals arising from chemo- and mechano-sensitive receptors responding to changing conditions during periods of breath holding and breathing. In amphibians in which the aquatic exchanger is a major avenue for CO2 elimination, [Formula: see text] levels in the lungs and blood do not change substantially in the latter stages of a breath hold. Under these conditions falling levels of oxygen may be the primary stimulus to terminate the breath hold and initiate breathing. There is, however, some interaction between the two gases since elevated CO2 levels affect the sensitivity of the predominantly O2-mediated response. Another major component in determining air-breathing patterns in these animals is their ability to delay the onset of breathing when certain behavioural activities take precedence over the need for additional gas exchange.