Mucus (, MEW-kəs) is a slippery gel produced by, and covering, mucous membranes. Mucus forms a protective barrier between epithelial cells on the internal and external surfaces of the body and the outside environment. Mucus is found in the linings of the respiratory, digestive, and urogenital systems, and in the eyes and ears. Mucus helps to trap and remove inhaled particles, and prevent the entry of pathogenic fungi, bacteria and viruses at different sites. The mucosal barrier is nonetheless selectively permeable, allowing the diffusion of small particles and blocking the entrance of larger particles. Mucus is typically produced from cells found in mucous glands, and may also originate from mixed glands which contain both serous and mucous cells. It is a viscous colloidal gel containing water (90–95%), inorganic salts, antimicrobial enzymes (such as lysozymes), antibodies especially immunoglobulin A, and glycoproteins such as lactoferrin and mucins, which are produced by goblet cells in the mucous membranes and submucosal glands. Most of the mucus in the body is produced in the gastrointestinal tract. Amphibians, fish, snails, slugs, and some other invertebrates also produce external mucus from their epidermis as protection against pathogens, to help in movement, and to line fish gills. Plants, and some microorganisms produce a similar substance called mucilage.
Properties Mucus is selectively permeable, allowing the diffusion of small particles and blocking the entrance of larger particles. This filtering is based on particle size and also on surface chemistry.
Respiratory system
In the human respiratory system, mucus is part of the airway surface liquid (ASL), also known as epithelial lining fluid (ELF), that lines most of the respiratory tract. The airway surface liquid consists of a sol layer termed the periciliary liquid layer and an overlying gel layer termed the mucus layer. The periciliary liquid layer is so named as it surrounds the cilia and lies on top of the surface epithelium. The periciliary liquid layer surrounding the cilia consists of a gel meshwork of cell-tethered mucins and polysaccharides. The mucus blanket aids in the protection of the lungs by trapping foreign particles before they can enter them, in particular through the nose during normal breathing. Mucus is made up of a fluid component of around 95% water, the mucins secreted from goblet cells, and submucosal glands (2–3% glycoproteins), proteoglycans (0.1–0.5%), lipids (0.3–0.5%), proteins, and DNA. The major mucins secreted – MUC5AC and MUC5B - are large polymers that give the mucus its rheologic or viscoelastic properties. MUC5AC is the main gel-forming mucin secreted by goblet cells, in the form of threads and thin sheets. MUC5B is a polymeric protein secreted from submucosal glands and some goblet cells, and this is in the form of strands. In the airways—the trachea, bronchi, and bronchioles—the lining of mucus is produced by specialized airway epithelial cells called 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 process, together with the continual movement of the cilia on the respiratory epithelium toward the oropharynx (mucociliary clearance), 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. Mucus is produced continuously in the respiratory tract. Mucociliary action carries it down from the nasal passages and up from the rest of the tract to the pharynx, with most of it being swallowed subconsciously. Sometimes in times of respiratory illness or inflammation, mucus can become thickened with cell debris, bacteria, and inflammatory cells. It is then known as phlegm which may be coughed up as sputum to clear the airway.
Respiratory tract
Increased mucus production in the upper respiratory tract is a symptom of many common ailments, such as the common cold, and influenza. 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, 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.
In the lower respiratory tract impaired mucociliary clearance due to conditions such as primary ciliary dyskinesia may result in mucus accumulation in the bronchi. The dysregulation of mucus homeostasis 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. The thinning of the mucus layer ultimately affects the periciliary liquid layer, which becomes dehydrated, compromising ciliary function, and impairing mucociliary clearance. A respiratory therapist can recommend airway clearance therapy which uses a number of clearance techniques to help with the clearance of mucus.
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