||It has been suggested that Colon (anatomy) be merged into this article. (Discuss) Proposed since March 2014.|
|This article needs additional citations for verification. (July 2011)|
|Front of abdomen, showing the large intestine, with the stomach and small intestine in gray outline.|
|Front of abdomen, showing surface markings for liver (red), and the stomach and large intestine (blue)|
|Gray's||subject #249 1177|
|Lymph||inferior mesenteric lymph nodes|
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.
Terminologia Anatomica, Medscape, and Gray's Anatomy define the large intestine as the combination of the cecum, colon, rectum, and anal canal. Other sources, such as Mosby's Medical Dictionary and the Oxford Dictionaries of Medicine and Biology exclude the anal canal. 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.
Parts of the large intestine are:
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
Locations along the colon are:
- The ascending colon
- The right colic flexure (hepatic)
- The transverse colon
- The transverse mesocolon
- The left colic flexure (splenic)
- The descending colon
- The sigmoid colon – the v-shaped region of the large intestine
In other animals
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.
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.
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.
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 which 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.
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.
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. The appendix can be removed with no apparent damage or consequence to the patient.
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.
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.
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.
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.
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'.
This article uses anatomical terminology; for an overview, see anatomical terminology.
- "large intestine". NCI Dictionary of Cancer Terms. National Cancer Institute, National Institutes of Health. Retrieved 2014-03-04.
- Kapoor, Vinay Kumar (13 Jul 2011). "Large Intestine Anatomy". In Gest, Thomas R. Medscape. WebMD LLC. Retrieved 2013-08-20.
- Gray, Henry (1918). Gray's Anatomy. Philadelphia: Lea & Febiger.
- "large intestine". Mosby's Medical Dictionary (8th ed.). Elsevier. 2009. ISBN 9780323052900.
- "intestine". Concise Medical Dictionary. Oxford University Press. 2010. ISBN 9780199557141.
- "large intestine". A Dictionary of Biology. Oxford University Press. 2013. ISBN 9780199204625.
- Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 351–354. ISBN 0-03-910284-X.
- 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.
- 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.
|Wikimedia Commons has media related to Large intestine.|
|Look up large intestine in Wiktionary, the free dictionary.|
- 09-118h. at Merck Manual of Diagnosis and Therapy Home Edition
- Large Intestine at the US National Library of Medicine Medical Subject Headings (MeSH)