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The mechanism by which gastric acid is secreted and how this secretion is neutralised in the small intestine - Essay Example

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Its secretion is mediated through hormonal and neural pathways. Stimulation of the vagus nerve acts as the neural effector while histamine and gastrin act as the hormonal effectors. Gastric acid production is the physiologic function closely associated with the stomach…
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The mechanism by which gastric acid is secreted and how this secretion is neutralised in the small intestine
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Download file to see previous pages Food acts as a physiologic stimulus for the production of gastric acid. Although gastric acid is produced in response to a meal, it plays a rather small role in digestion. However, it still does play a role in digestion. The stomach on the other hand has a well developed mucosal defensive mechanism which protects it from caustic injury. Parietal cells which are responsible for creating this concentrated acid environment within the lumen of the stomach are located at the centre of this elaborate system.The parietal cell has special histamine receptors known as H2 receptors. The stimulation of these receptors leads to increased acid secretion. Enterochromaffinlike (ECL) cells are special neuroendocrine cells of the stomach accredited with the production of histamine.ECL cells are mainly located in acid-secreting regions of the stomach. Rhoades and Bell (2009, p.503) point out that “The mechanisms that stimulate the ECL cells to release histamine are poorly understood.” The importance of histamine as an effector of gastric acid secretion has been indirectly demonstrated by the effectiveness of cimetidine which is a H2 blocker in reducing the secretion of acid.
Physiologic regulation of gastric acid secretion is controlled by three major stimulants namely; histamine, gastrin and acetylcholine. Parietal cells are innervated by intrinsic and extrinsic nerves of the parasympathetic nervous system. Gastrin is the primary endocrine stimulant of the parietal cell. It is synthesized, stored and secreted by G-cells of the antral mucosa. Endocrine cell activity is regulated by dietary or luminal nutrients, nerves and paracrine factors. With luminal acid being a potent physiologic inhibitor of hormone release, amino acids, peptides and amines are primary luminal stimulatory factors. Histamine acts on specific H2 receptors of the parietal cell. The H2 receptors subsequently activate adenylate cyclise through a G-protein linking mechanism. Raff (2003, p.166), states that this results in “an increase in cytoplasmic cAMP results in the activation of protein kinase A and phosphorylation of proteins that regulate the opening of CI- ion channels in the canalicular membrane.” Acetylcholine (Ach), released from vagus nerve terminals bind to muscarinic (M3) receptors and activates the parietal cell through the IP3-Ca++ pathway. Food stimulated increase in gastric acid secretion occurs in a sequential order divided into three main phases namely; cephalic phase, gastric phase and intestinal phase. Phase Stimulus Pathway Stimulus to Parietal Cell Cephalic Thought of food, smell, taste, chewing and swallowing. Vagus nerve to:Parietal cells G cells Acetylcholine (Ach), Gastrin Gastric Stomach distention Local (enteric) reflexes and vago-vagal reflexes to: Parietal cells G cells Acetylcholine (Ach), Gastrin Intestinal Protein digestion products in duodenum Amino acids in blood Amino acids Table 1: Cephalic, Gastric, and Intestinal phases of Stimulation of Acid Secretion after Ingesting a Meal. Cited in Rhoades and Bell, 2009, p.504. The cephalic phase is associated with the central nervous system. Smelling, chewing, swallowing or simply the thought of food sends impulses to the parietal and G cells in the stomach through the vagus nerves. The vagus ...Download file to see next pagesRead More
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