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The Human Body:
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Stomach
The next one, is the stomach. It serves 3 basic functions. Firstly, it is the most important function , to store food. When food enters stomach, the body of stomach enlarges to accommodate the food by a process called receptive relaxation.
Secondly, it is.Mixing. The presence of food in body & antrum increases contractile activity of stomach. It occurs as such, when the peristaltic contraction reaches the pyloric sphincter, the sphincter is tightly closed and no further emptying occurs. So, the chyme that was being propelled towards the closed sphincter, is tossed back into the antrum. The combination of peristalsis and retropulsion mixes food with stomach acid and enzymes, breaking it into smaller pieces with each peristaltic contraction .
Lastly, it is Emptying. It occurs as such, there will be a peristaltic contraction that occurs in the upper fundus which sweeps down towards the pyloric sphincter. The contraction becomes more rigous when it reaches the thickly muscled antrum. The strong antral peristaltic contraction propels the chyme forward. A small portion of the chyme is then pushed through the pyloric sphincter into small intestine by the antral area. The amount of chyme that is allowed into the duodenum from the stomach is controlled by duodenal hormones secreted in response to presence and type of chyme. The more the strength of the antral contraction, the more chyme is being forced through.
The stomach also secretes HCl from the gastric gland shown in diagram 2.2 which has many functions. The low pH (which can be as low as 2) in the stomach converts pepsinogen into the active form, pepsin. The active Pepsin begins protein digestion. The hydrochloric acid in the stomach can also cause the breakdown of connective tissue and muscle fibers, the denaturation of proteins, and the killing of most microorganisms.
Diagram 2.2 lateral view of the stomach wall
There are many gastric pits present in the stomach as shown in the section in diagram 2.2. It composes of many cells that aid in the functions of the stomach. Firstly, the exocrine cells, which are the mucous cells, the chief cells and the parietal cells. The mucous cells under the mechanical stimulation of the contents in the stomach secretes an alkaline mucous that protects mucosa under mechanical, pepsin and acid injuries. Whereas the chief cells under the influence of Ach and gastrin secretes pepsinogen, when activated starts protein digestion. The parietal cells under the effects of Ach, gastrin and histamine secretes HCl and an intrinsic factor that facilitates the absorption of vitamin B12. The other type of cells are the endocrine cells, G cells, D cells and ECL cells(Enterochromaffin-like cells). G cells are stimulated by protein products and Ach to secrete Gastrin which in turn stimulates parietal, chief and ECL cells. D cells are stimulated by acid and secretes somatostatin which inhibits parietal, G and ECL cells. ECL cells are affected by ACh and gastrin to secrete histamine that stimulates parietal cells. The important GI hormones mentioned earlier has the following functions. Gastrin increases the secretion of hydrochloric acid and pepsinogen and enhances gastric motility. Secretin inhibits gastric emptying and gastric secretion, stimulate the pancreas to produce sodium bicarbonate, and stimulates the liver to produce bile. CCK (cholecystokinin) inhibits gastric motility and secretion, stimulates pancreatic enzymes and signals the gallbladder to secrete bile. GIP (gastric inhibitory peptide) promotes metabolic processing of nutrients once they are absorbed.
The different phases in the gastric secretion of the stomach are divided into 3 main ones. The
Cephalic phase, Gastric phase and the Intestinal phase. The Cehalic phase is to prepare the stomach for arrival of food and is stimulated by the sight, smell, taste or thoughts of food. As seen, its mechanism is mainly neural via the preganglionic fibers in the vagus nerve and synapses in the submucosal plexus. Its duration is very short(minutes). The primary action is to raise the amount of gastric juice by causing the increased production of mucus, acid and enzymes. Secondly, it is to stimulate the release of gastrin.
The next phase is the Gastric phase which is to enhance secretion started in cephalic phase. It also initiates digestion of proteins and homogenize and acidify chyme. Its mechanism is neural whereby short reflexes are triggered by stimulation of stretch receptors when the stomach fills and chemoreceptors when pH elevates. It is also under the hormonal mechanism whereby stimulation of gastrin release is caused by parasympathetic activity and the presence of peptides or amino acids. This phase lasts for hours. Its aim is to increase acid and pepsinogen production as well as raising motility and initiation of mixing waves.
Lastly, the Intestinal phase is to control rate of chyme entry into duodenum. It is caused upon by short reflexes due to the distension of the duodenum and the primary hormonal control of the release of the GI hormones due to presence of acid, carbohydrates and lipids. The secondary hormonal stimualtion is the release of gastrin is stimulated by presence of peptides and undigested proteins. This phase also last for hours. It serves to provide a feedback inhibition of gastric acid and pepsin action and the reduction of gastric motility as it tries to inhibit the mechanism mentioned earlier.
It is to be noted that there is no absorption of nutrients in the stomach.
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