Respiratory responses of ducks to simulated altitude.

Domestic ducks were exposed to simulated altitudes of 0, 3000, 6000, and 9000 m in order to study the respiratory changes that take place. We found that the respiratory minute volume (VE,BTPS) increased with altitude, the increase being due to increased respiratory frequency while tidal volume (VT, BTPS) showed only minor changes. The quantity of air moved (VE, STPD), however, remained nearly unchanged with increasing altitude. The oxygen extraction, calculated as 1--(FIN2FEO2)/(FEN2FIO2), remained constant at about 0.28 up to 6000 m and declined to 0.17 at 9000 m. The fractional gas concentrations (FO2 and FCO2) in exhaled air and in the interclavicular and posterior thoracic air sacs changed only little up to 6000 m, but at 9000 m FO2 increased and FCO2 decreased. The relative constancy of expired and air sac gas up to 6000 m seems remarkable. However, when applied to current models of air flow in the avian respiratory system the results seem fully explainable and permit a detailed analysis of the functioning of the avian respiratory system.