Exercise performance of mammals: an allometric perspective.

We have examined aerobic exercise performance among the mammals with particular attention to the constraints that body size places on all aspects of muscle biomechanics, aerobic energetics, tissue oxygen diffusion, cardiovascular oxygen delivery, and pulmonary oxygen uptake. Several body-size-dependent patterns emerge that seemingly govern aerobic performance in mammals, with the caveat that at any given body size there is a range of aerobic capacities, the result of natural selection operating on the size-dependent "default values" of structure and function. Among these default values, the following apparent functional clusters surface: 1. In general, concentrations and pressures (e.g., of proteins and gases) are roughly independent of body size. Inspiratory and expiratory ventilation pressures, blood pressure and the partial pressures of O2 and CO2 in lungs, blood, and tissues do not vary with body size. Likewise, concentrations of hemoglobin, myoglobin, and hematocrit are independent of body size. 2. Most volumes and capacities scale linearly with body size (i.e., as a constant function of body mass). In addition to heart, lung, and total blood volumes, important examples relevant to exercise performance are the diffusing capacities for oxygen in the lung and, apparently, in the tissues. 3. Finally, most time-dependent variables related to oxygen delivery scale allometrically with body mass; they are of shorter duration in small animals than in large ones. Biological rates, for example, Vmax of working muscle, heart and respiratory rates, and transit times of blood through the muscles and lungs, all vary roughly as the -1/5 to -1/4 power of body mass.