Chapter 6: Stomach and Duodenum |
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Index |
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Acknowledgements |
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2.1 General Anatomy The stomach is the most capacious part of the GI tract and lies between the distal esophagus and the duodenum. It is situated entirely within the abdomen below the diaphragm (Figure 1). The body of the stomach lies slightly to the left of the midline, the antrum across the spinal vertebrae at the level of T10-L1 and the pylorus to the right of the vertebral column. The duodenum is predominantly retroperitoneal and comprises the cap, descending and distal portions. The greater curvature is some three or four times the length of the lesser curvature. A point known as the angulus or incisura may be defined on the lesser curvature. This point is relatively constant and marks a change from the prominent rugal folds of the body of the stomach to the smoother, less prominent folds of the antrum. The stomach and duodenum lie in close proximity to a number of important anatomic structures. Anterosuperiorly are the left diaphragm and left lobe of the liver, while the body and tail of the pancreas lie posteriorly. Laterally to the left are the hilum of the left kidney, the left adrenal gland and, above that, the spleen. These organs form the stomach bed and are separated from it by the lesser omentum and the lesser sac. The duodenum, apart from the cap, lies retroperitoneally. The second and distal parts surround the head of the pancreas while the cap, which is attached to the lesser omentum, lies anterior to the head of the pancreas.
The main arterial blood supply (Figure 2) arises from the celiac axis. The common hepatic artery gives rise to the gastroduodenal artery and the right gastric artery, which then anastomoses with the left gastric artery. The splenic artery gives rise to the short gastric arteries that supply the body along the greater curvature of the stomach. The right and left gastroepiploic arteries also form an anastomosis along the greater curvature. Venous drainage essentially follows the arterial supply but passes to the portal venous system and its tributaries, the splenic vein and the superior mesenteric vein. Veins from the fundus communicate with veins draining the lower third of the esophagus and form a connection between the systemic and portal venous systems. This connection assumes clinical importance if portal venous pressure rises, when venous flow is reversed through the esophageal veins, leading to esophageal or fundal varices. Lymphatic drainage is to the pancreaticosplenic nodes, the left gastric nodes and the pyloric nodes, and then via the celiac group to the preaortic lymph nodes and the cisterna chyli.
The nerve supply is both sympathetic and parasympathetic. The vagal supply arises via the anterior and posterior trunks, which pass through the diaphragm on either side of the esophagus before giving rise to the hepatic and celiac branches. The hepatic branch supplies further branches to the anterior surface of the body of the stomach and to the pyloric region, while the celiac branch passes to the celiac plexus and the posterior aspect of the body of the stomach. The vagal fibers anastomose with ganglion cells of the stomach within the muscle layers, forming Auerbach's plexus or, in the submucosa, forming Meissner's plexus. The sympathetic nerve supply arises from the spinal cord between T6 and T10 and passes to the sympathetic ganglia. The parasympathetic supply contracts the stomach, relaxes the pylorus and stimulates acid, pepsin and mucus secretion, whereas sympathetic stimulation constricts the blood supply and reduces gastric motor activity and secretion while the pylorus is contracted.
The stomach and duodenum comprise an outer serosal coat, a muscular layer, submucosa and mucus membrane. The mucosal surface is ridged by the rugal folds created by contractions of the muscularis mucosa, especially prominent in the body of the stomach and less obvious in the antrum. The glands of the stomach are of two main types - gastric and pyloric - both of which are closely packed in the columnar epithelium. The gastric glands (known as oxyntic glands) make up 70-80% of the total and are responsible for secreting mucus, pepsinogen, hydrochloric acid and intrinsic factor (Figure 3). The pyloric glands, which secrete mucus and gastrin, make up only about 15%. A line of demarcation can usually be seen between the gastric and pyloric glands in the region of the incisura. The gastric glands differ in cell type: the chief or peptic cells secrete pepsinogen, while the parietal or oxyntic cells secrete hydrochloric acid and intrinsic factor. The endocrine cells of the antrum secrete gastrin and 5-hydroxytryptamine. In the duodenum the first 4-5 cm of mucosa are smooth, but in the descending duodenum the mucosa is thrown into crescentic folds. The mucosa is lined with columnar, goblet, Paneth's and endocrine cells. The columnar cells line the villi and crypts, which increase in size in the second and third parts of the duodenum. A characteristic feature in the duodenal submucosa is the presence of Brunner's glands, which are similar to pyloric glands. |
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