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Mucus ( ) is a slippery aqueous secretion produced by, and covering,
mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers of Epithelium, epithelial cells overlying a layer of loose connecti ...
s. It is typically produced from cells found in
mucous gland Mucous gland, also known as muciparous glands, are found in several different parts of the body, and they typically stain (biology), stain lighter than serous glands during standard histology, histological preparation. Most are multicellular, but ...
s, although it may also originate from mixed glands, which contain both
serous In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physica ...
and mucous cells. It is a viscous
colloid A colloid is a mixture In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically combined. A mixture is the physical combination of two or more substances in which the identities are r ...

colloid
containing inorganic salts,
antimicrobial An antimicrobial is an agent that kills microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All orga ...
enzymes (such as
lysozyme Lysozyme, also known as muramidase or N-acetylmuramide glycanhydrolase, is an antimicrobial An antimicrobial is an agent that kills microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an ...

lysozyme
s),
immunoglobulin An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystal ...

immunoglobulin
s (especially
IgA Immunoglobulin A (IgA, also referred to as sIgA in its secretory form) is an that plays a crucial role in the of s. The amount of IgA produced in association with mucosal membranes is greater than all other types of antibody combined. In abs ...
), and
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
s such as
lactoferrin Lactoferrin (LF), also known as lactotransferrin (LTF), is a multifunctional protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perfor ...

lactoferrin
and
mucin Mucins () are a family of high molecular weight A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ph ...

mucin
s, which are produced by
goblet cell Goblet cells are simple columnar epithelial cell Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. ...
s in the mucous membranes and
submucosal glandsSubmucosal glands can refer to various racemose A raceme ( or ) or racemoid is an unbranched, indeterminate type of inflorescence An inflorescence is a group or cluster of flower A flower, sometimes known as a bloom or blossom, is t ...
. Mucus serves to protect
epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume ...
in the linings of the
respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans sever ...

respiratory
,
digestive
digestive
, and
urogenital system The genitourinary system, or urogenital system, are the organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categ ...
s, and structures in the
visual The visual system comprises the sensory organ A sense is a biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are determined b ...
and
auditory system The auditory system is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neur ...
s from
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
ic
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
,
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
and
viruses A virus is a wikt:submicroscopic, submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, incl ...
. Most of the mucus in the body is produced in the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
.
Amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s,
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
,
snail A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the pr ...

snail
s,
slugs Slug, or land slug, is a common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. ...

slugs
, and some other
invertebrates Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebrat ...
also produce external mucus from their
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
as protection against pathogens, and to help in movement and is also produced in fish to line their
gill A gill () is a respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
s. Plants produce a similar substance called
mucilage Mucilage is a thick, gluey substance produced by nearly all plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert ...
that is also produced by some
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
s.


Respiratory system

In the human
respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertainment, and media * ''Network'' (1976 film), a 1976 Ame ...

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 respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network which connects several biologically re ...

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 The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...
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
lung The lungs are the primary organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma ...

lung
s by trapping foreign particles before they enter them, in particular, through the nose, during normal breathing. Mucus is made up of a fluid component of around 95% water, the
mucin Mucins () are a family of high molecular weight A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ph ...

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. The major mucins secreted –
MUC5AC Mucin 5AC (Muc5AC) is a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzy ...
, and
MUC5B Mucin 5B (MUC5B/Muc5b) is a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including ...
are large polymers that give the mucus its , or
viscoelastic In and , viscoelasticity is the property of that exhibit both and characteristics when undergoing . Viscous materials, like water, resist and linearly with time when a is applied. Elastic materials strain when stretched and immediately ret ...
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 The trachea, also known as the windpipe, is a tube that connects the to the bronchi of the s, allowing the passage of , and so is present in almost all air- s with lungs. The trachea extends from the larynx and branches into the two primary . ...

trachea
,
bronchi A bronchus is a passage or airway in the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animal A ...

bronchi
, and
bronchiole The bronchioles or bronchioli are the smaller branches of the bronchial airways in the respiratory tract The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory sys ...

bronchiole
s, the lining of mucus is produced by specialized airway epithelial cells called
goblet cell Goblet cells are simple columnar epithelial cell Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. ...
s, and
submucosal glandsSubmucosal glands can refer to various racemose A raceme ( or ) or racemoid is an unbranched, indeterminate type of inflorescence An inflorescence is a group or cluster of flower A flower, sometimes known as a bloom or blossom, is t ...
. Small particles such as dust, particulate
pollutant A pollutant is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. A pollutant may cause long- or short-term damage by changing the growth rate of plant or animal spec ...
s, and
allergen An allergen is a type of antigen In immunology Immunology is a branch of biology that covers the study of immune systems in all organisms. Immunology charts, measures, and contextualizes the Physiology, physiological functioning of the im ...
s, as well as
infectious agent In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
s and
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

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 The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...
on the
respiratory epithelium The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are d ...
toward the
oropharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity The nasal cavity is a large, air-filled space above and behind the nose A nose is a protuberance in vertebrate Vertebrates () comprise all speci ...
(
mucociliary clearance Mucociliary clearance (MCC), mucociliary transport, or the mucociliary escalator, describes the self-clearing mechanism of the airways in the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biol ...
), 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 and airway epithelia from drying out. Mucus is produced continuously, in the
respiratory tract The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network which connects several biologically re ...

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 (; , ''phlégma'', "inflammation", "humour Humour (English in the Commonwealth of Nations, Commonwealth English) or humor (American English) is the tendency of experiences to provoke laughter and provide amusement. The term derives fro ...
which may be
cough A cough is a sudden expulsion of air through the large breathing passages that can help clear them of fluids, irritants, foreign particles and microbes A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an o ...
ed up as
sputum Sputum is mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...
to clear the airway.


Respiratory tract

Increased mucus production in the
upper respiratory tract The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animal ...
is a symptom of many common ailments, such as the
common cold The common cold, also known simply as a cold, is a viral infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplication, and the reaction of host (b ...
, and
influenza Influenza, commonly known as "the flu", is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplication, and the reaction of host (biology), ...

influenza
. Nasal mucus may be removed by blowing the nose or by using . Excess nasal mucus, as with a cold or
allergies Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity Hypersensitivity (also called hypersensitivity reaction or intolerance) refers to undesirable reactions produced by the normal immune system, i ...

allergies
, due to vascular engorgement associated with vasodilation and increased capillary permeability caused by histamines, may be treated cautiously with
decongestant A decongestant, or nasal decongestant, is a type of pharmaceutical drug A medication (also referred to as medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug t ...
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 The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network Network and networking may refer to: ...
impaired
mucociliary clearance Mucociliary clearance (MCC), mucociliary transport, or the mucociliary escalator, describes the self-clearing mechanism of the airways in the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biol ...
due to conditions such as
primary ciliary dyskinesia Primary ciliary dyskinesia (PCD), is a rare, ciliopathic, autosomal An autosome is any chromosome that is not a sex chromosome (an allosome). The members of an autosome pair in a diploid Ploidy () is the number of complete sets of chromosome ...

primary ciliary dyskinesia
may result in mucus accumulation in the bronchi. The dysregulation of mucus homeostasis is the fundamental characteristic of
cystic fibrosis Cystic fibrosis (CF) is a genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene In biology, a gene (from ''genos'' "...Wilhelm Johan ...
, an inherited disease caused by mutations in the ''
CFTR Cystic fibrosis transmembrane conductance regulator (CFTR) is a membrane protein and chloride channel in vertebrates that is encoded by the ''CFTR'' gene. The CFTR gene codes for an ABC transporter-class ion channel protein that conducts chlorid ...

CFTR
'' gene, which encodes a
chloride channel Chloride channels are a superfamily of poorly understood ion channels specific for chloride. These channels may conduct many different ions, but are named for chloride because its concentration ''in vivo'' is much higher than other anions. Several ...
. 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 A respiratory therapist is a specialized healthcare professional, healthcare practitioner trained in Intensive care medicine, critical care and cardio-pulmonary medicine in order to work therapeutically with people suffering from acute critical ...

respiratory therapist
can recommend airway clearance therapy which uses a number of clearance techniques to help with the clearance of mucus.


Mucus hypersecretion

In the
lower respiratory tract The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network Network and networking may refer to: ...
excessive mucus production in the bronchi and bronchioles is known as mucus hypersecretion. Chronic mucus hypersecretion results in the chronic productive cough of chronic bronchitis, and is generally synonymous with this. Excessive mucus can narrow the airways, limit airflow, and accelerate a decline in lung function.


Digestive system

In the
human digestive system The human digestive system consists of the human gastrointestinal tract, gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food in ...
, mucus is used as a lubricant for materials that must pass over membranes, e.g., food passing down the
esophagus The esophagus (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

esophagus
. Mucus is extremely important in the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
. 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. The layer of mucus of the
gastric mucosa The gastric mucosa is the mucous membrane layer of the stomach, which contains the glands and the gastric pits. In humans, it is about 1 mm thick, and its surface is smooth, soft, and velvety. It consists of simple columnar epithelium, lamina ...
lining the
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
is vital to protect the stomach lining from the highly acidic environment within it.


Reproductive system

In the human female reproductive system,
cervical mucus In anatomy, cervical is an adjective that has two meanings: # of or pertaining to any neck The neck is the part of the body on many vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellul ...
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 Ovulation is the release of egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, alth ...

ovulation
cervical mucus is clear, runny, and conducive to
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
; post-ovulation, mucus becomes thicker and is more likely to block sperm. Several
Fertility Awareness Fertility awareness (FA) refers to a set of practices used to determine the fertile Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of childr ...
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.


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 Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
or due to the body's reaction to infection. The green color of mucus comes from the heme group in the iron-containing enzyme
myeloperoxidase Myeloperoxidase (MPO) is a peroxidase enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substr ...

myeloperoxidase
secreted by white blood cells as a cytotoxic defense during a
respiratory burst Respiratory burst (or oxidative burst) is the rapid release of the reactive oxygen species Reactive oxygen species (ROS) are highly chemicals formed from O2. Examples of ROS include s, , , , and . The reduction of molecular oxygen (O2) produce ...
. In the case of bacterial infection, the bacterium becomes trapped in already-clogged
sinuses Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity The nasal cavity is a large, air-filled space above and behind the nose A nose is a protuberance in vertebrate Vertebrates () comprise all speci ...
, breeding in the moist, nutrient-rich environment.
Sinusitis Sinusitis, also known as rhinosinusitis, is inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogen In biology Biology is the natural science ...

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. Almost all sinusitis infections are viral and antibiotics are ineffective and not recommended for treating typical cases. In the case of a viral infection such as
cold , a common physiological response to cold, aiming to reduce the loss of body heat in a cold environment File:AntarcticaDomeCSnow.jpg, A photograph of the snow surface at Dome C Station, Antarctica a part of the notoriously cold Polar Platea ...
or
flu Influenza, commonly known as "the flu", is an infectious disease An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host A host is a person responsible ...

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.
Obstructive lung disease Obstructive lung disease is a category of respiratory disease Respiratory diseases, or lung diseases, are pathological conditions affecting the organs and tissues that make gas exchange Gas exchange is the physical process by which gases move pa ...
s often result from impaired mucociliary clearance that can be associated with mucus hypersecretion, and these are sometimes referred to as ''mucoobstructive lung diseases''. Techniques of airway clearance therapy can help to clear secretions, maintain respiratory health, and prevent inflammation in the airways. A unique umbilical cord lining epithelial stem cell expresses
MUC1 Mucin 1, cell surface associated (MUC1), also called polymorphic epithelial mucin (PEM) or epithelial membrane antigen or EMA, is a mucin encoded by the ''MUC1'' gene in humans. MUC1 is a glycoprotein with extensive O-linked glycosylation of its ex ...

MUC1
, termed (CLEC-muc). This has been shown to have good potential in the regeneration of the
cornea The cornea is the transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...

cornea
.


Properties of mucus


Tunable swelling capacity

Mucus is able to absorb water or dehydrate through variations. The swelling capacity of mucus stems from the bottlebrush structure of
mucin Mucins () are a family of high molecular weight A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ph ...

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 Polyelectrolytes are polymers A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** ...
s. Mucins, a kind of polyelectrolyte
proteoglycan Proteoglycans are protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958 ...
s, are the main component of mucus, which provides the polyelectrolyte effect in mucus. 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.


Mechanism of pH-tunable swelling

The ionic charges of mucin are mainly provided by acidic amino acids including aspartic acid (
pKa An acid dissociation constant, ''K''a, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme str ...
=3.9) and
glutamic acid Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compound ...
(pKa=4.2). The charges of acidic amino acids will change with environmental pH value due to 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 ''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, microaerophile, microaerophilic, spiral bacteria, spiral (helical) bacterium usually found in the stomach. Its helical shape (from which the genus name, ...

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.


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 Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for Electrokinetic phenomena, electro ...

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.


Other animals

Mucus is also produced by a number of other animals. All
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
are covered in mucus secreted from glands all over their bodies.
Invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s such as
snails A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the pr ...

snails
and
slug Slug, or land slug, is a common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. ...

slug
s secrete mucus called snail slime to enable movement, and to prevent their bodies from drying out. Their reproductive systems also make use of mucus for example in the covering of their
egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do not. eg ...

egg
s. In the unique mating ritual of ''
Limax maximus ''Limax maximus'' (literally, "biggest slug"), known by the common names great grey slug and leopard slug, is a species of slug in the family Limacidae, the keeled slugs.Marshall, B. (2014). Limax maximus Linnaeus, 1758. Accessed through: World R ...

Limax maximus
'' the mating slugs lower themselves from elevated locations by a mucus thread. Mucus is an essential constituent of
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, a ...

hagfish
slime used to deter predators. Mucus is produced by the
endostyle The endostyle is an anatomical feature found in invertebrate chordate A chordate () is an animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any ...
in some
tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time cal ...
s and larval
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

lamprey
s to help in filter feeding.


See also

*
Alkaline mucusAlkaline mucus is a thick fluid produced by animals which confers tissue protection in an acidic environment, such as in the stomach. Properties Mucus that serves a protective function against acidic environments generally has a high viscosity, th ...
*
Empty nose syndrome Empty nose syndrome (ENS), one form of secondary atrophic rhinitis, is a clinical syndrome in which people who have clear nasal passages experience a range of symptoms, most commonly feelings of nasal obstruction, nasal dryness and crusting, and a ...
* Mucoadhesion *Mucophagy *Sniffle *Spinnbarkeit *Feces


Notes


References

{{Authority control Body fluids Exocrine system Symptoms and signs: Respiratory system Excretion