Respiratory and cardiovascular responses to acute hypoxia and hyperoxia in internally pipped chicken embryos

During the first day of hatching, the developing chicken embryo internally pips the air cell and relies on both the lungs and chorioallantoic membrane (CAM) for gas exchange. Our objective in this study was to examine respiratory and cardiovascular responses to acute changes in oxygen at the air cell or the rest of the egg during internal pipping. We measured lung (V·_O2lung) and CAM (V·_O2CAM) oxygen consumption independently before and after 60 min exposure to combinations of hypoxia, hyperoxia, and normoxia to the air cell and the remaining egg. Significant changes in V·_O2total were only observed with combined egg and air cell hypoxia (decreased V·_O2total) or egg hyperoxia and air cell hypoxia (increased V·_O2total). In response to the different O₂ treatments, a change in V·_O2lung was compensated by an inverse change in V·_O2CAM of similar magnitude. To test for the underlying mechanism, we focused on ventilation and cardiovascular responses during hypoxic and hyperoxic air cell exposure. Ventilation frequency and minute ventilation (V_E) were unaffected by changes in air cell O₂, but tidal volume (V_T) increased during hypoxia. Both V_T and V_E decreased significantly in response to decreased P_CO2. The right-to-left shunt of blood away from the lungs increased significantly during hypoxic air cell exposure and decreased significantly during hyperoxic exposure. These results demonstrate the internally pipped embryo's ability to control the site of gas exchange by means of altering blood flow between the lungs and CAM.