Snails, slugs, certain other invertebrates, and hagfish, and also produce external mucus, which in addition to serving a protective function, can facilitate movement and play a role in communication.

Mucus also contains mucins, produced by goblet cells in the mucous membranes and submucosal glands, and inorganic salts suspended in water. The average human body produces about a litre of mucus per day.

Respiratory system

In the respiratory system mucus aids in the protection of the lungs by trapping foreign particles that enter, particularly through the nose, during normal breathing. "Phlegm" is a specialized term for mucus that is restricted to the respiratory tract, while the term "mucus" more globally describes secretions of the nasal passages as well.

Nasal mucus is produced by the nasal mucosa, and mucal tissues 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 such as 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. Additionally, 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 constitutively, with most of it swallowed unconsciously, even when it is driedFact: date=November 2008.

Increased mucus production in the respiratory tract is a symptom of many common illnesses, such as the common cold and influenza. Similarly, 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. Tears are also a component of nasal mucus.

Diseases involving mucus

Generally mucus is clear and thin, serving to filter air during inhalation. During times of infection, mucus can change color to yellow or green either as a result of trapped bacteria, or due to the body's reaction to viral infection. Such colored mucus or phlegm usually has an offensive putrid odour.Or: date=October 2008

In the case of bacterial infection, the bacterium becomes trapped in already clogged sinuses, breeding in the moist, nutrient-rich environment. Antibiotics may be used to treat the secondary infection in these cases, but will generally not help with the original cause.

In the case of a viral infection such as cold or flu, the first stage of infection causes the production of a clear, thin mucus in the nose or back of the throat. As the body begins to react to the virus (generally one to three days), mucus thickens and may turn yellow or green. In viral infections, antibiotics will not be useful, and are a major source of misuse. Treatment is generally symptom-based; the only cure is to allow the immune system to fight off the virus over time.