Any of the 300 million or so small air spaces in the lungs 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.
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