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 epithelial cells that line capillaries are routinely referred to as endothelial cells, and the capillary epithelium is called an Endothelium-a term reserved for the layer of flat cells lining blood vessels, lymph vessels, and other cavities.he epithelial cells that line capillaries are routinely referred to as endothelial cells, and the capillary epithelium is called an Endothelium-a term reserved for the layer of flat cells lining blood vessels, lymph vessels, and other cavities.