Young elephants, pandas, koalas, and hippos eat the faeces of their mother, probably to obtain the bacteria required to properly digest vegetation. When they are born, their intestines do not contain these bacteria (they are completely sterile). Without them, they would be unable to get any nutritional value from many plant components.


An  earthworm 's digestive system consists of a mouth,  pharynx ,  esophagus , crop,  gizzard , and  intestine . The mouth is surrounded by strong lips, which act like a hand to grab pieces of dead grass, leaves, and weeds, with bits of soil to help chew. The lips break the food down into smaller pieces. In the pharynx, the food is lubricated by mucus secretions for easier passage. The esophagus adds calcium carbonate to neutralize the acids formed by food matter decay. Temporary storage occurs in the crop where food and calcium carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass of food and dirt. When the churning is complete, the glands in the walls of the gizzard add enzymes to the thick paste, which helps chemically breakdown the organic matter. By  peristalsis , the mixture is sent to the intestine where friendly bacteria continue chemical breakdown. This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body.


In most  vertebrates , digestion is a multistage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:


Underlying the process is muscle movement throughout the system through swallowing and  peristalsis . Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances. Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids (lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc.).


The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by  gut  bacteria and for resorption of water from digests before excretion.


In  mammals , preparation for digestion begins with the  cephalic phase  in which saliva is produced in the mouth and  digestive enzymes  are produced in the  stomach .  Mechanical and chemical digestion begin in the mouth where food is  chewed , and mixed with saliva to begin enzymatic processing of  starches .  The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes.  Absorption  occurs in the stomach and  gastrointestinal tract , and the process finishes with defecation. [ 5 ]


The  human gastrointestinal tract  is around 9 metres (30 feet) long. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours. [ 20 ]


Digestion begins in the  mouth  with the secretion of saliva and its digestive enzymes. Food is formed into a  bolus  by the mechanical  mastication  and swallowed into the  esophagus  from where it enters the stomach through the action of  peristalsis .  Gastric juice  contains  hydrochloric acid  and  pepsin  which could damage the stomach lining, but  mucus  and bicarbonates are secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. Mainly proteins are digested in stomach. The partially digested food enters the  duodenum  as a thick semi-liquid  chyme . In the small intestine, the larger part of digestion takes place and this is helped by the secretions of  bile ,  pancreatic juice  and  intestinal juice .  The intestinal walls are lined with  villi , and their  epithelial cells  are covered with numerous  microvilli  to improve the absorption of nutrients by increasing the  surface area  of the intestine. Bile helps in emulsification of fats and also activates lipases.


In the large intestine, the passage of food is slower to enable fermentation by the  gut flora  to take place. Here, water is absorbed and waste material stored as  feces  to be removed by defecation via the  anal canal  and  anus .


Different  phases of digestion  take place including: the  cephalic phase ,  gastric phase , and  intestinal phase .


The cephalic phase occurs at the sight, thought and smell of food, which stimulate the  cerebral cortex . Taste and smell stimuli are sent to the  hypothalamus  and  medulla oblongata . After this it is routed through the  vagus nerve  and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and  G cell  (secretes gastrin) activity via  D cell  secretion of  somatostatin .


The gastric phase takes 3 to 4 hours. It is stimulated by  distension  of the stomach, presence of food in stomach and decrease in  pH . Distention activates long and myenteric reflexes. This activates the release of  acetylcholine , which stimulates the release of more gastric juices. As protein enters the stomach, it binds to  hydrogen  ions, which raises the pH of the stomach. Inhibition of gastrin and  gastric acid  secretion is lifted. This triggers G cells to release  gastrin , which in turn stimulates  parietal cells  to secrete gastric acid.  Gastric acid is about 0.5%  hydrochloric acid , which lowers the pH to the desired pH of 1–3.  Acid release is also triggered by  acetylcholine  and  histamine .


The intestinal phase has two parts, the excitatory and the inhibitory. Partially digested food fills the  duodenum . This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating  sympathetic fibers  causing the  pyloric sphincter  to tighten to prevent more food from entering, and inhibits local reflexes.


Protein digestion occurs in the stomach and  duodenum  in which 3 main enzymes, pepsin secreted by the stomach and  trypsin  and  chymotrypsin  secreted by the pancreas, break down food proteins into  polypeptides  that are then broken down by various  exopeptidases  and  dipeptidases  into  amino acids .  The digestive enzymes however are mostly secreted as their inactive precursors, the  zymogens .  For example, trypsin is secreted by pancreas in the form of  trypsinogen , which is activated in the duodenum by  enterokinase  to form trypsin.  Trypsin then cleaves  proteins  to smaller polypeptides.


Digestion of some fats can begin in the mouth where  lingual lipase  breaks down some short chain lipids into  diglycerides . However fats are mainly digested in the small intestine. [ 21 ]   The presence of fat in the small intestine produces hormones that stimulate the release of  pancreatic lipase  from the pancreas and  bile  from the liver which helps in the emulsification of fats for absorption of  fatty acids . [ 21 ]   Complete digestion of one molecule of fat (a  triglyceride ) results a mixture of fatty acids, mono- and di-glycerides, but no  glycerol . [ 21 ]


In humans, dietary starches are composed of  glucose  units arranged in long chains called amylose, a  polysaccharide . During digestion, bonds between glucose molecules are broken by salivary and pancreatic  amylase , resulting in progressively smaller chains of glucose.  This results in simple sugars glucose and  maltose  (2 glucose molecules) that can be absorbed by the small intestine.


Lactase  is an enzyme that breaks down the  disaccharide   lactose  to its component parts, glucose and  galactose .  Glucose and galactose can be absorbed by the small intestine.  Approximately 65 percent of the adult population produce only small amounts of lactase and are unable to eat  unfermented  milk-based foods.  This is commonly known as  lactose intolerance .  Lactose intolerance varies widely by genetic heritage;  more than 90 percent of peoples of east Asian descent are lactose intolerant, in contrast to about 5 percent of people of northern European descent. [ 22 ]


Sucrase  is an enzyme that breaks down the disaccharide  sucrose , commonly known as table sugar, cane sugar, or beet sugar. Sucrose digestion yields the sugars  fructose  and glucose which are readily absorbed by the small intestine.


DNA and RNA are broken down into  mononucleotides  by the  nucleases   deoxyribonuclease  and  ribonuclease  (DNase and RNase) from the pancreas.


Some nutrients are complex molecules (for example  vitamin B 12 ) which would be destroyed if they were broken down into their  functional groups .  To digest vitamin B 12  non-destructively,  haptocorrin  in saliva strongly binds and protects the B 12  molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes. [ 23 ]


After the B 12 -haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the B 12  molecules which rebind to  intrinsic factor  (IF).  These B 12 -IF complexes travel to the ileum portion of the small intestine where  cubilin  receptors enable  assimilation  and circulation of B 12 -IF complexes in the blood. [ 24 ]


There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. Connections to metabolic control (largely the glucose-insulin system) have been uncovered.


Digestion is a complex process controlled by several factors. pH plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx and esophagus, pH is typically about 6.8, very weakly acidic. Saliva controls pH in this region of the digestive tract.  Salivary amylase  is contained in saliva and starts the breakdown of carbohydrates into  monosaccharides . Most digestive enzymes are sensitive to pH and will denature in a high or low pH environment.


The stomach's high acidity inhibits the breakdown of  carbohydrates  within it. This acidity confers two benefits: it  denatures  proteins for further digestion in the small intestines, and provides  non-specific immunity , damaging or eliminating various  pathogens . [ 25 ]


In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralize the acidic conditions from the stomach, and the  pancreatic duct  empties into the duodenum, adding  bicarbonate  to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline with a pH of about 8.5. [ citation needed ]


The  digestive system of gastropods  has evolved to suit almost every kind of diet and feeding behavior.  Gastropods  ( snails  and  slugs ) as the largest taxonomic class of the  mollusca  are very diverse: the group includes  carnivores ,  herbivores ,  scavengers ,  filter feeders , and even  parasites .


In particular, the  radula  is often highly adapted to the specific diet of the various group of gastropods. Another distinctive feature of the digestive tract is that, along with the rest of the  visceral  mass, it has undergone  torsion , twisting around through 180 degrees during the larval stage, so that the  anus  of the animal is located above its head. [ 1 ]


A number of species have developed special adaptations to feeding, such as the "drill" of some  limpets , or the harpoon of the  neogastropod  genus  Conus . Filter feeders use the  gills , mantle lining, or nets of mucus to trap their prey, which they then pull into the mouth with the radula. The highly modified parasitic genus  Enteroxenos  has no digestive tract at all, and simply absorbs the blood of its host through the body wall. [ 1 ]


The digestive system usually has the following parts:


The buccal mass is the first part of the digestive system, and consists of the mouth and pharynx. The mouth includes a  radula , and in most cases, also a pair of jaws. The pharynx can be very large, especially in carnivorous species.


Many carnivorous species have developed a  proboscis , containing the oral cavity, radula, and part of the oesophagus. At rest, the proboscis is enclosed within a sac-like sheath, with an opening at the front of the animal that resembles a true mouth. When the animal feeds, it pumps blood into the proboscis, inflating it and pushing it out through the opening to grasp the gastropod's prey. A set of retractor muscles help pull the proboscis back inside the sheath once feeding is completed. [ 1 ]


The  radula  is a  chitinous  ribbon used for scraping or cutting food.


Several herbivorous species, as well as carnivores that prey on  sessile  animals, have also developed simple jaws, which help to hold the food steady while the radula works on it. The jaw is opposite to the  radula  and reinforces part of the  foregut . [ 2 ]


The more purely carnivorous the diet, the more the jaw is reduced. [ 2 ]


There are often pieces of food in the gut corresponding to the shape of the jaw. [ 2 ]


The jaw structure can be ribbed or smooth:


Some species have no jaw.


Salivary glands  plays primary role in the anatomical and physiological adaptations of the digestive system of predatory gastropods. [ 3 ]  Ducts from large salivary glands lead into the buccal cavity, and the oesophagus also supplies the digestive enzymes that help to break down the food. [ 1 ]  Salivary secretions lubricate the food and they also contain  bioactive compounds . [ 3 ]


The mouth of gastropods opens into an  oesophagus , which connects to the  stomach . Because of torsion, the oesophagus usually passes around the stomach, and opens into its posterior portion, furthest from the mouth. In species that have undergone de-torsion, however, the oesophagus may open into the anterior of the stomach, which is therefore reversed from the usual gastropod arrangement. [ 1 ]


In  Tarebia granifera , the  brood pouch  is above the oesophagus. [ 4 ]


There is available an extensive  rostrum  on the anterior part of the oesophagus in all carnivorous gastropods. [ 5 ]


Some basal gastropod clades have  oesophageal gland .


In most species, the stomach itself is a relatively simple sac, and is the main site of digestion. In many herbivores, however, the hind part of the oesophagus is enlarged to form a  crop , which, in terrestrial  pulmonates , may even replace the stomach entirely. In many aquatic herbivores, however, the stomach is adapted into a  gizzard  that helps to grind up the food. The gizzard may have a tough  cuticle , or may be filled with abrasive sand grains. [ 1 ]


In the most primitive gastropods, however, the stomach is a more complex structure. In these species, the hind part of the stomach, where the oesophagus enters, is  chitinous , and includes a sorting region lined with  cilia . [ 1 ]


In all gastropods, the portion of the stomach furthest from the oesophagus, called the "style sac", is lined with cilia. These beat in a rotary motion, pulling the food forward in a steady stream from the mouth. Usually, the food is embedded in a string of mucus produced in the mouth, creating a coiled conical mass in the style sac. This action, rather than muscular  peristalsis , is responsible for the movement of food through the gastropod digestive tract. [ 1 ]


Two diverticular glands open into the stomach, and secrete enzymes that help to break down the food. In the more primitive species, these glands may also absorb the food particles directly and digest them intracellularly. [ 1 ]


The hepatopancreas is the largest organ in  stylommatophoran  gastropods. [ 7 ]  It produces enzymes, and absorbs and stores nutrients.


The anterior portion of the stomach opens into a coiled  intestine , which helps to resorb water from the food, producing  faecal pellets . The anus opens above the head. [ 1 ]


Diosmectite  (brand names  Smecta ,  Smecdral ) is a natural  silicate  of  aluminium  and  magnesium  used as an  intestinal   adsorbent  in the treatment of several  gastrointestinal diseases , including infectious and non-infectious acute and chronic  diarrhoea , including  irritable bowel syndrome  diarrhea subtype. Other uses include: chronic diarrhea caused by radiation-induced, chemotherapy-induced, and  HIV/AIDS -associated chronic diarrhea. [ 1 ]


It is  insoluble  in water. It is usually taken orally as a suspension in warm water. In the field of geology, it is more commonly known as  montmorillonite . Diosmectite is a contraction of " dioctahedral   smectite ".


Its effectiveness in improving stool consistency is the result of its alleged ability to absorb bacteria, viruses and toxins as well as strengthening the intestinal mucus barrier to reduce  luminal   antigens  passing through the mucus barrier which in turn helps to reduce inflammation. [ 1 ]


Diosmectite may be a useful additive in the treatment of acute childhood diarrhoea. As evidence is of ‘low quality’, future research is needed with higher quality designs before any firm recommendations can be made.  [ 2 ]


The  duodenal bulb  (also  ampulla of duodenum ,  duodenal ampulla , or  duodenal cap ) is the initial, dilated portion of (the superior part of) the  duodenum [ 1 ]  just distal to the  stomach ; it begins at the  pylorus  and ends at the neck of the  gallbladder . It is normally about 5 centimeters long. [ 2 ]


It is located posterior to the liver and the gallbladder and superior to the  pancreatic head . [ citation needed ]  The  gastroduodenal artery , [ 3 ]   portal vein , and  common bile duct  are situated posterior to the duodenal bulb. The distal part of the bulb is located retroperitoneally. It is located immediately distal to the  pyloric sphincter . [ citation needed ]


The duodenal bulb is the site of duodenal ulcer occurrence. Duodenal ulcers are more common than gastric ulcers and - unlike gastric ulcers - are caused by increased gastric acid secretion. Duodenal ulcers are commonly located anteriorly, and rarely posteriorly. Anterior ulcers can be complicated by perforation, while the posterior ones bleed. The reason for that is explained by their location. The peritoneal or abdominal cavity is located anterior to the duodenum. Therefore, if the ulcer grows deep enough, it will perforate, whereas if a posterior ulcer grows deep enough, it will penetrate the  gastroduodenal artery  and bleed. [ citation needed ]


The  duodenojejunal flexure  or  duodenojejunal junction , also known as the  angle of Treitz , [ 1 ] [ 2 ]  is the border between the  duodenum  and the  jejunum .


The ascending portion of the  duodenum  ascends on the left side of the  aorta , as far as the level of the upper border of the second  lumbar vertebra . At this point, it turns abruptly forward to merge with the  jejunum , forming the duodenojejunal flexure. This forms the beginning of the jejunum. [ 3 ]  The duodenojejunal flexure is surrounded by the  suspensory muscle of the duodenum . [ 4 ] : 274   It is  retroperitoneal , so is less mobile than the jejunum that comes after it, helping to stabilise the jejunum. [ 5 ]


The duodenojejunal flexure lies in front of the left  psoas major muscle , the  left renal artery , and the  left renal vein . It is covered in front, and partly at the sides, by  peritoneum  continuous with the left portion of the  mesentery .


The  ligament of Treitz , a peritoneal fold, from the  right crus of diaphragm , is an identification point for the duodenojejunal flexure during  abdominal surgery . [ 6 ] : 85


This article incorporates text in the  public domain  from  page 1170  of the 20th edition of   Gray's Anatomy   (1918)


The  duodenum  is the first section of the  small intestine [ 3 ]  in most  higher vertebrates , including  mammals ,  reptiles , and  birds . In mammals, it may be the principal site for  iron  absorption.
The duodenum precedes the  jejunum  and  ileum  and is the shortest part of the small intestine.


In human beings, the duodenum is a hollow jointed tube about 25–38 centimetres (10–15 inches) long connecting the  stomach  to the  middle part of the small intestine . [ 4 ] [ 5 ]  It begins with the  duodenal bulb  and ends at the  suspensory muscle of duodenum . [ 6 ]  The duodenum can be divided into four parts: the first (superior), the second (descending), the third (transverse) and the fourth (ascending) parts. [ 5 ]


The duodenum is the first section of the  small intestine  in most  higher vertebrates , including  mammals ,  reptiles , and  birds . In  fish , the divisions of the small intestine are not as clear, and the terms  anterior intestine  or  proximal intestine  may be used instead of duodenum. [ 7 ]  In mammals the duodenum may be the principal site for  iron  absorption. [ 8 ]


In humans, the duodenum is a C-shaped hollow jointed tube, 25–38 centimetres (10–15 inches) in length, lying adjacent to the  stomach  (and connecting it to the small intestine). It is divided anatomically into four sections. The first part lies within the  peritoneum  but its other parts are  retroperitoneal . [ 9 ] : 273


The  first part , or  superior part , of the duodenum is a continuation from the  pylorus  to the transpyloric plane. It is superior (above) to the rest of the segments, at the  vertebral  level of  L1 . The  duodenal bulb , about  2 cm ( 3 ⁄ 4  in) long, is the first part of the duodenum and is slightly dilated. The duodenal bulb is a remnant of the mesoduodenum, a  mesentery  that suspends the organ from the posterior abdominal wall in fetal life. [ 10 ]  The first part of the duodenum is mobile, and connected to the liver by the  hepatoduodenal ligament  of the  lesser omentum . The first part of the duodenum ends at the corner, the  superior duodenal flexure . [ 9 ] : 273


Relations: [ citation needed ]


The  second part , or  descending part , of the duodenum begins at the superior duodenal flexure. It goes  inferior  to the lower border of vertebral body L3, before making a sharp turn  medially  into the  inferior duodenal flexure , the end of the descending part. [ 9 ] : 274


The  pancreatic duct  and  common bile duct  enter the descending duodenum, through the  major duodenal papilla . The second part of the duodenum also contains the minor duodenal papilla, the entrance for the  accessory pancreatic duct . The junction between the embryological  foregut  and  midgut  lies just below the major duodenal papilla. [ 9 ] : 274


The  third part , or  horizontal part  or  inferior part  of the duodenum is 10~12 cm in length. It begins at the  inferior duodenal flexure  and passes transversely to the left, passing in front of the  inferior vena cava ,  abdominal aorta  and the  vertebral column . The  superior mesenteric artery  and  vein  are  anterior  to the third part of the duodenum. [ 9 ] : 274   This part may be compressed between the aorta and SMA causing  superior mesenteric artery syndrome .


The  fourth part , or  ascending part , of the duodenum passes upward, joining with the  jejunum  at the  duodenojejunal flexure . The fourth part of the duodenum is at the vertebral level L3, and may pass directly on top, or slightly to the left, of the  aorta . [ 9 ] : 274


The duodenum receives arterial blood from two different sources. The transition between these sources is important as it demarcates the foregut from the midgut. Proximal to the 2nd part of the duodenum (approximately at the major duodenal papilla – where the bile duct enters) the arterial supply is from the  gastroduodenal artery  and its branch the  superior pancreaticoduodenal artery . Distal to this point (the midgut) the arterial supply is from the  superior mesenteric artery  (SMA), and its branch the  inferior pancreaticoduodenal artery  supplies the 3rd and 4th sections.
The  superior  and  inferior pancreaticoduodenal  arteries (from the gastroduodenal artery and SMA respectively) form an  anastomotic  loop between the  celiac trunk  and the SMA; so there is potential for collateral circulation here.


The venous drainage of the duodenum follows the arteries. Ultimately these veins drain into the  portal system , either directly or indirectly through the  splenic  or  superior mesenteric vein  and then to the portal vein.


The  lymphatic vessels  follow the arteries in a retrograde fashion. The anterior lymphatic vessels drain into the pancreatoduodenal  lymph nodes  located along the superior and inferior pancreatoduodenal arteries and then into the pyloric lymph nodes (along the gastroduodenal artery). The posterior lymphatic vessels pass posterior to the head of the pancreas and drain into the superior mesenteric lymph nodes. Efferent lymphatic vessels from the duodenal lymph nodes ultimately pass into the celiac lymph nodes.


Under  microscopy , the duodenum has a  villous   mucosa . This is distinct from the mucosa of the  pylorus , which directly joins the duodenum. Like other structures of the  gastrointestinal tract , the duodenum has a  mucosa ,  submucosa ,  muscularis externa , and  adventitia . Glands line the duodenum, known as  Brunner's glands , which secrete  mucus  and  bicarbonate  in order to neutralise stomach acids. These are distinct glands not found in the ileum or jejunum, the other parts of the small intestine. [ 11 ] : 274–275


The duodenum's close anatomical association with the pancreas creates differences in function based on the position and orientation of the organs. The congenital abnormality, annular pancreas, causes a portion of the pancreas to encircle the duodenum. In an extramural annular pancreas, the pancreatic duct encircles the duodenum which results in gastrointestinal obstruction. An intramural annular pancreas is characterized by pancreatic tissue that is fused with the duodenal wall, causing duodenal ulceration. [ 12 ]


About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal duodenum. [ 13 ] [ 14 ]  Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the duodenum. The corresponding specific proteins are expressed in the duodenal mucosa, and many of these are also expressed in the small intestine, such as  alanine aminopeptidase , a digestive enzyme,  angiotensin-converting enzyme , involved in controlling  blood pressure , and  RBP2 , a protein involved in the uptake of  vitamin A . [ 15 ]


The duodenum is largely responsible for the breakdown of food in the small intestine, using  enzymes . The duodenum also regulates the rate of emptying of the stomach via hormonal pathways.  Secretin  and  cholecystokinin  are released from cells in the duodenal  epithelium  in response to acidic and fatty stimuli present there when the  pylorus  opens and emits gastric  chyme  into the duodenum for further digestion. These cause the  liver  and  gallbladder  to release  bile , and the  pancreas  to release bicarbonate and digestive enzymes such as  trypsin ,  lipase  and  amylase  into the duodenum as they are needed. [ 16 ]


The duodenum is a critical contributor to the regulation of food intake [ 17 ]  and glycemic control. [ 18 ]  As the first part of the small intestine, the duodenum is the initial site of nutrient absorption in the gastrointestinal tract. The duodenum senses nutrient intake and composition, and signals to the liver, pancreas, adipose tissue and brain [ 19 ]  through the direct and indirect [ 20 ]  release of several key hormones and signaling molecules, including the  incretin  peptides  Glucose-dependent insulinotropic polypeptide  (GIP) and  Glucagon-like peptide-1  (GLP-1), [ 20 ]  as well as  Cholecystokinin  (CCK) and  Secretin .  The duodenum also signals to the brain directly via vagal afferents enabling neural control over food intake and glycemia. [ 21 ]   Intestinal secretion of GIP and GLP-1 stimulates glucose-dependent insulin secretion from pancreatic beta-cells, known as the incretin effect. [ 22 ]  Incretin peptides, principally GLP-1 and GIP, regulate islet hormone secretion, glucose concentrations, lipid metabolism, gut motility, appetite and body weight, and immune function. [ 23 ]


The villi of the duodenum have a leafy-looking appearance, which is a histologically identifiable structure.  Brunner's glands , which secrete  mucus , are only found in the duodenum. The duodenum wall consists of a very thin layer of cells that form the  muscularis mucosae .


Ulcers of the duodenum commonly occur because of infection by the bacteria  Helicobacter pylori . These bacteria, through a number of mechanisms, erode the protective mucosa of the duodenum, predisposing it to damage from gastric acids. The first part of the duodenum is the most common location of ulcers since it is where the acidic chyme meets the duodenal mucosa before mixing with the alkaline secretions of the duodenum. [ 24 ]  Duodenal ulcers may cause recurrent abdominal pain and  dyspepsia , and are often investigated using a  urea breath test  to test for the bacteria, and  endoscopy  to confirm ulceration and take a  biopsy . If managed, these are often managed through  antibiotics  that aim to eradicate the bacteria, and  proton-pump inhibitors  and  antacids  to reduce the gastric acidity. [ 25 ]


The  British Society of Gastroenterology  guidelines specify that a duodenal biopsy is required for the diagnosis of adult  celiac disease . The biopsy is ideally performed at a moment when the patient is on a gluten-containing diet. [ 26 ]


Duodenal cancer is a cancer in the first section of the small intestine. Cancer of the duodenum is relatively rare compared to  stomach cancer  and  colorectal cancer ; malignant tumors in the duodenum constitute only around 0.3% of all the gastrointestinal tract tumors but around half of cancerous tissues that develop in the small intestine. [ 27 ]  Its histology is often observed to be  adenocarcinoma , meaning that the cancerous tissue arises from glandular cells in the  epithelial tissue  lining the duodenum. [ 28 ]


A  western diet  induces duodenal mucosal hyperplasia and dysfunction that underlie insulin resistance, type 2 diabetes and obesity. [ 29 ] [ 30 ]  Diet-induced duodenal mucosal hyperplasia consists of increased mucosal mass, [ 31 ]  increased villus length, [ 29 ] [ 32 ] [ 33 ] [ 34 ]  decreased crypt density, [ 29 ]  proliferation of enteroendocrine cells, [ 35 ]  increased enterocyte mass, [ 36 ]  and an accumulation of lipid droplets in the mucosa. [ 37 ] [ 38 ]  Diet induced duodenal dysfunction  includes increased duodenal nutrient absorption, [ 32 ] [ 39 ] [ 40 ] [ 41 ]  altered duodenal hormone secretion, [ 29 ] [ 35 ]  and altered intestinal vagal afferent neuronal function. [ 42 ]


Inflammation of the duodenum is referred to as  duodenitis . There are multiple known causes. [ 43 ]  Celiac disease and inflammatory bowel disease are two of the known causes. [ 44 ]


The name  duodenum  is  Medieval Latin , short for  intestīnum duodēnum digitōrum , meaning intestine of twelve finger-widths (in length), genitive pl. of duodēnī, twelve each, from Latin duodeni "twelve each" (from duodecim "twelve"). Coined by Gerard of Cremona (d. 1187) in his Latin translation of "Canon Avicennae," "اثنا عشر" itself a loan-translation of Greek dodekadaktylon, literally "twelve fingers long." The intestine part was so called by the Greek physician  Herophilus  (c. 335–280 BCE) for its length, about equal to the breadth of 12 fingers. [ 45 ]


Many languages retain a similar etymology for this word. For example,  German   Zwölffingerdarm ,  Dutch   Twaalfvingerige darm  and  Turkish   Oniki parmak bağırsağı .


In  pharmacology , the  elimination  or  excretion  of a drug is understood to be any one of a number of processes by which a drug is eliminated (that is,  cleared  and  excreted ) from an organism either in an unaltered form (unbound molecules) or modified as a metabolite. The  kidney  is the main excretory organ although others exist such as the  liver , the  skin , the  lungs  or  glandular  structures, such as the  salivary glands  and the  lacrimal glands . These organs or structures use specific routes to expel a drug from the body, these are termed  elimination pathways :


Drugs are excreted from the kidney by  glomerular filtration  and by active  tubular secretion  following the same steps and mechanisms as the products of intermediate metabolism. Therefore, drugs that are filtered by the  glomerulus  are also subject to the process of passive  tubular reabsorption .  Glomerular filtration will only remove those drugs or metabolites that are not bound to proteins present in blood plasma (free fraction) and many other types of drugs (such as the organic acids) are actively secreted.  In the  proximal  and   distal convoluted tubules , non-ionised acids and weak bases are reabsorbed both actively and passively. Weak acids are excreted when the tubular fluid becomes too alkaline and this reduces passive reabsorption. The opposite occurs with weak bases. Poisoning treatments use this effect to increase elimination, by alkalizing the urine causing  forced diuresis  which promotes excretion of a weak acid, rather than it getting reabsorbed. As the acid is  ionised , it cannot pass through the  plasma membrane  back into the blood stream and instead gets excreted with the urine. Acidifying the urine has the same effect for weakly basic drugs.


On other occasions drugs combine with bile juices and enter the intestines. In the intestines the drug will join with the unabsorbed fraction of the administered dose and be eliminated with the faeces or it may undergo a new process of absorption to eventually be eliminated by the kidney.


The other elimination pathways are less important in the elimination of drugs, except in very specific cases, such as the respiratory tract for alcohol or anaesthetic gases. The case of mother's milk is of special importance. The liver and kidneys of newly born  infants  are relatively undeveloped and they are highly sensitive to a drug's  toxic effects . For this reason it is important to know if a drug is likely to be eliminated from a woman's body if she is breast feeding in order to avoid this situation.


Pharmacokinetics studies the manner and speed with which drugs and their metabolites are eliminated by the various excretory organs. This elimination will be proportional to the drug's plasmatic concentrations. In order to model these processes a working definition is required for some of the concepts related to excretion.


The plasma half-life or  half life of elimination  is the time required to eliminate 50% of the absorbed dose of a drug from an organism. Or put another way, the time that it takes for the plasma concentration to fall by half from its maximum levels.


The difference in a drug's concentration in arterial blood (before it has circulated around the body) and venous blood (after it has passed through the body's organs) represents the amount of the drug that the body has eliminated or  cleared . Although clearance may also involve other organs than the kidney, it is almost synonymous with  renal clearance  or  renal plasma clearance .  Clearance is therefore expressed as the plasma volume totally free of the drug per unit of time, and it is measured in units of volume per units of time.
Clearance can be determined on an overall, organism level («systemic clearance») or at an organ level (hepatic clearance, renal clearance etc.). The equation that describes this concept is:


Where:

 C

 L

 o

 {\displaystyle CL_{o}}

  is the organ's clearance rate,

 C

 A

 {\displaystyle C_{A}}

  is the drug's plasma concentration in arterial blood,

 C

 V

 {\displaystyle C_{V}}

  is the drug's plasma concentration in venous blood and

 Q

 {\displaystyle Q}

  an organ's blood flow.


Each organ will have its own specific clearance conditions, which will relate to its mode of action. The «renal clearance» rate will be determined by factors such as the degree of  plasma protein binding  as the drug will only be filtered out if it is in the unbound free form, the degree of  saturation  of the transporters (active secretion depends on transporter proteins that can become saturated) or the number of functioning  nephrons  (hence the importance of factors such as  kidney failure ).


As «hepatic clearance» is an active process it is therefore determined by factors that alter an organism's metabolism such as the number of functioning  hepatocytes , this is the reason that  liver failure  has such clinical importance.


The steady state or  stable concentration  is reached when the drug's supply to the blood plasma is the same as the rate of elimination from the plasma. It is necessary to calculate this concentration in order to decide the period between doses and the amount of drug supplied with each dose in prolonged treatments.


Other parameters of interest include a drug's  bioavailability  and the  apparent volume of distribution .


For elimination via bile please see: Estimation of Biliary Excretion of Foreign Compounds Using Properties of Molecular Structure. 2014. Sharifi M., Ghafourian T. AAPS J. 16(1) 65–78.


The  embryological origin of the mouth and anus  is an important characteristic, and forms the morphological basis for separating  bilaterian  animals into two  natural groupings : the  protostomes  and  deuterostomes .


In animals at least as complex as an  earthworm , a dent forms in one side of the early,  spheroidal   embryo . This dent, the  blastopore , 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 the embryo until it reaches the other side, forming an opening that becomes the mouth. [ 1 ]  It was originally thought that the blastopore of the  protostomes  formed the mouth, and the anus formed second when the gut tunneled through the embryo. More recent research has shown that our understanding of protostome mouth formation is somewhat less secure than we had thought. [ 2 ]   Acoelomorpha , which form a sister group to the rest of the bilaterian animals, have a single mouth that leads into a blind gut (with no anus). The genes employed in the embryonic construction of the flatworm mouth are the same as those expressed for the protostome and deuterostome mouth, which suggests that the structures are equivalent  homologous , and that the older ideas [ which? ]  about  protostome  mouth formation were correct. [ 1 ]  An alternative way to develop two openings from the blastopore during  gastrulation , called amphistomy, appears to exist in some animals, such as  nematodes . [ 3 ] [ 4 ]


In humans, the perforation of the mouth and anus happen at four weeks and eight weeks respectively. [ 5 ]


Xenacoelomorpha


Protostomia


Deuterostomia


Bilaterians likely evolved from an ancestor that was radially symmetrical. There have been suggestions that the blastopore started out as the digestive surface on a radial organism that became elongated (and thus bilaterally symmetrical) before its sides closed over to leave a mouth at the front and an anus at the rear. [ 1 ]  This matches with the "flaps-folding-over" model of gut formation, but an alternative view is that the original blastopore migrated forwards to one end of the ancestral organism before deepening to become a blind gut. [ 1 ]  This is consistent with living  Xenacoelomorpha , which are the sister taxon to protostomes and deuterostomes. [ 6 ] [ 7 ]  The story is a little more complex, because the blastopore itself does not directly give rise to the mouth of these worms. [ 1 ]  This suggests that the  last common ancestor of bilaterians  had a similar gut configuration, and that the anus evolved after the mouth. [ 1 ]


Exactly how a through gut formed from this blind gut is somewhat harder to tell. The genetic mechanisms responsible for anus formation are quite variable, which might suggest that the anus evolved several times in different groups. Scientists are currently looking into this matter to generate a more complete picture. [ 1 ] [ 8 ]


In humans (a deuterostome), the development proceeds differently. The  buccopharyngeal membrane  is created in the  foregut  and is perforated during the fourth week of human development, creating the primitive mouth, whereas the  cloacal membrane  is created in the  hindgut  and is perforated during the eighth week of human development, creating the primitive anus after the mouth opening has already been created. [ 5 ]


Enteral respiration , also referred to as  cloacal respiration  or  intestinal respiration , [ 1 ]  is a form of  respiration  in which  gas exchange  occurs across the  epithelia  of the  enteral system , usually in the caudal cavity ( cloaca ). This is used in various species as an alternative respiration mechanism in hypoxic environments as a means to supplement  blood oxygen .


Some  turtles , especially those specialized in diving, are highly reliant on  cloacal   respiration  during dives. [ 2 ]  They accomplish this by having a pair of accessory air bladders connected to the cloaca which can absorb oxygen from the water. [ 3 ]


Various  fish , as well as  polychaete  worms and even  crabs , are specialized to take advantage of the constant flow of water through the cloacal respiratory tree of  sea cucumbers  while simultaneously gaining the protection of living within the sea cucumber itself. At night, many of these species emerge from the anus of the sea cucumber in search of food. [ 4 ]


The  pond loach  is able to respond to the periodic drying in their native habitats by burrowing into the mud and exchanging gas through the posterior end of their alimentary canal. [ 5 ] [ 6 ]


Studies have shown that mammals are capable of performing intestinal respiration to a limited degree in a laboratory setting. [ 1 ]  Mice were subjected to hypoxic conditions and supplied oxygen through their intestines survived an average of 18 minutes compared to 11 minutes in the control group. In 2024  Ig Nobel Prize  an award in physiology has been given to a study proving that pigs are capable of this as well. [ 7 ]  When the intestinal lining was abraded before oxygen was introduced, most of the animals survived for at least 50 minutes. Investigations are planned regarding the effectiveness of the strategy, the safety of this application of  perfluorocarbons , and the feasibility of application to humans. [ 8 ]  It has potential application to people with a  respiratory disease  or lung damage.


Enterochromaffin-like cells  or  ECL cells  are a type of  neuroendocrine cell  found in the gastric glands of the  gastric mucosa  beneath the  epithelium , in particular in the vicinity of  parietal cells , that aid in the production of  gastric acid  via the release of  histamine . They are also considered a type of  enteroendocrine cell . [ 1 ]


ECL cells synthesize and secrete  histamine .  These cells are stimulated by the hormones  gastrin  (not depicted in the adjacent diagram) and  pituitary adenylyl cyclase-activating peptide .  G cells are stimulated by  vagal  stimulation through the neurotransmitter  gastrin-releasing peptide ; this causes the G cells to secrete gastrin, which in turn stimulates ECL cells to release histamine.  Note that this circuit is not activated by acetylcholine (ACh), which is of particular importance because the administration of  atropine  will not block the vagal stimulation of the G cells, as ACh is not the neurotransmitter for these cells.


However, ECL cells are activated directly by ACh on M1 receptors from direct vagal innervation leading to histamine release. This pathway will be inhibited by atropine. [ 2 ]


Gastrin is transferred from a specific type of  G cell  in the gastric epithelium to the ECL cells by  blood . Histamine and gastrin act synergistically as the most important stimulators of  hydrochloric acid  secretion from parietal cells and stimulators of secretion of pepsinogen from  chief cells .  The most important inhibitor of the ECL cell is somatostatin from oxyntic  D cells .


Enterochromaffin-like cells also produce  pancreastatin  and probably other  peptide hormones  and  growth factors .


A prolonged stimulation of these cells causes their  hyperplasia . This is especially important in  gastrinoma  (the  tumors  in which there is an excessive secretion of gastrin), as this is one of the factors contributing to  Zollinger–Ellison's syndrome . It was once believed that tumors of ECL origin form after a prolonged inhibition of gastric acid secretion, however there is no data to support this conclusion and proton pump inhibitors are not thought to contribute to gastric cancer. [ 3 ]


The name is derived from their location in the enteric system and their  chromaffin -like staining pattern in  histologic sections , which is characterized by  silver staining .


Enterocytes , or  intestinal absorptive cells , are  simple columnar   epithelial cells  which line the inner surface of the  small  and  large intestines . A  glycocalyx  surface coat contains  digestive enzymes .  Microvilli  on the apical surface increase its surface area. This facilitates transport of numerous small molecules into the enterocyte from the intestinal  lumen . These include broken down  proteins ,  fats , and  sugars , as well as water,  electrolytes ,  vitamins , and  bile salts . Enterocytes also have an  endocrine  role, secreting hormones such as  leptin .


The major functions of enterocytes include: [ 1 ]


Intestinal stem cell  aging has been studied in  Drosophila  as a model for understanding the biology of stem cell/niche aging. [ 4 ]   Using knockdown mutants defective in various genes that function in the  DNA damage  response in enterocytes, it was shown that deficiency in the DNA damage response accelerates intestinal stem cell aging, thus providing a better understanding of the molecular mechanisms of this aging process. [ 4 ]


An  enterogastrone  is any  hormone  secreted by the  mucosa  of the  duodenum  in the lower  gastrointestinal tract  in response to dietary  lipids  that inhibits the caudal (or "forward, analward") motion of the contents of  chyme . The function of enterogastrone is almost the same as gastric inhibitor peptide, it inhibits gastric secretion and motility of the stomach.


Examples include:


Enterohepatic circulation  is the circulation of  biliary acids ,  bilirubin , drugs or other substances from the  liver  to the  bile , followed by entry into the  small intestine , absorption by the  enterocyte  and transport back to the liver. Enterohepatic circulation is an especially important concept in the field of  toxicology  as many  lipophilic   xenobiotics  undergo this process causing repeated liver damage.


Hepatocytes   metabolize   cholesterol  to  cholic acid  and  chenodeoxycholic acid . These  lipid - soluble   bile acids  are conjugated (reversibly attached) mainly to  glycine  or  taurine  molecules to form  water   soluble  primary conjugated bile acids, sometimes called "bile salts".  These bile acids travel to the gall bladder during the interdigestive phase for storage and to the descending part of the  duodenum  via the  common bile duct  through the major duodenal
papilla during digestion.  95% of the bile acids which are delivered to the duodenum will be recycled by the enterohepatic circulation.


Due to the pH of the small intestine, most of the bile acids are ionized and mostly occur as their sodium salts which are then called “primary conjugated bile salts.”  In the lower small intestine and  colon , bacteria dehydroxylate some of the primary bile salts to form secondary conjugated bile salts (which are still water-soluble). Along the proximal and distal  ileum , these conjugated primary bile salts are reabsorbed actively into hepatic portal circulation. Bacteria deconjugate some of the primary and secondary conjugated bile salts back to lipid-soluble bile acids, which are passively absorbed into hepatic portal circulation. Finally, the conjugated bile acids which remained un-ionized conjugated bile acids are passively absorbed.


Venous blood from the ileum goes straight into the portal vein and then into the  liver sinusoids .  There, hepatocytes extract bile acids very efficiently, and little escapes the healthy liver into systemic circulation.


The net effect of enterohepatic recirculation is that each bile salt molecule is reused about 20 times, often multiple times during a single digestive phase.


The presence of biliary acids in the intestines helps in  absorption  of fats and other substances. [ 1 ]


Bilirubin  is conjugated with  glucuronic acid  in the liver by the enzyme  glucuronyltransferase , making it soluble in water. Much of it goes into the bile and thus out into the small intestine. Although 20% of the secreted bilirubinoid bile is reabsorbed by the small intestine, [ 2 ]  conjugated bilirubin is not reabsorbed in small intestine. All conjugated bilirubin in the large intestine is metabolised by colonic bacteria to  urobilinogen , which is then further oxidized to  urobilin  and  stercobilin . Urobilin, stercobilin and their degradation products give feces its brown color.   [ 3 ]   However, just like bile, some of the urobilinogen  reabsorbed is resecreted in the bile which is also part of enterohepatic circulation. The rest of the reabsorbed urobilinogen is excreted in the urine where it is converted to an oxidized form,  urobilin , which gives urine its characteristic yellow color.


Chloramphenicol ,  aspirin ,  paracetamol ,  diazepam ,  lorazepam ,  morphine ,  metronidazole . Not only drugs but also endogenous substrates like bilirubin,  steroidal hormones  and  thyroxine  utilize this pathway.


Enterohepatic circulation of drugs describes the process by which drugs are conjugated to glucuronic acid in the liver, excreted into bile, metabolized back into the free drug by intestinal bacteria, and the drug is then reabsorbed into plasma. For many drugs that undergo this process, lower doses of drugs can be therapeutically effective because elimination is reduced by the 'recycling' of the drug. But for a small number of drugs that are very toxic to the intestine (e.g.  irinotecan ), these molecules which would not otherwise be very toxic can become so because of this process, and therefore inhibition of this step can be protective. For the majority of drugs which undergo enterohepatic circulation that are not toxic to the intestine, inhibition of this process leads to a reduction of the levels of drug and reduced therapeutic effect. For example, antibiotics that kill gut bacteria often reduce enterohepatic drug circulation and this requires a temporary increase of the drug's dose until the antibiotic use is discontinued and the gut repopulates with bacteria. This effect of antibiotics on enterohepatic circulation of other drugs is one of several types of drug interactions.


Pharmacokinetic  models of the enterohepatic circulation process has been summarized in a recent article. [ 4 ] [ further explanation needed ]


Enterostatin  is a  pentapeptide [ 1 ]  derived from a  proenzyme  in the  gastrointestinal tract  called  procolipase . It reduces food intake, in particular fat intake, [ 2 ]  when given peripherally or into the brain. [ 3 ]


Enterostatin has the sequence Val-Pro-Asp-Pro-Arg in most mammals, but Val-Pro-Gly-Pro-Arg or Val-Pro-Asp-Pro-Arg in rodents and Ala-Pro-Gly-Pro-Arg in humans. [ 4 ]  The sequence that it necessary for its anorexic effects is X-pro-Y-pro-arg and conserved among several vertebrate species. [ 5 ] [ 6 ]


Enterostatin has been detected in gut endocrine cells. [ 7 ]  It is created in the intestine by pancreatic procolipase, the other colipase serving as an obligatory cofactor for  pancreatic lipase  during fat digestion. Enterostatin can be created in the gastric mucosa and the mucosal epithelia in the small intestine. A high fat diet will cause the procolipase gene transcription and enterostatin to release into the gastrointestinal lumen. Enterostatin appears in the lymph and circulation after a meal. Enterostatin has been shown to selectively reduce fat intake during a normal meal. The testing has been successful with different species. [ 8 ]


The signaling pathway of the peripheral mechanism uses afferent vagal to hypothalamic centers. The central responses are mediated through a pathway including serotonergic and opioidergic components. [ 9 ]  Enterostatin cuts fat intake, bodyweight, and body fat. This reaction may involve multiple metabolic effects of enterostatin, which include a decrease of insulin secretion, [ 10 ]  a growth in sympathetic drive to brown adipose tissue, and the stimulation of adrenal corticosteroid secretion. It has been demonstrated that enterostatin stimulates neurons in the amygdala, the arcuate nucleus, the lateral and the ventromedial hypothalamus that have anatomic and functional projections to the paraventricular nucleus (PVN) of the hypothalamus. [ 11 ]  Additionally, enterostatin regulates the expression of  Agouti-related peptide  (AgRP) in a complex manner. [ 12 ]


A possible pathophysiological role is indicated by studies that have associated low enterostatin output and/or responsiveness to breeds of rat that become obese and prefer dietary fat. Humans with obesity also exhibit a lower secretion from pancreatic procolipase after a test meal, compared with persons of normal weight. [ 3 ]


Its effects include a reduction of  insulin  secretion, an increase in sympathetic drive to  brown adipose tissue , and the stimulation of adrenal  corticosteroid  secretion. At the end level, it initiates a sensation of fullness of  stomach  which could be the reason for its role in regulation of fat intake and reduction of  body weight . For enterostatin to be utilized it needs the presence of CCK A receptors. Studies based on rats who lack these receptors have found them to be un-responsive to enterostatin. [ 13 ]


When rats have been injected with high doses of enterostatin into the brain the rats ate progressively less food as the dose was increased. [ 14 ] : 969
In rats, examination of experiments involving the effects of peripheral or intracerebroventricular administration of enterostatin show this selectively slows down fat consumption. [ 15 ] : 8


Although enterostatin-like immunoreactivities exist in blood, brain, and gut, and exogenous enterostatins decrease fat appetite and insulin secretion in rats, the roles of these peptides in human obesity remain to be examined. [ 16 ]  It has been hypothesized that resistance to enterostatin impairs its effects in obesity. [ 16 ]


In  biochemistry ,  enzymatic hydrolysis  is a process in which  enzymes  facilitate the  cleavage of bonds  in  molecules  with the addition of the elements of  water  (i.e.  hydrolysis ). It plays an important role in the  digestion  of food. [ 1 ]


It may be used to help provide  renewable energy , as with  cellulosic ethanol . [ 2 ]


This  molecular  or  cell biology  article is a  stub . You can help Wikipedia by  expanding it .


This  enzyme -related article is a  stub . You can help Wikipedia by  expanding it .


The  epiglottis  ( pl. :  epiglottises  or  epiglottides ) is a leaf-shaped flap in the  throat  that prevents food and water from entering the trachea and the lungs. It stays open during breathing, allowing air into the larynx. During  swallowing , it closes to prevent  aspiration  of food into the lungs, forcing the swallowed liquids or food to go along the  esophagus  toward the stomach instead. It is thus the valve that diverts passage to either the  trachea  or the esophagus.


The epiglottis is made of  elastic cartilage  covered with a  mucous membrane , attached to the entrance of the  larynx . It projects upwards and backwards behind the  tongue  and the  hyoid bone .


The epiglottis may be inflamed in a condition called  epiglottitis , which is most commonly due to the vaccine-preventable bacterium  Haemophilus influenzae . Dysfunction may cause the inhalation of food, called aspiration, which may lead to  pneumonia  or  airway obstruction . The epiglottis is also an important landmark for  intubation .


The epiglottis has been identified as early as Aristotle, and gets its name from being above the  glottis  ( epi-  +  glottis ).


The epiglottis sits at the entrance of the  larynx . It is shaped like a leaf of  purslane  and has a free upper part that rests behind the tongue, and a lower stalk ( Latin :  petiolus ). [ 2 ]  The stalk originates from the back surface of the  thyroid cartilage , connected by a  thyroepiglottic ligament . At the sides, the stalk is connected to the  arytenoid cartilages  at the walls of the larynx by  folds . [ 2 ]


The epiglottis originates at the entrance of the  larynx , and is attached to the hyoid bone. From there, it projects upwards and backwards behind the  tongue . [ 3 ]  The space between the epiglottis and the tongue is called the  vallecula . [ 3 ]


The epiglottis has two surfaces; a forward-facing  anterior  surface, and a  posterior  surface facing the larynx. [ 2 ]  The forward-facing surface is covered with several layers of thin cells ( stratified squamous epithelium ), and is  not covered with keratin , the same surface as the back of the tongue. [ 2 ]  The back surface is covered in a layer of  column-shaped cells  with  cilia , similar to the rest of the  respiratory tract . It also has  mucus -secreting  goblet cells . [ 2 ]  There is an intermediate zone between these surfaces that contains cells that transition in shape. [ 4 ]  The body of the epiglottis consists of  elastic cartilage . [ 2 ]


The epiglottis arises from the fourth  pharyngeal arch . It can be seen as a distinct structure later than the other cartilage of the pharynx, visible around the fifth month of development. [ 1 ]  The position of the epiglottis also changes with ageing. In infants, it touches the  soft palate , whereas in adults, its position is lower. [ 3 ]


A high-rising epiglottis is a normal anatomical variation, visible during an examination of the mouth. It does not cause any serious problem apart from maybe a mild sensation of a foreign body in the throat. It is seen more often in children than adults and does not need any medical or surgical intervention. [ 5 ]  The front surface of the epiglottis is occasionally notched. [ 2 ]


The epiglottis is normally pointed upward during  breathing  with its underside functioning as part of the  pharynx . [ 2 ]  There are  taste buds  on the epiglottis. [ 6 ]


During  swallowing , the epiglottis bends backwards, folding over the entrance to the trachea, and preventing food from going into it. [ 2 ]  The folding backwards is a complex movement the causes of which are not completely understood. [ 2 ]  It is likely that during swallowing the  hyoid bone  and the  larynx  move upwards and forwards, which increases passive pressure from the back of the tongue; the  aryepiglottic muscles  contract; the passive weight of the food pushes down; and the laryngeal and thyroarytenoid muscles contract. [ 2 ]  The consequence of this is that during swallowing the bent epiglottis blocks off the  trachea , preventing food from going into it; food instead travels down the  esophagus , which is behind it. [ 3 ]


In many languages, the epiglottis is not essential for producing sounds. [ 2 ]  In some languages, the epiglottis is used to produce  epiglottal consonant  speech sounds, though this sound-type is rather rare. [ 7 ]


Inflammation of the epiglottis is known as epiglottitis. Epiglottitis is mainly caused by  Haemophilus influenzae . A person with epiglottitis may have a fever, sore throat, difficulty swallowing, and difficulty breathing. For this reason, acute epiglottitis is considered a  medical emergency , because of the risk of obstruction of the pharynx. Epiglottitis is often managed with  antibiotics , inhaled aerosolised  epinephrine  to act as a  bronchodilator , and may require  tracheal intubation  or a  tracheostomy  if breathing is difficult. [ 8 ]


The incidence of epiglottitis has decreased significantly in countries where  vaccination  against  Haemophilus influenzae  is administered. [ 9 ] [ 10 ]


When food or other objects travel down the respiratory tract rather than down the esophagus to the stomach, this is called  aspiration . This can lead to  the obstruction of airways ,  inflammation of lung tissue , and  aspiration pneumonia ; and in the long term,  atelectasis  and  bronchiectasis . [ 3 ]  One reason aspiration can occur is because of failure of the epiglottis to close completely. [ 2 ] [ 3 ]


If food or liquid enters the airway due to the epiglottis failing to close properly, throat-clearing or a cough reflex may occur to protect the respiratory system and expel material from the airway. [ 3 ] [ 11 ]  Where there is impairment in laryngeal vestibule sensation, silent aspiration (entry of material to the airway that does not result in a cough reflex) may occur. [ 3 ] [ 12 ]


The epiglottis and vallecula are important anatomical landmarks in intubation. [ 13 ]  Abnormal positioning of the epiglottis is a rare cause of  obstructive sleep apnoea . [ 14 ]


The epiglottis is present in  mammals , [ 15 ]  including land mammals and  cetaceans , [ 16 ]  also as a cartilaginous structure. [ 17 ]  Like in humans, it functions to prevent entry of food into the trachea during swallowing. [ 17 ]  The position of the larynx is flat in mice and other rodents, as well as rabbits. [ 4 ]  For this reason, because the epiglottis is located behind the soft palate in rabbits, they are obligate nose breathers, [ 18 ] [ 19 ]  as are mice and other rodents. [ 4 ]  In rodents and mice, there is a unique pouch in front of the epiglottis, and the epiglottis is commonly injured by inhaled substances, particularly at the transition zone between the flattened and cuboidal epithelium. [ 20 ] [ 4 ]  It is also common to see taste buds on the epiglottis in these species. [ 4 ]


The epiglottis was noted by  Aristotle , [ 15 ]  although the epiglottis' function was first defined by  Vesalius  in 1543. [ 21 ]  The word has Greek roots. [ 22 ]  The epiglottis gets its name from being above ( Ancient Greek :  ἐπί ,  romanized :  epi- ) the  glottis  ( Ancient Greek :  γλωττίς ,  romanized :  glottis ,  lit.   'tongue'). [ 23 ]


The  esophageal glands  are glands that are part of the digestive system of various animals, including humans.


In humans the glands are known as the  esophageal submucosal glands  and are a part of the  human digestive system . [ 1 ]  They are a small compound  racemose   exocrine glands  of the  mucous  type. [ citation needed ]


There are two types:


Each opens upon the surface by a long excretory  duct . [ citation needed ]


The esophageal gland is enlarged in large  monoplacophoran  species. [ 3 ]


The esophageal gland or  oesophageal pouch  is a part of the  digestive system of some gastropods . The esophageal gland or pouch is a common feature in so-called basal gastropod clades, including  Patelloidea ,  Vetigastropoda ,  Cocculiniformia ,  Neritimorpha  and  Neomphalina . [ 4 ]


The size of the esophageal gland of the  scaly-foot gastropod   Chrysomallon squamiferum  (family  Peltospiridae  within Neomphalina) is about two orders of magnitude over the usual size. [ 4 ]  The scaly-foot gastropod houses endosymbiotic bacteria in the esophageal gland. [ 4 ]   Chrysomallon squamiferum  was thought to be the only species of Peltospiridae, that has an enlarged esophageal gland, but later it was shown that both species  Gigantopelta  the gland also enlarged. [ 5 ]  In other peltospirids, the posterior portion of the oesophagus forms a pair of blind mid-oesophageal pouches or gutters extending only to the anterior end of the foot ( Rhynchopelta ,  Peltospira ,  Nodopelta ,  Echinopelta ,  Pachydermia ). [ 4 ]  The same situation is in  Melanodrymia  within the family  Melanodrymiidae . [ 4 ]   Bathyphytophilidae  and  Lepetellidae  are also known to have enlarged esophageal pouches, however, though not to the extent of  Chrysomallon . [ 4 ]  Both are known to house endosymbiotic bacteria, in the case of bathyphytophilids most likely also in the esophageal glands but in the lepetellids the endosymbionts are spread in the  hemocoel . [ 4 ]


This article incorporates text in the  public domain  from  page 1146  of the 20th edition of   Gray's Anatomy   (1918) .
This article incorporates Creative Commons (CC-BY-4.0) text from the reference [ 4 ]


Esophageal spasm  is a disorder of  motility  of the  esophagus . [ 2 ]


There are two types of esophageal spasm: [ 2 ]


Both conditions are linked to  gastroesophageal reflux disease  (GERD). [ 2 ]  DES and nutcracker esophagus present similarly and can may require  esophageal manometry  for differentiation. [ 3 ]


When the coordinated  muscle contraction  are irregular or uncoordinated, this condition may be called diffuse esophageal spasm. These spasms can prevent food from reaching the stomach where food gets stuck in the esophagus. At other times the coordinated muscle contraction is very powerful, which is called nutcracker esophagus. These contractions move food through the esophagus but can cause severe pain. [ 4 ]


The symptoms may include  trouble swallowing ,  regurgitation ,  chest pain , [ 5 ]   heartburn , [ 6 ]   globus pharyngis  (which is a feeling that something is stuck in the  throat ) or a dry  cough . [ 7 ]


It is not clear what causes esophageal spasms. [ 1 ] [ 7 ]  Sometimes esophageal spasms start when someone eats hot or cold foods or drinks. However, they can also occur without eating or drinking. [ 7 ]  The increased release of  acetylcholine  may also be a factor, but the triggering event is not known. [ 6 ]  Spasms may also be the result of a food intolerance.


The diagnosis is generally confirmed by  esophageal manometry . [ 2 ]  DES is present when more than a fifth of swallows results in distal esophageal contractions. [ 2 ]  NE is present if the average strength of the contractions of the distal esophagus is greater than 180 mmHg but the contraction of the esophagus is otherwise normal. [ 2 ]


Often, symptoms that may suggest esophageal spasm are the result of another condition such as food intolerance,  gastroesophageal reflux disease  (GERD) or  achalasia . [ 4 ]  The symptoms can commonly be mistaken as heart palpitations.


Since esophageal spasms are often associated with other disorders, management in these cases involve attempts to correct the underlying problem. Medications may include use of  calcium channel blockers  (CCBs) and nitrates.  Tricyclic antidepressants  (TCA) and  sildenafil  can be used as alternative treatment options. If caused by food allergy, an elimination diet may be necessary.


There have been reports of treatment of distal esophageal spasm in 8 patients using low amounts of  peppermint oil  (five drops of peppermint oil in 10ml of water), [ 8 ]  and a report of treatment of an elderly patient with the same diagnosis. [ 9 ]


If medical therapy fails either  botulinum toxin  injection or surgical  myotomy  may be tried in distal esophageal spasms. [ 10 ]


Distal esophageal spasm is rare. [ 10 ]


The  esophagus  ( American English ),  oesophagus  ( British English ), or œsophagus ( archaic spelling ) ( see spelling difference ) all  / iː ˈ s ɒ f ə ɡ ə s ,   ɪ -/ ; [ 1 ]   pl. :  ((o)e)(œ)sophagi  or ( (o)e)(œ)sophaguses ), colloquially known also as the  food pipe ,  food tube , or  gullet , is an  organ  in  vertebrates  through which  food  passes, aided by  peristaltic contractions , from the  pharynx  to the  stomach . The esophagus is a  fibromuscular  tube, about 25 cm (10 in) long in adults, that travels behind the  trachea  and  heart , passes through the  diaphragm , and empties into the uppermost region of the  stomach . During swallowing, the  epiglottis  tilts backwards to prevent food from going down the  larynx  and lungs. The word  oesophagus  is from Ancient Greek οἰσοφάγος (oisophágos), from οἴσω (oísō), future form of φέρω (phérō, "I carry") + ἔφαγον (éphagon, "I ate").


The wall of the esophagus from the  lumen  outwards consists of  mucosa ,  submucosa  (connective tissue),  layers of muscle fibers  between layers of  fibrous tissue , and an outer layer of connective tissue. The mucosa is a  stratified squamous epithelium  of around three layers of squamous cells, which contrasts to the  single layer of columnar cells  of the stomach. The transition between these two types of epithelium is visible as a zig-zag line. Most of the muscle is  smooth muscle  although  striated muscle  predominates in its upper third. It has two muscular rings or  sphincters  in its wall, one at the top and one at the bottom. The lower sphincter helps to prevent reflux of acidic stomach content. The esophagus has a rich blood supply and venous drainage. Its smooth muscle is innervated by involuntary nerves ( sympathetic nerves  via the  sympathetic trunk  and  parasympathetic nerves  via the  vagus nerve ) and in addition voluntary nerves ( lower motor neurons ) which are carried in the vagus nerve to innervate its striated muscle.


The esophagus passes through the thoracic cavity into the  diaphragm  into the stomach.


The esophagus may be affected by  gastric reflux ,  cancer , prominent dilated blood vessels called  varices  that can bleed heavily,  tears , constrictions, and disorders of motility. Diseases may cause difficulty swallowing ( dysphagia ), painful swallowing ( odynophagia ),  chest pain , or cause no symptoms at all. Clinical investigations include  X-rays  when swallowing  barium sulfate ,  endoscopy , and  CT scans . Surgically,
the esophagus is difficult to access in part due to its position between critical organs and directly between the sternum and spinal column. [ 2 ]


The esophagus is one of the upper parts of the  digestive system . There are  taste buds  on its upper part. [ 3 ]  It begins at the back of the mouth, passing downward through the rear part of the  mediastinum , through the  diaphragm , and into the stomach. In humans, the esophagus generally starts around the level of the sixth  cervical vertebra  behind the  cricoid cartilage  of the  trachea , enters the diaphragm at about the level of the  tenth thoracic vertebra , and ends at the  cardia  of the stomach, at the level of the  eleventh thoracic vertebra . [ 4 ]  The esophagus is usually about 25 cm (10 in) in length. [ 5 ]


Many  blood vessels  serve the esophagus, with blood supply varying along its course. The upper parts of the esophagus and the upper esophageal sphincter receive blood from the  inferior thyroid artery , the parts of the esophagus in the thorax from the  bronchial arteries  and branches directly from the  thoracic aorta , and the lower parts of the esophagus and the lower esophageal sphincter receive blood from the  left gastric artery  and the left  inferior phrenic artery . [ 6 ] [ 7 ]  The venous drainage also differs along the course of the esophagus. The upper and middle parts of the esophagus drain into the  azygos  and  hemiazygos veins , and blood from the lower part drains into the  left gastric vein . All these veins drain into the  superior vena cava , with the exception of the left gastric vein, which is a branch of the  portal vein . [ 6 ]  Lymphatically, the upper third of the esophagus drains into the  deep cervical lymph nodes , the middle into the superior and posterior mediastinal lymph nodes, and the lower esophagus into the  gastric  and  celiac lymph nodes . This is similar to the lymphatic drainage of the abdominal structures that arise from the  foregut , which all drain into the celiac nodes. [ 6 ]


The upper esophagus lies at the back of the mediastinum behind the  trachea , adjoining along the  tracheoesophageal stripe , and in front of the  erector spinae muscles  and the  vertebral column . The lower esophagus lies behind the  heart  and curves in front of the  thoracic aorta . From the  bifurcation of the trachea  downwards, the esophagus passes behind the right  pulmonary artery ,  left main bronchus , and  left atrium . At this point, it passes through the diaphragm. [ 4 ]


The  thoracic duct , which drains the majority of the body's  lymph , passes behind the esophagus, curving from lying behind the esophagus on the right in the lower part of the esophagus, to lying behind the esophagus on the left in the upper esophagus. The esophagus also lies in front of parts of the  hemiazygos veins  and the  intercostal veins  on the right side. The  vagus nerve  divides and covers the esophagus in a  plexus . [ 4 ]


The esophagus has four points of constriction. When a corrosive substance, or a solid object is swallowed, it is most likely to lodge and damage one of these four points. These constrictions arise from particular structures that compress the esophagus. These constrictions are: [ 8 ]


The esophagus is surrounded at the top and bottom by two muscular rings, known respectively as the upper esophageal sphincter and the lower esophageal sphincter. [ 4 ]  These  sphincters  act to close the esophagus when food is not being swallowed. The upper esophageal sphincter is an anatomical sphincter, which is formed by the lower portion of the inferior pharyngeal constrictor, also known as the cricopharyngeal sphincter due to its relation with  cricoid cartilage  of the  larynx  anteriorly. However, the lower esophageal sphincter is not an anatomical but rather a functional sphincter, meaning that it acts as a sphincter but does not have a distinct thickening like other sphincters.


The upper esophageal sphincter surrounds the upper part of the esophagus. It consists of  skeletal muscle  but is not under  voluntary control . Opening of the upper esophageal sphincter is triggered by the  swallowing reflex . The primary muscle of the upper esophageal sphincter is the  cricopharyngeal part  of the  inferior pharyngeal constrictor . [ 9 ]


The lower esophageal sphincter, or gastroesophageal sphincter, surrounds the lower part of the esophagus at the junction between the esophagus and the stomach. [ 10 ]  It is also called the cardiac sphincter or cardioesophageal sphincter, named from the adjacent part of the stomach, the  cardia . Dysfunction of the gastroesophageal sphincter causes gastroesophageal  reflux , which causes  heartburn , and, if it happens often enough, can lead to  gastroesophageal reflux disease , with damage of the esophageal mucosa. [ 11 ]


The esophagus is innervated by the vagus nerve and the cervical and thoracic  sympathetic trunk . [ 6 ]  The vagus nerve has a  parasympathetic  function, supplying the muscles of the esophagus and stimulating glandular contraction. Two sets of nerve fibers travel in the vagus nerve to supply the muscles. The upper striated muscle, and upper esophageal sphincter, are supplied by neurons with bodies in the  nucleus ambiguus , whereas fibers that supply the smooth muscle and lower esophageal sphincter have bodies situated in the  dorsal motor nucleus . [ 6 ]  The vagus nerve plays the primary role in initiating  peristalsis . [ 12 ]  The sympathetic trunk has a  sympathetic  function. It may enhance the function of the vagus nerve, increasing peristalsis and glandular activity, and causing sphincter contraction. In addition, sympathetic activation may relax the muscle wall and cause blood vessel constriction. [ 6 ]  Sensation along the esophagus is supplied by both nerves, with gross sensation being passed in the vagus nerve and pain passed up the sympathetic trunk. [ 4 ]


The gastroesophageal junction (also known as the esophagogastric junction) is the junction between the esophagus and the stomach, at the lower end of the esophagus. [ 13 ]  The pink color of the esophageal mucosa contrasts to the deeper red of the gastric mucosa, [ 6 ] [ 14 ]  and the mucosal transition can be seen as an irregular zig-zag line, which is often called the z-line. [ 15 ]  Histological examination reveals abrupt transition between the  stratified squamous epithelium  of the esophagus and the  simple columnar epithelium  of the  stomach . [ 16 ]  Normally, the  cardia  of the stomach is immediately distal to the z-line [ 17 ]  and the z-line coincides with the upper limit of the gastric folds of the cardia; however, when the anatomy of the mucosa is distorted in  Barrett's esophagus  the true gastroesophageal junction can be identified by the upper limit of the gastric folds rather than the mucosal transition. [ 18 ]  The functional location of the lower oesophageal sphincter is generally situated about  3 cm ( 1 + 1 ⁄ 4  in) below the z-line. [ 6 ]


The human esophagus has a  mucous membrane  consisting of a tough  stratified squamous epithelium  without  keratin , a smooth  lamina propria , and a  muscularis mucosae . [ 6 ]  The epithelium of the esophagus has a relatively rapid turnover and serves a protective function against the abrasive effects of food. In many animals, the epithelium contains a layer of keratin, representing a coarser diet. [ 19 ]  There are two types of glands, with mucus-secreting  esophageal glands  being found in the  submucosa  and esophageal cardiac glands, similar to  cardiac glands  of the stomach, located in the lamina propria and most frequent in the terminal part of the organ. [ 19 ] [ 20 ]  The  mucus  from the glands gives a good protection to the lining. [ 21 ]  The submucosa also contains the  submucosal plexus , a network of  nerve cells  that is part of the  enteric nervous system . [ 19 ]


The  muscular layer  of the esophagus has two types of muscle. The upper third of the esophagus contains  striated muscle , the lower third contains  smooth muscle , and the middle third contains a mixture of both. [ 6 ]  Muscle is arranged in two layers: one in which the muscle fibers run longitudinal to the esophagus, and the other in which the fibers encircle the esophagus. These are separated by the  myenteric plexus , a tangled network of nerve fibers involved in the secretion of mucus and in peristalsis of the smooth muscle of the esophagus. The outermost layer of the esophagus is the  adventitia  in most of its length, with the abdominal part being covered in  serosa . This makes it distinct from many other structures in the gastrointestinal tract that only have a serosa. [ 6 ]


In early  embryogenesis , the esophagus develops from the  endodermal   primitive gut tube . The ventral part of the embryo abuts the  yolk sac . During the second week of embryological development, as the embryo grows, it begins to surround parts of the sac. The enveloped portions form the basis for the adult gastrointestinal tract. [ 22 ]  The sac is surrounded by a network of  vitelline arteries . Over time, these arteries consolidate into the three main arteries that supply the developing gastrointestinal tract: the  celiac artery ,  superior mesenteric artery , and  inferior mesenteric artery . The areas supplied by these arteries are used to define the  midgut ,  hindgut  and  foregut . [ 22 ]


The surrounded sac becomes the primitive gut. Sections of this gut begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus,  stomach , and  intestines . [ 22 ]  The esophagus develops as part of the foregut tube. [ 22 ]  The innervation of the esophagus develops from the  pharyngeal arches . [ 4 ]


Food is  ingested  through the  mouth  and when  swallowed  passes first into the  pharynx  and then into the esophagus. The esophagus is thus one of the first components of the  digestive system  and the  gastrointestinal tract . After food passes through the esophagus, it enters the stomach. [ 10 ]  When food is being swallowed, the  epiglottis  moves backward to cover the  larynx , preventing food from entering the  trachea . At the same time, the upper esophageal sphincter relaxes, allowing a  bolus  of  food  to enter.  Peristaltic  contractions of the esophageal muscle push the food down the esophagus. These rhythmic contractions occur both as a reflex response to food that is in the mouth, and also as a response to the sensation of food within the esophagus itself. Along with peristalsis, the lower esophageal sphincter relaxes. [ 10 ]


The stomach produces  gastric acid , a strongly  acidic  mixture consisting of  hydrochloric acid  (HCl) and  potassium  and  sodium  salts to enable food  digestion . Constriction of the upper and lower esophageal sphincters helps to prevent reflux (backflow) of gastric contents and acid into the esophagus, protecting the esophageal mucosa. The acute  angle of His  and the lower  crura of the diaphragm  also help this sphincteric action. [ 10 ] [ 23 ]


About 20,000 protein-coding genes are expressed in human cells and nearly 70% of these genes are expressed in the normal esophagus. [ 24 ] [ 25 ]  Some 250 of these genes are more specifically expressed in the esophagus with less than 50 genes being highly specific. The corresponding esophagus-specific proteins are mainly involved in squamous differentiation such as  keratins   KRT13 ,  KRT4  and  KRT6C . Other specific proteins that help lubricate the inner surface of esophagus are  mucins  such as  MUC21  and MUC22. Many genes with elevated expression are also shared with skin and other organs that are composed of  squamous epithelia . [ 26 ]


The main conditions affecting the esophagus are described here. For a more complete list, see  esophageal disease .


Inflammation of the esophagus is known as  esophagitis .  Reflux  of  gastric acids  from the stomach, infection, substances ingested (for example,  corrosives ), some  medications  (such as  bisphosphonates ), and  food allergies  can all lead to esophagitis.  Esophageal candidiasis  is an infection of the yeast  Candida albicans  that may occur when a person is  immunocompromised . As of 2021 [update]  the causes of some forms of esophagitis, such as  eosinophilic esophagitis , are not well-characterized, but may include  Th2 -mediated  atopies  or genetic factors. There appear to be correlations between eosinophilic esophagitis,  asthma  (itself with an  eosinophilic  component),  eczema , and  allergic rhinitis , though it is not clear whether these conditions contribute to eosinophilic esophagitis or vice versa, or if they are symptoms of mutual underlying factors. [ 27 ]  Esophagitis can cause  painful swallowing  and is usually treated by managing the cause of the esophagitis - such as managing reflux or treating infection. [ 5 ]


Prolonged esophagitis, particularly from gastric reflux, is one factor thought to play a role in the development of  Barrett's esophagus . In this condition, there is  metaplasia  of the lining of the lower esophagus, which changes from  stratified squamous epithelia  to  simple columnar epithelia . Barrett's esophagus is thought to be one of the main contributors to the development of  esophageal cancer . [ 5 ]


There are two main types of  cancer of the esophagus .  Squamous cell carcinoma  is a  carcinoma  that can occur in the squamous cells lining the esophagus. This type is much more common in  China  and  Iran . The other main type is an  adenocarcinoma  that occurs in the glands or columnar tissue of the esophagus. This is most common in  developed countries  in those with Barrett's esophagus, and occurs in the cuboidal cells. [ 5 ]


In its early stages, esophageal cancer may not have any symptoms at all. When severe, esophageal cancer may eventually cause obstruction of the esophagus, making swallowing of any solid foods very difficult and causing weight loss. The progress of the cancer is  staged using a system  that measures how far into the esophageal wall the cancer has invaded, how many  lymph nodes  are affected, and whether there are any  metastases  in different parts of the body. Esophageal cancer is often managed with radiotherapy, chemotherapy, and may also be managed by  partial surgical removal of the esophagus . Inserting a  stent  into the esophagus, or inserting a  nasogastric tube , may also be used to ensure that a person is able to digest enough food and water. As of 2014 [update] , the prognosis for esophageal cancer is still poor, so  palliative therapy  may also be a focus of treatment. [ 5 ]


Esophageal varices  are swollen twisted branches of the  azygous vein  in the lower third of the esophagus. These blood vessels  anastomose  (join up) with those of the  portal vein  when  portal hypertension  develops. [ 28 ]  These blood vessels are engorged more than normal, and in the worst cases may partially obstruct the esophagus. These blood vessels develop as part of a collateral circulation that occurs to drain blood from the  abdomen  as a result of  portal hypertension , usually as a result of  liver diseases  such as  cirrhosis . [ 5 ] : 941–42   This collateral circulation occurs because the lower part of the esophagus drains into the left gastric vein, which is a branch of the portal vein. Because of the extensive venous plexus that exists between this vein and other veins, if portal hypertension occurs, the direction of blood drainage in this vein may reverse, with blood draining from the portal venous system, through the plexus. Veins in the plexus may engorge and lead to varices. [ 6 ] [ 7 ]


Esophageal varices often do not have symptoms until they rupture. A ruptured varix is considered a medical emergency because varices can bleed a lot. A bleeding varix may cause a person  to vomit blood , or suffer  shock . To deal with a ruptured varix, a band may be placed around the bleeding blood vessel, or a small amount of a clotting agent may be injected near the bleed. A surgeon may also try to use a small inflatable balloon to apply pressure to stop the wound.  IV fluids  and  blood products  may be given in order to prevent  hypovolemia  from excess blood loss. [ 5 ]


Several disorders affect the motility of food as it travels down the esophagus. This can cause difficult swallowing, called  dysphagia , or painful swallowing, called  odynophagia .  Achalasia  refers to a failure of the lower esophageal sphincter to relax properly, and generally develops later in life. This leads to progressive enlargement of the esophagus, and possibly eventual  megaesophagus . A  nutcracker esophagus  refers to swallowing that can be extremely painful.  Diffuse esophageal spasm  is a spasm of the esophagus that can be one cause of chest pain. Such  referred pain  to the wall of the upper chest is quite common in esophageal conditions. [ 29 ]  Sclerosis of the esophagus, such as with  systemic sclerosis  or in  CREST syndrome  may cause hardening of the walls of the esophagus and interfere with peristalsis. [ 5 ]


Esophageal strictures  are usually benign and typically develop after a person has had  reflux  for many years. Other strictures may include  esophageal webs  (which can also be congenital) and damage to the esophagus by radiotherapy, corrosive ingestion, or eosinophilic esophagitis. A  Schatzki ring  is fibrosis at the gastroesophageal junction. Strictures may also develop in chronic  anemia , and  Plummer-Vinson  syndrome. [ 5 ]


Two of the most common  congenital  malformations affecting the esophagus are an  esophageal atresia  where the esophagus ends in a blind sac instead of connecting to the stomach; and an  esophageal fistula  – an abnormal connection between the esophagus and the trachea. [ 30 ]  Both of these conditions usually occur together. [ 30 ]  These are found in about 1 in 3500 births. [ 31 ]  Half of these cases may be part of a  syndrome  where other abnormalities are also present, particularly of the  heart  or  limbs . The other cases occur singly. [ 32 ]


An  X-ray  of  swallowed barium  may be used to reveal the size and shape of the esophagus, and the presence of any masses. The esophagus may also be  imaged  using a flexible camera inserted into the esophagus, in a procedure called an  endoscopy . If an endoscopy is used on the stomach, the camera will also have to pass through the esophagus. During an endoscopy, a  biopsy  may be taken. If cancer of the esophagus is being investigated, other methods, including a  CT scan , may also be used. [ 5 ]


The word  esophagus  ( British English :  oesophagus ), comes from the  Greek :  οἰσοφάγος  ( oisophagos ) meaning  gullet . It derives from  two roots   (eosin)  to carry and ( phagos ) to eat. [ 33 ]  The use of the word oesophagus, has been documented in anatomical literature since at least the time of  Hippocrates , who noted that "the oesophagus ... receives the greatest amount of what we consume." [ 34 ]  Its existence in other animals and its relationship with the  stomach  was documented by the  Roman   naturalist   Pliny the Elder  (AD23–AD79), [ 35 ]  and the  peristaltic  contractions of the esophagus have been documented since at least the time of  Galen . [ 36 ]


The first attempt at surgery on the esophagus focused in the neck, and was conducted in dogs by  Theodore Billroth  in 1871. In 1877  Czerny  carried out surgery in people. By 1908, an operation had been performed by Voeckler to remove the esophagus, and in 1933 the first surgical removal of parts of the lower esophagus, (to control  esophageal cancer ), had been conducted. [ 37 ]


The  Nissen fundoplication , in which the stomach is wrapped around the lower  esophageal sphincter  to stimulate its function and control  reflux , was first conducted by  Rudolph Nissen  in 1955. [ 37 ]


In  tetrapods , the pharynx is much shorter, and the esophagus correspondingly longer, than in fish. In the majority of vertebrates, the esophagus is simply a connecting tube, but in some  birds , which regurgitate components to feed their young, it is extended towards the lower end to form a  crop  for storing food before it enters the true stomach. [ 39 ] [ 40 ]  In  ruminants , animals with four chambered stomachs, a groove called the  sulcus reticuli  is often found in the esophagus, allowing milk to drain directly into the hind stomach, the  abomasum . [ 41 ]  In the  horse  the esophagus is about 1.2 to 1.5 m (4 to 5 ft) in length, and carries food to the stomach. A muscular ring, called the cardiac sphincter, connects the stomach to the esophagus. This sphincter is very well developed in horses. This and the oblique angle at which the esophagus connects to the stomach explains why horses cannot  vomit . [ 42 ]  The esophagus is also the area of the digestive tract where horses may have the condition known as  choke .


The esophagus of  snakes  is remarkable for the distension it undergoes when swallowing prey. [ 43 ]


In most fish, the esophagus is extremely short, primarily due to the length of the pharynx (which is associated with the  gills ). However, some fish, including  lampreys ,  chimaeras , and  lungfish , have no true stomach, so that the esophagus effectively runs from the pharynx directly to the  intestine , and is therefore somewhat longer. [ 39 ]


In many vertebrates, the esophagus is lined by  stratified squamous epithelium  without glands. In fish, the esophagus is often lined with  columnar epithelium , [ 40 ]  and in  amphibians ,  sharks  and  rays , the esophageal epithelium is  ciliated , helping to wash food along, in addition to the action of muscular peristalsis. [ 39 ]  In addition, in the  bat   Plecotus auritus , fish and some amphibians, glands secreting  pepsinogen  or  hydrochloric acid  have been found. [ 40 ]


The muscle of the esophagus in many mammals is initially striated but then becomes smooth muscle in the caudal third or so. In  canines  and  ruminants , however, it is entirely striated to allow regurgitation to feed young (canines) or regurgitation to chew cud (ruminants). It is entirely smooth muscle in amphibians, reptiles and birds. [ 40 ]


Contrary to popular belief, [ 44 ]  an adult  human body  would not be able to pass through the esophagus of a  whale , which generally measures less than 10 cm (4 in) in diameter, although in larger  baleen whales  it may be up to 25 cm (10 in) when fully distended. [ 45 ]


A structure with the same name is often found in invertebrates, including  molluscs  and  arthropods , connecting the oral cavity with the stomach. [ 46 ]  In terms of the  digestive system of snails and slugs , the mouth opens into an esophagus, which connects to the stomach. Because of  torsion , which is the rotation of the main body of the animal during larval development, the esophagus usually passes around the stomach, and opens into its back, furthest from the mouth. In species that have undergone de-torsion, however, the esophagus may open into the anterior of the stomach, which is the reverse of the usual gastropod arrangement. [ 47 ]  There is an extensive  rostrum  at the front of the esophagus in all carnivorous snails and slugs. [ 48 ]  In the freshwater snail species  Tarebia granifera , the brood pouch is above the esophagus. [ 49 ]


In the  cephalopods , the brain often surrounds the esophagus. [ 50 ]


Excretion  is elimination of  metabolic waste , which is an essential process in all  organisms . In  vertebrates , this is primarily carried out by the  lungs ,  kidneys , and  skin . [ 1 ]  This is in contrast with  secretion , where the substance may have specific tasks after leaving the  cell . For example,  placental mammals  expel  urine  from the  bladder  through the  urethra , [ 2 ]  which is part of the  excretory system .  Unicellular organisms  discharge waste products directly through the surface of the cell.


During life activities such as  cellular respiration , several chemical reactions take place in the body. These are known as  metabolism . These chemical reactions produce waste products such as  carbon dioxide , water,  salts ,  urea  and  uric acid . Accumulation of these wastes beyond a level inside the body is harmful to the body. The excretory organs remove these wastes. This process of removal of metabolic waste from the body is known as excretion.


Green  plants  excrete carbon dioxide and water as respiratory products. In green plants, the  carbon dioxide  released during respiration gets used during photosynthesis.  Oxygen  is a byproduct generated during  photosynthesis , and exits through  stomata , root cell walls, and other routes. Plants can get rid of excess water by transpiration and  guttation . It has been shown that the leaf acts as an 'excretophore' and, in addition to being a primary organ of photosynthesis, is also used as a method of excreting toxic wastes via diffusion. Other waste materials that are exuded by some plants —  resin , saps,  latex , etc. are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells. These latter processes do not need added energy, they act passively. However, during the pre-abscission phase, the metabolic levels of a leaf are high. [ 3 ] [ 4 ]  Plants also excrete some waste substances into the soil around them. [ 5 ]


In animals, the main excretory products are  carbon dioxide ,  ammonia  (in  ammoniotelics ),  urea  (in  ureotelics ),  uric acid  (in  uricotelics ),  guanine  (in  Arachnida ), and  creatine . The liver and kidneys  clear  many substances from the  blood  (for example, in  renal excretion ), and the cleared substances are then excreted from the body in the  urine  and  feces . [ 6 ]


Aquatic animals  usually excrete  ammonia  directly into the external environment, as this compound has high  solubility  and there is ample water available for dilution. In  terrestrial animals , ammonia-like compounds are converted into other nitrogenous materials, i.e.  urea , that are less harmful as there is less water in the environment and ammonia itself is  toxic . This process is called detoxification. [ 7 ]


Birds  excrete their nitrogenous wastes as  uric acid  in the form of a paste. Although this process is metabolically more expensive, it allows more efficient  water  retention and it can be stored more easily in the  egg . Many avian species, especially  seabirds , can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the  beak .


In  insects , a system involving  Malpighian tubules  is used to excrete  metabolic waste . Metabolic waste diffuses or is actively transported into the tubule, which transports the wastes to the intestines. The metabolic waste is then released from the body along with fecal matter.


The excreted material may be called  ejecta . [ 8 ]  In  pathology  the word ejecta is more commonly used. [ 9 ]


Fart  is a word in the  English language  most commonly used in reference to  flatulence  that can be used as a  noun  or a  verb . [ 1 ]  The immediate roots are in the Middle English words  ferten ,  feortan  and  farten , kin of the Old High German word  ferzan .  Cognates are found in  Old Norse , Slavic and also Greek and Sanskrit. The word  fart  has been incorporated into the  colloquial  and  technical speech  of a number of occupations, including computing. It is often considered unsuitable in formal situations as it may be considered vulgar or offensive.


The English word  fart  is one of the oldest words in the English lexicon.  Its  Indo-European  origins are confirmed by the many  cognate  words in some other  Indo-European languages :  It is cognate with  Greek  verb πέρδομαι (perdomai), [ 2 ]  as well as the  Latin   pēdĕre ,  Sanskrit   pardate ,  Ashkun   pidiṅ , [ 3 ]   Avestan   pərəδaiti ,  Italian   fare un peto ,  French  "péter",  Russian  пердеть (perdet') and  Polish  " pierd " <<  PIE  * perd  [break wind loudly] or * pezd  [the same, softly], all of which mean the same thing.  Like most Indo-European roots in the  Germanic languages , it was altered under  Grimm's law , so that Indo-European  /p/  >  /f/ , and  /d/  >  /t/ , as the  German  cognate  furzen  also manifests. [ 4 ] [ 5 ] [ 6 ] [ 7 ]


In certain circles the word is considered merely a common  profanity  with an often  humorous   connotation . For example, a person may be referred to as a 'fart', or an 'old fart', not necessarily depending on the person's age.  This may convey the sense that a person is boring or unduly fussy and be intended as an insult, mainly when used in the second or third person.  For example, '"he's a boring old fart!"  However the word may be used as a colloquial term of  endearment  or in an attempt at humorous self-deprecation (e.g., in such phrases as "I know I'm just an old fart" or "you do like to fart about!").  'Fart' is often only used as a term of endearment when the subject is personally well known to the user.