Editing Large intestine (section)

==Function==
[[File:Large intestine histology.jpg|thumb|Histological section.]]
The large intestine absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum. The colon absorbs vitamins that are created by the colonic bacteria, such as [[thiamine]], [[riboflavin]], and [[vitamin K]] (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate [[blood coagulation]]).<ref>{{Cite journal |last1=Sellers |first1=Rani S. |last2=Morton |first2=Daniel |date=2014 |title=The Colon: From Banal to Brilliant |journal=Toxicologic Pathology |volume=42 |issue=1 |pages=67–81 |doi=10.1177/0192623313505930 |pmid=24129758 |s2cid=20465985 }}</ref>{{Citation needed|date=May 2018}}<ref>{{Cite journal |last=Booth |first=Sarah |date=April 2012 |title=Vitamin K: Food Consumption and Dietary Intakes |url=https://foodandnutritionresearch.net/index.php/fnr/article/view/505 |journal=[[Food Nutrition Research]] |volume=56|doi=10.3402/fnr.v56i0.5505 |pmid=22489217 |pmc=3321250 }}</ref> It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the [[anus]] in [[defecation]].

The large intestine also secretes K+ and Cl-. Chloride secretion increases in cystic fibrosis.
Recycling of various nutrients takes place in the colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling.<ref>{{Cite news|url=http://www.news-medical.net/health/What-Does-the-Large-Intestine-Do.aspx|title=The Large Intestine (Human)|date=2009-11-17|work=News-Medical.net|access-date=2017-03-15}}</ref>{{Citation needed|date=March 2014}}

The [[appendix (anatomy)|appendix]] contains a small amount of [[mucosa-associated lymphoid tissue]] which gives the appendix an undetermined role in immunity. However, the appendix is known to be important in fetal life as it contains [[endocrine]] cells that release biogenic amines and peptide hormones important for [[homeostasis]] during early growth and development.<ref name="SAAppendix">{{cite journal | url=http://www.scientificamerican.com/article/what-is-the-function-of-t/ | title=What is the function of the human appendix? Did it once have a purpose that has since been lost? | journal=Scientific American | date=1999-10-21 | access-date=2014-03-03 | author=Martin, Loren G.}}</ref>

By the time the [[chyme]] has reached this tube, most [[nutrient]]s and 90% of the water have been absorbed by the body. Indeed, as demonstrated by the commonality of [[ileostomy]] procedures, it is possible for many people to live without large portions of their large intestine, or even without it completely. At this point only some [[electrolyte]]s like [[sodium]], [[magnesium]], and [[chloride]] are left as well as indigestible parts of ingested food (e.g., a large part of ingested [[amylose]], starch which has been shielded from digestion heretofore, and [[dietary fiber]], which is largely indigestible [[carbohydrate]] in either soluble or insoluble form). As the chyme moves through the large intestine, most of the remaining [[water]] is removed, while the chyme is mixed with [[mucus]] and [[bacteria]] (known as [[gut flora]]), and becomes feces. The [[ascending colon]] receives fecal material as a liquid. The muscles of the colon then move the watery waste material forward and slowly absorb all the excess water, causing the stools to gradually solidify as they move along into the [[descending colon]].<ref>[http://ccnt.hsc.usc.edu/colorectal/functioncolon.aspx La función de la] [http://www.hidroterapiadecolon.es/ hidroterapia de colon] Retrieved on 2010-01-21</ref>

The bacteria break down some of the [[fiber]] for their own nourishment and create [[acetate]], [[propionate]], and [[butyrate]] as waste products, which in turn are used by the cell lining of the colon for nourishment.<ref>{{cite journal |author1=Terry L. Miller |author2=Meyer J. Wolin |year=1996 |title=Pathways of Acetate, Propionate, and Butyrate Formation by the Human Fecal Microbial Flora |journal=[[Applied and Environmental Microbiology]] |volume=62 |issue=5 |pages=1589–1592 |url= |doi=10.1128/AEM.62.5.1589-1592.1996 |pmid=8633856 |pmc=167932 |bibcode=1996ApEnM..62.1589M }}</ref> No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet;<ref>{{cite journal|last=McNeil|first=NI|title=The contribution of the large intestine to energy supplies in man|journal=The American Journal of Clinical Nutrition|year=1984|volume=39|issue=2|pages=338–342|pmid=6320630|doi=10.1093/ajcn/39.2.338}}</ref> in other animals, including other apes and primates, who have proportionally larger colons, more is made available, thus permitting a higher portion of plant material in the diet.  The large intestine<ref>{{Cite news|url=http://www.maglenia.com/what-side-is-your-appendix-located/|title=What Side is Your Appendix Located?|last=lorriben|date=2016-07-09|newspaper=Maglenia|access-date=2016-10-23|archive-url=https://web.archive.org/web/20161009115416/http://www.maglenia.com/what-side-is-your-appendix-located/|archive-date=2016-10-09|url-status=dead}}</ref> produces no digestive [[enzyme]]s — [[chemical digestion]] is completed in the [[small intestine]] before the chyme reaches the large intestine. The [[pH]] in the colon varies between 5.5 and 7 (slightly [[acid]]ic to neutral).<ref>[http://jamesallred.com/function-of-the-colon Function Of The Large Intestine] {{webarchive|url=https://web.archive.org/web/20131105044630/http://jamesallred.com/function-of-the-colon |date=2013-11-05 }} Retrieved on 2010-01-21</ref>

===Standing gradient osmosis===
Water absorption at the colon typically proceeds against a [[Route of administration#Application location|transmucosal]] [[osmotic pressure]] [[Osmosis#Osmotic gradient|gradient]]. The '''standing gradient osmosis''' is the reabsorption of water against the osmotic gradient in the intestines. Cells occupying the intestinal lining pump sodium ions into the intercellular space, raising the osmolarity of the intercellular fluid. This [[Tonicity|hypertonic]] fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis via [[tight junction]]s and adjacent cells, which then in turn moves across the [[Basal lamina|basement membrane]] and into the [[Capillary|capillaries]], while more sodium ions are pumped again into the intercellular fluid.<ref>{{cite web | url=http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_water.html | title=Absorption of Water and Electrolytes}}</ref> Although water travels down an osmotic gradient in each individual step, overall, water usually travels against the osmotic gradient due to the pumping of sodium ions into the intercellular fluid. This allows the large intestine to absorb water despite the blood in capillaries being [[Tonicity|hypotonic]] compared to the fluid within the intestinal lumen.

===Gut flora===
{{main|Gut microbiota}}

The large intestine houses over 700 species of [[bacteria]] that perform a variety of functions, as well as [[fungi]], [[protozoa]], and [[archaea]]. Species diversity varies by geography and diet.<ref>{{cite journal | last1 = Yatsunenko | first1 = Tanya | display-authors = etal | year = 2012 | title = Human gut microbiome viewed across age and geography | journal = Nature | volume = 486 | issue = 7402| pages = 222–227 | pmc = 3376388 | pmid = 22699611 | doi = 10.1038/nature11053 | bibcode = 2012Natur.486..222Y }}</ref> The microbes in a human distal gut often number in the vicinity of 100 trillion, and can weigh around 200 grams (0.44 pounds). This mass of mostly symbiotic microbes has recently been called the latest human organ to be "discovered" or in other words, the "forgotten organ".<ref>{{cite journal | last1 = O'Hara | first1 = Ann M. | last2 = Shanahan | first2 = Fergus | year = 2006 | title = The gut flora as a forgotten organ | journal = EMBO Reports | volume = 7 | issue = 7| pages = 688–693 | doi=10.1038/sj.embor.7400731| pmid = 16819463 | pmc = 1500832 }}</ref>

The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested [[polysaccharides]] (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed by [[passive diffusion]]. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids.<ref>{{Cite journal|title = The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism|journal = Journal of Lipid Research|date = 2013-09-01|issn = 0022-2275|pmc = 3735932|pmid = 23821742|pages = 2325–2340|volume = 54|issue = 9|doi = 10.1194/jlr.R036012 |doi-access=free |first1 = Gijs|last1 = den Besten|first2 = Karen|last2 = van Eunen|first3 = Albert K.|last3 = Groen|first4 = Koen|last4 = Venema|first5 = Dirk-Jan|last5 = Reijngoud|first6 = Barbara M.|last6 = Bakker}}</ref>

These bacteria also produce large amounts of [[vitamins]], especially [[vitamin K]] and [[biotin]] (a [[B vitamin]]), for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual who depends on absorption of vitamins formed by bacteria in the large intestine may become vitamin-deficient if treated with [[antibiotics]] that inhibit the vitamin producing species of bacteria as well as the intended disease-causing bacteria.<ref>{{Cite journal|title = Time to Recognize Our Fellow Travellers|journal = Journal of General Internal Medicine|date = 2012-12-01|issn = 0884-8734|pmc = 3509308|pmid = 22588826|pages = 1704–1706|volume = 27|issue = 12|doi = 10.1007/s11606-012-2105-6|first1 = Travis B.|last1 = Murdoch|first2 = Allan S.|last2 = Detsky}}</ref>

Other bacterial products include gas ([[flatulence|flatus]]), which is a mixture of [[nitrogen]] and [[carbon dioxide]], with small amounts of the gases [[hydrogen]], [[methane]], and [[hydrogen sulfide]]. Bacterial [[Fermentation (biochemistry)|fermentation]] of undigested [[polysaccharides]] produces these. Some of the fecal odor is due to [[indole]]s, metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and [[lymphatics]].{{Citation needed|date=March 2014}}

They are also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that are also effective against related pathogens, thereby preventing infection or invasion.

The two most prevalent phyla of the colon are [[Bacillota]] and [[Bacteroidota]]. The ratio between the two seems to vary widely as reported by the Human Microbiome Project.<ref name="Structure, function and diversity of the healthy human microbiome">{{cite journal|last1=Human Microbiome Project Consortium|title=Structure, function and diversity of the healthy human microbiome|journal=Nature|date=Jun 14, 2012|volume=486|issue=7402|pages=207–214|doi=10.1038/nature11234|pmid=22699609|pmc=3564958|bibcode=2012Natur.486..207T}}</ref> [[Bacteroides]] are implicated in the initiation of [[colitis]] and [[colon cancer]]. [[Bifidobacteria]] are also abundant, and are often described as 'friendly bacteria'.<ref>{{Cite journal|title = Commensal Bacteroides species induce colitis in host-genotype-specific fashion in a mouse model of inflammatory bowel disease|journal = Cell Host & Microbe|date = 2011-05-19|issn = 1931-3128|pmc = 3241010|pmid = 21575910|pages = 390–403|volume = 9|issue = 5|doi = 10.1016/j.chom.2011.04.009|first1 = Seth M.|last1 = Bloom|first2 = Vinieth N.|last2 = Bijanki|first3 = Gerardo M.|last3 = Nava|first4 = Lulu|last4 = Sun|first5 = Nicole P.|last5 = Malvin|first6 = David L.|last6 = Donermeyer|first7 = W. Michael|last7 = Dunne|first8 = Paul M.|last8 = Allen|first9 = Thaddeus S.|last9 = Stappenbeck}}</ref><ref>{{Cite journal|title = Diversity, ecology and intestinal function of bifidobacteria|journal = Microbial Cell Factories|date = 2014-08-29|issn = 1475-2859|pmc = 4155821|pmid = 25186128|pages = S4|volume = 13|issue = Suppl 1|doi = 10.1186/1475-2859-13-S1-S4|first1 = Francesca|last1 = Bottacini|first2 = Marco|last2 = Ventura|first3 = Douwe|last3 = van Sinderen|first4 = Mary|last4 = O'Connell Motherway | doi-access=free }}</ref>

A [[mucus]] layer protects the large intestine from attacks from colonic [[commensal bacteria]].<ref>{{Cite journal|title = The gastrointestinal mucus system in health and disease|journal = Nature Reviews. Gastroenterology & Hepatology|date = 2013-06-01|issn = 1759-5045|pmc = 3758667|pmid = 23478383|pages = 352–361|volume = 10|issue = 6|doi = 10.1038/nrgastro.2013.35|first1 = Malin E.V.|last1 = Johansson|first2 = Henrik|last2 = Sjövall|first3 = Gunnar C.|last3 = Hansson}}</ref>