Saliva

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Saliva on a baby's lips Saliva Baby.jpg
Saliva on a baby's lips

Saliva (commonly referred to as spit or drool) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be extracted), enzymes (such as lipase and amylase), and antimicrobial agents (such as secretory IgA, and lysozymes). [1]

Contents

The enzymes found in saliva are essential in beginning the process of digestion of dietary starches and fats. These enzymes also play a role in breaking down food particles entrapped within dental crevices, thus protecting teeth from bacterial decay. [2] Saliva also performs a lubricating function, wetting food and permitting the initiation of swallowing, and protecting the oral mucosa from drying out. [3]

Saliva has specialized purposes for a variety of animal species beyond predigestion. Certain swifts construct nests with their sticky saliva. The foundation of bird's nest soup is an aerodramus nest. Venomous saliva injected by fangs is used by cobras, vipers, and certain other members of the venom clade to hunt. Some caterpillars use modified salivary glands to store silk proteins, which they then use to make silk fiber. [4]

Composition

Produced in salivary glands, human saliva comprises 99.5% water, but also contains many important substances, including electrolytes, mucus, antibacterial compounds and various enzymes. [1] Medically, constituents of saliva can noninvasively provide important diagnostic information related to oral and systemic diseases. [5]

Daily salivary output

Experts debate the amount of saliva that a healthy person produces. Production is estimated at 1500ml per day and researchers generally accept that during sleep the amount drops significantly. [3] [9] In humans, the submandibular gland contributes around 70 to 75% of secretions, while the parotid gland secretes about 20 to 25%; small amounts are secreted from the other salivary glands. [10]

Functions

Saliva contributes to the digestion of food and to the maintenance of oral hygiene. Without normal salivary function the frequency of dental caries, gum disease (gingivitis and periodontitis), and other oral problems increases significantly.[ citation needed ] Saliva limits the growth of bacterial pathogens and is a major factor in sustaining systemic and oral health through the prevention of tooth decay and the removal of sugars and other food sources for microbes. [11]

Lubricant

Saliva coats the oral mucosa mechanically protecting it from trauma during eating, swallowing, and speaking. Mouth soreness is very common in people with reduced saliva (xerostomia) and food (especially dry food) sticks to the inside of the mouth.

Digestion

The digestive functions of saliva include moistening food and helping to create a food bolus. The lubricative function of saliva allows the food bolus to be passed easily from the mouth into the esophagus. Saliva contains the enzyme amylase, also called ptyalin, which is capable of breaking down starch into simpler sugars such as maltose and dextrin that can be further broken down in the small intestine. About 30% of starch digestion takes place in the mouth cavity. Salivary glands also secrete salivary lipase (a more potent form of lipase) to begin fat digestion. Salivary lipase plays a large role in fat digestion in newborn infants as their pancreatic lipase still needs some time to develop. [12]

Role in taste

Saliva is very important in the sense of taste. It is the liquid medium in which chemicals are carried to taste receptor cells (mostly associated with lingual papillae). People with little saliva often complain of dysgeusia (i.e. disordered taste, e.g. reduced ability to taste, or having a bad, metallic taste at all times). A rare condition identified to affect taste is that of 'Saliva Hypernatrium' , or excessive amounts of sodium in saliva that is not caused by any other condition (e.g., Sjögren syndrome ), causing everything to taste 'salty'.

Other

Production

The production of saliva is stimulated both by the sympathetic nervous system and the parasympathetic. [16]

Sympathetic stimulation of saliva is to facilitate respiration, whereas parasympathetic stimulation is to facilitate digestion.

Parasympathetic stimulation leads to acetylcholine (ACh) release onto the salivary acinar cells. ACh binds to muscarinic receptors, specifically M3, and causes an increased intracellular calcium ion concentration (through the IP3/DAG second messenger system). Increased calcium causes vesicles within the cells to fuse with the apical cell membrane leading to secretion. ACh also causes the salivary gland to release kallikrein, an enzyme that converts kininogen to lysyl-bradykinin. Lysyl-bradykinin acts upon blood vessels and capillaries of the salivary gland to generate vasodilation and increased capillary permeability, respectively. The resulting increased blood flow to the acini allows the production of more saliva. In addition, Substance P can bind to Tachykinin NK-1 receptors leading to increased intracellular calcium concentrations and subsequently increased saliva secretion. Lastly, both parasympathetic and sympathetic nervous stimulation can lead to myoepithelium contraction which causes the expulsion of secretions from the secretory acinus into the ducts and eventually to the oral cavity.

Sympathetic stimulation results in the release of norepinephrine. Norepinephrine binding to α-adrenergic receptors will cause an increase in intracellular calcium levels leading to more fluid vs. protein secretion. If norepinephrine binds β-adrenergic receptors, it will result in more protein or enzyme secretion vs. fluid secretion. Stimulation by norepinephrine initially decreases blood flow to the salivary glands due to constriction of blood vessels but this effect is overtaken by vasodilation caused by various local vasodilators.

Saliva production may also be pharmacologically stimulated by the so-called sialagogues. It can also be suppressed by the so-called antisialagogues.

Behaviour

Spitting

A building being renovated in the Carrollton section of New Orleans OakDoNotSpitOnFloor.jpg
A building being renovated in the Carrollton section of New Orleans

Spitting is the act of forcibly ejecting saliva or other substances from the mouth. In many parts of the world, it is considered rude and a social taboo, and has sometimes been outlawed. In some countries, for example, it has been outlawed for reasons of public decency and attempting to reduce the spread of disease. These laws may not strictly enforced, but in Singapore, the fine for spitting may be as high as SGD$2,000 for multiple offenses, and one can even be arrested.[ citation needed ] In China, expectoration is more socially acceptable (even if officially disapproved of or illegal), and spittoons are still a common appearance in some cultures. Some animals, even humans in some cases, use spitting as an automatic defensive maneuver. Camels are well known for doing this, though most domestic camels are trained not to.

Spitting by an infected person (for example, one with SARS-CoV-2) whose saliva contains large amounts of virus, [17] is a health hazard to the public.

Glue to construct bird nests

Many birds in the swift family, Apodidae, produce a viscous saliva during nesting season to glue together materials to construct a nest. [18] Two species of swifts in the genus Aerodramus build their nests using only their saliva, the base for bird's nest soup. [19]

Wound licking

A common belief is that saliva contained in the mouth has natural disinfectants, which leads people to believe it is beneficial to "lick their wounds". Researchers at the University of Florida at Gainesville have discovered a protein called nerve growth factor (NGF) in the saliva of mice. Wounds doused with NGF healed twice as fast as untreated and unlicked wounds; therefore, saliva can help to heal wounds in some species. NGF has been found in human saliva, as well as antibacterial agents as secretory mucin, IgA, lactoferrin, lysozyme and peroxidase. [20] [21] It has not been shown that human licking of wounds disinfects them, but licking is likely to help clean the wound by removing larger contaminants such as dirt and may help to directly remove infective bodies by brushing them away. Therefore, licking would be a way of wiping off pathogens, useful if clean water is not available to the animal or person.

Classical conditioning

In Pavlov's experiment, dogs were conditioned to salivate in response to a ringing bell; this stimulus is associated with a meal or hunger. Salivary secretion is also associated with nausea. Saliva is usually formed in the mouth through an act called gleeking, which can be voluntary or involuntary.

Making alcoholic beverages

Some old cultures chewed grains to produce alcoholic beverages, such as chicha, kasiri or sake.

Substitutes

A number of commercially available saliva substitutes exist. [22]

See also

Related Research Articles

<span class="mw-page-title-main">Pancreas</span> Organ of the digestive system and endocrine system of vertebrates

The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e., it has both an endocrine and a digestive exocrine function. 99% of the pancreas is exocrine and 1% is endocrine. As an endocrine gland, it functions mostly to regulate blood sugar levels, secreting the hormones insulin, glucagon, somatostatin and pancreatic polypeptide. As a part of the digestive system, it functions as an exocrine gland secreting pancreatic juice into the duodenum through the pancreatic duct. This juice contains bicarbonate, which neutralizes acid entering the duodenum from the stomach; and digestive enzymes, which break down carbohydrates, proteins and fats in food entering the duodenum from the stomach.

<span class="mw-page-title-main">Stomach</span> Digestive organ

The stomach is a muscular, hollow organ in the upper gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach is involved in the gastric phase of digestion, following the cephalic phase in which the sight and smell of food and the act of chewing are stimuli. In the stomach a chemical breakdown of food takes place by means of secreted digestive enzymes and gastric acid.

<span class="mw-page-title-main">Exocrine gland</span> Gland that secretes substances onto an epithelial surface by way of a duct

Exocrine glands are glands that secrete substances onto an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one of two types of glands in the human body, the other being endocrine glands, which secrete their products directly into the bloodstream. The liver and pancreas are both exocrine and endocrine glands; they are exocrine glands because they secrete products—bile and pancreatic juice—into the gastrointestinal tract through a series of ducts, and endocrine because they secrete other substances directly into the bloodstream. Exocrine sweat glands are part of the integumentary system; they have eccrine and apocrine types.

Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Mechanical digestion takes place in the mouth through mastication and in the small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small compounds that the body can use.

<span class="mw-page-title-main">Salivary gland</span> Exocrine glands that produce saliva through a system of ducts

The salivary glands in many vertebrates including mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands, as well as hundreds of minor salivary glands. Salivary glands can be classified as serous, mucous, or seromucous (mixed).

<span class="mw-page-title-main">Parotid gland</span> Major salivary gland in many animals

The parotid gland is a major salivary gland in many animals. In humans, the two parotid glands are present on either side of the mouth and in front of both ears. They are the largest of the salivary glands. Each parotid is wrapped around the mandibular ramus, and secretes serous saliva through the parotid duct into the mouth, to facilitate mastication and swallowing and to begin the digestion of starches. There are also two other types of salivary glands; they are submandibular and sublingual glands. Sometimes accessory parotid glands are found close to the main parotid glands.

<span class="mw-page-title-main">Gastric acid</span> Digestive fluid formed in the stomach

Gastric acid or stomach acid is the acidic component – hydrochloric acid of gastric juice, produced by parietal cells in the gastric glands of the stomach lining. With a pH of between one and three, gastric acid plays a key role in the digestion of proteins by activating digestive enzymes, which together break down the long chains of amino acids of proteins. Gastric acid is regulated in feedback systems to increase production when needed, such as after a meal. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring a regulated pH. These cells also produce mucus – a viscous barrier to prevent gastric acid from damaging the stomach. The pancreas further produces large amounts of bicarbonate and secretes bicarbonate through the pancreatic duct to the duodenum to neutralize gastric acid passing into the digestive tract.

<span class="mw-page-title-main">Submandibular gland</span> Human salivary gland

The paired submandibular glands are major salivary glands located beneath the floor of the mouth. In adult humans, they each weigh about 15 grams and contribute some 60–67% of unstimulated saliva secretion; on stimulation their contribution decreases in proportion as parotid gland secretion rises to 50%. The average length of the normal adult human submandibular salivary gland is approximately 27 mm, while the average width is approximately 14.3 mm.

<span class="mw-page-title-main">Digestive enzyme</span> Class of enzymes

Digestive enzymes take part in the chemical process of digestion, which follows the mechanical process of digestion. Food consists of macromolecules of proteins, carbohydrates, and fats that need to be broken down chemically by digestive enzymes in the mouth, stomach, pancreas, and duodenum, before being able to be absorbed into the bloodstream. Initial breakdown is achieved by chewing (mastication) and the use of digestive enzymes of saliva. Once in the stomach further mechanical churning takes place mixing the food with secreted gastric acid. Digestive gastric enzymes take part in some of the chemical process needed for absorption. Most of the enzymatic activity, and hence absorption takes place in the duodenum.

<span class="mw-page-title-main">Xerostomia</span> Medical condition

Xerostomia, also known as dry mouth, is a subjective complaint of dryness in the mouth, which may be associated with a change in the composition of saliva, or reduced salivary flow, or have no identifiable cause.

<span class="mw-page-title-main">Epidermal growth factor</span> Protein that stimulates cell division and differentiation

Epidermal growth factor (EGF) is a protein that stimulates cell growth and differentiation by binding to its receptor, EGFR. Human EGF is 6-kDa and has 53 amino acid residues and three intramolecular disulfide bonds.

Pancreatic juice is a liquid secreted by the pancreas, which contains a number of digestive enzymes, including trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase, nucleases and amylase. The pancreas is located in the visceral region, and is a major part of the digestive system required for proper digestion and subsequent assimilation of macronutrient substances required for living.

In physiology, serous fluid or serosal fluid is any of various body fluids resembling serum, that are typically pale yellow or transparent and of a benign nature. The fluid fills the inside of body cavities. Serous fluid originates from serous glands, with secretions enriched with proteins and water. Serous fluid may also originate from mixed glands, which contain both mucous and serous cells. A common trait of serous fluids is their role in assisting digestion, excretion, and respiration.

<span class="mw-page-title-main">Sialolithiasis</span> Medical condition

Sialolithiasis is a crystallopathy where a calcified mass or sialolith forms within a salivary gland, usually in the duct of the submandibular gland. Less commonly the parotid gland or rarely the sublingual gland or a minor salivary gland may develop salivary stones.

A sialogogue is a substance, especially a medication, that increases the flow rate of saliva. The definition focuses on substances that promote production or secretion of saliva rather than any food that is mouthwatering.

<span class="mw-page-title-main">Head and neck anatomy</span> Structure of the human head and neck

This article describes the anatomy of the head and neck of the human body, including the brain, bones, muscles, blood vessels, nerves, glands, nose, mouth, teeth, tongue, and throat.

Gastrointestinal physiology is the branch of human physiology that addresses the physical function of the gastrointestinal (GI) tract. The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products. The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion. The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.

<span class="mw-page-title-main">Salivary gland disease</span> Medical condition

Salivary gland diseases (SGDs) are multiple and varied in cause. There are three paired major salivary glands in humans: the parotid glands, the submandibular glands, and the sublingual glands. There are also about 800–1,000 minor salivary glands in the mucosa of the mouth. The parotid glands are in front of the ears, one on side, and secrete mostly serous saliva, via the parotid ducts, into the mouth, usually opening roughly opposite the second upper molars. The submandibular gland is medial to the angle of the mandible, and it drains its mixture of serous and mucous saliva via the submandibular duct into the mouth, usually opening in a punctum in the floor of mouth. The sublingual gland is below the tongue, on the floor of the mouth; it drains its mostly mucous saliva into the mouth via about 8–20 ducts, which open along the plica sublingualis, a fold of tissue under the tongue.

<span class="mw-page-title-main">Human digestive system</span> Digestive system in humans

The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion. Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.

Heterocrine glands are the glands which function as both exocrine gland and endocrine gland. These glands exhibit a unique and diverse secretory function encompassing the release of proteins and non-proteinaceous compounds, endocrine and exocrine secretions into both the bloodstream and ducts respectively. This duality allows them to serve crucial roles in regulating various physiological processes and maintaining homeostasis. These include the gonads, pancreas and salivary glands.

References

  1. 1 2 Nosek, Thomas M. Essentials of Human Physiology, Section 6, Chapter 4. Archived from the original on 2016-01-17.
  2. ↑ Fejerskov, O.; Kidd, E. (2007). Dental Caries: The Disease and Its Clinical Management (2nd ed.). Wiley-Blackwell. ISBN   978-1-4051-3889-5.
  3. 1 2 Edgar, M.; Dawes, C.; O'Mullane, D. (2004). Saliva and Oral Health (3 ed.). British Dental Association. ISBN   978-0-904588-87-3.
  4. ↑ "Insect-produced silk" (PDF). Archived from the original (PDF) on 2017-01-31. Retrieved 2019-04-13.
  5. ↑ Nonaka, Taichiro; Wong, David T.W. (13 June 2022). "Saliva Diagnostics". Annual Review of Analytical Chemistry. 15 (1): 107–121. Bibcode:2022ARAC...15..107N. doi:10.1146/annurev-anchem-061020-123959. ISSN   1936-1327. PMC   9348814 . PMID   35696523. Archived from the original on 16 September 2023. Retrieved 28 June 2022.
  6. ↑ Venturi, Sebastiano (January 2021). "Cesium in Biology, Pancreatic Cancer, and Controversy in High and Low Radiation Exposure Damage—Scientific, Environmental, Geopolitical, and Economic Aspects". International Journal of Environmental Research and Public Health. 18 (17): 8934. doi: 10.3390/ijerph18178934 . PMC   8431133 . PMID   34501532. CC-BY icon.svg Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License Archived 2017-10-16 at the Wayback Machine .
  7. ↑ Venturi, Sebastiano (December 2022). "Correlation of Diabetes, Salivary gland cancer and Pancreatic Cancer with Iodine and Cesium Radionuclides".
  8. 1 2 3 4 Boron, Walter F. (2003). Medical Physiology: A Cellular And Molecular Approach. Elsevier/Saunders. p. 928. ISBN   978-1-4160-2328-9.
  9. ↑ Dawes, C. (1972). "Circadian rhythms in human salivary flow rate and composition". Journal of Physiology. 220 (3): 529–545. doi:10.1113/jphysiol.1972.sp009721. PMC   1331668 . PMID   5016036.
  10. ↑ "Salivary Gland Disease and Tumors | Cedars-Sinai". Cedars-Sinai. Archived from the original on 20 October 2020. Retrieved 28 April 2018.
  11. ↑ Cohen-Brown, Gwen; Ship, Jonathan A. (2004). "Diagnosis and Treatment of Salivary Gland Disorders" (PDF). Quintessence International. 35 (2): 108–123. PMID   15000634. Archived from the original (PDF) on 19 November 2021. Retrieved 19 November 2021.
  12. ↑ Maton, Anthea (1993). Human Biology and Health . Prentice Hall. ISBN   978-0-13-981176-0.
  13. ↑ Manuel Ramos-Casals; Haralampos M. Moutsopoulos; John H. Stone. Sjogren's syndrome: Diagnosis and Therapeutics. Springer, 2011. p. 522.
  14. ↑ Herbst RS (2004). "Review of epidermal growth factor receptor biology". International Journal of Radiation Oncology, Biology, Physics. 59 (2 Suppl): 21–6. doi: 10.1016/j.ijrobp.2003.11.041 . PMID   15142631.
  15. ↑ Venturi S, Venturi M (2009). "Iodine in evolution of salivary glands and in oral health". Nutrition and Health. 20 (2): 119–134. doi:10.1177/026010600902000204. PMID   19835108. S2CID   25710052.
  16. ↑ Nosek, Thomas M. "Section 6/6ch4/s6ch4_7". Essentials of Human Physiology. Archived from the original on 2016-03-24.
  17. ↑ To, Kelvin Kai-Wang; Tsang, Owen Tak-Yin; Yip, Cyril Chik-Yan; Chan, Kwok-Hung; et al. (12 February 2020). "Consistent Detection of 2019 Novel Coronavirus in Saliva". Clinical Infectious Diseases . 71 (15). Oxford University Press: 841–843. doi:10.1093/cid/ciaa149. PMC   7108139 . PMID   32047895.
  18. ↑ Ramel, Gordon, "Digestion", The Amazing World of Birds, Earthlife Web, archived from the original on 2017-02-01, retrieved 2012-07-29
  19. ↑ "Swiftlet". 2011-12-27. Archived from the original on 2016-04-14. Retrieved 2012-07-29.
  20. ↑ Grewal, JS; Bordoni, B; Ryan, J (2020), "article-36176", Anatomy, Head and Neck, Sublingual Gland, This book is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits use, duplication, adaptation, distribution, and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, a link is provided to the Creative Commons license, and any changes made are indicated., Treasure Island (FL): StatPearls Publishing, PMID   30571047, archived from the original on 2021-02-17, retrieved 2020-03-28
  21. ↑ Jorma (2002). "Antimicrobial Agents in Saliva—Protection for the Whole Body". Journal of Dental Research. 81 (12): 807–809. doi:10.1177/154405910208101202. PMID   12454092. S2CID   45194203.
  22. ↑ Myers, Eugene N.; Ferris, Robert L. (2007). Salivary Gland Disorders. Springer Science & Business Media. p. 191. ISBN   9783540470724. Archived from the original on 2023-09-16. Retrieved 2017-09-11.

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