Figure 3b. Distribution of gas and tissue along cephalocaudal axis. (a) In normal lung, volume of gas was larger than volume of tissue. (b) At baseline, volume of tissue increased and volume of gas decreased in all levels of lung. Gas was located predominantly in upper lung, whereas tissue was predominant in caudal lung regions. (c) One hour after prone positioning, compared with findings at baseline, volume of gas decreased in cephalic regions, whereas it increased in caudal regions. At the same time, lung volume below diaphragm was expanded. (d) Return to supine position restored distribution of gas and tissue to similar pattern at baseline. DD = dome of diaphragm.