The hypothesis that, in biological organisms, structural design is matched to functional demand is difficult to test because it is largely based on anecdotal evidence suggesting economic design. The hypothesis of symmorphosis postulates a quantitative match of design and function parameters within a defined functional system; because of its stringency it is refutable and can, therefore, be subjected to empirical test, for example, by assessing whether the structures that support the pathway for oxygen from the lung to the consumer in muscle cells are quantitatively adjusted to the limit of functional performance of the respiratory system. The study of allometric and adaptive variation leads to the conclusion that the hypothesis of symmorphosis is acceptable for all internal compartments of the respiratory system (blood, heart, muscle capillaries, and mitochondria), whereas it must be refuted for the lung that forms the interface to the environment.