The<unk>  is one of the upper parts of the digestive system. There are taste buds on its upper part. 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<unk>  generally starts around the level of the sixth cervical vertebra behind the cricoid cartilage of the trachea,<unk> nters the diaphragm at about the level of the tenth thoracic vertebra, and<unk> nds at the cardia of the stomach, at the level of the<unk> leventh thoracic vertebra. The<unk>  is usually about 25 cm (10 in) in length. Many blood vessels serve the<unk> , with blood supply varying along its course. The upper parts of the<unk>  and the upper<unk><unk> ageal sphincter receive blood from the inferior thyroid artery, the parts of the<unk>  in the thorax from the bronchial arteries and branches directly from the thoracic aorta, and the lower parts of the<unk>  and the lower<unk><unk> ageal sphincter receive blood from the left gastric artery and the left inferior phrenic artery. The venous drainage also differs along the course of the<unk> . The upper and middle parts of the<unk>  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<unk> xception of the left gastric vein, which is a branch of the portal vein. Lymphatically, the upper third of the<unk>  drains into the deep cervical lymph nodes, the middle into the superior and posterior mediastinal lymph nodes, and the lower<unk>  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. ;Position The upper<unk>  lies at the back of the mediastinum behind the trachea, adjoining along the tracheoesophageal stripe, and in front of the<unk> rector spinae muscles and the vertebral column. The lower<unk>  lies behind the heart and curves in front of the thoracic aorta. From the bifurcation of the trachea downwards, the<unk>  passes behind the right pulmonary artery, left main bronchus, and left atrium. At this point, it passes through the diaphragm. The thoracic duct, which drains the majority of the body's lymph, passes behind the<unk> , curving from lying behind the<unk>  on the right in the lower part of the<unk> , to lying behind the<unk>  on the left in the upper<unk> . The<unk>  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<unk>  in a plexus. ;Constrictions alt=Diagram showing the four constrictions of the<unk> . The<unk>  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<unk> . These constrictions are: * At the start of the<unk> , where the laryngopharynx joins the<unk> , behind the cricoid cartilage * Where it is crossed on the front by the aortic arch in the superior mediastinum * Where the<unk>  is compressed by the left main bronchus in the posterior mediastinum * The<unk><unk> ageal hiatus, where it passes through the diaphragm in the posterior mediastinum Sphincters The<unk>  is surrounded at the top and bottom by two muscular rings, known respectively as the upper<unk><unk> ageal sphincter and the lower<unk><unk> ageal sphincter. These sphincters act to close the<unk>  when food is not being swallowed. The upper<unk><unk> ageal 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<unk><unk> ageal 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<unk><unk> ageal sphincter surrounds the upper part of the<unk> . It consists of skeletal muscle but is not under voluntary control. Opening of the upper<unk><unk> ageal sphincter is triggered by the swallowing reflex. The primary muscle of the upper<unk><unk> ageal sphincter is the cricopharyngeal part of the inferior pharyngeal constrictor. The lower<unk><unk> ageal sphincter, or gastroesophageal sphincter, surrounds the lower part of the<unk>  at the junction between the<unk>  and the stomach. 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<unk> nough, can lead to gastroesophageal reflux disease, with damage of the<unk><unk> ageal mucosa. Nerve supply The<unk>  is innervated by the vagus nerve and the cervical and thoracic sympathetic trunk. The vagus nerve has a parasympathetic function, supplying the muscles of the<unk>  and stimulating glandular contraction. Two sets of nerve fibers travel in the vagus nerve to supply the muscles. The upper striated muscle, and upper<unk><unk> ageal sphincter, are supplied by neurons with bodies in the nucleus ambiguus, whereas fibers that supply the smooth muscle and lower<unk><unk> ageal sphincter have bodies situated in the dorsal motor nucleus. The vagus nerve plays the primary role in initiating peristalsis. The sympathetic trunk has a sympathetic function. It may<unk> nhance 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. Sensation along the<unk>  is supplied by both nerves, with gross sensation being passed in the vagus nerve and pain passed up the sympathetic trunk. Gastro-esophageal junction The gastro-esophageal junction (also known as the<unk><unk> agogastric junction) is the junction between the<unk>  and the stomach, at the lower<unk> nd of the<unk> . The pink color of the<unk><unk> ageal mucosa contrasts to the deeper red of the gastric mucosa, and the mucosal transition can be seen as an irregular zig-zag line, which is often called the z-line. Histological<unk> xamination reveals abrupt transition between the stratified squamous<unk> pithelium of the<unk>  and the simple columnar<unk> pithelium of the stomach. Normally, the cardia of the stomach is immediately distal to the z-line 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<unk>  the true gastro-eshophageal junction can be identified by the upper limit of the gastric folds rather than the mucosal transition. The functional location of the lower<unk> ageal sphincter is generally situated about below the z-line. Microanatomy The human<unk>  has a mucous membrane consisting of a tough stratified squamous<unk> pithelium without keratin, a smooth lamina propria, and a muscularis mucosae. The<unk> pithelium of the<unk>  has a relatively rapid turnover and serves a protective function against the abrasive<unk> ffects of food. In many animals, the<unk> pithelium contains a layer of keratin, representing a coarser diet. There are two types of glands, with mucus-secreting<unk><unk> ageal glands being found in the submucosa and<unk><unk> ageal cardiac glands, similar to cardiac glands of the stomach, located in the lamina propria and most frequent in the terminal part of the organ. The mucus from the glands gives a good protection to the lining. The submucosa also contains the submucosal plexus, a network of nerve cells that is part of the<unk> nteric nervous system. The muscular layer of the<unk>  has two types of muscle. The upper third of the<unk>  contains striated muscle, the lower third contains smooth muscle, and the middle third contains a mixture of both. Muscle is arranged in two layers: one in which the muscle fibers run longitudinal to the<unk> , and the other in which the fibers<unk> ncircle the<unk> . 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<unk> . The outermost layer of the<unk>  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.
Esophagus
Epigastrium
Endobronchial valve
Palatine tonsil
Oropharyngeal airway
Salve
Indium lung
Subclavian loop