Any of the 300 million or so small air spaces in the lung s where carbon dioxide leaves the blood and oxygen enters it.
Alveoli form clusters (alveolar sacs) connected by alveolar ducts to the bronchioles. Their thin walls contain numerous capillaries, supported by a mesh of elastic and collagenous fibres; gas exchange between them occurs by diffusion . A film of fatty substances (surfactant) over the walls reduces surface tension, keeping the alveoli from collapsing and making it easier to expand the lungs. Alveolar macrophages (see leukocyte ; lymphoid tissue ) act as mobile scavengers, engulfing foreign particles in the lungs.