Foregut




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The foregut consists of the distal end of the esophagus, the stomach, and a portion of the duodenum. In addition, the pancreas, liver, and gallbladder form embryologically from the foregut and thus also are included in this discussion. The celiac trunk is the principal (but not exclusive) artery supplying the foregut. The celiac trunk arises from the abdominal aorta.




Esophagus



The distal end of the esophagus enters the abdominal cavity in the upper left quadrant by traversing the diaphragm at the T10 vertebral level. The esophagus immediately transitions into the stomach, with the cardiac sphincter serving as the transition boundary. Coursing parallel to the esophagus are the anterior and posterior vagal trunks. The vagal trunks consist of visceral motor and sensory parasympathetic fibers from the left and right vagus nerves, respectively.



A function of the stomach is the production of hydrochloric acid. If the cardiac sphincter fails to contain the acidic chyme produced by the stomach, the acid moves into the esophagus, irritating its mucosal lining and causing gastro-esophageal reflux disease (GERD). The irritation presents as an uncomfortable, perhaps burning sensation in the region of the esophagus, deep to the heart. As a result, this condition is also referred to as “heart burn.”




Stomach



The stomach is a dilated, J-shaped portion of the foregut, juxtaposed between the esophagus and the duodenum (Figure 9-1A and B). The stomach is located in the upper left quadrant of the abdomen, with the spleen, pancreas, and aorta located deep to the stomach body.




Figure 9-1



A. Parts of the stomach and duodenum. B. Anterior view of the foregut; the lesser omentum is partially removed.




Gastric secretions are churned in the stomach, with food, into a semifluid mixture (chyme) that is eventually transported from the stomach into the duodenum.



The stomach is partitioned into the following four regions:




  • Cardia. Surrounds the gastroesophageal opening.
  • Fundus. Dome-shaped region superior to the cardia.
  • Body. Largest region of the stomach that consists of a lesser curvature and a greater curvature, where the lesser omentum and greater omentum attach, respectively.
  • Pylorus. Distal end of the stomach containing the pyloric sphincter, which is located in the transpyloric plane at the L1 vertebral level.



The distal end of the esophagus and gastric fundus can herniate through the esophageal hiatus of the diaphragm into the thoracic cavity. This is known as a hiatal hernia and results from conditions such as strain on the diaphragm due to childbirth or to congenital defects in the diaphragm.



When food enters the stomach, it begins to expand and stretch, resulting in a vagovagal reflex. Visceral sensory neurons from the vagus nerve [cranial nerve (CN) X] relay the stretching of the stomach to the brainstem. In response, the brainstem relays impulses via the vagus nerve, inhibiting the tone of the muscularis externa (smooth muscle of the stomach). In this way, the wall of the stomach progressively expands to accommodate greater quantities of food. The stomach can hold up to 1.5 L of food before pressure within the stomach lumen increases.




Duodenum



The duodenum is approximately 25-cm long and curves around the pancreatic head. It is divided into the following four parts (Figure 9-1A and B):




  • Part one (superior). The stomach is an intraperitoneal organ as is the first part of the duodenum. The first part of the duodenum contains the duodenal cap, a dilation of the proximal duodenum that is easily identifiable on radiographs. In contrast, the second part of the duodenum courses deep to the parietal peritoneum and thus is retroperitoneal, as are the third and fourth parts of the duodenum. However, the distal portion of the fourth part is transitional, from retroperitoneal to intraperitoneal in the region of the duodenojejunal junction.
  • Part two (descending). Courses deep to the transverse colon and anterior to the right kidney. The common bile duct enters its posterior wall. Within the duodenal wall, the common bile duct receives the main pancreatic duct (of Wirsung). Immediately after the junction, there is an enlargement called the major duodenal papilla (ampulla of Vater). The papilla is surrounded by smooth muscle called the sphincter of Oddi. An accessory pancreatic duct (of Santorini) may enter the duodenum proximal to the main pancreatic duct.
  • Part three (horizontal). Turns left and courses horizontally across the inferior vena cava, the aorta, and the vertebral column. In addition, the superior mesenteric artery and vein course anteriorly to the third part of the duodenum.
  • Part four (ascending). Ascends anterior to the aorta at the L2 vertebral level.



The duodenum receives its blood supply from branches of both the celiac trunk (superior pancreaticoduodenal arteries) and the superior mesenteric trunk (inferior pancreaticoduodenal arteries).




Liver and Gallbladder



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In addition to its numerous metabolic activities, the liver secretes bile. Bile is transported to the gallbladder, where it is stored. When food reaches the duodenum, the gallbladder releases bile, which emulsifies fat in the duodenum.


Dec 29, 2018 | Posted by in ANESTHESIA | Comments Off on Foregut
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