BSC 2086 1st Edition Lecture 20Outline of Last Lecture I. Layers of Digestive TractII. Movement of Materials in Digestive Tract III. Control of Digestive FunctionsIV. Oral CavityV. PharynxVI. EsophagusVII. StomachOutline of Current Lecture I. StomachII. Small IntestineIII. PancreasIV. Liver V. GallbladderVI. Coordination of Secretion and AbsorptionVII. Large Intestine Current LectureI. Stomach a. Digestion and Absorptioni. Pepsin responsible for preliminary digestion of proteins ii. Nutrients NOT absorbed here iii. Stomach contents become more fluid1. Some carb digestion by salivary amylase2. Some lipid digestion by lingual lipase3. As pH approaches 2.0,a. Pepsin activity increases as protein assembly beginsb. Amylase/lipase activity increasesiv. 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 smallintestines have 3. Gastric lining is relatively impermeable to water4. 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 absorbedII. Small Intestinea. Key roles in digestion and absorption of nutrients i. 90% of absorption occurs hereb. Duodenumi. Segment closest to stomachii. 25 cm (10 in.) longiii. “Mixing bowl” gets chyme from stomach and digestive secretions frompancreas and liveriv. Functions: 1. Receive chyme2. Neutralize acids before they can damage absorptive surfaces of small intestinec. Jejunum i. Middle segment of small intestineii. 2.5 meters (8.2 feet) longiii. Where most chemical digestion and nutrient absorption occursiv. Portion is removed in weight loss surgeryd. Ileum i. Final segmentii. 3.5 meters (11.48 feet) longiii. 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 fillsii. Intestinal villi: series of fingerlike projections in mucosa 1. Covered by simple columnar epithelium a. Covered by microvillib. Carbs and amino acids absorbed across epithelia into bloodc. 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 surfaces2. Brush border enzymes: integral membrane proteins found on surfaces of intestinal microvilli (maltase, sucrase, lactase)a. Break down materials in contact with the brush borderb. Lactose intolerance: mucosa stops making lactase3. Enteroendocrine cells: make intestinal hormones such as gastrin, cholecystokinin (CCK) and secretin f. Intestinal secretionsi. Watery intestinal juice, 1.8 L/day enters intestinal lumen1. Moistens chyme2. Assists in buffering acids3. Keeps digestive enzymes and products of digestion in solution g. Intestinal Movementsi. Chyme arrives at the duodenum 1. Distention of the duodenum causes stimulation of stretch receptors to activate enterogastric reflexa. This reflex inhibits gastrin production, gastric contraction and contracts pyloric sphincterii. Chyme moves slowly toward the jejunum due to weak peristaltic contractions 1. Controlled by myenteric reflexes, not under CNS control2. Don’t confuse enterogastric reflex with the gastroenteric reflex h. Central reflexesi. Triggered by stimulation of stretch receptors in stomach wall as stomach fillsii. Gastroenteric reflex1. Stimulates motility and secretion along the entire small intestine2. Can be stimulated by caffeine or alcoholiii. Gastroileal reflex1. Triggers relaxation of ileocecal valve2. Allows materials to pass from small intestine into large intestine III. Pancreasa. Found posterior to stomach, from duodenum toward spleenb. Lobules of the pancreas are separated by connective tissue partitions c. Each lobule:i. Has pancreatic ducts that branch repeatedlyii. Ends in blind pockets 1. Pancreatic acini: endocrine tissues of pancreas located thered. Functions i. Endocrine cells of pancreatic islets secrete insulin and glucagon into the bloodstreamii. Exocrine cells 1. Acinar cells and epithelial cells of duct system secrete pancreatic juice2. Controlled by hormones (CCK) from duodenum3. Contains pancreatic enzymese. Pancreatic Enzymes i. Pancreatic alpha-amylase: carbohydrase similar to salivary amylase, breaks down starchesii. Pancreatic lipase: breaks down complex lipids, releases products such as fatty acids that are easily absorbed iii. Proteolytic enzymes: break certain products apart1. Responsible for 70% of pancreatic enzyme production2. Secreted as inactive proenzymes, activated in small intestine a. Proteases: break down large protein complexesb. Peptidases: break small peptides into amino acids iv. Nucleases: break down nucleic acidsf. Pancreatitisi. Inflammation of pancreas caused by:1. Blockage of excretory duct2. Bacterial/viral infection3. Drug reactionsii. Lysosymes in injured exocrine cells activate proenzymes which cause autolysis, or cell deathiii. This releases proteolytic enzymes to surrounding undamaged cells which activates their enzymes, causing a chain reactioniv. 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 IV. Liver a. Largest visceral organi. 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 proper2. 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 liver1. Each of the lobules are divided by connective tissue into about 100,000 liver lobulesa. Hexagonal in cross sectionb. 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 ductii. Hepatocytes: adjusts circulating levels of nutrients through selective absorption and secretionc. Cirrhosis: replacement of lobule with fibrous tissue i. Occurs sometimes with hepatitisii. May cause liver failure d. Hepatocytesi. As blood flows through the sinusoids, hepatocytes absorb solutes from the plasma and secrete materials such as plasma proteins ii.
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