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NORMAL STOMACH
 The stomach lies between the oesophagus and the duodenum, and acts primarily as an initial mixing chamber for digestion of ingested food. It comprises several distinct zones. The cardia is situated just below the oesophageal opening, and the fundus constitutes that part cephalad to the oesophagogastric junction. The main part of the stomach is the body, which has a shorter, lesser curve and a longer dependent greater curve. The mucosa of the body and fundus is thrown into folds, or rugae. The distal third of the stomach, the antrum, is smooth with no rugae, and is demarcated proximally by the incisura angularis and distally by the pylorus. The pyloric sphincter itself is formed from a concentration of circular muscle.
Occasionally the fundus will hang down across the body of the stomach; this normal variant is known, from its radiological appearance, as the cup and spill' deformity.
The stomach is innervated by parasympathetic nerves supplied by the vagus, and by sympathetic nerves that originate in the coeliac plexus. The vagus is divided into anterior and posterior trunks; the anterior trunk is further divided into an anterior gastric division that supplies the anterior wall of the stomach, and a hepatic division that supplies the proximal duodenum. The posterior gastric wall is supplied by the posterior gastric division of the posterior vagal trunk.
In addition to afferent fibres, the vagus contains three types of efferent fibre - cholinergic stimulatory, adrenergic inhibitory, and nonadrenergic inhibitory - and these play a part in the control of secretion and motility.
The mucosa of the fundus and body of the stomach is covered by regular columnar epithelium which extends for a short distance into the gastric pits. The gastric glands open into the base of the gastric pits. The glands of the body incorporate two types of specialized secretory cells: the parietal cells, which produce hydrochloric acid and vitamin B12-binding intrinsic factor; and pepsin-producing chief cells. Antral (pyloric) glands do not contain acid- or pepsin-producing cells, but are mucus secreting. The majority of gastrin-producing (G') cells are found in the antrum, interspersed between the mucus cells.
The stomach is lined by a layer of mucus, a gelatinous material composed of proteins, glycoproteins and mucopolysaccharides secreted by the surface epithelium. The mucus appears to protect the gastric mucosa against surface injury by physical irritants, and to buffer gastric acid under basal conditions, although its effect in buffering stimulated acid secretion is negligible. The mucus layer is also important in allowing colonization by Helicobacter pylori (see below).
The vagus acts on parietal cells to stimulate acid production, and on G cells to stimulate gastrin release, in both cases via the action of acetylcholine. In addition, acetylcholine potentiates the parietal cell response to other secretagogues. Gastrin is also released directly by exposure to peptides and amino acids, and by antral distension.
Although stimulation of H2 receptors on the parietal cells by histamine also stimulates acid production, the final common pathway for gastric acid secretion is now known to be via the hydrogen-potassium ATPase system. Both histamine and the gastrin analogue, pentagastrin, can, however, provoke maximal acid production (the basis of the augmented histamine-pentagastrin test for assessment of acid secretory capacity).
Although the volume of the resting stomach is 50ml or less, receptive relaxation of the body of the stomach occurs as food and liquid are ingested, so that there is little rise in intragastric pressure. Peristaltic waves are initiated by the gastric pacemaker in the fundus of the stomach, and then occur at a rate of about 3 per minute. These gradually propel the viscous gastric contents into the distal antrum. Unlike the cardia, in the resting state the pylorus is always open, and it closes only during peristalsis.
The rate at which contents pass into the duodenum depends on physical and chemical composition: solids, lipids, and hypertonic fluids all empty at a slower rate than isotonic fluids. Scintigraphy using separate labels for solid and liquid phases demonstrates this readily. As pressure rises in the antrum, a proportion of the antral contents passes through the open pylorus into the duodenum.
Partly as a result of duodenal receptors which contribute negative feedback, the pylorus then abruptly contracts, causing antral pressure to increase and thus propel contents back into the body of the stomach. This antral pump' therefore ensures the thorough mixing of gastric contents and digestive juices, and a controlled emptying of the stomach.
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