Carbon dioxide transport and acid-base balance in chickens before and after hatching

Abstract

During the late stages of incubation, when the CO₂ increases within the shell due to higher metabolic rate and a fixed shell conductance, the increases in standard CO₂ content, Haldane effect, CO₂ capacitance coefficient and hematocrit augment CO₂ transport and buffer capacity. The buildup of CO₂ results in respiratory disturbances of acid-base balance, but the decrease in pH is mitigated by an increase in non-respiratory bicarbonate. In the final stages of the prenatal period, a metabolic change in pH, partially due to lactate buildup, becomes dominant over the respiratory change.

After the chick pips internally and begins pulmonary ventilation that promotes CO₂ loss, standard bicarbonate of Van Slyke decreases. Blood $P_{{O_2}}^{}$ varies widely among embryos and the average $P_{{O_2}}^{}$ is not significantly different from the final stages of prenatal period. Hence, the average pH decreases further due to a fall in non-respiratory bicarbonate. The contribution of pulmonary ventilation to the acid-base status is variable, probably in conformity with an individual established degree of ventilatory movement.

Finally the chick pips externally and undergoes respiratory compensation and a rise in pH. After hatching, the blood $P_{{O_2}}^{}$ further decreases by the large convective conductance of pulmonary ventilation but the pH change is small due to a corresponding fall in non-respiratory bicarbonate. This is immediately followed by a gradual increase in blood $P_{{O_2}}^{}$ and precipitous rise in non-respiratory bicarbonate synchronizing with the initiation of feeding. The postnatal pH results in a light positive overawing during 2–3 d and stabilizes within 1–2 weeks.