LARGE INTESTINE - By vikas lodhi

Large intestine

The large intestine is the last part of the digestive system in vertebrate animals. Its function is to absorb water from the 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.

Structure

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.

Appearance

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.

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 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.

Function

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.

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 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.

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. No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet; 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. 

 

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