The small intestine or small bowel is an

organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an electronic keyboard instrument ** Hammond ...

in the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...

where most of the

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

s from food takes place. It lies between the stomach and large intestine, and receives

bile Bile (from Latin ''bilis''), or gall, is a dark-green-to-yellowish-brown fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver bile ...

and pancreatic juice through the pancreatic duct to aid in digestion. The small intestine is about long and folds many times to fit in the abdomen. Although it is longer than the large intestine, it is called the small intestine because it is narrower in diameter. The small intestine has three distinct regions – the

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 m ...

, jejunum, and ileum. The duodenum, the shortest, is where preparation for absorption through small finger-like protrusions called villi begins. The jejunum is specialized for the absorption through its lining by enterocytes: small nutrient particles which have been previously digested by

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

in the duodenum. The main function of the ileum is to absorb vitamin B₁₂, bile salts, and whatever products of digestion that were not absorbed by the jejunum.

Structure

Size

The length of the small intestine can vary greatly, from as short as to as long as , also depending on the measuring technique used. The typical length in a living person is . The length depends both on how tall the person is and how the length is measured. Taller people generally have a longer small intestine and measurements are generally longer after death and when the bowel is empty. It is approximately in diameter in newborns after 35 weeks of gestational age, and in diameter in adults. On abdominal X-rays, the small intestine is considered to be abnormally dilated when the diameter exceeds 3 cm. On

CT scan A computed tomography scan (CT scan; formerly called computed axial tomography scan or CAT scan) is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers ...

s, a diameter of over 2.5 cm is considered abnormally dilated. The surface area of the human small intestinal mucosa, due to enlargement caused by folds, villi and microvilli, averages .

Parts

The small intestine is divided into three structural parts. * The

is a short structure ranging from in length, and shaped like a "C". It surrounds the head of the pancreas. It receives gastric

chyme Chyme or chymus (; from Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenumpancreas ( digestive enzymes) and the liver (

). The digestive enzymes break down proteins and bile emulsifies fats into micelles. The

contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in gastric chyme. * The jejunum is the midsection of the small intestine, connecting the duodenum to the ileum. It is about long, and contains the

circular folds The circular folds (also known as valves of Kerckring, valves of Kerchkring, plicae circulares, ''plicae circulae, and'' ''valvulae conniventes'') are large valvular flaps projecting into the lumen of the small intestine. Structure The entire sm ...

, and intestinal villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The suspensory muscle of duodenum marks the division between the duodenum and the jejunum. * The ileum: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients. The ileum joins to the cecum of the large intestine at the

ileocecal junction 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 c ...

. The jejunum and ileum are suspended in the

abdominal cavity The abdominal cavity is a large body cavity in humans and many other animals that contains many organs. It is a part of the abdominopelvic cavity. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the th ...

mesentery 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 intestines ...

. The mesentery is part of the peritoneum. Arteries, veins, lymph vessels and nerves travel within the mesentery.

Blood supply

The small intestine receives a blood supply from the

celiac trunk The celiac () artery (also spelled ''coeliac''), also known as the celiac trunk or truncus coeliacus, is the first major branch of the abdominal aorta. It is about 1.25 cm in length. Branching from the aorta at thoracic vertebra 12 (T12) i ...

and the superior mesenteric artery. These are both branches of the aorta. The duodenum receives blood from the

coeliac trunk The celiac () artery (also spelled ''coeliac''), also known as the celiac trunk or truncus coeliacus, is the first major branch of the abdominal aorta. It is about 1.25 cm in length. Branching from the aorta at thoracic vertebra 12 (T12) in ...

via the superior pancreaticoduodenal artery and from the superior mesenteric artery via the inferior pancreaticoduodenal artery. These two arteries both have anterior and posterior branches that meet in the midline and anastomose. The jejunum and ileum receive blood from the superior mesenteric artery. Branches of the superior mesenteric artery form a series of arches within the mesentery known as

arterial arcades 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 pul ...

, which may be several layers deep. Straight blood vessels known as vasa recta travel from the arcades closest to the ileum and jejunum to the organs themselves.

Microanatomy

The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The parts of the intestine are as follows: Cross Section of a Small Intestine

Gene and protein expression

About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal duodenum. Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the small intestine. The corresponding specific proteins are expressed in glandular cells of the mucosa, such as

fatty acid binding protein The fatty-acid-binding proteins (FABPs) are a family of transport proteins for fatty acids and other lipophilic substances such as eicosanoids and retinoids. These proteins are thought to facilitate the transfer of fatty acids between extra- and ...

FABP6. Most of the more specifically expressed genes in the small intestine are also expressed in the duodenum, for example FABP2 and the

DEFA6 Defensin, alpha 6 (DEFA6) also known as human alpha defensin 6 (HD6) is a human protein that is encoded by the ''DEFA6'' gene. ''DEFA6'' is expressed in the Paneth cells of the ileum. Function The alpha defensins are a family of microbicida ...

protein expressed in secretory granules of Paneth cells.

Development

The small intestine develops from the midgut of the

primitive gut tube Primitive may refer to: Mathematics * Primitive element (field theory) * Primitive element (finite field) * Primitive cell (crystallography) * Primitive notion, axiomatic systems * Primitive polynomial (disambiguation), one of two concepts * Prim ...

. By the fifth week of

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

logical life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the

primary intestinal loop Primary or primaries may refer to: Arts, entertainment, and media Music Groups and labels * Primary (band), from Australia * Primary (musician), hip hop musician and record producer from South Korea * Primary Music, Israeli record label Works * ...

. The loop grows so fast in length that it outgrows the abdomen and protrudes through the umbilicus. By week 10, the loop retracts back into the abdomen. Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo. It rotates a further 180 degrees after it has moved back into the abdomen. This process creates the twisted shape of the large intestine. File:Sobo 1906 399.png, First stage of the development of the intestinal canal and the peritoneum, seen from the side (diagrammatic). From colon 1 the ascending and transverse colon will be formed and from colon 2 the descending and sigmoid colons and the rectum. File:Sobo 1906 400.png, Second stage of development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The liver has been removed and the two layers of the ventral mesogastrium (lesser omentum) have been cut. The vessels are represented in black and the peritoneum in the reddish tint. File:Sobo 1906 401.png, Third state of the development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The mode of preparation is the same as in Fig 400

Function

Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter.

Digestion

The small intestine is where most chemical digestion takes place. Many of the digestive enzymes that act in the small intestine are secreted by the pancreas and liver and enter the small intestine via the pancreatic duct. Pancreatic enzymes and

from the gallbladder enter the small intestine in response to the hormone cholecystokinin, which is produced in the response to the presence of nutrients.

Secretin Secretin is a hormone that regulates water homeostasis throughout the body and influences the environment of the duodenum by regulating secretions in the stomach, pancreas, and liver. It is a peptide hormone produced in the S cells of the duode ...

, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach. The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates: * Proteins are degraded into small peptides and amino acids before absorption. Chemical breakdown begins in the stomach and continues in the small intestine. Proteolytic enzymes, including trypsin and

chymotrypsin Chymotrypsin (, chymotrypsins A and B, alpha-chymar ophth, avazyme, chymar, chymotest, enzeon, quimar, quimotrase, alpha-chymar, alpha-chymotrypsin A, alpha-chymotrypsin) is a digestive enzyme component of pancreatic juice acting in the duodenu ...

, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products. * Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the

secreted by the liver and stored in the gall bladder. Bile salts attach to triglycerides to help

emulsify An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...

them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption. * Some carbohydrates are degraded into simple sugars, or

monosaccharide Monosaccharides (from Greek ''monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. They are usually colorless, water-solub ...

s (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...

) into oligosaccharides. Other carbohydrates pass undigested into the large intestine and further handling by

intestinal bacteria Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut mic ...

. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase. Lactase is absent in some adult humans and, for them, lactose (a disaccharide), as well as most polysaccharides, is not digested in the small intestine. Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria from the large intestine.

Absorption

Digested food is now able to pass into the blood vessels in the wall of the intestine through either diffusion or active transport. The small intestine is the site where most of the nutrients from ingested food are absorbed. The inner wall, or mucosa, of the small intestine, is lined with simple columnar epithelial tissue. Structurally, the mucosa is covered in wrinkles or flaps called

, which are considered permanent features in the mucosa. They are distinct from

rugae In anatomy, rugae are a series of ridges produced by folding of the wall of an organ. Most commonly rugae refers to the gastric rugae of the internal surface of the stomach. Function A purpose of the gastric rugae is to allow for expansion of ...

which are considered non-permanent or temporary allowing for distention and contraction. From the circular folds project microscopic finger-like pieces of tissue called villi ( Latin for "shaggy hair"). The individual epithelial cells also have finger-like projections known as

microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, ...

. The functions of the circular folds, the villi, and the microvilli are to increase the amount of surface area available for the absorption of

s, and to limit the loss of said nutrients to intestinal fauna. Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body. The material that remains undigested and unabsorbed passes into the large intestine. Absorption of glucose in the small intestine

Absorption of glucose in the small intestine

Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions: * Iron is absorbed in the duodenum. * Folate (Vitamin B9) is absorbed in the duodenum and jejunum. * Vitamin B12 and bile salts are absorbed in the terminal ileum. Vitamin B12 will only be absorbed by the ileum after binding to a protein known as intrinsic factor. * Water is absorbed by osmosis and lipids by passive diffusion throughout the small intestine. *

Sodium bicarbonate Sodium bicarbonate (IUPAC name: sodium hydrogencarbonate), commonly known as baking soda or bicarbonate of soda, is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cation ( Na+) and a bicarbonate anion ( HCO3−) ...

is absorbed by active transport and glucose and amino acid co-transport *

Fructose Fructose, or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galacto ...

is absorbed by

facilitated diffusion Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembra ...

Immunological

The small intestine supports the body's immune system. The presence of

gut flora Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut mi ...

appears to contribute positively to the host's immune system. Peyer's patches, located within the ileum of the small intestine, are an important part of the digestive tract's local immune system. They are part of the lymphatic system, and provide a site for antigens from potentially harmful bacteria or other microorganisms in the digestive tract to be sampled, and subsequently presented to the immune system.

Clinical significance

The small intestine is a complex organ, and as such, there are a very large number of possible conditions that may affect the function of the small bowel. A few of them are listed below, some of which are common, with up to 10% of people being affected at some time in their lives, while others are vanishingly rare. * Small intestine obstruction or obstructive disorders ** Meconium ileus ** Paralytic ileus ** Volvulus ** Hernia **

Intussusception Intussusception may refer to: * Intussusception (medical disorder) Intussusception is a medical condition in which a part of the intestine folds into the section immediately ahead of it. It typically involves the small bowel and less commonly t ...

** Adhesions ** Obstruction from external pressure ** Obstruction by masses in the lumen (

foreign bodies A foreign body (FB) is any object originating outside the body of an organism. In machinery, it can mean any unwanted intruding object. Most references to foreign bodies involve propulsion through natural orifices into hollow organs. Foreign bo ...

, bezoar, gallstones) *

Infectious diseases An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...

Giardiasis Giardiasis is a parasitic disease caused by ''Giardia duodenalis'' (also known as ''G. lamblia'' and ''G. intestinalis''). Infected individuals who experience symptoms (about 10% have no symptoms) may have diarrhea, abdominal pain, and weight los ...

Ascariasis Ascariasis is a disease caused by the parasitic roundworm ''Ascaris lumbricoides''. Infections have no symptoms in more than 85% of cases, especially if the number of worms is small. Symptoms increase with the number of worms present and may in ...

** Tropical sprue ** Tape worm (''

Diphyllobothrium latum ''Diphyllobothrium'' is a genus of tapeworms which can cause diphyllobothriasis in humans through consumption of raw or undercooked fish. The principal species causing diphyllobothriasis is ''D. latum'', known as the broad or fish tapeworm, or b ...

'', '' Taenia solium'', ''

Hymenolepis nana Dwarf tapeworm (''Hymenolepis nana'', also known as ''Rodentolepis nana'', ''Vampirolepis nana'', ''Hymenolepis fraterna'', and ''Taenia nana'') is a cosmopolitan species though most common in temperate zones, and is one of the most common ces ...

'') ** Hookworm (e.g. '' Necator americanus'', ''

Ancylostoma duodenale ''Ancylostoma'' is a genus of nematodes that includes some species of hookworms. Species include: : ''Ancylostoma braziliense'', commonly infects cats, popularly known in Brazil as ''bicho-geográfico'' : ''Ancylostoma caninum'', commonly infec ...

'') ** Nematodes (e.g. '' Ascaris lumbricoides'') ** Other Protozoa (e.g. ''

Cryptosporidium parvum ''Cryptosporidium parvum'' is one of several species that cause cryptosporidiosis, a parasitic disease of the mammalian intestinal tract. Primary symptoms of ''C. parvum'' infection are acute, watery, and nonbloody diarrhea. ''C. parvum'' infect ...

'', ''

Cyclospora ''Cyclospora'' is a genus of apicomplexan parasites. It includes the species ''Cyclospora cayetanensis'', the causative agent of cyclosporiasis. Members of ''Cyclospora'' are characterized as having oocysts with two Apicomplexan life cycle, spor ...

'', '' Microsporidia'', '' Entamoeba histolytica'') ** Bacterial infections *** Enterotoxigenic '' Escherichia coli'' *** '' Salmonella enterica'' *** ''

Campylobacter ''Campylobacter'' (meaning "curved bacteria") is a genus of Gram-negative bacteria. ''Campylobacter'' typically appear comma- or s-shaped, and are motile. Some ''Campylobacter'' species can infect humans, sometimes causing campylobacteriosis, a d ...

'' *** '' Shigella'' *** '' Yersinia'' *** ''

Clostridium difficile ''Clostridioides difficile'' (syn. ''Clostridium difficile'') is a bacterium that is well known for causing serious diarrheal infections, and may also cause colon cancer. Also known as ''C. difficile'', or ''C. diff'' (), is Gram-positive spec ...

'' (antibiotic-associated colitis, ''

Pseudomembranous 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) ...

'') *** ''

Mycobacterium ''Mycobacterium'' is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis ('' M. tuberculosis'') and ...

'' (

Mycobacterium avium paratuberculosis ''Mycobacterium avium'' subspecies ''paratuberculosis'' (MAP) is an obligate pathogenic bacterium in the genus '' Mycobacterium''. It is often abbreviated ''M. paratuberculosis'' or ''M. avium'' ssp. ''paratuberculosis''. It is the causative ag ...

, disseminated ''

Mycobacterium tuberculosis ''Mycobacterium tuberculosis'' (M. tb) is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, ''M. tuberculosis'' has an unusual, waxy coating on its c ...

'') *** Whipple's disease *** '' Vibrio'' (

cholera Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...

) *** Enteric (typhoid) fever ('' Salmonella enterica'' var. typhii) and paratyphoid fever *** '' Bacillus cereus'' *** '' Clostridium perfringens'' ( gas gangrene) ** Viral infections *** Rotavirus *** Norovirus *** Astrovirus ***

Adenovirus Adenoviruses (members of the family ''Adenoviridae'') are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. Their name derives from the ...

*** Calicivirus * Neoplasms (cancers) **

Adenocarcinoma Adenocarcinoma (; plural adenocarcinomas or adenocarcinomata ) (AC) is a type of cancerous tumor that can occur in several parts of the body. It is defined as neoplasia of epithelial tissue that has glandular origin, glandular characteristics, or ...

** Carcinoid ** Gastrointestinal stromal tumor (GIST) ** Lymphoma ** Sarcoma **

Leiomyoma A leiomyoma, also known as a fibroid, is a benign smooth muscle tumor that very rarely becomes cancer (0.1%). They can occur in any organ, but the most common forms occur in the uterus, small bowel, and the esophagus. Polycythemia may occur due to ...

** Metastatic tumors, especially SCLC or

melanoma Melanoma, also redundantly known as malignant melanoma, is a type of skin cancer that develops from the pigment-producing cells known as melanocytes. Melanomas typically occur in the skin, but may rarely occur in the mouth, intestines, or eye ( ...

* Developmental, congenital or genetic conditions ** Duodenal (intestinal) atresia ** Hirschsprung's disease ** Meckel's diverticulum ** Pyloric stenosis **

Pancreas divisum Pancreatic divisum is a congenital anomaly in the anatomy of the ducts of the pancreas in which a single pancreatic duct is not formed, but rather remains as two distinct dorsal and ventral ducts. Most individuals with pancreas divisum remain with ...

** Ectopic pancreas **

Enteric duplication cyst Enteric duplication cysts, sometimes simply called duplication cysts, are rare congenital malformations of the gastrointestinal tract. They most frequently occur in the small intestine, particularly the ileum, but can occur anywhere along the gastro ...

** Situs inversus **

Cystic fibrosis Cystic fibrosis (CF) is a rare genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. O ...

** Malrotation ** Persistent

urachus The urachus is a fibrous remnant of the allantois, a canal that drains the urinary bladder of the fetus that joins and runs within the umbilical cord. The fibrous remnant lies in the space of Retzius, between the transverse fascia anteriorly and t ...

** Omphalocele ** Gastroschisis **

Disaccharidase Disaccharidases are glycoside hydrolases, enzymes that break down certain types of sugars called disaccharides into simpler sugars called monosaccharides. In the human body, disaccharidases are made mostly in an area of the small intestine's wall ca ...

( lactase) deficiencies ** Primary bile acid malabsorption ** Gardner syndrome ** Familial adenomatous polyposis syndrome (FAP) * Other conditions **

Crohn's disease Crohn's disease is a type of inflammatory bowel disease (IBD) that may affect any segment of the gastrointestinal tract. Symptoms often include abdominal pain, diarrhea (which may be bloody if inflammation is severe), fever, abdominal distension ...

, and the more general

inflammatory bowel disease Inflammatory bowel disease (IBD) is a group of inflammation, inflammatory conditions of the colon (anatomy), colon and small intestine, Crohn's disease and ulcerative colitis being the principal types. Crohn's disease affects the small intestine a ...

** Typhlitis (

neutropenic Neutropenia is an abnormally low concentration of neutrophils (a type of white blood cell) in the blood. Neutrophils make up the majority of circulating white blood cells and serve as the primary defense against infections by destroying bacteria ...

colitis in the

immunosuppressed Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...

** Coeliac disease (sprue or non-tropical sprue) **

Mesenteric ischemia Intestinal ischemia is a medical condition in which injury to the large or small intestine occurs due to not enough blood supply. It can come on suddenly, known as acute intestinal ischemia, or gradually, known as chronic intestinal ischemia. The ...

** Embolus or thrombus of the superior mesenteric artery or the

superior mesenteric vein In human anatomy, the superior mesenteric vein (SMV) is a blood vessel that drains blood from the small intestine (jejunum and ileum). Behind the neck of the pancreas, the superior mesenteric vein combines with the splenic vein to form the hepati ...

Arteriovenous malformation Arteriovenous malformation is an abnormal connection between arteries and veins, bypassing the capillary system. This vascular anomaly is widely known because of its occurrence in the central nervous system (usually cerebral AVM), but can appea ...

** Gastric dumping syndrome ** Irritable bowel syndrome ** Duodenal (peptic) ulcers ** Gastrointestinal perforation ** Hyperthyroidism **

Diverticulitis Diverticulitis, specifically colonic diverticulitis, is a gastrointestinal disease characterized by inflammation of abnormal pouches—diverticula—which can develop in the wall of the large intestine. Symptoms typically include lower abdominal ...

Radiation enterocolitis In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

Mesenteric cyst A mesenteric cyst () is a cyst in the mesenterium, and is one of the rarest abdominal tumors, with approximately 822 cases reported since 1507. The incidence is between 1 per 100,000 to 1 per 250,000 hospital admissions. Tillaux's triad named aft ...

s ** Peritoneal Infection **

Sclerosing Sclerotherapy (the word reflects the Greek ''skleros'', meaning ''hard'') is a procedure used to treat blood vessel malformations ( vascular malformations) and also malformations of the lymphatic system. A medicine is injected into the vessels, ...

retroperitonitis **

Small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO), also termed bacterial overgrowth, or small bowel bacterial overgrowth syndrome (SBBOS), is a disorder of excessive bacterial growth in the small intestine. Unlike the colon (or large bowel), which is r ...

Endometriosis Endometriosis is a disease of the female reproductive system in which cells similar to those in the endometrium, the layer of tissue that normally covers the inside of the uterus, grow outside the uterus. Most often this is on the ovaries, f ...

Other animals

The small intestine is found in all tetrapods and also in teleosts, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of ''pyloric caeca'', small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the

rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...

being marked only by the end of the digestive epithelium. In tetrapods, the

ileocaecal 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 cr ...

is always present, opening into the colon. The length of the small intestine is typically longer in tetrapods than in teleosts, but is especially so in

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

s, as well as in mammals and birds, which have a higher metabolic rate than

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

s or

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s. The lining of the small intestine includes microscopic folds to increase its surface area in all vertebrates, but only in mammals do these develop into true villi. The boundaries between the duodenum, jejunum, and ileum are somewhat vague even in humans, and such distinctions are either ignored when discussing the anatomy of other animals, or are essentially arbitrary. There is no small intestine as such in non-teleost fish, such as sharks,

sturgeon Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretace ...

s, and lungfish. Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods. In lampreys, the spiral valve is extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions.

Society and culture

In traditional Chinese medicine, the

small intestine 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 p ...

is a

yang Yang may refer to: * Yang, in yin and yang, one half of the two symbolic polarities in Chinese philosophy * Korean yang, former unit of currency of Korea from 1892 to 1902 * YANG, a data modeling language for the NETCONF network configuration pr ...

organ.

Additional images

File:Sobo 1906 408.png, Small intestine in situ, greater omentum folded upwards. File:Tissue layers of the small intestine (mucosa, submucosa & muscularis).jpg, Tissue layers (mucosa, submucosa & muscularis)

References

Bibliography

* * Solomon et al. (2002) Biology Sixth Edition, Brooks-Cole/Thomson Learning * Townsend et al. (2004) Sabiston Textbook of Surgery, Elsevier * *

External links

Small intestine
at th
Human Protein Atlas
{{DEFAULTSORT:Small Intestine Abdomen Digestive system Organs (anatomy)