Acinus is the name applied to each of the minute sacs of which secreting glands are composed, and which usually cluster around the branches of the gland-duct like grapes on their stem. (See GLAND.)
n. (pl. acini) 1. a small sac or cavity surrounded by the secretory cells of a gland. Some authorities regard the term as synonymous with *alveolus, but others distinguish an acinus by the possession of a narrow passage (lumen) leading from the sac. 2. (in the lung) the tissue supplied with air by one terminal *bronchiole. *Emphysema is classified by the part of the acinus involved (i.e. centriacinar, panacinar, or periacinar). —acinous adj.
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.... alveolus
n. a subdivision of the bronchial tree that does not contain cartilage or mucous glands in its wall. Bronchioles open from the fifth or sixth generation of bronchi and extend for up to 20 more generations before reaching the terminal bronchioles. Each terminal bronchiole divides into a number of respiratory bronchioles, from which the *alveoli open. Each terminal bronchiole conducts air to an acinus in the *lung. —bronchiolar adj.... bronchiole
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