PET3322 Exam 4 Study GuideGastrointestinal PhysiologyAlimentary canal/GI tract: mouth, pharynx, esophagus, stomach, small intestine, large intestineAccessory digestive organs: salivary glands, gallbladder, liver, pancreasHistology of Alimentary Canal ->Digestive Process- Ingestion, propulsion, mechanicaldigestion, chemical digestion,absorption, defecationPropulsion: swallowing and peristalsisMechanical Digestion: chewing, mixing,churning foodChemical dig.: catabolic breakdown offoodAbsorption: movement of nutrients fromGI tract to blood or lymphDefecation: elimination of indigestiblesolid wastesGI Tract:- Has 2 muscle layerso Except stomach, which has three - External environment for dig. process- Regulation of dig. involves:o Mechanical and chemical stimuli – stretch receptors, osmolarity, presence of substrate in lumeno Extrinsic control by CNS Long reflexes arising within or outside GI tract Involve CNS centers and extrinsic autonomic nerveso Intrinsic control by local centers Nerve plexuses near GI tract initiate short reflexes Short reflexes are mediated by local enteric plexuses (gut brain)- Receptorso Mechano- and chemoreceptors respond to: Stretch, osmolarity, pH Presence of substrate, and end products of digestion Initiate reflexes that:- Activate/inhibit dig. glands- Mix lumen contents and move them alongEnteric NS- Composed of 2 major nerve plexuseso Submucosal NP – regulates glands, smooth muscle in mucosao Myenteric NP – controls GI tract mobility- Segmentation and peristalsis – largely autonomic, involving local reflex arcsSalivary Glands- 3 pairs of extrinsic glands: parotid, submandibular, sublingual- Intrinsic salivary glands (buccal glands):o Produce/secrete saliva: Cleanses mouth, moistens food, enzymes break down starch (CHO)- Saliva:o Secreted from serous and mucous cells of salivary glandso 97-99.5% water, hypo-osmotic, slightly acidic solution containing electrolytes and enzyme salivary amylaseStomach- Chemical breakdown of proteins begins, foodconverted to chyme- Fundus: dome-shaped area beneath diaphragm- Pylorus is continuous with duodenum throughpyloric sphincter- Layers of stomach wall:o Outer: mucosao Middle: submucosao Inner: muscularis externa Allows stomach to churn food Oblique, circular, longitudinal- Glands of fundus and bodyo Parietal cells secrete HCl and intrinsic factoro Chief cells produce pepsinogen, which is converted to pepsin by HCl OR pepsin itself via positive feedback mechanismo Enteroendocrine cells secrete gastrin, endorphins, serotonin, cholecystokinin (CCK), andsomatostatino G cells gastrin Increased gastric pH, vagus nerve stimulation, presence of proteino Chief cells pepsinogen- Protein dig. in stomacho Pepsin initiates processo Pepsin is created from pepsinogen in the presence of pH-lowering HClo Newly produced pepsin molecules catalyze production of more pepsino Pepsin molecules begin to break down proteins into peptides- Stomach Peristalsiso Food enters, distends stomach; stretch receptors activate enteric reflexes that promote peristaltic movements (mixing waves) which forced chyme toward/through pyloric sphinctero Digestive movements stimulated by parasympathetic NSo Enterogastric reflex triggered when more and more chyme leaves stomach, distending stretch reflexes in duodenum Inhibits excessive amts of chyme from entering duodenum Reduces intestinal cell erosion by limiting inflow of gastric acid Increases duration of digestion of chyme before it moves to small intestine Motor impulses of this reflex are sympatheticThe Duodenum - Intestinal phase of dig. begins when chymeenters duodenum- Acidity of chyme can damage intestinalmucosao S cells in intestinal mucosaproduce secretin- Secretin stimulates pancreatic acinar cells,stimulates pancreas to produce/delivermore HCO3- to SIo Buffers acidity, protects tissue- Secretin also targets parietal cells to inhibit secretion of gastric acid; lowers the acidity of chyme moving into duodenum- Cholecystokinin (CCK)o If fatty chyme enters duodenum, enteroendocrine cells are stimulated to produce CCKo CCK targets pancreatic acinar cells and biliary systemo Increases delivery of pancreatic lipases and bile to SI Greater amounts of fat in chyme ar digestedo Regulates gastric emptying Distended duodenum and fatty acids/undigested proteins in chyme promote enterogastric reflex and secretion of CCK CCK triggers closing of pyloric sphincter, inhibiting gastric emptyingIntestinal Juice- Secreted by intestinal glands in response to distension or irritation or mucosa- Slightly alkaline/basic- Largely water, enzyme-poor, but contains mucusPancreas- Exocrine functiono Secretes pancreatic juice which breaks down all types of food- Endocrine functiono Release of insulin and glucagon- Acinus (clusters of secretory cells) contain zymogen granules with digestive enzymes- Pancreatic juiceo Water solution of enzymes and electrolytes (primarily HCO3-) Neutralizes acid chymeo Enzymes (amylase, lipases, proteolitic) are released in inactive form and activated in duodenum Trypsinogen is activated to trypsin Amylase and lipases require ions or bile for optimal activity- Carbohydrate digestiono Pancreas secretes amylase into duodenumo In small intestine, bicarbonate ions from pancreatic juice neutralize gastric acido Amylase continues breakdown of starches and glycogen into maltose, maltotriose, and alpha-dextrins Does not act on cellulose, an indigestible plant fibero Enzymes used: salivary amylase, pancreatic amylase, brush border enzymes- Carbohydrate absorption in pancreaso Absorbed via cotransport with Na+, and facilitated diffusion Enter the capillary bed in villi Transported to liver via hepatic portal vein- Lipid Dig. in Pancreaso Chyme entering duodenum met with bile salts and pancreatic juice Bile salts cling to mono-, di-, triglycerides of fat globules which then break up into triglyceride emulsion droplets Pancreatic lipase attaches to molecules of the emulsion droplets Each molecules breaks up into monoglycerides and fatty acidsThe Small Intestine- Carb absorption in SIo All end products (glucose, fructose, galactose) are absorbed as monosaccharideso Ultimately absorbed into capillaries of villio Fructose absorbed by facilitated diffusiono Glucose and galactose absorbed by secondary active transport- Protein absorption in SIo Three mechanisms: Active transport (most AA) Na+-dependent
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