Intestinal Phase of the Response to a Meal.Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7DigestionAbsorptionSlide 10Intestinal Phase Regulated by the composition of the chyme.Intestinal Feedback to StomachCarbohydratesDigestion of CarbohydratesCarbohydrates Are Absorbed as MonosaccharidesBrush-Border EnzymesBrush Border Enzymes Needed for Carbohydrate AbsorptionDisorders of Carbohydrate Digestion and AbsorptionSlide 19Protein Digestion and AbsorptionProtein DigestionAmino Acid Absorption by the EnterocyteSequence for Protein DigestionPancreatic Proteases: Activation and TargetsSlide 25Absorption of Whole ProteinM cell and Peyer’s patchesLipid Digestion and AbsorptionSlide 29BileSlide 31Slide 32Slide 33Slide 34Synthesis of Bile Acids and Conversion to Bile SaltsDisorders of Lipid Digestion and AbsorptionIntestinal Phase of the Response to a Meal.•Digestion and Absorption are the primary functions of the small intestine.•Duodenum, jejunum, ileum.•Accessary organs: pancreas, liver, gallbladder.•Digestion: the chemical breakdown of macromolecules into component subunits.Secretions of the Digestive SystemFigure 21-5The exocrine pancreasBrunner’s glandsHydrolysisDigestionThe chemical breakdown of macromolecules(Carbohydrates, Proteins, Lipids)Digestion1. The chemical breakdown of macromolecules into their component building blocks.2. Accomplished by the digestive enzymes.3. The digestive enzymes are secreted in the salivary, gastric and pancreatic juice, or enzymes attached to the brush border.4. Amylase, disaccharidases, proteases, lipases.Absorption•Movement of nutrients, water, and electrolytes from the lumen of the intestine into the blood.•Two paths of absorption: transcellular and paracellular.•Transcellular pathway requires transport across luminal membrane, enter the intestinal epithelial cell, and then be extruded from the cell across the basolateral membrane.Intestinal Phase Regulated by the composition of the chyme.•Following a meal a full stomach will slowly empty into the small intestine.•Rate of empting is regulated and dependent upon the nature of the meal.•Carbohydrate > protein > fatIntestinal Feedback to Stomach•Chyme entering duodenum generates feedback signals that control the rate of gastric emptying.•Neural and hormonal pathways•Vagal afferents respond to nutrients, H+, hyperosmotic content.•CCK inhibits gastric emptying (among other actions).•The volume of chyme in duodenum decreases as it moves down the intestine, reducing the strength of the feedback inhibition.Carbohydrates•Monosaccharides (monomers)•Glucose, fructose, galactose•Disaccharides•Maltose, sucrose, lactose•Oligosaccharides (short polymers)•Polysaccharides (long polymers)•Amylose: straight-chain glucose polymer•α-1,4 linear linkage•Amylopectin: massive branched glucose polymer•Starch (plant): α-1,6 branch points•Glycogen (animal starch): many more α-1,6 branch pointsDigestion of Carbohydrates•Straight chain: Amylose•Amylase  Maltotriose, Maltose•Branched chain: Amylopectin•Amylase  Maltotriose, Maltose and α-Limit dextrinsCarbohydrates Are Absorbed as MonosaccharidesStarch, Glycogen, and Cellulose are the most common carbohydrate polymers in our dietsAmylose, a(1->4) linkageCellulose, b(1->4) linkagea-AmylaseCellulaseBrush-Border Enzymes•Digestive enzymes of the intestine are attached to the brush-border of the enterocytes.•Maltase, sucrase, lactaseBrush Border Enzymes Needed for Carbohydrate Absorption•Lactase: Lactose  Glucose + Galactose•Maltase: Maltose  2 Glucose•Sucrase: Sucrose  Glucose + Fructose•Isomaltase: α-limit dextrins, maltotriose, maltose  Glucose•Sucrase and Isomaltase exists as a complex: sucrase-isomaltaseDisorders of Carbohydrate Digestion and Absorption•Failure to break down ingested carbohydrate to an absorbable form.•Non-absorbed carbohydrate remains in the lumen with water.•Osmotic diarrhea•Lactose intolerance•75% of adults world wideAbsorption of Glucose, Galactose, and Fructose by the EnterocyteProtein Digestion and Absorption•Proteins require hydrolysis to oligopeptides or amino acids before absorption in the small intestine•Pancreatic proteasesProtein Digestion•Protein digestion occurs in multiple steps. •Proteins hydrolyzed to long peptides by acidic pH of gastric lumen.•Four phases of enzymatic action:•Pepsin in gastric lumen•Pancreatic proteases•Brush border peptidases•Cytosolic peptidase of enterocytesAmino Acid Absorption by the EnterocyteAt least seven different amino acid transporters have been describedSequence for Protein DigestionPancreatic Proteases:Activation and TargetsTri- and Di-peptide Absorption dependent upon sodium and proton gradientAbsorption of Whole Protein•Neonatal period•Apical Pinocytosis•Passive immunity from mother to child•Ceases after six months: called “closure”•Corticosteroids promote early closure•Adult •Small amount of protein and polypetides•Route(s) not certain:•Endocytosis by enterocytes•Paracellular•M-cellsM cell and Peyer’s patchesPeyer’s patches are follicles of lymphoid tissue in the lamina propria of the small intestinePeyer’s patchesLipid Digestion and Absorption•Dietary lipids include triglycerides, cholesterol, and phospholipids•Hydrophobic•Salivary and gastric lipase begins digestion (10%), but bulk of the work is done in the small intestine.•Most import role of stomach in the digestion of lipid is to control release of chyme into duodenum.Triglycerides are digested to monoglyceride and free fatty acidsBile•Watery mixture of organic and inorganic compounds.•Phosphatidylcholine (lecithin)•Conjugated bile acids (the bile salts)•Amphipatic: contain both polar (hydroxyl) and nonpolar(methyl) groups•Emulsifying agent. The bodies detergent.Figure 21-20aFigure 21-20bFigure 21-19 - OverviewBile Salt Recycling or Enterohepatic CirculationSynthesis of Bile Acids and Conversion to Bile SaltsDuring enterohepatic circulation, primary and secondary, bile acids may be conjugated with glycine or taurine. The conjugated forms are usually referred to as the bile salts and are more water soluble than the unconjugated forms.Disorders of Lipid Digestion and Absorption•Pancreatic insufficiency•Acidity of duodenal contents (inadequate bicarbonate)•Ileal resection•Bacterial overgrowth -> de-congugates bile salts from glycine or taurine ->