The purpose of this paper is to explore further the possibilities of a theory of structure (form) as determined by function. The paper is preliminary in nature and emphasis is placed more on the methodological approach than on immediately verifiable conclusions. An approximate mathematical treatment of diffusion between alveoli and blood is given, taking into account the periodic variations of the concentration of gases in the lung and in the bloodstream due to the periodicity of breathing. Expressions for the blood concentration and the alveolar concentration of oxygen as functions of time are derived in terms of the frequency of breathing and of some structural geometric parameters of the lung. It is then shown what tentative conclusions can be drawn from such considerations in regard to the structure of the lung.