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In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. It is a viscous colloid containing antiseptic enzymes (such as lysozyme) and immunoglobulins that serves to protect epithelial cells in the respiratory, gastrointestinal, urogenital, visual, and auditory systems in mammals; the epidermis in amphibians; and the gills in fish.
Snails, slugs, bony fish, hagfish and some other invertebrates 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 liter of mucus per day.
In the respiratory system mucus aids in the protection of the lungs by trapping foreign particles that enter it, 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 dried.
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 nasal 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.
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 and also the last stage of the 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 avenue for misuse. Treatment is generally symptom-based; the only cure is to allow the immune system to fight off the virus over time.
- Main article: Cystic fibrosis
Cystic fibrosis is an inherited disease that affects the entire body, but symptoms begin mostly in the lungs with extremely viscous (thick) production of mucus which is difficult to expel.
Mucus as a medical symptom
Increased mucus production in the upper respiratory tract is a symptom of many common ailments, such as the common cold. Nasal mucus may be removed by blowing the nose, picking the nose, or by using traditional methods of nasal irrigation. Excess nasal mucus, as with a cold or allergies may be treated cautiously with decongestant medications. Excess mucus production in the bronchi and bronchioles, as may occur in asthma, bronchitis or influenza, may be treated with anti-inflammatory medications as a means of reducing the airway inflammation which triggers mucus over-production. Thickening of mucus as a "rebound" effect following over-use of decongestants may produce nasal or sinus drainage problems and circumstances that promote infection. Mucus with any color other than clear or white is generally an indicator of an infection of the nasal mucosa, the paranasal sinus or, if produced via a productive cough, of a lower respiratory tract infection.
Cold weather and mucus
During cold weather, the cilia which normally sweep mucus away from the nostrils and towards the back of the throat (see respiratory epithelium) become sluggish or completely cease functioning. This results in mucus running down the nose and dripping (a runny nose). Mucus also thickens in cold weather; when an individual comes in from the cold, the mucus thaws and begins to run before the cilia begin to work again.
In the digestive system, mucus is used as a lubricant for materials which must pass over membranes, e.g., food passing down the esophagus. A layer of mucus along the inner walls of the stomach is vital to protect the cell linings of that organ from the highly acidic environment within it. The same protective layer of mucus is what comes out when you sneeze. Mucus does not digest in the intestinal tract, so mucus commonly appears in fecal matter whether its origin is from the intestines, or swallowed.
In the female reproductive system, cervical mucus prevents infection. The consistency of cervical mucus varies depending on the stage of a woman's menstrual cycle. At ovulation cervical mucus is clear, runny, and conducive to sperm; post-ovulation, mucus becomes thicker and is more likely to block sperm.
- Empty nose syndrome
- What's a Booger?. KidsHealth.
- (2008) Structure and function of the polymeric mucins in airways mucus. Annual Review of Physiology (44): 459–486.
- Gates, Stefan (2006). "Boogers" Gastronaut: Adventures in Food for the Romantic, the Foolhardy, and the Brave, 68, 69.
- Runny Nose (with green or yellow mucus). Get Smart: Know When Antibiotics Work. Centers for Disease Control and Prevention.
- Yellow-green Phlegm and Other Myths. University of Arizona campus health services. URL accessed on 2007-10-22.
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