Intestine
Encyclopedia
In human anatomy
, the intestine (or bowel) 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 humans, the small intestine is further subdivided into the duodenum
, jejunum
and ileum
while the large intestine is subdivided into the cecum
and colon
.
The structure and function can be described both as gross anatomy
and at a microscopic level.
The intestinal tract can be broadly divided into two different parts, the small and large intestine. .
People will have different sized intestines according to their size and age.
The lumen
is the cavity where digested food passes through and from where nutrients are absorbed. Both intestines share a general structure with the whole gut, and are composed of several layers. Going from inside the lumen radially outwards, one passes the mucosa (glandular epithelium and muscularis mucosa), submucosa, muscularis externa (made up of inner circular and outer longitudinal), and lastly serosa.
The large intestine hosts several kinds of bacteria
that deal with molecules the human body is not able to break down itself. This is an example of symbiosis
. These bacteria also account for the production of gases inside our intestine (this gas is released as flatulence
when eliminated through the anus). However the large intestine is mainly concerned with the absorption of water from digested material (which is regulated by the hypothalamus
) and the reabsorption of sodium
, as well as any nutrients that may have escaped primary digestion in the ileum.
that is a source of milk
, a corresponding rennet
is obtained from the intestines of milk-fed . Pig
and calf
intestines are eaten, and pig intestines are used as sausage casings. Calf intestines supply Calf Intestinal Alkaline Phosphatase (CIP)
, and are used to make Goldbeater's skin
.
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
, the intestine (or bowel) is the segment of the alimentary canal
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
extending from the pyloric sphincter of the stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
to 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,...
and, in humans and other mammals, consists of two segments, 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...
and the large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
. In humans, the small intestine is further subdivided into the duodenum
Duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...
, jejunum
Jejunum
The jejunum is the middle section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms middle intestine or mid-gut may be used instead of jejunum.The jejunum lies between the duodenum...
and ileum
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
while the large intestine is subdivided into 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...
.
Structure and function
The structure and function can be described both as gross anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
and at a microscopic level.
The intestinal tract can be broadly divided into two different parts, the small and large intestine. .
People will have different sized intestines according to their size and age.
The lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
is the cavity where digested food passes through and from where nutrients are absorbed. Both intestines share a general structure with the whole gut, and are composed of several layers. Going from inside the lumen radially outwards, one passes the mucosa (glandular epithelium and muscularis mucosa), submucosa, muscularis externa (made up of inner circular and outer longitudinal), and lastly serosa.
- Along the whole length of the gut in the glandular epithelium are goblet cellGoblet cellGoblet 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. These secrete mucusMucusIn 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...
which lubricates the passage of food along and protects it from digestive enzymes. In the small intestine, villi are vaginations (folds) of the mucosaMucous membraneThe mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...
and increase the overall surface area of the intestine while also containing a lactealLactealA lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine.Triglycerides are emulsified by bile and hydrolyzed by the enzyme lipase, resulting in a mixture of fatty acids and monoglycerides. These then pass from the intestinal lumen into the enterocyte, where...
, which is connected to the lymph systemLymphatic systemThe lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated...
and aids in the removal of lipidLipidLipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s and tissue fluid from the blood supply. Microvilli are present on the epithelium of a villus and further increase the surface area over which absorption can take place. Pocket-like invaginations into the underlying tissue are termed Crypts of LieberkühnCrypts of LieberkühnIn histology, an intestinal crypt is a gland found in the epithelial lining of the small intestine and colon...
. In the large intestines, villi are absent and a flat surface with thousands of crypts is observed. - Underlying the epithelium is the lamina propriaLamina propriaThe lamina propria is a constituent of the moist linings known as mucous membranes or mucosa, which line various tubes in the body ....
, which contains myofibroblasts, blood vessels, nerves, and several different immune cells. - The next layer is the muscularis mucosa which is a layer of smooth muscle that aids in the action of continued peristalsisPeristalsisPeristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar...
and catastalsisCatastalsisCatastalsis is the rhythmic contraction and relaxation of the muscle of the intestines. It resembles ordinary peristalsis but is not preceded by a zone of inhibition....
along the gut. The submucosa contains nerves (e.g. Meissner's plexus), blood vessels and elastic fibre with collagen that stretches with increased capacity but maintains the shape of the intestine. - Surrounding this is the muscularis externa which comprises longitudinal and circular smooth muscle that again helps with continued peristalsis and the movement of digested material out of and along the gut. In between the two layers of muscle lies Auerbach's plexusAuerbach's plexusA part of the enteric nervous system, Auerbach's plexus , exists between the longitudinal and circular layers of muscularis externa in the gastrointestinal tract and provides motor innervation to both layers of the mucosa, having both parasympathetic and sympathetic input, whereas Meissner's plexus...
. - Lastly there is the serosaSerous membraneIn anatomy, serous membrane is a smooth membrane consisting of a thin layer of cells which secrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid which reduces friction from muscle movement...
which is made up of loose connective tissue and coated in mucus so as to prevent friction damage from the intestine rubbing against other tissue. Holding all this in place are the mesenteriesMesenteryIn anatomy, the mesentery is the double layer of peritoneum that suspends the jejunum and ileum from the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity.-Mesentery :The...
which suspend the intestine in the abdominal cavity and stop it being disturbed when a person is physically active.
The large intestine hosts several kinds of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that deal with molecules the human body is not able to break down itself. This is an example of symbiosis
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
. These bacteria also account for the production of gases inside our intestine (this gas is released as flatulence
Flatulence
Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The medical term for the mixture of gases is flatus, informally known as a fart, or simply gas...
when eliminated through the anus). However the large intestine is mainly concerned with the absorption of water from digested material (which is regulated by the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
) and the reabsorption of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, as well as any nutrients that may have escaped primary digestion in the ileum.
Diseases and disorders
- GastroenteritisGastroenteritisGastroenteritis is marked by severe inflammation of the gastrointestinal tract involving both the stomach and small intestine resulting in acute diarrhea and vomiting. It can be transferred by contact with contaminated food and water...
is an inflammation of the intestines. It is the most common disease of all the intestines. - IleusIleusIleus is a disruption of the normal propulsive ability of the gastrointestinal tract.Ileus is commonly defined simply as bowel obstruction. However, authoritative sources define it as decreased motor activity of the GI tract due to non-mechanical causes...
is a blockage of the intestines. - IleitisIleitisIleitis is an inflammation of the ileum, a portion of the small intestine. Crohn's ileitis is a type of Crohn's disease affecting the ileum. Ileitis is caused by the bacteria Lawsonia intracellularis....
is an inflammation of the ileumIleumThe ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
. - ColitisColitisIn 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...
is an inflammation of the large intestineLarge intestineThe large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
. - AppendicitisAppendicitisAppendicitis 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 inflammation of the vermiform appendixVermiform appendixThe appendix is a blind-ended tube connected to the cecum , from which it develops embryologically. The cecum is a pouchlike structure of the colon...
located at the caecum. This is a potentially fatal disease if left untreated; most cases of appendicitis require surgical intervention. - Coeliac diseaseCoeliac diseaseCoeliac disease , is an autoimmune disorder of the small intestine that occurs in genetically predisposed people of all ages from middle infancy onward...
is a common form of malabsorptionMalabsorptionMalabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal tract.Impairment can be of single or multiple nutrients depending on the abnormality...
, affecting up to 1% of people of northern European descent. An autoimmune response is triggered in intestinal cells by digestion of gluten proteins. Ingestion of proteins found in wheat, barley and rye, causes villous atrophy in the small intestine. Life-long dietary avoidance of these foodstuffs in a gluten-free diet is the only treatment. - Crohn's diseaseCrohn's diseaseCrohn's disease, also known as regional enteritis, is a type of inflammatory bowel disease that may affect any part of the gastrointestinal tract from mouth to anus, causing a wide variety of symptoms...
and ulcerative colitisUlcerative colitisUlcerative colitis is a form of inflammatory bowel disease . Ulcerative colitis is a form of colitis, a disease of the colon , that includes characteristic ulcers, or open sores. The main symptom of active disease is usually constant diarrhea mixed with blood, of gradual onset...
are examples of inflammatory bowel diseaseInflammatory bowel diseaseIn medicine, inflammatory bowel disease is a group of inflammatory conditions of the colon and small intestine. The major types of IBD are Crohn's disease and ulcerative colitis.-Classification:...
. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as an autoimmune diseaseAutoimmunityAutoimmunity is the failure of an organism to recognize its own constituent parts as self, which allows an immune response against its own cells and tissues. Any disease that results from such an aberrant immune response is termed an autoimmune disease...
. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such. (See List of autoimmune diseases). - Enteroviruses are named by their transmission-route through the intestine (enteric meaning intestinal), but their symptoms aren't mainly associated with the intestine.
- Irritable bowel syndromeIrritable bowel syndromeIrritable bowel syndrome is a diagnosis of exclusion. It is a functional bowel disorder characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any detectable organic cause. In some cases, the symptoms are relieved by bowel movements...
(IBS) is the most common functional disorderFunctional disorderA functional disorder is a medical condition that impairs the normal function of a bodily process, but where every part of the body looks completely normal under examination, dissection or even under a microscope...
of the intestine. Functional constipation and chronic functional abdominal painChronic functional abdominal painChronic functional abdominal pain is the ongoing presence of abdominal pain for which there is no known medical explanation. It is quite similar to, but less common than, irritable bowel syndrome , and many of the same treatments for IBS can also be of benefit to those with CFAP...
are other disorders of the intestine that have physiological causes, but do not have identifiable structural, chemical, or infectious pathologies. They are aberrations of normal bowel function but not diseases. - Diverticular disease is a condition that is very common in older people in industrialized countries. It usually affects the large intestine but has been known to affect the small intestine as well. DiverticulosisDiverticulosisDiverticulosis also known as "diverticular disease" is the condition of having diverticula in the colon, which are outpocketings of the colonic mucosa and submucosa through weaknesses of muscle layers in the colon wall. These are more common in the sigmoid colon, which is a common place for...
occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as diverticulitisDiverticulitisDiverticulitis is a common digestive disease particularly found in the large intestine. Diverticulitis develops from diverticulosis, which involves the formation of pouches on the outside of the colon...
. - EndometriosisEndometriosisEndometriosis is a gynecological medical condition in which cells from the lining of the uterus appear and flourish outside the uterine cavity, most commonly on the ovaries. The uterine cavity is lined by endometrial cells, which are under the influence of female hormones...
can affect the intestines, with similar symptoms to IBS. - Bowel twist (or similarly, bowel strangulation) is a comparatively rare event (usually developing sometime after major bowel surgery). It is, however, hard to diagnose correctly, and if left uncorrected can lead to bowel infarctionInfarctionIn medicine, infarction refers to tissue death that is caused by a local lack of oxygen due to obstruction of the tissue's blood supply. The resulting lesion is referred to as an infarct.-Causes:...
and death. (The singer Maurice GibbMaurice GibbMaurice Ernest Gibb, CBE was a musician, singer-songwriter and record producer. He was born on the Isle of Man, the twin brother of Robin Gibb, and younger brother to Barry. He is best known as a member of the singing/songwriting trio the Bee Gees, formed with his brothers...
is understood to have died from this.) - AngiodysplasiaAngiodysplasiaIn medicine , angiodysplasia is a small vascular malformation of the gut. It is a common cause of otherwise unexplained gastrointestinal bleeding and anemia. Lesions are often multiple, and frequently involve the cecum or ascending colon, although they can occur at other places...
of the colon - Chronic functional abdominal painChronic functional abdominal painChronic functional abdominal pain is the ongoing presence of abdominal pain for which there is no known medical explanation. It is quite similar to, but less common than, irritable bowel syndrome , and many of the same treatments for IBS can also be of benefit to those with CFAP...
- Colorectal cancerColorectal cancerColorectal cancer, commonly known as bowel cancer, is a cancer caused by uncontrolled cell growth , in the colon, rectum, or vermiform appendix. Colorectal cancer is clinically distinct from anal cancer, which affects the anus....
- ConstipationConstipationConstipation refers to bowel movements that are infrequent or hard to pass. Constipation is a common cause of painful defecation...
- DiarrheaDiarrheaDiarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...
- Hirschsprung's diseaseHirschsprung's diseaseHirschsprung's disease , or congenital aganglionic megacolon is a serious medical problem where the enteric nervous system is missing from the end of the bowel. The enteric nervous system is a complex network of neurons and glia that controls most aspects of intestinal function...
(aganglionosis) - IntussusceptionIntussusception (medical disorder)An intussusception is a medical condition in which a part of the intestine has invaginated into another section of intestine, similar to the way in which the parts of a collapsible telescope slide into one another. This can often result in an obstruction...
- Polyp (medicine)Polyp (medicine)A polyp is an abnormal growth of tissue projecting from a mucous membrane. If it is attached to the surface by a narrow elongated stalk, it is said to be pedunculated. If no stalk is present, it is said to be sessile. Polyps are commonly found in the colon, stomach, nose, sinus, urinary bladder...
(see also Colorectal polypColorectal polypA colorectal polyp is a polyp occurring on the lining of the colon or rectum. Untreated colorectal polyps can develop into colorectal cancer....
) - Pseudomembranous colitisPseudomembranous colitisPseudomembranous colitis, a cause of antibiotic-associated diarrhea , is an infection of the colon. It is often, but not always, caused by the bacterium Clostridium difficile. Because of this, the informal name C. difficile colitis is also commonly used. The illness is characterized by...
- Ulcerative colitisUlcerative colitisUlcerative colitis is a form of inflammatory bowel disease . Ulcerative colitis is a form of colitis, a disease of the colon , that includes characteristic ulcers, or open sores. The main symptom of active disease is usually constant diarrhea mixed with blood, of gradual onset...
and toxic megacolonToxic megacolonToxic megacolon is an acute form of colonic distension. It is characterized by a very dilated colon , accompanied by abdominal distension , and sometimes fever, abdominal pain, or shock....
In non-human animals
Animal intestines have multiple uses. From each species of livestockLivestock
Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...
that is a source of milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
, a corresponding rennet
Rennet
Rennet is a complex of enzymes produced in any mammalian stomach to digest the mother's milk, and is often used in the production of cheese. Rennet contains many enzymes, including a proteolytic enzyme that coagulates the milk, causing it to separate into solids and liquid...
is obtained from the intestines of milk-fed . Pig
Pig
A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...
and calf
Calf
Calves are the young of domestic cattle. Calves are reared to become adult cattle, or are slaughtered for their meat, called veal.-Terminology:...
intestines are eaten, and pig intestines are used as sausage casings. Calf intestines supply Calf Intestinal Alkaline Phosphatase (CIP)
Calf Intestinal Alkaline Phosphatase (CIP)
Calf-intestinal alkaline phosphatase is an enzyme that catalyzes the 5' phosphate group's removal, dephosphorylation, from DNA . This enzyme is frequently used in DNA sub-cloning, because DNA fragments that lack the 5' phosphoryl termini cannot self-ligate...
, and are used to make Goldbeater's skin
Goldbeater's skin
Goldbeater's skin — the outer membrane of calf's intestine — is a parchment traditionally used in the process of making gold leaf by beating, reducing gold into mere 1μm-thick leaves....
.
See also
- Inflammatory bowel diseaseInflammatory bowel diseaseIn medicine, inflammatory bowel disease is a group of inflammatory conditions of the colon and small intestine. The major types of IBD are Crohn's disease and ulcerative colitis.-Classification:...
(or "IBD") - DiarrheaDiarrheaDiarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...
- ConstipationConstipationConstipation refers to bowel movements that are infrequent or hard to pass. Constipation is a common cause of painful defecation...
- Human liquorHuman liquorHuman liquor means waste content from bowels isolated to investigate specific enzymes and peptidases involved in enteric contraction and digestion of compounds....