The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane that covers internal organs is called a visceral membrane; while the one that covers the cavity wall is called the parietal membrane. Between the two opposing serosal surfaces is often a potential space, mostly empty except for the small amount of serous fluid.