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In the alveoli, oxygen from the air is absorbed into the blood. Carbon dioxide, a waste product of metabolism, travels from the blood to the alveoli, where it can be exhaled. Between the alveoli is a thin layer of cells called the interstitium, which contains blood vessels and cells that help support the alveoli.
1. Alveoli are composed of epithelial layers and extracellular matrix enclosed in capillaries while alveolar sacs are the distal ends of alveolar ducts. 2. The alveoli sacs are formed by a group or cluster of alveoli, and it is there where they communicate while the alveoli are made up of collagen and elastic fibers.. Alveoli are composed of epithelial layers and extracellular matrix enclosed in capillaries while alveolar sacs are the distal ends of alveolar ducts. 2. The alveoli sacs are formed by a group or cluster of alveoli, and it is there where they communicate while the alveoli are made up of collagen and elastic fibers.