|This article needs additional citations for verification. (December 2007)|
|The esophagus relations to pharynx and mouth|
|Digestive organs (esophagus is #1)|
|Gray's||subject #245 1144|
|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 œsophagus, which derives from the Greek word oisophagos, lit. "entrance for eating."
In humans the esophagus is continuous with the laryngeal part of the pharynx at the level of the C6 vertebra. The esophagus passes through posterior mediastinum in the thorax and enters abdomen through a hole in the diaphragm at the level of the tenth thoracic vertebrae (T10). It is usually about 25cm, but extreme variations have been recorded ranging 10–50 cm long depending on individual height. It is divided into cervical, thoracic and abdominal parts. Due to the inferior pharyngeal constrictor muscle, the entry to the esophagus opens only when swallowing or vomiting.
In animals, food is ingested through the mouth. During swallowing, food passes from the mouth through the pharynx into the esophagus. The epiglottis folds down to a more horizontal position so as to prevent food from going into the trachea, instead directing it to the esophagus. Once in the esophagus, the bolus travels down to the stomach via rhythmic contraction and relaxation of muscles known as peristalsis.
At rest, the esophagus is closed at both ends by the upper esophageal sphincter at the top, and the lower esophageal sphincter at the bottom.
The upper esophageal sphincter (also called the pharyngoesophageal sphincter) refers to the superior portion of the esophagus. Unlike the lower esophageal sphincter, it consists of skeletal muscle and yet, is not under conscious control. Opening of the UES is triggered by the swallow reflex. The primary muscle of the UES is the cricopharyngeus portion of the inferior pharyngeal constrictor. During swallowing, the upper esophageal sphincter opens so the bolus can pass into the esophagus. A secondary role of the UES is to reduce backflow from the esophagus into the pharynx. It also makes the sound of eructation.
The lower oesophageal sphincter is a muscular sphincter surrounding te lower part of the oesophagus. The junction between the esophagus and the stomach (the gastroesophageal junction or GE junction) is controlled by the lower esophageal sphincter (LES), which remains constricted at all times other than during swallowing and vomiting to prevent the contents of the stomach from entering the esophagus. As the esophagus does not have the same protection from acid as the stomach, any failure of the LES can lead to heartburn.
Normally, the esophagus has three anatomic constrictions at the following levels:
- At the esophageal inlet, where the pharynx joins the esophagus, behind the cricoid cartilage (14–16 cm from the incisor teeth).
- Where its anterior surface is crossed by the aortic arch and the left bronchus (25–27 cm from the incisor teeth).
- Where it pierces the diaphragm (36–38 cm from the incisor teeth).
The distances from the incisor teeth are important as is useful for diagnostic endoscopic procedures.
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.
Section from the middle of the human esophagus.; a. Fibrous covering.; b. Divided fibers of longitudinal muscular coat.; c. Transverse muscular fibers.; d. Submucous or areolar layer.; e. Muscularis mucosae.; f. Mucous membrane, with vessels and part of a lymphoid nodule.; g. Stratified epithelial lining.; h. Mucous gland.; i. Gland duct.; m’. Striated muscular fibers cut across.
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 oesophageal 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.
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In the esophagus, as in much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called a bolus) down and through the LES into the stomach.
The stomach generates strong acids (HCl) and enzymes (such as pepsin) to aid in food digestion. This digestive mixture is called gastric juice. The inner lining of the stomach has several mechanisms to resist the effect of gastric juice on itself, but the mucosa of the esophagus does not. The esophagus is normally protected from these acids by a one-way valve mechanism at its junction with the stomach. This one-way valve is called the esophageal sphincter (ES), and this, along with the angle of His formed here, prevents gastric juice from flowing back into the esophagus.
During peristalsis, the oesophageal sphincter allows the food bolus to pass into the stomach. It prevents chyme, a mixture of bolus, stomach acid, and digestive enzymes, from returning up the esophagus. The oesophageal sphincter is aided in the task of keeping the flow of materials in one direction by the diaphragm.
The oesophageal sphincter is a functional sphincter but not an anatomical sphincter. That is to say, though there is no thickening of the smooth muscle, as in the pyloric sphincter, chyme is (usually) prevented from travelleling back from the stomach up the esophagus. The lower crus of the diaphragm helps this sphincteric action. 
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Deficiencies in the strength or the efficiency of the LES lead to various medical problems involving acid damage on the esophagus.
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In other animals
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 tetrapods, the pharynx is much shorter, and the esophagus correspondingly longer, than in fish. In amphibians, sharks and rays, the esophageal epithelium is ciliated, helping to wash food along, in addition to the action of muscular peristalsis. In the majority of vertebrates, the esophagus is simply a connecting tube, but in birds, it is extended towards the lower end to form a crop for storing food before it enters the true stomach.
- Physiology at MCG 6/6ch2/s6ch2_30
- Mu L, Wang J, Su H, Sanders I (2007). "Adult human upper esophageal sphincter contains specialized muscle fibers expressing unusual myosin heavy chain isoforms". J. Histochem. Cytochem. 55 (3): 199–207. doi:10.1369/jhc.6A7084.2006. PMID 17074861.
- BU Histology Learning System: 10801loa
- BU Histology Learning System: 11101loa
- Barrett KE, "Chapter 7. Esophageal Motility" (Chapter). Barrett KE: Gastrointestinal Physiology: http://www.accessmedicine.com/content.aspx?aID=2307248
- Gastrointestinal disease: an endoscopic approach, By Anthony J. DiMarino, p.166
- GI Motility online (2006-05-16). "Esophagus - anatomy and development : GI Motility online". Nature.com. Retrieved 2013-04-24.
- USA (2013-03-25). "Neuromuscular Anatomy of Esophagus and Lower Esophageal Sphincter - Motor Function of the Pharynx, Esophagus, and its Sphincters - NCBI Bookshelf". Ncbi.nlm.nih.gov. Retrieved 2013-04-24.
- Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 344–345. ISBN 0-03-910284-X.
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