Large intestine
Encyclopedia
The large intestine is the third-to-last part of the digestive system — — in vertebrate
animal
s. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material
from the body. This article is primarily about the human gut, though the information about its processes are directly applicable to most mammals.
The large intestine consists of the cecum
and colon
. It starts in the right iliac region of the pelvis
, just at or below the right waist
, where it is joined to the bottom end of the small intestine
. From here it continues up the abdomen
, then across the width of the abdominal cavity, and then it turns down, continuing to its endpoint at the anus
.
The large intestine is about 4.9 feet (1.5 m) long, which is about one-fifth of the whole length of the intestinal canal
.
In Terminologia Anatomica
the large intestine includes the cecum, colon, rectum, and anal canal. However, some sources exclude the anal canal.
in defecation
.
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 cell
s, they are abundant in the large intestine.
The appendix
is attached to its inferior surface of the cecum. It contains the least of lymphoid tissue. It is a part of mucosa-associated lymphoid tissue
, which gives the appendix an important role in immunity. Appendicitis
is the result of a blockage that traps infectious material in the lumen. 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 4.9 ft 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.
Cecum
– the first part of the large intestine
Locations along the colon are:
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 that 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.
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'.
A mucus
layer protects the large intestine from attacks from colonic commensal bacteria.
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 amniote
s, 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 carnivora
ns or ruminant
s. The rectum of mammals (other than monotreme
s) 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
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.
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
from the body. This article is primarily about the human gut, though the information about its processes are directly applicable to most mammals.
The large intestine consists of the cecum
Cecum
The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...
and colon
Colon (anatomy)
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 fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
. It starts in the right iliac region of the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, just at or below the right waist
Waist
The waist is the part of the abdomen between the rib cage and hips. On proportionate people, the waist is the narrowest part of the torso....
, where it is joined to the bottom end of the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
. From here it continues up the abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
, then across the width of the abdominal cavity, and then it turns down, continuing to its endpoint at the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
.
The large intestine is about 4.9 feet (1.5 m) long, which is about one-fifth of the whole length of the intestinal canal
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
.
In Terminologia Anatomica
Terminologia Anatomica
Terminologia Anatomica is the international standard on human anatomic terminology. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998. It supersedes the previous standard, Nomina...
the large intestine includes the cecum, colon, rectum, and anal canal. However, some sources exclude the anal canal.
Function and relation to other organs
The large intestine takes about 16 hours to finish up the remaining processes of the digestive system. Food is no longer broken down at this stage of digestion. The colon absorbs vitamins which are created by the colonic bacteria - poo 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 anusAnus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
in defecation
Defecation
Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...
.
The large intestine differs in physical form from the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
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
Intestinal epithelium
The intestinal epithelium is the epithelium that covers the small and large intestine. It is simple columnar and nonciliated.They primarily take part in the digestive system. However, they also express TLR 4 receptors, and are thus a part of the immune system, both as a barrier and as a first-line...
. 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 cell
Goblet cell
Goblet cells are glandular simple columnar epithelial cells whose sole function is to secrete mucin, which dissolves in water to form mucus. They use both apocrine and merocrine methods for secretion....
s, they are abundant in the large intestine.
The appendix
Appendix
Appendix may refer to:In documents:*Addendum, any addition to a document, such as a book or legal contract*Bibliography, a systematic list of books and other works...
is attached to its inferior surface of the cecum. It contains the least of lymphoid tissue. It is a part of mucosa-associated lymphoid tissue
Mucosa-associated lymphoid tissue
The mucosa-associated lymphoid tissue is the diffusion system of small concentrations of lymphoid tissue found in various sites of the body, such as the gastrointestinal tract, thyroid, breast, lung, salivary glands, eye, and skin.MALT is populated by lymphocytes such as T cells and B cells, as...
, which gives the appendix an important role in immunity. Appendicitis
Appendicitis
Appendicitis is a condition characterized by inflammation of the appendix. It is classified as a medical emergency and many cases require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, mortality is high, mainly because of the risk of rupture leading to...
is the result of a blockage that traps infectious material in the lumen. 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 4.9 ft 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.
Parts and location
Parts of the large intestine are:Cecum
Cecum
The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...
– the first part of the large intestine
- Taeniae coli – three bands of smooth muscle
- HaustraHaustraThe haustra of the colon are the small pouches caused by sacculation, which give the colon its segmented appearance. The taenia coli runs the length of the large intestine...
– bulges caused by contraction of taeniae coli - Epiploic appendages – small fat accumulations on the viscera
Locations along the colon are:
- The ascending colonAscending colonThe ascending colon is smaller in caliber than the cecum.It passes upward, from its commencement at the cecum, opposite the colic valve, to the under surface of the right lobe of the liver, on the right of the gall-bladder, where it is lodged in a shallow depression, the colic impression; here it...
- The right colic flexure (hepatic)
- The transverse colonTransverse colonThe transverse colon, the longest and most movable part of the colon, passes with a downward convexity from the right hypochondrium region across the abdomen, opposite the confines of the epigastric and umbilical zones, into the left hypochondrium region, where it curves sharply on itself beneath...
- The transverse mesocolonTransverse mesocolonThe transverse mesocolon is a broad, meso-fold of peritoneum, which connects the transverse colon to the posterior wall of the abdomen.It is continuous with the two posterior layers of the greater omentum, which, after separating to surround the transverse colon, join behind it, and are continued...
- The left colic flexure (splenic)
- The descending colonDescending colonThe descending colon of humans passes downward through the left hypochondrium and lumbar regions, along the lateral border of the left kidney....
- The sigmoid colonSigmoid colonThe sigmoid colon is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about 40 cm...
– the v-shaped region of the large intestine
Bacterial flora
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
Vitamin K
Vitamin K is a group of structurally similar, fat soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone derivatives...
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 that 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
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, with small amounts of the gases hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, and hydrogen sulphide. Bacterial fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
of undigested polysaccharides produces these.
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
Bacteroides
Bacteroides is a genus of Gram-negative, bacillus bacteria. Bacteroides species are non-endospore-forming, anaerobes, and may be either motile or non-motile, depending on the species. The DNA base composition is 40-48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids...
, which have been implicated in the initiation of colitis
Colitis
In medicine, colitis refers to an inflammation of the colon and is often used to describe an inflammation of the large intestine .Colitides may be acute and self-limited or chronic, i.e...
and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.
A mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
layer protects the large intestine from attacks from colonic commensal bacteria.
In other animals
The large intestine is truly distinct only in tetrapodTetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
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 amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s, 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 carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
ns or ruminant
Ruminant
A ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again...
s. The rectum of mammals (other than monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s) is derived from the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
of other vertebrates, and is, therefore, not truly homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
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
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, this includes a rectal gland that secretes salt to help the animal maintain osmotic
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
balance with the seawater. The gland somewhat resembles a caecum in structure, but is not a homologous structure.