Spherical outcropping of the respiratory bronchioles and primary site of gas exchange with the blood. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates[WP]. [ http://en.wikipedia.org/wiki/Pulmonary_alveolus ]

Synonyms: respiratory alveolus pulmonary alveolus alveolus pulmonis lung alveolus

This is just here as a test because I lose it