Large Intestine 

   The large intestine consists of the cecum, ascending, transverse, descending and sigmoid colon, rectum and anus. The longitudinal muscle of the muscularis  externa is concentrated into 3 bands called teniae coli. These 3 bands of muscle form pouches (haustra) because they are shorter than the rest of the colon. The epithelium of the mucosa is mostly gobet cells, and while it has a numerous 

amount of crypts, there are no villi. The ileocecal valve (really a sphincter) prevents materials from moving back into the ileum In the Proximal, or upper half of the large intestine, enzymes from the small intestine complete the digestive process and bacteria produces the B vitamins (B12, thiamin, and riboflavin) as well as vitamin K.

   The large intestine's primary function is to absorb water and electrolytes from digestive residues (which in humans usually takes about 24 - 30 hours) and storage of fecal matter untill it can be expelled.The large intestine is the last attraction in digestive tube and the location of the terminal phases of digestion. In comparison to other regions of the tube, there are huge differences among species in the relative size and complexity of the large intestine. Nonetheless, in all species it functions in three processes: 

? Recovery of water and electrolytes from ingesta: By the time ingesta reaches the terminal ileum, roughly 90% of its water has been absorbed, but considerable water and electrolytes like sodium and chloride remain and must be recovered by absorption in the large gut.

? Formation and storage of feces: As ingesta is moved through the large intestine, it is dehydrated, mixed with bacteria and mucus, and formed into feces. The craftsmanship (for want of a better term) with which this is carried out varies among species.

? Microbial fermentation: The large intestine of all species teems with microbial life. Those microbes produce enzymes capable of digesting many of molecules that to vertebrates are indigestible, cellulose being a premier example. The extent and benefit of fermentation also varies greatly among species. 

   Around 500-1500 ml of chyme enters the large intestine per day but less than 200 ml is lost in feces. The bacteria in the large intestine produces some vitamins which are also absorbed. The colon secretes some potassium, which may create a problem of potassium depletion during severe diarrhea.Churning movements of the intestine gradually exposes digestion residue to the absorbing walls.

   In primarily vegetarian animals, the large intestine is usually longer. The immature frog (or tadpole), for example, eats mainly plant matter and has a long, highly coiled large intestine. As the frog matures and begins to eat mostly of insects, it's intestine becomes considerably shorter. High protein food, such as meat, can readily be digested by the small intestine although there is a need of much more chemical action and agitation to reduce the tough cellulose fiber of plant cells. The large intestine performs this function with it's slow digestive process.

   In newborn humans, the large intestine does not contain the bacteria essential for production of vitamin K, lack of which may cause excessive bleeding. Infant diets should be supplemented with this vitamin for a few weeks until the infant is capable of producing its own supply. Common afflictions of the large intestine include inflammation, such as colitis; diverticulosis; and abnormal growths, such as benign or malignant tumours.

   A wide variety of diseases and disorders occur in the large intestine. Imperfect fetal development may result in an anus that has no opening, a defect that requires major plastic surgery to correct. Abnormal rotation of the colon is fairly frequent and occasionally leads to disorders. Unusually long mesenteries (the supporting tissues of the large intestine) may permit recurrent twisting, cutting off the blood supply to the involved loop. The loop itself may be completely obstructed by rotation. Such complications are usually seen in elderly patients and particularly in those with a long history of constipation.
 

   The most frequent and common disease of the large intestine is Crohn's disease. The cause of Crohn's disease is unknown. Apart from the greater tendency for fistulas to form and for the wall of the intestine to thicken until the channel is obstructed, it is distinguishable from ulcerative colitis by microscopic findings. In Crohn's disease, the maximum damage occurs beneath the mucosa, and lymphoid conglomerations, known as granulomata, are formed in the submucosa. Crohn's disease attacks the perianal tissues more often than does ulcerative colitis. Although these two diseases are not common, they are disabling.

   Because there is no specific etiology, a combination of anti-inflammatory drugs, including corticosteroids and aminosalicylic acid compounds, is used to treat Crohn's disease. The drugs are effective both in treating acute episodes and in suppressing the disease over the long term. Depending on the circumstances, hematinics, vitamins, high-protein diets, and blood transfusions are also used. Surgical resection of the portion of the large bowel affected is often done. The entire colon may have to be removed and the small intestine brought out to the skin as an ileostomy an opening to serve as a substitute for the anus. In ulcerative colitis, as opposed to Crohn's disease, the rectal muscle may be preserved and the ileum brought through it and joined to the anus.

   Tumours of the colon are usually polyps (growths from the mucous membranes) or cancers. The tendency of some persons to form polyps is strikingly exemplified in the rare disorder known as familial polyposis, in which the colon may be studded with hundreds or thousands of small polyps. Because a colon that produces so many polyps eventually produces cancers as well, these colons should be removed surgically as soon as the diagnosis is made. 

   The rectum may be left, but a visual examination of the residual mucosa must be made twice yearly to detect signs of early cancerous change. Another peculiar form of polyp is the villous adenoma often a slowly growing, fernlike structure that spreads along the surface of the colon for some distance. It can recur after being locally resected, or it can develop into a cancer.

   Several studies both within and between countries suggest a pathogenic association between total fat intake and cancer of the colon or rectum and that both wheat fibre and brassica vegetables may protect against it. Meat consumption is usually correlated with fat but is less consistently related to cancer of the large intestine. Some types of beer have been associated with cancer of the rectum (the lower end of the large intestine). There have, however, been variations and inconsistencies in all the epidemiological studies. Most cancers of the large intestine appear to originate in polyps (precancerous adenomas), which are often multiple
 

   The arterial blood supply to the large intestine is supplied by branches of the superior and inferior mesenteric arteries (both of which are branches of the abdominal aorta) and the hypogastric branch of the internal iliac (which supplies blood to the pelvic walls and viscera, the genital organs, the buttocks, and the inside of the thighs). The vessels form a continuous row of arches from which vessels arise to enter the large intestine. Venous blood is drained from the colon from branches that form venous arches similar to those of the arteries. These eventually drain into the superior and inferior mesenteric veins, which ultimately join with the splenic vein to form the portal vein.

                                                                  LINKS
 
 

http://ibscrohns.about.com/health/ibscrohns/library/glossary/bldef-largeintestine.htm

 http://tqjunior.thinkquest.org/4245/intestine.htm

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 http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/largegut/index.html

 http://www.sghms.ac.uk/ssm/alimpath/colon/colon.htm

 http://views.vcu.edu/~mikuleck/gi3/tsld036.htm

 http://www.med.sc.edu/hightower/GI8noBG/sld047.htm

 http://www.bayerpharma-na.com/children/RON/digest/chrd0106.asp

 http://www.britannica.com/bcom/eb/article/9/0,5716,48299+1+47200,00.html

 http://about.ferris.edu/htmls/academics/course.offerings/physbo/biology/courses/biol232/outlines/dg_colon.htm