Gastrointestinal tract

Thegastrointestinal tract(GI tract,digestive tract,alimentary canal) is the tract or passageway of thedigestive systemthat leads from themouthto theanus. The GI tract contains all the majororgansof the digestive system, in humans and other animals, including theesophagus,stomach, andintestines. Food taken in through the mouth isdigestedto extractnutrientsand absorbenergy, and the waste expelled at the anus asfeces.Gastrointestinalis an adjective meaning of or pertaining to the stomach and intestines.
Most animalshave a "through-gut" or complete digestive tract. Exceptions are more primitive ones:spongeshave small pores (ostia) throughout their body for digestion and a larger dorsal pore (osculum) for excretion,comb jellieshave both a ventral mouth and dorsal anal pores, whilecnidariansandacoelshave a single pore for both digestion and excretion.
The human gastrointestinal tract consists of theesophagus,  stomach, and intestines, and is divided into the upper and lower gastrointestinal tracts.The GI tract includes all structures between themouthand theanus,forming a continuous passageway that includes the main organs of digestion, namely, thestomach,small intestine, andlarge intestine. The completehuman digestive systemis made up of the gastrointestinal tract plus the accessory organs of digestion (thetongue,salivary glands,pancreas,liverandgallbladder).The tract may also be divided intoforegut,midgut, andhindgut, reflecting theembryologicalorigin of each segment. The whole human GI tract is about nine meters (30 feet) long atautopsy. It is considerably shorter in the living body because the intestines, which are tubes ofsmooth muscle tissue, maintain constantmuscle tonein a halfway-tense state but can relax in spots to allow for local distention andperistalsis.
The gastrointestinal tract contains thegut microbiota, with some 1,000 differentstrainsofbacteriahaving diverse roles in the maintenance ofimmune healthandmetabolism, and many othermicroorganisms.Cells of the GI tract releasehormonesto help regulate the digestive process. Thesedigestive hormones, includinggastrin,secretin,cholecystokinin, andghrelin, are mediated through eitherintracrineorautocrinemechanisms, indicating that the cells releasing these hormones are conserved structures throughoutevolution.
The structure and function can be described both asgross anatomyand asmicroscopic anatomyorhistology. The tract itself is divided into upper and lower tracts, and the intestinessmallandlargeparts.
The upper gastrointestinal tract consists of themouth,pharynx,esophagus,stomach, andduodenum.The exact demarcation between the upper and lower tracts is thesuspensory muscle of the duodenum. This differentiates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describegastrointestinal bleedingas being of either "upper" or "lower" origin. Upondissection, the duodenum may appear to be a unified organ, but it is divided into four segments based on function, location, and internal anatomy. The four segments of the duodenum are as follows (starting at the stomach, and moving toward the jejunum):bulb, descending, horizontal, and ascending. The suspensory muscle attaches the superior border of the ascending duodenum to thejejunum.
The suspensory muscle is an important anatomical landmark that shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively.This is a thin muscle which is derived from theembryonicmesoderm.
The lower gastrointestinal tract includes most of thesmall intestineand all of thelarge intestine.Inhuman anatomy, theintestine(bowelorgut; Greek:éntera) is the segment of the gastrointestinal tract extending from the pyloric sphincter of thestomachto theanusand as in other mammals, consists of two segments: thesmall intestineand thelarge intestine. In humans, the small intestine is further subdivided into theduodenum,jejunum, andileumwhile the large intestine is subdivided into thececum, ascending, transverse, descending, and sigmoidcolon,rectum, andanal canal.
Thesmall intestinebegins at theduodenumand is a tubular structure, usually between 6 and 7 m long.Itsmucosalarea in an adult human is about 30 m(320 sq ft).The combination of thecircular folds, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a total area of about 250 m(2,700 sq ft) for the entire small intestine.Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions:
Thelarge intestineforms an arch starting at thececumand ending at therectumandanal canal. It also includes theappendix, which is attached to thececum. Its length is about 1.5 m, and the area of the mucosa in an adult human is about 2 m(22 sq ft).The longest part of the large intestine is thecolonwhose main function is to absorb water and salts.
The large intestine begins at the cecum, where theappendixis located. This is also the start of the colon as theascending colonin the back wall of the abdomen. At theright colic flexure(hepaticflexure) (the flexed portion of the ascending andtransverse colon) it runs across the abdomen in the transverse colon, passing below the diaphragm. At theleft colic flexure(splenicflexure) the flexed portion of the transverse anddescending colon, it descends down the left side of the abdomen.
It reaches thesigmoid colonwhich is a loop of the colon closest to the rectum and continues to the rectum andanal canal.
The gut is anendoderm-derived structure. At approximately the sixteenth day of human development, theembryobegins to foldventrally(with the embryo's ventral surface becomingconcave) in two directions: the sides of the embryo fold in on each other and the head and tail fold toward one another. The result is that a piece of theyolk sac, anendoderm-lined structure in contact with theventralaspect of the embryo, begins to be pinched off to become the primitive gut. The yolk sac remains connected to the gut tube via thevitelline duct. Usually, this structure regresses during development; in cases where it does not, it is known asMeckel's diverticulum.
Duringfetallife, the primitive gut is gradually patterned into three segments:foregut,midgut, andhindgut. Although these terms are often used in reference to segments of the primitive gut, they are also used regularly to describe regions of the definitive gut as well.
Each segment of the gut is further specified and gives rise to specific gut and gut-related structures in later development. Components derived from the gut proper, including thestomachandcolon, develop as swellings or dilatations in the cells of the primitive gut. In contrast, gut-related derivatives — that is, those structures that derive from the primitive gut but are not part of the gut proper, in general, develop as out-pouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.
The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy.The GI tract can be divided into four concentric layers in the following order:
The mucosa is the innermost layer of the gastrointestinal tract. The mucosa surrounds thelumen, or open space within the tube. This layer comes in direct contact with digested food (chyme). The mucosa is made up of:
The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium.
The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa andmuscularis externa. It contains thesubmucosal plexus, anenteric nervous plexus, situated on the inner surface of themuscularis externa.
Themuscular layerconsists of an inner circular layer and alongitudinalouter layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch.Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food. The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer, and the outer longitudinal layer.
Between the circular and longitudinal muscle layers is themyenteric plexus. This controls peristalsis. Activity is initiated by the pacemaker cells, (myentericinterstitial cells of Cajal). The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. The rate can be modulated by the rest of theautonomic nervous system.
The coordinated contractions of these layers is calledperistalsisand propels the food through the tract. Food in the GI tract is called a bolus (ball of food) from the mouth down to the stomach. After the stomach, the food is partially digested and semi-liquid, and is referred to aschyme. In the large intestine, the remaining semi-solid substance is referred to asfaeces.
The outermost layer of the gastrointestinal tract consists of several layers ofconnective tissue.
Intraperitonealparts of the GI tract are covered withserosa. These include most of thestomach, first part of theduodenum, all of thesmall intestine,caecumandappendix,transverse colon,sigmoid colonandrectum. In these sections of the gut, there is a clear boundary between the gut and the surrounding tissue. These parts of the tract have amesentery.
Retroperitonealparts are covered withadventitia. They blend into the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through thetranspyloric plane. These include theesophagus,pylorusof the stomach, distalduodenum,ascending colon,descending colonandanal canal. In addition, theoral cavityhas adventitia.
Approximately 20,000 protein coding genes are expressed in human cells and 75% of these genes are expressed in at least one of the different parts of the digestive organ system.Over 600 of these genes are more specifically expressed in one or more parts of the GI tract and the corresponding proteins have functions related to digestion of food and uptake of nutrients. Examples of specific proteins with such functions arepepsinogen PGCand thelipase LIPF, expressed inchief cells, and gastricATPase ATP4Aandgastric intrinsic factor GIF, expressed inparietal cellsof the stomach mucosa. Specific proteins expressed in the stomach and duodenum involved in defence includemucinproteins, such asmucin 6andintelectin-1.
The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender.Several techniques have been used to measure transit time, including radiography following abarium-labeled meal, breathhydrogenanalysis,scintigraphicanalysis following aradiolabeledmeal,and simple ingestion and spotting ofcorn kernels.It takes 2.5 to 3 hours for 50% of the contents to leave the stomach.The rate of digestion is also dependent of the material being digested, as food composition from the same meal may leave the stomach at different rates.Total emptying of the stomach takes around 4–5 hours, and transit through the colon takes 30 to 50 hours.
The gastrointestinal tract forms an important part of theimmune system.
The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court.With such a large exposure (more than three times larger than theexposed surface of the skin), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems.Fundamental components of this protection are provided by theintestinal mucosal barrier, which is composed of physical, biochemical, and immune elements elaborated by the intestinal mucosa.Microorganisms also are kept at bay by an extensive immune system comprising thegut-associated lymphoid tissue(GALT)
There are additional factors contributing to protection from pathogen invasion. For example, lowpH(ranging from 1 to 4) of the stomach is fatal for manymicroorganismsthat enter it.Similarly,mucus(containingIgAantibodies) neutralizes many pathogenic microorganisms.Other factors in the GI tract contribution to immune function includeenzymessecreted in thesalivaandbile.
Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example,Clostridia, one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system.It has been demonstrated that the intake of a high fiber diet could be responsible for the induction ofT-regulatory cells(Tregs). This is due to the production ofshort-chain fatty acidsduring the fermentation of plant-derived nutrients  such asbutyrateandpropionate.  Basically, the butyrate induces the differentiation of Treg cells by enhancinghistone H3acetylationin the promoter and conserved non-coding sequence regions of theFOXP3locus, thus regulating theT cells, resulting in the reduction of the inflammatory response and allergies.
The large intestine contains multiple types ofbacteriathat can break down molecules the human body cannot process alone,demonstrating asymbioticrelationship. These bacteria are responsible for gas production athost–pathogen interface, which is released asflatulence. Intestinal bacteria can also participate in biosynthesis reactions. For example, certain strains in the large intestine produce vitamin B12;an essential compound in humans for things like DNA synthesis and red blood cell production.However, the primary function of the large intestine is water absorption from digested material (regulated by thehypothalamus) and the reabsorption ofsodiumand nutrients.
Beneficialintestinal bacteriacompete with potentially harmfulbacteriafor space and "food", as the intestinal tract has limited resources. A ratio of 80–85% beneficial to 15–20% potentially harmful bacteria is proposed for maintaininghomeostasis.An imbalanced ratio results indysbiosis.
Enzymessuch asCYP3A4, along with theantiporteractivities, are also instrumental in the intestine's role ofdrug metabolismin the detoxification ofantigensandxenobiotics.
In mostvertebrates, includingamphibians,birds,reptiles,egg-laying mammals, and somefish, the gastrointestinal tract ends in acloacaand not ananus. In the cloaca, theurinary systemis fused with the genito-anal pore.Therians(all mammals that do not lay eggs, including humans) possess separate anal and uro-genital openings. The females of the subgroupPlacentaliahave even separate urinary and genital openings.
Duringearly development, the asymmetric position of the bowels and inner organs is initiated (see alsoaxial twist theory).
Ruminantsshow many specializations for digesting andfermentingtough plant material, consisting ofadditional stomach compartments.
Many birds and other animals have a specialised stomach in the digestive tract called agizzardused for grinding up food.
Another feature found in a range of animals is thecrop. In birds this is found as a pouch alongside the esophagus.
In 2020, the oldest known fossil digestive tract, of an extinct wormlike organism in theCloudinidaewas discovered; it lived during the lateEdiacaranperiodabout 550 million years ago.
A through-gut (one with both mouth and anus) is thought to have evolved within thenephrozoanclade ofBilateria, after their ancestral ventral orifice (single, as incnidariansandacoels; re-evolved in nephrozoans likeflatworms) stretched antero-posteriorly, before the middle part of the stretch would get narrower and closed fully, leaving an anterior orifice (mouth) and a posterior orifice (anus plusgenital opening). A stretched gut without the middle part closed is present in another branch of bilaterians, the extinctproarticulates. This and theamphistomicdevelopment (when both mouth and anus develop from the gut stretch in the embryo) present in some nephrozoans (e.g.roundworms) are considered to support this hypothesis.
There are many diseases and conditions that can affect the gastrointestinal system, includinginfections,inflammationandcancer.
Variouspathogens, such asbacteriathat causefoodborne illnesses, can inducegastroenteritiswhich results frominflammationof the stomach and small intestine.Antibioticsto treat such bacterial infections can decrease themicrobiomediversity of the gastrointestinal tract, and further enable inflammatory mediators.Gastroenteritis is the most common disease of the GI tract.
Diverticular diseaseis 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.Diverticulosisoccurs when pouches form on the intestinal wall.  Once the pouches become inflamed it is known asdiverticulitis.
Inflammatory bowel diseaseis an inflammatory condition affecting the bowel walls, and includes the subtypesCrohn's diseaseandulcerative colitis. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as anautoimmune disease. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such.
Functional gastrointestinal disordersthe most common of which isirritable bowel syndrome. Functional constipation andchronic functional abdominal painare other functional disorders of the intestine that have physiological causes but do not have identifiable structural, chemical, or infectious pathologies.
Several symptoms can indicate problems with the gastrointestinal tract, including:
Gastrointestinal surgerycan often be performed in the outpatient setting.  In the United States in 2012, operations on the digestive system accounted for 3 of the 25 most common ambulatory surgery procedures and constituted 9.1 percent of all outpatient ambulatory surgeries.
Various methods of imaging the gastrointestinal tract include theupperandlower gastrointestinal series:
Intestines from animals other than humans are used in a number of ways.  From each species oflivestockthat is a source ofmilk, a correspondingrennetis obtained from the intestines of milk-fedcalves.Pigandcalfintestines are eaten, and pig intestines are used assausagecasings. Calf intestines supplycalf-intestinal alkaline phosphatase(CIP), and are used to makegoldbeater's skin.
Other uses are: