Interstitial edema in the alveolar septa is the first morphologically recognisable change to be observed in cases of shock. It is brought about by the altered function of the membranes of the damaged epithelium and endothelium in the alveolar wall. At the same time there is an impairment of gaseous exchange, which is rendered more difficult by the exudative process in the interstitium. Pari passu with these events there is injury to the cells of both the alveolar epithelium and the alveolar capillary endothelium. Both these processes are still reversible. The point of irreversibility appears to be reached--so far as time is concerned--at the end of the first week, after which the injurious effects on the cell are established, since the thin alveolar wall necessary for the exchange of gases becomes overgrown with bulky alveocytes (Tpye II), and the fibroblasts in thealveolar interstitium push the capillaries away from the surface of the alveolus. In most of the advanced cases of shock this process of thickening of the alveolar wall exceeds the critical value, and respiratory exchange is so impaired that satisfactory functioning of the lungs is no longer possible.