The human gastrointestinal tract, or GI tract, or GIT is an organ system responsible for consuming and digesting foodstuffs, absorbing nutrients, and expelling waste.
The tract consists of the stomach and intestines, and is divided into the upper and lower gastrointestinal tracts. However, by the broadest definition, the GI tract includes all structures between the mouth and the anus. On the other hand, the digestive system is a broader term that includes other structures, including the digestive organs and their accessories. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment.
The whole digestive tract is about nine metres (30 feet) long.
The GI tract releases hormones from enzymes to help regulate the digestive process. These hormones, including gastrin, secretin, cholecystokinin, and ghrelin, are mediated through either intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution.
The structure and function can be described both as gross anatomy and as microscopic anatomy or histology. The tract itself is divided into upper and lower tracts, and the intestines small and large parts.
Upper gastrointestinal tract
The upper gastrointestinal tract consists of the buccal cavity, esophagus, stomach, and duodenum. The exact demarcation between the upper and lower tracts is the suspensory ligament of the duodenum (also known as the Ligament of Treitz). This delineates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of "upper" or "lower" origin. Upon dissection, the duodenum may appear to be a unified organ, but it is divided into four segments based upon 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 ligament attaches the superior border of the ascending duodenum to the diaphragm.
The suspensory muscle of duodenum is an important anatomical landmark which 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 the embryonic mesoderm.
Lower gastrointestinal tract
The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. In human anatomy, the intestine (or bowel, hose or gut) is the segment of the gastrointestinal tract 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, colon, rectum, and anal canal.
The small intestine begins at the duodenum, which receives food from the stomach. It is a tubular structure, usually between 6 and 7 m long. The area of the human, adult small intestinal mucosa is about 30 m2. Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. It has three major divisions:
- Duodenum: A short structure (about 20â"25Â cm long ) which receives gastric chyme from the stomach, together with digestive juices from the pancreas (digestive enzymes) and the gall bladder (bile). The digestive enzymes break down proteins and bile and emulsify fats into micelles. The duodenum contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralizes the stomach acids contained in gastric chyme.
- Jejunum: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m long, and contains the plicae circulares, and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.
- 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 large intestine consists of the cecum, colon, rectum, and anal canal. It also includes the vermiform appendix, which is attached to the cecum. The colon is further divided into:
- Vermiform appendix and Caecum (first portion of the colon)
- Ascending colon (ascending in the back wall of the abdomen)
- Right colic flexure (flexed portion of the ascending and transverse colon apparent to the liver
- Transverse colon (passing below the diaphragm)
- Left colic flexure (flexed portion of the transverse and descending colon apparent to the spleen
- Descending colon (descending down the left side of the abdomen)
- Sigmoid Flexure
The main function of the large intestine is to absorb water. The area of the human, adult large intestinal mucosa is about 2 m2 (Helander HF, FÃ¤ndriks L. Surface area of the digestive tract â" revisited. Scand J Gastroenterol 49: 681-9, 2014.)
The gut is an endoderm-derived structure. At approximately the sixteenth day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming concave) 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 the yolk sac, an endoderm-lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut. The yolk sac remains connected to the gut tube via the vitelline duct. Usually this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum.
During fetal life, the primitive gut is gradually patterned into three segments: foregut, midgut, and hindgut. 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 the stomach and colon, develop as swellings or dilatations in the cells of the primitive gut. In contrast, gut-related derivatives â" 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:
- Muscularis externa (the external muscular layer)
- Adventitia or serosa
The mucosa is the innermost layer of the gastrointestinal tract. that is surrounding the lumen, or open space within the tube. This layer comes in direct contact with digested food (chyme). The mucosa is made up of:
- Epithelium - innermost layer. Responsible for most digestive, absorptive and secretory processes.
- Lamina propria - a layer of connective tissue. Unusually cellular compared to most connective tissue
- Muscularis mucosae - a thin layer of smooth muscle that aids the passing of material and enhances the interaction between the epithelial layer and the contents of the lumen by agitation and peristalsis.
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 and muscularis externa. It contains Meissner's plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.
The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. The circular muscle 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.
The coordinated contractions of these layers is called peristalsis and 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 as chyme. In the large intestine the remaining semi-solid substance is referred to as faeces.
Between the two muscle layers are the myenteric or Auerbach's plexus. This controls peristalsis. Activity is initiated by the pacemaker cells (interstitial cells of Cajal). The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. The rate can of course be modulated by the rest of the autonomic nervous system.
Adventitia and serosa
The outermost layer of the gastrointestinal tract consists of several layers of connective tissue.
Intraperitoneal parts of the GI tract are covered with serosa. These include most of the stomach, first part of the duodenum, all of the small intestine, caecum and appendix, transverse colon, sigmoid colon and rectum. In these sections of the gut there is clear boundary between the gut and the surrounding tissue. These parts of the tract have a mesentery.
Retroperitoneal parts are covered with adventitia. They blend into the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through the transpyloric plane. These include the esophagus, pylorus of the stomach, distal duodenum, ascending colon, descending colon and anal canal. In addition, the oral cavity has adventitia.
The time taken for food or other ingested objects to transit through the gastrointestinal tract varies depending on many factors, but roughly, it takes less than an hour after a meal for 50% of stomach contents to empty into the intestines and total emptying of the stomach takes around 2 hours. Subsequently, 50% emptying of the small intestine takes 1 to 2 hours. Finally, transit through the colon takes 12 to 50 hours with wide variation between individuals.
The gastrointestinal tract is also a prominent part of the immune system. The surface area of the digestive tract is estimated to be the surface area of a football field. With such a large exposure, the immune system must work hard to prevent pathogens from entering into blood and lymph.[WP:V]
The low pH (ranging from 1 to 4) of the stomach is fatal for many microorganisms that enter it. Similarly, mucus (containing IgA antibodies) neutralizes many of these microorganisms. Other factors in the GI tract help with immune function as well, including enzymes in saliva and bile. Enzymes such as Cyp3A4, along with the antiporter activities, also are instrumental in the intestine's role of detoxification of antigens and xenobiotics, such as drugs, involved in first phase metabolism.
Health-enhancing intestinal bacteria of the gut flora serve to prevent the overgrowth of potentially harmful bacteria in the gut. These two types of bacteria compete for space and "food," as there are limited resources within the intestinal tract. A ratio of 80-85% beneficial to 15-20% potentially harmful bacteria generally is considered normal within the intestines. Microorganisms also are kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT).
Immune system homeostasis
Beneficial bacteria also can contribute to the gastrointestinal system homeostasis. A case in point is the relationship between human gut and Clostridia, one of the most predominant bacterial groups in the gastrointestinal tract. Clostridia play an important role influencing the dynamics of our immune system in the gut. It has been demonstrated that the intake of a high fiber diet could be the responsible for the induction of Treg cells. This is due to the production of short-chain fatty acids during the fermentation of plant derived nutrients such as butyrate and propionate. Basically, the butyrate induces the differentiation of Treg cells by enhancing histone H3 acetylation in the promoter and conserved non-coding sequence regions of the Foxp3 locus, and thus regulating the T cells, having as a result the reduction of the inflammatory response and allergies.
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 at host-pathogen interface, 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 re absorption of sodium, as well as any nutrients that may have escaped primary digestion in the ileum.
There are a number of diseases and conditions affecting the gastrointestinal system, including:
- Infection. Gastroenteritis is an inflammation of the intestines. It occurs more frequently than any other disease of the intestines.
- Cancer may occur at any point in the gastrointestinal tract, and includes mouth cancer, tongue cancer, oesophageal cancer, stomach cancer, and colorectal cancer.
- Inflammatory conditions. Ileitis is an inflammation of the ileum, Colitis is an inflammation of the large intestine.
- Appendicitis is inflammation of the vermiform appendix located at the caecum. This is a potentially fatal condition if left untreated; most cases of appendicitis require surgical intervention.
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. Diverticulosis occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as diverticulitis.
Inflammatory bowel disease is an inflammatory condition affecting the bowel walls, and includes the subtypes Crohn's disease and ulcerative 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 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).
Several symptoms are used to indicate problems with the gastrointestinal tract:
- Vomiting, which may include regurgitation of food or the vomiting of blood
- Diarrhoea, or the passage of liquid or more frequent stools
- Constipation, which refers to the passage of fewer and hardened stools
- Blood in stool, which includes fresh red blood, maroon-coloured blood, and tarry-coloured blood
Surgery of the gastrointestinal tract can 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 are used:
- Radioopaque dyes may be swallowed to produce a Barium swallow
- Parts of the tract may be visualised by camera. This is known as endoscopy if examining the upper gastrointestinal tract, and colonoscopy or sigmoidoscopy if examining the lower gastrointestinal tract. Capsule endoscopy is where a capsule is swallowed in order to examine the tract. Biopsies may also be taken when examined.
- An abdominal x-ray may be used to examine the lower gastrointestinal tract.
- Celiac Disease
- Enteric duplication cyst
- Irritable bowel syndrome
- Peptic ulcer disease
- Yellow Fever
- Helicobacter pylori is a gram-negative spiral bacterium. Over half the world's population infected, mainly during childhood, it is not certain as to how the disease is transmitted. It colonizes the gastrointestinal system, predominantly the stomach. The bacterium has specific survival conditions that our gastric microenvironment: it is both capnophilic and microaerophilic. Helicobacter also exhibits a tropism for gastric epithelial lining and the gastric mucosal layer about it. Gastric colonization of this bacterium triggers a robust immune response leading to moderate to severe inflammation. This inflammatory response triggers a cascade of mucosal changes that can persist from chronic gastritis, duodenal cancer, metaplasia, dysplasia, carcinoma, to mucosal associated lymphoid tissue lymphoma (MALT lymphoma). Many individuals go through life without realizing they are infected because they were exposed young and their body sees it as normal flora. However, signs and symptoms are gastritis, burning abdominal pain, weight loss, loss of appetite, bloating, burping, nausea, bloody vomit, and black tarry stools. Infection is easy enough to detect: GI X-rays, endoscopy, blood tests for anti-Helicobacter antibodies, a stool test, and a urease breath test (which is a by-product of the bacteria.) Luckily if caught soon enough there is a treatment: it is three doses of different proton pump inhibitors as well as two antibiotics and it takes about a week to cure. If not caught soon enough surgery may be required.
- Intestinal Pseudo-Obstruction is a syndrome caused by a malformation of the digestive system, characterized by a severe impairment in the ability of the intestines to push and assimilate. Symptoms include daily abdominal and stomach pain, nausea, severe distension, vomiting, heartburn, dysphagia, diarrhea, constipation, dehydration and malnutrition. There is no cure for intestinal pseudo-obstruction. Different types of surgery and treatment managing life-threatening complications such as ileus and volvulus, intestinal stasis which lead to bacterial overgrowth, and resection of affected or dead parts of the gut may be needed. Many patients require parenteral nutrition.
- Ileus is a blockage of the intestines.
- Coeliac disease is a common form of malabsorption, 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. Lifelong dietary avoidance of these foodstuffs in a gluten-free diet is the only treatment.
- 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 syndrome (IBS) is the most common functional disorder of the intestine. Functional constipation and chronic functional abdominal pain 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.
- Endometriosis 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 infarction and death. (The singer Maurice Gibb is understood to have died from this.)
- Angiodysplasia of the colon
- Chronic functional abdominal pain
- Hirschsprung's disease (aganglionosis)
- Polyp (medicine) (see also Colorectal polyp)
- Pseudomembranous colitis
- Ulcerative colitis and toxic megacolon
In other animals
Animal intestines have multiple uses. From each species of livestock that is a source of milk, a corresponding rennet is obtained from the intestines of milk-fed calves. 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.
- Gastrointestinal hormone
- Major systems of the human body
- Host-pathogen interface
- Anatomy and Physiology
- Pediatric Overview of Human Digestive System
- Anatomy atlas of the Digestive System
- Overview at Colorado State University
- Your Digestive System and How It Works at National Institutes of Health