Editing Digestion (section)

{{short description|Biological process of breaking down food}}
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{{for-multi|the industrial process|Anaerobic digestion|the treatment of precipitates in analytical chemistry|Precipitation (chemistry)#Digestion|the journal|Digestion (journal)|the term in alchemy|Digestion (alchemy)}}
{{Infobox anatomy
|Name         = Digestive system
|Latin        = systema digestorium
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'''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 enzyme]]s. Mechanical digestion takes place in the [[mouth]] through [[Chewing|mastication]] and in the small intestine through [[segmentation contractions]]. In '''chemical digestion''', [[enzyme]]s break down food into the small compounds that the body can use.

In the [[human digestive system]], food enters the mouth and mechanical digestion of the food starts by the action of mastication (chewing), a form of mechanical digestion, and the wetting contact of [[saliva]]. Saliva, a liquid secreted by the [[salivary glands]], contains [[salivary amylase]], an enzyme which starts the digestion of [[starch]] in the food.<ref>{{Cite book |title=The Digestive System |last=Avraham |first=Regina |publisher=Chelsea House |year=1989 |isbn=0-7910-0015-X |location=New York |pages=[https://archive.org/details/digestivesyste00avra/page/48/mode/2up 49] |url=https://archive.org/details/digestivesyste00avra |others=Introduction by C. Everett Koop |ol=2055854M |access-date=2024-03-20}}</ref> The saliva also contains [[mucus]], which lubricates the food; the [[electrolyte]] hydrogencarbonate ({{chem2|HCO-3|link=Bicarbonate#Physiological role}}), which provides the ideal conditions of pH for amylase to work; and other electrolytes ({{chem2|Na+|link=Sodium in biology}}, {{chem2|K+|link=Potassium in biology}}, {{chem2|Cl-|link=Chloride#Role in biology}}).<ref>{{Cite book |title=Principles of Physiology |last1=Berne |first1=Robert M. |publisher=[[Mosby (imprint)|Mosby]] |year=2000 |isbn=0-323-00813-5 |edition=3rd |location=St. Louis |pages=[https://archive.org/details/principlesofphys00cvmo/page/372/mode/2up 373-374] |url=https://archive.org/details/principlesofphys00cvmo |last2=Levy |first2=Matthew N. |ol=9840795M |author-link=Robert M. Berne |author-link2=Matthew N. Levy |access-date=2024-03-20}}</ref> About 30% of starch is [[hydrolyzed]] into [[disaccharide]] in the oral cavity (mouth). After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a [[Bolus (digestion)|bolus]]. It will then travel down the [[esophagus]] and into the [[stomach]] by the action of [[peristalsis]]. [[Gastric juice]] in the stomach starts [[proteolysis|protein digestion]]. Gastric juice mainly contains [[hydrochloric acid]] and [[pepsin]]. In [[infants]] and [[toddlers]], gastric juice also contains [[rennin]] to digest milk proteins. As the first two chemicals may damage the stomach wall, mucus and bicarbonates are secreted by the stomach. They provide a slimy layer that acts as a shield against the damaging effects of chemicals like concentrated hydrochloric acid while also aiding lubrication.<ref>{{Cite journal|last1=Allen|first1=Adrian|last2=Flemström|first2=Gunnar|date=January 2005|title=Gastroduodenal mucus bicarbonate barrier: protection against acid and pepsin|url=https://pubmed.ncbi.nlm.nih.gov/15591243/|journal=American Journal of Physiology. Cell Physiology|volume=288|issue=1|pages=C1–19|doi=10.1152/ajpcell.00102.2004|issn=0363-6143|pmid=15591243}}</ref> Hydrochloric acid provides acidic pH for pepsin. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which is waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. Pepsin breaks down proteins into [[peptides]] or [[proteose]]s, which is further broken down into dipeptides and [[amino acid]]s by enzymes in the small intestine. Studies suggest that increasing the number of chews per bite increases relevant gut hormones and may decrease self-reported hunger and food intake.<ref>{{Cite journal|last1=Miquel-Kergoat|first1=Sophie|last2=Azais-Braesco|first2=Veronique|last3=Burton-Freeman|first3=Britt|last4=Hetherington|first4=Marion M.|date=2015-11-01|title=Effects of chewing on appetite, food intake and gut hormones: A systematic review and meta-analysis|journal=Physiology & Behavior|volume=151|pages=88–96|doi=10.1016/j.physbeh.2015.07.017|issn=1873-507X|pmid=26188140|doi-access=free}}</ref>

When the [[Pylorus|pyloric sphincter valve]] opens, partially digested food ([[chyme]]) enters the [[duodenum]] where it mixes with digestive enzymes from the [[pancreas]] and bile juice from the [[liver]] and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of nutrient absorption occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the [[Colon (anatomy)|colon]] (large intestine) where the pH is slightly acidic (about 5.6 ~ 6.9). Some vitamins, such as [[biotin]] and [[vitamin K]] (K<sub>2</sub>MK7) produced by bacteria in the colon are also absorbed into the blood in the colon. Absorption of water, simple sugar and alcohol also takes place in stomach. Waste material ([[feces]]) is eliminated from the [[rectum]] during [[defecation]].<ref name=Maton>{{cite book| last = Maton| first = Anthea| author2 = Jean Hopkins| author3 = Charles William McLaughlin| author4 = Susan Johnson| author5 = Maryanna Quon Warner| author6 = David LaHart| author7 = Jill D. Wright| title = Human Biology and Health| publisher = Prentice Hall| year = 1993| location = Englewood Cliffs, NJ| isbn = 978-0-13-981176-0| oclc = 32308337| url-access = registration| url = https://archive.org/details/humanbiologyheal00scho}}</ref>