Mucus is produced by the specialized membranes that line the organs of the body cavities that open to the exterior, such as those found in the respiratory, digestive and reproductive systems. Although mucus is a secretion that is most commonly known for its presence in the nasal and oral cavities, it is also produced in the stomach, esophagus and in the male seminal vesicles. The secretion of mucus plays a vital role as a protective and transport medium in the internal functions of the digestive, respiratory and reproductive systems of humans and other vertebrates.
The mucus produced in the human respiratory system protects the lungs by limiting the foreign particles that reach them. Mucus in the nasal and airway passages traps small particulates, such as dust, allergens and bacteria, before they can enter the system. Phlegm refers to a specific kind of mucus that is normally confined to the respiratory tract. Nasal mucus is produced by the tissues that line the airway organs, such as those lining the trachea and bronchioles. A continuous production of nasal mucus moisturizes inhaled air and also prevents airway and nasal tissues from becoming dry.
In the human digestive system, mucus prevents the cell walls of the stomach from being damaged by the high concentration of acid present within the organ. The esophagus makes use of mucus as a lubricant to aid in the passage of food on its way to the stomach.