Digestive SystemHuman Anatomy & PhysiologyUniversity of Washington PMT

Digestive System FunctionAcquires nutrients from environmentAnabolismUses raw materials to synthesize essential compoundsCatabolismDecomposes substances to provide energy cells need to function

Digestive (GI) Tract

Actions of Digestive (GI) TractIngestionOccurs when material enters via the mouthMechanical ProcessingCrushing / Shearing makes material easier to move through the tractDigestionChemical breakdown of food into small organic compounds for absorptionSecretionRelease of water acids, buffers, enzymes & salts by epithelium of GI tract and glandular organsAbsorptionMovement of organic substrates, electrolytes, vitamins & water across digestive epitheliumExcretionRemoval of waste products from body fluids

Digestive (GI) TractThe Digestive Organs and the PeritoneumLined with serous membrane consisting ofSuperficial mesothelium covering a layer of areolar tissueSerosa, or visceral peritoneum:covers organs within peritoneal cavityParietal peritoneum:lines inner surfaces of body wall

Histological Structure of the Digestive (GI) Tract

Movement of Digestive MaterialsBy muscular layers of digestive tractConsist of visceral smooth muscleAlong digestive tract:Has rhythmic cycles of activities (PERISTALSIS)Consists of waves of muscular contractionsMove a bolus along the length of the tractControlled by pacesetter cellsSurrounding the lumen of the tractCells undergo spontaneous depolarizationTriggering wave of contraction through entire muscular sheet

Peristalsis

Functions of Oral CavitySensory analysisOf material before swallowingMechanical processingThrough actions of teeth, tongue, and palatal surfacesLubricationMixing with mucus and salivary gland secretionsLimited digestionOf carbohydrates and lipids

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 Is innervated by fibers from the esophageal plexus

Stomach FunctionMajor 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 intestine

Gastric Anatomy

Digestion in the StomachStomach performs preliminary digestion of proteins by pepsinSome digestion of carbohydrates (by salivary amylase)Lipids (by lingual lipase)

Stomach contentsBecome more fluidpH approaches 2.0Pepsin activity increasesProtein disassembly begins

Although digestion occurs in the stomach, nutrients are not absorbed there

Small Intestine90% of absorption occurs in the small intestine

Small IntestineThe Duodenum The segment of small intestine closest to stomach25 cm (10 in.) longMixing 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 intestine

Small IntestineThe Jejunum Is the middle segment of small intestine2.5 meters (8.2 ft) longIs the location of mostChemical digestionNutrient absorptionHas few plicae circularesSmall villi

Small IntestineThe IleumThe final segment of small intestine3.5 meters (11.48 ft) long Ends at the ileocecal valve, a sphincter that controls flow of material from the ileum into the large intestine

Small Intestine

Small IntestineIntestinal SecretionsWatery intestinal juice1.8 liters per day enter intestinal lumenMoisten chymeAssist in buffering acidsKeep digestive enzymes and products of digestion in solutionIntestinal MovementsChyme arrives in duodenumWeak peristaltic contractions move it slowly toward jejunumMyenteric reflexesNot under CNS controlParasympathetic stimulation accelerates local peristalsis and segmentation

PancreasLies posterior to stomachFrom duodenum toward spleenIs bound to posterior wall of abdominal cavityIs wrapped in thin, connective tissue capsuleFunctions of the PancreasEndocrine cells of the pancreatic islets:Secrete insulin and glucagon into bloodstreamExocrine cells:Acinar cells and epithelial cells of duct system secrete pancreatic juice

PancreasPancreatic Enzymes Pancreatic alpha-amylaseA carbohydraseBreaks down starchesSimilar to salivary amylase Pancreatic lipaseBreaks down complex lipidsReleases products (e.g., fatty acids) that are easily absorbedPancreatic Enzymes NucleasesBreak down nucleic acids Proteolytic enzymesBreak certain proteins apartProteases break large protein complexesPeptidases break small peptides into amino acids70% of all pancreatic enzyme productionSecreted as inactive proenzymesActivated after reaching small intestine

Liver

LiverHepatocytesAre liver cellsAdjust circulating levels of nutrientsThrough selective absorption and secretion In a liver lobule form a series of irregular plates arranged like wheel spokesMany Kupffer cells (stellate reticuloendothelial cells) are located in sinusoidal liningAs blood flows through sinusoidsHepatocytes absorb solutes from plasmaAnd secrete materials such as plasma proteins

Liver FunctionThe Physiology of the LiverMetabolic regulationHematological regulationBile production

Liver FunctionMetabolic RegulationThe liver regulates:Composition of circulating bloodNutrient metabolism (carbohydrate, lipid & amino acid)Waste product removalVitamin Storage (A, D, E & K)Nutrient storage (iron) Drug inactivation

Liver FunctionComposition 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 activities

Liver FunctionHematological RegulationLargest blood reservoir in the bodyReceives 25% of cardiac outputFunctions of Hematological Regulation Phagocytosis and antigen presentationSynthesis of plasma proteinsRemoval of circulating hormonesRemoval of antibodiesRemoval or storage of toxinsSynthesis and secretion of bile

Liver FunctionThe 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 salts

Liver

GallbladderIs a pear-shaped, muscular sacStores and concentrates bile prior to excretion into small intestineIs located in the fossa on the posterior surface of the livers right lobeThe Cystic DuctExtends from gallbladderUnion with common hepatic duct forms common bile duct

GallbladderFunctions of the GallbladderStores bileReleases bile into duodenum, but only under stimulation of 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 gallbladder

Coordination of Secretion & Absorption

Coordination of Secretion & AbsorptionIntestinal Absorption It takes about 5 hours for materials to pass from duodenum to end of ileum Movements of the mucosa increases absorptive effectivenessStir and mix intestinal contentsConstantly change environment around epithelial cells

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.) wide

Large Intestine FunctionsReabsorption of water Compaction of intestinal contents into fecesAbsorption of important vitamins produced by bacteriaStorage of fecal material prior to defecation

Parts of Large IntestineThe CecumIs an expanded pouch Receives material arriving from the ileumStores materials and begins compactionAppendixAlso called vermiform appendixIs a slender, hollow appendage about 9 cm (3.6 in.) longIs dominated by lymphoid nodules (a lymphoid organ)

Parts of 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 colon

Parts of ColonAscending 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)Transverse 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 omentum

Parts of ColonThe Descending Colon Proceeds inferiorly along left side to the iliac fossa (inner surface of left ilium)Is retroperitoneal, firmly attached to abdominal wall The Sigmoid 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

Parts of Colon

Parts of 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 AnusAlso called anal orificeIs exit of the anal canalHas keratinized epidermis like skin

Physiology of 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 wastes

Physiology of 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 metabolismPantothenic acid: B5 (water soluble):Required in manufacture of steroid hormones and some neurotransmitters

Physiology of the Large IntestineOrganic Wastes Bacteria convert bilirubin to urobilinogens and stercobilinogensBacteria break down peptides in feces and generateAmmonia, Indole & skatole, hydrogen sulfideBacteria feed on indigestible carbohydrates (complex polysaccharides)Produce flatus, or intestinal gas, in large intestine

Movements of the Large IntestineGastroileal & 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 haustra

Movements 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 rectum

DigestionDigestive system handles each nutrient differentlyLarge organic moleculesMust be digested before absorption can occurWater, electrolytes, and vitaminsCan be absorbed without processingMay require special transport

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 glycerides

DigestionWater AbsorptionCells cannot actively absorb or secrete waterAll movement of water across lining of digestive tractInvolves passive water flow down osmotic gradients

Catabolic ReactionsRequire two essential ingredients:OxygenOrganic molecules broken down by intracellular enzymes:e.g., carbohydrates, fats, and proteins

*Peritoneal FluidIs produced by serous membrane liningProvides essential lubricationSeparates parietal and visceral surfacesAllows sliding without friction or irritation

*The MucosaIs the inner lining of digestive tractIs a mucous membrane consisting ofEpithelium, moistened by glandular secretionsLamina propria of areolar tissueThe Digestive Epithelium Mucosal epithelium is simple or stratifiedDepending on location, function, and stresses: oral cavity, pharynx, and esophagus:mechanical stresseslined by stratified squamous epitheliumstomach, small intestine, and most of large intestine:absorptionsimple columnar epithelium with mucous (goblet) cellsThe Digestive Epithelium Enteroendocrine cellsAre scattered among columnar cells of digestive epitheliumSecrete hormones that:coordinate activities of the digestive tract and accessory glands

*Peristaltic MotionCircular muscles contract behind bolus:While circular muscles ahead of bolus relaxLongitudinal muscles ahead of bolus contract:Shortening adjacent segmentsWave of contraction in circular muscles:Forces bolus forward

*Salivary GlandsThree pairs secrete into oral cavity Each pair has distinctive cellular organizationAnd produces saliva with different propertiesSalivary GlandsProduce 1.01.5 liters of saliva each daySaliva99.4% water0.6% includesElectrolytes (Na+, Cl-, and HCO3-)BuffersGlycoproteins (mucins)AntibodiesEnzymesWaste productsMuscles of Mastication Close the jawsSlide or rock lower jaw from side to side Chewing involves mandibularElevation and depressionProtraction and retractionMedial and lateral movement

*SwallowingAlso called deglutitionCan be initiated voluntarilyProceeds automaticallyIs divided into three phasesBuccal phasePharyngeal phaseEsophageal phase

*Regions of the StomachCardiaFundus BodyPylorus Constantly being replaced, covered thick mucus, same simple columnar epitheliumPyloric Sphincter regulates gastric emptying

Gastric GlandsIn fundus and body of stomachExtend deep into underlying lamina propriaEach gastric pit communicates with several gastric glandsParietal cells Chief cellsParietal CellsSecrete intrinsic factor and hydrochloric acid (HCl)Chief CellsSecrete hydrochloric acid (HCl)Are most abundant near base of gastric glandSecrete pepsinogen (inactive proenzyme)PepsinogenIs converted by HCl in the gastric lumenTo pepsin (active proteolytic enzyme)

**Brush Border Enzymes Integral membrane proteins On surfaces of intestinal microvilli Break down materials in contact with brush border Intestinal GlandsEnteropeptidaseA brush border enzymeActivates pancreatic proenzyme trypsinogen Enteroendocrine cells Produce intestinal hormones such as gastrin, cholecystokinin, and secretinDuodenal GlandsAlso called submucosal glands or Brunner glands Produce copious quantities of mucusWhen chyme arrives from stomach

*Pancreatic AciniBlind pocketsAre lined with simple cuboidal epithelium Contain scattered pancreatic isletsPancreatic IsletsEndocrine tissues of pancreasScattered (1% of pancreatic cells)

*Is 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 functionsAnatomy of the LiverIs wrapped in tough fibrous capsuleIs covered by visceral peritoneum Is divided into lobes

*Metabolic Activities of the LiverCarbohydrate metabolismLipid metabolismAmino acid metabolismWaste product removalVitamin storageMineral storageDrug inactivation

*Removes damaged / old red blood cellsPlasma proteins: albumin contrbutes to osmotic concentrationReabsorped Epinephrine, norepinephrine, insulin, thyroid & steroid hormonesRemoves antibodies and converts to amino acidsTraps some lipid-soluble toxins (DDT) or breaks down and removes from bloodBile production

*The Bile Duct SystemLiver secretes bile fluidInto a network of narrow channels (bile canaliculi)Between opposing membranes of adjacent liver cellsRight 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 eitherThe common bile duct, which empties into duodenal ampullaThe cystic duct, which leads to gallbladderThe Common Bile Duct Is formed by union ofCystic ductCommon hepatic ductPasses within the lesser omentum toward stomachPenetrates wall of duodenumMeets pancreatic duct at duodenal ampulla

*Physiology of the GallbladderFull gallbladder contains 4070 mL bileBile composition gradually changes in gallbladderWater is absorbedBile salts and solutes become concentrated

*Secretin Is released when chyme arrives in duodenumIncreases secretion of bile and buffers by liver and pancreasCholecystokinin (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 duodenum Gastric Inhibitory Peptide (GIP) Is secreted when fats and carbohydrates enter small intestineVasoactive Intestinal Peptide (VIP)Stimulates secretion of intestinal glandsDilates regional capillariesInhibits acid production in stomachGastrin Is secreted by G cells in duodenumWhen exposed to incompletely digested proteinsPromotes increased stomach motilityStimulates acids and enzyme productionEnterocrininIs released when chyme enters small intestineStimulates mucin production by submucosal glands of duodenum

*Secretin Is released when chyme arrives in duodenumIncreases secretion of bile and buffers by liver and pancreasCholecystokinin (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 duodenum Gastric Inhibitory Peptide (GIP) Is secreted when fats and carbohydrates enter small intestineVasoactive Intestinal Peptide (VIP)Stimulates secretion of intestinal glandsDilates regional capillariesInhibits acid production in stomachGastrin Is secreted by G cells in duodenumWhen exposed to incompletely digested proteinsPromotes increased stomach motilityStimulates acids and enzyme productionEnterocrininIs released when chyme enters small intestineStimulates mucin production by submucosal glands of duodenum

*Functions of the Large IntestineReabsorption of water Compaction of intestinal contents into fecesAbsorption of important vitamins produced by bacteriaStorage of fecal material prior to defecation

**Physiology of the Large IntestineLess than 10% of nutrient absorption occurs in large intestinePrepares fecal material for ejection from the body

*Vitamins Are organic molecules Important as cofactors or coenzymes in metabolismNormal bacteria in colon make three vitamins that supplement diet

*Vitamins Are organic molecules Important as cofactors or coenzymes in metabolismNormal bacteria in colon make three vitamins that supplement diet

*The Processing and Absorption of NutrientsBreaks down physical structure of foodDisassembles component molecules Molecules released into bloodstream areAbsorbed by cellsBroken down to provide energy for ATP synthesisOr used to synthesize carbohydrates, proteins, and lipids

*Digestive Enzymes Are secreted bySalivary glandsTongueStomachPancreas Digestive Enzymes Brush border enzymes break nucleotides intoSugarsPhosphatesNitrogenous bases

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