The respiratory system, that is the set of compartments interposed between the cells and the environment, is best studied among mammals. Across the compartments there exist net fluxes of O2 and CO2, the intensities of which depend on the animal's size. In steady state, at rest, the relation between O2 flux and body size has the form of an allometric equation in which the O2 consumption, MO2, is proportional to body mass with an exponent or scaling factor of about 0.75 (Brody, 1945; Kleiber, 1961). The principal experimental methods to force animals to maximal MO2 are cold exposure and exercise. According to the concepts of physiological time, strenuous exercise to reach maximal MO2 should be shorter in small animals than in large ones. The scaling factor for maximal MO2 seems to be higher than standard conditions; in other terms, large exercising mammals may increase their maximal MO2 more than small animals. The comparative approach of maximal MO2 is a powerful tool for finding where in the respiratory system the strictures or bottlenecks are which may limit O2 consumption. According to the ambient or organismal conditions (pressure, temperature, posture, nutrition), the site of the stricture may vary. As far as is known, the O2 consumption of the whole human body, in standard conditions as well as in maximal activity, and for a given mass, is such that, compared to other mammals, the human exhibits no species-specific characteristics.

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