The mechanics of lung ventilation and the effects of hypercapnia on respiration in Bufo marinus

Abstract

Respiratory and circulatory changes associated with exposure to elevated ambient carbon dioxide (CO₂) levels were studied in free-moving Bufo marinus. The buccal cavity, lungs, and systemic arch were cannulated singly or in various combinations. Simultaneous recordings from the lungs and buccal cavity permitted analysis of respiratory rate and pattern, while systemic cannulae provided constant blood-pressure measurements. All experiments were conducted at 22 °C.Normal breathing patterns consisted of continuous buccal 'oscillations' with irregularly occurring lung 'ventilations' and occasional high pressure 'inflations.' On exposure to increased ambient CO₂ levels, oscillations decreased, ventilations increased, and inflations predominated, resulting in an overall increase in lung minute-volume. Systemic pressure decreased as ambient CO₂ approached 3%, but returned to normal, with an associated increase in heart rate, at the 5% level. It is postulated that the ventilatory and concomitant circulatory responses are mediated by receptors functioning in both the chemoreceptive and baroreceptive modes.