In order to illustrate the alveoli more precisely, we only focused on several of them. Twenty-one single alveoli were chosen from different part of the lung. The three-dimensional images of the alveoli were shown in Figure 5A . The maximum diameter and the minimum diameter were measured in Figure 5B . The ratio of maximum and minimum diameters reflects the morphology of the alveoli to some extent. If this ratio approximates one, the shape of the alveolus is closer to a spheroid. The alveoli provide an enormous surface area for respiratory exchange; the change of alveolar surface area is closely related to lung health. For this reason, we calculated the surface area of each alveolus, as shown in Figure 5C .