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