Lung alveoli in pneumonia, illustration. In diseases such as pneumonia, a thick mucus blocks the lung alveoli, reducing lung capacity. In severe cases, this can be fatal. A bundle of alveoli are shown here, sectioned at left to show the mucus (yellow). At right, the capillary bed is seen, where gas exchange normally takes place with the air in the alveoli. The capillary bed arises from an arteriole (upper right), while the alveoli arise from an alveolar duct that leads to a bronchiole and eventually to the main lung airways.