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.
The inner layer covers organs in bodily cavities and is called the visceral membrane; this inner layer of epithelial cells is followed by a connective tissue; after the connective tissue is the second layer of epithelial cells of the serous membrane called the parietal layer.The Latin anatomical name is tunica serosa.he inner layer covers organs in bodily cavities and is called the visceral membrane; this inner layer of epithelial cells is followed by a connective tissue; after the connective tissue is the second layer of epithelial cells of the serous membrane called the parietal layer. The Latin anatomical name is tunica serosa.