Ventilatory pressure loading at constant pulmonary FCO2 in Gallus domesticus

Seven White Leghorn roosters were unidirectionally ventilated at constant flows and CO2 concentrations. The birds were awake and stood or crouched in a plethysmograph. A servo system clamped the pressure in the air sacs at constant values from -10 to +10 cm H2O in 2 cm H2O increments. Therefore, the animals could inflate or deflate the air sacs with breathing movements without affecting intrapulmonary pressures. Decreasing air sac pressure less than atmospheric caused inspiratory duration (TI), expiratory duration (TE), total period (TTOT) and tidal volume (VR) to decrease, and the ratio, TI/TE to increase. Increasing air sac pressures to 6 cm H2O above atmospheric caused, TE to increase, TI and TI/TE to decrease and VT and TTOT to change very little. After bilateral vagotomy air sac pressure changes caused little or no changes in TI, TE, TTOT or TI/TE, but produced percentage changes in VT similar to before vagotomy. Comparison of end expiratory volumes with apneic volumes (produced by lowering CO2 in the insulfating gas) over the range of air sac pressures clamped shows: (1) chickens actively exhale at pressures as low as -10 cm H2O, and (2) the change of mean air sac volume due to imposed pressure is less during breathing than during apnea. These findings, we believe, are due to a reflex initiated by mechanoreceptors with projections in the vagus nerves.