A membrane made from one or several layers of cells that lines body cavities and internal organs. Some of the cells secrete mucous so that the membrane is always kept moist - this is why it always has a slightly shiny appearance. Mucous membranes are found in the gastrointestinal tract, the respiratory tract, the reproductive tract and the urinary tract. They protect the tissue beneath from external influences (e.g. by removing particles of dust or pathogens from the air that is breathed in) and from internal influences (e.g. protecting the tissue of the bladder from the accumulating urine).