Editing Stomach (section)

==Function==
===Digestion===
{{Further|Human digestive system}}
{{See also|Gastric acid}}
In the [[human digestive system]], a [[Bolus (digestion)|bolus]] (a small rounded mass of [[mastication|chewed up]] food) enters the stomach through the esophagus via the [[Esophagus#Sphincters|lower esophageal sphincter]]. The stomach releases [[proteases]] (protein-digesting [[enzyme]]s such as [[pepsin]]), and [[hydrochloric acid]], which kills or inhibits [[bacteria]] and provides the acidic [[pH]] of 2 for the proteases to work. Food is churned by the stomach through [[Peristalsis|peristaltic]] muscular contractions of the wall – reducing the volume of the bolus, before looping around the fundus<ref>{{cite book |author1=Richard M. Gore |author2=Marc S. Levine. |title=Textbook of Gastrointestinal Radiology |publisher=Saunders |location=Philadelphia, PA. |year= 2007|isbn=978-1-4160-2332-6}}</ref> and the [[body of stomach]] as the boluses are converted into [[chyme]] (partially digested food). Chyme slowly passes through the [[pyloric sphincter]] and into the [[duodenum]] of the [[small intestine]], where the extraction of nutrients begins.

[[Gastric juice]] in the stomach contains [[pepsinogen]] and [[gastric acid]], ([[hydrochloric acid]]) which activates this inactive form of enzyme into the active form, pepsin. Pepsin breaks down proteins into [[polypeptides]].

====Mechanical digestion====
Within a few moments after food enters the stomach, mixing waves begin to occur at intervals of approximately 20 seconds. A mixing wave is a unique type of [[peristalsis]] that mixes and softens the food with gastric juices to create chyme. The initial mixing waves are relatively gentle, but these are followed by more intense waves, starting at the body of the stomach and increasing in force as they reach the pylorus. 

The pylorus, which holds around 30 mL of chyme, acts as a filter, permitting only liquids and small food particles to pass through the mostly, but not fully, closed pyloric sphincter. In a process called [[gastric emptying]], rhythmic mixing waves force about 3 mL of chyme at a time through the pyloric sphincter and into the duodenum. Release of a greater amount of chyme at one time would overwhelm the capacity of the small intestine to handle it. The rest of the chyme is pushed back into the body of the stomach, where it continues mixing. This process is repeated when the next mixing waves force more chyme into the duodenum.

Gastric emptying is regulated by both the stomach and the duodenum. The presence of chyme in the duodenum activates receptors that inhibit gastric secretion. This prevents additional chyme from being released by the stomach before the duodenum is ready to process it.<ref name="Openstax Anatomy & Physiology attribution">{{CC-notice|cc=by4|url=https://openstax.org/books/anatomy-and-physiology/pages/23-4-the-stomach}} {{cite book|last1=Betts|first1=J Gordon|last2=Desaix|first2=Peter|last3=Johnson|first3=Eddie|last4=Johnson|first4=Jody E|last5=Korol|first5=Oksana|last6=Kruse|first6=Dean|last7=Poe|first7=Brandon|last8=Wise|first8=James|last9=Womble|first9=Mark D|last10=Young|first10=Kelly A|title=Anatomy & Physiology|location=Houston|publisher=OpenStax CNX|isbn=978-1-947172-04-3|date=September 13, 2023|at=23.4 The Stomach}}</ref>

====Chemical digestion====
The fundus stores both undigested food and gases that are released during the process of chemical digestion. Food may sit in the fundus of the stomach for a while before being mixed with the chyme. While the food is in the fundus, the digestive activities of [[salivary amylase]] continue until the food begins mixing with the acidic chyme. Ultimately, mixing waves incorporate this food with the chyme, the acidity of which inactivates salivary amylase and activates [[lingual lipase]]. Lingual lipase then begins breaking down triglycerides into free fatty acids, and mono- and diglycerides.

The breakdown of protein begins in the stomach through the actions of hydrochloric acid, and the enzyme [[pepsin]].

The stomach can also produce [[gastric lipase]], which can help digesting fat.

The contents of the stomach are completely emptied into the duodenum within two to four hours after the meal is eaten. Different types of food take different amounts of time to process. Foods heavy in carbohydrates empty fastest, followed by high-protein foods. Meals with a high triglyceride content remain in the stomach the longest. Since enzymes in the small intestine digest fats slowly, food can stay in the stomach for 6 hours or longer when the duodenum is processing fatty chyme. However, this is still a fraction of the 24 to 72 hours that full digestion typically takes from start to finish.<ref name="Openstax Anatomy & Physiology attribution"/>

=== Absorption ===
Although the absorption in the human digestive system is mainly a function of the small intestine, some absorption of certain small molecules nevertheless does occur in the stomach through its lining. This includes:
* Water, if the body is [[Dehydration|dehydrated]]
* Medication, such as [[aspirin]]
* [[Amino acids]]<ref>{{Cite book|title = Amino Acids in Animal Nutrition|edition = 2nd|editor-first = J.P.F.|editor-last = D'Mello|chapter = Absorption of Amino acids and Peptides|last1 = Krehbiel|first1 = C.R.|chapter-url = http://www.fcav.unesp.br/Home/departamentos/zootecnia/lucianohauschild/085199654x_chap03.pdf|first2 = J.C.|last2 = Matthews|pages = 41–70|access-date = 2015-04-25|archive-date = 2015-07-15|archive-url = https://web.archive.org/web/20150715082016/http://www.fcav.unesp.br/Home/departamentos/zootecnia/lucianohauschild/085199654x_chap03.pdf|url-status = dead}}</ref>
* 10–20% of ingested [[ethanol]] (e.g. from alcoholic beverages)<ref name=intox>{{cite web|title=Alcohol and the Human Body|url=http://www.intox.com/t-physiology.aspx|publisher=Intoximeters, Inc.|access-date=30 July 2012}}</ref>
* [[Caffeine]]<ref>{{Cite book|title = Coffee and Health|last = Debry|first = Gérard|publisher = John Libbey Eurotext|year = 1994|isbn = 9782742000371|location = Montrouge|url = https://books.google.com/books?id=uZaaCk-8s9sC&pg=PA129|access-date = 2015-04-26|format = PDF (eBook)|page = 129}}</ref>
* To a small extent water-soluble [[vitamin]]s (most are absorbed in the small intestine)<!-- See page 419--><ref>{{Cite book|title = Nutritional Sciences: From Fundamentals to Food|last1=McGuire|first1=Michelle|last2=Beerman|first2=Kathy|publisher=Cengage Learning|edition=3|url = https://books.google.com/books?id=bYAJAAAAQBAJ|isbn=978-1133707387|date=2012-01-01|page=419}}</ref>

The [[parietal cell]]s of the human stomach are responsible for producing [[intrinsic factor]], which is necessary for the absorption of [[vitamin B12]]. B12 is used in cellular metabolism and is necessary for the production of [[red blood cell]]s, and the functioning of the [[nervous system]].

=== Control of secretion and motility ===
[[Image:Stomach emptying into duodenum.svg|thumb|center|600px|Emptying of stomach chyme into the duodenum through the pyloric sphincter]]

Chyme from the stomach is slowly released into the [[duodenum]] through coordinated [[peristalsis]] and opening of the pyloric sphincter. The movement and the flow of chemicals into the stomach are controlled by both the [[autonomic nervous system]] and by the various [[Digestion#Digestive hormones|digestive hormones]] of the digestive system:

{| class="wikitable"
| [[Gastrin]] || The hormone ''gastrin'' causes an increase in the secretion of HCl from the parietal cells and pepsinogen from chief cells in the stomach. It also causes increased motility in the stomach. Gastrin is released by [[G cell]]s in the stomach in response to distension of the antrum and digestive products (especially large quantities of incompletely digested proteins). It is inhibited by a [[pH]] normally less than 4(high acid), as well as the hormone [[somatostatin]].
 |-
 | [[Cholecystokinin]] || ''Cholecystokinin'' (CCK) has most effect on the [[gall bladder]], causing gall bladder contractions, but it also decreases gastric emptying and increases release of [[Pancreas|pancreatic]] juice, which is alkaline and neutralizes the chyme. CCK is synthesized by I-cells in the mucosal epithelium of the small intestine.
 |-
 | [[Secretin]] || In a different and rare manner, ''secretin'', which has the most effects on the pancreas, also diminishes acid secretion in the stomach. Secretin is synthesized by [[S cell|S-cells]], which are located in the duodenal mucosa as well as in the jejunal mucosa in smaller numbers.
 |-
 | [[Gastric inhibitory polypeptide]] || ''Gastric inhibitory polypeptide'' (GIP) decreases both gastric acid release and motility. GIP is synthesized by K-cells, which are located in the duodenal and jejunal mucosa.
 |-
 | [[Enteroglucagon]] || ''Enteroglucagon'' decreases both gastric acid and motility.
 |-
 |}

Other than gastrin, these hormones all act to turn off the stomach action. This is in response to food products in the [[liver]] and gall bladder, which have not yet been absorbed. The stomach needs to push food into the small intestine only when the intestine is not busy. While the intestine is full and still digesting food, the stomach acts as storage for food.

===Other===
;Effects of EGF
[[Epidermal growth factor]] (EGF) results in cellular proliferation, differentiation, and survival.<ref name="Herbst">{{cite journal | author = Herbst RS | title = Review of epidermal growth factor receptor biology | journal = International Journal of Radiation Oncology, Biology, Physics | volume = 59 | issue = 2 Suppl | pages = 21–6 | year = 2004 | pmid = 15142631 | doi = 10.1016/j.ijrobp.2003.11.041 | doi-access = free }}</ref>  EGF is a low-molecular-weight polypeptide first purified from the mouse submandibular gland, but since then found in many human tissues including the [[submandibular gland]], and the [[parotid gland]]. Salivary EGF, which also seems to be regulated by dietary inorganic [[iodine]], also plays an important physiological role in the maintenance of oro-esophageal and gastric tissue integrity. The biological effects of salivary EGF include healing of oral and gastroesophageal ulcers, inhibition of gastric acid secretion, stimulation of DNA synthesis, and mucosal protection from intraluminal injurious factors such as gastric acid, bile acids, pepsin, and trypsin and from physical, chemical, and bacterial agents.<ref>{{cite journal|author = Venturi S.|author2 = Venturi M.|year = 2009|title = Iodine in evolution of salivary glands and in oral health|journal = Nutrition and Health|volume = 20|pages = 119–134|pmid = 19835108|issue = 2|doi=10.1177/026010600902000204|s2cid = 25710052}}</ref>

;Stomach as nutrition sensor
The human stomach has  receptors responsive to [[sodium glutamate]]<ref>{{cite journal | last1 = Uematsu | first1 = A | last2 = Tsurugizawa | first2 = T | last3 = Kondoh | first3 = T | last4 = Torii | first4 = K. | year = 2009 | title = Conditioned flavor preference learning by intragastric administration of L-glutamate in rats | journal = Neurosci. Lett. | volume = 451 | issue = 3| pages = 190–3 | doi = 10.1016/j.neulet.2008.12.054 | pmid = 19146916 | s2cid = 21764940 }}</ref> and this information is passed to the [[lateral hypothalamus]] and [[limbic system]] in the [[brain]] as a [[palatability]] signal through the [[vagus nerve]].<ref>{{cite journal | last1 = Uematsu | first1 = A | last2 = Tsurugizawa | first2 = T | last3 = Uneyama | first3 = H | last4 = Torii | first4 = K. | year = 2010 | title = Brain-gut communication via vagus nerve modulates conditioned flavor preference | journal = Eur J Neurosci | volume = 31 | issue = 6| pages = 1136–43 | doi = 10.1111/j.1460-9568.2010.07136.x | pmid = 20377626 | s2cid = 23319470 }}</ref> The stomach can also sense, independently of tongue and oral taste receptors, [[glucose]],<ref name="Araujo">{{cite journal| pmid= 18367093 | doi=10.1016/j.neuron.2008.01.032| title= Food Reward in the Absence of Taste Receptor Signaling| year= 2008| last1= De Araujo| first1= Ivan E.| last2= Oliveira-Maia| first2= Albino J.| last3= Sotnikova| first3= Tatyana D.| last4= Gainetdinov| first4= Raul R.| last5= Caron| first5= Marc G.| last6= Nicolelis| first6= Miguel A.L.| last7= Simon| first7= Sidney A.| journal= Neuron| volume= 57| issue= 6| pages= 930–41| s2cid=47453450| doi-access= free}}</ref> [[carbohydrate]]s,<ref name="Perez">{{cite journal | last1 = Perez | first1 = C. | last2 = Ackroff | first2 = K. | last3 = Sclafani | first3 = A. | year = 1996 | title = Carbohydrate- and protein conditioned flavor preferences: effects of nutrient preloads | journal = Physiol. Behav. | volume = 59 | issue = 3| pages = 467–474 | doi = 10.1016/0031-9384(95)02085-3 | pmid = 8700948 | s2cid = 23422504 }}</ref>  [[protein]]s,<ref name="Perez"/> and [[fat]]s.<ref>{{cite journal | last1 = Ackroff | first1 = K. | last2 = Lucas | first2 = F. | last3 = Sclafani | first3 = A. | year = 2005 | title = Flavor preference conditioning as a function of fat source | journal = Physiol. Behav. | volume = 85 | issue = 4| pages = 448–460 | doi = 10.1016/j.physbeh.2005.05.006 | pmid = 15990126 | s2cid = 7875868 }}</ref> This allows the brain to link [[nutritional]] value of foods to their tastes.<ref name="Araujo"/>

;Thyrogastric syndrome  
This syndrome defines the association between thyroid disease and chronic gastritis, which was first described in the 1960s.<ref>{{cite journal |pages=605–25 |title=Autoimmunity in pernicious anemia and thyroiditis: a family study. |year=1965 |last1=Doniach  |first1=D. |last2=Roitt |first2=I.M. |last3=Taylor |first3=K.B. |journal=Ann N Y Acad Sci |volume=124 |issue=2 |pmid=5320499|doi=10.1111/j.1749-6632.1965.tb18990.x |bibcode=1965NYASA.124..605D |s2cid=39456072 }}</ref> This term was coined also to indicate the presence of thyroid autoantibodies or autoimmune thyroid disease in patients with pernicious anemia, a late clinical stage of atrophic gastritis.<ref>{{cite journal |pages=771–81 |title=An analysis of gastric parietal cell antibodies and thyroid cell antibodies in patients with pernicious anaemia and thyroid disorders. |year=1968 |last1=Cruchaud |first1=A. |last2=Juditz |first2=E.|journal=Clin Exp Immunol |volume=3|issue=8 |pmid=4180858|pmc=1578967 }}</ref> In 1993, a more complete investigation on the stomach and thyroid was published,<ref>{{cite journal |pages=17–23 |title=A new hypothesis: iodine and gastric cancer. |year=1993 |last1=Venturi |first1=S. |last2=Venturi |first2=A. |last3= Cimini |first3=D., Arduini, C; Venturi, M; Guidi, A.|journal=Eur J Cancer Prev|volume=2 |issue=1|pmid=8428171|doi=10.1097/00008469-199301000-00004 }}</ref> reporting that the thyroid is, embryogenetically and  phylogenetically, derived from a primitive stomach, and that the thyroid cells, such as primitive gastroenteric cells, migrated and specialized in uptake of iodide and in storage and elaboration of iodine compounds during vertebrate evolution. In fact, the stomach and thyroid share iodine-concentrating ability and many morphological and functional similarities, such as cell polarity and apical microvilli, similar organ-specific antigens and associated autoimmune diseases, secretion of glycoproteins (thyroglobulin and mucin) and peptide hormones, the digesting and readsorbing ability, and lastly, similar ability to form iodotyrosines by peroxidase activity, where iodide acts as an electron donor in the presence of H<sub>2</sub>O<sub>2</sub>. In the following years, many researchers published reviews about this syndrome.<ref>{{cite journal|title=Thyro-entero-gastric autoimmunity: Pathophysiology and implications for patient management. A review.|year=2019 |last1=Lahner|first1=E. |last2=Conti |first2=L. |last3=Cicone |first3=F. ; Capriello, S; Cazzato, M; Centanni, M; Annibale, B; Virili, C.|journal=Best Pract Res Clin Endocrinol Metab |volume=33 |issue=6 |pages=101373 |pmid=31864909|doi=10.1016/j.beem.2019.101373 |s2cid=209446096 }}</ref>