Critics have claimed that, without avian air sacs, modest improvements in a few aspects of a modern reptile's circulatory and respiratory systems would enable the reptile to achieve 50% to 70% of the oxygen flow of a mammal of similar size, and that lack of avian air sacs would not prevent the development of endothermy.
Os críticos afirmaron que, sen sacos aéreos como os das aves, pequenas melloras nuns poucos aspectos nos sistemas respiratorio e circulatorio dun réptil moderno permitiríanlles aos réptiles obter do 50 ao 70% do fluxo de oxíxeno dun mamífero de similar tamaño, e que a falta de sacos aéreos non impediría o desenvolvemento da endotermia.