(1) The minute divisions of glands and the air sacs of the lungs.
(2) The sockets of the teeth in the jawbone.
n. (pl. alveoli) 1. (in the *lung) a blind-ended air sac of microscopic size. About 30 alveoli open out of each alveolar duct, which leads from a respiratory *bronchiole. The alveolar walls, which separate alveoli, contain capillaries. The alveoli are lined by a single layer of *pneumocytes, which thus form a very thin layer between air and blood so that exchange of oxygen and carbon dioxide is normally rapid and complete. Children are born with about 20 million alveoli. The adult number of about 300 million is reached around the age of eight. 2. the part of the upper or lower jawbone that supports the roots of the teeth (see also mandible; maxilla). After tooth extraction it is largely absorbed. 3. the sac of a *racemose gland (see also acinus). 4. any other small cavity, depression, or sac. —alveolar adj.
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