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scending 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 18 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 main article sigmoid colon 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 main article rectum the rectum is the last section of the large intestine it holds the formed feces awaiting elimination via defecation it is about 12 cm or 4 7 in long 19 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 20 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 21 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 middle rectal veins are an exception delivering blood to inferior vena cava and bypassing the liver 22 lymphatic drainage edit lymphatic drainage from the ascending colon and proximal two thirds of the transverse colon is to the ileocolic 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 ileocolic 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 sacral plexus s2 s4 citation needed development edit see also development of the digestive system the endoderm mesoderm and ectoderm are germ layers that develop in a process called gastrulation gastrulation occurs early in human development the gastrointestinal tract is derived from these layers 25 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 26 27 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 28 microanatomy edit further information gastrointestinal wall colonic crypts edit colonic crypts intestinal glands within four tissue sections the cells have been stained to show a brown orange color if the cells produce the mitochondrial protein cytochrome c oxidase subunit i ccoi and the nuclei of the cells located at the outer edges of the cells lining the walls of the crypts are stained blue gray with haematoxylin panels a b were cut across the long axes of the crypts and panels c d were cut parallel to the long axes of the crypts in panel a the bar shows 100 μm and allows an estimate of the frequency of crypts in the colonic epithelium panel b includes three crypts in cross section each with one segment deficient for ccoi expression and at least one crypt on the right side undergoing fission into two crypts panel c shows on the left side a crypt fissioning into two crypts panel d shows typical small clusters of two and three ccoi deficient crypts the bar shows 50 μm the images were made from original photomicrographs but panels a b and d were also included in an article 29 and illustrations were published with creative commons attribution noncommercial license allowing re use the wall of the large intestine is lined with simple columnar epithelium with invaginations the invaginations are called the intestinal glands or colonic crypts micrograph of normal large instestinal crypts anatomy of normal large intestinal crypts the colon crypts are shaped like microscopic thick walled test tubes with a central hole down the length of the tube the crypt lumen four tissue sections are shown here two cut across the long axes of the crypts and two cut parallel to the long axes in these images the cells have been stained by immunohistochemistry to show a brown orange color if the cells produce a mitochondrial protein called cytochrome c oxidase subunit i ccoi the nuclei of the cells located at the outer edges of the cells lining the walls of the crypts are stained blue gray with haematoxylin as seen in panels c and d crypts are about 75 to about 110 cells long baker et al 30 found that the average crypt circumference is 23 cells thus by the images shown here there are an average of about 1 725 to 2 530 cells per colonic crypt nooteboom et al 31 measuring the number of cells in a small number of crypts reported a range of 1 500 to 4 900 cells per colonic crypt cells are produced at the crypt base and migrate upward along the crypt axis before being shed into the colonic lumen days later 30 there are 5 to 6 stem cells at the bases of the crypts 30 as estimated from the image in panel a there are about 100 colonic crypts per square millimeter of the colonic epithelium 32 since the average length of the human colon is 160 5 cm 11 and the average inner circumference of the colon is 6 2 cm 32 the inner surface epithelial area of the human colon has an average area of about 995 cm 2 which includes 9 950 000 close to 10 million crypts in the four tissue sections shown here many of the intestinal glands have cells with a mitochondrial dna mutation in the ccoi gene and appear mostly white with their main color being the blue gray staining of the nuclei as seen in panel b a portion of the stem cells of three crypts appear to have a mutation in ccoi so that 40 to 50 of the cells arising from those stem cells form a white segment in the cross cut area overall the percent of crypts deficient for ccoi is less than 1 before age 40 but then increases linearly with age 29 colonic crypts deficient for ccoi in women reaches on average 18 in women and 23 in men by 80 84 years of age 29 crypts of the colon can reproduce by fission as seen in panel c where a crypt is fissioning to form two crypts and in panel b where at least one crypt appears to be fissioning most crypts deficient in ccoi are in clusters of crypts clones of crypts with two or more ccoi deficient crypts adjacent to each other see panel d 29 mucosa edit about 150 of the many thousands of protein coding genes expressed in the large intestine some are specific to the mucous membrane in different regions and include ceacam7 33 function edit histological section the large intestine absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum 34 the colon absorbs vitamins that are created by the colonic bacteria such as thiamine riboflavin and vitamin k 35 36 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 potassium and chloride recycling of various nutrients takes place in the colon examples include fermentation of carbohydrates short chain fatty acids and urea cycling 34 the appendix 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 37 by the time chyme enters the large intestine the small intestine has absorbed nearly all digestible nutrients and approximately 90 of the ingested water based on volume measurements showing that only about 1 2 litres of fluid pass into the colon from a daily intestinal load of roughly 9 10 litres 38 39 indeed as demonstrated by the commonality of ileostomy procedures it is possible for many people to live without large portions of their large intestine or even without it completely at this point only 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 causing the stools to gradually solidify as they move along into the descending colon 34 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 40 no protein is made available in humans perhaps 10 of the undigested carbohydrate thus becomes available though this may vary with diet 41 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 34 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 cells occupying the intestinal lining pump sodium ions into the intercellular space raising the osmolarity of the intercellular fluid 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 while more sodium ions are pumped again into the intercellular fluid 42 although water travels down an osmotic gradient in each individual step overall water usually travels against the osmotic gradient due to the pumping of sodium ions into the intercellular fluid this allows the large intestine to absorb water despite the blood in capillaries being hypotonic compared to the fluid within the intestinal lumen gut flora edit main article gut microbiota the large intestine houses over 700 species of bacteria that perform a variety of functions as well as fungi protozoa and archaea species diversity varies by geography and diet 43 the microbes in a human distal gut often number in the vicinity of 100 trillion and can weigh around 200 grams 0 44 pounds this mass of mostly symbiotic microbes has recently been called the latest human organ to be discovered or in other words the forgotten organ 44 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 45 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 the vitamin producing species of bacteria as well as the intended disease causing bacteria 46 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 sulfide bacterial fermentation of undigested polysaccharides produces these some of the fecal odor is due to indoles 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 two most prevalent phyla of the colon are bacillota and bacteroidota the ratio between the two seems to vary widely as reported by the human microbiome project 47 bacteroides are implicated in the initiation of colitis and colon cancer bifidobacteria are also abundant and are often described as ...
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