Anatomy and Physiology VERTEBRATES left the water and became air breathers approximately one billion years ago. This transition gave rise to a whole host of new problems, one of which was maintenance of the terminal spaces for gas exchange in a relatively dry state. Previously, it had been required to maintain an adequate volume flow of water and to keep gas-exchange units persistently wet. It was now required to keep these spaces persistently dry. This necessity called for an elaborate series of structural and functional adaptations to prevent flooding and preserve the integrity of pulmonary gas exchange. Failure of these.
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