The relationship between structure and function of the pulmonary gas‐exchange apparatus and its comparative aspects is demonstrated in four sections: (1) the lung as gas exchanger, (2) the gas‐exchange structures of the human lung, (3) comparative aspects of structure‐function correlation, and (4) analysis of structure‐function discordancy. Physical activity and body size are described to influence the structural dimensions of the pulmonary gas exchange apparatus in different ways. As far as the inhalation of toxic substances is concerned, different kinds of effects must therefore be taken into consideration, depending on an animal's size and metabolism; hence, experiments of comparative anatomy and physiology are desirable in order to find an appropriate animal model that permits extrapolation to humans.