Illustration of the<unk> . The colon of the<unk>  is the last part of the digestive system. It has a segmented appearance due to a series of saccules called haustra. It extracts water and salt from solid wastes before they are eliminated from the body and is the site in which the fermentation of unabsorbed material by the gut microbiota occurs. Unlike the small<unk> , the colon does not play a major role in absorption of foods and nutrients. About 1.5<unk> tres or 45 ounces of water arrives in the colon each day. The colon is the longest part of the<unk>  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. Sections Inner diameters of colon sections In mammals, the<unk>  consists of the cecum (including the appendix), colon (the longest part), rectum, and anal canal. 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. Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal. 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. The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging in diameter. Diameters<unk> r than certain thresholds for each colonic section can be diagnostic for megacolon. 3D File generated from computed tomography of<unk>  Cecum and appendix The cecum is the first section of the<unk>  and is involved in digestion, while the 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 The ascending colon is the first of four main sections of the<unk> . It is connected to the small<unk>  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<unk>  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 via Gerlach's valve. In ruminants, the ascending colon is known as the spiral colon. Taking into account all ages and sexes, colon cancer occurs here most often (41%). Transverse colon 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<unk> ver) 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<unk>  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 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. 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 The sigmoid colon is the part of the<unk>  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. Rectum The rectum is the last section of the<unk> . It holds the formed feces awaiting elimination via defecation. It is about 12 cm long. Appearance The cecum – the first part of the<unk>  * 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<unk> . Because the taenia coli are shorter than the<unk>  bowel itself, the colon becomes sacculated, forming the haustra of the colon which are the shelf-like intraluminal projections. Blood supply 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<unk> terature questions the existence of this vessel, with some experts calling for the abolition of these terms from future medical<unk> terature. 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<unk> ver. Lymphatic drainage Lymphatic drainage from the ascending colon and proximal two-thirds of the transverse colon is to the ileocolic lymph nodes and the superior mesenteric lymph nodes, which drain into the cisterna chyli. 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. The lower rectum to the anal canal above the pectinate<unk> ne drain to the internal ileocolic nodes. The anal canal below the pectinate<unk> ne drains into the superficial inguinal nodes. The pectinate<unk> ne only roughly marks this transition. Nerve supply Sympathetic supply : Superior & inferior mesenteric ganglia Parasympathetic supply : Vagus & pelvic nerves Development 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. 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.
Large intestine
Small intestine
Stomach
Gastrointestinal tract
Liver
Gastric mucosa
Lung
Kidney