The pulmonary alveoli are minute air-filled sacs in the vertebrate lung, thin walled and surrounded by blood vessels.
Small polyhedral outpouchings along the walls of the alveolar sacs, alveolar ducts and terminal bronchioles through the walls of which gas exchange between alveolar air and pulmonary capillary blood takes place. ... (12 Dec 1998) ... 
The pulmonary alveoli are minute air-filled sacs in the vertebrate lung, thin walled and surrounded by blood vessels.