Effects of barometric pressure and abnormal gas mixtures on gaseous exchange by the avian embryo

Abstract

Gas moves through the pores of the egg shell by diffusion in the gas phase. The gas flux is therefore determined by the product of the effective conductance of the shell and the partial pressure gradient of the gas between the ambient air and the inner side of the shell. The partial pressure gradient of oxygen is decreased by a reduction of the oxygen partial pressure in the ambient air. This can be achieved by reducing barometric pressure at normal ambient oxygen concentration or by reducing ambient oxygen concentration at standard barometric pressure. Both methods are reported to decrease oxygen consumption of the embryo but to a different degree. At the same ambient oxygen pressure the reduction is less in eggs exposed to a reduced barometric pressure. In an attempt to explain this difference, chicken embryos aged 16-19 days were exposed to various oxygen concentrations and carbon dioxide production was measured. At subnormal oxygen concentrations carbon dioxide output diminished as the oxygen concentration was lowered and the duration of exposure was prolonged. At oxygen concentrations above normal a small but significant increase in carbon dioxide production was found. Finally the results are compared with those in the literature on the diverse effects of a continuous reduction of barometric pressure and ambient oxygen concentration. This difference is ascribed to the fact that a reduction of barometric pressure not only decreases oxygen partial pressure in the ambient air but also increases effective conductance of the egg shell, the latter being inversely proportional to the barometric pressure. © 1980 by the American Society of Zoologists.