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The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s. Water is absorbed here and the remaining waste material is stored in the rectum as feces before being removed by defecation. The colon is the longest portion of the large intestine, and the terms are often used interchangeably but most sources define the large intestine as the combination of the
cecum The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, to which it is joined). The wo ...
, colon, rectum, and anal canal. Some other sources exclude the anal canal. In humans, the large intestine begins in the right
iliac region The ilium () (plural ilia) is the uppermost and largest part of the hip bone, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bon ...
of the pelvis, just at or below the
waist The waist is the part of the abdomen between the rib cage and hips. On people with slim bodies, the waist is the narrowest part of the torso. ''Waistline'' refers to the horizontal line where the waist is narrowest, or to the general appear ...
, where it is joined to the end of the small intestine at the cecum, via the
ileocecal valve The ileocecal valve (ileal papilla, ileocaecal valve, Tulp's valve, Tulpius valve, Bauhin's valve, ileocecal eminence, valve of Varolius or colic valve) is a sphincter muscle valve that separates the small intestine and the large intestine. Its ...
. It then continues as the colon ascending the
abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...
, across the width of the abdominal cavity as the
transverse colon In human anatomy, the transverse colon is the longest and most movable part of the colon. Anatomical position It crosses the abdomen from the ascending colon at the right colic flexure (hepatic flexure) with a downward convexity to the descend ...
, and then descending to the rectum and its endpoint at the anal canal. Overall, in humans, the large intestine is about long, which is about one-fifth of the whole length of the human gastrointestinal tract.


Structure

The colon of the large intestine is the last part of the digestive system. It has a segmented appearance due to a series of saccules called haustra. It extracts
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
and
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
from solid wastes before they are eliminated from the body and is the site in which the fermentation of unabsorbed material by the gut microbiota occurs. Unlike the small intestine, the colon does not play a major role in absorption of foods and nutrients. About 1.5 litres or 45 ounces of water arrives in the colon each day. The colon is the longest part of the large intestine and its average length in the adult human is 65 inches or 166 cm (range of 80 to 313 cm) for males, and 61 inches or 155 cm (range of 80 to 214 cm) for females.


Sections

In mammals, the large intestine consists of the
cecum The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, to which it is joined). The wo ...
(including the appendix), colon (the longest part), rectum, and anal canal. The four sections of the colon are: the ascending colon,
transverse colon In human anatomy, the transverse colon is the longest and most movable part of the colon. Anatomical position It crosses the abdomen from the ascending colon at the right colic flexure (hepatic flexure) with a downward convexity to the descend ...
, descending colon, and sigmoid colon. These sections turn at the
colic flexures In the anatomy of the human digestive tract, there are two colic flexures, or curvatures in the transverse colon. The right colic flexure is also known as the hepatic flexure, and the left colic flexure is also known as the splenic flexure. Note ...
. The pats of the colon are either intraperitoneal or behind it in the
retroperitoneum The retroperitoneal space (retroperitoneum) is the anatomical space (sometimes a potential space) behind (''retro'') the peritoneum. It has no specific delineating anatomical structures. Organs are retroperitoneal if they have peritoneum on their ...
. Retroperitoneal organs, in general, do not have a complete covering of
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of meso ...
, so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile. Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal. This is important as it affects which organs can be easily accessed during surgery, such as a
laparotomy A laparotomy is a surgical procedure involving a surgical incision through the abdominal wall to gain access into the abdominal cavity. It is also known as a celiotomy. Origins and history The first successful laparotomy was performed without ane ...
. In terms of diameter, the cecum is the widest, averaging slightly less than 9 cm in healthy individuals, and the transverse colon averages less than 6 cm in diameter. The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging in diameter. Diameters larger than certain thresholds for each colonic section can be diagnostic for
megacolon Megacolon is an abnormal dilation of the colon (also called the large intestine). This leads to hypertrophy of the colon. The dilation is often accompanied by a paralysis of the peristaltic movements of the bowel. In more extreme cases, the feces ...
.


Cecum and appendix

The
cecum The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, to which it is joined). The wo ...
is the first section of the large intestine and is involved in digestion, while the appendix which develops embryologically from it, is not involved in digestion and is considered to be part of the gut-associated lymphoid tissue. The function of the appendix is uncertain, but some sources believe that it has a role in housing a sample of the gut microbiota, and is able to help to repopulate the colon with microbiota if depleted during the course of an immune reaction. The appendix has also been shown to have a high concentration of lymphatic cells.


Ascending colon

The ascending colon is the first of four main sections of the large intestine. It is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs upwards through the abdominal cavity 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. As the waste material exits the small intestine through the
ileocecal valve The ileocecal valve (ileal papilla, ileocaecal valve, Tulp's valve, Tulpius valve, Bauhin's valve, ileocecal eminence, valve of Varolius or colic valve) is a sphincter muscle valve that separates the small intestine and the large intestine. Its ...
, it will move into the cecum and then to the ascending colon where this process of extraction starts. The waste material is pumped upwards toward the transverse colon by
peristalsis Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whic ...
. The ascending colon is sometimes attached to the appendix via Gerlach's valve. In ruminants, the ascending colon is known as the spiral colon. Taking into account all ages and sexes, colon cancer occurs here most often (41%).


Transverse colon

The transverse colon is the part of the colon from the hepatic flexure, also known as the right colic, (the turn of the colon by the
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...
) to the
splenic flexure In the anatomy of the human digestive tract, there are two colic flexures, or curvatures in the transverse colon. The right colic flexure is also known as the hepatic flexure, and the left colic flexure is also known as the splenic flexure. Note ...
also known as the left colic, (the turn of the colon by the
spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .
). The transverse colon hangs off the
stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
, attached to it by a large fold of
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of meso ...
called the
greater omentum The greater omentum (also the great omentum, omentum majus, gastrocolic omentum, epiploon, or, especially in animals, caul) is a large apron-like fold of visceral peritoneum that hangs down from the stomach. It extends from the greater curvature ...
. On the posterior side, the transverse colon is connected to the posterior abdominal wall by a mesentery known as the
transverse mesocolon The mesentery is an organ that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum. It helps in storing fat and allowing blood vessels, lymphatics, and nerves to supply the intestine ...
. The transverse colon is encased in
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of meso ...
, and is therefore mobile (unlike the parts of the colon immediately before and after it). The proximal two-thirds of the transverse colon is perfused by the middle colic artery, a branch of the
superior mesenteric artery In human anatomy, the superior mesenteric artery (SMA) is an artery which arises from the anterior surface of the abdominal aorta, just inferior to the origin of the celiac trunk, and supplies blood to the intestine from the lower part of the du ...
(SMA), while the latter third is supplied by branches of the inferior mesenteric artery (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between the
midgut The midgut is the portion of the embryo from which most of the intestines develop. After it bends around the superior mesenteric artery, it is called the "midgut loop". It comprises the portion of the alimentary canal from the end of the foregut a ...
and hindgut, is an area sensitive to ischemia.


Descending colon

The descending 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 The retroperitoneal space (retroperitoneum) is the anatomical space (sometimes a potential space) behind (''retro'') the peritoneum. It has no specific delineating anatomical structures. Organs are retroperitoneal if they have peritoneum on thei ...
in two-thirds of humans. In the other third, it has a (usually short) mesentery. 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

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 Sigmoid means resembling the lower-case Greek letter sigma (uppercase Σ, lowercase σ, lowercase in word-final position ς) or the Latin letter S. Specific uses include: * Sigmoid function, a mathematical function * Sigmoid colon, part of the l ...
; cf.
sigmoid sinus The sigmoid sinuses (sigma- or s-shaped hollow curve), also known as the , are venous sinuses within the skull that receive blood from posterior dural venous sinus veins. Structure The sigmoid sinus is a dural venous sinus situated within the du ...
). 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 The sigmoid arteries are 2-5 branches of the inferior mesenteric artery that are distributed to the distal descending colon and the sigmoid colon. Anatomy Course and relations The sigmoid arteries course obliquely inferior-ward and to the lef ...
, a branch of the IMA. The IMA terminates as the
superior rectal artery The superior rectal artery (superior hemorrhoidal artery) is an artery that descends into the pelvis to supply blood to the rectum. Structure The superior rectal artery is the continuation of the inferior mesenteric artery. It descends into the p ...
.
Sigmoidoscopy Sigmoidoscopy (from the Greek term for letter " s/ς" + "eidos" + "scopy": namely, to look inside an "s"/"ς"-like object) is the minimally invasive medical examination of the large intestine from the rectum through to the nearest part of the co ...
is a common diagnostic technique used to examine the sigmoid colon.


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 long.


Appearance

The
cecum The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, to which it is joined). The wo ...
– 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 An artery (plural arteries) () is a blood vessel in humans and most animals that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pu ...
supply to the colon comes from branches of the
superior mesenteric artery In human anatomy, the superior mesenteric artery (SMA) is an artery which arises from the anterior surface of the abdominal aorta, just inferior to the origin of the celiac trunk, and supplies blood to the intestine from the lower part of the du ...
(SMA) and inferior mesenteric artery (IMA). Flow between these two systems communicates via the
marginal artery of the colon In human anatomy, the marginal artery of the colon, also known as the marginal artery of Drummond, the artery of Drummond, and simply as the marginal artery, is an artery that connects the inferior mesenteric artery with the superior mesenteric ar ...
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.
Venous Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...
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 The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...
.


Lymphatic drainage

Lymphatic drainage from the ascending colon and proximal two-thirds of the
transverse colon In human anatomy, the transverse colon is the longest and most movable part of the colon. Anatomical position It crosses the abdomen from the ascending colon at the right colic flexure (hepatic flexure) with a downward convexity to the descend ...
is to the ileocolic lymph nodes and the superior mesenteric lymph nodes, which drain into the
cisterna chyli The cisterna chyli (or cysterna chyli, and etymologically more correct, receptaculum chyli) is a dilated sac at the lower end of the thoracic duct in most mammals into which lymph from the intestinal trunk and two lumbar lymphatic trunks flow. It ...
. The lymph from the distal one-third of the
transverse colon In human anatomy, the transverse colon is the longest and most movable part of the colon. Anatomical position It crosses the abdomen from the ascending colon at the right colic flexure (hepatic flexure) with a downward convexity to the descend ...
, the descending colon, the sigmoid colon, and the upper rectum drain into the inferior mesenteric and colic lymph nodes. The lower rectum to the anal canal above the pectinate line drain to the internal ileocolic nodes. The anal canal below the pectinate line drains into the superficial inguinal nodes. The pectinate line only roughly marks this transition.


Nerve supply

Sympathetic supply: superior & inferior mesenteric ganglia; parasympathetic supply: vagus & pelvic nerves


Development

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.


Variation

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 A volvulus is when a loop of intestine twists around itself and the mesentery that supports it, resulting in a bowel obstruction. Symptoms include abdominal pain, abdominal bloating, vomiting, constipation, and bloody stool. Onset of symptoms m ...
occurs, resulting in obstruction and requiring immediate medical attention. A significant indirect health consequence is that use of a standard adult
colonoscope Colonoscopy () or coloscopy () is the endoscopic examination of the large bowel and the distal part of the small bowel with a CCD camera or a fiber optic camera on a flexible tube passed through the anus. It can provide a visual diagnosis (''e. ...
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.


Microanatomy


Colonic crypts

The wall of the large intestine is lined with simple columnar epithelium with
invagination Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination is a mechanism that takes place during gastrulation. This mechanism or cell movement happens mostly in the vegeta ...
s. The invaginations are called the
intestinal gland In histology, an intestinal gland (also crypt of Lieberkühn and intestinal crypt) is a gland found in between villi in the intestinal epithelium lining of the small intestine and large intestine (or colon). The glands and intestinal villi are co ...
s or colonic crypts. File:Micrograph of normal large intestinal crypts.jpg, Micrograph of normal large instestinal crypts. File:Anatomy of normal large intestinal crypts.jpg, 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. 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. 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. There are 5 to 6 stem cells at the bases of the crypts. As estimated from the image in panel A, there are about 100 colonic crypts per square millimeter of the colonic epithelium. Since the average length of the human colon is 160.5 cm and the average inner circumference of the colon is 6.2 cm, the inner surface epithelial area of the human colon has an average area of about 995 cm2, 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. Colonic crypts deficient for CCOI in women reaches, on average, 18% in women and 23% in men by 80–84 years of age. 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).


Mucosa

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.


Function

The large intestine absorbs 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 thiamine, riboflavin, and
vitamin K Vitamin K refers to structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-synthesis modification of certain proteins that are required for blood coagulation ...
(especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation). 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 the colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling. The appendix contains a small amount of
mucosa-associated lymphoid tissue The mucosa-associated lymphoid tissue (MALT), also called mucosa-associated lymphatic tissue, is a diffuse system of small concentrations of lymphoid tissue found in various submucosal membrane sites of the body, such as the gastrointestinal tr ...
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 In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
during early growth and development. 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 Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
,
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
, and
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
are left as well as indigestible parts of ingested food (e.g., a large part of ingested
amylose Amylose is a polysaccharide made of α-D-glucose units, bonded to each other through α(1→4) glycosidic bonds. It is one of the two components of starch, making up approximately 20–30%. Because of its tightly packed helical structure, amylose ...
, starch which has been shielded from digestion heretofore, and dietary fiber, which is largely indigestible
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...
in either soluble or insoluble form). As the chyme moves through the large intestine, most of the remaining
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
is removed, while the chyme is mixed with
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
and
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
(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. The bacteria break down some of the
fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorpora ...
for their own nourishment and create acetate,
propionate Propionic acid (, from the Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is a liqu ...
, 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
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s — 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).


Standing gradient osmosis

Water absorption at the colon typically proceeds against a transmucosal osmotic pressure
gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...
. 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 junction Tight junctions, also known as occluding junctions or ''zonulae occludentes'' (singular, ''zonula occludens''), are multiprotein junctional complexes whose canonical function is to prevent leakage of solutes and water and seals between the epith ...
s and adjacent cells, which then in turn moves across the basement membrane and into the
capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
, while more sodium ions are pumped again into the intercellular fluid. 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 In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane-imp ...
compared to the fluid within the intestinal lumen.


Gut flora

The large intestine houses over 700 species of
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
that perform a variety of functions, as well as
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
, protozoa, and archaea. Species diversity varies by geography and diet. 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". The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested
polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
(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 A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient that an organism needs in small quantities for the proper functioning of its metabolism. Essential nutrien ...
, especially
vitamin K Vitamin K refers to structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-synthesis modification of certain proteins that are required for blood coagulation ...
and biotin (a
B vitamin B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism and synthesis of red blood cells. Though these vitamins share similar names (B1, B2, B3, etc.), they are chemically distinct compounds that often coexist ...
), 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 An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention o ...
that inhibit the vitamin producing species of bacteria as well as the intended disease-causing bacteria. Other bacterial products include gas (
flatus Flatulence, in humans, is the expulsion of gas from the intestines via the anus, commonly referred to as farting. "Flatus" is the medical word for gas generated in the stomach or bowels. A proportion of intestinal gas may be swallowed environm ...
), which is a mixture of
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
, with small amounts of the gases
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
,
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
, and hydrogen sulfide. Bacterial fermentation of undigested
polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
produces these. Some of the fecal odor is due to
indole Indole is an aromatic heterocyclic organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other c ...
s, metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and
lymphatics The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessel ...
. 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 The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earl ...
and Bacteroidota. The ratio between the two seems to vary widely as reported by the Human Microbiome Project. Bacteroides are implicated in the initiation of
colitis Colitis is swelling or inflammation of the large intestine ( colon). Colitis may be acute and self-limited or long-term. It broadly fits into the category of digestive diseases. In a medical context, the label ''colitis'' (without qualification ...
and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'. A
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
layer protects the large intestine from attacks from colonic
commensal bacteria Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...
.


Clinical significance


Disease

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


Colonoscopy

Colonoscopy is the
endoscopic An endoscopy is a procedure used in medicine to look inside the body. The endoscopy procedure uses an endoscope to examine the interior of a hollow organ or cavity of the body. Unlike many other medical imaging techniques, endoscopes are insert ...
examination of the large intestine and the distal part of the
small bowel The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pa ...
with a CCD camera or a
fiber optic An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...
camera on a flexible tube passed through the anus. It can provide a visual diagnosis (e.g.
ulceration An ulcer is a discontinuity or break in a bodily membrane that impedes normal function of the affected organ. According to Robbins's pathology, "ulcer is the breach of the continuity of skin, epithelium or mucous membrane caused by sloughing o ...
,
polyps A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are roughly cylindrical in shape and elongated at the axis of the vase-shaped body. In solitary polyps, the aboral (opposite to oral) end i ...
) and grants the opportunity for
biopsy A biopsy is a medical test commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a dise ...
or removal of suspected colorectal cancer lesions. Colonoscopy can remove polyps as small as one millimetre or less. Once polyps are removed, they can be studied with the aid of a microscope to determine if they are precancerous or not. It takes 15 years or less for a polyp to turn cancerous. Colonoscopy is similar to
sigmoidoscopy Sigmoidoscopy (from the Greek term for letter " s/ς" + "eidos" + "scopy": namely, to look inside an "s"/"ς"-like object) is the minimally invasive medical examination of the large intestine from the rectum through to the nearest part of the co ...
—the difference being related to which parts of the colon each can examine. A colonoscopy allows an examination of the entire colon (1200–1500 mm in length). A sigmoidoscopy allows an examination of the distal portion (about 600 mm) of the colon, which may be sufficient because benefits to cancer survival of colonoscopy have been limited to the detection of lesions in the distal portion of the colon.as PDF
A sigmoidoscopy is often used as a screening procedure for a full colonoscopy, often done in conjunction with a stool-based test such as a fecal occult blood test (FOBT), fecal immunochemical test (FIT), or multi-target stool DNA test (Cologuard) or blood-based test, SEPT9 DNA methylation test (Epi proColon). About 5% of these screened patients are referred to colonoscopy.as PDF
Virtual colonoscopy, which uses 2D and 3D imagery reconstructed from computed tomography (CT) scans or from
nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
(MR) scans, is also possible, as a totally
non-invasive A medical procedure is defined as ''non-invasive'' when no break in the skin is created and there is no contact with the mucosa, or skin break, or internal body cavity beyond a natural or artificial body orifice. For example, deep palpation and p ...
medical test, although it is not standard and still under investigation regarding its diagnostic abilities. Furthermore, virtual colonoscopy does not allow for therapeutic maneuvers such as polyp/tumour removal or biopsy nor visualization of lesions smaller than 5 millimeters. If a growth or polyp is detected using CT colonography, a standard colonoscopy would still need to be performed. Additionally, surgeons have lately been using the term
pouchoscopy Pouchoscopy is a minimally invasive endoscopic procedure to examine an ileo-anal pouch, a replacement for the colon / rectum which is surgically created from the small intestine (ileum) as treatment for ulcerative colitis, a preventive measure in ...
to refer to a colonoscopy of the ileo-anal pouch.


Other animals

The large intestine is truly distinct only in
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s, 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
ruminant Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
s. 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
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s, 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.


Additional images

Image:illu_intestine.jpg, Intestines File:Slide12bek.JPG, Colon. Deep dissection. Anterior view.


See also

*
Large intestine (Chinese medicine) The large intestine ( Chinese: 大肠/大腸: pinyin: ''dà cháng'') is one of the '' fu'' organs stipulated by traditional Chinese medicine (TCM). As distinct from the Western medical concept of large intestine, this concept from TCM is more a way ...
*
Colectomy Colectomy ('' col-'' + '' -ectomy'') is bowel resection of the large bowel ( colon). It consists of the surgical removal of any extent of the colon, usually segmental resection (partial colectomy). In extreme cases where the entire large intest ...
* Colonic ulcer


References


External links

* {{Portal bar, Anatomy Digestive system Organs (anatomy)