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Alligator Breath
Birds have a unidirectional system of airflow within their lungs that has been attributed to the peculiarities of flight. However, Farmer and Sanders (p. 338) provide evidence that this unidirectional and more or less continuous flow of air also occurs through parts of the alligator lung; in contrast to the tidal, biphasic system in mammals. By analyzing lung and tracheal structures, the similarities of the alligator lungs were compared with those of birds. The data suggest that the unusual properties of bird lungs originated before the divergence of the alligator line from the dinosaur or avian line.
Abstract
The lungs of birds move air in only one direction during both inspiration and expiration through most of the tubular gas-exchanging bronchi (parabronchi), whereas in the lungs of mammals and presumably other vertebrates, air moves tidally into and out of terminal gas-exchange structures, which are cul-de-sacs. Unidirectional flow purportedly depends on bellowslike ventilation by air sacs and may have evolved to meet the high aerobic demands of sustained flight. Here, we show that air flows unidirectionally through parabronchi in the lungs of the American alligator, an amphibious ectotherm without air sacs, which suggests that this pattern dates back to the basal archosaurs of the Triassic and may have been present in their nondinosaur descendants (phytosaurs, aetosaurs, rauisuchians, crocodylomorphs, and pterosaurs) as well as in dinosaurs.
