Gas exchange by a bird's embryo

Current knowledge about the mechanism of chick embryo gas exchange is outlined. The overall gas exchange pathway between ambient air and chorioallantoic capillary blood is divided into two components and each are discussed separately. The first component, comprised of the shell and outer shell membrane, separates ambient air and air cell gas. Equations are given for transport of O₂, CO₂ and water vapor across this barrier. Evidence is presented which shows that transport of these gases is by diffusion. This means that the relationship between the embryo's metabolic rate and the permeability of the shell is important in determining the P_O2 and P_CO2 the air cell. The second component, comprised of the inner shell membrane and a thin tissue layer, separates air cell gas and allantoic capillary blood. Little is known about transport across this barrier. Separate measurements of air cell and allantoic venous blood P_OO2 and P_CO2, for the same age embryo, are compared. These data indicate that O₂ transport by this barrier is inefficient but CO₂ is transported readily. Possible explanations for these observations are briefly discussed.