What made you want to look up frenum? Please tell us where you read or heard it (including the quote, if possible).
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. Surfactant layer that is lining the alveolar surface. 2. the alveolar epithelium. 3 the basement membrane of the alveolar epithelium. 4. extremely thin interstitial space. 5 the basement membrane of the capillary endothelium. 6. The capillary endothelium.