Pulmonary constraints limit prolonged performance of some highly trained endurance athletes during extreme exercise. Diffusion limitation is probably the most significant among the interrelated pulmonary deficiencies that cause reduced arterial hemoglobin oxygen saturation. Highly trained athletes are affected because they have developed adaptations in muscle tissue and cardiovascular function which surpass those of the less‐adaptive pulmonary system. Untrained people have not accrued muscle and cardiovascular adaptations, and therefore cannot achieve an exercise stress that exceeds the usual pulmonary system functional reserve.