The lung of the emu, Dromaius novaehollandiae: a microscopic and morphometric study.

Qualitative and quantitative characteristics suggest that the lung of the emu is poorly adapted for gas exchange when compared with that of other birds. The granular epithelial cells extend over the air capillaries, and the squamous epithelial cells have microvilli indicating a poor differentiation of the epithelium of the exchange tissue. The surface area of the blood-gas tissue barrier per unit body mass was only 5.4 cm2/g, the volume of the pulmonary capillary blood per unit body mass was only 0.93 cm3/kg, and the tissue barrier was unusually thick (0.232 micron). These parameters produce a relatively small total morphometric pulmonary diffusing capacity for oxygen of 0.014 ml O2/sec/mbar/kg. The findings conform to the evolution of a very large flightless bird in a warm environment lacking effective predators.