Bile acids BAs are not only digestive surfactants but also important cell signaling molecules, which stimulate several signaling pathways to regulate some important biological processes. Pharynx Muscles Spaces peripharyngeal retropharyngeal parapharyngeal retrovisceral danger prevertebral Pterygomandibular raphe Pharyngeal raphe Buccopharyngeal fascia Pharyngobasilar fascia Piriform sinus. ASBT apical sodium-dependent bile salt transporter. 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. These juices help to digest food and allow the body to absorb nutrients. It is also the site of the appendix attachment.
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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. 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.
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. 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 doesn't 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.
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. 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 CKK 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.
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. Partially digested food starts to arrive in the small intestine as semi-liquid chyme , one hour after it is eaten.
After two hours the stomach has emptied. 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.
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.
The cecum is a pouch marking the division between the small intestine and 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. In the large intestine ,  the passage of the digesting food in the colon is a lot slower, taking from 12 to 50 hours until it is removed by defecation. 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 via defecation from the anus.
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. 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. 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 enteric nervous system consists of some one hundred million neurons  that are embedded in the peritoneum , the lining of the gastrointestinal tract extending from the esophagus to the anus.
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. 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. 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.
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.
Crohn's disease is a common chronic inflammatory bowel disease IBD , which can affect any part of the GI tract,  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 coliltis is the most common of the IBDs . 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. 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. Giardiasis is the most common pathogenic parasitic infection in humans. There are diagnostic tools mostly involving the ingestion of barium sulphate to investigate disorders of the GI tract. 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. From Wikipedia, the free encyclopedia. Redirected from Digestive system. See also gastrointestinal tract. For digestive systems of non-human animals, see Digestion. Development of the digestive system.
Retrieved 1 October Human Biology and Health. Human Physiology Third ed. Dorland's illustrated medical dictionary 32nd ed. Regulation of Front-line Body Defenses". Essential Haematology 5e Essential. Textbook of Medical Physiology. Mitchell; illustrations by Richard; Richardson, Paul Gray's anatomy for students. Retrieved 22 May Journal of parenteral and enteral nutrition. Principles for Clinical Medicine 3rd ed. Guyton and Hal Textbook of Medical Physiology 12th ed. Sensory Nerves, Brendan J.
Larsen's human embryology Thoroughly rev. Robbins and Cotran pathologic basis of disease. The Journal of the American Medical Association. Retrieved 12 June The New England Journal of Medicine, The Functional Gastrointestinal Disorders. Diagnosis, Pathophysiology and Treatment. Anatomy of the mouth. Vermilion border Frenulum of lower lip Labial commissure of mouth Philtrum. Hard palate Soft palate Palatine raphe Incisive papilla. Parotid gland duct Submandibular gland duct Sublingual gland duct.
Oropharynx fauces Plica semilunaris of the fauces Uvula Palatoglossal arch Palatopharyngeal arch Tonsillar fossa Palatine tonsil. Anatomy of the gastrointestinal tract , excluding the mouth. Muscles Spaces peripharyngeal retropharyngeal parapharyngeal retrovisceral danger prevertebral Pterygomandibular raphe Pharyngeal raphe Buccopharyngeal fascia Pharyngobasilar fascia Piriform sinus.
Sphincters upper lower glands. Suspensory muscle Major duodenal papilla Minor duodenal papilla Duodenojejunal flexure Brunner's glands. Ileocecal valve Peyer's patches Microfold cell. Ascending colon Hepatic flexure Transverse colon Splenic flexure Descending colon Sigmoid colon Continuous taenia coli haustra epiploic appendix.
Anatomy of the liver, pancreas and biliary tree. Physiology of the gastrointestinal system. Submucous plexus Myenteric plexus. Segmentation contractions Migrating motor complex Borborygmus Defecation. Diseases of the digestive system primarily K20—K93 , — Gastritis Atrophic Ménétrier's disease Gastroenteritis Peptic gastric ulcer Cushing ulcer Dieulafoy's lesion Dyspepsia Pyloric stenosis Achlorhydria Gastroparesis Gastroptosis Portal hypertensive gastropathy Gastric antral vascular ectasia Gastric dumping syndrome Gastric volvulus.
Coeliac Tropical sprue Blind loop syndrome Small bowel bacterial overgrowth syndrome Whipple's Short bowel syndrome Steatorrhea Milroy disease Bile acid malabsorption. Abdominal angina Mesenteric ischemia Angiodysplasia Bowel obstruction: Proctitis Radiation proctitis Proctalgia fugax Rectal prolapse Anismus. Upper Hematemesis Melena Lower Hematochezia. Peritonitis Spontaneous bacterial peritonitis Hemoperitoneum Pneumoperitoneum.
Stomodeum Buccopharyngeal membrane Rathke's pouch Tracheoesophageal septum Pancreatic bud Hepatic diverticulum. Urorectal septum Proctodeum Cloaca Cloacal membrane. Esophagogastroduodenoscopy Barium swallow Upper gastrointestinal series. Bariatric surgery Duodenal switch Jejunoileal bypass Bowel resection Ileostomy Intestine transplantation Jejunostomy Partial ileal bypass surgery Strictureplasty.
Appendicectomy Colectomy Colonic polypectomy Colostomy Hartmann's operation. Anal sphincterotomy Anorectal manometry Lateral internal sphincterotomy Rubber band ligation Transanal hemorrhoidal dearterialization.
Colonoscopy Anoscopy Capsule endoscopy Enteroscopy Proctoscopy Sigmoidoscopy Abdominal ultrasonography Defecography Double-contrast barium enema Endoanal ultrasound Enteroclysis Lower gastrointestinal series Small-bowel follow-through Transrectal ultrasonography Virtual colonoscopy. Fecal fat test Fecal pH test Stool guaiac test. Artificial extracorporeal liver support Bioartificial liver devices Liver dialysis Hepatectomy Liver biopsy Liver transplantation Portal hypertension Transjugular intrahepatic portosystemic shunt [TIPS] Distal splenorenal shunt procedure.
Frey's procedure Pancreas transplantation Pancreatectomy Pancreaticoduodenectomy Puestow procedure. Diagnostic peritoneal lavage Intraperitoneal injection Laparoscopy Omentopexy Paracentesis Peritoneal dialysis. Inguinal hernia surgery Femoral hernia repair.