Editing Large intestine (section)

==Structure==
[[File:Blausen 0604 LargeIntestine2.png|thumb|Illustration of the large intestine.]]
The '''colon''' of the large intestine is the last part of the [[Digestion|digestive system]]. It has a segmented appearance due to a series of saccules called [[haustra]].<ref>{{Cite journal|last1=Azzouz|first1=Laura|last2=Sharma|first2=Sandeep|title=Physiology, Large Intestine|url=https://www.ncbi.nlm.nih.gov/books/NBK507857/|website=[[NCBI Bookshelf]]|year=2020|pmid=29939634}}</ref> It extracts [[water]] and [[salt]] from [[feces|solid wastes]] before they are [[defecation|eliminated]] from the body and is the site in which the [[fermentation]] of unabsorbed material by the [[gut microbiota]] occurs. Unlike the [[small intestine]], the colon does not play a major role in absorption of foods and nutrients. About 1.5 litres or 45 ounces of water arrives in the colon each day.<ref name="Krogh2010">{{citation|author=David Krogh|title=Biology: A Guide to the Natural World|url=https://books.google.com/books?id=Ph7NSAAACAAJ|date=2010|publisher=Benjamin-Cummings Publishing Company|isbn=978-0-321-61655-5|page=597}}</ref>

The colon is the longest part of the large intestine and its average length in the adult human is 65 inches or 166&nbsp;cm (range of 80 to 313&nbsp;cm) for males, and 61 inches or 155&nbsp;cm (range of 80 to 214&nbsp;cm) for females.<ref name="Hounnou">{{cite journal |vauthors=Hounnou G, Destrieux C, Desmé J, Bertrand P, Velut S |title=Anatomical study of the length of the human intestine |journal=Surg Radiol Anat |volume=24 |issue=5 |pages=290–294 |year=2002 |pmid=12497219 |doi=10.1007/s00276-002-0057-y |s2cid=33366428 }}</ref>

===Sections===
[[File:Diameters of the large intestine.svg|thumb|Inner diameters of colon sections]]
In [[mammal]]s, the large intestine consists of the [[cecum]] (including the [[appendix (anatomy)|appendix]]), colon (the longest part), [[rectum]], and [[anal canal]].<ref name='NCILargeIntestineDef'/>

The four sections of the colon are: the [[ascending colon]], [[transverse colon]], [[descending colon]], and [[sigmoid colon]]. These sections turn at the [[colic flexures]].

The parts of the colon are either intraperitoneal or behind it in the [[retroperitoneum]]. Retroperitoneal organs, in general, do not have a complete covering of [[peritoneum]], so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile.<ref>{{cite web |url=http://www.mananatomy.com/digestive-system/peritoneum |title=Peritoneum |publisher=Mananatomy.com |date=2013-01-18 |access-date=2013-02-07 |archive-date=2018-10-08 |archive-url=https://web.archive.org/web/20181008195347/http://www.mananatomy.com/digestive-system/peritoneum |url-status=dead }}</ref> Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal.<ref>{{cite web|url=http://www.ucd.ie/vetanat/ga-subject/abdomen/ab13.html|title=Untitled}}</ref> This is important as it affects which organs can be easily accessed during surgery, such as a [[laparotomy]].

In terms of diameter, the cecum is the widest, averaging slightly less than 9&nbsp;cm in healthy individuals, and the transverse colon averages less than 6&nbsp;cm in diameter.<ref name=":0">{{Cite journal|last1=Horton|first1=K. M.|last2=Corl|first2=F. M.|last3=Fishman|first3=E. K.|date=March 2000|title=CT evaluation of the colon: inflammatory disease|journal=Radiographics |volume=20|issue=2|pages=399–418|doi=10.1148/radiographics.20.2.g00mc15399|issn=0271-5333|pmid=10715339|doi-access=}}</ref> The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging {{convert|4–5|cm|in|abbr=on}} in diameter.<ref name=":0" /><ref>{{Citation|last=Rossini|first=Francesco Paolo|chapter=The normal colon|date=1975|pages=46–55|editor-last=Rossini|editor-first=Francesco Paolo|publisher=Springer New York|doi=10.1007/978-1-4615-9650-9_12|isbn=9781461596509|title=Atlas of coloscopy}}</ref> Diameters larger than certain thresholds for each colonic section can be diagnostic for [[megacolon]].
[[File:3DPX-002736 Large intestine Nevit Dilmen.stl|thumb|3D file generated from [[computed tomography]] of large intestine]]

====Cecum and appendix====
{{Main|Cecum|Appendix (anatomy)}}
The [[cecum]] is the first section of the large intestine and is involved in digestion, while the [[appendix (anatomy)|appendix]] which develops embryologically from it, is not involved in digestion and is considered to be part of the [[gut-associated lymphoid tissue]]. The function of the appendix is uncertain, but some sources believe that it has a role in housing a sample of the [[gut microbiota]], and is able to help to repopulate the colon with microbiota if depleted during the course of an immune reaction. The appendix has also been shown to have a high concentration of lymphatic cells.

====Ascending colon====
{{Main|Ascending colon}}
The ascending colon is the first of four main sections of the large intestine. It is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs upwards through the abdominal cavity toward the transverse colon for approximately eight inches (20&nbsp;cm).

One of the main functions of the colon is to remove the water and other key nutrients from waste material and recycle it. As the waste material exits the small intestine through the [[ileocecal valve]], it will move into the cecum and then to the ascending colon where this process of extraction starts. The waste material is pumped upwards toward the transverse colon by [[peristalsis]]. The ascending colon is sometimes attached to the [[appendix (anatomy)|appendix]] via [[Joseph von Gerlach|Gerlach's valve]]. In [[ruminants]], the ascending colon is known as the '''spiral colon'''.<ref>{{citation
| url = http://medical-dictionary.thefreedictionary.com/spiral+colon
| title = Medical dictionary}}</ref><ref>{{citation
| url = http://faculty.ucc.edu/biology-potter/Pig_Digestive_System/sld009.htm
| title = Spiral colon and caecum
| access-date = 2014-04-02
| archive-url = https://web.archive.org/web/20160304022928/http://faculty.ucc.edu/biology-potter/Pig_Digestive_System/sld009.htm
| archive-date = 2016-03-04
| url-status = dead
}}</ref><ref>{{cite web|url=http://www.answers.com/topic/colon-9|title=Answers – The Most Trusted Place for Answering Life's Questions|website=[[Answers.com]]}}</ref>
Taking into account all ages and sexes, colon cancer occurs here most often (41%).<ref name="Colorectal cancer statistics, 2017">{{cite journal|vauthors=Siegel RL, Miller KD, Fedewa SA, Ahnen DJ, Meester RG, Barzi A, Jemal A |title=Colorectal cancer statistics|journal=CA Cancer J. Clin.|volume=67|issue=3|pages=177–193|date=March 1, 2017|doi=10.3322/caac.21395|pmid=28248415|doi-access=free}}</ref>

====Transverse colon====
{{Main|Transverse colon}}
The transverse colon is the part of the colon from the [[Colic flexures|hepatic flexure]], also known as the right colic, (the turn of the colon by the [[liver]]) to the [[Colic flexures|splenic flexure]] also known as the left colic, (the turn of the colon by the [[spleen]]). The transverse colon hangs off the [[stomach]], attached to it by a large fold of [[peritoneum]]  called the [[greater omentum]]. On the posterior side, the transverse colon is connected to the posterior abdominal wall by a [[mesentery]] known as the [[transverse mesocolon]].

The transverse colon is encased in [[peritoneum]], and is therefore mobile (unlike the parts of the colon immediately before and after it).

The proximal two-thirds of the transverse colon is perfused by the [[middle colic artery]], a branch of the [[superior mesenteric artery]] (SMA), while the latter third is supplied by branches of the [[inferior mesenteric artery]] (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between the [[midgut]] and [[hindgut]], is an area sensitive to [[ischemia]].

====Descending colon====
{{Main|Descending colon}}
The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. One function of the descending colon in the digestive system is to store feces that will be emptied into the rectum. It is [[retroperitoneal]] in two-thirds of humans. In the other third, it has a (usually short) mesentery.<ref>{{Cite journal|last1=Smithivas|first1=T.|last2=Hyams|first2=P. J.|last3=Rahal|first3=J. J.|date=1971-12-01|title=Gentamicin and ampicillin in human bile|journal=The Journal of Infectious Diseases|volume=124 Suppl|pages=S106–108|issn=0022-1899|pmid=5126238|doi=10.1093/infdis/124.supplement_1.s106}}</ref> The arterial supply comes via the [[left colic artery]]. The descending colon is also called the ''distal gut'', as it is further along the gastrointestinal tract than the proximal gut. Gut flora are very dense in this region.

====Sigmoid colon====
{{Main|Sigmoid colon}}
The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name ''sigmoid'' means S-shaped (see [[wiktionary:sigmoid|sigmoid]]; cf. [[sigmoid sinus]]). The walls of the sigmoid colon are muscular and contract to increase the pressure inside the colon, causing the [[feces|stool]] to move into the rectum.

The sigmoid colon is supplied with blood from several branches (usually between 2 and 6) of the [[sigmoid arteries]], a branch of the IMA. The IMA terminates as the [[superior rectal artery]].

[[Sigmoidoscopy]] is a common diagnostic technique used to examine the sigmoid colon.

====Rectum====
{{Main|Rectum}}
The [[rectum]] is the last section of the large intestine. It holds the formed feces awaiting elimination via defecation.
It is about 12&nbsp;cm long.<ref>{{Cite web|title=Anatomy of Colon and Rectum {{!}} SEER Training|url=https://training.seer.cancer.gov/colorectal/anatomy/|access-date=2021-04-14|website=training.seer.cancer.gov}}</ref>

===Appearance===
The [[cecum]] – the first part of the large intestine
* [[Taeniae coli]] – three bands of smooth muscle
* [[Haustra]] – bulges caused by contraction of taeniae coli
* [[Epiploic appendages]] – small fat accumulations on the viscera

The [[taenia coli]] run the length of the large intestine.  Because the taenia coli are shorter than the large bowel itself, the colon becomes ''[[:wikt:sacculated|sacculated]]'', forming the [[haustra]] of the colon which are the shelf-like intraluminal projections.<ref>Anatomy at a Glance by Omar Faiz and David Moffat</ref>

===Blood supply===
[[Artery|Arterial]] supply to the colon comes from branches of the [[superior mesenteric artery]] (SMA) and [[inferior mesenteric artery]] (IMA). Flow between these two systems communicates via the [[marginal artery of the colon]] that runs parallel to the colon for its entire length. Historically, a structure variously identified as the arc of Riolan or meandering mesenteric artery (of Moskowitz) was thought to connect the [[Anatomical terms of location#Proximal and distal|proximal]] SMA to the proximal IMA. This variably present structure would be important if either vessel were occluded. However, at least one review of the literature questions the existence of this vessel, with some experts calling for the abolition of these terms from future medical literature.<ref>{{cite journal |last1=Lange |first1=Johan F. |last2=Komen |first2=Niels |last3=Akkerman |first3=Germaine |last4=Nout |first4=Erik |last5=Horstmanshoff |first5=Herman |last6=Schlesinger |first6=Frans |last7=Bonjer |first7=Jaap |last8=Kleinrensink |first8=Gerrit-Jan |title=Riolan's arch: confusing, misnomer, and obsolete. A literature survey of the connection(s) between the superior and inferior mesenteric arteries |journal=Am J Surg |date=June 2007 |volume=193 |issue=6 |pages=742–748 |doi=10.1016/j.amjsurg.2006.10.022|pmid=17512289 }}</ref>

[[Vein|Venous]] drainage usually mirrors colonic arterial supply, with the [[inferior mesenteric vein]] draining into the [[splenic vein]], and the [[superior mesenteric vein]] joining the splenic vein to form the [[hepatic portal vein]] that then enters the [[liver]]. [[Middle rectal veins]] are an exception, delivering blood to [[inferior vena cava]] and bypassing the liver.<ref>{{cite journal |last1=van Hoogdalem |first1=Edward |last2=de Boer |first2=Albertus G. |last3=Breimer |first3=Douwe D. |title=Pharmacokinetics of rectal drug administration, Part I. General considerations and clinical applications of centrally acting drugs |journal=Clinical Pharmacokinetics |date=July 1991 |volume=21 |issue=1 |page=14 |doi=10.2165/00003088-199121010-00002 |pmid=1717195 |url=https://pubmed.ncbi.nlm.nih.gov/1717195/ |access-date=18 March 2024 |issn=0312-5963 |quote=The superior rectal vein, perfusing the upper part of the rectum, drains into the portal vein and subsequently into the liver On the other hand, the middle and inferior rectal veins drain the lower part of the rectum and venous blood is returned to the inferior vena cava.}}</ref>

===Lymphatic drainage===
[[Lymphatic system|Lymphatic drainage]] from the ascending colon and proximal two-thirds of the [[transverse colon]] is to the [[ileocolic lymph node]]s and the [[superior mesenteric lymph node]]s, which drain into the [[cisterna chyli]].<ref name="Little, Brown, and Company">{{cite book|last1=Snell|first1=Richard S.|title=Clinical Anatomy for Medical Students|date=1992|publisher=Little, Brown, and Company|location=Boston|pages=53–54|edition=4th}}</ref> The lymph from the distal one-third of the [[transverse colon]], the [[descending colon]], the sigmoid colon, and the upper [[rectum]] drain into the inferior mesenteric and colic lymph nodes.<ref name="Little, Brown, and Company"/> The lower rectum to the anal canal above the pectinate line drain to the internal ileocolic nodes.<ref name="First Aid for the USMLE Step 1">{{cite book|last1=Le|first1=Tao|title=First Aid for the USMLE Step 1|date=2014|publisher=McGraw-Hill Education|page=196|display-authors=etal}}</ref> The anal canal below the pectinate line drains into the superficial [[Groin|inguinal]] nodes.<ref name="First Aid for the USMLE Step 1"/> The [[pectinate line]] only roughly marks this transition.

===Nerve supply===
Sympathetic supply: superior & inferior mesenteric ganglia;
parasympathetic supply: vagus & sacral plexus (S2-S4){{citation needed|date=September 2024}}

===Development===
{{See also|Development of the digestive system}}
The endoderm, mesoderm and ectoderm are germ layers that develop in a process called gastrulation. Gastrulation occurs early in human development. The gastrointestinal tract is derived from these layers.<ref>{{Citation |last1=Wilson |first1=Danielle J. |title=Embryology, Bowel |date=2022 |url=http://www.ncbi.nlm.nih.gov/books/NBK545247/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31424831 |access-date=2022-05-27 |last2=Bordoni |first2=Bruno}}</ref>

===Variation===
One variation on the normal anatomy of the colon occurs when extra loops form, resulting in a colon that is up to five metres longer than normal. This condition, referred to as '''redundant colon''', typically has no direct major health consequences, though rarely [[volvulus]] occurs, resulting in obstruction and requiring immediate medical attention.<ref>{{cite web|url=http://www.mayoclinic.com/health/redundant-colon/AN00239/rss=1 |title=Redundant colon: A health concern? |access-date=2007-06-11 |date=2006-10-13 |author=Mayo Clinic Staff |work=Ask a Digestive System Specialist |publisher=MayoClinic.com |archive-url=https://web.archive.org/web/20070929110621/http://www.mayoclinic.com/health/redundant-colon/AN00239/rss%3D1 |archive-date=2007-09-29 |url-status=dead }}</ref><ref>{{cite web |url= http://www.riversideonline.com/health_reference/Questions-Answers/AN00239.cfm |title= Redundant colon: A health concern? (Above with active image links) |work= riversideonline.com |author= Mayo Clinic Staff |access-date= 8 November 2013 |archive-date= 9 November 2013 |archive-url= https://web.archive.org/web/20131109023215/http://www.riversideonline.com/health_reference/Questions-Answers/AN00239.cfm |url-status= dead }}</ref> A significant indirect health consequence is that use of a standard adult [[colonoscope]] is difficult and in some cases impossible when a redundant colon is present, though specialized variants on the instrument (including the pediatric variant) are useful in overcoming this problem.<ref>{{cite journal | last=Lichtenstein | first=Gary R. |author2=Peter D. Park |author3=William B. Long |author4=Gregory G. Ginsberg |author5=Michael L. Kochman  | title=Use of a Push Enteroscope Improves Ability to Perform Total Colonoscopy in Previously Unsuccessful Attempts at Colonoscopy in Adult Patients | journal=The American Journal of Gastroenterology | volume=94 | issue=1 | pages=187–190 | date=18 August 1998 | doi=10.1111/j.1572-0241.1999.00794.x | pmid=9934753 | s2cid=24536782 }} ''Note: single use PDF copy provided free by [[Blackwell Publishing]] for purposes of Wikipedia content enrichment.''</ref>