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.
Endothelium is the internal lining of your blood vessels 10. Viscera a. A delicate layer called a serous membrane lines the walls of these internal cavities and covers the surfaces of the enclosed viscera. 11. Thoracic Cavity a. Each pleural cavity, which contains a lung, is lined by a shiny, slippery serous membrane that reduces the friction as the lung expands and recoils during respiration. b. The serous membrane lining a pleural cavity is called pleura. The visceral pleura covers the outer surfaces of the lung, the parietal pleura covers the inner surfaces of the lung. c.