Biochemical and anatomical adaptation of the lung to oxygen-induced injury

A review of anatomical and biochemical responses of the lung to high concentrations of oxygen leads us to postulate a biphasic adaptive response. The early phase entails a defense against life-threatening pulmonary edema engendered by destruction of oxygen susceptible cells forming most of the air-blood interface. This defense is brought about by type II alveolar cell replication to reform a continuous epithelial layer in the alveoli; its success would depend upon the rapidly with which this continuity can be reestablished. Factors favoring a successful defense would include an initial large population of type II cells or the ability of type II cells to divide fast enough to reestablish continuity before of oxygen-sensitive cells (type 1 alveolar epithelial and endothelial cells) proceeds to fatal pulmonary edema; both conditions probably exist in young animals, which are known to be more resistant to hyperoxia than old animals. The second phase of adaptation would require the development of increased tolerance of previously susceptible cells to continued exposure to high oxygen concentrations to prevent their total destruction. We postulate that here the development of new biochemical defenses or the augmentation of those previously present would play a major role.