Mucous membrane: Lines the digestive, respiratory, urinary, and reproductive tracts. it lines all body cavities that open to the outside. It is usually ciliated. Its most obvious function is to produce mucus, but it also concentrates bile in the gallbladder. In the intestines, it secretes enzymes for the digestion of food and nutrients before absorption. Mucous membranes protect, absorb nutrients, and secretes mucus, enzymes, and bile salts.