<P> Bony fish, hagfish, snails, slugs, and some other invertebrates also produce external mucus . In addition to serving a protective function against infectious agents, such mucus provides protection against toxins produced by predators, can facilitate movement and may play a role in communication . </P> <P> In the human respiratory system, mucus, also known as airway surface liquid (ASL), aids in the protection of the lungs by trapping foreign particles that enter them, in particular, through the nose, during normal breathing . Further distinction exists between the superficial and cell - lining layers of ASL, which are known as mucus layer (ML) and pericilliary liquid layer (PCL), respectively . "Phlegm" is a specialized term for mucus that is restricted to the respiratory tract, whereas the term "nasal mucus" describes secretions of the nasal passages . </P> <P> Nasal mucus is produced by the nasal mucosa; and mucus lining the airways (trachea, bronchus, bronchioles) is produced by specialized airway epithelial cells (goblet cells) and submucosal glands . Small particles such as dust, particulate pollutants, and allergens, as well as infectious agents and bacteria are caught in the viscous nasal or airway mucus and prevented from entering the system . This event along with the continual movement of the respiratory mucus layer toward the oropharynx, helps prevent foreign objects from entering the lungs during breathing . This explains why coughing often occurs in those who smoke cigarettes . The body's natural reaction is to increase mucus production . In addition, mucus aids in moisturizing the inhaled air and prevents tissues such as the nasal and airway epithelia from drying out . Nasal and airway mucus is produced continuously, with most of it swallowed subconsciously, even when it is dried . </P> <P> Increased mucus production in the respiratory tract is a symptom of many common illnesses, such as the common cold and influenza . Hypersecretion of mucus can occur in inflammatory respiratory diseases such as respiratory allergies, asthma, and chronic bronchitis . The presence of mucus in the nose and throat is normal, but increased quantities can impede comfortable breathing and must be cleared by blowing the nose or expectorating phlegm from the throat . </P>

Where is mucus secreted in the digestive system