alveolus

An alveolus (plural alveoli) is one of the tiny, thin-walled air sacs at the very end of the respiratory path, where the bronchioles open out. Each sac is wrapped in capillaries, and the wall between the air and the blood is thin enough for gas exchange to happen by diffusion: the arriving blood holds less oxygen than the air, so oxygen moves from the air into the blood, and it holds more carbon dioxide than the air, so carbon dioxide moves from the blood into the air. The alveoli are where the path's work is done; everything before them — nose, pharynx, larynx, trachea, bronchi, bronchioles — warms, filters and delivers. Because the sac wall is thin and the capillary touches it, breathing and blood flow are coupled: air that never reaches the alveoli exchanges nothing, which is why every airway before them — the nose to the smallest bronchioles — is called the conducting zone.