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Knowing About Large Intestine

Large intestine

By Wikipedia The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces before being removed by defecation.^[1]

The colon^[2] is the largest portion of the large intestine, so many mentions of the large intestine and colon overlap in meaning whenever precision is not the focus. Most sources define the large intestine as the combination of the cecum, colon, rectum, and anal canal.^[3][4] Some other sources exclude the anal canal.^[5][6][7]

In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve. It then continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, and then descending to the rectum and its endpoint at the anal canal.^[8] 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 gastrointestinal tract.^[9]

Structure

The colon is the last part of the digestive system. It extracts water and salt from solid wastes before they are eliminated from the body and is the site in which flora-aided (largely bacterial) fermentation of unabsorbed material 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.^[10]

The length of the average adult human colon 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]

Sections[edit]

Sections of the colon

In mammals, the colon consists of six sections: the cecum plus 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.^[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 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.^[14] The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging 4–5 cm in diameter.^[14][15] Diameters larger than certain thresholds for each colonic section can be diagnostic for megacolon.

Cecum and appendix[edit]

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.

Ascending colon[edit]

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.^[16][17][18] Taking into account all ages and sexes, colon cancer occurs here most often (41%).^[19]

Transverse colon[edit]

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.

Descending colon[edit]

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.^[20] 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[edit]

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.

Rectum[edit]

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

Appearance[edit]

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.^[21]

Blood supply[edit]

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.^[22]

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[edit]

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.^[23] 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.^[23] The lower rectum to the anal canal above the pectinate line drain to the internal iliac nodes.^[24] The anal canal below the pectinate line drains into the superficial inguinal nodes.^[24] The pectinate line only roughly marks this transition.

Nerve supply[edit]

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