Chapter 24



Chapter 24 The Digestive SystemAn Introduction to the Digestive SystemLearning Outcomes24-1Identify the organs of the digestive system, list their major functions, describe the histology of the digestive tract, and outline the mechanisms that regulate digestion.24-2Discuss the anatomy of the oral cavity, and list the functions of its major structures and regions.24-3 Describe the structure and functions of the pharynx.24-4 Describe the structure and functions of the.An Introduction to the Digestive SystemLearning Outcomes24-5 Describe the anatomy of the stomach, including its histological features, and discuss its roles in digestion and absorption.24-6 Describe the anatomical and histological characteristics of the small intestine, explain the functions and regulation of intestinal secretions, and describe the structure, functions, and regulation of the accessory digestive organs.An Introduction to the Digestive SystemLearning Outcomes24-7Describe the gross and histological structure of the large intestine, including its regional specializations and role in nutrient absorption.24-8List the nutrients required by the body, describe the chemical events responsible for the digestion of organic nutrients, and describe the mechanisms involved in the absorption of organic and inorganic nutrients.An Introduction to the Digestive SystemLearning Outcomes24-9 Summarize the effects of aging on the digestive system.24-10Give examples of interactions between the digestive system and other organ systems studied so far.An Introduction to the Digestive SystemThe Digestive SystemAcquires nutrients from environmentAnabolismUses raw materials to synthesize essential compoundsCatabolismDecomposes substances to provide energy cells need to functionAn Introduction to the Digestive SystemCatabolic ReactionsRequire two essential ingredientsOxygenOrganic molecules broken down by intracellular enzymesFor example, carbohydrates, fats, and proteins 24-1 The Digestive TractThe Digestive Tract Also called gastrointestinal (GI) tract or alimentary canalIs a muscular tubeExtends from oral cavity to anus Passes through pharynx, esophagus, stomach, and small and large intestines24-1 The Digestive TractSix Functions of the Digestive System Ingestion Mechanical processing Digestion Secretion Absorption Excretion24-1 The Digestive TractIngestion Occurs when materials enter digestive tract via the mouthMechanical Processing Crushing and shearingMakes materials easier to propel along digestive tractDigestion The chemical breakdown of food into small organic fragments for absorption by digestive epithelium24-1 The Digestive TractSecretionIs the release of water, acids, enzymes, buffers, and salts By epithelium of digestive tractBy glandular organs24-1 The Digestive TractAbsorption Movement of organic substrates, electrolytes, vitamins, and waterAcross digestive epitheliumInto interstitial fluid of digestive tractExcretion Removal of waste products from body fluidsProcess called defecation removes feces24-1 The Digestive TractThe Lining of the Digestive Tract Safeguards surrounding tissues against:Corrosive effects of digestive acids and enzymesMechanical stresses, such as abrasionBacteria either ingested with food or that reside in digestive tract24-1 The Digestive TractThe Digestive Organs and the PeritoneumLined with serous membrane consisting of:Superficial mesothelium covering a layer of areolar tissueSerosa, or visceral peritoneumCovers organs within peritoneal cavityParietal peritoneumLines inner surfaces of body wall24-1 The Digestive TractPeritoneal FluidIs produced by serous membrane liningProvides essential lubricationSeparates parietal and visceral surfacesAllows sliding without friction or irritationAbout 7 liters produced and absorbed daily, but very little in peritoneal cavity at one timeAscites – excess peritoneal fluid causing abdominal swelling24-1 The Digestive TractMesenteriesAre double sheets of peritoneal membraneSuspend portions of digestive tract within peritoneal cavity by sheets of serous membraneThat connect parietal peritoneumWith visceral peritoneum 24-1 The Digestive TractMesenteriesAreolar tissue between mesothelial surfacesProvides an access route to and from the digestive tractFor passage of blood vessels, nerves, and lymphatic vessels Stabilize positions of attached organsPrevent intestines from becoming entangled24-1 The Digestive TractMesentery DevelopmentDuring embryonic developmentDigestive tract and accessory organs are suspended in peritoneal cavity by:Dorsal mesenteryVentral mesenteryLater disappears along most of digestive tract except at the lesser omentum and at the falciform ligament24-1 The Digestive TractThe Lesser OmentumStabilizes position of stomachProvides access route for blood vessels and other structures entering or leaving liverThe Falciform Ligament Helps stabilize position of liverRelative to diaphragm and abdominal wall24-1 The Digestive TractThe Dorsal Mesentery Enlarges to form an enormous pouch, called the greater omentumExtends inferiorly between:The body wall and the anterior surface of small intestineHangs like an apronFrom lateral and inferior borders of stomach24-1 The Digestive TractAdipose Tissue in Greater OmentumConforms to shapes of surrounding organsPads and protects surfaces of abdomenProvides insulation to reduce heat lossStores lipid energy reserves24-1 The Digestive TractThe Mesentery Proper Is a thick mesenterial sheetProvides stabilityPermits some independent movementSuspends all but first 25 cm (10 in.) of small intestineIs associated with initial portion of small intestine (duodenum) and pancreasFuses with posterior abdominal wall, locking structures in position24-1 The Digestive TractThe Mesocolon A mesentery associated with a portion of the large intestineTransverse mesocolon supports transverse colonSigmoid mesocolon supports sigmoid colonDuring development, mesocolon of ascending colon, descending colon, and the rectum:Fuse to dorsal body wallLock regions in place24-1 The Digestive TractHistological Organization of the Digestive TractFour major layers of the digestive tract Mucosa Submucosa Muscularis externa Serosa24-1 The Digestive TractThe MucosaIs the inner lining of digestive tractIs a mucous membrane consisting of:Epithelium, moistened by glandular secretionsLamina propria of areolar tissue24-1 The Digestive TractThe Digestive Epithelium Mucosal epithelium is simple or stratifiedDepending on location, function, and stresses 24-1 The Digestive TractThe Digestive EpitheliumOral cavity, pharynx, and esophagusMechanical stressesLined by stratified squamous epitheliumStomach, small intestine, and most of large intestineAbsorptionSimple columnar epithelium with mucous cells24-1 The Digestive TractThe Digestive Epithelium Enteroendocrine cellsAre scattered among columnar cells of digestive epitheliumSecrete hormones that:Coordinate activities of the digestive tract and accessory glands24-1 The Digestive TractLining of Digestive TractFolding increases surface area for absorption Longitudinal folds, disappear as digestive tract fillsPermanent transverse folds (plicae circulares)24-1 The Digestive TractThe Lamina PropriaConsists of a layer of areolar tissue that contains:Blood vesselsSensory nerve endingsLymphatic vesselsSmooth muscle cellsScattered areas of lymphatic tissue24-1 The Digestive TractThe Lamina PropriaMuscularis mucosaeNarrow band of smooth muscle and elastic fibers in lamina propriaSmooth muscle cells arranged in two concentric layersInner layer encircles lumen (circular muscle)Outer layer contains muscle cells parallel to tract (longitudinal layer)24-1 The Digestive TractThe SubmucosaIs a layer of dense irregular connective tissueSurrounds muscularis mucosaeHas large blood vessels and lymphatic vesselsMay contain exocrine glandsSecrete buffers and enzymes into digestive tract24-1 The Digestive TractSubmucosal Plexus Also called Meissner’s plexus Innervates the mucosa and submucosaContains:Sensory neuronsParasympathetic ganglionic neuronsSympathetic postganglionic fibers24-1 The Digestive TractThe Muscularis Externa Is dominated by smooth muscle cellsAre arranged in:Inner circular layerOuter longitudinal layer24-1 The Digestive TractThe Muscularis Externa Involved in:Mechanical processingMovement of materials along digestive tractMovements coordinated by enteric nervous system (ENS)Sensory neuronsInterneuronsMotor neurons 24-1 The Digestive TractThe Muscularis Externa ENSInnervated primarily by parasympathetic division of ANSSympathetic postganglionic fibersThe mucosaThe myenteric plexus (Auerbach’s plexus)24-1 The Digestive TractThe SerosaSerous membrane covering muscularis externaExcept in oral cavity, pharynx, esophagus, and rectumWhere adventitia, a dense sheath of collagen fibers, firmly attaches the digestive tract to adjacent structures24-1 The Digestive TractThe Movement of Digestive MaterialsBy muscular layers of digestive tractConsist of visceral smooth muscle tissueAlong digestive tractHas rhythmic cycles of activityControlled by pacesetter cellsCells undergo spontaneous depolarizationTriggering wave of contraction through entire muscular sheet24-1 The Digestive TractPacesetter CellsLocated in muscularis mucosae and muscularis externaSurrounding lumen of digestive tractPeristalsisConsists of waves of muscular contractionsMoves a bolus along the length of the digestive tract24-1 The Digestive TractPeristaltic Motion Circular muscles contract behind bolusWhile circular muscles ahead of bolus relax Longitudinal muscles ahead of bolus contractShortening adjacent segments Wave of contraction in circular musclesForces bolus forward24-1 The Digestive TractSegmentationCycles of contractionChurn and fragment the bolusMix contents with intestinal secretionsDoes not follow a set patternDoes not push materials in any one direction24-1 The Digestive TractControl of Digestive FunctionsLocal FactorsProstaglandins, histamine, and other chemicals released into interstitial fluidMay affect adjacent cells within small segment of digestive tractCoordinate response to changing conditionsFor example, variations in local pH, chemical, or physical stimuliAffect only a portion of tract24-1 The Digestive TractControl of Digestive FunctionsNeural MechanismsControlMovement of materials along digestive tractSecretory functionsMotor neuronsControl smooth muscle contraction and glandular secretionLocated in myenteric plexus24-1 The Digestive TractNeural MechanismsShort reflexesAre responsible for local reflexesControl small segments of digestive tractOperate entirely outside of CNS controlSensory neuronsMotor neuronsInterneurons24-1 The Digestive TractNeural MechanismsLong reflexesHigher level control of digestive and glandular activitiesControl large-scale peristaltic wavesInvolve interneurons and motor neurons in CNSMay involve parasympathetic motor fibers that synapse in the myenteric plexusGlossopharyngeal, vagus, or pelvic nerves 24-1 The Digestive TractHormonal MechanismsAt least 18 peptide hormones that affect:Most aspects of digestive functionActivities of other systemsAre produced by enteroendocrine cells in digestive tractReach target organs after distribution in bloodstream24-2 The Oral CavityFunctions of the Oral Cavity Sensory analysisOf material before swallowing Mechanical processingThrough actions of teeth, tongue, and palatal surfaces LubricationMixing with mucus and salivary gland secretions Limited digestionOf carbohydrates and lipids24-2 The Oral CavityOral MucosaLining of oral cavityHas stratified squamous epitheliumOf cheeks, lips, and inferior surface of tongueIs relatively thin, nonkeratinized, and delicate Inferior to tongue is thin and vascular enough to rapidly absorb lipid-soluble drugs Cheeks are supported by pads of fat and the buccinator muscles24-2 The Oral CavityLabiaAlso called lipsAnteriorly, the mucosa of each cheek is continuous with that of the lipsVestibule Space between the cheeks (or lips) and the teeth24-2 The Oral CavityGingivae (Gums) Ridges of oral mucosaSurround base of each tooth on alveolar processes of maxillary bones and mandible24-2 The Oral CavityThe Tongue Manipulates materials inside mouthFunctions of the tongue Mechanical processing by compression, abrasion, and distortionManipulation to assist in chewing and to prepare material for swallowingSensory analysis by touch, temperature, and taste receptorsSecretion of mucins and the enzyme lingual lipase24-2 The Oral CavitySalivary GlandsThree pairs secrete into oral cavity Parotid salivary glands Sublingual salivary glands Submandibular salivary glandsEach pair has distinctive cellular organizationAnd produces saliva with different properties24-2 The Oral CavityParotid Salivary Glands Inferior to zygomatic archProduce serous secretionEnzyme salivary amylase (breaks down starches) Drained by parotid duct Which empties into vestibule at second molar 24-2 The Oral CavitySublingual Salivary Glands Covered by mucous membrane of floor of mouthProduce mucous secretionActs as a buffer and lubricantSublingual ductsEither side of lingual frenulum24-2 The Oral CavitySubmandibular Salivary Glands In floor of mouthWithin mandibular groove Secrete buffers, glycoproteins (mucins), and salivary amylaseSubmandibular ducts Open immediately posterior to teethEither side of lingual frenulum 24-2 The Oral CavitySalivaGlands produce 1.0–1.5 liters of saliva each day70 percent by submandibular glands25 percent by parotids5 percent by sublingual glands24-2 The Oral CavitySaliva99.4 percent water0.6 percent includes:Electrolytes (Na+, Cl?, and HCO3?)BuffersGlycoproteins (mucins)AntibodiesEnzymesWaste products24-2 The Oral CavityFunctions of SalivaLubricating the mouthMoistening and lubricating materials in the mouthDissolving chemicals that stimulate taste buds and provide sensory informationInitiating digestion of complex carbohydrates by the enzyme salivary amylase (ptyalin or alpha-amylase)24-2 The Oral CavityControl of Salivary SecretionsBy autonomic nervous systemParasympathetic and sympathetic innervation Parasympathetic accelerates secretion by all salivary glandsSalivatory nuclei of medulla oblongata influenced by:Other brain stem nucleiActivities of higher centers24-2 The Oral CavityThe TeethTongue movements pass food across occlusal surfaces of teethChew (masticate) food24-2 The Oral CavityDentinA mineralized matrix similar to that of boneDoes not contain cells Pulp CavityReceives blood vessels and nerves through the root canal24-2 The Oral CavityRootOf each tooth sits in a bony socket (alveolus) A layer of cementum covers dentin of the rootProviding protection and anchoring periodontal ligamentCrownExposed portion of toothProjects beyond soft tissue of gingivaDentin covered by layer of enamel24-2 The Oral CavityAlveolar ProcessesOf the maxillaeForm maxillary arcade (upper dental arch)Of the mandibleForm mandibular arcade (lower dental arch)24-2 The Oral CavityTypes of Teeth Incisors Cuspids (canines) Bicuspids (premolars) Molars24-2 The Oral CavityIncisors Blade-shaped teeth Located at front of mouthUsed for clipping or cuttingHave a single root24-2 The Oral CavityCuspids (Canines)ConicalSharp ridgelinePointed tipUsed for tearing or slashingHave a single root24-2 The Oral CavityBicuspids (Premolars)Flattened crownsProminent ridgesUsed to crush, mash, and grindHave one or two roots24-2 The Oral CavityMolars Very large, flat crownsWith prominent ridgesUsed for crushing and grindingHave three or more roots24-2 The Oral CavityDental SuccessionDuring embryonic development, two sets of teeth formPrimary dentition, or deciduous teethSecondary dentition, or permanent dentition24-2 The Oral CavityDeciduous TeethAlso called primary teeth, milk teeth, or baby teeth20 temporary teeth of primary dentition Five on each side of upper and lower jaws2 incisors1 cuspid2 deciduous molars24-2 The Oral CavitySecondary DentitionAlso called permanent dentitionReplaces deciduous teeth32 permanent teethEight on each side, upper and lower2 incisors1 cuspid5 molars24-2 The Oral CavityMasticationAlso called chewingFood is forced from oral cavity to vestibule and back Crossing and recrossing occlusal surfaces 24-2 The Oral CavityMuscles of Mastication Close the jawsSlide or rock lower jaw from side to side Chewing involves mandibular:Elevation and depressionProtraction and retractionMedial and lateral movement24-3 The PharynxThe Pharynx (Throat)A common passageway for solid food, liquids, and airRegions of the pharynx:NasopharynxOropharynxLaryngopharynxFood passes through the oropharynx and laryngopharynx to the esophagus24-4 The EsophagusThe EsophagusA hollow muscular tubeAbout 25 cm (10 in.) long and 2 cm (0.80 in.) wideConveys solid food and liquids to the stomachBegins posterior to cricoid cartilage Enters abdominopelvic cavity through the esophageal hiatusIs innervated by fibers from the esophageal plexus24-4 The EsophagusResting Muscle Tone In the circular muscle layer in the superior 3 cm (1.2 in.) of esophagus prevents air from entering Histology of the EsophagusWall of esophagus has three layersMucosalSubmucosalMuscularis 24-4 The EsophagusHistology of the EsophagusMucosa contains:Nonkeratinized and stratified squamous epithelium Mucosa and submucosa form:Large folds that extend the length of the esophagusMuscularis mucosae consists of:Irregular layer of smooth muscle24-4 The EsophagusHistology of the EsophagusSubmucosa contains esophageal glandsWhich produce mucous secretionReduces friction between bolus and esophageal lining Muscularis externa has:Usual inner circular and outer longitudinal layers24-4 The EsophagusSwallowingAlso called deglutitionCan be initiated voluntarilyProceeds automaticallyIs divided into three phasesBuccal phasePharyngeal phaseEsophageal phase24-5 The StomachMajor Functions of the StomachStorage of ingested foodMechanical breakdown of ingested foodDisruption of chemical bonds in food material by acid and enzymesProduction of intrinsic factor, a glycoprotein required for absorption of vitamin B12 in small intestine24-5 The StomachAnatomy of the Stomach The stomach is shaped like an expanded JShort lesser curvature forms medial surfaceLong greater curvature forms lateral surfaceAnterior and posterior surfaces are smoothly rounded Shape and size vary from individual to individual and from one meal to the nextStomach typically extends between levels of vertebrae T7 and L324-5 The StomachRegions of the Stomach Cardia Fundus Body Pylorus 24-5 The StomachSmooth MuscleMuscularis mucosae and muscularis externaContain extra layers of smooth muscle cells Oblique layer in addition to circular and longitudinal layers24-5 The StomachHistology of the StomachSimple columnar epithelium lines all portions of stomachEpithelium is a secretory sheetProduces mucus that covers interior surface of stomachGastric pits, shallow depressions that open onto the gastric surfaceMucous cells, at the base, or neck, of each gastric pit, actively divide, replacing superficial cells24-5 The StomachGastric GlandsIn fundus and body of stomachExtend deep into underlying lamina propriaEach gastric pit communicates with several gastric glandsParietal cells Chief cells24-5 The StomachParietal CellsSecrete intrinsic factor and hydrochloric acid (HCl)Chief CellsAre most abundant near base of gastric glandSecrete pepsinogen (inactive proenzyme)24-5 The StomachPepsinogenIs converted by HCl in the gastric lumenTo pepsin (active proteolytic enzyme) 24-5 The StomachPyloric GlandsLocated in the pylorusProduce mucous secretionScattered with enteroendocrine cellsG cells produce gastrin D cells release somatostatin, a hormone that inhibits release of gastrin24-5 The StomachRegulation of Gastric ActivityProduction of acid and enzymes by the gastric mucosa can be: Controlled by the CNSRegulated by short reflexes of ENSRegulated by hormones of digestive tract Three phases of gastric control Cephalic phase Gastric phase Intestinal phase24-5 The StomachDigestion and Absorption in the StomachStomach performs preliminary digestion of proteins by pepsinSome digestion of carbohydrates (by salivary amylase)Lipids (by lingual lipase)24-5 The StomachDigestion and Absorption in the StomachStomach contentsBecome more fluidpH approaches 2.0Pepsin activity increasesProtein disassembly begins Although digestion occurs in the stomach, nutrients are not absorbed there24-6 The Small IntestineThe Small IntestinePlays key role in digestion and absorption of nutrients90 percent of nutrient absorption occurs in the small intestine24-6 The Small IntestineThe Duodenum The segment of small intestine closest to stomach25 cm (10 in.) long“Mixing bowl” that receives chyme from stomach and digestive secretions from pancreas and liverFunctions of the duodenum: To receive chyme from stomachTo neutralize acids before they can damage the absorptive surfaces of the small intestine24-6 The Small IntestineThe Jejunum Is the middle segment of small intestine2.5 meters (8.2 ft) longIs the location of most:Chemical digestionNutrient absorptionHas few plicae circularesSmall villi 24-6 The Small IntestineThe IleumThe final segment of small intestine3.5 meters (11.48 ft) long Ends at the ileocecal valveA sphincter that controls flow of material from the ileum into the cecum of the large intestine24-6 The Small IntestineHistology of the Small IntestineCircular folds (plicae circulares)Transverse folds in intestinal liningAre permanent featuresDo not disappear when small intestine fills Intestinal villi A series of fingerlike projections in mucosa of small intestineCovered by simple columnar epitheliumCovered with microvilli 24-6 The Small IntestineHistology of the Small IntestineIntestinal glands (Crypts of Lieberkühn)Mucous cells between columnar epithelial cellsEject mucins onto intestinal surfacesOpenings from intestinal glandsTo intestinal lumen at bases of villi Entrances for brush border enzymes24-6 The Small IntestineIntestinal Glands Brush border enzymes Integral membrane proteins On surfaces of intestinal microvilli Break down materials in contact with brush border 24-6 The Small IntestineIntestinal GlandsEnteropeptidaseA brush border enzymeActivates pancreatic proenzyme trypsinogen Enteroendocrine cells Produce intestinal hormones such as gastrin, cholecystokinin, and secretin24-6 The Small IntestineIntestinal Glands Duodenal glandsAlso called submucosal glands or Brunner’s glands Produce copious quantities of mucusWhen chyme arrives from stomach24-6 The Small IntestineIntestinal SecretionsWatery intestinal juice1.8 liters per day enter intestinal lumenMoisten chymeAssist in buffering acidsKeep digestive enzymes and products of digestion in solution 24-6 The Small IntestineIntestinal MovementsChyme arrives in duodenumWeak peristaltic contractions move it slowly toward jejunumMyenteric reflexesNot under CNS controlParasympathetic stimulation accelerates local peristalsis and segmentation24-6 The Small IntestineThe Gastroenteric Reflex Stimulates motility and secretionAlong entire small intestineThe Gastroileal Reflex Triggers relaxation of ileocecal valveAllows materials to pass from small intestine into large intestine24-6 The PancreasThe PancreasLies posterior to stomachFrom duodenum toward spleenIs bound to posterior wall of abdominal cavityIs wrapped in thin, connective tissue capsule 24-6 The PancreasRegions of the PancreasHeadBroadIn loop of duodenum Body SlenderExtends toward spleenTailShort and rounded24-6 The PancreasHistological OrganizationLobules of the pancreasAre separated by connective tissue partitions (septa)Contain blood vessels and tributaries of pancreatic ducts In each lobule:Ducts branch repeatedly End in blind pockets (pancreatic acini)24-6 The PancreasHistological OrganizationPancreatic aciniBlind pocketsAre lined with simple cuboidal epithelium Contain scattered pancreatic isletsPancreatic isletsEndocrine tissues of pancreasScattered (1 percent of pancreatic cells)24-6 The PancreasFunctions of the PancreasEndocrine cells of the pancreatic isletsSecrete insulin and glucagon into bloodstreamExocrine cellsAcinar cells and epithelial cells of duct system secrete pancreatic juice24-6 The PancreasPhysiology of the Pancreas1000 mL (1 qt) pancreatic juice per dayControlled by hormones from duodenumContain pancreatic enzymes24-6 The PancreasPancreatic Enzymes Include:Pancreatic alpha-amylasePancreatic lipaseNucleasesProteolytic enzymes24-6 The PancreasPancreatic Alpha-AmylaseA carbohydraseBreaks down starchesSimilar to salivary amylase Pancreatic LipaseBreaks down complex lipidsReleases products (e.g., fatty acids) that are easily absorbed24-6 The PancreasNucleasesBreak down nucleic acids Proteolytic EnzymesBreak certain proteins apartProteases break large protein complexesPeptidases break small peptides into amino acids70 percent of all pancreatic enzyme productionSecreted as inactive proenzymesActivated after reaching small intestine24-6 The LiverThe LiverIs the largest visceral organ (1.5 kg; 3.3 lb)Lies in right hypochondriac and epigastric regionsExtends to left hypochondriac and umbilical regionsPerforms essential metabolic and synthetic functions24-6 The LiverAnatomy of the LiverWrapped in tough fibrous capsuleCovered by visceral peritoneum Divided into lobes24-6 The LiverHepatic Blood Supply1/3 of blood supplyArterial blood from hepatic artery proper2/3 venous blood from hepatic portal vein, originating at:EsophagusStomachSmall intestineMost of large intestine 24-6 The LiverHistological Organization of the LiverLiver lobulesThe basic functional units of the liverEach lobe is divided by connective tissueAbout 100,000 liver lobules1 mm diameter eachHexagonal in cross sectionWith six portal areas (portal triads)One at each corner of lobule24-6 The LiverA Portal AreaContains three structuresBranch of hepatic portal veinBranch of hepatic artery properSmall branch of bile duct24-6 The LiverHepatocytesAdjust circulating levels of nutrientsThrough selective absorption and secretion In a liver lobule form a series of irregular plates arranged like wheel spokes24-6 The LiverHepatocytesMany Kupffer cells (stellate reticuloendothelial cells) are located in sinusoidal liningAs blood flows through sinusoids:Hepatocytes absorb solutes from plasma and secrete materials such as plasma proteins24-6 The LiverThe Bile Duct SystemLiver secretes bile fluidInto a network of narrow channels (bile canaliculi)Between opposing membranes of adjacent liver cells24-6 The LiverRight and Left Hepatic Ducts Collect bile from all bile ducts of liver lobesUnite to form common hepatic duct that leaves the liverBile FlowFrom common hepatic duct to either:The common bile duct, which empties into duodenal ampullaThe cystic duct, which leads to gallbladder24-6 The LiverThe Common Bile Duct Is formed by union of:Cystic ductCommon hepatic ductPasses within the lesser omentum toward stomachPenetrates wall of duodenumMeets pancreatic duct at duodenal ampulla24-6 The LiverThe Physiology of the Liver Metabolic regulation Hematological regulation Bile production24-6 The LiverMetabolic RegulationThe liver regulates:Composition of circulating bloodNutrient metabolismWaste product removalNutrient storage Drug inactivation24-6 The LiverComposition of Circulating Blood All blood leaving absorptive surfaces of digestive tractEnters hepatic portal systemFlows into the liverLiver cells extract nutrients or toxins from bloodBefore they reach systemic circulation through hepatic veins Liver removes and stores excess nutrientsCorrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities24-6 The LiverMetabolic Activities of the LiverCarbohydrate metabolismLipid metabolismAmino acid metabolismWaste product removalVitamin storageMineral storageDrug inactivation24-6 The LiverHematological RegulationLargest blood reservoir in the bodyReceives 25 percent of cardiac output24-6 The Liver Functions of Hematological Regulation Phagocytosis and antigen presentationSynthesis of plasma proteinsRemoval of circulating hormonesRemoval of antibodiesRemoval or storage of toxinsSynthesis and secretion of bile24-6 The LiverThe Functions of BileDietary lipids are not water solubleMechanical processing in stomach creates large drops containing lipids Pancreatic lipase is not lipid solubleInteracts only at surface of lipid droplet Bile salts break droplets apart (emulsification)Increases surface area exposed to enzymatic attack Creates tiny emulsion droplets coated with bile salts24-6 The Gallbladder The GallbladderIs a hollow, pear-shaped, muscular sacStores and concentrates bile prior to excretion into small intestineIs located in the fossa on the posterior surface of the liver’s right lobe 24-6 The Gallbladder Regions of the Gallbladder Fundus Body Neck24-6 The Gallbladder The Cystic DuctExtends from gallbladderUnion with common hepatic duct forms common bile duct 24-6 The Gallbladder Physiology of the GallbladderStores bileReleases bile into duodenum, but only under stimulation of intestinal hormone cholecystokinin (CCK) CCKHepatopancreatic sphincter remains closedBile exiting liver in common hepatic duct cannot flow through common bile duct into duodenum Bile enters cystic duct and is stored in gallbladder24-6 The Gallbladder Physiology of the GallbladderFull gallbladder contains 40–70 mL bileBile composition gradually changes in gallbladderWater is absorbedBile salts and solutes become concentrated24-6 Coordination of Secretion and AbsorptionNeural and Hormonal Mechanisms Coordinate activities of digestive glandsRegulatory mechanisms center around duodenumWhere acids are neutralized and enzymes added24-6 Coordination of Secretion and AbsorptionNeural Mechanisms of the CNS Prepare digestive tract for activity (parasympathetic innervation)Inhibit gastrointestinal activity (sympathetic innervation)Coordinate movement of materials along digestive tract (the enterogastric, gastroenteric, and gastroileal reflexes)Motor neuron synapses in digestive tract release neurotransmitters24-6 Coordination of Secretion and AbsorptionIntestinal Hormones Intestinal tract secretes peptide hormones with multiple effectsIn several regions of digestive tractIn accessory glandular organs24-6 Coordination of Secretion and AbsorptionHormones of Duodenal Enteroendocrine CellsCoordinate digestive functionsGastrin SecretinGastric inhibitory peptide (GIP) Cholecystokinin (CCK) Vasoactive intestinal peptide (VIP) Enterocrinin24-6 Coordination of Secretion and AbsorptionGastrin Is secreted by G cells in duodenumWhen exposed to incompletely digested proteinsPromotes increased stomach motilityStimulates acids and enzyme productionSecretin Is released when chyme arrives in duodenumIncreases secretion of bile and buffers by liver and pancreas24-6 Coordination of Secretion and AbsorptionGastric Inhibitory Peptide (GIP) Is secreted when fats and carbohydrates enter small intestineCholecystokinin (CCK) Is secreted in duodenumWhen chyme contains lipids and partially digested proteinsAccelerates pancreatic production and secretion of digestive enzymesRelaxes hepatopancreatic sphincter and gallbladderEjecting bile and pancreatic juice into duodenum24-6 Coordination of Secretion and AbsorptionVasoactive Intestinal Peptide (VIP)Stimulates secretion of intestinal glandsDilates regional capillariesInhibits acid production in stomachEnterocrininIs released when chyme enters small intestineStimulates mucin production by submucosal glands of duodenum24-6 Coordination of Secretion and AbsorptionIntestinal Absorption It takes about five hours for materials to pass from duodenum to end of ileum Movements of the mucosa increase absorptive effectivenessStir and mix intestinal contentsConstantly change environment around epithelial cells 24-7 The Large IntestineThe Large IntestineIs horseshoe shaped Extends from end of ileum to anusLies inferior to stomach and liverFrames the small intestineAlso called large bowel Is about 1.5 meters (4.9 ft) long and 7.5 cm (3 in.) wide24-7 The Large IntestineFunctions of the Large IntestineReabsorption of water Compaction of intestinal contents into fecesAbsorption of important vitamins produced by bacteriaStorage of fecal material prior to defecation24-7 The Large IntestineParts of the Large Intestine Cecum The pouchlike first portion Colon The largest portion Rectum The last 15 cm (6 in.) of digestive tract24-7 The Large IntestineThe CecumIs an expanded pouch Receives material arriving from the ileumStores materials and begins compaction 24-7 The Large IntestineAppendixAlso called vermiform appendixIs a slender, hollow appendage about 9 cm (3.6 in.) longIs dominated by lymphoid nodules (a lymphoid organ) Is attached to posteromedial surface of cecumMesoappendix connects appendix to ileum and cecum 24-7 The Large IntestineThe ColonHas a larger diameter and thinner wall than small intestine The wall of the colonForms a series of pouches (haustra)Haustra permit expansion and elongation of colon24-7 The Large IntestineColon Muscles Three longitudinal bands of smooth muscle (taeniae coli)Run along outer surfaces of colon Deep to the serosaSimilar to outer layer of muscularis externaMuscle tone in taeniae coli creates the haustra24-7 The Large IntestineSerosa of the ColonContains numerous teardrop-shaped sacs of fatFatty appendices or epiploic appendages24-7 The Large IntestineFour Regions of the Colon Ascending colon Transverse colon Descending colon Sigmoid colon24-7 The Large IntestineAscending Colon Begins at superior border of cecum Ascends along right lateral and posterior wall of peritoneal cavity To inferior surface of the liver and bends at right colic flexure (hepatic flexure)24-7 The Large IntestineTransverse ColonCrosses abdomen from right to left; turns at left colic flexure (splenic flexure)Is supported by transverse mesocolonIs separated from anterior abdominal wall by greater omentum24-7 The Large IntestineDescending Colon Proceeds inferiorly along left side to the iliac fossa (inner surface of left ilium)Is retroperitoneal, firmly attached to abdominal wall 24-7 The Large IntestineSigmoid Colon Is an S-shaped segment, about 15 cm (6 in.) longStarts at sigmoid flexureLies posterior to urinary bladderIs suspended from sigmoid mesocolonEmpties into rectum 24-7 The Large IntestineBlood Supply of the Large IntestineReceives blood from tributaries of:Superior mesenteric and inferior mesenteric arteriesVenous blood is collected from:Superior mesenteric and inferior mesenteric veins24-7 The Large IntestineThe RectumForms last 15 cm (6 in.) of digestive tractIs an expandable organ for temporary storage of fecesMovement of fecal material into rectum triggers urge to defecate The Anal Canal Is the last portion of the rectumContains small longitudinal folds called anal columns 24-7 The Large IntestineAnusAlso called anal orificeIs exit of the anal canalHas keratinized epidermis like skin24-7 The Large IntestineAnal SphinctersInternal anal sphincterCircular muscle layer of muscularis externaHas smooth muscle cells, not under voluntary controlExternal anal sphincterEncircles distal portion of anal canalA ring of skeletal muscle fibers, under voluntary control24-7 The Large IntestineHistology of the Large IntestineLacks villiAbundance of mucous cellsPresence of distinctive intestinal glandsAre deeper than glands of small intestineAre dominated by mucous cells24-7 The Large IntestineHistology of the Large IntestineDoes not produce enzymes Provides lubrication for fecal material Large lymphoid nodules are scattered throughout the lamina propria and submucosaThe longitudinal layer of the muscularis externa is reduced to the muscular bands of taeniae coli24-7 The Large IntestinePhysiology of the Large IntestineLess than 10 percent of nutrient absorption occurs in large intestinePrepares fecal material for ejection from the body 24-7 The Large IntestineAbsorption in the Large IntestineReabsorption of water Reabsorption of bile saltsIn the cecum Transported in blood to liver Absorption of vitamins produced by bacteriaAbsorption of organic wastes24-7 The Large IntestineVitamins Are organic molecules Important as cofactors or coenzymes in metabolismNormal bacteria in colon make three vitamins that supplement diet24-7 The Large IntestineThree Vitamins Produced in the Large Intestine Vitamin K (fat soluble)Required by liver for synthesizing four clotting factors, including prothrombin Biotin (water soluble)Important in glucose metabolism Vitamin B5 (pantothenic acid) (water soluble)Required in manufacture of steroid hormones and some neurotransmitters24-7 The Large IntestineOrganic Wastes Bacteria convert bilirubin to urobilinogens and stercobilinogensUrobilinogens absorbed into bloodstream are excreted in urineUrobilinogens and stercobilinogens in colon convert to urobilins and stercobilins by exposure to oxygen24-7 The Large IntestineOrganic Wastes Bacteria break down peptides in feces and generate:AmmoniaAs soluble ammonium ionsIndole and skatoleNitrogen compounds responsible for odor of fecesHydrogen sulfideGas that produces “rotten egg” odor24-7 The Large IntestineOrganic Wastes Bacteria feed on indigestible carbohydrates (complex polysaccharides)Produce flatus, or intestinal gas, in large intestine24-7 The Large IntestineMovements of the Large Intestine Gastroileal and gastroenteric reflexesMove materials into cecum while you eatMovement from cecum to transverse colon is very slow, allowing hours for water absorption Peristaltic waves move material along length of colonSegmentation movements (haustral churning) mix contents of adjacent haustra24-7 The Large IntestineMovements of the Large Intestine Movement from transverse colon through rest of large intestine results from powerful peristaltic contractions (mass movements) Stimulus is distension of stomach and duodenum; relayed over intestinal nerve plexusesDistension of the rectal wall triggers defecation reflexTwo positive feedback loopsBoth loops triggered by stretch receptors in rectum24-7 The Large IntestineTwo Positive Feedback Loops Short reflexTriggers peristaltic contractions in rectum Long reflexCoordinated by sacral parasympathetic systemStimulates mass movements24-7 The Large IntestineRectal Stretch Receptors Also trigger two reflexes important to voluntary control of defecation A long reflexMediated by parasympathetic innervation in pelvic nervesCauses relaxation of internal anal sphincter A somatic reflexMotor commands carried by pudendal nerves Stimulates contraction of external anal sphincter (skeletal muscle)24-7 The Large IntestineElimination of Feces Requires relaxation of internal and external anal sphinctersReflexes open internal sphincter, close external sphincterOpening external sphincter requires conscious effort24-8 DigestionNutrientsA balanced diet contains:CarbohydratesLipidsProteinsVitaminsMineralsWater24-8 DigestionThe Processing and Absorption of NutrientsBreaks down physical structure of foodDisassembles component molecules Molecules released into bloodstream are:Absorbed by cellsBroken down to provide energy for ATP synthesisOr used to synthesize carbohydrates, proteins, and lipids24-8 DigestionDigestive Enzymes Are secreted by:Salivary glandsTongueStomachPancreas24-8 DigestionDigestive Enzymes Break molecular bonds in large organic moleculesCarbohydrates, proteins, lipids, and nucleic acidsIn a process called hydrolysisAre divided into classes by targetsCarbohydrases break bonds between simple sugarsProteases break bonds between amino acidsLipases separate fatty acids from glycerides24-8 DigestionDigestive Enzymes Brush border enzymes break nucleotides into:SugarsPhosphatesNitrogenous bases24-8 DigestionWater AbsorptionCells cannot actively absorb or secrete waterAll movement of water across lining of digestive tract:Involves passive water flow down osmotic gradients 24-8 DigestionIon Absorption Osmosis does not distinguish among solutesDetermined only by total concentration of solutesTo maintain homeostasis:Concentrations of specific ions must be regulatedSodium ion absorptionRate increased by aldosterone (steroid hormone from adrenal cortex)24-8 DigestionIon Absorption Calcium ion absorptionInvolves active transport at epithelial surfaceRate increased by parathyroid hormone (PTH) and calcitriolPotassium ion concentration increases:As other solutes move out of lumenOther ions diffuse into epithelial cells along concentration gradient 24-8 DigestionIon Absorption Cation absorption (magnesium, iron)Involves specific carrier proteinsCell must use ATP to transport ions to interstitial fluidAnions (chloride, iodide, bicarbonate, and nitrate)Are absorbed by diffusion or carrier-mediated transportPhosphate and sulfate ionsEnter epithelial cells by active transport24-8 DigestionVitaminsAre organic compounds required in very small quantitiesAre divided into two major groupsFat-soluble vitaminsWater-soluble vitamins24-9 Effects of Aging on the Digestive SystemAge-Related ChangesDivision of epithelial stem cells declinesDigestive epithelium becomes more susceptible to damage by abrasion, acids, or enzymesSmooth muscle tone and general motility decreasePeristaltic contractions become weakerCumulative damage from toxins (alcohol, other chemicals) Absorbed by digestive tract and transported to liver for processing24-9 Effects of Aging on the Digestive SystemAge-Related ChangesRates of colon cancer and stomach cancer rise with ageOral and pharyngeal cancers common among elderly smokersDecline in olfactory and gustatory sensitivitiesLeads to dietary changes that affect entire body ................
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