alveoli

Meaning of alveoli: The alveoli present an enormous surface area within the lungs, a feature essential to allowing efficient gas exchange with each breath.

Definition of alveoli

Alveoli are tiny, thin walled air sacs located at the very end of the airway branches within the lungs, called bronchioles. Each alveolus is surrounded by a dense network of tiny blood vessels called capillaries, and it is across this thin shared surface that oxygen and carbon dioxide are exchanged between inhaled air and the bloodstream.

With each breath, oxygen from inhaled air passes across the alveolar wall into the surrounding capillaries, while carbon dioxide carried in the blood passes in the opposite direction into the alveoli to be exhaled. The lungs contain an enormous number of alveoli, giving them a very large internal surface area available for this exchange despite their compact overall size.

Because alveoli are thin walled and directly involved in gas exchange, several lung diseases affect them directly, including conditions that cause fluid or inflammation to accumulate within the alveoli or that damage and destroy alveolar walls, both of which impair the lungs' ability to move oxygen and carbon dioxide normally.