Determinants of maximal oxygen consumption.

This article lays out the determinants of maximal O2 consumption (VO2 max) achieved during high intensity endurance exercise. It is not a traditional topical review but rather an educational essay that intertwines chance observations made during an unrelated research project with a subsequent program of stepwise thought, analysis and experimentation to reveal how O2 is delivered to and used by the mitochondria. The centerpiece is the recognition that O2 is delivered by an inter-dependent system of transport components functioning as a "bucket brigade", made up of the lungs, heart, blood and circulation, and the muscles themselves, each of which affects O2 transport by similar amounts as they change. There is thus no single "limiting factor" to VO2 max. Moreover, each component is shown to quantitatively affect the performance of the others. Mitochondrial respiration is integrated into the O2 transport system analysis to reveal its separate contribution to VO2 max, and to show that mitochondrial PO2 at VO2 max must be extremely low. Clinical application of the O2 transport systems analysis is described to separate central cardiopulmonary from peripheral tissue contributions to exercise limitation, illustrated by a study of patients with COPD. Finally, a short discussion of why muscles operating maximally must endure an almost anoxic state is offered. The hope is that in sum, both the increased understanding of O2 transport and the scientific approach to achieving that understanding described in the review can serve as a model for solving other complex problems going forward.