Mucous cells of the stomach lining secrete mucus (pink) into the lumen

Mucus (s/ MEW-kəs) is a polymer. It is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is a viscous colloid containing inorganic salts, antiseptic enzymes (such as lysozymes), immunoglobulins, and glycoproteins such as lactoferrin[1] and mucins, which are produced by goblet cells in the mucous membranes and submucosal glands. 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, against infectious agents such as fungi, bacteria[2] and viruses. Most of the mucus produced is in the gastrointestinal tract.

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.

Respiratory system

Illustration depicting the movement of mucus in the respiratory tract

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.[3] 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.[4] "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.

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.[5] Nasal and airway mucus is produced continuously, with most of it swallowed subconsciously, even when it is dried.[6]

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.[5] 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.

Diseases involving mucus

In general, 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[7] or due to the body's reaction to viral infection. The green color of mucus comes from the heme group in the iron-containing enzyme myeloperoxidase secreted by white blood cells as a cytotoxic defense during a respiratory burst.

In the case of bacterial infection, the bacterium becomes trapped in already-clogged sinuses, breeding in the moist, nutrient-rich environment. Sinusitis is an uncomfortable condition which may include congestion of mucus. A bacterial infection in sinusitis will cause discolored mucus and would respond to antibiotic treatment; viral infections typically resolve without treatment.[8] Almost all sinusitis infections are viral and antibiotics are ineffective and not recommended for treating typical cases.[9]

In the case of a viral infection such as cold or flu, the first stage and also the last stage of the infection cause 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. Viral infections cannot be treated with antibiotics, and are a major avenue for their misuse. Treatment is generally symptom-based; often it is sufficient to allow the immune system to fight off the virus over time.[10]

Upper respiratory tract

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 or by using nasal irrigation. Excess nasal mucus, as with a cold or allergies, due to vascular engorgement associated with vasodilation and increased capillary permeability caused by histamines,[11] may be treated cautiously with decongestant medications. Thickening of mucus as a "rebound" effect following overuse of decongestants may produce nasal or sinus drainage problems and circumstances that promote infection.

During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined. As a result, the nasal cavity can fill up with mucus. At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils. This causes an excess amount of water to build up inside nasal cavities. In these cases, the excess fluid usually spills out externally through the nostrils.[12]

Lower respiratory tract

Excess mucus production in the bronchi and bronchioles, as may occur in asthma, bronchitis or influenza, results from chronic airway inflammation, and hence may be treated with anti-inflammatory medications. Impaired mucociliary clearance due to conditions such as primary ciliary dyskinesia may also result in its accumulation in the bronchi.[4] The dysregulation of mucus homesotasis is the fundamental characteristic of cystic fibrosis, an inherited disease caused by mutations in the CFTR gene, which encodes a chloride channel. This defect leads to the altered electrolyte composition of mucus, which triggers its hyperabsorption and dehydration. Such low-volume, viscous, acidic mucus has a reduced antimicrobial function, which facilitates bacterial colonisation.[13] The thinning of the mucus layer ultimately affects the PCL, which becomes dehydrated, compromising ciliary function and impairing mucociliary clearance.[4][13]

Other Languages
العربية: مخاط
Aymar aru: Jurma
বাংলা: শ্লেষ্মা
беларуская: Слізь
bosanski: Sluz
brezhoneg: Mic'hi
català: Mucositat
Cebuano: Kugmo
čeština: Hlen
chiShona: Dzihwa
Cymraeg: Mwcws
dansk: Slim
Deutsch: Schleim
eesti: Lima
Ελληνικά: Βλέννα
español: Moco
Esperanto: Muko
euskara: Muki
فارسی: مخاط
français: Mucus
galego: Moco
한국어: 점액
Ido: Muko
íslenska: Slím
italiano: Muco
עברית: ריר
Latina: Mucus
lietuvių: Gleivės
മലയാളം: ശ്ലേഷ്മം
Bahasa Melayu: Mukus
Nederlands: Slijm
日本語: 粘液
norsk: Snørr
norsk nynorsk: Slim
ਪੰਜਾਬੀ: ਬਲਗਮ
português: Muco
русский: Слизь
sicilianu: Mòccaru
Simple English: Mucus
српски / srpski: Šlajm
suomi: Räkä
svenska: Slem
Tagalog: Uhog
ไทย: เมือก
Türkçe: Sümük
українська: Слиз
اردو: مخاط
Tiếng Việt: Dịch nhầy
ייִדיש: שליים
中文: 黏液