In vertebrates, mucus (/mjuːkəs/ MYOO-kəss; adjectival form: "mucous") is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is rich in glycoproteins and water and is typically produced from cells found in mucous glands. Mucous fluid may also originate from mixed glands, which contain both serous and mucous cells. It is a viscous colloid containing antiseptic enzymes (such as lysozymes), immunoglobulins, inorganic salts, proteins such as lactoferrin, and glycoproteins known as mucins that are produced by goblet cells in the mucous membranes and submucosal glands. This mucus serves to protect epithelial cells (that line the tubes) in the respiratory, gastrointestinal, urogenital, visual, and auditory systems; the epidermis in amphibians; and the gills in fish. A major function of this mucus is to protect against infectious agents such as fungi, bacteria and viruses. The average human nose produces about a liter of mucus per day. Most of the mucus produced is in the gastrointestinal tract.