How toads breathe: control of air flow to and from the lungs by the nares in Bufo marinus.

The external nares rarely close completely during positive pressure lung filling in Bufo marinus. Simultaneous measurement of airflow at the nares and transnarial pressure revealed continuous airflow (out) during lung emptying (nares open) and the subsequent lung filling (nares partially closed). Expiratory minute volume (VE) measured at the nares is consistently greater than VE from the lungs because the buccal contents are being simultaneously forced into the lungs and out the partially closed nares. The relation between narial resistance and ventilatory parameters during hypercapnia was investigated to determine if control of the narial aperture is an important variable related to the volume of air transferred to and from the lungs and buccal chamber. Buccal frequency decreased (from 73 to 30/min) and buccal TE (time for expiratory air flow at the nares) increased (from 0.28 to 0.41 sec) when breathing 2% CO2. Mean narial resistance was 0.40 cm H2O/L . min, maximum narial resistance was 0.44 cm H2O/L . min, and both were uninfluenced by CO2. Lung ventilation frequency increased with 2% CO2 (from 5.3 to 20.8/min). TE of lung ventilation cycles remained constant (about 0.45 sec) while VTE (expiratory tidal volume at the nares) increased from 0.73 to 0.90 ml/100 g. Mean narial resistance during lung ventilations decreased from 3.80 to 1.76 cm H2O/L . min with 2% CO2. Maximum narial resistance during lung ventilations was unchanged. It is concluded that the nares may remain open for a longer proportion of each lung ventilation cycle during hypercapnic hyperpnea.