This informative post by Nick Mark explaining what VO₂ max is and how it varies from humans, sled dogs, pronghorn, bats, hummingbirds, and bumblebees.
VO₂max – the maximum rate of oxygen consumption during peak exercise – is one of the most informative numbers in physiology. Among athletes, physiologists, and intensivists alike, it functions as a kind of summary statistic for the entire oxygen delivery cascade: how well the lungs extract oxygen, how efficiently hemoglobin carries it, how powerfully the heart pumps it, how densely the capillaries deliver it, and how effectively mitochondria consume it. Each step is potentially rate-limiting, and VO₂max tells you how well the entire oxygen cascade functions. VO₂max also happens to be one of the strongest independent predictors of all-cause mortality.
VO₂max is best understood not as a single trait, but as the product of a chain, as described by the Fick equation:
VO₂max = Cardiac Output × Arteriovenous O₂ Difference
Or, expanding the Fick equation more fully: pulmonary ventilation × extraction efficiency × hemoglobin concentration × cardiac output (stroke volume × heart rate) × capillary density × mitochondrial oxidative capacity. Each of these steps can be a bottleneck. In the short term, training, and in the long term evolution, can widen any these bottlenecks.