Stage 3 (11-20 days) involves progressive repair of the lung, with a decreased number of alveoli. There is compensatory hypertrophy of the remaining alveoli, and hypertrophy of bronchial-wall muscle and glands. Regenerating clear cells are seen, along with exudation of alveolar macrophages and histiocytes into airways. Airtrapping, pulmonary hyperinflation, tracheomegaly, tracheomalacia, interstitial edema, and ciliary dysfunction may also be present, (E-Medicine, 2009).