A hypothesis, proposed by Ewald Weibel and Charles Richard Taylor in 1981, postulating that biological systems adhere to an ‘economy of design’ giving a close match between their various structural and functional parameters. Hence, no single parameter in the system has unnecessary excess capacity, beyond the requirements of the system. The hypothesis was tested initially by analysing the mammalian respiratory system. Here it was found that, except for the lungs, the structures of the oxygen-transfer system, including blood, heart, muscle capillaries, and mitochondria, are well matched to the functional capacity of the system. However, components often serve in more than one physiological system; for example, the blood and blood vessels are also parts of the excretory system. Therefore, apparent spare capacity in one system might be needed for another system.