Digestive organs (esophagus is #1)
|System||Part of the Digestive system|
|Nerve||Celiac ganglia, vagus|
The esophagus (oesophagus, commonly known as the gullet) is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. The word esophagus is derived from the Latin oesophagus, which derives from the Greek word oisophagos, lit. "entrance for eating."
- 1 Structure
- 2 Function
- 3 Clinical significance
- 4 History
- 5 Other animals
- 6 Additional images
- 7 References
- 8 External links
The esophagus is one of the upper parts of the gastrointestinal system. At the mouth end, it is continuous with the back of the oral cavity. It passes from the back of the oral cavity downwards and through the back part of the mediastinum, through the diaphragm, and into the stomach. In humans, the esophagus generally starts at about the level of the C6 vertebrae, behind the cricoid cartilage, enters the diaphragm at about T10, and ends at the cardia of the stomach, at T11. :192 The esophagus is usually about 25cm in length.
The upper esophagus lies at the back of the mediastinum behind the trachea, and in front of the erector spinae muscles and the vertebral column. The lower esophagus lies behind the heart and curves in front of 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. :192
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.:192-4
The esophagus is surrounded at the top and bottom by two muscular rings, the upper and lower esophageal sphincters. :192 These are known as the upper esophageal sphincter and lower esophageal sphincter, and act to close the esophagus when food not being swallowed. The esophageal sphincters are functional, but not anatomical sphincters, meaning that they are sections of the oesophageal wall that act as a sphincter, but do not have distinct thickenings like other sphincters. 
The upper esophageal sphincter surrounds the upper part of the esophagus. It consists of skeletal muscle, but is not under conscious control. Opening of the upper esophageal sphincter is triggered by the swallow reflex. The primary muscle of the upper esophageal sphincter is the cricopharyngeus portion of the inferior pharyngeal constrictor.
The lower esophageal sphincter surrounds the lower part of the esophagus. This sphincter surrounds the junction between the esophagus and the stomach. 
The esophagus has four points of constriction. When corrosive substances are ingested, or a solid object is swallowed, it is most likely to lodge and damage these four points. These constrictions are because of particular structures that compress on the esophagus. These constrictions are::192
- At the start of the esophagus, where the pharynx joins the esophagus, behind the cricoid cartilage
- Where it is crossed on the front by the aortic arch and the left main bronchus
- Where it passes through the diaphragm
Many vessels supply blood to the esophagus. These include branches of the thoraic aorta, bronchial arteries, and the ascending branch of the left gastric artery. Blood is drained into the azygos vein, hemiazygos vein, and the left gastric vein. :192-4
The esophagus receives a parasympathetic supply from the vagus nerve and a sympathetic supply from the sympathetic trunk behind it. Sensation along the esophagus is supplied by both the vagus nerve and sympathetic trunk, with gross sensation being passed in the vagus nerve and pain passed up the sympathetic trunk. The sympathetic trunk the striated muscle of the upper esophagus and the vagus nerve supplies the smooth muscle of the lower esophagus. :194
The esophagus has a mucosa consisting of a stratified squamous epithelium without keratin, a smooth lamina propria, and a muscularis mucosae of smooth muscle. The submucosa contains the mucous secreting glands (esophageal glands), and connective structures termed papillae. The muscularis externa has a unique composition, varying over the length of the esophagus. The upper third of the muscularis is striated muscle, the middle third both smooth muscle and striated muscle, and the lower third predominantly smooth muscles. The esophagus also has an adventitia.
On histological examination, the junction can be identified the transition between nonkeratinized stratified squamous epithelium in the esophagus, and simple columnar epithelium in the stomach. The z-line is defined as this point, and the cardia of the stomach as the area immediately distal to the z-line. This junction usually occurs in the area of the oesphagus surrounded by the lower esophageal sphincter, and is characterised macroscopically by changes from salmon pink to a deeper red. But, a study correlating manometric and endoscopic localization of the LES (z-line) found that the functional location of LES was 3 cm distal to the z-line.
In Barrett's esophagus, the epithelial distinction may vary, so the histological border may not be identical with the functional border. The cardiac glands can be seen in this region. They can be distinguished from other stomach glands (fundic glands and pyloric glands) because the glands are shallow and simple tubular.
Microscopic section of the esophagus, showing a fibrous coat (a); Longitudinal (b) and transverse (c) muscles fibres; the submucosa (d) and muscularis mucosa (e); Mucous membrane (f), with vessels and part of a lymphoid nodule.; the stratified squamous epithelia (g); Mucous gland (h) and Gland duct (i) and Striated muscular fibers (m).
The esophagus fails to develop or there is a fistula between the trachea and esophagus in about 1 in 3500 births. Conventional classification divides such fistulas into five types, based on whether the esophagus is a continuous tube or not, and which end of the esophagus (proximal, distal, both or neither) connects to the trachea. About half the time, these abnormalities occurs with additional abnormalities in other parts of the body, especially affecting the heart.
The esophagus develops from the embryonic gut tube, an endodermal structure.Early in development, the human embryo has three layers and abuts an embryonic yolk sac. During the second week of embryological development, as the embryo grows, it begins to surround and envelop portions of this sac. The enveloped portions form the basis for the adult gastrointestinal tract. Sections of this gut begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach, and intestines.
The sac is surrounded by a network of arteries called the vitelline plexus. Over time, these arteries consolitate into three main arteries that supply the developing gastrointestinal tract: the coeliac artery, superior mesenteric artery, and inferior mesenteric artery. The areas supplied by these arteries are used to define the midgut, hindgut and foregut.
In humans and other animals, food is ingested through the mouth. During swallowing, food passes from the mouth through the pharynx into the esophagus. The esophagus is thus one of the first components of the human digestive system and the human gastrointestinal tract. After food passes through the esophagus, it enters the stomach.
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. Rhythmic contraction of the esophageal muscle called peristalsis push the food down the esophagus. These rhythmic contractions occur both as a reflex response to food that's in the mouth, and also as a response to the sensation of food within the esophagus itself. Along with the peristalsis, the lower esophageal sphincter relaxes.
Preventing gastric reflux
The stomach generates strong acids, including hydrochloric acid (HCl), and enzymes to aid in food digestion. This digestive mixture is called gastric juice. Constriction of the upper and lower esophageal sphincters help to prevent reflux of gastric contents and juices into the esophagus, protecting the esophageal mucosa. In addition, the acute angle of His and the lower crus of the diaphragm helps this sphincteric action.
Inflammation of the esophagus is known as esophagitis. Substances ingested (for example, corrosives), some medications (such as Bisphosphinates), food allergies, infection, and reflux of gastric acids from the stomach can all lead to esophagitis. As of 2014[update] the cause of some forms of esophagitis, such as eosinophilic esophagitis, is not known. Esophagitis can cause painful swallowing and is usually treated by managing the cause of the oesophagitis - such as managing reflux or treating infection. :863–867
Prolonged oesophagitis, particularly from gastric reflux, is one factor thought to play a role in the development of Barrett's oesophagus. In this condition, the lining of the oesophagus changes from multiple layers of flat cells to a single layer of taller, secretory cells. The new cells are known as cuboidal epithelia and are the same as the lining of the stomach. Barrett's oesophagus is thought to be one of the main contributors to the development of esophageal cancer. :863–867
Cancer of the oesophagus differs geographically. In some countries, such as China and Iran, cancer of the flat cells lining the oesophagus is common. In Western countries, oesophageal cancer is more common in people who already have Barrett's esophagus, and occurs in the cuboidal cells. In its early stages, oesophageal cancer may not have any symptoms at all. When severe, esophageal cancer may eventually cause obstruction of the oesophagus, 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 oesophageal wall the cancer has invaded, how many lymph nodes possess the cancer, and whether there are any metastases in different parts of the body. Oesophageal cancer is often managed with radiotherapy, chemotherapy, and may also be managed by partial surgical removal of the esophagus. Inserting a stent into the oesophagus, or inserting a nasogastric tube, may also be used to ensure that a person is able to digest enough food and water. Unfortunately, As of 2014[update] the prognosis for oesophageal cancer is still poor, so palliative therapy may also be a focus of treatment. :869–870
Oesophageal varices refer to engorged blood vessels present within the oesophageal walls. These blood vessels are engorged more than normal, and in the worst cases may partially obstruct the oesophagus. 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. Oesophageal varices often do not have symptoms until they rupture. A ruptured varice is considered a medical emergency, because varices can bleed a lot. A bleeding varice may cause a person to throw up blood, or may cause symptoms of hypovolemic shock. To deal with a ruptured varice, a band may be placed around the bleeding bood 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. A person with a bleeding varice may also receive IV fluids to ensure that they don't become dehydrated. :941–942
An X-ray of swalloed barium may be used to reveal the size and shape of the esophagus, and whether there are any masses. The oesophagus may also be imaged using a flexible camera inserted into the esophagus, called an endoscopy. If the stomach undergoes endoscopy, the camera will also have to pass through the esophagus. During an endoscopy, a biopsy maybe taken. If cancer of the oesphagus is being investigated, other tests, such as a CT scan, may also be used. :860–870
Deficiencies in the strength or the efficiency of the LES lead to various medical problems involving acid damage on the esophagus.
The word esophagus (British English: oesophagus) comes from Greek: οἰσοφάγος (oisophagos), from "to carry" (oesin) and "to eat" (phagos). The Greek term oesophagus, from which the English term has been derived, has been documented in anatomical literature since at least the time of the Hippocrates, who noted that "the esophagus... receives the greatest amount of what we consume."  :59 Its existence in other animals and relationship with the Stomach was documented by the Ancient Roman philosopher and physician Pliny the Elder, and the peristaltic contractions of the Oesophagus has been documented at least by the time of Galen.
Surgically, the first attempt at surgery focused on the oesophagus in the neck, and was first conducted in dogs by Theodore Billroth in 1871, and in people by Czerny in 1877. By 1908, an operation was performed by Voeckler to remove the oesophagus, and in 1933 the first surgical removal of parts of the lower oesophagus, in order to control oesophageal carcinoma, had been conducted. :744–746
The Nissen fundoplication, in which the stomach is wrapped around the lower esophageal sphincter in order to stimulates its function and control reflux, was first conducted by Rudolph Nissen in 1955. :744–746
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. In ruminants, animals with four stomachs, a groove called the sulcus reticuli is often found in the esophagus, allowing milk to drain directly into the hind stomach, the abomasum.  The esophagus of snakes is remarkable for the distension it undergoes when swallowing prey. Cundall D, Tuttman C, Close M. (Mar 2014). "A model of the anterior esophagus in snakes, with functional and developmental implications". Anat Rec 297 (3): 586–98. doi:10.1002/ar.22860 (inactive 2014-04-15).
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.
In many vertebrates, the esophagus is lined by stratified squamous epithelia without glands. In fish, the esophagus is often lined with columnar epithelia, and in in amphibians, sharks and rays, the esophageal epithelium is ciliated, helping to wash food along, in addition to the action of muscular peristalsis.  In addition, in the bat Plecotus auritus, fish and some amphibians, glands secreting pepsinogen or hydrochloric acid have been found. 
The muscle of the esophagus is always striated in mammals and smooth in amphibians, reptiles and birds, and varies in mammals.
Contrary to popular belief, a person would not be able to pass through the esophagus of a whale, which generally measure less than 10cm in diameter, although in larger baleen whales may be up to ten inches when fully distended. 
A structure with the same name is often found in invertebrates, including molluscs and arthropods, connecting the oral cavity with the stomach.In the cephalopods, the brain often surrounds the esophagus.
The mouth of gastropods opens into an esophagus, 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.
There is available an extensive rostrum on the anterior part of the oesophagus in all carnivorous gastropods.
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