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, hagfish, and certain 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.
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 mucus lining the airways
) 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
become caught in the viscous nasal or airway mucus. 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.
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.
Mucus is produced by submucosal cells as well as goblet cells found in the airway epithelium in the respiratory tract. It consists of mucin
, a highly glycosylated peptide
. Upon stimulation, MARCKS (myristylated alanine-rich C kinase substrate) protein coordinates the secretion of mucin from mucin filled vesicles
within the specialized epithelial cells. Fusion of the vesicles to the plasma membrane
causes release of the mucin, which as it exchanges Ca2+
expands up to 600 fold. The result is a viscoelastic
product of interwoven molecules which, combined with other secretions from the airway epithelium
and the submucosal glands
, is called mucus.
Diseases involving mucus
Generally mucus is clear and thin, serving to filter air during inhalation. During times of infection, mucus can change colour to yellow or green either as a result of trapped bacteria
, or due to the body's reaction to viral
infection. Such coloured mucus or phlegm usually has an offensive putrid odour.
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 fruitfully 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.
is an inherited disease that affects the entire body, but symptoms begin mostly in the lungs with excess production of mucus which is difficult to expel.
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.
As a medical symptom
Increased mucus production in the upper respiratory tract is a symptom of many common aliments, 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
, 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.
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.
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.
In the male reproductive system, the seminal vesicles contribute up to 60% of the total volume of the semen and contain mucus, amino acids, and fructose as the main energy source for the sperm.