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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.
The walls of the ventral body cavity and outer covering of its organs contain a thin covering called the serosa (also called serous membrane). It is a double-layered membrane made up of two parts called the â parietal serosa â (lines the cavity walls) and â visceral serosa â (covers organs in the cavity). The serous membranes are separated by a thin layer of fluid called â serous fluid â. Serous fluid is secreted by both membranes and acts as a lubricant, allowing organs to slide in the cavity without causing friction. Typically, the serous membranes are named according to the cavity and organ they associate with.