Although it is often stated that the structure of the human lung is ideally suited to its gas exchange function, the lung is very vulnerable under some abnormal conditions. One example is the postoperative period in a patient with an otherwise normal lung where retained secretions can rapidly cause unventilated areas or atelectasis and substantial impairment of gas exchange. Some pulmonologists may be surprised to learn that evolution has provided a very different, and arguably superior, lung design in the bird. Here the gas exchange and ventilatory functions of the lung are separated. Gas exchange occurs in relatively rigid parabronchi which are more robust than the delicate alveoli in the human lung, and ventilation is carried out by highly expandable air sacs. A comparison of these two completely divergent evolutionary paths throws light on some of the problems of the human lung.