Y khoa, y dược - The digestive system: Part C

23 The Digestive System: Part CPancreasLocationMostly retroperitoneal, deep to the greater curvature of the stomachHead is encircled by the duodenum; tail abuts the spleenPancreasEndocrine functionPancreatic islets secrete insulin and glucagonExocrine functionAcini (clusters of secretory cells) secrete pancreatic juice Zymogen granules of secretory cells contain digestive enzymesFigure 23.26aSmallductAcinar cellsBasementmembraneZymogengranulesRoughendoplasmicreticulum(a) Pancreatic JuiceWatery alkaline solution (pH 8) neutralizes chymeElectrolytes (primarily HCO3–) EnzymesAmylase, lipases, nucleases are secreted in active form but require ions or bile for optimal activityProteases secreted in inactive form Pancreatic JuiceProtease activation in duodenumTrypsinogen is activated to trypsin by brush border enzyme enteropeptidaseProcarboxypeptidase and chymotrypsinogen are activated by trypsinFigure 23.27StomachPancreasEpithelialcellsTrypsinogen(inactive)Chymotrypsinogen(inactive)Procarboxypeptidase(inactive)TrypsinChymotrypsinCarboxypeptidaseMembrane-boundenteropeptidaseRegulation of Bile SecretionBile secretion is stimulated byBile salts in enterohepatic circulation Secretin from intestinal cells exposed to HCl and fatty chymeRegulation of Bile SecretionGallbladder contraction is stimulated byCholecystokinin (CCK) from intestinal cells exposed to proteins and fat in chymeVagal stimulation (minor stimulus)CKK also causes the hepatopancreatic sphincter to relaxRegulation of Pancreatic SecretionCCK induces the secretion of enzyme-rich pancreatic juice by aciniSecretin causes secretion of bicarbonate-rich pancreatic juice by duct cellsVagal stimulation also causes release of pancreatic juice (minor stimulus)Figure 23.28 Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. CCK (red dots) and secretin (yellow dots) enter the bloodstream. CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.Slide 1123456Figure 23.28, step 1 Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells.1Figure 23.28, step 2 Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. CCK (red dots) and secretin (yellow dots) enter the bloodstream.12Figure 23.28, step 3 Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. CCK (red dots) and secretin (yellow dots) enter the bloodstream. CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice.123Figure 23.28, step 4 Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. CCK (red dots) and secretin (yellow dots) enter the bloodstream. CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly.1234Figure 23.28, step 5 Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. CCK (red dots) and secretin (yellow dots) enter the bloodstream. CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum.12345Figure 23.28, step 6 Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. CCK (red dots) and secretin (yellow dots) enter the bloodstream. CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.123456Digestion in the Small IntestineChyme from stomach containsPartially digested carbohydrates and proteins Undigested fatsRequirements for Digestion and Absorption in the Small IntestineSlow delivery of hypertonic chymeDelivery of bile, enzymes, and bicarbonate from the liver and pancreasMixingMotility of the Small IntestineSegmentationInitiated by intrinsic pacemaker cells Mixes and moves contents slowly and steadily toward the ileocecal valveIntensity is altered by long and short reflexes Wanes in the late intestinal (fasting) phaseFigure 23.3b(b)MicrovilliAbsorptivecell Motility of the Small IntestinePeristalsisInitiated by motilin in the late intestinal phase Each wave starts distal to the previous (the migrating motility complex)Meal remnants, bacteria, and debris are moved to the large intestineFigure 23.3aFrom mouth(a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.Motility of the Small IntestineLocal enteric neurons coordinate intestinal motilityCholinergic sensory neurons may activate the myenteric plexusCauses contraction of the circular muscle proximally and of longitudinal muscle distallyForces chyme along the tractMotility of the Small IntestineIleocecal sphincter relaxes and admits chyme into the large intestine whenGastroileal reflex enhances the force of segmentation in the ileumGastrin increases the motility of the ileumIleocecal valve flaps close when chyme exerts backward pressureLarge IntestineUnique featuresTeniae coliThree bands of longitudinal smooth muscle in the muscularisHaustraPocketlike sacs caused by the tone of the teniae coliEpiploic appendagesFat-filled pouches of visceral peritoneumLarge IntestineRegionsCecum (pouch with attached vermiform appendix)ColonRectumAnal canalFigure 23.29aLeft colic(splenic) flexureTransversemesocolon Epiploicappendages Descendingcolon Teniae coliSigmoidcolon Cut edge ofmesentery External anal sphincterRectumAnal canal(a)Right colic(hepatic) flexureTransversecolon SuperiormesentericarteryHaustrumAscendingcolon IIeumIIeocecal valveVermiform appendixCecumColonAscending colon and descending colon are retroperitoneal Transverse colon and sigmoid colon are anchored via mesocolons (mesenteries)Figure 23.30cTransverse colonGreater omentumDescending colonJejunumMesenteryTransversemesocolonSigmoidmesocolonSigmoid colonIleum(c)Figure 23.30d(d)PancreasLiverLesser omentumStomachDuodenumTransversemesocolonGreater omentumMesenteryJejunumVisceral peritoneumUrinary bladderTransverse colonIleumParietal peritoneumRectumRectum and AnusRectumThree rectal valves stop feces from being passed with gasAnal canalThe last segment of the large intestineSphinctersInternal anal sphincter—smooth muscleExternal anal sphincter—skeletal muscleFigure 23.29b(b)Rectal valveRectumAnal canalLevator animuscle AnusAnal sinusesAnal columnsInternal analsphincterExternal analsphincterHemorrhoidalveinsPectinate lineLarge Intestine: Microscopic AnatomyMucosa of simple columnar epithelium except in the anal canal (stratified squamous)Abundant deep crypts with goblet cellsSuperficial venous plexuses of the anal canal form hemorrhoids if inflamedBacterial FloraEnter from the small intestine or anus Colonize the colonFerment indigestible carbohydratesRelease irritating acids and gases Synthesize B complex vitamins and vitamin KFunctions of the Large IntestineVitamins, water, and electrolytes are reclaimedMajor function is propulsion of feces toward the anusColon is not essential for lifeMotility of the Large IntestineHaustral contractionsSlow segmenting movements Haustra sequentially contract in response to distensionMotility of the Large IntestineGastrocolic reflexInitiated by presence of food in the stomachActivates three to four slow powerful peristaltic waves per day in the colon (mass movements)DefecationMass movements force feces into rectumDistension initiates spinal defecation reflexParasympathetic signalsStimulate contraction of the sigmoid colon and rectumRelax the internal anal sphincterConscious control allows relaxation of external anal sphincter Figure 23.31Impulses fromcerebral cortex(consciouscontrol)Voluntary motornerve to externalanal sphincterExternal analsphincter(skeletal muscle)Internal anal sphincter(smooth muscle)Sensorynerve fibersInvoluntary motor nerve(parasympathetic division)Stretch receptors in wallRectumSigmoidcolon 312 Distension, or stretch, of therectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter. If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so that feces may pass.Chemical DigestionCatabolicEnzymaticHydrolysisChemical Digestion and Absorption of CarbohydratesDigestive enzymesSalivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)Chemical Digestion and Absorption of CarbohydratesAbsorptionSecondary active transport (cotransport) with Na+Facilitated diffusion of some monosaccharidesEnter the capillary beds in the villiTransported to the liver via the hepatic portal veinFigure 23.32 (1 of 4)Carbohydrate digestion• Glucose and galactose are absorbed via cotransport with sodium ions.• Fructose passes via facilitated diffusion.• All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.Starch and disaccharidesOligosaccharidesand disaccharidesLactoseMaltoseSucroseGlucoseFructoseSalivaryamylase MouthPancreaticamylase Brush borderenzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)Smallintestine Smallintestine FoodstuffGalactosePath of absorptionEnzyme(s)and sourceSite ofactionChemical Digestion and Absorption of ProteinsEnzymes: pepsin in the stomachPancreatic proteasesTrypsin, chymotrypsin, and carboxypeptidaseBrush border enzymesAminopeptidases, carboxypeptidases, and dipeptidasesAbsorption of amino acids is coupled to active transport of Na+ Figure 23.33AbsorptiveepithelialcellApical membrane (microvilli)Aminoacid carrierCapillaryLumen of intestinePancreaticproteasesAmino acids of protein fragmentsBrush border enzymesNa+Na+1 Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.2 The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis).3 The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts.Active transportPassive transportFigure 23.32 (2 of 4)Protein digestion• Amino acids are absorbed by cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells.+ • Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.Smallintestine Smallintestine StomachFoodstuffProteinLarge polypeptidesPepsin(stomach glands)in presence of HClSmall polypeptides,small peptidesPancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)Amino acids(some dipeptidesand tripeptides)Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)Path of absorptionEnzyme(s)and sourceSite ofactionChemical Digestion and Absorption of LipidsPre-treatment—emulsification by bile saltsEnzymes—pancreatic lipaseAbsorption of glycerol and short chain fatty acidsAbsorbed into the capillary blood in villiTransported via the hepatic portal veinChemical Digestion and Absorption of LipidsAbsorption of monoglycerides and fatty acidsCluster with bile salts and lecithin to form micellesReleased by micelles to diffuse into epithelial cellsCombine with proteins to form chylomicronsEnter lacteals and are transported to systemic circulationFigure 23.34Epithelialcells ofsmallintestineFat dropletscoated withbile saltsFat globuleLactealBile saltsMicelles made up of fatty acids, monoglycerides,and bile salts1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.Figure 23.32 (3 of 4)Fat digestionSmall intestineSmall intestineFoodstuffUnemulsifiedfatsEmulsification by the detergent action of bile salts ductedin from the liverPancreatic lipasesMonoglyceridesand fatty acidsGlycerolandfatty acids Path of absorptionEnzyme(s)and sourceSite ofaction• Fatty acids and monoglycerides enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis.• The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.Chemical Digestion and Absorption of Nucleic AcidsEnzymesPancreatic ribonuclease and deoxyribonucleaseAbsorptionActive transport Transported to liver via hepatic portal veinFigure 23.32 (4 of 4)Nucleic acid digestion• Units enter intestinal cells by active transport via membrane carriers.• Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.Smallintestine Smallintestine FoodstuffNucleic acidsPancreatic ribo-nuclease and deoxyribonucleaseBrush borderenzymes(nucleosidasesand phosphatases)Pentose sugars,N-containing bases, phosphate ionsPath of absorptionEnzyme(s)and sourceSite ofactionVitamin AbsorptionIn small intestineFat-soluble vitamins (A, D, E, and K) are carried by micelles and then diffuse into absorptive cellsWater-soluble vitamins (vitamin C and B vitamins) are absorbed by diffusion or by passive or active transporters.Vitamin B12 binds with intrinsic factor, and is absorbed by endocytosis Vitamin AbsorptionIn large intestineVitamin K and B vitamins from bacterial metabolism are absorbedElectrolyte AbsorptionMostly along the length of small intestineIron and calcium are absorbed in duodenum Na+ is coupled with absorption of glucose and amino acidsIonic iron is stored in mucosal cells with ferritinK+ diffuses in response to osmotic gradientsCa2+ absorption is regulated by vitamin D and parathyroid hormone (PTH)Water Absorption95% is absorbed in the small intestine by osmosisNet osmosis occurs whenever a concentration gradient is established by active transport of solutes Water uptake is coupled with solute uptakeMalabsorption of NutrientsCausesAnything that interferes with delivery of bile or pancreatic juice Damaged intestinal mucosa (e.g., bacterial infection)Malabsorption of NutrientsGluten-sensitive enteropathy (celiac disease)Gluten damages the intestinal villi and brush borderTreated by eliminating gluten from the diet (all grains but rice and corn)Developmental AspectsIn the third week Endoderm has folded and foregut and hindgut have formedMidgut is open and continuous with the yolk sacMouth and anal openings are nearly formedIn the eighth weekAccessory organs are budding from endodermFigure 23.35StomodeumForegutSite ofliverdevelopmentMidgutSpinal cordHindgutProctodeumEndodermBrainOralmembrane HeartYolk sacCloacalmembraneBodystalk (a)Lung budLiverGall-bladderCystic ductVentral pancreatic budDorsalpancreaticbudDuodenumStomach(b)BileductDevelopmental AspectsFetal nutrition is via the placenta, but the GI tract is stimulated to mature by amniotic fluid swallowed in uteroThe newborn’s rooting reflex helps the infant find the nipple; the sucking reflex aids in swallowingDevelopmental AspectsDuring old age GI tract activity declines, absorption is less efficient, and peristalsis is slowedDiverticulosis, fecal incontinence, and cancer of the GI tract CancerStomach and colon cancers rarely have early signs or symptomsMetastasized colon cancers frequently cause secondary liver cancerPreventionRegular dental and medical examination