PET3322 Exam 4 Study Guide Gastrointestinal Physiology Alimentary canal GI tract mouth pharynx esophagus stomach small intestine large intestine Accessory digestive organs salivary glands gallbladder liver pancreas Histology of Alimentary Canal Digestive Process Ingestion propulsion mechanical digestion chemical digestion absorption defecation Propulsion swallowing and peristalsis Mechanical Digestion chewing mixing churning food Chemical dig catabolic breakdown of food Absorption movement of nutrients from GI tract to blood or lymph Defecation elimination of indigestible solid wastes GI Tract Has 2 muscle layers o Except stomach which has three External environment for dig process Regulation of dig involves in lumen o Extrinsic control by CNS o Mechanical and chemical stimuli stretch receptors osmolarity presence of substrate Long reflexes arising within or outside GI tract Involve CNS centers and extrinsic autonomic nerves o Intrinsic control by local centers Nerve plexuses near GI tract initiate short reflexes Short reflexes are mediated by local enteric plexuses gut brain Receptors o 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 along Enteric NS Composed of 2 major nerve plexuses o Submucosal NP regulates glands smooth muscle in mucosa o Myenteric NP controls GI tract mobility Segmentation and peristalsis largely autonomic involving local reflex arcs 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 o Secreted from serous and mucous cells of salivary glands o 97 99 5 water hypo osmotic slightly acidic solution containing electrolytes and enzyme salivary amylase Salivary Glands Saliva Stomach Chemical breakdown of proteins begins food converted to chyme Fundus dome shaped area beneath diaphragm Pylorus is continuous with duodenum through pyloric sphincter Layers of stomach wall o Outer mucosa o Middle submucosa o Inner muscularis externa Allows stomach to churn food Oblique circular longitudinal Glands of fundus and body o Parietal cells secrete HCl and intrinsic factor o Chief cells produce pepsinogen which is converted to pepsin by HCl OR pepsin itself via positive feedback mechanism o Enteroendocrine cells secrete gastrin endorphins serotonin cholecystokinin CCK and somatostatin o G cells gastrin o Chief cells pepsinogen Protein dig in stomach Increased gastric pH vagus nerve stimulation presence of protein o Pepsin initiates process o Pepsin is created from pepsinogen in the presence of pH lowering HCl o Newly produced pepsin molecules catalyze production of more pepsin o Pepsin molecules begin to break down proteins into peptides Stomach Peristalsis o Food enters distends stomach stretch receptors activate enteric reflexes that promote peristaltic movements mixing waves which forced chyme toward through pyloric sphincter o Digestive movements stimulated by parasympathetic NS o 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 sympathetic The Duodenum Intestinal phase of dig begins when chyme enters duodenum Acidity of chyme can damage intestinal mucosa o S cells in intestinal mucosa produce secretin Secretin stimulates pancreatic acinar cells stimulates pancreas to produce deliver more HCO3 to SI o 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 CCK o CCK targets pancreatic acinar cells and biliary system o Increases delivery of pancreatic lipases and bile to SI Greater amounts of fat in chyme ar digested o 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 emptying Secreted by intestinal glands in response to distension or irritation or mucosa Slightly alkaline basic Largely water enzyme poor but contains mucus Intestinal Juice Pancreas Exocrine function o Secretes pancreatic juice which breaks down all types of food Endocrine function o Release of insulin and glucagon Acinus clusters of secretory cells contain zymogen granules with digestive enzymes Pancreatic juice o Water solution of enzymes and electrolytes primarily HCO3 Neutralizes acid chyme o Enzymes amylase lipases proteolitic are released in inactive form and activated in duodenum Carbohydrate digestion Trypsinogen is activated to trypsin Amylase and lipases require ions or bile for optimal activity o Pancreas secretes amylase into duodenum o In small intestine bicarbonate ions from pancreatic juice neutralize gastric acid o Amylase continues breakdown of starches and glycogen into maltose maltotriose and alpha dextrins Does not act on cellulose an indigestible plant fiber o Enzymes used salivary amylase pancreatic amylase brush border enzymes Carbohydrate absorption in pancreas o Absorbed via cotransport with Na and facilitated diffusion Enter the capillary bed in villi Transported to liver via hepatic portal vein Lipid Dig in Pancreas o 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 acids The Small Intestine Carb absorption in SI o All end products glucose fructose galactose are absorbed as monosaccharides o Ultimately absorbed into capillaries of villi o Fructose absorbed by facilitated diffusion o Glucose and galactose absorbed by secondary active transport Protein absorption in SI o Three mechanisms Lipid Absorption in SI epithelial cells Active transport most AA Na dependent secondary AT some AA H dependent secondary AT dipeptides and tripeptides o Requires bile salts which form micelles that ferry fatty acids and monoglycerides to o
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