A pulmonary alveolus, also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where pulmonary gas exchange takes place. Oxygen is exchanged for carbon dioxide at the blood–air barrier between the alveolar air and the pulmonary capillary. Alveoli make up the functional tissue of the mammalian lungs known as the lung parenchyma, which takes up 90 percent of the total lung volume.
A histologic slide of a human alveolar sac
Cryptococcosis of lung in patient with AIDS. Mucicarmine stain. Histopathology of lung shows widened alveolar septum containing a few inflammatory cells and numerous yeasts of Cryptococcus neoformans. The inner layer of the yeast capsule stain red.
Parenchyma is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology, it is the name for the tissue that fills the interior of flatworms.
Lung parenchyma showing damage due to large subpleural bullae.