Thelarge intestine, also known as thelarge bowel, is the last part of thegastrointestinal tract and of thedigestive system intetrapods. Water is absorbed here and the remaining waste material is stored in therectum asfeces before being removed bydefecation.[1] Thecolon (progressing from theascending colon to thetransverse, thedescending and finally thesigmoid colon) is the longest portion of the large intestine, and the terms "large intestine" and "colon" are often used interchangeably, but most sources define the large intestine as the combination of thececum, colon,rectum, andanal canal.[1][2][3] Some other sources exclude the anal canal.[4][5][6]
In humans, the large intestine begins in the rightiliac region of thepelvis, just at or below thewaist, where it is joined to the end of thesmall intestine at the cecum, via theileocecal valve. It then continues as the colonascending theabdomen, across the width of theabdominal cavity as thetransverse colon, and thendescending to the rectum and its endpoint at theanal canal.[7] Overall, in humans, the large intestine is about 1.5 metres (5 ft) long, which is about one-fifth of the whole length of the human gastrointestinal tract.[8]
Thecolon of the large intestine is the last part of thedigestive system. It has a segmented appearance due to a series of saccules calledhaustra.[9] It extractswater andsalt fromsolid wastes before they areeliminated from the body and is the site in which thefermentation of unabsorbed material by thegut microbiota occurs. Unlike thesmall 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.[10]
The colon is the longest part of the large intestine and its average length in the adult human is 65 inches or 166 cm (range of 80 to 313 cm) for males, and 61 inches or 155 cm (range of 80 to 214 cm) for females.[11]
The parts of the colon are either intraperitoneal or behind it in theretroperitoneum. Retroperitoneal organs, in general, do not have a complete covering ofperitoneum, so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile.[12] Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal.[13] This is important as it affects which organs can be easily accessed during surgery, such as alaparotomy.
In terms of diameter, the cecum is the widest, averaging slightly less than 9 cm in healthy individuals, and the transverse colon averages less than 6 cm in diameter.[14] The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging 4–5 cm (1.6–2.0 in) in diameter.[14][15] Diameters larger than certain thresholds for each colonic section can be diagnostic formegacolon.
Thececum is the first section of the large intestine and is involved in digestion, while theappendix which develops embryologically from it, is not involved in digestion and is considered to be part of thegut-associated lymphoid tissue. The function of the appendix is uncertain, but some sources believe that it has a role in housing a sample of thegut 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.
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 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 theileocecal 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 byperistalsis. The ascending colon is sometimes attached to theappendix viaGerlach's valve. Inruminants, the ascending colon is known as thespiral colon.[16][17][18]Taking into account all ages and sexes, colon cancer occurs here most often (41%).[19]
The transverse colon is the part of the colon from thehepatic flexure, also known as the right colic, (the turn of the colon by theliver) to thesplenic flexure also known as the left colic, (the turn of the colon by thespleen). The transverse colon hangs off thestomach, attached to it by a large fold ofperitoneum called thegreater omentum. On the posterior side, the transverse colon is connected to the posterior abdominal wall by amesentery known as thetransverse mesocolon.
The transverse colon is encased inperitoneum, 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 themiddle colic artery, a branch of thesuperior mesenteric artery (SMA), while the latter third is supplied by branches of theinferior mesenteric artery (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between themidgut andhindgut, is an area sensitive toischemia.
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 isretroperitoneal in two-thirds of humans. In the other third, it has a (usually short) mesentery.[20] The arterial supply comes via theleft colic artery. The descending colon is also called thedistal gut, as it is further along the gastrointestinal tract than the proximal gut. Gut flora are very dense in this region.
The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The namesigmoid means S-shaped (seesigmoid; cf.sigmoid sinus). The walls of the sigmoid colon are muscular and contract to increase the pressure inside the colon, causing thestool to move into the rectum.
The sigmoid colon is supplied with blood from several branches (usually between 2 and 6) of thesigmoid arteries, a branch of the IMA. The IMA terminates as thesuperior rectal artery.
Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon.
Thetaenia coli run the length of the large intestine. Because the taenia coli are shorter than the large bowel itself, the colon becomessacculated, forming thehaustra of the colon which are the shelf-like intraluminal projections.[22]
Arterial supply to the colon comes from branches of thesuperior mesenteric artery (SMA) andinferior mesenteric artery (IMA). Flow between these two systems communicates via themarginal 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 theproximal 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.[23]
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.[27]
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 asredundant colon, typically has no direct major health consequences, though rarelyvolvulus occurs, resulting in obstruction and requiring immediate medical attention.[28][29] A significant indirect health consequence is that use of a standard adultcolonoscope 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.[30]
Colonic crypts (intestinal glands) within four tissue sections. The cells have beenstained to show a brown-orange color if the cells produce themitochondrial proteincytochrome c oxidase subunit I (CCOI), and thenuclei of the cells (located at the outer edges of the cells lining the walls of the crypts) are stained blue-gray withhaematoxylin. Panels A, B were cut across the long axes of the crypts and panels C, D were cut parallel to the long axes of the crypts. In panel A the bar shows 100 μm and allows an estimate of the frequency of crypts in the colonic epithelium. Panel B includes three crypts in cross-section, each with one segment deficient for CCOI expression and at least one crypt, on the right side, undergoing fission into two crypts. Panel C shows, on the left side, a crypt fissioning into two crypts. Panel D shows typical small clusters of two and three CCOI deficient crypts (the bar shows 50 μm). The images were made from original photomicrographs, but panels A, B and D were also included in an article[31] and illustrations were published with Creative Commons Attribution-Noncommercial License allowing re-use.
The colon crypts are shaped like microscopic thick walled test tubes with a central hole down the length of the tube (the cryptlumen). Four tissue sections are shown here, two cut across the long axes of the crypts and two cut parallel to the long axes. In these images the cells have been stained byimmunohistochemistry to show a brown-orange color if the cells produce amitochondrial protein calledcytochrome c oxidase subunit I (CCOI). Thenuclei of the cells (located at the outer edges of the cells lining the walls of the crypts) are stained blue-gray withhaematoxylin. As seen in panels C and D, crypts are about 75 to about 110 cells long. Baker et al.[32] found that the average crypt circumference is 23 cells. Thus, by the images shown here, there are an average of about 1,725 to 2,530 cells per colonic crypt. Nooteboom et al.[33] measuring the number of cells in a small number of crypts reported a range of 1,500 to 4,900 cells per colonic crypt. Cells are produced at the crypt base and migrate upward along the crypt axis before being shed into the coloniclumen days later.[32] There are 5 to 6 stem cells at the bases of the crypts.[32]
As estimated from the image in panel A, there are about 100 colonic crypts per square millimeter of the colonic epithelium.[34] Since the average length of the human colon is 160.5 cm[11] and the average inner circumference of the colon is 6.2 cm,[34] the inner surface epithelial area of the human colon has an average area of about 995 cm2, which includes 9,950,000 (close to 10 million) crypts.
In the four tissue sections shown here, many of the intestinal glands have cells with amitochondrial DNA mutation in theCCOI gene and appear mostly white, with their main color being the blue-gray staining of the nuclei. As seen in panel B, a portion of the stem cells of three crypts appear to have a mutation inCCOI, so that 40% to 50% of the cells arising from those stem cells form a white segment in the cross cut area.
Overall, the percent of crypts deficient for CCOI is less than 1% before age 40, but then increases linearly with age.[31] Colonic crypts deficient for CCOI in women reaches, on average, 18% in women and 23% in men by 80–84 years of age.[31]
Crypts of the colon can reproduce by fission, as seen in panel C, where a crypt is fissioning to form two crypts, and in panel B where at least one crypt appears to be fissioning. Most crypts deficient in CCOI are in clusters of crypts (clones of crypts) with two or more CCOI-deficient crypts adjacent to each other (see panel D).[31]
About 150 of the many thousands ofprotein coding genes expressed in the large intestine, some are specific to the mucous membrane in different regions and includeCEACAM7.[35]
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 asthiamine,riboflavin, andvitamin K (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequateblood coagulation).[36][citation needed][37] It also compacts feces, and stores fecal matter in the rectum until it can be discharged via theanus indefecation.
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.[38][citation needed]
Theappendix contains a small amount ofmucosa-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 containsendocrine cells that release biogenic amines and peptide hormones important forhomeostasis during early growth and development.[39]
By the time thechyme has reached this tube, mostnutrients and 90% of the water have been absorbed by the body. Indeed, as demonstrated by the commonality ofileostomy 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 someelectrolytes likesodium,magnesium, andchloride are left as well as indigestible parts of ingested food (e.g., a large part of ingestedamylose, starch which has been shielded from digestion heretofore, anddietary fiber, which is largely indigestiblecarbohydrate in either soluble or insoluble form). As the chyme moves through the large intestine, most of the remainingwater is removed, while the chyme is mixed withmucus andbacteria (known asgut flora), and becomes feces. Theascending 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 thedescending colon.[40]
The bacteria break down some of thefiber for their own nourishment and createacetate,propionate, andbutyrate as waste products, which in turn are used by the cell lining of the colon for nourishment.[41] No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet;[42] 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[43] produces no digestiveenzymes —chemical digestion is completed in thesmall intestine before the chyme reaches the large intestine. ThepH in the colon varies between 5.5 and 7 (slightlyacidic to neutral).[44]
Water absorption at the colon typically proceeds against atransmucosalosmotic pressuregradient. Thestanding 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. Thishypertonic fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis viatight junctions and adjacent cells, which then in turn moves across thebasement membrane and into thecapillaries, while more sodium ions are pumped again into the intercellular fluid.[45] 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 beinghypotonic compared to the fluid within the intestinal lumen.
The large intestine houses over 700 species ofbacteria that perform a variety of functions, as well asfungi,protozoa, andarchaea. Species diversity varies by geography and diet.[46] 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".[47]
The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigestedpolysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed bypassive diffusion. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids.[48]
These bacteria also produce large amounts ofvitamins, especiallyvitamin K andbiotin (aB 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 withantibiotics that inhibit the vitamin producing species of bacteria as well as the intended disease-causing bacteria.[49]
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 areBacillota andBacteroidota. The ratio between the two seems to vary widely as reported by the Human Microbiome Project.[50]Bacteroides are implicated in the initiation ofcolitis andcolon cancer.Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.[51][52]
Colonoscopy is theendoscopic examination of the large intestine and thedistal part of thesmall bowel with aCCD camera or afiber optic camera on a flexible tube passed through theanus. It can provide a visual diagnosis (e.g.ulceration,polyps) and grants the opportunity forbiopsy or removal of suspectedcolorectal cancer lesions. Colonoscopy can remove polyps as small as one millimetre or less. Once polyps are removed, they can be studied with the aid of a microscope to determine if they are precancerous or not. It takes 15 years or fewer for a polyp to turn cancerous.
Colonoscopy is similar tosigmoidoscopy—the difference being related to which parts of the colon each can examine. A colonoscopy allows an examination of the entire colon (1200–1500 mm in length). A sigmoidoscopy allows an examination of the distal portion (about 600 mm) of the colon, which may be sufficient because benefits to cancer survival of colonoscopy have been limited to the detection of lesions in the distal portion of the colon.[54][55][56]
A sigmoidoscopy is often used as a screening procedure for a full colonoscopy, often done in conjunction with a stool-based test such as a fecal occult blood test (FOBT), fecal immunochemical test (FIT), or multi-target stool DNA test (Cologuard) or blood-based test, SEPT9 DNA methylation test (Epi proColon).[57] About 5% of these screened patients are referred to colonoscopy.[58]
Virtual colonoscopy, which uses 2D and 3D imagery reconstructed fromcomputed tomography (CT) scans or fromnuclear magnetic resonance (MR) scans, is also possible, as a totallynon-invasive medical test, although it is not standard and still under investigation regarding its diagnostic abilities. Furthermore, virtual colonoscopy does not allow for therapeutic maneuvers such as polyp/tumour removal or biopsy nor visualization of lesions smaller than 5 millimeters. If a growth or polyp is detected using CT colonography, a standard colonoscopy would still need to be performed. Additionally, surgeons have lately been using the termpouchoscopy to refer to a colonoscopy of theileo-anal pouch.
The large intestine is truly distinct only intetrapods, in which it is almost always separated from the small intestine by anileocaecal valve. In most vertebrates, however, it is a relatively short structure running directly to the anus, although noticeably wider than the small intestine. Although the caecum is present in mostamniotes, only in mammals does the remainder of the large intestine develop into a true colon.[59]
In some small mammals, the colon is straight, as it is in other tetrapods, but, in the majority of mammalian species, it is divided into ascending and descending portions; a distinct transverse colon is typically present only inprimates. However, the taeniae coli and accompanying haustra are not found in eithercarnivorans orruminants. The rectum of mammals (other thanmonotremes) is derived from thecloaca of other vertebrates, and is, therefore, not trulyhomologous with the "rectum" found in these species.[59]
In some fish, there is no true large intestine, but simply a short rectum connecting the end of the digestive part of the gut to the cloaca. Insharks, this includes arectal gland that secretes salt to help the animal maintainosmotic balance with the seawater. The gland somewhat resembles a caecum in structure but is not a homologous structure.[59]
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