Metabolic control of pulmonary ventilation in the developing chick embryo

In birds, during the period from the breaking of the air cell by the beak (internal pipping) to hatching, pulmonary ventilation (VE) begins and gas exchange is jointly provided by the lungs and the chorioallantoic membrane (CAM). We asked to what extent, during this phase of two concurrent gas exchange organs, changes in the embryo's metabolic needs were accompanied by changes in VE. The carbon dioxide and oxygen exchange rates (VCO2, VO2) through lungs and CAM were separately, but simultaneously, measured in chicken embryos at 20-21 days of incubation, while VE was calculated from the measurements of pressure oscillations in the air cell during breathing. During the last 24 h of incubation, lung VO2 and VCO2 gradually progressed as the corresponding CAM values declined. An increase in egg temperature (T) from 33 to 39 degrees C increased the embryo's total metabolic rate, especially when the lungs were the predominant gas exchange route. Whether metabolism increased because of the embryo's development or because of the increase in T, VE was linearly proportional to lung VO2 and VCO2, and not to the embryo's total metabolic rate. Hence, in the developing chick embryo, VE control mechanisms sense the peripheral tissue requirements via the gaseous component of cellular metabolism.