diges&on in the small intes&ne -...
TRANSCRIPT
Chapter 23: Digestive System 1
Diges&onintheSmallIntes&ne
• Par$allydigestedmaterialentersduodenum
• Proteins,starches,fatsmixedwith
• Bile‐gallbladderandliver
• Diges$veenzymes‐pancreas
• Mucus‐Brunnersglands/gobletcells
• Brushborderenzymes‐epithelialcells
Mo&lityintheSmallIntes&ne• Twotypes
• Segmenta$on
• Mixeschymeanddiges$vejuices
• Enhancescontactbetweenfoodstuffsandmucosa
• Maymovecontentstowardtheileocecalvalve
• Propulsion
• AGernutrientshavebeenabsorbed
• Beginswitheachwavestar$ngdistaltotheprevious
• Ini$atedbyintrinsicpacemakercells(Cajalcells)
• Mealremnants,bacteria,mucosalcells,anddebrisaremovedintothelargeintes$ne
Chapter 23: Digestive System 2
ControlofMo&lity
• Gastroilealreflex
• Increasedmo$lityandsecre$oninstomach
• Resultsinsegmenta$oncontrac$onsintheileumandopeningofileocecalsphincter
• LocalentericneuronsoftheGItractcoordinateintes$nalmo$lity
• Cholinergicneuronscause:
• Contrac$onandshorteningofthecircularmusclelayer
• Shorteningoflongitudinalmuscle
• Distensionoftheintes$ne
ChemicalDiges&on:Bile• Bile
• Emulsifiesfats
• Increasessurfaceareaforlipaseac$on
• Enterohepa$ccircula$on
• 90‐95%bilesaltsreleasedtoduodenum
• Reabsorbedintheileumandreturnedtotheliverviathehepa$cportalsystem
• Secre$n
• S$mulatesbilesecre$on/produc$on
• Increasesbilesaltsinblood
• Cholecystokinin(CCK)
• Causesgallbalddercontrac$onandhepatopanrea$csphincterrelaxa$on
Chapter 23: Digestive System 3
ChemicalDiges&on
• Pancrea$cjuice
• Asdescribedearlier
• Intes$nalenzymes‐brushborderenzymes
• Disaccharases‐three
• Pep$dases
• Finalbreakdownofsmallpep$destoaminoacids
• Aminopep$dase,carboxypep$dase,dipep$dase
• Enterokinase
• Ac$vatesallpancrea$cenzymes
ChemicalDiges&on:Carbohydrates
• Carbohydrates
• Typicallyreducedtomaltoseby$meofarrivalinsmallintes$ne
• Maltose=2glucosemolecules
• Sucroseandlactoseingestedindisaccharideform
• Oligosaccharides(2‐8monomers)‐ifs$llpresent
• Convertedtomaltosebydextranaseandglucoamylase
• Disaccharides
• Lactase,maltase,sucrase
• Digestdisaccharidestomonosaccharides
• Lactose,maltose,sucrosetogalactose,glucose,fructose
Chapter 23: Digestive System 4
ChemicalDiges&on:Proteins
• Startsinthestomach
• Pepsin
• Inthesmallintes$ne
• Pancrea$cenzymes–trypsin,chymotrypsin,andcarboxypep$dase
• Brushborderenzymes–aminopep$dases,carboxypep$dases,dipep$dases,tripep$dase
• Finishofftoaminoacids
ChemicalDiges&on:Fats• Occursprimarilyinsmallintes$ne
• Inadults
• Instomachofinfantsusingrenninandgastriclipase
• Bilesaltsbreakfatglobulesintodroplets
• Emulsifica$onincreasessurfaceareaforlipases
• Pancrea$clipase
• Hydrolyzestriglyceridesintoamonoglycerideandtwofa_yacids
Chapter 23: Digestive System 5
ChemicalDiges&on:Fats
Figure 23.35
Absorp&on:Carbohydrates
• Ac$vetransportintoepithelium
• Monosaccharides‐glucoseandgalactose
• Diffusetocapillaries
• Fructose
• Movesbyfacilitateddiffusionthroughepithelialcells
Chapter 23: Digestive System 6
Absorp&on:Proteins
• Absorbedasaminoacids
• Viaac$vetransportintomucosalcells
• Diffusetocapillaries
• Remnantdipep$desandtripep$desarehydrolyzedtoaminoacidsinmucosalcells
• Thendiffusedtocapillaries
ChemicalDiges&onandAbsorp&on:Proteins
Figure 23.34
Chapter 23: Digestive System 7
LipidAbsorp&on
• Micellesformed
• Bilesalts,monoglycerides,fa_yacids,cholesterol,lecithin
• Micellesapproachmucosalcells
• Monoglyceridesandfreefa_yacidsmoveintomucosalcellbydiffusion
• Insidecell,monoglyceridesandfreefa_yacidsarerecombinedintotriglycerides
• InERtriglyceridescombinewithcholesterol,phospholipids,fa_yacids,andprotein
• Packagesingolgiaschylomicrons‐withawatersolubleproteinskin
• Chylomicronsenterlactealandtravelinlympha$csystemtothoracicducttoenterbloodstream
Fa@yAcidAbsorp&on
Figure 23.36
Chapter 23: Digestive System 8
VitaminAbsorp&on
• VitaminsA,D,E,andK
• Fatsoluble
• Associatedwithmicellesandfollowmicellepathofabsorp$on
• VitaminsC,andB(notB12)
• Watersoluble
• Diffuseintomucosalcellsandtocapillaries
• B12needsintrinsicfactorfromparietalcells
ElectrolyteAbsorp&on
• Mostionsareac$velyabsorbedalongthelengthofsmallintes$ne
• Na+iscoupledwithabsorp$onofglucoseandaminoacids(ac$ve)
• Canalsodiffusealone
• K+viasimplediffusionandosmo$cpressures
• Cl‐,I‐,NO3‐
• Passivelyfollowsodiumorareac$velytransported
• Ca2+
• Ac$velytransportedbutneedPTHandvitD
• Fe+
• Ac$velytransportedintomucosalcells
• Bindstoferri$n
Chapter 23: Digestive System 9
WaterAbsorp&on
• 200‐400mlperhour
• 95%ofwaterisabsorbedinthesmallintes$nesbyosmosis
• Movesinbothdirec$onsacrossintes$nalmucosa
• Netosmosisoccurs
• Wheneveraconcentra$ongradientisestablishedbyac$vetransportofsolutesintothemucosalcells
LargeIntes&ne
• Fromileocecalvalvetoanus
• ~5’long;2.5”diameter
• Mesocolona_achestoposteriorwall
• Fourparts
• Cecum
• Colon
• Rectum
• Anus
cecum
anusrectum
colon
Chapter 23: Digestive System 10
LargeIntes&neGrossAnatomy• Ileoceccalvalve
• Controlsmovementfromileumintocecum
• Cecum
• 2.5”pouchhangingbelowileocecalvalve
• Vermiformappendix
• Filledwithlymphoid$ssue
• A_achedtocecum
• Colon‐fourparts
• Ascendingcolon⇒rightcolicflexure⇒transversecolon⇒leGcolicflexure⇒descendingcolon⇒sigmoidcolon⇒rectum
• Rectum
• ~8”long,anteriortosacrum
• Final1”isanalcanal
LargeIntes&ne
Figure 23.29a
Chapter 23: Digestive System 11
ValvesandSphinctersoftheRectumandAnus
• Analcolumns
• Longitudinalfoldsofmucosa
• Analsinusesbetweencolumns
• Producemucusandcona$nsbloodvessels
• Theanushastwosphincters:
• Internalanalsphinctercomposedofsmoothmuscle
• Externalanalsphinctercomposedofskeletalmuscle
StructureoftheAnalCanal
Figure 23.29b
Chapter 23: Digestive System 12
LargeIntes&ne:Histology
• Simplecolumnarepithelium
• exceptintheanalcanal‐stra$fiedsquamous
• Novilli,brushborder,orplicacirculares
• Hasplicasemilunares
• Numerousdeepcryptslinedwithgobletcells
• Teniaecoli
• Remnantsofmuscularisexterna
• Reducedsheetswithlongitudinalcordsofsmoothmuscle
• Toniccontrac$onscompressescolonintopouchescalledhoustra
Diges&veProcessesinLargeIntes&ne• Gastroilealreflex(gastrocolic)
• AGermeal‐relaxa$onofileocecalsphincter
• Increasedperistalsisinileum
• Chymeenterscecum
• Propulsion
• Haustralchurning
• Whilefillinghaus_raarerelaxedanddistended
• Oncefull,wallscontractandmovematerialtonexthoustra
• Peristalsis
• Slowerrate@~3‐12perminute
• Massperistalsis
• Strongwaveorigina$nginmidtransversecolonmovingcontentstowardrectum
• 3or4$mesadayduringorfollowingmeals
Chapter 23: Digestive System 13
Diges&veProcessesinLargeIntes&ne
• Absorp$on
• Water,electrolytes,andvitaminsKandB
• KandBmadebyintes$nalbacteria
• Defaca$on
• Feces‐undigested/absorbedfood,mucus,bacteria,epithelialcells,water
• Massperistalsispushesfecestorectumcausingdistension
• Setsoffdefeca$onreflex
• Wallsofsigmoidcolonandrectumcontract
• Andlongitudinalmuscle
• Relaxa$onofinternalanalsphinctertoawaitvoluntaryrelaxa$onofexternalanalsphincter
Defeca&on
Figure 23.32