View source for Mucus

{{Short description|Secretion produced by mucous membranes}}
{{Other use}}
[[File:Mucus cells.png|thumb|Mucous cells of the stomach lining secrete mucus (pink) into the lumen]]

'''Mucus''' ({{IPAc-en|ˈ|m|j|uː|k|ə|s}}, {{respell|MEW|kəs}}) is a slippery aqueous secretion produced by, and covering, [[mucous membrane]]s. It is typically produced from cells found in [[mucous gland]]s, although it may also originate from mixed glands, which contain both [[Serous fluid|serous]] and mucous cells. It is a viscous [[colloid]] containing [[inorganic ions|inorganic salts]], [[antimicrobial]] enzymes (such as [[lysozyme]]s), [[Antibody|immunoglobulins]] (especially [[Immunoglobulin A|IgA]]), and [[glycoprotein]]s such as [[lactoferrin]]<ref>{{cite journal | last1 = Singh | first1 = PK | last2 = Parsek | first2 = MR | last3 = Greenberg | first3 = EP | last4 = Welsh | first4 = MJ |date=May 2002 | title = A component of innate immunity prevents bacterial biofilm development | journal = [[Nature (journal)|Nature]] | volume = 417 | issue = 6888 | pages = 552–555 | pmid = 12037568 | doi = 10.1038/417552a| bibcode = 2002Natur.417..552S | s2cid = 4423528 }}</ref> and [[mucin]]s, which are produced by [[goblet cell]]s in the mucous membranes and [[submucosal glands]]. Mucus covers the [[Epithelium|epithelial cells]] that interact with outside environment,<ref>{{Cite journal |last=Bansil |first=Rama |last2=Turner |first2=Bradley S. |date=2018-01-15 |title=The biology of mucus: Composition, synthesis and organization |url=https://www.sciencedirect.com/science/article/abs/pii/S0169409X17302041 |journal=Advanced Drug Delivery Reviews |series=Technological strategies to overcome the mucus barrier in mucosal drug delivery |volume=124 |pages=3–15 |doi=10.1016/j.addr.2017.09.023 |issn=0169-409X|url-access=subscription }}</ref> serves to protect the linings of the [[respiratory system|respiratory]], [[Digestion#Digestive system|digestive]], and  [[Genitourinary system|urogenital systems]], and structures in the [[Visual system|visual]] and [[auditory system]]s from [[pathogen]]ic [[Fungus|fungi]], [[bacteria]]<ref>Barr et al. [http://www.pnas.org/content/110/26/10771 Bacteriophage adhering to mucus provide a non–host-derived immunity] PNAS 2013</ref> and [[viruses]]. Most of the mucus in the body is produced in the [[gastrointestinal tract]].

[[Amphibian]]s, [[fish]], [[Land snail|snail]]s, [[Slug|slugs]], and some other [[Invertebrate|invertebrates]] also produce external mucus from their [[Epidermis (zoology)|epidermis]] as protection against pathogens, to help in movement, and to line fish [[gill]]s. Plants produce a similar substance called [[mucilage]] that is also produced by some [[microorganism]]s.<ref>{{Cite web|url=http://www.biologydiscussion.com/protists/modes-of-locomotion-in-protists-5-modes/52116|title=Modes of locomotion in protists: 5 modes|date=2016-09-06|website=biology discussion}}</ref>

==Respiratory system==
{{Further|Mucociliary clearance}}
[[File:Blausen 0766 RespiratoryEpithelium.png|thumb|Illustration depicting the movement of mucus in the respiratory tract]]

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 [[Cilium|cilia]] and lies on top of the surface epithelium.<ref name="Atanasova">{{cite journal |last1=Atanasova |first1=KR |last2=Reznikov |first2=LR |title=Strategies for measuring airway mucus and mucins. |journal=Respiratory Research |date=21 November 2019 |volume=20 |issue=1 |pages=261 |doi=10.1186/s12931-019-1239-z |pmid=31752894|pmc=6873701 |doi-access=free }}</ref><ref>{{Cite journal|last=Althaus|first=M|date=March 2013|title=ENaC inhibitors and airway re-hydration in cystic fibrosis: state of the art.|journal=Curr Mol Pharmacol|volume=6|issue=1|pages=3–12|pmid=23547930|doi=10.2174/18744672112059990025}}</ref><ref name="Hiemstra">{{cite journal |last1=Hiemstra |first1=PS |last2=McCray PB |first2=Jr |last3=Bals |first3=R |title=The innate immune function of airway epithelial cells in inflammatory lung disease. |journal=The European Respiratory Journal |date=April 2015 |volume=45 |issue=4 |pages=1150–1162 |doi=10.1183/09031936.00141514 |pmid=25700381|pmc=4719567 }}</ref> The periciliary liquid layer surrounding the cilia consists of a gel meshwork of cell-tethered mucins and polysaccharides.<ref name="Ohar">{{cite journal |last1=Ohar |first1=JA |last2=Donohue |first2=JF |last3=Spangenthal |first3=S |title=The Role of Guaifenesin in the Management of Chronic Mucus Hypersecretion Associated with Stable Chronic Bronchitis: A Comprehensive Review. |journal=Chronic Obstructive Pulmonary Diseases |date=23 October 2019 |volume=6 |issue=4 |pages=341–349 |doi=10.15326/jcopdf.6.4.2019.0139 |pmid=31647856|pmc=7006698 }}</ref> The mucus blanket aids in the protection of the [[lung]]s by trapping foreign particles before they can enter them, in particular through the nose during normal breathing.<ref>{{cite journal|last1=Lillehoj|first1=ER|last2=Kim|first2=KC|title=Airway mucus: its components and function|journal=Archives of Pharmacal Research|date=December 2002|volume=25|issue=6|pages=770–780|pmid=12510824|doi=10.1007/bf02976990|s2cid=21467111}}</ref>

Mucus is made up of a fluid component of around 95% water, the [[mucin]] secretions from the goblet cells, and the submucosal glands (2–3% glycoproteins), proteoglycans (0.1–0.5%), lipids (0.3–0.5%), proteins, and DNA.<ref name="Ohar"/> The major mucins secreted –  [[Mucin 5AC|MUC5AC]] and [[Mucin 5B|MUC5B]] - are large polymers that give the mucus its [[rheology|rheologic]] or [[viscoelasticity|viscoelastic]] properties.<ref name="Ohar"/><ref name="Atanasova"/> 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.<ref name="Ostedgaard">{{cite journal |last1=Ostedgaard |first1=LS |last2=Moninger |first2=TO |last3=McMenimen |first3=JD |last4=Sawin |first4=NM |last5=Parker |first5=CP |last6=Thornell |first6=IM |last7=Powers |first7=LS |last8=Gansemer |first8=ND |last9=Bouzek |first9=DC |last10=Cook |first10=DP |last11=Meyerholz |first11=DK |last12=Abou Alaiwa |first12=MH |last13=Stoltz |first13=DA |last14=Welsh |first14=MJ |title=Gel-forming mucins form distinct morphologic structures in airways. |journal=Proceedings of the National Academy of Sciences of the United States of America |date=27 June 2017 |volume=114 |issue=26 |pages=6842–6847 |doi=10.1073/pnas.1703228114 |pmid=28607090|pmc=5495256 |bibcode=2017PNAS..114.6842O |doi-access=free }}</ref><ref name="Shen">{{cite journal |last1=Shen |first1=Y |title=Management of airway mucus hypersecretion in chronic airway inflammatory disease: Chinese expert consensus (English edition). |journal=International Journal of Chronic Obstructive Pulmonary Disease |date=2018 |volume=13 |pages=399–407 |doi=10.2147/COPD.S144312 |pmid=29430174|pmc=5796802 |doi-access=free }}</ref>

In the airways—the [[trachea]], [[bronchus|bronchi]], and [[bronchiole]]s—the lining of mucus is produced by specialized airway epithelial cells called [[goblet cell]]s, and [[submucosal glands]]. Small particles such as dust, particulate [[pollutant]]s, and [[allergen]]s, as well as [[Pathogen|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 [[cilium|cilia]] on the [[respiratory epithelium]] toward the [[Pharynx|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 [[Tissue (biology)|tissues]] such as the [[Nasal cavity|nasal]] and [[respiratory epithelium|airway epithelia]] from drying out.<ref name="annu-rev-physiol">{{cite journal
  | last1 = Thorton
  | first1 = DJ
  | last2 = Rousseau
  | first2 = K
  | last3 = MucGuckin
  | first3 = MA
  | title = Structure and function of the polymeric mucins in airways mucus
  | journal = Annual Review of Physiology
  | volume =70
  | issue = 44
  | pages = 459–486
  | year = 2008
  | doi = 10.1146/annurev.physiol.70.113006.100702
  | pmid = 17850213 }}</ref>

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#Phlegm|phlegm]] which may be [[cough]]ed up as [[sputum]] to clear the airway.<ref name="Rubin">{{cite journal |last1=Rubin |first1=BK |title=The role of mucus in cough research. |journal=Lung |date=January 2010 |volume=188 |issue=Suppl 1 |pages=S69–72 |doi=10.1007/s00408-009-9198-7 |pmid=19936981|s2cid=33857446 }}</ref><ref name="Hall">{{cite book |last1=Hall |first1=John |title=Guyton and Hall textbook of medical physiology |date=2011 |isbn=9781416045748 |page=473 |publisher=Saunders/Elsevier |edition=Twelfth}}</ref>

===Respiratory tract===
{{Main|Rhinorrhea|Post-nasal drip}}
Increased mucus production in the [[Respiratory tract|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 [[Allergy|allergies]], due to vascular engorgement associated with vasodilation and increased capillary permeability caused by [[Histamine|histamines]],<ref>
{{cite journal |vauthors=Monroe EW, Daly AF, Shalhoub RF |title=Appraisal of the validity of histamine-induced wheal and flare to predict the clinical efficacy of antihistamines |journal=J. Allergy Clin. Immunol. |volume=99 |issue=2 |pages=S798–806 |date=February 1997 |pmid=9042073 |doi=10.1016/s0091-6749(97)70128-3|doi-access=free }}</ref> 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.<ref>{{cite web|publisher=NPR|url=https://www.npr.org/templates/story/story.php?storyId=99844567|title=Why Does Cold Weather Cause Runny Noses?|access-date=2011-09-22}}</ref>

[[File: Respiratory Tract.jpg|thumb|3D render showing accumulated mucus in the airways]]
In the [[Respiratory tract|lower respiratory tract]] impaired [[mucociliary clearance]] due to conditions such as [[primary ciliary dyskinesia]] may result in mucus accumulation in the bronchi.<ref name="Mall">{{cite journal|last1=Mall|first1=MA|title=Role of Cilia, Mucus, and Airway Surface Liquid in Mucociliary Dysfunction: Lessons from Mouse Models|journal=Journal of Aerosol Medicine and Pulmonary Drug Delivery|date=March 2008|volume=21|issue=1|pages=13–24|doi=10.1089/jamp.2007.0659|pmid=18518828|s2cid=18362681}}</ref> The dysregulation of mucus homeostasis is the fundamental characteristic of [[cystic fibrosis]], an inherited disease caused by mutations in the ''[[Cystic fibrosis transmembrane conductance regulator|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.<ref name="Haq">{{cite journal|last1=Haq|first1=Iram J|last2=Gray|first2=Michael A|last3=Garnett|first3=James P|last4=Ward|first4=Christopher|last5=Brodlie|first5=Malcolm|title=Airway surface liquid homeostasis in cystic fibrosis: pathophysiology and therapeutic targets|journal=Thorax|date=March 2016|volume=71|issue=3|pages=284–287|doi=10.1136/thoraxjnl-2015-207588|pmid=26719229|doi-access=free}}</ref> The thinning of the mucus layer ultimately affects the periciliary liquid layer, which becomes dehydrated, compromising ciliary function, and impairing mucociliary clearance.<ref name="Mall" /><ref name="Haq" /> A [[respiratory therapist]] can recommend [[airway clearance therapy]] which uses a number of clearance techniques to help with the clearance of mucus.<ref name="Volsko">{{cite journal |last1=Volsko |first1=TA |title=Airway clearance therapy: finding the evidence. |journal=Respiratory Care |date=October 2013 |volume=58 |issue=10 |pages=1669–1678 |doi=10.4187/respcare.02590 |pmid=24064626|doi-access=free }}</ref>

====Mucus hypersecretion====
In the [[Respiratory tract|lower respiratory tract]] excessive mucus production in the bronchi and bronchioles is known as '''mucus hypersecretion'''.<ref name="Shen"/> Chronic mucus hypersecretion results in the chronic productive cough of [[Bronchitis#Chronic bronchitis|chronic bronchitis]],<ref name=GOLD2019>{{cite book |title=Global Initiative for Chronic Obstructive Lung Disease – GOLD |date=2018 |pages=15|url=https://goldcopd.org/wp-content/uploads/2018/11/GOLD-2019-v1.7-FINAL-14Nov2018-WMS.pdf |access-date=10 November 2019}}</ref> and is generally synonymous with this.<ref name="Novartis">{{cite book |last1=Novartis Foundation |title=Mucus hypersecretion in respiratory disease. |date=2002 |publisher=Wiley |isbn=0470844787 |page=3}}</ref> Excessive mucus can narrow the airways, limit airflow, and accelerate a decline in lung function.<ref name=Shen/>

==Digestive system==
[[File:Mucus in the Stomach.svg|thumb|Gastric glands are composed of epithelial cells (B), [[Gastric chief cell|chief cell]]s (D), and [[parietal cell]]s (E). The chief and parietal cells produce and secrete mucus (F) to protect the lining of the stomach (C) against the harsh pH of stomach acid. The mucus is basic, while the stomach acid (A) is acidic.]]

In the [[human digestive system]], mucus is used as a lubricant for materials that must pass over membranes, e.g., food passing down the [[esophagus]]. Mucus is extremely important in the [[gastrointestinal tract]]. It forms an essential layer in the colon and in the small intestine that helps reduce intestinal inflammation by decreasing bacterial interaction with intestinal epithelial cells.<ref>{{Cite journal|title = Bacteria penetrate the inner mucus layer before inflammation in the dextran sulfate colitis model|journal = PLOS ONE|date = 2010-01-01|issn = 1932-6203|pmc = 2923597|pmid = 20805871|pages = e12238|volume = 5|issue = 8|doi = 10.1371/journal.pone.0012238|first1 = Malin E. V.|last1 = Johansson|first2 = Jenny K.|last2 = Gustafsson|first3 = Karolina E.|last3 = Sjöberg|first4 = Joel|last4 = Petersson|first5 = Lena|last5 = Holm|first6 = Henrik|last6 = Sjövall|first7 = Gunnar C.|last7 = Hansson|bibcode = 2010PLoSO...512238J|doi-access = free}}</ref> The layer of mucus of the [[gastric mucosa]] lining the [[stomach]] is vital to protect the stomach lining from the highly acidic environment within it.<ref name=acids>{{cite web |last=Purves |first=William |title=Why don't our digestive acids corrode our stomach linings? |url=http://www.scientificamerican.com/article.cfm?id=why-dont-our-digestive-ac |work=[[Scientific American]] |access-date=6 December 2012 |quote=Second, HCl in the lumen doesn't digest the mucosa because goblet cells in the mucosa secrete large quantities of protective mucus that line the mucosal surface.}}</ref>

==Reproductive system==
In the human female reproductive system, [[Cervix#Cervical mucus|cervical mucus]] prevents infection and provides lubrication during sexual intercourse. 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. Several [[fertility awareness]] methods rely on observation of cervical mucus, as one of three primary fertility signs, to identify a woman's fertile time at the mid-point of the cycle.  Awareness of the woman's fertile time allows a couple to time intercourse to improve the odds of pregnancy.  It is also proposed as a method to avoid pregnancy.<ref>{{cite journal|last1=Han|first1=Leo|last2=Taub|first2=Rebecca|last3=Jensen|first3=Jeffrey T.|title=Cervical mucus and contraception: what we know and what we don't|journal=Contraception|date=November 2017|volume=96|issue=5|pages=310–321|doi=10.1016/j.contraception.2017.07.168|pmid=28801053}}</ref>

==Clinical significance==
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]]<ref>{{cite web|url=https://www.cdc.gov/drugresistance/community/files/GetSmart_RunnyNose.htm|title=Runny Nose (with green or yellow mucus)|publisher=[[Centers for Disease Control and Prevention]]|work=Get Smart: Know When Antibiotics Work |archive-url=https://web.archive.org/web/20080308233950/http://www.cdc.gov/drugresistance/community/files/GetSmart_RunnyNose.htm|archive-date=March 8, 2008|date=March 9, 2006}}</ref> or due to the body's reaction to [[virus|viral]] infection.  For example, ''[[Staphylococcus aureus]]'' infection may turn the mucus yellow.<ref>{{Citation |last=Taylor |first=Tracey A. |title=Staphylococcus aureus Infection |date=2024 |work=StatPearls |url=http://www.ncbi.nlm.nih.gov/books/NBK441868/ |access-date=2024-05-31 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=28722898 |last2=Unakal |first2=Chandrashekhar G.}}</ref> 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 [[Paranasal sinuses|sinuses]], breeding in the moist, nutrient-rich environment. [[Sinusitis]] is an uncomfortable condition that 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.<ref>{{Citation |author1 = Consumer Reports |author2 = American Academy of Family Physicians |author1-link = Consumer Reports |author2-link = American Academy of Family Physicians |date = April 2012 |title = Treating sinusitis: Don't rush to antibiotics |publisher = [[Consumer Reports]] |work = Choosing Wisely: an initiative of the [[ABIM Foundation]] |url = http://consumerhealthchoices.org/wp-content/uploads/2012/04/ChoosingWiselySinusitisAAFP.pdf |access-date = August 17, 2012 |archive-date = June 11, 2012 |archive-url = https://web.archive.org/web/20120611205432/http://consumerhealthchoices.org/wp-content/uploads/2012/04/ChoosingWiselySinusitisAAFP.pdf |url-status = dead }}</ref> Almost all sinusitis infections are viral and antibiotics are ineffective and not recommended for treating typical cases.<ref>{{Citation |author1 = American Academy of Family Physicians |author1-link = American Academy of Family Physicians |others = presented by [[ABIM Foundation]] |title = Five Things Physicians and Patients Should Question |publisher = [[American Academy of Family Physicians]] |work = Choosing Wisely: an initiative of the [[ABIM Foundation]] |url = http://choosingwisely.org/wp-content/uploads/2012/04/5things_12_factsheet_Amer_Acad_Fam_Phys.pdf |access-date = August 14, 2012 |url-status = dead |archive-url = https://web.archive.org/web/20120624075453/http://choosingwisely.org/wp-content/uploads/2012/04/5things_12_factsheet_Amer_Acad_Fam_Phys.pdf |archive-date = June 24, 2012 }}</ref>

In the case of a viral infection such as [[common cold|cold]] or [[influenza|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. 

[[Obstructive lung disease]]s often result from impaired mucociliary clearance that can be associated with mucus hypersecretion, and these are sometimes referred to as ''mucoobstructive lung diseases''.<ref name="Lewis">{{cite journal |last1=Lewis |first1=BW |last2=Patial |first2=S |last3=Saini |first3=Y |title=Immunopathology of Airway Surface Liquid Dehydration Disease. |journal=Journal of Immunology Research |date=2019 |volume=2019 |pages=2180409 |doi=10.1155/2019/2180409 |pmid=31396541|pmc=6664684 |doi-access=free }}</ref> Techniques of [[airway clearance therapy]] can help to clear secretions, maintain respiratory health, and prevent inflammation in the airways.<ref name="Volsko"/>

A unique [[cord lining|umbilical cord lining]] epithelial stem cell expresses [[Mucin short variant S1|MUC1]], termed (CLEC-muc). This has been shown to have good potential in the regeneration of the [[cornea]].<ref name="Saleh">{{cite journal |last1=Saleh |first1=R |last2=Reza |first2=HM |title=Short review on human umbilical cord lining epithelial cells and their potential clinical applications. |journal=Stem Cell Research & Therapy |date=10 October 2017 |volume=8 |issue=1 |pages=222 |doi=10.1186/s13287-017-0679-y |pmid=29017529|pmc=5634865 |doi-access=free }}</ref><ref name="Bains">{{cite journal |last1=Bains |first1=KK |display-authors=etal|title=Recovering vision in corneal epithelial stem cell deficient eyes. |journal=Contact Lens & Anterior Eye |date=August 2019 |volume=42 |issue=4 |pages=350–358 |doi=10.1016/j.clae.2019.04.006 |pmid=31047800|pmc=6611221 }}</ref>

== Properties of mucus ==
=== Tunable swelling capacity ===
Mucus is able to absorb water or dehydrate through [[pH]] variations. The swelling capacity of mucus stems from the [[Polymer brush|bottlebrush structure]]<ref>{{cite journal | last1 = Chremos | first1 = A | last2 = Douglas | first2 = JF | title = A comparative study of thermodynamic, conformational, and structural properties of bottlebrush with star and ring polymer melts | journal  = J. Chem. Phys. | volume = 149 | issue = 4 | pages = 044904 | date = 2018  | doi=10.1063/1.5034794| pmid = 30068167 | bibcode = 2018JChPh.149d4904C | s2cid = 51894232 | pmc = 11446256 }}</ref> of [[mucin]] within which hydrophilic segments provide a large surface area for water absorption. Moreover, the tunability of swelling effect is controlled by polyelectrolyte effect.

==== Polyelectrolyte effect in mucus ====
Polymers with charged molecules are called [[polyelectrolyte]]s. Mucins, a kind of polyelectrolyte [[proteoglycan]]s, are the main component of mucus, which provides the polyelectrolyte effect in mucus.<ref>{{Cite journal|last1=Sircar|first1=S.|last2=Keener|first2=J. P.|last3=Fogelson|first3=A. L.|date=2013-01-07|title=The effect of divalent vs. monovalent ions on the swelling of Mucin-like polyelectrolyte gels: Governing equations and equilibrium analysis|journal=The Journal of Chemical Physics|volume=138|issue=1|pages=014901|doi=10.1063/1.4772405|pmid=23298059|issn=0021-9606|pmc=3555968|bibcode=2013JChPh.138a4901S}}</ref> The process of inducing this effect comprises two steps: attraction of counter-ions and water compensation. When exposed in physiological ionic solution, the charged groups in the polyelectrolytes attract counter-ions with opposite charges, thereby leading to a solute concentration gradient. An osmotic pressure is introduced to equalize the concentration of solute throughout the system by driving water to flow from the low concentration areas to the high concentration areas. In short, the influx and outflux of water within mucus, managed by the polyelectrolyte effect, contribute to mucus' tunable swelling capacity.<ref>{{Cite journal|last1=Harding|first1=S. E.|last2=Creeth|first2=J. M.|date=1983-07-28|title=Polyelectrolyte behaviour in mucus glycoproteins|journal=Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology|volume=746|issue=1–2|pages=114–119|issn=0006-3002|pmid=6871229|doi=10.1016/0167-4838(83)90017-1}}</ref>

==== Mechanism of pH-tunable swelling ====
The ionic charges of mucin are mainly provided by acidic amino acids including [[aspartic acid]] ([[Acid dissociation constant|pKa]]=3.9) and [[glutamic acid]] (pKa=4.2). The charges of acidic amino acids will change with environmental pH value due to [[Acid dissociation constant|acid dissociation]] and association. Aspartic acid, for example, has a negative side chain when the pH value is above 3.9, while a neutrally charged side chain will be introduced as pH value drops below 3.9. Thus, the number of negative charges in mucus is influenced by the pH value of surrounding environment. That is, the polyelectrolyte effect of mucus is largely affected by the pH value of solution due to the charge variation of acidic amino acid residues on the mucin backbone. For instance, the charged residue on mucin is protonated at a normal pH value of the stomach, approximately pH 2. In this case, there is scarcely polyelectrolyte effect, thereby causing compact mucus with little swelling capacity. However, a kind of bacteria, ''[[Helicobacter pylori]]'', is prone to producing base to elevate the pH value in stomach, leading to the deprotonation of aspartic acids and glutamic acids, i.e., from neutral to negative-charged. The negative charges in the mucus greatly increase, thus inducing the polyelectrolyte effect and the swelling of the mucus. This swelling effect increases the pore size of the mucus and decreases mucus' viscosity, which allows bacteria to penetrate and migrate into the mucus and cause disease.<ref>{{Cite journal|last1=Celli|first1=Jonathan P.|last2=Turner|first2=Bradley S.|last3=Afdhal|first3=Nezam H.|last4=Keates|first4=Sarah|last5=Ghiran|first5=Ionita|last6=Kelly|first6=Ciaran P.|last7=Ewoldt|first7=Randy H.|last8=McKinley|first8=Gareth H.|last9=So|first9=Peter|date=2009-08-25|title=Helicobacter pylori moves through mucus by reducing mucin viscoelasticity|journal=Proceedings of the National Academy of Sciences|language=en|volume=106|issue=34|pages=14321–14326|doi=10.1073/pnas.0903438106|issn=0027-8424|pmid=19706518|pmc=2732822|bibcode=2009PNAS..10614321C|doi-access=free}}</ref>

=== Charge selectivity ===
The high selective permeability of mucus plays a crucial role in the healthy state of human beings by limiting the penetration of molecules, nutrients, pathogens, and drugs. The charge distribution within mucus serves as a charge selective diffusion barrier, thus significantly affecting the transportation of agents. Among particles with various surface [[zeta potential]]s, cationic particles tend to have a low depth of penetration, neutral ones possess medium penetration, and anionic ones have the largest penetration depth. Furthermore, the effect of charge selectivity changes when the status of the mucus varies, i.e., native mucus has a threefold higher potential to limit agent penetration than purified mucus.<ref>{{Cite journal|last1=Crater|first1=Jason S.|last2=Carrier|first2=Rebecca L.|date=2010-12-08|title=Barrier Properties of Gastrointestinal Mucus to Nanoparticle Transport|journal=Macromolecular Bioscience|language=en|volume=10|issue=12|pages=1473–1483|doi=10.1002/mabi.201000137|pmid=20857389|s2cid=20013702|issn=1616-5195}}</ref>

==Other animals==
Mucus is also produced by a number of other animals.<ref>{{cite journal |last1=Rühs |first1=Patrick A. |last2=Bergfreund |first2=Jotam |last3=Bertsch |first3=Pascal |last4=Gstöhl |first4=Stefan J. |last5=Fischer |first5=Peter |title=Complex fluids in animal survival strategies |journal=Soft Matter |date=2021 |volume=17 |issue=11 |pages=3022–3036 |doi=10.1039/D1SM00142F | arxiv=2005.00773 |pmid=33729256 |bibcode=2021SMat...17.3022R |doi-access=free }}</ref> All [[fish]] are covered in mucus secreted from glands all over their bodies.<ref name="Dash">{{cite journal |last1=Dash |first1=S |last2=Das |first2=SK |last3=Samal |first3=J |last4=Thatoi |first4=HN |title=Epidermal mucus, a major determinant in fish health: a review. |journal=Iranian Journal of Veterinary Research |date=Spring 2018 |volume=19 |issue=2 |pages=72–81 |pmid=30046316|pmc=6056142 }}</ref> [[Invertebrate]]s such as [[Land snail|snails]] and [[slug]]s secrete mucus called [[snail slime]] to enable movement, and to prevent their bodies from drying out. Their [[Reproductive system of gastropods|reproductive systems]] also make use of mucus for example in the covering of their [[egg]]s. In the unique mating ritual of ''[[Limax maximus]]'' the mating slugs lower themselves from elevated locations by a mucus thread. Mucus is an essential constituent of [[hagfish]] slime used to deter predators.<ref>{{cite journal |last1=Böni |first1=Lukas |last2=Fischer |first2=Peter |last3=Böcker |first3=Lukas |last4=Kuster |first4=Simon |last5=Rühs |first5=Patrick A. |title=Hagfish slime and mucin flow properties and their implications for defense |journal=Scientific Reports |date=September 2016 |volume=6 |issue=1 |pages=30371 |doi=10.1038/srep30371 |pmid=27460842 |pmc=4961968 |bibcode=2016NatSR...630371B |doi-access=free }}</ref> Mucus is produced by the [[endostyle]] in some [[tunicate]]s and larval [[lamprey]]s to help in filter feeding.

==See also==
{{Portal|Biology|Medicine}}
* [[Alkaline mucus]]
* [[Empty nose syndrome]]
* [[Feces]]
* [[Lung flute]]
* [[Mucoadhesion]]
* [[Mucophagy]]
* [[Sniffle]]
* [[Spinnbarkeit]]

==References==
{{reflist|2}}

{{Gastrointestinal physiology}}
{{Respiratory physiology}}
{{Authority control}}

[[Category:Body fluids]]
[[Category:Excretion]]
[[Category:Exocrine system]]
[[Category:Symptoms and signs: Respiratory system]]