BSC 2086 1st Edition Lecture 20 Outline of Last Lecture I II III Layers of Digestive Tract Movement of Materials in Digestive Tract Control of Digestive Functions IV Oral Cavity V Pharynx VI Esophagus VII Stomach Outline of Current Lecture I Stomach II Small Intestine III Pancreas IV Liver V VI VII Gallbladder Coordination of Secretion and Absorption Large Intestine Current Lecture I Stomach a Digestion and Absorption i Pepsin responsible for preliminary digestion of proteins ii Nutrients NOT absorbed here iii Stomach contents become more fluid 1 Some carb digestion by salivary amylase 2 Some lipid digestion by lingual lipase 3 As pH approaches 2 0 II a Pepsin activity increases as protein assembly begins b Amylase lipase activity increases iv Reasons why absorption doesn t occur 1 Epithelial cells not directly exposed to chyme because they are covered by alkaline mucus 2 Epithelial cells don t have transport mechanisms that cells in small intestines have 3 Gastric lining is relatively impermeable to water 4 By the time chyme leaves the stomach digestion is not fully complete v Some lipid soluble substances like alcohol and certain drugs including aspirin can be absorbed Small Intestine a Key roles in digestion and absorption of nutrients i 90 of absorption occurs here b Duodenum i Segment closest to stomach ii 25 cm 10 in long iii Mixing bowl gets chyme from stomach and digestive secretions from pancreas and liver iv Functions 1 Receive chyme 2 Neutralize acids before they can damage absorptive surfaces of small intestine c Jejunum i Middle segment of small intestine ii 2 5 meters 8 2 feet long iii Where most chemical digestion and nutrient absorption occurs iv Portion is removed in weight loss surgery d Ileum i Final segment ii 3 5 meters 11 48 feet long iii Ends at ileocecal valve sphincter that controls flow of materials from ileum into cecum of large intestine e Histology i Plicae circularis transverse folds in intestinal lining 1 Permanent features that don t disappear when the small intestine fills ii Intestinal villi series of fingerlike projections in mucosa 1 Covered by simple columnar epithelium a Covered by microvilli III b Carbs and amino acids absorbed across epithelia into blood c Lipids from micelles enter epithelia and forms chylomicron and enters lacteal iii Intestinal glands mucous cells between columnar epithelial cells 1 Eject mucins onto the intestinal surfaces 2 Brush border enzymes integral membrane proteins found on surfaces of intestinal microvilli maltase sucrase lactase a Break down materials in contact with the brush border b Lactose intolerance mucosa stops making lactase 3 Enteroendocrine cells make intestinal hormones such as gastrin cholecystokinin CCK and secretin f Intestinal secretions i Watery intestinal juice 1 8 L day enters intestinal lumen 1 Moistens chyme 2 Assists in buffering acids 3 Keeps digestive enzymes and products of digestion in solution g Intestinal Movements i Chyme arrives at the duodenum 1 Distention of the duodenum causes stimulation of stretch receptors to activate enterogastric reflex a This reflex inhibits gastrin production gastric contraction and contracts pyloric sphincter ii Chyme moves slowly toward the jejunum due to weak peristaltic contractions 1 Controlled by myenteric reflexes not under CNS control 2 Don t confuse enterogastric reflex with the gastroenteric reflex h Central reflexes i Triggered by stimulation of stretch receptors in stomach wall as stomach fills ii Gastroenteric reflex 1 Stimulates motility and secretion along the entire small intestine 2 Can be stimulated by caffeine or alcohol iii Gastroileal reflex 1 Triggers relaxation of ileocecal valve 2 Allows materials to pass from small intestine into large intestine Pancreas a Found posterior to stomach from duodenum toward spleen b Lobules of the pancreas are separated by connective tissue partitions c Each lobule i Has pancreatic ducts that branch repeatedly ii Ends in blind pockets 1 Pancreatic acini endocrine tissues of pancreas located there IV d Functions i Endocrine cells of pancreatic islets secrete insulin and glucagon into the bloodstream ii Exocrine cells 1 Acinar cells and epithelial cells of duct system secrete pancreatic juice 2 Controlled by hormones CCK from duodenum 3 Contains pancreatic enzymes e Pancreatic Enzymes i Pancreatic alpha amylase carbohydrase similar to salivary amylase breaks down starches ii Pancreatic lipase breaks down complex lipids releases products such as fatty acids that are easily absorbed iii Proteolytic enzymes break certain products apart 1 Responsible for 70 of pancreatic enzyme production 2 Secreted as inactive proenzymes activated in small intestine a Proteases break down large protein complexes b Peptidases break small peptides into amino acids iv Nucleases break down nucleic acids f Pancreatitis i Inflammation of pancreas caused by 1 Blockage of excretory duct 2 Bacterial viral infection 3 Drug reactions ii Lysosymes in injured exocrine cells activate proenzymes which cause autolysis or cell death iii This releases proteolytic enzymes to surrounding undamaged cells which activates their enzymes causing a chain reaction iv In most cases only a small portion of the pancreas is affected and conditions subside in a few days v Diabetes mellitus can result if islet cells die Liver a Largest visceral organ i Performs essential metabolic and synthetic functions ii Covered by visceral peritoneum iii Divided into lobes iv Hepatic blood supply 1 1 3 is arterial blood from hepatic artery proper 2 2 3 is venous blood from hepatic portal vein originating from esophagus stomach small intestine most of large intestine b Histology i Liver lobules basic functional units of liver 1 Each of the lobules are divided by connective tissue into about 100 000 liver lobules a Hexagonal in cross section b Contains 6 portal areas portal triads one at each corner of lobule i Has a branch of the hepatic portal vein hepatic artery proper and bile duct ii Hepatocytes adjusts circulating levels of nutrients through selective absorption and secretion c Cirrhosis replacement of lobule with fibrous tissue i Occurs sometimes with hepatitis ii May cause liver failure d Hepatocytes i As blood flows through the sinusoids hepatocytes absorb solutes from the plasma and secrete materials such as plasma proteins ii Phagocytes that are part of the monocyte macrophage system known as Kupffer cells are found in sinusoidal lining 1 They engulf a
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