Large intestine

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Large Intestine
Intestine-diagram.svg
Front of abdomen, showing the large intestine, with the stomach and small intestine in gray outline.
Gray1223.png
Front of abdomen, showing surface markings for liver (red), and the stomach and large intestine (blue)
Latin intestinum crassum
Gray's p.1177
Lymph inferior mesenteric lymph nodes
Dorlands
/Elsevier
Large intestine

The large intestine is the last part of the digestive system in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body.[1]

Terminologia Anatomica, Medscape, and Gray's Anatomy define the large intestine as the combination of the cecum, colon, rectum, and anal canal.[2][3] Other sources, such as Mosby's Medical Dictionary and the Oxford Dictionaries of Medicine and Biology exclude the anal canal.[4][5][6] In humans, it 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. It then continues up the abdomen, across the width of the abdominal cavity, and then down to its endpoint at the anus. Overall, in humans, the large intestine is about 1.5 metres (4.9 ft) long, which is about one-fifth of the whole length of the intestinal canal.[citation needed]

Structure[edit]

Illustration of the large intestine.

The colon is the last part of the digestive system in most vertebrates. It extracts water and salt from solid wastes before they are eliminated from the body and is the site in which flora-aided (large bacterial) fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a major role in absorption of foods and nutrients. However, the colon does absorb water, sodium and some fat soluble vitamins.[7]

Sections[edit]

Sections of the colon

In mammals, the colon consists of four sections: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon (the proximal colon usually refers to the ascending colon and transverse colon). The cecum, colon, rectum and anal canal make up the large intestine.[1]

Locations along the colon are:

The parts of the colon are either in the abdominal cavity (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.[8] Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the caecum, appendix, transverse colon and sigmoid colon are intraperitoneal.[9] This is important as it affects which organs can be easily accessed during surgery, such as a laparotomy.

Ascending colon[edit]

The ascending colon is one part of four sections of the large intestine. This first section of the large intestine is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs through the abdominal cavity, upwards 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 back into the body. As the waste material exits the small intestine it will move into the cecum and then the ascending colon where this process of extracting starts. The waste material is moved upwards toward the transverse section of the colon by a process known as peristalsis.

In ruminants, this is known as the spiral colon.[10] The cecum receives the solid wastes of digestion from the ileum via the Ileocecal valve.[11][12]

Transverse colon[edit]

The transverse colon is the part of the colon from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). The transverse colon hangs off the stomach, attached to it by a wide band of tissue 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). Cancers form more frequently further along the large intestine as the contents become more solid (water is removed) in order to form feces.

The proximal two-thirds of the transverse colon is perfused by the middle colic artery, a branch of SMA, while the latter third is supplied by branches of the 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. The function of the descending colon in the digestive system is to store faeces 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.

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.

Appearance[edit]

Cecum – the first part of the large intestine

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.[13]

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 a "marginal artery" that runs parallel to the colon for its entire length. Historically, it has been believed that the arc of Riolan, or the meandering mesenteric artery (of Moskowitz), is a variable vessel connecting the proximal SMA to the proximal IMA that can be extremely important if either vessel is occluded. However, recent studies conducted with improved imaging technology have questioned the actual existence of this vessel, with some experts calling for the abolition of the terms from future medical literature.

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 entire colon and proximal two-thirds of the rectum is to the paraaortic lymph nodes that then drain into the cisterna chyli. The lymph from the remaining rectum and anus can either follow the same route, or drain to the internal iliac and superficial inguinal nodes. The pectinate line only roughly marks this transition.

Variation[edit]

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.[14][15] 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.[16]

Function[edit]

The large intestine takes about 16 hours to finish the digestion of the food. It removes 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 as vitamin K (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation), vitamin B12, thiamine and riboflavin. It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation. The large intestine also secretes K+ and Cl-. Chloride secretion increases in cystic fibrosis. Recycling of various nutrients takes place in colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling.[citation needed]

The large intestine differs in physical form from the small intestine in being much wider and in showing the longitudinal layer of the muscularis have been reduced to 3 strap-like structures known as the taeniae coli. The wall of the large intestine is lined with simple columnar epithelium. Instead of having the evaginations of the small intestine (villi), the large intestine has invaginations (the intestinal glands). While both the small intestine and the large intestine have goblet cells, they are abundant in the large intestine.[citation needed]

The appendix is attached to the inferior surface of the cecum, and contains a small amount of mucosa-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 contains endocrine cells that release biogenic amines and peptide hormones important for homeostasis during early growth and development.[17] The appendix can be removed with no apparent damage or consequence to the patient.[citation needed]

The large intestine extends from the ileocecal junction to the anus and is about 1.5 m long. On the surface, bands of longitudinal muscle fibers called taeniae coli, each about 1/5 in wide, can be identified. There are three bands, and they start at the base of the appendix and extend from the cecum to the rectum. Along the sides of the taeniae, tags of peritoneum filled with fat, called epiploic appendages (or appendices epiploicae) are found. The sacculations, called haustra, are characteristic features of the large intestine, and distinguish it from the small intestine.[citation needed]

The large intestine comes after the small intestine in the digestive tract and measures approximately 1.5 meters in length in adult humans. There are differences in the large intestine between different organisms. The large intestine is mainly responsible for storing waste, reclaiming water, maintaining the water balance, absorbing some vitamins, such as vitamin K, and providing a location for flora-aided fermentation.

By the time the chyme has reached this tube, most nutrients and 90% of the water have been absorbed by the body. At this point some electrolytes like sodium, magnesium, and chloride are left as well as indigestible parts of ingested food (e.g., a large part of ingested amylose, starch which has been shielded from digestion heretofore, and dietary fiber, which is largely indigestible carbohydrate in either soluble or insoluble form). As the chyme moves through the large intestine, most of the remaining water is removed, while the chyme is mixed with mucus and bacteria (known as gut flora), and becomes feces. The ascending 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. The stools gradually solidify as they move along into the descending colon.[18]

The bacteria break down some of the fiber for their own nourishment and create acetate, propionate, and butyrate as waste products, which in turn are used by the cell lining of the colon for nourishment.[19] No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet;[20] 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 produces no digestive enzymes -— chemical digestion is completed in the small intestine before the chyme reaches the large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral).[21]

Standing gradient osmosis[edit]

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. 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.

Bacterial flora[edit]

The large intestine houses over 700 species of bacteria that perform a variety of functions.

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.[citation needed]

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 other species of bacteria as well as the disease-causing bacteria.[citation needed]

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 sulphide. Bacterial fermentation of undigested polysaccharides produces these. Some of the fecal odor is due to indole compounds, metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.[citation needed]

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 most prevalent bacteria are the bacteroides, which have been implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.[citation needed]

A mucus layer protects the large intestine from attacks from colonic commensal bacteria.[22]

Clinical significance[edit]

Disease[edit]

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

Colonoscopy[edit]

In other animals[edit]

The large intestine is truly distinct only in tetrapods, in which it is almost always separated from the small intestine by an ileocaecal 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 most amniotes, only in mammals does the remainder of the large intestine develop into a true colon.[23]

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 in primates. However, the taeniae coli and accompanying haustra are not found in either carnivorans or ruminants. The rectum of mammals (other than monotremes) is derived from the cloaca of other vertebrates, and is, therefore, not truly homologous with the "rectum" found in these species.[23]

In 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. In sharks, this includes a rectal gland that secretes salt to help the animal maintain osmotic balance with the seawater. The gland somewhat resembles a caecum in structure, but is not a homologous structure.[23]

Additional images[edit]

See also[edit]

This article uses anatomical terminology; for an overview, see anatomical terminology.

References[edit]

This article incorporates text from a public domain edition of Gray's Anatomy.

  1. ^ a b "large intestine". NCI Dictionary of Cancer Terms. National Cancer Institute, National Institutes of Health. Retrieved 2014-03-04. 
  2. ^ Kapoor, Vinay Kumar (13 Jul 2011). "Large Intestine Anatomy". In Gest, Thomas R. Medscape. WebMD LLC. Retrieved 2013-08-20. 
  3. ^ Gray, Henry (1918). Gray's Anatomy. Philadelphia: Lea & Febiger. 
  4. ^ "large intestine". Mosby's Medical Dictionary (8th ed.). Elsevier. 2009. ISBN 9780323052900. 
  5. ^ "intestine". Concise Medical Dictionary. Oxford University Press. 2010. ISBN 9780199557141. 
  6. ^ "large intestine". A Dictionary of Biology. Oxford University Press. 2013. ISBN 9780199204625. 
  7. ^ Colon Function And Health Information Retrieved on 2010-01-21
  8. ^ "Peritoneum". Mananatomy.com. 2013-01-18. Retrieved 2013-02-07. 
  9. ^ http://www.ucd.ie/vetanat/ga-subject/abdomen/ab13.html
  10. ^ Medical dictionary 
  11. ^ Spiral colon and caecum 
  12. ^ Veterinary Dictionary
  13. ^ Anatomy at a Glance by Omar Faiz and David Moffat
  14. ^ Mayo Clinic Staff (2006-10-13). "Redundant colon: A health concern?". Ask a Digestive System Specialist. MayoClinic.com. Archived from the original on 2007-09-29. Retrieved 2007-06-11. 
  15. ^ Mayo Clinic Staff. "Redundant colon: A health concern? (Above with active image links)". riversideonline.com. Retrieved 8 November 2013. 
  16. ^ Lichtenstein, Gary R.; Peter D. Park, William B. Long, Gregory G. Ginsberg, Michael L. Kochman (18 August 1998). "Use of a Push Enteroscope Improves Ability to Perform Total Colonoscopy in Previously Unsuccessful Attempts at Colonoscopy in Adult Patients". The American Journal of Gastroenterology 94 (1): 187–90. doi:10.1111/j.1572-0241.1999.00794.x. PMID 9934753.  Note: single use PDF copy provided free by Blackwell Publishing for purposes of Wikipedia content enrichment.
  17. ^ Martin, Loren G. (1999-10-21). "What is the function of the human appendix? Did it once have a purpose that has since been lost?". Scientific American. Retrieved 2014-03-03. 
  18. ^ The Function Of The Colon Retrieved on 2010-01-21
  19. ^ Terry L. Miller and Meyer J. Wolin (1996). "Pathways of Acetate, Propionate, and Butyrate Formation by the Human Fecal Microbial Flora". Applied and Environmental Microbiology 62 (5): 1589–1592. 
  20. ^ McNeil, NI (1984). "The contribution of the large intestine to energy supplies in man". The American journal of clinical nutrition 39 (2): 338–342. PMID 6320630. 
  21. ^ Function Of The Large Intestine Retrieved on 2010-01-21
  22. ^ Stremmel, W; Merle, U; Zahn, A; Autschbach, F; Hinz, U; Ehehalt, R (2005). "Retarded release phosphatidylcholine benefits patients with chronic active ulcerative colitis". Gut 54 (7): 966–971. doi:10.1136/gut.2004.052316. PMC 1774598. PMID 15951544. 
  23. ^ a b c Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 351–354. ISBN 0-03-910284-X. 

External links[edit]