When the hydrogen peroxide is dripped on to the guaiac paper, it oxidizes the alpha-guaiaconic acid to a blue colored  quinone . [ 11 ] [ 12 ]  Normally, when no blood and no peroxidases or catalases from vegetables are present, this oxidation occurs very slowly.  Heme, a component of hemoglobin found in blood, catalyzes this reaction, giving a result in about two seconds.  Therefore, a positive test result is one where there is a quick and intense blue color change of the film.


The guaiac test can often be  false-positive  which is a positive test result when there is in fact no source of bleeding. This is particularly common if the recommended dietary preparation is not followed, as the heme in red meat or the peroxidase or catalase activity in vegetables, especially if uncooked, can cause analytical false positives.


Vitamin C  can cause analytical  false negatives  due to its anti-oxidant properties inhibiting the color reaction. [ 13 ] [ 14 ]


If the card has not been promptly developed, the water content of the feces decreases, and this can reduce the detection of blood. [ 15 ]  Although rehydration of stored samples can reverse this effect [ 16 ]  this is not recommended because the test becomes unduly analytically sensitive and thus much less specific. [ 17 ] [ 18 ]


Some stool specimens have a high bile content that causes a green color to show after applying the developer drops. If entirely green, such samples are negative, but if questionably green to blue, such samples are designated positive. [ 19 ]


The package insert guidelines from the manufacturers, for example Hemoccult SENSA, [ 20 ]  recommend that nonsteroidal anti-inflammatory drugs ( NSAID ), such as ibuprofen and aspirin, and iron supplements be discontinued for at least several days before the tests.  There is a concern that these agents may irritate the body and cause biologically positive tests even in the absence of a more substantial illness, [ 21 ] [ 22 ]  but there is some doubt about how frequently this occurs with NSAID medication. [ 23 ] [ 24 ] [ 25 ]  Although both iron and bismuth containing products such as antacids and antidiarrheals can cause dark stools that are occasionally confused as containing blood, actual bleeding from iron is unusual. [ 26 ] [ 27 ]


There is no consensus on whether to stop warfarin before a guaiac test. [ 28 ]  Even when using anticoagulants a high proportion of positive guaiac tests were found to be due to diagnosable lesions, suggesting anticoagulants may not cause bleeding unless there is an abnormality. [ 29 ] [ 30 ]


The article  fecal occult blood  (FOB) provides an expanded consideration of the clinical application of FOB tests generally, including other clinical methods, and the comments here are those that relate specifically to the guaiac gFOBT method.


One major use of stool testing for blood is detection of  colorectal cancer . However, other possible positive results include: gastroesophageal cancer, GI bleeds,  diverticulae ,  hemorrhoids ,  anal fissures ,  colon polyps ,  ulcerative colitis ,  Crohn's disease ,  celiac disease ,  GERD ,  esophagitis ,  peptic ulcers ,  gastritis ,  inflammatory bowel disease ,  vascular ectasias ,  portal hypertensive gastropathy ,  aortoenteric fistulas ,  hemobilia ,  endometriosis , and  trauma .


The stool guaiac test was originally the principal colon cancer screening technology available, but modern tests which look for  globin  or DNA are now also available.  Several recent colon cancer screening guidelines have recommended replacing any older low-sensitivity, guaiac-based fecal occult blood testing (gFOBT) with either newer high-sensitivity guaiac-based fecal occult blood testing (gFOBT) or fecal immunochemical testing (FIT), which tests for  globin  rather than the  heme  detected by the guaiac method. The US Multisociety Task Force (MSTF) looked at 6 studies that compared high sensitivity gFOBT (Hemoccult SENSA) to FIT, and  concluded that there were no clear difference in overall performance between these methods, [ 31 ]  and a similar recommendation was made by the National Guideline Clearinghouse (NGC). [ 32 ]


Results of a single fecal sample should be interpreted cautiously, as there is a high rate of false negativity associated with the test. [ 33 ] [ 10 ]  Using three cards, each on a different day, is recommended to improve sensitivity. [ 34 ]  The Centers for Disease Control and Prevention (CDC) in a 2006–2007 survey found extensive inappropriate use of low sensitivity gFOBT and of single specimens; it is unclear if these widespread suboptimal approaches have since declined. [ 35 ]   The Current Procedural Terminology (CPT) coding was changed in January 2006 to include CPT code 82270, which indicates that consecutive collection of three stool samples has occurred, either as three single cards or a single triple card. Since January 2007, the US Medicare program reimburses for colorectal cancer screening with gFOBT only when this code is used. [ 36 ]


The stool guaiac test method may be preferable to fecal immunochemical testing (FIT) if there is a clinical concern about possible gastric or proximal upper intestinal bleeding. [ 37 ]  However, although  heme  breakdown is less than  globin  during intestinal transit, false negative results can be seen with the stool guaiac tests due to degradation of the peroxidase-activity. This can cause false negative results in upper gastrointestinal bleeding sources, or in right colon adenomas and cancers that have comparable blood losses to positively testing left colon lesions. [ 38 ]  A positive gFOBT with subsequent negative colonoscopy may lead to an upper endoscopy. [ 39 ]  It is unclear whether this is an effective intervention if there is a positive gFOBT but no anemia. [ 40 ] [ 41 ] [ 42 ]  Endoscopy when there is a positive gFOBT along with iron deficiency anemia, or iron deficiency anemia on its own, has a higher rate of finding problems. [ citation needed ]


String sign , or  gastrointestinal string sign  (also called string sign of Kantour), is a medical term for a radiographic finding on an  upper GI series , in which the patient is given a  radio-opaque  material, such as barium, to drink.    X-rays  are then taken of the patient's stomach and intestines.


The gastrointestinal string sign represents a severe narrowing of loop of bowel, in which a thin stripe of contrast within the lumen looks like a string.


It may be seen in  Crohn's disease ,  hypertrophic pyloric stenosis ,  carcinoid  tumor and  colon cancer . In people with Crohn's Disease, the string sign is caused by incomplete filling of the intestinal lumen, which results from irritability and spasm associated with severe ulceration. In such cases, the string sign is most frequently seen at the terminal ileum.


In infants with hypertrophic pyloric stenosis, the pylorus is narrowed and the radio-opaque material will take on the appearance of a thin string as it passes through this narrowed channel.  Often, there are several of these strings seen (called the "railroad track sign"). The use of the upper GI series for the diagnosis of HPS, which was the primary diagnostic tool for this condition in the 1980s and 1990s, has been largely replaced by the use of  ultrasound , which is less invasive and can visualize the thickened pylorus, giving actual measurements of this thickening.


An  upper gastrointestinal series , also called a  barium swallow ,  barium study , or  barium meal , is a series of  radiographs  used to examine the  gastrointestinal tract  for abnormalities. A  contrast medium , usually a  radiocontrast agent  such as  barium sulfate  mixed with water, is ingested or instilled into the gastrointestinal tract, and  X-rays  are used to create radiographs of the regions of interest. The barium enhances the visibility of the relevant parts of the gastrointestinal tract by coating the inside wall of the tract and appearing white on the film. This in combination with other plain radiographs allows for the imaging of parts of the upper gastrointestinal tract such as the  pharynx ,  larynx ,  esophagus ,  stomach , and  small intestine  such that the inside wall lining, size, shape, contour, and patency are visible to the examiner. With  fluoroscopy , it is also possible to visualize the functional movement of examined organs such as  swallowing ,  peristalsis , or  sphincter  closure. Depending on the organs to be examined, barium radiographs can be classified into "barium swallow", "barium meal", "barium follow-through", and "enteroclysis" ("small bowel enema"). To further enhance the quality of images, air or gas is sometimes introduced into the gastrointestinal tract in addition to barium, and this procedure is called double-contrast imaging. In this case the gas is referred to as the negative contrast medium. Traditionally the images produced with barium contrast are made with plain-film radiography, but  computed tomography  is also used in combination with barium contrast, in which case the procedure is called " CT enterography ". [ 1 ]


Various types of barium X-ray examinations are used to examine different parts of the gastrointestinal tract. These include barium swallow, barium meal, barium follow-through, and  barium enema . [ 2 ]  The barium swallow, barium meal, and barium follow-through are together also called an upper gastrointestinal series (or study), whereas the barium enema is called a  lower gastrointestinal series  (or study). [ 3 ]  In upper gastrointestinal series examinations, the barium sulfate is mixed with water and swallowed orally, whereas in the lower gastrointestinal series (barium enema), the barium  contrast agent  is administered as an  enema  through a small tube inserted into the  rectum . [ 2 ]


Barium X-ray examinations are useful tools for the study of appearance and function of the parts of the gastrointestinal tract. They are used to diagnose and monitor  esophageal reflux ,  dysphagia ,  hiatus hernia ,  strictures ,  diverticula ,  pyloric stenosis ,  gastritis ,  enteritis ,  volvulus ,  varices ,  ulcers ,  tumors , and gastrointestinal dysmotility, as well as to detect  foreign bodies . [ 3 ] [ 6 ]  Although barium X-ray examinations are increasingly being replaced by more modern techniques, such as  computer tomography ,  magnetic resonance imaging ,  ultrasound imaging ,  endoscopy  and  capsule endoscopy , [ 7 ]  barium contrast imaging remains in common use because it offers the advantages of greater affordability, wider availability, [ 1 ] [ 5 ]  and better resolution in assessing superficial mucosal lesions. [ 7 ] [ 8 ]


Barium sulfate is swallowed and is a  radio opaque  substance that does not allow the passage of X-rays. As a result, areas coated by barium sulfate will appear white on an X-ray film. The passage of barium sulfate through the gastrointestinal tract is observed by a radiologist using a  fluoroscope  attached to a TV monitor. The radiologist takes a series of individual X-ray images at timed intervals depending on the areas to be studied. Sometimes medication which produces gas in the gastrointestinal tract is administered together with the Barium sulfate. This gas distends the gastrointestinal lumen, providing better imaging conditions and in this case the procedure is called double-contrast imaging. [ 9 ]


Clinical status and relevant medical history are reviewed prior to the studies. [ 10 ]   Patient consent  is required. [ 3 ]


A barium swallow study is also known as a  barium  esophagram  and needs little if any preparations for the study of the larynx, pharynx, and esophagus when studied alone. [ 11 ] [ 12 ]


Amongst the uses of barium swallow are: persistent  dysphagia  and  odynophagia  despite negative  esophagogastroduodenoscopy  (OGDS) findings, failed OGDS,  esophageal motility disorder ,  globus pharyngis , assessment of  tracheoesophageal fistula , and timed barium swallow to monitor the progress of  esophageal achalasia  therapy. [ 13 ]   Barium sulfate suspension  such as 100 ml or more of E-Z HD 200 to 250% concentration and Baritop 100% can be used. Water-soluble contrast agent such as Gastrografin ( diatrizoate ) and Conray ( Iotalamic acid ) is used instead of barium if oesophageal perforation is suspected. Low osmolar contrast medium with concentration of 300 mg/ml is used instead of gastrografin if there is risk of  aspiration  or there is tracheoesophageal fistula. [ 13 ]


A thick barium mixture is swallowed in supine position and fluoroscopic images of the swallowing process are made. Then several swallows of a thin barium mixture are taken and the passage is recorded by fluoroscopy and standard radiographs. The procedure is repeated several times with the examination table tilted at various angles. A total of 350–450 mL of barium is swallowed during the process. [ 14 ] [ 15 ]  Normally, 90% of ingested fluid should have passed into the stomach after 15 seconds. [ 16 ]


Right anterior oblique (RAO) view is to see the oesophagus clearly, away from overlapping spine. [ 13 ]  AP (anterior-posterior) view is also done to visualise the  gastroesophageal junction . [ 13 ]  AP and lateral views are also done to visualise the  hypopharynx  during swallowing at a frame rate of 3–4 per second. Left posterior oblique (LPO) position is used to identify hernias, mucosal rings, and varices. [ 13 ]


Intravenous injection of Buscopan ( Hyoscine butylbromide ) 20 mg or  glucagon  0.3 mg is used to distend the stomach and slow down the emptying of the contrast into the duodenum. [ 13 ]


Right anterior oblique (RAO) view is used to demonstrate antrum and greater curve of stomach. Supine position is to demonstrate antrum and body of stomach. Left anterior oblique (LAO) view is used to see the lesser curve of stomach en face. This position is also used to check for gastroesophageal reflux when patient is asked to cough or swallow (water siphon test). Left lateral tilted with head up 45 degrees is used to demonstrate the fundus of the stomach. [ 13 ]  To demonstrate the duodenal loop, the subject can lie down in prone position on a compression pad to prevent excessive barium flowing into the duodenal loop. Anterior view of duodenal loop can be seen at RAO position. [ 13 ]  Duodenal cap can be visualised by taking images when subject lie down in prone position, RAO, supine, and then LAO positions or it can be seen on erect position with RAO and steep LAO views. [ 13 ]  Total mucosal coating of the stomach is done by asking the subject to roll to the right side into a complete circle until RAO position. Arae gastriae in the antrum (fine reticular network of grooves) is visible if good coating is achieved. [ 13 ]


Indications to do this procedure are: unexplained chronic abdominal pain with weight loss, unexplained diarrhea, anemia which is caused by gastrointestinal bleeding or dependent on  blood transfusion  where the cause cannot be explained despite OGDS or colonoscopy investigations, partial obstruction of bowel/small bowel adhesive obstruction suspected, and unexplained  malabsorption  of nutrients. [ 13 ]  For barium follow-through examinations, a 6-hour period of fasting is observed prior to the study. [ 10 ]


Barium is administered orally, sometimes mixed with  diatrizoic acid  (gastrografin) to reduce transit time in the bowel. Intravenous  metoclopramide  is sometimes also added to the mixture to enhance gastric emptying. [ 17 ]  600 ml of 0.5% methylcellulose can be given orally, after barium meal is given, to improve the images of small bowel follow-through by reducing the time taken for barium to pass through the small intestines, and increase the transparency of the contrast-filled small bowels. [ 18 ]  Other methods to reduce transit time are to add ice cold normal saline after the administration of barium saline mixture [ 19 ]  or to give a dry meal. [ 20 ]


X-ray images are then taken in a supine position at intervals of 20–30 minutes. Real-time fluoroscopy is used to assess bowel motility. The radiologist may press or palpate the abdomen during images to separate intestinal loops. The total time necessary for the test depends on the speed of bowel motility or transit time and may vary between 1 and 3 hours. [ 17 ]


Enteroclysis is also known as small bowel enema. [ 21 ]  It has been largely replaced by  magnetic resonance enterography/enteroclysis [ 13 ]  and  computed tomography enterography/enteroclysis . [ 22 ]


In addition to fasting for 8 hours prior to examination, a  laxative  may also be necessary for bowel preparation and cleansing. [ 12 ]  The main aim of this study is to distend the proximal bowel through infusion of large amount of barium suspension. Otherwise, the distension of distal small bowel is generally similar with small bowel follow-through. Therefore, there is a need to pass a tube through the nose into the  jejunum  (nasojejunal tube) to administer large amount of contrast. This can be unpleasant to the subject, requires more staff, longer procedural time, and higher radiation dose when compared to small bowel follow-through. The indications for enteroclysis are generally similar to small bowel follow-through. Barium suspensions such as diluted E-Z Paque 70% and Baritop 100% can be used. After that, 600 ml of 0.5% methylcellulose is administered after 500 ml of 70% barium suspension is given. Bilbao-Dotter tube and Silk tube can be used to administer barium suspension. The subject should be fasted overnight, any  antispasmodic  drugs should be stopped one day before the examination, and  Tetracaine  lozenges can be used 30 minutes before the procedure to numb the throat for nasojejunal tube insertion. [ 13 ]


The filling of the small intestines can be viewed continuously using  fluoroscopy , or viewed as standard radiographs taken at frequent intervals. The technique is a double-contrast procedure that allows detailed imaging of the entire small intestine. However, the procedure may take 6 hours or longer to complete and is quite uncomfortable to undergo. [ 23 ]


Complete  gastrointestinal obstruction  is a contraindication for barium studies. [ 17 ]


Barium sulfate as a contrast medium was evolved from the prior use of bismuth preparations which were too toxic. The use of bismuth preparations had been described as early as 1898. Barium sulfate as a contrast medium in medical practice was introduced largely as a result of the works of Krause a director of the Bonn Polyclinic, now the medical faculty of the  University of Bonn  and his colleagues Bachem and Gunther. In a paper read in 1910 at the radiological congress they advocated for the use of barium sulfate as an opaque contrast medium in medicine. [ 36 ]


D-xylose absorption test  is a medical test performed to diagnose conditions that present with  malabsorption  of the proximal  small intestine [ 1 ]  due to defects in the integrity of the gastrointestinal mucosa. [ 2 ]   D-xylose  is a  monosaccharide , or simple sugar, that does not require enzymes for digestion prior to absorption. Its absorption requires an intact mucosa only. In contrast,  polysaccharides  require enzymes, such as  amylase , to break them down so that they can eventually be absorbed as monosaccharides. This test was previously in use but has been made redundant by  antibody tests . [ citation needed ]


In normal individuals, a 25 g oral dose of D-xylose will be absorbed and excreted in the urine at approximately 4.5 g in 5 hours. A decreased urinary excretion of D-xylose is seen in conditions involving the gastrointestinal mucosa, such as  small intestinal bacterial overgrowth  and  Whipple's disease . In cases of bacterial overgrowth, the values of D-xylose absorption return to normal after treatment with  antibiotics . In contrast, if the D-xylose urinary excretion is not normal after a course of antibiotics, then the problem must be due to a non-infectious cause of malabsorption (i.e.,  celiac disease ). [ citation needed ]


The  human digestive system  consists of the  gastrointestinal tract  plus the accessory organs of digestion (the  tongue ,  salivary glands ,  pancreas ,  liver , and  gallbladder ).  Digestion  involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the  cephalic phase , the  gastric phase , and the  intestinal phase .


The first stage, the cephalic phase of digestion, begins with secretions from  gastric glands  in response to the sight and smell of food. This stage includes the mechanical breakdown of food by  chewing , and the chemical breakdown by  digestive enzymes , that takes place in the  mouth .  Saliva  contains the digestive enzymes  amylase , and  lingual lipase , secreted by the salivary and  serous glands  on the tongue. Chewing, in which the food is mixed with saliva, begins the mechanical process of digestion. This produces a  bolus  which is swallowed down the  esophagus  to enter the  stomach .


The second stage, the gastric phase, happens in the stomach. Here, the food is further broken down by mixing with  gastric acid  until it passes into the  duodenum , the first part of the  small intestine .


The third stage, the intestinal phase, begins in the duodenum. Here, the partially digested food is mixed with a number of  enzymes  produced by the pancreas.


Digestion is helped by the chewing of food carried out by the  muscles of mastication , the tongue, and the  teeth , and also by the  contractions  of  peristalsis , and  segmentation . Gastric acid, and the production of  mucus  in the stomach, are essential for the continuation of digestion.


Peristalsis is the rhythmic contraction of  muscles  that begins in the esophagus and continues along the wall of the stomach and the rest of the gastrointestinal tract. This initially results in the production of  chyme  which when fully broken down in the small intestine is absorbed as  chyle  into the  lymphatic system . Most of the digestion of food takes place in the small intestine. Water and some  minerals  are reabsorbed back into the blood in the colon of the  large intestine . The waste products of digestion ( feces ) are  defecated  from the  rectum  via the  anus .


There are several organs and other components involved in the digestion of food. The organs known as the  accessory digestive organs  are the  liver ,  gall bladder  and  pancreas . Other components include the  mouth ,  salivary glands ,  tongue ,  teeth  and  epiglottis .


The largest structure of the digestive  system  is the  gastrointestinal tract  (GI tract). This starts at the mouth and ends at the  anus , covering a distance of about nine metres (30 ft). [ 1 ]


A major digestive organ is the  stomach . Within its  mucosa  are millions of embedded  gastric glands . Their secretions are vital to the functioning of the organ.


Most of the digestion of food takes place in the  small intestine  which is the longest part of the GI tract.


The largest part of the GI tract is the colon or  large intestine . Water is absorbed here and the remaining waste matter is stored prior to  defecation . [ 2 ]


There are many specialised  cells  of the GI tract. These include the various cells of the gastric glands,  taste cells ,  pancreatic duct cells ,  enterocytes  and  microfold cells .


Some parts of the digestive system are also part of the  excretory system , including the large intestine. [ 2 ]


The  mouth  is the first part of the  upper gastrointestinal tract  and is equipped with several structures that begin the first processes of digestion. [ 3 ]  These include salivary glands, teeth and the tongue. The mouth consists of two regions; the vestibule and the oral cavity proper. The vestibule is the area between the teeth, lips and cheeks, [ 4 ]  and the rest is the oral cavity proper. Most of the oral cavity is lined with  oral mucosa , a  mucous membrane  that produces a lubricating  mucus , of which only a small amount is needed. Mucous membranes vary in structure in the different regions of the body but they all produce a lubricating mucus, which is either secreted by surface cells or more usually by underlying glands. The mucous membrane in the mouth continues as the thin mucosa which lines the bases of the teeth. The main component of mucus is a  glycoprotein  called  mucin  and the type secreted varies according to the region involved. Mucin is  viscous, clear, and clinging. Underlying the mucous membrane in the mouth is a thin layer of  smooth muscle tissue  and the loose connection to the membrane gives it its great elasticity. [ 5 ]  It covers the cheeks, inner surfaces of the  lips , and floor of the mouth, and the mucin produced is highly protective against  tooth decay . [ 6 ]


The roof of the mouth is termed the  palate  and it separates the oral cavity from the nasal cavity. The palate is hard at the front of the mouth since the overlying mucosa is covering a plate of  bone ; it is softer and more pliable at the back being made of muscle and connective tissue, and  it can move to swallow food and liquids. The  soft palate  ends at the  uvula . [ 7 ]  The surface of the  hard palate  allows for the pressure needed in eating food, to leave the nasal passage clear. [ 8 ]  The opening between the lips is termed the oral fissure, and the opening into the throat is called the  fauces . [ 9 ]


At either side of the soft palate are the  palatoglossus muscles  which also reach into regions of the tongue. These muscles raise the back of the tongue and also close both sides of the fauces to enable food to be swallowed. [ 10 ] : 1208   Mucus helps in the mastication of food in its ability to soften and collect the food in the formation of the bolus.


There are three pairs of main  salivary glands  and between 800 and 1,000 minor salivary glands, all of which mainly serve the digestive process, and also play an important role in the maintenance of dental health and general mouth lubrication, without which speech would be impossible. [ 11 ]  The main glands are all  exocrine glands , secreting via ducts. All of these glands terminate in the mouth. The largest of these are the  parotid glands —their secretion is mainly  serous . The next pair are underneath the jaw, the  submandibular glands , these produce both serous fluid and mucus. The serous fluid is produced by  serous glands  in these salivary glands which also produce  lingual lipase . They produce about 70% of the oral cavity saliva. The third pair are the  sublingual glands  located underneath the tongue and their secretion is mainly mucous with a small percentage of saliva.


Within the oral mucosa, and also on the tongue, palates, and floor of the mouth, are the minor salivary glands; their secretions are mainly mucous and they are innervated by the  facial nerve  ( CN7 ). [ 12 ]  The glands also secrete  amylase  a first stage in the breakdown of food acting on the carbohydrate in the food to transform the starch content into maltose. There are other serous glands on the surface of the tongue that encircle  taste buds  on the back part of the tongue and these also produce lingual  lipase . Lipase is a  digestive enzyme  that catalyses the  hydrolysis  of  lipids  (fats). These glands are termed  Von Ebner's glands  which have also been shown to have another function in the secretion of  histatins  which offer an early defense (outside of the immune system) against microbes in food, when it makes contact with these glands on the tongue tissue. [ 11 ] [ 13 ]  Sensory information can stimulate the secretion of saliva providing the necessary fluid for the tongue to work with and also to ease swallowing of the food.


Saliva  moistens and softens food, and along with the chewing action of the teeth, transforms the food into a smooth  bolus . The bolus is further helped by the lubrication provided by the saliva in its passage from the mouth into the esophagus. Also of importance is the presence in saliva of the digestive enzymes amylase and lipase. Amylase starts to work on the  starch  in  carbohydrates , breaking it down into the simple  sugars  of  maltose  and  dextrose  that can be further broken down in the small intestine. Saliva in the mouth can account for 30% of this initial starch digestion. Lipase starts to work on breaking down  fats . Lipase is further produced in the pancreas where it is released to continue this digestion of fats. The presence of salivary lipase is of prime importance in young babies whose pancreatic lipase has yet to be developed. [ 14 ]


As well as its role in supplying digestive enzymes, saliva has a cleansing action for the teeth and mouth. [ 15 ]  It also has an  immunological  role in supplying antibodies to the system, such as  immunoglobulin A . [ 16 ]  This is seen to be key in preventing  infections  of the salivary glands, importantly that of  parotitis .


Saliva also contains a glycoprotein called  haptocorrin  which is a binding protein to vitamin B 12 . [ 17 ]  It binds with the vitamin in order to carry it safely through the acidic content of the stomach. When it reaches the duodenum, pancreatic enzymes break down the glycoprotein and free the vitamin which then binds with  intrinsic factor .


Food enters the mouth where the first stage in the digestive process takes place, with the action of the  tongue  and the secretion of saliva. The tongue is a fleshy and  muscular   sensory organ , and the first sensory information is received via the taste buds in the  papillae  on its surface. If the taste is agreeable, the tongue will go into action, manipulating the food in the mouth which stimulates the secretion of saliva from the salivary glands. The liquid quality of the saliva will help in the softening of the food and its enzyme content will start to break down the food whilst it is still in the mouth. The first part of the food to be broken down is the starch of carbohydrates (by the enzyme amylase in the saliva).


The tongue is attached to the floor of the mouth by a ligamentous band called the  frenum [ 5 ]  and this gives it great mobility for the manipulation of food (and  speech ); the range of manipulation is optimally controlled by the action of several muscles and limited in its external range by the stretch of the frenum. The tongue's two sets of muscles, are four  intrinsic muscles  that originate in the tongue and are involved with its shaping, and four  extrinsic muscles  originating in bone that are involved with its movement.


Taste  is a form of  chemoreception  that takes place in the specialised  taste receptors , contained in structures called  taste buds  in the mouth. Taste buds are mainly on the upper surface (dorsum) of the tongue. The function of taste perception is vital to help prevent harmful or rotten foods from being consumed. There are also taste buds on the  epiglottis  and upper part of the  esophagus . The taste buds are innervated by a branch of the facial nerve the  chorda tympani , and the  glossopharyngeal nerve . Taste messages are sent via these  cranial nerves  to the  brain . The brain can distinguish between the chemical qualities of the food. The five  basic tastes  are referred to as those of  saltiness ,  sourness ,  bitterness ,  sweetness , and   umami . The detection of saltiness and sourness enables the control of salt and acid balance. The detection of bitterness warns of poisons—many of a plant's defences are of poisonous compounds that are bitter. Sweetness guides to those foods that will supply energy; the initial breakdown of the energy-giving carbohydrates by salivary amylase creates the taste of sweetness since simple sugars are the first result. The taste of umami is thought to signal protein-rich food. Sour tastes are acidic which is often found in bad food. The brain has to decide very quickly whether the food should be eaten or not. It was the findings in 1991, describing the first  olfactory  receptors that helped to prompt the research into taste. The olfactory receptors are located on cell surfaces in the  nose  which bind to chemicals enabling the detection of smells. It is assumed that signals from taste receptors work together with those from the nose, to form an idea of complex food flavours. [ 18 ]


Teeth  are complex structures made of materials specific to them. They are made of a bone-like material called  dentin , which is covered by the hardest tissue in the body— enamel . [ 8 ]  Teeth have different shapes to deal with different aspects of  mastication  employed in tearing and chewing pieces of food into smaller and smaller pieces. This results in a much larger surface area for the action of digestive enzymes.
The teeth are named after their particular roles in the process of mastication— incisors  are used for cutting or biting off pieces of food;  canines , are used for tearing,  premolars  and  molars  are used for chewing and grinding. Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be  swallowed  to make its way down the  upper gastrointestinal tract  to the stomach. [ 19 ]
The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles. [ 20 ] [ 15 ]


The  epiglottis  is a flap of  elastic cartilage  attached to the entrance of the  larynx . It is covered with a mucous membrane and there are taste buds on its lingual surface which faces into the mouth. [ 21 ]  Its laryngeal surface faces into the larynx. The epiglottis functions to guard the entrance of the  glottis , the opening between the  vocal folds . It is normally pointed upward during breathing with its underside functioning as part of the pharynx, but during swallowing, the epiglottis folds down to a more horizontal position, with its upper side functioning as part of the pharynx. In this manner it prevents food from going into the trachea and instead directs it to the esophagus, which is behind. During swallowing, the backward motion of the tongue forces the epiglottis over the glottis' opening to prevent any food that is being swallowed from entering the larynx which leads to the lungs; the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter produces a strong  cough reflex  in order to protect the lungs.


The  pharynx  is a part of the  conducting zone  of the  respiratory system  and also a part of the digestive system. It is the part of the throat immediately behind the  nasal cavity  at the back of the mouth and above the esophagus and  larynx . The pharynx is made up of three parts. The lower two parts—the  oropharynx  and the  laryngopharynx  are involved in the digestive system. The laryngopharynx connects to the esophagus and it serves as a passageway for both air and food. Air enters the larynx anteriorly but anything swallowed has priority and the passage of air is temporarily blocked. The pharynx is innervated by the  pharyngeal plexus of the vagus nerve . [ 10 ] : 1465    Muscles in the pharynx  push the food into the esophagus. The pharynx joins the esophagus at the oesophageal inlet which is located behind the  cricoid cartilage .


The  esophagus , commonly known as the foodpipe or gullet, consists of a muscular tube through which food passes from the pharynx to the stomach. The esophagus is continuous with the laryngopharynx. It passes through the posterior  mediastinum  in the  thorax  and enters the  stomach  through a hole in the  thoracic diaphragm —the  esophageal hiatus , at the level of the tenth  thoracic vertebra  (T10). Its length averages 25 cm, varying with an individual's height. It is divided into cervical,  thoracic  and  abdominal  parts. The pharynx joins the esophagus at the esophageal inlet which is behind the  cricoid cartilage .


At rest the esophagus is closed at both ends, by the  upper and lower esophageal sphincters . The opening of the upper sphincter is triggered by the  swallowing reflex  so that food is allowed through. The sphincter also serves to prevent back flow from the esophagus into the pharynx. The esophagus has a mucous membrane and the epithelium which has a protective function is continuously replaced due to the volume of food that passes inside the esophagus. During swallowing, food passes from the mouth through the pharynx into the esophagus. The epiglottis folds down to a more horizontal position to direct the food into the esophagus, and away from the  trachea .


Once in the esophagus, the bolus travels down to the stomach via rhythmic contraction and relaxation of muscles known as  peristalsis . The lower esophageal sphincter is a muscular sphincter surrounding the lower part of the esophagus. The  gastroesophageal junction  between the esophagus and the stomach is controlled by the lower esophageal sphincter, which remains constricted at all times other than during swallowing and vomiting to prevent the contents of the stomach from entering the esophagus. As the esophagus does not have the same protection from acid as the stomach, any failure of this sphincter can lead to heartburn.


The  diaphragm  is an important part of the body's digestive system. The muscular diaphragm separates the  thoracic cavity  from the  abdominal cavity  where most of the digestive organs are located. The  suspensory muscle  attaches the ascending duodenum to the diaphragm. This muscle is thought to be of help in the digestive system in that its attachment offers a wider angle to the  duodenojejunal flexure  for the easier passage of digesting material. The diaphragm also attaches to, and anchors the liver at its  bare area . The esophagus enters the abdomen through a  hole in the diaphragm  at the level of  T10 .


The  stomach  is a major organ of the gastrointestinal tract and digestive system. It is a consistently J-shaped organ joined to the esophagus at its upper end and to the duodenum at its lower end.
 Gastric acid  (informally  gastric juice ), produced in the stomach plays a vital role in the digestive process, and mainly contains  hydrochloric acid  and  sodium chloride . A  peptide hormone ,  gastrin ,  produced by  G cells  in the  gastric glands , stimulates the production of gastric juice which activates the digestive enzymes.  Pepsinogen  is a precursor enzyme ( zymogen ) produced by the  gastric chief cells , and gastric acid activates this to the enzyme  pepsin  which begins the digestion of  proteins . As these two chemicals would damage the stomach wall, mucus is secreted by innumerable gastric glands in the stomach, to provide a slimy protective layer against the damaging effects of the chemicals on the inner layers of the stomach.


At the same time that protein is being digested, mechanical churning occurs through the action of peristalsis, waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes.  Gastric lipase  secreted by the chief cells in the fundic glands in the gastric mucosa of the stomach, is an acidic lipase, in contrast with the alkaline pancreatic lipase. This breaks down fats to some degree though is not as efficient as the pancreatic lipase.


The  pylorus , the lowest section of the stomach which attaches to the  duodenum  via the  pyloric canal , contains countless glands which secrete digestive enzymes including gastrin. After an hour or two, a thick semi-liquid called  chyme  is produced. When the  pyloric sphincter , or valve opens, chyme enters the duodenum where it mixes further with digestive enzymes from the pancreas, and then passes through the small intestine, where digestion continues.


The  parietal cells  in the fundus of the stomach, produce a glycoprotein called intrinsic factor which is essential for the absorption of  vitamin B12 . Vitamin B12 (cobalamin), is carried to, and through the stomach, bound to a glycoprotein secreted by the salivary glands –  transcobalamin I  also called haptocorrin, which protects the acid-sensitive vitamin from the acidic stomach contents. Once in the more neutral duodenum, pancreatic enzymes break down the protective glycoprotein. The freed vitamin B12 then binds to intrinsic factor which is then absorbed by the enterocytes in the ileum.


The stomach is a distensible organ and can normally expand to hold about one litre of food. [ 22 ]  This expansion is enabled by a series of  gastric folds  in the inner walls of the stomach. The stomach of a newborn baby will only be able to expand to retain about 30 ml.


The  spleen  is the largest lymphoid organ in the body but has other functions. [ 23 ]  It breaks down both red and white  blood cells  that are  spent . This is why it is sometimes known as the 'graveyard of red blood cells'. [ 23 ]  A product of this  digestion  is the pigment  bilirubin , which is sent to the  liver  and secreted in the  bile . Another product is  iron , which is used in the formation of new blood cells in the  bone marrow . [ 5 ]   Medicine  treats the spleen solely as belonging to the  lymphatic system , though it is acknowledged that the full range of its important functions is not yet understood. [ 10 ] : 1751


The  liver  is the second largest organ (after the  skin ) and is an accessory digestive gland which plays a role in the body's  metabolism . The liver has many functions some of which are important to digestion. The liver can detoxify various  metabolites ; synthesise proteins and produce  biochemicals  needed for digestion. It regulates the storage of  glycogen  which it can form from  glucose  ( glycogenesis ). The liver can also synthesise glucose from certain  amino acids . Its digestive functions are largely involved with the breaking down of carbohydrates. It also maintains protein metabolism in its synthesis and degradation. In lipid metabolism it synthesises  cholesterol . Fats are also produced in the process of  lipogenesis . The liver synthesises the bulk of lipoproteins. The liver is located in the upper right quadrant of the abdomen and below the diaphragm to which it is attached at one part, the bare area of the liver. This is to the right of the stomach and it overlies the gall bladder. The liver synthesises  bile acids  and  lecithin  to promote the digestion of fat. [ 24 ]


Bile  produced by the liver is made up of water (97%),  bile salts , mucus and  pigments , 1% fats and inorganic salts. [ 25 ]   Bilirubin  is its major pigment. Bile acts partly as a  surfactant  which lowers the surface tension between either two liquids or a solid and a liquid and helps to  emulsify  the fats in the chyme. Food fat is dispersed by the action of bile into smaller units called  micelles . The breaking down into micelles creates a much larger surface area for the pancreatic enzyme, lipase to work on. Lipase digests the  triglycerides  which are broken down into two  fatty acids  and a  monoglyceride . These are then absorbed by  villi  on the intestinal wall. If fats are not absorbed in this way in the small intestine problems can arise later in the large intestine which is not equipped to absorb fats. Bile also helps in the absorption of  vitamin K  from the diet.
Bile is collected and delivered through the  common hepatic duct . This duct joins with the  cystic duct  to connect in a  common bile duct  with the gallbladder.
Bile is stored in the gallbladder for release when food is discharged into the duodenum and also after a few hours. [ 5 ]


The  gallbladder  is a hollow part of the  biliary tract  that sits just beneath the liver, with the gallbladder body resting in a small depression. [ 26 ]  It is a small organ where the bile produced by the liver is stored, before being released into the small intestine. Bile flows from the liver through the  bile ducts  and into the gall bladder for storage. The bile is released in response to  cholecystokinin  (CCK), a peptide hormone released from the duodenum. The production of CCK (by endocrine cells of the duodenum) is stimulated by the presence of fat in the duodenum. [ 27 ]


It is divided into three sections, a fundus, body and neck. The neck tapers and connects to the biliary tract via the  cystic duct , which then joins the common hepatic duct to form the common bile duct. At this junction is a mucosal fold called  Hartmann's pouch , where  gallstones  commonly get stuck. The  muscular layer  of the body is of smooth muscle tissue that helps the gallbladder contract, so that it can discharge its bile into the bile duct. The gallbladder needs to store bile in a natural, semi-liquid form at all times.  Hydrogen ions  secreted from the inner lining of the gallbladder keep the bile acidic enough to prevent hardening. To dilute the bile, water and  electrolytes  from the digestion system are added. Also, salts attach themselves to cholesterol molecules in the bile to keep them from  crystallising . If there is too much cholesterol or bilirubin in the bile, or if the gallbladder does not empty properly the systems can fail. This is how gallstones form when a small piece of calcium gets coated with either cholesterol or bilirubin and the bile crystallises and forms a gallstone. The main purpose of the gallbladder is to store and release bile, or  gall . Bile is released into the small intestine in order to help in the digestion of fats by breaking down larger molecules into smaller ones. After the fat is absorbed, the bile is also absorbed and transported back to the liver for reuse.


The  pancreas  is a major organ functioning as an accessory digestive gland in the digestive system. It is both an  endocrine gland  and an  exocrine gland . [ 28 ]  The endocrine part secretes  insulin  when the  blood sugar  becomes high; insulin moves glucose from the blood into the muscles and other tissues for use as energy. The endocrine part releases  glucagon  when the blood sugar is low; glucagon allows stored sugar to be broken down into glucose by the liver in order to re-balance the sugar levels. The pancreas produces and releases important digestive enzymes in the  pancreatic juice  that it delivers to the duodenum. [ 24 ]  The pancreas lies below and at the back of the stomach. It connects to the duodenum via the  pancreatic duct  which it joins near to the bile duct's connection where both the bile and pancreatic juice can act on the chyme that is released from the stomach into the duodenum. Aqueous pancreatic secretions from  pancreatic duct cells  contain  bicarbonate  ions which are alkaline and help with the bile to neutralise the acidic chyme that is churned out by the stomach.


The pancreas is also the main source of enzymes for the digestion of fats and proteins. Some of these are released in response to the production of cholecystokinin in the duodenum. (The enzymes that digest polysaccharides, by contrast, are primarily produced by the walls of the intestines.) The cells are filled with secretory granules containing the precursor digestive enzymes. The major  proteases , the pancreatic enzymes which work on proteins, are  trypsinogen  and  chymotrypsinogen .  Elastase  is also produced. Smaller amounts of lipase and amylase are secreted. The pancreas also secretes  phospholipase A2 ,  lysophospholipase , and cholesterol  esterase . The precursor  zymogens , are inactive variants of the enzymes; which avoids the onset of  pancreatitis  caused by autodegradation. Once released in the intestine, the enzyme  enteropeptidase  present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin; further cleavage results in chymotripsin.


The lower gastrointestinal tract (GI), includes the  small intestine  and all of the  large intestine . [ 29 ]  The intestine is also called the bowel or the gut. The lower GI starts at the pyloric sphincter of the stomach and finishes at the anus. The small intestine is subdivided into the  duodenum , the  jejunum  and the  ileum . The cecum marks the division between the small and large intestine. The large intestine includes the rectum and  anal canal . [ 2 ]


Partially digested food starts to arrive in the  small intestine  as semi-liquid  chyme , one hour after it is eaten. [ citation needed ]  The stomach is half empty after an average of 1.2 hours. [ 30 ]  After four or five hours the stomach has emptied. [ 31 ]


In the small intestine, the  pH  becomes crucial; it needs to be finely balanced in order to activate digestive enzymes. The chyme is very acidic, with a low pH, having been released from the stomach and needs to be made much more alkaline. This is achieved in the  duodenum  by the addition of bile from the gall bladder combined with the  bicarbonate  secretions from the pancreatic duct and also from secretions of bicarbonate-rich mucus from duodenal glands known as  Brunner's glands . The chyme arrives in the intestines having been released from the stomach through the opening of the  pyloric sphincter . The resulting alkaline fluid mix neutralises the gastric acid which would damage the lining of the intestine. The mucus component lubricates the walls of the intestine.


When the digested food particles are reduced enough in size and composition, they can be absorbed by the intestinal wall and carried to the bloodstream. The first receptacle for this chyme is the  duodenal bulb . From here it passes into the first of the three sections of the small intestine, the duodenum (the next section is the  jejunum  and the third is the  ileum ). The duodenum is the first and shortest section of the small intestine. It is a hollow, jointed C-shaped tube connecting the stomach to the jejunum. It starts at the duodenal bulb and ends at the  suspensory muscle of duodenum . The attachment of the suspensory muscle to the diaphragm is thought to help the passage of food by making a wider angle at its attachment.


Most food digestion takes place in the small intestine.  Segmentation contractions  act to mix and move the chyme more slowly in the small intestine allowing more time for absorption (and these continue in the large intestine). In the duodenum, pancreatic lipase is secreted together with a  co-enzyme ,  colipase  to further digest the fat content of the chyme. From this breakdown, smaller particles of emulsified fats called  chylomicrons  are produced. There are also digestive cells called  enterocytes  lining the intestines (the majority being in the small intestine). They are unusual cells in that they have  villi  on their surface which in turn have innumerable  microvilli  on their surface. All these villi make for a greater surface area, not only for the absorption of chyme but also for its further digestion by large numbers of digestive enzymes present on the microvilli.


The chylomicrons are small enough to pass through the enterocyte villi and into their  lymph  capillaries called  lacteals . A milky fluid called  chyle , consisting mainly of the emulsified fats of the chylomicrons, results from the absorbed mix with the lymph in the lacteals. [ clarification needed ]  Chyle is then transported through the  lymphatic system  to the rest of the body.


The suspensory muscle marks the end of the duodenum and the division between the upper gastrointestinal tract and the lower GI tract. The digestive tract continues as the jejunum which continues as the ileum. The jejunum, the midsection of the small intestine contains  circular folds , flaps of doubled mucosal membrane which partially encircle and sometimes completely encircle the  lumen  of the intestine. These folds together with villi serve to increase the surface area of the jejunum enabling an increased absorption of digested sugars, amino acids and fatty acids into the bloodstream. The circular folds also slow the passage of food giving more time for nutrients to be absorbed.


The last part of the small intestine is the ileum. This also contains villi and  vitamin B12 ; bile acids and any residue nutrients are absorbed here. When the chyme is exhausted of its nutrients the remaining waste material changes into the semi-solids called  feces , which pass to the large intestine, where bacteria in the  gut flora  further break down residual proteins and starches. [ 32 ]


Transit time through the small intestine is an average of 4 hours. Half of the food residues of a meal have emptied from the small intestine by an average of 5.4 hours after ingestion. Emptying of the small intestine is complete after an average of 8.6 hours. [ 30 ]


The  cecum  is a pouch marking the division between the small intestine and the large intestine. It lies below the  ileocecal valve  in the  lower right quadrant  of the abdomen. [ 33 ]  The cecum receives chyme from the last part of the small intestine, the  ileum , and connects to the  ascending colon  of the large intestine. At this junction there is a sphincter or valve, the ileocecal valve which slows the passage of chyme from the ileum, allowing further digestion. It is also the site of the  appendix  attachment. [ 33 ]


In the  large intestine , [ 2 ]  the passage of the digesting food in the  colon  is a lot slower, taking from 30 to 40 hours until it is removed by  defecation . [ 31 ]  The colon mainly serves as a site for the fermentation of digestible matter by the  gut flora . The time taken varies considerably between individuals. The remaining semi-solid waste is termed  feces  and is removed by the coordinated contractions of the intestinal walls, termed  peristalsis , which propels the  excreta  forward to reach the  rectum  and exit through the  anus  via defecation. The wall has an outer layer of longitudinal muscles, the  taeniae coli , and an inner layer of circular muscles. The circular muscle keeps the material moving forward and also prevents any back flow of waste. Also of help in the action of peristalsis is the  basal electrical rhythm  that determines the frequency of contractions. [ 34 ]  The taeniae coli can be seen and are responsible for the bulges ( haustra ) present in the colon. Most parts of the GI tract are covered with  serous membranes  and have a  mesentery . Other more muscular parts are lined with  adventitia .


The digestive system is supplied by the  celiac artery . The celiac artery is the first major branch from the  abdominal aorta , and is the only major artery that nourishes the digestive organs.


There are three main divisions – the  left gastric artery , the  common hepatic artery  and the  splenic artery .


The celiac artery supplies the liver, stomach, spleen and the upper 1/3 of the duodenum (to the  sphincter of Oddi ) and the pancreas with oxygenated blood. Most of the blood is returned to the liver via the  portal venous system  for further processing and detoxification before returning to the  systemic circulation  via the  hepatic veins .


The next branch from the abdominal aorta is the  superior mesenteric artery , which supplies the regions of the digestive tract derived from the midgut, which includes the distal 2/3 of the duodenum, jejunum, ileum, cecum, appendix, ascending colon, and the proximal 2/3 of the transverse colon.


The final branch which is important for the digestive system is the  inferior mesenteric artery , which supplies the regions of the digestive tract derived from the hindgut, which includes the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum, and the anus above the  pectinate line .


Blood flow to the digestive tract reaches its maximum 20–40 minutes after a meal and lasts for 1.5–2 hours. [ 35 ]


The  enteric nervous system  consists of some one hundred million  neurons [ 36 ]  that are embedded in the  peritoneum , the lining of the  gastrointestinal tract  extending from the esophagus to the anus. [ 37 ]  These neurons are collected into two  plexuses  – the  myenteric (or Auerbach's) plexus  that lies between the longitudinal and the smooth muscle layers, and the  submucosal (or Meissner's) plexus  that lies between the circular smooth muscle layer and the mucosa. [ 38 ] [ 39 ] [ 40 ]


Parasympathetic innervation  to the  ascending colon  is supplied by the  vagus nerve .  Sympathetic innervation  is supplied by the  splanchnic nerves  that join the  celiac ganglia . Most of the digestive tract is innervated by the two large celiac ganglia, with the upper part of each ganglion joined by the  greater splanchnic nerve  and the lower parts joined by the  lesser splanchnic nerve . It is from these ganglia that many of the  gastric plexuses  arise.


Early in  embryonic development , the  embryo  has three  germ layers  and abuts a  yolk sac . During the second week of development, the embryo grows and begins to surround and envelop portions of this sac. The enveloped portions form the basis for the adult gastrointestinal tract. Sections of this  foregut  begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach, and intestines. [ 41 ]


During the fourth week of development, the stomach rotates. The stomach, originally lying in the midline of the embryo, rotates so that its body is on the left. This rotation also affects the part of the gastrointestinal tube immediately below the stomach, which will go on to become the duodenum. By the end of the fourth week, the developing duodenum begins to spout a small outpouching on its right side, the  hepatic diverticulum , which will go on to become the  biliary tree . Just below this is a second outpouching, known as the  cystic diverticulum , that will eventually develop into the gallbladder. [ 41 ]


Each part of the digestive system is subject to a wide range of disorders many of which can be  congenital .  Mouth diseases  can also be caused by  pathogenic   bacteria ,  viruses ,  fungi  and as a side effect of some  medications . Mouth diseases include  tongue diseases  and  salivary gland diseases . A common  gum  disease in the mouth is  gingivitis  which is caused by bacteria in  plaque . The most common viral infection of the mouth is  gingivostomatitis  caused by  herpes simplex . A common  fungal  infection is  candidiasis  commonly known as  thrush  which affects the  mucous membranes  of the mouth.


There are a number of  esophageal diseases  such as the development of  Schatzki rings  that can restrict the passageway, causing difficulties in swallowing. They can also completely block the esophagus. [ 42 ]


Stomach diseases  are often chronic conditions and include  gastroparesis ,  gastritis , and  peptic ulcers .


A number of problems including  malnutrition  and  anemia  can arise from  malabsorption , the abnormal absorption of nutrients in the GI tract. Malabsorption can have many causes ranging from  infection , to enzyme deficiencies such as  exocrine pancreatic insufficiency . It can also arise as a result of other gastrointestinal diseases such as  coeliac disease . Coeliac disease is an  autoimmune  disorder of the small intestine. This can cause  vitamin deficiencies  due to the improper absorption of nutrients in the small intestine. The  small intestine can also be  obstructed  by a  volvulus , a loop of intestine that becomes twisted enclosing its attached  mesentery . This can cause  mesenteric ischemia  if severe enough.


A common disorder of the bowel is  diverticulitis .  Diverticula  are small pouches that can form inside the bowel wall, which can become inflamed to give diverticulitis. This disease can have complications if an inflamed diverticulum bursts and infection sets in. Any infection can spread further to the lining of the abdomen ( peritoneum ) and cause potentially fatal  peritonitis . [ 43 ]


Crohn's disease  is a common chronic  inflammatory bowel disease  (IBD), which can affect any part of the GI tract, [ 44 ]  but it mostly starts in the  terminal ileum .


Ulcerative colitis , an ulcerative form of  colitis , is the other major inflammatory bowel disease which is restricted to the colon and rectum. Both of these IBDs can give an increased risk of the development of  colorectal cancer . Ulcerative colitis is the most common of the IBDs [ 45 ]


Irritable bowel syndrome  (IBS) is the most common of the  functional gastrointestinal disorders . These are  idiopathic  disorders that the  Rome process  has helped to define. [ 46 ]


Giardiasis  is a disease of the small intestine caused by a  protist   parasite   Giardia lamblia . This does not spread but remains confined to the lumen of the small intestine. [ 47 ]  It can often be  asymptomatic , but as often can be indicated by a variety of symptoms. Giardiasis is the most common  pathogenic   parasitic infection  in humans. [ 48 ]


There are diagnostic tools mostly involving the ingestion of  barium sulphate  to investigate disorders of the GI tract. [ 49 ]  These are known as  upper gastrointestinal series  that enable  imaging  of the pharynx, larynx, oesophagus, stomach and small intestine [ 50 ]  and  lower gastrointestinal series  for imaging of the colon.


Gestation  can predispose for certain digestive disorders.  Gestational diabetes  can develop in the mother as a result of  pregnancy  and while this often presents with few symptoms it can lead to  pre-eclampsia . [ 51 ]


In the early 11th century, the Islamic medical philosopher  Avicenna  wrote extensively on many subjects including medicine. Forty of these treatises on medicine survive, and in the most famous one titled the  Canon of Medicine  he discusses "rising gas". Avicenna believed that digestive system dysfunction was responsible for the overproduction of gas in the gastrointestinal tract. He suggested lifestyle changes and a compound of herbal drugs for its treatment. [ 52 ]


In 1497,  Alessandro Benedetti  viewed the stomach as an unclean organ separated off by the diaphragm. This view of the stomach and intestines as being base organs was generally held until the mid-17th century. [ 53 ]


In the Renaissance of the 16th century,  Leonardo da Vinci  produced some early drawings of the stomach and intestines. He thought that the digestive system aided the respiratory system. [ 53 ]   Andreas Vesalius  provided some early anatomical drawings of the abdominal organs in the 16th century.


In the middle of the 17th century, a Flemish physician  Jan Baptist van Helmont  offered the first  chemical account of digestion  which was later described as being very close to the later conceptualised enzyme. [ 53 ]


In 1653,  William Harvey  described the intestines in terms of their length, their blood supply, the mesenteries, and fat (adenylyl cyclase). [ 53 ]


In 1823,  William Prout  discovered  hydrochloric acid  in the gastric juice. [ 54 ]  In 1895,  Ivan Pavlov  described its secretion as being stimulated by a neurologic reflex with the  vagus nerve  having a crucial role. Black in the 19th century suggested an association of histamine with this secretion. In 1916, Popielski described histamine as a gastric secretagogue of hydrochloric acid.


William Beaumont  was an army surgeon who in 1825, was able to observe digestion as it took place in the stomach. [ 55 ]  This was made possible by experiments on a man with a stomach wound that did not fully heal leaving an opening into the stomach. The churning motion of the stomach was described among other findings. [ 53 ]


In the 19th century, it was accepted that chemical processes were involved in the process of digestion.  Physiological research  into secretion and the gastrointestinal tract was pursued with experiments undertaken by Claude Bernard, Rudolph Heidenhain and Ivan Pavlov.


The rest of the 20th century was dominated by research into enzymes. The first to be discovered was  secretin  by  Ernest Starling  in 1902, with ensuing results from John Edkins in 1905 who first suggested  gastrin  with its structure being determined in 1964. [ 54 ]  Andre Latarjet and Lester Dragstedt found a role for  acetylcholine  in the digestive system. [ 54 ]  In 1972,  H2 receptor agonists  were described by J. Black, that block the action of histamine and decrease the production of hydrochloric acid. In 1980,  proton pump inhibitors  were described by Sachs. In 1983, the role of  Helicobacter pylori  in the formation of ulcers was described by  Barry Marshall , and  Robin Warren . [ 56 ]


Art historians have often noted that  banqueters  on iconographic records of ancient Mediterranean societies almost always appear to be lying down on their left sides. One possible explanation could lie in the anatomy of the stomach and in the digestive mechanism. When lying on the left, the food has room to expand because the curvature of the stomach is enhanced in that position. [ 57 ]


The  abomasum , also known as the  maw , [ 1 ]   rennet-bag , [ 1 ]  or  reed tripe , [ 1 ]  is the fourth and final  stomach  compartment in  ruminants . It secretes  rennet , which is used in  cheese  creation.


The word  abomasum  ( ab-  "away from" +  omasum  " intestine  of an  ox ") is from  Neo-Latin  and it was first used in  English  in 1706. It is possibly from the  Gaulish language .


The abomasum's normal anatomical location is along the ventral midline. It is a  secretory  stomach similar in  anatomy  and function to the  monogastric  stomach. It serves primarily in the  acid   hydrolysis  of  microbial  and dietary  protein , preparing these protein sources for further  digestion  and  absorption  in the  small intestine . The abomasum is lined with glands to release  hydrochloric acid  and digestive enzymes needed to break down food. It is very similar to the nonruminant stomach.


Dairy cattle on high production diets are susceptible to a number of pathologies, most commonly after calving. A gas-filled abomasum can move into an abnormal location and become  displaced . If the abomasum displaces to the right, it is at risk of torsion. A displaced abomasum will cause cows to present all or some of the following signs: loss of appetite, decrease rumen contractions, decrease cud-chewing, and drop in milk production. While a displaced abomasum is not immediately life-threatening, veterinary care is required for surgical correction. Milder cases can be corrected by rolling the cow over or forcing her to run up a steep hill. [ 2 ]   Abomasitis  is a relatively rare, but serious, disease of the abomasum whose causes are currently unknown.


The abomasum is used to make the  lampredotto , a typical dish of  Florence . It is also fried and eaten with onions as part of the Korean dish  Makchang gui . Another dish made with the abomasum is the Persian  Sirabi-Shirdan  (borrowed in Turkey as  Şırdan ). Chusta (literally abomasum) is a famous dish in Bihar, India. In Japan, it is grilled and served as  horumonyaki , this particular type of horumon is known as Akasen.


Accessory pancreas  is a rare condition in which small groups of pancreatic cells are separate from the  pancreas . They may occur in the  mesentery  of the  small intestine , the wall of the  duodenum , the upper part of the  jejunum , or more rarely, in the wall of the  stomach ,  ileum ,  gallbladder  or  spleen . The condition was first described by Klob in 1859. [ 1 ]


Accessory pancreas is a small cluster of pancreas cells detached from the pancreas and sometimes found in the wall of the stomach or intestines.


Pancreatic disorders are often accompanied by weakness and fatigue. The past Medical history may reveal previous disorders of the  biliary tract  or  duodenum , abdominal trauma or surgery, and metabolic disorders such as  diabetes mellitus . The medication history should be detailed and specifically include the use of  thiazides ,  furosemide ,  estrogens ,  corticosteroids , sulfonamides, and opiates. Note a family history of pancreatic disorders. In the review of systems, obtain a complete description of any pain in the upper abdomen or  epigastric  area. Symptoms that may be important in relation to pancreatic disorders are  pruritus , abdominal pain,  dyspnea , nausea, and vomiting. The functional assessment includes data about the patient's dietary habits and use of alcohol.


Note any restlessness, flushing, or  diaphoresis  during the examination. Vital signs may disclose low-grade fever,  tachypnea ,  tachycardia , and hypotension. Inspect the skin for jaundice. Assess the abdomen for distention, tenderness, discoloration, and diminished bowel sounds.


Tests and procedures used to diagnose pancreatic disorders include laboratory analyses of blood, urine, stool, and pancreatic fluid, and imaging studies. Specific blood studies used to assess pancreatic function include measurements of serum amylase, lipase, glucose, calcium, and  triglyceride levels . Urine amylase and renal amylase clearance tests may also be ordered. Stool specimens may be analyzed for fat content. The secretin stimulation test measures the bicarbonate concentration of pancreatic fluid after secretin is given intravenously to stimulate the production of pancreatic fluid.


Treatment of accessory pancreas depends on the location and extent of the injured tissue. Surgery may be an option, or some physicians order  prophylactic antibiotics .


Alkaline mucus  is a thick fluid produced by animals which confers tissue protection in an  acidic  environment, such as in the  stomach . [ 1 ]


Mucus  that serves a protective function against acidic environments generally has a high  viscosity , though the thickness and viscosity of the mucus layer can vary due to several factors. For example, alkaline mucus in the stomach increases in thickness when the stomach is distended. [ 2 ]  The  pH  level of the mucus also plays a role in its viscosity, as higher pH levels tend to alter the thickness of the mucus, making it less viscous. [ 1 ]  Because of this, invading agents such as  Helicobacter pylori , a bacterium that causes stomach ulcers, can alter the pH of the mucus to make the mucus pliable enough to move through. [ 3 ]  Exposure to atmospheric air also tends to increase the pH level of alkaline mucus. [ 4 ]


In humans, alkaline mucus is present in several organs and provides protection by way of its alkalinity and high  viscosity . Alkaline mucus exists in the human  eye ,  stomach ,  saliva , and  cervix . [ 5 ]


In the stomach, alkaline mucus is secreted by gastric glands in the  gastric mucosa  of the stomach wall. [ 6 ]  Secretion of alkaline mucus is necessary to protect the  mucous membrane  of the stomach from acids released during digestion. [ 6 ]  Ulcers can develop as a result of damage caused to the  gastric mucosal barrier . [ 7 ]  Duodenal ulcers have been shown to develop in sites that are in direct contact with pepsin and acids. [ 8 ]  To prevent damage and protect the mucus epithelium, alkaline mucus secretions increase in the digestive system when food is being eaten. [ 6 ]


In the cervix, alkaline mucus has been shown to possess  bactericidal  properties to protect the cervix, uterus, peritoneal cavity, and vagina from microbes. [ 9 ]


Alkaline tide (mal del puerco)  refers to a condition, normally encountered after eating a meal, where during the production of  hydrochloric acid  by the  parietal cells  in the stomach, the parietal cells secrete  bicarbonate ions  across their basolateral membranes and into the  blood , causing a temporary increase in blood pH. [ 1 ]


During hydrochloric acid secretion in the stomach, the gastric  parietal cells  extract chloride anions, carbon dioxide, water and sodium cations from the blood plasma and in turn release bicarbonate back into the plasma after forming it from carbon dioxide and water constituents. This is to maintain the plasma's electrical balance, as the chloride anions have been extracted. The bicarbonate content causes the venous blood leaving the stomach to be more alkaline than the arterial blood delivered to it.


The alkaline tide is neutralised by a secretion of H +  into the blood during HCO 3 −  secretion in the pancreas. [ 2 ]


Postprandial  (i.e., after a meal) alkaline tide lasts until the acids in food absorbed in the small intestine reunite with the bicarbonate that was produced when the food was in the stomach. Thus, alkaline tide is self-limited and normally lasts less than two hours.


Postprandial alkaline tide has also been shown to be a causative agent of  calcium oxalate   urinary stones  in cats, [ 3 ]  and potentially in other species. [ 4 ]


A more pronounced alkaline tide results from  vomiting , which stimulates hyperactivity of gastric  parietal cells  to replace lost stomach acid. [ verification needed ]  Thus, protracted vomiting can result in  metabolic alkalosis . [ 5 ]


This  cardiovascular system  article is a  stub . You can help Wikipedia by  expanding it .


The  anal canal  is the part that connects the  rectum  to the  anus , located below the level of the  pelvic diaphragm . [ 1 ]  It is located within the  anal triangle  of the  perineum , between the right and left  ischioanal fossa . As the final functional segment of the  bowel , it functions to regulate release of  excrement  by two muscular  sphincter  complexes. The anus is the aperture at the terminal portion of the anal canal.


In humans, the anal canal is approximately 2.5 to 4 cm (0.98 to 1.57 in) long, from the anorectal junction to the  anus . [ 2 ] [ 3 ] [ 4 ]  It is directed downwards and backwards. It is surrounded by inner involuntary and outer voluntary sphincters which keep the  lumen  closed in the form of an anteroposterior slit.


The canal is differentiated from the rectum by a transition along the internal surface from  endodermal  to skin-like   ectodermal  tissue.


The anal canal is traditionally divided into two segments, upper and lower, separated by the  pectinate line  (also known as the dentate line):


The  anal verge  refers to the  distal  end of the anal canal, a transitional zone between the  epithelium  of the anal canal and the  perianal skin . It should not be confused with the pectinate line between the upper and lower zones within the anal canal.


The  anal gland  secretes lymphal discharge and built-up  fecal matter  from the  colon  lining. In some animals this gland expungement can be done routinely every 24–36 months to prevent infection and  fistula  formation.


The  external anal sphincter  muscle is the voluntary muscle that surrounds and adheres to the anus at the lower margin of the anal canal. This muscle is in a state of  tonic contraction , but during  defecation , it relaxes to allow the release of  feces .


Movement of the feces is also controlled by the involuntarily controlled internal anal sphincter, which is an extension of the circular muscle surrounding the anal canal. It relaxes to expel feces from the rectum and anal canal.


Anal columns  ( Columns of Morgagni  or less commonly  Morgagni's columns ) are a number of vertical folds, produced by an infolding of the  mucous membrane  and some of the muscular tissue in the upper half of the  lumen  of the  anal canal . They are named after  Giovanni Battista Morgagni , who has several other  eponyms named after him .


This article incorporates text in the  public domain  from  page 1185  of the 20th edition of   Gray's Anatomy   (1918)


An  anal sinus  ( rectal sinus ) is a furrow formed between any two adjacent  anal columns  of the  anal canal . An anal sinus is limited inferiorly by an  anal valve  (which unites the inferior ends of a pair of adjacent anal columns). The  anal glands  open into the anal sinuses. [ 1 ]


An  anal valve  is a small horizontal fold uniting the inferior ends of any two adjacent  anal columns  of the  anal canal . An anal valve forms the inferior boundary of an  anal sinus . The anal valves demarcate the level of the  pectinate line . [ 1 ]


An  anastomosis  ( / ə ˌ n æ s t ə ˈ m oʊ s ɪ s / ,  pl. :  anastomoses ) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between  blood vessels ,  leaf veins , or  streams . Such a connection may be normal (such as the  foramen ovale in a fetus' heart ) or abnormal (such as the  patent foramen ovale  in an adult's heart); it may be acquired (such as an  arteriovenous fistula ) or innate (such as the arteriovenous shunt of a  metarteriole ); and it may be natural (such as the aforementioned examples) or artificial (such as a  surgical anastomosis ). The reestablishment of an anastomosis that had become blocked is called a reanastomosis. Anastomoses that are abnormal, whether  congenital  or acquired, are often called  fistulas .


The term is used in  medicine , [ 1 ]   biology ,  mycology ,  geology , and  geography .


Anastomosis: medical or Modern Latin, from Greek ἀναστόμωσις, anastomosis, "outlet, opening", Greek ana- "up, on, upon", stoma "mouth", "to furnish with a mouth". [ 2 ]  Thus the  -stom-  syllable is cognate with that of  stoma  in botany  or  stoma  in medicine .


An anastomosis is the connection of two normally divergent structures. [ 3 ]  It refers to connections between  blood vessels  or between other tubular structures such as loops of  intestine .


In  circulatory anastomoses , many arteries naturally anastomose with each other; for example, the  inferior epigastric artery  and  superior epigastric artery , or the anterior and/or posterior communicating arteries in the  Circle of Willis  in the brain. The circulatory anastomosis is further divided into arterial and venous anastomosis. Arterial anastomosis includes actual arterial anastomosis (e.g.,  palmar arch ,  plantar arch ) and potential arterial anastomosis (e.g.  coronary arteries  and cortical branch of  cerebral arteries ). Anastomoses also form alternative routes around  capillary  beds in areas that do not need a large blood supply, thus helping regulate  systemic blood flow . [ citation needed ]


Surgical anastomosis  occurs when segments of  intestine , blood vessel, or any other structure are connected together surgically (anastomosed). Examples include arterial anastomosis in  bypass surgery , intestinal anastomosis after a piece of intestine has been resected,  Roux-en-Y anastomosis  and  ureteroureterostomy .  Surgical anastomosis techniques include linear stapled anastomosis, [ 4 ]  hand sewn anastomosis, [ 4 ]  end-to-end anastomosis (EEA). [ 5 ]  Anastomosis can be performed by hand or with an anastomosis assist device. [ 6 ]  Studies have been performed comparing various anastomosis approaches taking into account surgical "time and cost, postoperative anastomotic bleeding, leakage, and stricture". [ 7 ]


Pathological  anastomosis results from  trauma  or  disease  and may involve  veins ,  arteries , or  intestines . These are usually referred to as  fistulas . In the cases of veins or arteries, traumatic fistulas usually occur between artery and vein. Traumatic intestinal fistulas usually occur between two loops of intestine (entero-enteric fistula) or intestine and  skin  (enterocutaneous fistula).  Portacaval anastomosis , by contrast, is an anastomosis between a vein of the  portal circulation  and a vein of the  systemic circulation , which allows blood to bypass the  liver  in patients with  portal hypertension , often resulting in  hemorrhoids ,  esophageal varices , or  caput medusae . [ citation needed ]


In  evolution , anastomosis is a recombination of evolutionary lineage. Conventional accounts of evolutionary lineage present themselves as the branching out of species into novel forms.  Under anastomosis, species might recombine after initial branching out, such as in the case of recent research that shows that ancestral populations along human and chimpanzee lineages may have interbred after an initial branching event. [ 8 ]  The concept of anastomosis also applies to the theory of  symbiogenesis , in which new  species  emerge from the formation of novel symbiotic relationships. [ citation needed ]


In  mycology , anastomosis is the fusion between branches of the same or different  hyphae . [ 9 ]  Hence the bifurcating fungal hyphae can form true reticulating networks. By sharing materials in the form of dissolved  ions ,  hormones , and  nucleotides , the fungus maintains bidirectional communication with itself. The fungal network might begin from several origins; several spores (i.e. by means of  conidial anastomosis tubes ), several points of penetration, each a spreading circumference of absorption and assimilation. Once encountering the tip of another expanding, exploring self, the tips press against each other in  pheromonal  recognition or by an unknown recognition system, fusing to form a genetic singular clonal colony that can cover hectares called a  genet  or just microscopical areas. [ 10 ]


For fungi, anastomosis is also a component of reproduction. In some fungi, two different  haploid  mating types – if compatible – merge.  Somatically , they form a morphologically similar mycelial wave front that continues to grow and explore. The significant difference is that each  septated  unit is binucleate, containing two unfused  nuclei , i.e. one from each parent that eventually undergoes  karyogamy  and meiosis to complete the sexual cycle. [ citation needed ]


Also the term "anastomosing" is used for mushroom gills which interlink and separate to form a network. [ 11 ]


The growth of a  strangler fig  around a host tree, with tendrils fusing together to form a mesh, is called anastomosing. [ 12 ]


In  geology ,  veins  of  quartz  (or other) minerals can display anastomosis. [ 13 ]


Ductile  shear zones  frequently show anastomosing geometries of highly- strained  rocks around lozenges of less-deformed material. [ 14 ]


Molten  lava flows  sometimes flow in anastomosed  lava channels [ 15 ]  or  lava tubes . [ 16 ]


In cave systems, anastomosis is the splitting of cave passages that later reconnect. [ 17 ]


Anastomosing rivers ,  anastomosing streams  consist of multiple channels that divide and reconnect and are separated by semi-permanent banks formed of cohesive material, such that they are unlikely to migrate from one channel position to another. They can be confused with  braided rivers  based on their planforms alone, but braided rivers are much shallower and more dynamic than anastomosing rivers. Some definitions require that an anastomosing river be made up of interconnected channels that enclose floodbasins, [ 18 ]  again in contrast with braided rivers.


Rivers with anastomosed  reaches  include the  Magdalena River  in  Colombia , [ 19 ]  the upper  Columbia River  in  British Columbia , Canada, [ 20 ]  the  Drumheller Channels  of the  Channeled Scablands  of the state of Washington, US, and the upper  Narew River  in  Poland . [ 21 ]  The term  anabranch  has been used for segments of anastomosing rivers.


Braided streams  show anastomosing channels around  channel bars  of  alluvium . [ 22 ]


The  angle of His , also known as the  esophagogastric angle , is the acute angle created between the  cardia  at the entrance to the  stomach , and the  esophagus . It helps to prevent acid reflux of stomach acid into the esophagus. It is commonly undeveloped in  infants , making acid reflux more common.


The angle of His is the  acute angle  between the  stomach  and the  esophagus . [ 1 ]  It is created by the collar sling fibres and the circular muscles around this gastroesophageal junction. [ 2 ]


The angle of His is normally undeveloped in  infants , with the esophagus making a vertical junction with the stomach. [ 3 ]  As a result, reflux of stomach contents is common. [ 3 ]


The angle of His forms an anatomical sphincter. This prevents the reflux of  stomach acid ,  digestive enzymes , and duodenal  bile  from entering the esophagus. [ 4 ] [ 5 ]  This is important in preventing  gastroesophageal reflux disease  and  inflammation  of the esophagus. [ 5 ]


The angle of His is also known as the esophagogastric angle. [ 6 ]


The  angular incisure  (or  angular notch ) is a small notch on the  stomach . It is located on the  lesser curvature of the stomach  near the  pyloric  end. Its location varies depending on how distended the stomach is. [ 1 ]


The angular incisure is used as a separation point between the right and left portions of the stomach, the body and the pylorus. [ 2 ]  An imaginary line drawn perpendicular to the lesser curvature of the stomach through the angular incisure makes up the boundary between the  body of the stomach  and pylorus.


The  anocutaneous line , also called the  Hilton white line  or  intersphincteric groove , is a boundary in the  anal canal . [ 1 ]


Below the anocutaneous line, lymphatic drainage is to the  superficial inguinal nodes . [ 2 ]


The anocutaneous line is slightly below the  pectinate line  and a landmark for the intermuscular border between  internal  and  external anal sphincter  muscles.


The anocutaneous line represents the transition point from non-keratinized  stratified squamous epithelium  of the anal canal to  keratinized  stratified squamous epithelium of the  anus  and  perianal skin . [ 3 ]


In live persons, the color of the line is white, hence the alternative name. It is named for  John Hilton . [ 4 ]


In  mammals ,  invertebrates  and most  fish , [ 1 ] [ 2 ]  the  anus  ( pl. :  anuses  or  ani ; from  Latin , 'ring' or 'circle') is the external  body orifice  at the  exit  end of the  digestive tract  ( bowel ), i.e. the opposite end from the  mouth . Its function is to facilitate the  expulsion  of wastes that remain after  digestion .


Bowel contents that pass through the anus include the gaseous  flatus  and the semi-solid  feces , which (depending on the type of animal) include: indigestible matter such as  bones ,  hair   pellets ,  endozoochorous   seeds  and  digestive rocks ; [ 3 ]  residual  food  material after the digestible  nutrients  have been extracted, for example  cellulose  or  lignin ; ingested matter which would be toxic if it remained in the digestive tract;  excreted   metabolites  like  bilirubin -containing  bile ; and dead  mucosal   epithelia  or excess  gut bacteria  and other  endosymbionts . Passage of feces through the anus is typically controlled by  muscular   sphincters , and failure to stop unwanted passages results in  fecal incontinence .


Amphibians ,  reptiles  and  birds  use a similar orifice (known as the  cloaca ) for excreting liquid and solid wastes, for  copulation  and  egg-laying .  Monotreme  mammals also have a cloaca, which is thought to be a feature inherited from the earliest  amniotes .  Marsupials  have a single orifice for excreting both solids and liquids and, in females, a separate  vagina  for reproduction. Female  placental  mammals have completely separate orifices for  defecation ,  urination , and reproduction; males have one opening for defecation and  another for both urination and reproduction , although the channels flowing to that orifice are almost completely separate.


The development of the anus was an important stage in the evolution of multicellular animals. It appears to have happened at least twice, following different paths in  protostomes  and  deuterostomes . This accompanied or facilitated other important evolutionary developments: the  bilaterian body plan , the  coelom , and  metamerism , in which the body was built of repeated "modules" which could later specialize, such as the heads of most  arthropods , which are composed of fused, specialized segments.


In  comb jellies , there are species with one and sometimes two permanent anuses, species like the  warty comb jelly  grows an anus, which then disappear when it is no longer needed. [ 4 ]


In animals at least as complex as an  earthworm , the  embryo  forms a dent on one side, the  blastopore , which deepens to become the  archenteron , the first phase in the growth of the  gut . In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it was thought that in their embryos the dent formed the mouth first ( proto–  meaning "first") and the anus was formed later at the opening made by the other end of the gut. Research from 2001 shows the edges of the dent close up in the middles of protosomes, leaving openings at the ends which become the mouths and anuses. [ 5 ]


In the  anatomy  of humans and homologous primates, the  ascending colon  is the part of the  colon  located between the  cecum  and the  transverse colon .


The ascending colon is smaller in calibre than the cecum from where it starts. It passes upward, opposite the  colic valve , to the under surface of the  right lobe of the liver , on the right of the  gall-bladder , where it is lodged in a shallow depression, the  colic impression ; here it bends abruptly forward and to the left, forming the  right colic flexure  (hepatic) where it becomes the transverse colon.


It is retained in contact with the posterior wall of the abdomen by the peritoneum, which covers its anterior surface and sides, its posterior surface being connected by  loose areolar tissue  with the  iliacus ,  quadratus lumborum ,  aponeurotic  origin of  transversus abdominis , and with the front of the lower and lateral part of the right kidney.


Sometimes the  peritoneum  completely invests it and forms a distinct but narrow  mesocolon .


It is in relation, in front, with the convolutions of the  ileum  and the abdominal walls.


Parasympathetic innervation  to the ascending colon is supplied by the  vagus nerve .  Sympathetic innervation  is supplied by the thoracic  splanchnic nerves .


The ascending colon is on the right side of the body (barring any malformations). The term  right colon  is  hypernymous  to  ascending colon  in precise use; many casual mentions of the right colon chiefly concern the ascending colon.


This article incorporates text in the  public domain  from  page 1180  of the 20th edition of   Gray's Anatomy   (1918)


The  myenteric plexus  (or  Auerbach 's plexus ) provides motor innervation to both layers of the  muscular layer  of the gut, having both parasympathetic and sympathetic input (although present ganglion cell bodies belong to  parasympathetic  innervation, fibers from sympathetic innervation also reach the plexus), whereas the  submucous plexus  provides  secretomotor  innervation to the  mucosa  nearest the  lumen  of the gut.


It arises from cells in the  vagal trigone  also known as the nucleus ala cinerea, the parasympathetic nucleus of origin for the tenth cranial nerve ( vagus nerve ), located in the  medulla oblongata . The fibers are carried by both the anterior and posterior vagal nerves. The myenteric plexus is the major nerve supply to the  gastrointestinal  tract and controls GI tract  motility . [ 1 ]


According to preclinical studies, 30% of myenteric plexus' neurons are  enteric sensory neurons , thus Auerbach's plexus has also a sensory component. [ 2 ] [ 3 ]


A part of the  enteric nervous system , the myenteric plexus exists between the longitudinal and circular layers of muscularis externa in the gastrointestinal tract.  It is found in the muscles of the esophagus, stomach, and intestine. [ citation needed ]


The ganglia have properties similar to the  central nervous system  (CNS). These properties include presence of glia, interneurons, a small extracellular space, dense synaptic neuropil, isolation from blood vessels, multiple synaptic mechanisms and multiple neurotransmitters.


The myenteric plexus originates in the  medulla oblongata  as a collection of neurons from the ventral part of the brain stem. The vagus nerve then carries the axons to their destination in the gastrointestinal tract. [ 4 ]


They contain  Dogiel cells . [ 5 ]


The myenteric plexus functions as a part of the enteric nervous system (digestive system). The enteric nervous system can and does function autonomously, but normal digestive function requires communication links between this intrinsic system and the central nervous system. The ENS contains sensory receptors, primary afferent neurons, interneurons, and motor neurons. The events that are controlled, at least in part, by the ENS are multiple and include motor activity, secretion, absorption, blood flow, and interaction with other organs such as the gallbladder or pancreas. These links take the form of parasympathetic and sympathetic fibers that connect either the central and enteric nervous systems or connect the central nervous system directly with the digestive tract. Through these cross connections, the gut can provide sensory information to the CNS, and the CNS can affect gastrointestinal function. Connection to the central nervous system also means that signals from outside of the digestive system can be relayed to the digestive system: for instance, the sight of appealing food stimulates secretion in the stomach. [ 6 ]


The enteric nervous system makes use of over 30 different neurotransmitters, most similar to those of the CNS such as  acetylcholine ,  dopamine , and  serotonin . More than 90% of the body's serotonin lies in the gut; as well as about 50% of the body's dopamine, which is currently being studied to further our understanding of its utility in the brain. [ 7 ]
The heavily studied  neuropeptide  known as  substance P  is present in significant levels and may help facilitate the production of saliva, smooth muscle contractions, and other tissue responses.


Since many of the same neurotransmitters are found in the ENS as the brain, it follows that myenteric neurons can express receptors for both peptide and non-peptide (amines, amino acids, purines) neurotransmitters. Generally, expression of a receptor is limited to a subset of myenteric neurons, with probably the only exception being expression of nicotinic cholinergic receptors on all myenteric neurons. One receptor that has been targeted for therapeutic reasons has been the  5-hydroxytryptamine (5-HT 4 ) receptor . Activating this pre-synaptic receptor enhances cholinergic neurotransmission and can stimulate gastrointestinal motility. [ 8 ]


The enteric nervous system exhibits taste receptors similar to the ones in the tongue. The taste receptor  TAS1R3  and the taste G protein  gustducin  are two of the most common. These receptors sense "sweetness" on the tongue and sense glucose in the enteric nervous system. These receptors help regulate the secretion of insulin and other hormones that are responsible for controlling blood sugar levels. [ 9 ]


Hirschsprung's disease  is a congenital disorder of the colon in which nerve cells of the myenteric plexus in its walls, also known as ganglion cells, are absent. Hirschsprung's disease is a form of functional low bowel obstruction due to failure of caudal migration of  neuroblasts  within developing bowel – this results in an absence of parasympathetic intrinsic ganglion cells in both Auerbach's and Meissner's plexuses. The distal large
bowel from the point of neuronal arrest to the anus is continuously aganglionic. It is a rare disorder (1:5000), with prevalence among males being four times that of females. [ 10 ]


Achalasia  is a motor disorder of the esophagus characterized by decrease in ganglion cell density in the myenteric plexus. The cause of the lesion is unknown. [ 11 ]


Myenteric plexi destruction has been found to be secondary to Chagas disease (T. cruzi infection sequelae). Destruction occurs in the esophagus, intestines, and ureters. This denervation can lead to secondary achalasia (lower esophageal sphincter will not open; loss of inhibitory neurons), megacolon, and megaureter, respectively.


Because the ENS is known as the "brain of the gut", due to its similarities with the CNS, researchers have been using colonic biopsies of Parkinson's patients to help better understand and manage  Parkinson's disease . [ 12 ]  PD patients are known to experience severe constipation due to GI tract dysfunction years before the onset of motor movement complications, which characterises Parkinson's disease. [ 13 ]


Leopold Auerbach , a neuropathologist, was one of the first to further research the nervous system using histological staining methods. [ citation needed ]


The  basal  or  basic electrical rhythm  ( BER ) or  electrical control activity  ( ECA ) is the spontaneous depolarization and repolarization of pacemaker cells known as  interstitial cells of Cajal  (ICCs) in the  smooth muscle  of the stomach, small intestine, and large intestine. This electrical rhythm is spread through  gap junctions  in the smooth muscle of the  GI tract . [ 1 ]  These pacemaker cells, also called the ICCs, control the frequency of contractions in the gastrointestinal tract. The cells can be located in either the circular or longitudinal layer of the smooth muscle in the GI tract; circular for the small and large intestine, longitudinal for the stomach. [ 2 ]  The frequency of contraction differs at each location in the GI tract beginning with 3 per minute in the  stomach , then 12 per minute in the  duodenum , 9 per minute in the  ileum , and a normally low one contraction per 30 minutes in the  large intestines  that increases 3 to 4 times a day due to a phenomenon called  mass movement . [ 2 ]  The basal electrical rhythm controls the frequency of contraction but additional neuronal and hormonal controls regulate the strength of each contraction.


Smooth muscle  within the GI tract causes the involuntary  peristaltic  motion that moves consumed food down the  esophagus  and towards the  rectum . [ 1 ]   The smooth muscle throughout most of the GI tract is divided into two layers: an outer longitudinal layer and an inner circular layer. [ 1 ]   Both layers of muscle are located within the  muscularis externa .  The stomach has a third layer: an innermost oblique layer.


The physical contractions of the smooth muscle cells can be caused by  action potentials  in  efferent   motor neurons  of the  enteric nervous system , or by receptor mediated calcium influx. [ 1 ]  These efferent motor neurons of the enteric nervous system are cholinergic and adrenergic neurons. [ 2 ]  The inner circular layer is innervated by both excitatory and inhibitory motor neurons, while the outer longitudinal layer is innervated by mainly excitatory neurons.  These action potentials cause the smooth muscle cells to contract or relax, depending on the particular stimulation the cells receive.  Longitudinal muscle fibers depend on calcium influx into the cell for  excitation-contraction coupling , while circular muscle fibers rely on intracellular calcium release.  Contraction of the smooth muscle can occur when the BER reaches its plateau (an absolute value less than -45mV) [ citation needed ]  while a simultaneous stimulatory action potential occurs.  A contraction will not occur unless an action potential occurs.  Generally, BER waves stimulate action potentials and action potentials stimulate contractions.