Why doesn't exercise grow the lungs when other factors do?

Exercise training enhances every component of the O(2) transport and metabolic system--except the lungs. Consequently, the lungs can contribute to the limitation of VO(2)max. Only hypoxia early in life and substantial lung resection reproducibly stimulate growth in normal lungs across species. Possible pathways involve genes activated by hypoxia or mechanical strain, or both, including growth factors, hormones, nitric oxide, and retinoids.