The initiation of pulmonary respiration in a bird embryo: tidal volume and frequency.

Pulmonary ventilation in embryos and hatchlings of the Wedge-tailed Shearwater (Puffinus pacificus), measured with a barometric plethysmograph, revealed a progressive rise in tidal volume (VT) and minute volume (VE) during the paranatal period to achieve hatchling levels. VT and VE in internally pipped eggs (penetration of air cell) was 0.06 ml +/- 0.03 (SD) and 3.11 ml . min-1 +/- 1.80, respectively. Ventilation was significantly higher (P less than 0.05) in eggs with pip-holes (VT = 0.15 ml +/- 0.04 and VE = 7.09 ml . min-1 +/- 2.76). A significant difference (P less than 0.05) was also obtained for VT and VE between embryos and hatchling chicks. The respiratory frequency (f) was approximately 47 breaths . min-1 for both embryos and chicks. Acute changes in ventilation were examined in response to 2% CO2, 5% CO2, and 10% CO2 in air. For embryos, VT increased significantly (P less than 0.05) only with 10% CO2 and was accompanied by a significant decrease (P less than 0.05) in f. Hatchling VT increased significantly (P less than 0.05) with each test gas and VE increased significantly (P less than 0.05) with acute exposure to 5% CO2, from 16.5 ml . min-1 +/- 2.0 in normoxic air to 53.9 ml . min-1 +/- 8.6. The shearwater embryo is relatively insensitive to high levels of CO2, suggesting a respiratory adaptation to naturally inspired air cell gas concentrations during the internal pipping phase and rebreathing.