Real animals were not meant to sit still and patiently let a physiologist measure their “basal” metabolic rates. Animals eat, drink, sleep, run, chase, mate, and play. When their physical exertion is maximal, parts of the system, such as lungs and heart, must perform at a maximal level. Therefore, the limits on maximal performance are much more informative about animal design and much more interesting than the resting or idling level. Think of a racing car or an airplane sitting still with the engine running; the idling speed gives little information about maximal performance.

Maximal performance

During heavy physical work, such as running at top speed, oxygen is taken up in the lungs at a maximal rate and diffuses into the red blood cells of the lung capillaries, where it binds to the hemoglobin. The heart pumps the blood to the muscles, where oxygen diffuses from the capillaries to the cells and the mitochondria, which serve as the final oxygen sink. At each point in this chain, the flow rate of oxygen must equal the rate at which it is consumed in the sink.

Carbon dioxide, produced at a rate corresponding to the oxygen consumed, traverses the same pathway but in the opposite direction. At each step, the flow rate must equal the rate of production as CO2 flows from the mitochondria into the capillaries, is circulated to the lungs, diffuses into the alveoli, and is dumped to the outside atmosphere.