Meeting Abstract

65.1  Jan. 7  Unidirectional flow in the lungs of archosaurs FARMER, CG; Utah Artificial Heart Institute

"A satisfactory answer has not been found to the question of the benefit of unidirectional air flow in bird lungs" (Meyer et al., 1981). Studies on pulmonary air flow of the sister group to birds, crocodilians, may provide insight into this question. American alligators have two very cavernous sac-like lobes of the lung immediately dorsal to the heart and great vessels. These lobes physically adhere to the pericardium. With each beat of the heart a mechanical tugging on these lobes occurs, which could agitate the lungs and the gases contained therein. This morphology, along with the striking similarity between the spiraling tapered tubes of the crocodilian lung with the dorso- and ventrobronchi of bird lungs, has lead me to investigate the possibility that unidirectional cardiogenic flow exists in crocodilians. Measurements of gas movements within the lungs have been made using hot bead anemometers both during apnea and during ventilation. During apnea, a small unidirectional pulse of air moves in the lung coincident with each heartbeat. Unidirectional flow also appears to occur during ventilation. The presence of unidirectional flow in the lungs of crocodilians indicates that the most parsimonious scenario for the evolution of this trait is that unidirectional flow arose long before the evolution of Aves in the common ancestor to birds and crocodilians. This common ancestor was in many respects more akin to extant crocodilians than to birds. For example these animals were terrestrial quadrupeds. Thus unidirectional flow in these animals could not have been an adaptation for flight. This ancestral stock probably had an ectothermic physiology and an intermittent pattern of breathing. Unidirectional flow may therefore have arisen in this lineage to take advantage of mixing of lung gases during apnea that can occur with the beating heart.