In summary, we have shown that the design of the pulmonary system from the architectural capacities of the lung parenchyma and respiratory muscles to the remarkable, multi-level neural integration of breathing pattern and respiratory muscle recruitment is clearly intended for the exercising state. Furthermore, the system shows remarkable capability for true adaptation, both phylogenetically and even within only a few generations within a species, when preservation of the organism's ability to survive and function is at stake. At the same time there are limits to the system's homeostatic capabilities, and these appear in instances other than the "usual" ones, where the capabilities for gas transport and utilization beyond the lung (i.e., by the cardiovascular and musculo-skeletal systems) surpass those of the lung and chest wall, such as during exercise in certain pulmonary disease states or in alien environments or in the highly trained. Exercise-induced hypoxemia in the thoroughbred horse is a different type of dominance of the superior locomotor control system, because their extraordinary capability to produce and sustain a very high limb velocity dictates requirements for airway flow rates which may surpass the mechanical capabilities of the lung and perhaps even the chest wall. So this hypothesis does indeed suggest that the healthy pulmonary system may become a so-called "limiting" factor to oxygen transport and utilization and to CO2 transport and elimination, at least during short-term maximum exercise in the highly trained. On the one hand, the idea is especially appealing in a philosophical sense because of its conceptual tidiness and its confirmation of the premise that no organ system has limitless functional capacity; on the other hand, given the long list of our still untested speculations, we could use a bit more data.