Large intestine

Sections of the colon

In mammals, the colon consists of six sections: the cecum, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum.[1]

Sections of the colon are:

• The ascending colon including the cecum and appendix
• The transverse colon including the colic flexures and transverse mesocolon
• The descending colon
• The sigmoid colon – the s-shaped region of the large intestine
• The rectum

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.[13] Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal.[14] 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 cm in healthy individuals, and the transverse colon averages less than 6 cm in diameter.[15] The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging 4–5 cm in diameter.[15][16] Diameters larger than certain thresholds for each colonic section can be diagnostic for megacolon.

The cecum is the first section of the colon and involved in the digestion, while the appendix which develops embryologically from it, is a structure of the colon, not involved in digestion and considered to be part of the gut-associated lymphoid tissue. The function of the appendix is uncertain, but some sources believe that the appendix has a role in housing a sample of the colon's microflora, and is able to help to repopulate the colon with bacteria if the microflora has been damaged 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 the ileocecal valve, it will move into the cecum and then to the ascending colon where this process of extraction starts. The unwanted waste material is moved upwards toward the transverse colon by the action of peristalsis. The ascending colon is sometimes attached to the appendix via Gerlach's valve. In ruminants, the ascending colon is known as the spiral colon.[17][18][19] Taking into account all ages and sexes, colon cancer occurs here most often (41%).[20]

The transverse colon is the part of the colon from the hepatic flexure, also known as the right colic, (the turn of the colon by the liver) to the 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.

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.[21] 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.

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 sigmoid; cf. sigmoid sinus). The walls of the sigmoid colon are muscular, and contract to increase the pressure inside the colon, causing the 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.

The rectum is the last section of the large intestine. It holds the formed feces awaiting elimination via defecation. It is about 13 cm long.

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 sacculated, forming the haustra of the colon which are the shelf-like intraluminal projections.[22]

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 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.[23]

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.

Lymphatic drainage from the ascending colon and proximal two-thirds of the transverse colon is to the colic lymph nodes and the superior mesenteric lymph nodes, which drain into the cisterna chyli.[24] 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.[24] The lower rectum to the anal canal above the pectinate line drain to the internal iliac nodes.[25] The anal canal below the pectinate line drains into the superficial inguinal nodes.[25] The pectinate line only roughly marks this transition.

Sympathetic supply : Superior & inferior mesenteric ganglia Parasympathetic supply : Vagus & pelvic nerves

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.[26][27] 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.[28]

Colonic crypts (intestinal glands) within four tissue sections. The cells have been stained to show a brown-orange color if the cells produce the mitochondrial protein cytochrome c oxidase subunit I (CCOI), and the nuclei of the cells (located at the outer edges of the cells lining the walls of the crypts) are stained blue-gray with haematoxylin. 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[29] and illustrations were published with Creative Commons Attribution-Noncommercial License allowing re-use.

The wall of the large intestine is lined with simple columnar epithelium with invaginations. The invaginations are called the intestinal glands or colonic crypts.

• 
  Micrograph of normal large instestinal crypts.
• 
  Anatomy of normal large intestinal crypts

The colon crypts are shaped like microscopic thick walled test tubes with a central hole down the length of the tube (the crypt lumen). 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 by immunohistochemistry to show a brown-orange color if the cells produce a mitochondrial protein called cytochrome c oxidase subunit I (CCOI). The nuclei of the cells (located at the outer edges of the cells lining the walls of the crypts) are stained blue-gray with haematoxylin. As seen in panels C and D, crypts are about 75 to about 110 cells long. Baker et al.[30] 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.[31] 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 colonic lumen days later.[30] There are 5 to 6 stem cells at the bases of the crypts.[30]

As estimated from the image in panel A, there are about 100 colonic crypts per square millimeter of the colonic epithelium.[32] Since the average length of the human colon is 160.5 cm[12] and the average inner circumference of the colon is 6.2 cm,[32] 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 a mitochondrial DNA mutation in the CCOI 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 in CCOI, 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.[29] Colonic crypts deficient for CCOI in women reaches, on average, 18% in women and 23% in men by 80–84 years of age.[29]

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).[29]

About 150 of the many thousands of protein coding genes expressed in the large intestine, some are specific to the mucous membrane in different regions and include CEACAM7.[33]

Water absorption at the colon typically proceeds against a transmucosal osmotic pressure 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 hypertonic fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis via tight junctions and adjacent cells, which then in turn moves across the basement membrane and into the capillaries, while more sodium ions are pumped again into the intercellular fluid.[41] 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 hypotonic compared to the fluid within the intestinal lumen.

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.[42] 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".[43]

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.[44]

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.[45]

Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of the gases hydrogen, methane, and hydrogen sulfide. Bacterial fermentation of undigested polysaccharides produces these. Some of the fecal odor is due to indoles, metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.

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 firmicutes and bacteroidetes. The ratio between the two seems to vary widely as reported by the Human Microbiome Project.[46] Bacteroides are implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.[47][48]

A mucus layer protects the large intestine from attacks from colonic commensal bacteria.[49]

Following are the most common diseases or disorders of the colon:

Colonoscopy image, splenic flexure,
normal mucosa. You can see spleen through it : the black part

Colonoscopy is the endoscopic examination of the large intestine and the distal part of the small bowel with a CCD camera or a fiber optic camera on a flexible tube passed through the anus. It can provide a visual diagnosis (e.g. ulceration, polyps) and grants the opportunity for biopsy or removal of suspected colorectal 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 less for a polyp to turn cancerous.

Colonoscopy is similar to sigmoidoscopy—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.[50][51][52]

A sigmoidoscopy is often used as a screening procedure for a full colonoscopy, often done in conjunction with stool based test such as fecal occult blood test (FOBT), Fecal immunochemical test (FIT)or A multi-target stool DNA test (Cologuard) or blood-based test, SEPT9 DNA methylation test (Epi proColon).[53] About 5% of these screened patients are referred to colonoscopy.[54]

Virtual colonoscopy, which uses 2D and 3D imagery reconstructed from computed tomography (CT) scans or from nuclear magnetic resonance (MR) scans, is also possible, as a totally non-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 term pouchoscopy to refer to a colonoscopy of the ileo-anal pouch.