Acute regulation of hematocrit and acid–base balance in chicken embryos in response to severe intrinsic hypercapnic hypoxia

• Water submersion produced intrinsic hypercapnic hypoxia in chick embryos.
• There was a progressive hysteresis in acid–base disturbance during submersion and recovery in air.
• Swift blood lactate changes due to anaerobic glycolysis drive acid–base disturbances.
• Swift Hct changes result from proportional increases in red blood cell volume and concentration.
• A short severe intrinsic hypercapnic hypoxia does not compromise viability of embryos.

The regulation of blood acid–base balance and hematology in day 15 chicken embryos in response to partial water submersion (with egg's air cell in air) and complete submersion producing severe intrinsic hypercapnic hypoxia and recovery in air was studied. The acid–base disturbance during submersion was characterized by initial rapid respiratory changes and then superseded by metabolic processes, resulting in a large progressive hysteresis. Throughout submersion and recovery, blood lactate concentration changed swiftly along with the changes in bicarbonate concentration ([HCO3−]), indicating that anaerobic glycolysis determined overall acid–base disturbances. Both partial and complete submersion produced large, rapid increases in hematocrit through proportional increases in mean corpuscular volume and red blood cell concentration. Death ensued once the internal pool of O2 was exhausted and/or the acid–base disturbance became too severe for survival (i.e., [HCO3−]a < ∼10 mmol L−1). However, embryos recovered from acid–base and hematological disturbances within 120 min recovery in air after short bouts of complete (20 min) or partial (60 min) submersion, suggesting that shorter severe intrinsic hypercapnic hypoxia does not compromise viability of embryos.