The regulated release of mucus by the mucosa. Mucus is a viscous slimy secretion consisting of mucins and various inorganic salts dissolved in water, with suspended epithelial cells and leukocytes. The mucosa, or mucous membrane, is the membrane covered with epithelium that lines the tubular organs of the body. Mucins are carbohydrate-rich glycoproteins that have a lubricating and protective function.