Limitations to exercise capacity and endurance: pulmonary system.

The response of the pulmonary system to exercise is well-documented to be quite precise in its homeostatic regulation, highly efficient in terms of its physiologic cost, and operating well within its maximum reserves. Two exceptions to these generalizations are documented here in the highly-trained athlete: 1) the arterial hypoxemia observed in short-term work at a VO2 greater than 4-5 l . min-1, and 2) the tachypneic hyperventilation of long-term, heavy exercise in varying environmental conditions. The potential causes and consequences of these responses are discussed with reference to so-called exercise "limitations". The trained state as a cause of pulmonary limitations is emphasized.