Anatomy Digestive System 1st proff 2022-2023.pdf

Anatomy Digestive System 1st proff 2022-2023.pdf

1. Alimentary System
Pharm D 1
st
Prof
Bahauddin Zakariya University Multan
Waseem Ashraf

2. Recommended Books:
•Fundamentals of Anatomy & Physiology. Martini F, Nath JL, Bartholomew EF. 10th Edition
•Principles of Anatomy and Physiology. Tortora GJ, Derrickson B. 13th Edition

3. Digestive System
Thedigestivesystemconsistsofamusculartube,thedigestivetract,alsocalledthegastrointestinal(GI)tractoralimentary
canal,andvariousaccessoryorgans.Theoralcavity(mouth),pharynx(throat),esophagus,stomach,smallintestine,and
largeintestinemakeupthedigestivetract.Accessorydigestiveorgansincludetheteeth,tongue,andvariousglandular
organs,suchasthesalivaryglands,liver,gallbladder,andpancreas.
TheDigestiveOrgansandthePeritoneum
Theabdominopelviccavitycontainstheperitonealcavity,whichislinedbyaserousmembrane.Thismembraneconsistsof
asuperficialmesotheliumcoveringalayerofareolartissue.Thisserousmembraneisdividedintotwoparts.
Theserosaorvisceralperitoneumcoversorgansthatprojectintotheperitonealcavity.
Theparietalperitoneumlinestheinnersurfacesofthebodywall.
Theserousmembraneliningtheperitonealcavitycontinuouslyproducesperitonealfluid,whichlubricatesthesurfaces.
Becauseathinlayerofperitonealfluidseparatesthem,theparietalandvisceralsurfacescanslidewithoutfrictionand
resultingirritation.Themembranesecretesandreabsorbsabout7litersoffluideachday,butthevolumewithinthe
peritonealcavityatanyonetimeisverysmall.

5. Mesenteries
Portionsofthedigestivetractaresuspendedwithintheperitonealcavitybysheetsofserousmembranethatconnectthe
parietalperitoneumwiththevisceralperitoneum.Thesemesenteriesaredoublesheetsofperitonealmembrane.Theareolar
tissuebetweenthemesothelialsurfacesprovidesaroutetoandfromthedigestivetractforbloodvessels,nerves,and
lymphaticvessels.
Mesenteriesstabilizethepositionsoftheattachedorgans.Themesenteriesalsopreventtheintestinesfrombecoming
entangledduringdigestivemovementsorsuddenchangesinbodyposition.
Duringembryonicdevelopment,thedigestivetractandaccessoryorgansaresuspendedwithintheperitonealcavityby
dorsalandventralmesenteries.
Theventralmesenterylaterdisappearsalongmostofthedigestivetract.Itpersistsinadultsinonlytwoplaces:onthe
ventralsurfaceofthestomach,betweenthestomachandtheliver(thelesseromentum);andbetweentheliverandthe
anteriorabdominalwall(thefalciformligament).
Thelesseromentum(omentum,fatskin)stabilizesthepositionofthestomachandprovidesanaccessrouteforblood
vesselsandotherstructuresenteringorleavingtheliver.Thefalciform(falx,sickle+forma,form)ligamenthelpsstabilize
thepositionoftheliverrelativetothediaphragmandabdominalwall.

6. Mesenteries
Asthedigestivetractelongates,ittwistsandturnswithinthecrowdedperitonealcavity.Thedorsalmesenteryofthe
stomachbecomesgreatlyenlargedandformsanenormouspouchthatextendsinferiorlybetweenthebodywallandthe
anteriorsurfaceofthesmallintestine.Thispouchisthegreateromentum.Ithangslikeanapronfromthelateraland
inferiorbordersofthestomach.Adiposetissueinthegreateromentumconformstotheshapesofthesurroundingorgans,
providingpaddingandprotectionacrosstheanteriorandlateralsurfacesoftheabdomen.Whenanindividualgainsweight,
thisadiposetissuecontributestothecharacteristic“beerbelly.”Thelipidsintheadiposetissueareanimportantenergy
reserve.Thegreateromentumalsoinsulatestoreduceheatlossacrosstheanteriorabdominalwall.
Allbutthefirst25cm(10in.)ofthesmallintestineissuspendedbythemesenteryproper,athickmesenterialsheet.It
providesstability,butpermitssomeindependentmovement.Themesenteryassociatedwiththeinitialportionofthesmall
intestine(theduodenum)andthepancreasfuseswiththeposteriorabdominalwallandlocksthosestructuresinplace.Only
theiranteriorsurfacesremaincoveredbyperitoneum.Theseorgansaredescribedasretroperitoneal(retro,behind)because
theirmassliesposteriorto,ratherthansurroundedby,theperitonealcavity.Amesocolonisamesenteryassociatedwitha
portionofthelargeintestine.Duringnormaldevelopment,themesocolonoftheascendingcolon,thedescendingcolon,
andtherectumofthelargeintestinefusetotheposteriorbodywall.transversemesocolon,whichsupportsthetransverse
colon,
andthesigmoidmesocolon,whichsupportsthesigmoid

7. Mesenteries
Theseregionsbecomelockedinplace.Thereafter,theseorgansareretroperitoneal.
Thevisceralperitoneumcoversonlytheiranteriorsurfacesandportionsoftheirlateralsurfaces.Thetransverse
mesocolon,whichsupportsthetransversecolon,andthesigmoidmesocolon,whichsupportsthesigmoidcolon,areallthat
remainoftheoriginalembryonicmesocolon.

10. Histology of the Digestive Tract
Thefourmajorlayersofthedigestivetractare(1)themucosa,(2)thesubmucosa,(3)themuscularisexterna,and(4)the
serosa.
Thestructureoftheselayersvariesbyregion.Itmostcloselyresemblesthesmallintestine,thelongestsegmentofthe
digestivetract.
1.TheMucosa
Theinnerlining,ormucosa,ofthedigestivetractisamucousmembrane.Itconsistsofanepithelium,moistenedby
glandularsecretions,andalaminapropriaofareolartissue.
TheDigestiveEpithelium.Themucosalepitheliumiseithersimpleorstratified,dependingonitslocationandthestresses
placedonit.Mechanicalstressesaremostsevereintheoralcavity,pharynx,andesophagus.Thesestructuresarelinedbya
stratifiedsquamousepithelium.Incontrast,thestomach,thesmallintestine,andalmosttheentirelengthofthelarge
intestine(whereabsorptionoccurs)haveasimplecolumnarepitheliumthatcontainsmucouscells.Scatteredamongthe
columnarcellsareenteroendocrinecells.Theysecretehormonesthatcoordinatetheactivitiesofthedigestivetractandthe
accessoryglands.Theliningofthedigestivetractappearsaslongitudinalfolds,whichdisappearasthetractfills.Thelining
alsohaspermanenttransversefoldscalledcircularfolds,orplicae(folds)circulares.

11. Histology of the Digestive Tract
Thefoldingincreasesthesurfaceareaavailableforabsorption.Thesecretionsofglandcellsinthemucosaand
submucosa—orinaccessoryglandularorgans—arecarriedtotheepithelialsurfacesbyducts.
TheLaminaPropria.Thelaminapropriaisalayerofareolartissuethatalsocontainsbloodvessels,sensorynerveendings,
lymphaticvessels,smoothmusclecells,andscatteredlymphatictissue.Intheoralcavity,pharynx,esophagus,stomach,and
duodenum(theproximalportionofthesmallintestine),thelaminapropriaalsocontainsthesecretorycellsofmucous
glands.
Inmostareasofthedigestivetract,thelaminapropriacontainsanarrowsheetofsmoothmuscleandelasticfibers.This
sheetiscalledthemuscularismucosae.Thesmoothmusclecellsinthemuscularismucosaearearrangedintwoconcentric
layers.
Theinnerlayerencirclesthelumen(thecircularmuscle),and
theouterlayercontainsmusclecellsarrangedparalleltothelongaxisofthetract(thelongitudinallayer).
Contractionsintheselayersaltertheshapeofthelumenandmovetheepithelialpleatsandfolds.

12. Histology of the Digestive Tract
2.TheSubmucosa
Thesubmucosaisalayerofdenseirregularconnectivetissuethatbindsthemucosatothemuscularisexterna.
Thesubmucosahasnumerousbloodvesselsandlymphaticvessels.Insomeregionsitalsocontainsexocrineglandsthat
secretebuffersandenzymesintothelumenofthedigestivetract.Alongitsoutermargin,thesubmucosacontainsanetwork
ofintrinsicnervefibersandscatteredneurons.Thisnetworkisthesubmucosalplexus,orMeissner’splexus.Itcontains
sensoryneurons,parasympatheticganglionicneurons,andsympatheticpostganglionicfibersthatinnervatethemucosaand
submucosa.
TheMuscularisexterna
Thesubmucosalplexusliesalongtheinnerborderofthemuscularisexterna,alsocalledthemuscularis.Smoothmuscle
cellsdominatethisregion.Likethesmoothmusclecellsinthemuscularismucosae,thoseinthemuscularisexternaare
arrangedinaninnercircularlayerandanouterlongitudinallayer.Theselayersplayanessentialroleinmechanical
processingandinmovingmaterialsalongthedigestivetract.Themovementsofthedigestivetractarecoordinated
primarilybythesensoryneurons,interneurons,andmotorneuronsoftheentericnervoussystem(ENS).

13. Histology of the Digestive Tract
TheENSisprimarilyinnervatedbytheparasympatheticdivisionoftheANS.Sympatheticpostganglionicfibersalso
synapsehere.Manyofthesefiberscontinueonwardtoinnervatethemucosaandthemyentericplexus(mys,muscle+
enteron,intestine),orAuerbach’splexus.Thisplexusisanetworkofparasympatheticganglia,sensoryneurons,
interneurons,andsympatheticpostganglionicfibers.Itliessandwichedbetweenthecircularandlongitudinalmusclelayers.
Ingeneral,parasympatheticstimulationincreasesmuscletoneandactivity.Sympatheticstimulationdecreasesmuscletone
andactivity.
TheSerosa
Aserousmembraneknownastheserosacoversthemuscularisexternaalongmostportionsofthedigestivetractinsidethe
peritonealcavity.
Thereisnoserosacoveringthemuscularisexternaoftheoralcavity,pharynx,esophagus,andrectum.Instead,adense
networkofcollagenfibersfirmlyattachesthedigestivetracttoadjacentstructures.Thisfibroussheathiscalledan
adventitia.

15. Esophagus
Theesophagusisahollowmusculartubethatconveyssolidfoodandliquidstothestomach.Itslengthisapproximately25
cm(10in.)anditsdiameterisabout2cm(0.80in.)atitswidestpoint.
Theesophagusbeginsposteriortothecricoidcartilage,atthelevelofvertebraC6.Itisnarrowestatthispoint.The
esophagusdescendstowardthethoraciccavityposteriortothetrachea,continuinginferiorlyalongtheposteriorwallofthe
mediastinum.Itthenenterstheabdominopelviccavitythroughtheesophagealhiatus,anopeninginthediaphragm.The
esophagusemptiesintothestomachanteriortovertebraT7.
Theesophagusisinnervatedbyparasympatheticandsympatheticfibersfromtheesophagealplexus.Restingmuscletone
inthecircularmusclelayerinthesuperior3cm(1.2in.)oftheesophagusnormallypreventsairfromenteringthe
esophagus.Acomparablezoneattheinferiorendoftheesophagusnormallyremainsinastateofactivecontraction.This
statepreventsthebackflow(reflux)ofmaterialsfromthestomachintotheesophagus.Neitherregionhasawell-defined
sphinctermuscle.Nevertheless,weoftenusethetermsupperesophagealsphincterandloweresophagealsphincter
(cardiacsphincter)todescribetheseregions,whicharesimilarinfunctiontoothersphincters

16. Histology
Thewalloftheesophaguscontainsmucosal,submucosal,andmuscularislayerscomparabletothoseshowninFigure24–3.
Distinctivefeaturesoftheesophagealwallincludethefollowing(Figure24–10):
Themucosaoftheesophaguscontainsanonkeratinized,stratifiedsquamousepitheliumsimilartothatofthepharynxand
oralcavity.
Themucosaandsubmucosaarepackedintolargefoldsthatextendthelengthoftheesophagus.Thesefoldsallowfor
expansionduringthepassageofalargebolus.Muscletoneinthewallskeepsthelumenclosed,exceptwhenyouswallow.
Themuscularismucosaeconsistsofanirregularlayerofsmoothmuscle.
Thesubmucosacontainsscatteredesophagealglands.Theyproducemucusthatreducesfrictionbetweenthebolusandthe
esophageallining.
Themuscularisexternahastheusualinnercircularandouterlongitudinallayers.However,inthesuperiorone-thirdof
theesophagus,theselayerscontainskeletalmusclefibers.Themiddlethirdcontainsamixtureofskeletalandsmooth
muscletissue.Alongtheinferiorthird,onlysmoothmuscleoccurs.
Thereisnoserosa,butanadventitiaofconnectivetissueoutsidethemuscularisexternaanchorstheesophagustothe
posteriorbodywall.Overthe1–2cm(0.4–0.8in.)betweenthediaphragmandstomach,theesophagusisretroperitoneal.
Peritoneumcoverstheanteriorandleftlateralsurfaces

18. Stomach
ThestomachhastheshapeofanexpandedJ.Ashortlessercurvatureformsthemedialsurfaceoftheorgan,andalong
greatercurvatureformsthelateralsurface.Theanteriorandposteriorsurfacesaresmoothlyrounded.Theshapeandsize
ofthestomachcanvarygreatlyfromindividualtoindividualandevenfromonemealtothenext.Inan“average”stomach,
thelessercurvatureisapproximately10cm(4in.)long,andthegreatercurvaturemeasuresabout40cm(16in.).
ThestomachtypicallyextendsbetweenthelevelsofvertebraeT7andL3.
Wedividethestomachintofourregions:
1.TheCardia.Thecardiaisthesmallestpartofthestomach.Itconsistsofthesuperior,medialportionofthestomach
within3cm(1.2in.)ofthejunctionbetweenthestomachandtheesophagus.Thecardiacontainsabundantmucousglands.
Theirsecretionscoattheconnectionwiththeesophagusandhelpprotectthattubefromtheacidandenzymesofthe
stomach.
2.TheFundus.Thefundusistheportionofthestomachthatissuperiortothejunctionbetweenthestomachandthe
esophagus.Thefunduscontactstheinferior,posteriorsurfaceofthediaphragm.
3.TheBody.TheareaofthestomachbetweenthefundusandthecurveoftheJisthebody,thelargestregionofthe
stomach.Thebodyactsasamixingtankforingestedfoodandsecretionsproducedinthestomach.Gastric(stomach)glands
inthefundusandbodysecretemostoftheacidandenzymesinvolvedingastricdigestion.

19. Stomach
4.ThePylorus.ThepylorusformsthesharpcurveoftheJ.Thepylorusisdividedintoapyloricantrum(antron,cavity),
whichisconnectedtothebody,andapyloriccanal,whichemptiesintotheduodenum,theproximalsegmentofthesmall
intestine.Asmixingmovementstakeplaceduringdigestion,thepylorusfrequentlychangesshape.Amuscularpyloric
sphincterregulatesthereleaseofchymeintotheduodenum.Glandsinthepylorussecretemucusandimportantdigestive
hormones,includinggastrin,ahormonethatstimulatesgastricglands.
Whenthestomachisrelaxed(empty),themucosahasprominentfoldscalledrugae(wrinkles).Thesetemporaryfeatureslet
thegastriclumenexpand(Figure24–12b).Thestomachcanstretchupto50timesitsemptysize.Asthestomachfills,the
rugaegraduallyflattenoutuntil,atmaximumdistension,theyalmostdisappear.
Themuscularismucosaeandmuscularisexternaofthestomachcontainextralayersofsmoothmusclecellsinaddition
totheusualcircularandlongitudinallayers.Themuscularismucosaegenerallycontainanouter,circularlayerofmuscle
cells.Themuscularisexternahasaninner,obliquelayerofsmoothmuscle(Figure24–12b).Theextralayersofsmooth
musclestrengthenthestomachwallandassistinthemixingandchurningessentialtotheformationofchyme.

22. Histology
Simplecolumnarepitheliumlinesallportionsofthestomach(Figure24–13a).Theepitheliumisasecretorysheet,which
producesacarpetofmucusthatcoverstheinteriorsurfaceofthestomach.Thealkalinemucouslayerprotectsepithelial
cellsagainsttheacidandenzymesinthegastriclumen.Shallowdepressionscalledgastricpitsopenontothegastricsurface
(Figure24–13b).Themucouscellsatthebase,orneck,ofeachgastricpitactivelydivide,replacingsuperficialcellsthatare
shedintothechyme.Atypicalgastricepithelialcellhasalifespanofthreetosevendays.Exposuretoalcoholorother
chemicalsthatdamageorkillepithelialcellsincreasescellturnover.
Gastricglands
Inthefundusandbodyofthestomach,eachgastricpitcommunicateswithseveralgastricglands,whichextenddeepinto
theunderlyinglaminapropria(Figure24–13b).Gastricglandsaredominatedbytwotypesofsecretorycells:parietalcells
andchiefcells.Together,theysecreteabout1500ml(1.6qt.)ofgastricjuiceeachday.Parietalcellsareespeciallycommon
alongtheproximalportionsofeachgastricgland(Figure24–13b).Thesecellssecreteintrinsicfactor.Thisglycoprotein
helpsabsorbvitaminB12acrosstheintestinallining.Parietalcellsalsosecretehydrochloricacid(HCl).

23. Histology
Chiefcells,alsocalledzymogeniccells,aremostabundantnearthebaseofagastricgland(Figure24–13b).Thesecells
secretepepsinogen,aninactiveproenzyme.Acidinthegastriclumenconvertspepsinogentopepsin,anactiveproteolytic,
orprotein-digesting,enzyme.PepsinfunctionsmosteffectivelyatastronglyacidicpHof1.5–2.0.Inaddition,thestomachs
ofnewborninfants(butnotofadults)producerennin,alsoknownaschymosin,andgastriclipase.Theseenzymesare
importantforthedigestionofmilk.Rennincoagulatesmilkproteins.Gastriclipaseinitiatesthedigestionofmilkfats.
Pyloricglands
Glandsinthepylorusproduceprimarilyamucoussecretion,ratherthanenzymesoracid.Inaddition,severaltypesof
enteroendocrinecellsarescatteredamongthemucus-secretingcells.Theseenteroendocrinecellsproduceatleastseven
hormones,mostnotablygastrin.GastrinisproducedbyGcells,whicharemostabundantinthegastricpitsofthepyloric
antrum.Gastrinstimulatessecretionbybothparietalandchiefcells,aswellascontractionsofthegastricwallthatmixand
stirthegastriccontents.ThepyloricglandsalsocontainDcells,whichreleasesomatostatin,ahormonethatinhibitsthe
releaseofgastrin.DcellscontinuouslyreleasetheirsecretionsintotheinterstitialfluidadjacenttotheGcells.

25. Small Intestine
Thesmallintestineplaysthekeyroleinthedigestionandabsorptionofnutrients.Ninetypercentofnutrientabsorption
takesplaceinthesmallintestine.Mostoftherestoccursinthelargeintestine.Thesmallintestineaverages6m(19.7ft)in
length(range:4.5–7.5m;14.8–24.6ft).Itsdiameterrangesfrom4cm(1.6in.)atthestomachtoabout2.5cm(1in.)atthe
junctionwiththelargeintestine.Itoccupiesallabdominalregionsexcepttherightandlefthypochondriacandepigastric
regions.Thesmallintestinehasthreesegments:theduodenum,thejejunum,andtheileum.
Theduodenum,25cm(10in.)inlength,,isthesegmentclosesttothestomach.Thisportionofthesmallintestineisa
“mixingbowl.”Itreceiveschymefromthestomachanddigestivesecretionsfromthepancreasandliver.Fromits
connectionwiththestomach,theduodenumcurvesinaCthatenclosesthepancreas.Exceptfortheproximal2.5cm(1in.),
theduodenumisinaretroperitonealpositionbetweenvertebraeL1andL4.
Aratherabruptbendmarkstheboundarybetweentheduodenumandthejejunum.Atthisjunction,thesmallintestine
reenterstheperitonealcavity,supportedbyasheetofmesentery.Thejejunumisabout2.5meters(8.2ft)long.Thebulkof
chemicaldigestionandnutrientabsorptionoccursthere.
Theileum,thefinalsegmentofthesmallintestine,isalsothelongest.Itaverages3.5meters(11.5ft)inlength.Theileum
endsattheileocecalvalve.Thissphinctercontrolstheflowofmaterialfromtheileumintothececumofthelargeintestine.

28. Histology
Theintestinallininghasaseriesoftransversefoldscalledcircularfoldsorplicaecirculares.Unliketherugaeinthe
stomach,thecircularfoldsarepermanentfeatures.Theydonotdisappearwhenthesmallintestinefills.Thesmallintestine
containsabout800circularfolds—roughly2percentimeter.Theygreatlyincreasethesurfaceareaavailableforabsorption.
IntestinalVilli
Themucosaofthesmallintestinehasaseriesoffingerlikeprojections,theintestinalvilli.Thevilliarecoveredbysimple
columnarepitheliumthatiscarpetedwithmicrovilli.Thecellsaresaidtohaveabrushborderbecausethemicrovilli
projectfromtheepitheliumlikethebristlesonabrush.
Thelaminapropriaofeachvilluscontainsanextensivenetworkofcapillariesthatoriginateinavascularnetworkwithinthe
submucosa(Figure24–17c).
Inadditiontocapillariesandnerveendings,eachvilluscontainsalymphaticcapillarycalledalacteal(lacteus,milky)
(Figure24–17b,c).lactealstransportmaterialsthatcannotenterbloodcapillaries.Forexample,absorbedfattyacidsare
assembledintoprotein–lipidpackagesthataretoolargetodiffuseintothebloodstream.Thesepackets,called
chylomicrons,reachthevenouscirculationthroughthethoracicduct,whichdeliverslymphintotheleftsubclavianvein.
Thenamelactealreferstothepale,milkyappearanceoflymphthatcontainslargequantitiesoflipids.

29. Histology
Intestinalglands
Mucouscellsbetweenthecolumnarepithelialcellsejectmucinsontotheintestinalsurfaces(Figure24–17c,d).Atthebases
ofthevilliaretheentrancestotheintestinalglands,orcryptsofLieberkühn.
Theduodenumhasnumerousmucousglands,bothintheepitheliumanddeeptoit.Inadditiontointestinalglands,it
submucosacontainsduodenalglands,alsocalledsubmucosalglandsorBrunner’sglands.
Thedistalportionsoftheileumlackcircularfolds.Thelaminapropriatherecontains20–30massesoflymphoidtissue
calledaggregatedlymphoidnodules,orPeyer’spatches.

31. Large Intestine
Thehorseshoe-shapedlargeintestinebeginsattheendoftheileumandendsattheanus.Thelargeintestineliesinferiorto
thestomachandliverandalmostcompletelyframesthesmallintestine(Figure24–1).Thelargeintestinestoresdigestive
wastesandreabsorbswater.Residentbacteriainthelargeintestineareanimportantsourceofvitamins,especiallyvitamin
K,biotin,andvitaminB5.
Thelargeintestine,alsoknownasthelargebowel,hasanaveragelengthofabout1.5meters(4.9ft)andawidthof7.5cm
(3in.).Wecandivideitintothreeparts:(1)thepouchlikececum,thefirstportionofthelargeintestine;(2)thecolon,the
largestportion;and(3)therectum,thelast15cm(6in.)ofthelargeintestineandtheendofthedigestivetract.
TheCecum
Materialarrivingfromtheileumfirstentersanexpandedpouchcalledthececum.Theileumattachestothemedialsurface
ofthececumandopensintothececumattheileocecalvalve(Figure24–24a,b).Thececumcollectsandstoresmaterials
fromtheileumandbeginstheprocessofcompaction.Theslender,hollowappendix,orvermiformappendix(vermis,
worm),isattachedtotheposteromedialsurfaceofthececum(Figure24–24a,b).Theappendixisnormallyabout9cm(3.6
in.)long,butitssizeandshapearequitevariable.Asmallmesenterycalledthemesoappendixconnectstheappendixtothe
ileumandcecum.lymphoidnodulesdominatethemucosaandsubmucosaoftheappendix.

32. Large Intestine
TheColon
Thecolonhasalargerdiameterandathinnerwallthanthesmallintestine.Distinctivefeaturesofthecolonincludethe
following
•Thewallofthecolonformsaseriesofpouches,orhaustra.Thecreasesbetweenthehaustraaffectthemucosalliningas
well,producingaseriesofinternalfolds.Haustrapermitthecolontoexpandandelongate.
•Threeseparatelongitudinalbandsofsmoothmuscle—calledtheteniaecolirunalongtheoutersurfacesofthecolon
justdeeptotheserosa.Thesebandscorrespondtotheouterlayerofthemuscularisexternainotherportionsofthe
digestivetract.Muscletonewithintheteniaecoliiswhatcreatesthehaustra.
•Theserosaofthecoloncontainsnumerousteardropshapedsacsoffatcalledomentalappendices,alsocalledfattyor
epiploicappendages.
Thecoloncanbedividedintofourregions:theascendingcolon,transversecolon,descendingcolon,andsigmoidcolon.

33. Large Intestine
Theascendingcolonbeginsatthesuperiorborderofthececumandascendsalongtherightlateralandposteriorwallofthe
peritonealcavitytotheinferiorsurfaceoftheliver.There,thecolonbendssharplytotheleftattherightcolicflexure,or
hepaticflexure.Thisbendmarkstheendoftheascendingcolonandthebeginningofthetransversecolon.
Thetransversecoloncurvesanteriorlyfromtherightcolicflexureandcrossestheabdomenfromrighttoleft.The
transversecolonissupportedbythetransversemesocolon.Itisseparatedfromtheanteriorabdominalwallbythelayersof
thegreateromentum.Attheleftsideofthebody,thetransversecolonpassesinferiortothegreatercurvatureofthestomach.
Nearthespleen,thecolonmakesa90°turnattheleftcolicflexure,orsplenicflexure,andbecomesthedescendingcolon.
Thedescendingcolonproceedsinferiorlyalongtheleftsideuntilreachingtheiliacfossaformedbytheinnersurfaceofthe
leftilium.Thedescendingcolonisretroperitonealandfirmlyattachedtotheabdominalwall.Attheiliacfossa,the
descendingcoloncurvesatthesigmoidflexureandbecomesthesigmoidcolon.
Thesigmoidflexureisthestartofthesigmoidcolon(sigmeidos,theGreekletterS),anS-shapedsegmentthatisonlyabout
15cm(6in.)long.Thesigmoidcolonliesposteriortotheurinarybladder,suspendedfromthesigmoidmesocolon.The
sigmoidcolonemptiesintotherectum.

36. Large Intestine
TheRectum
Therectumformsthelast15cm(6in.)ofthedigestivetract(Figure24–24a,c).Itisanexpandableorganforthetemporary
storageoffeces.Themovementoffecalmaterialintotherectumtriggerstheurgetodefecate(expelfeces).Thelastportion
oftherectum,theanalcanal,containssmalllongitudinalfoldscalledanalcolumns.Thedistalmarginsofthesecolumnsare
joinedbytransversefoldsthatmarktheboundarybetweenthecolumnarepitheliumoftheproximalrectumanda
stratifiedsquamousepitheliumlikethatintheoralcavity.
Theanus,oranalorifice,istheexitoftheanalcanal.There,theepidermisbecomeskeratinizedandidenticaltothesurface
oftheskin.
Thecircularmusclelayerofthemuscularisexternainthisregionformstheinternalanalsphincter(Figure24–24c).The
smoothmusclecellsofthissphincterarenotundervoluntarycontrol.Theexternalanalsphincter,whichguardstheanus,
consistsofaringofskeletalmusclefibersthatencirclesthedistalportionoftheanalcanal.Thissphincterconsistsof
skeletalmuscleandisundervoluntarycontrol.
Thelaminapropriaandsubmucosaoftheanalcanalcontainanetworkofveins.Ifvenouspressuresthererisetoohighdue
tostrainingduringdefecationorpregnancy,theveinscanbecomedistended,producinghemorrhoids.

38. Histology
Thediameterofthecolonisaboutthreetimesthatofthesmallintestine,butitswallismuchthinner.Themajor
characteristicsofthecolonarethelackofvilli,theabundanceofmucouscells,andthepresenceofdistinctiveintestinal
glands.
Theglandsinthelargeintestinearedeeperthanthoseofthesmallintestineandaredominatedbymucouscells.Themucosa
ofthelargeintestinedoesnotproduceenzymes.Anydigestionthatoccursresultsfromenzymesintroducedinthesmall
intestineorfrombacterialaction.Themucusprovideslubricationasthefecalmaterialbecomesdrierandmorecompact.
Mucusissecretedaslocalstimuli,suchasfrictionorexposuretoharshchemicals,triggershortreflexesinvolvinglocal
nerveplexuses.largelymphoidnodulesarescatteredthroughoutthelaminapropriaandsubmucosa.
Themuscularisexternaofthelargeintestineisunusual,becausethelongitudinallayerhasbeenreducedtothe
muscularbandsoftheteniaecoli.However,themixingandpropulsivecontractionsofthecolonresemblethoseofthe
smallintestine.

39. The Liver
Theliveristhelargestvisceralorgan.Itisoneofourmostversatileorgansandthecenterformetabolicregulationinthe
body.Mostofitsmassliesintherighthypochondriacandepigastricregions,butitmayextendintothelefthypochondriac
andumbilicalregionsaswell.Theliverweighsabout1.5kg(3.3lb).Thislarge,firm,reddish-brownorganperforms
essentialmetabolicandsyntheticfunctions.
AnatomyoftheLiver
Theliveriswrappedinatoughfibrouscapsuleandiscoveredbyalayerofvisceralperitoneum.Ontheanteriorsurface,
thefalciformligamentmarksthedivisionbetweentheorgan’sleftandrightlobes(Figure24–19a,b).Athickeninginthe
posteriormarginofthefalciformligamentistheroundligament,orligamentumteres.Thisfibrousbandmarksthepathof
thefetalumbilicalvein.
Ontheposteriorsurfaceoftheliver,theimpressionleftbytheinferiorvenacavamarksthedivisionbetweentherightlobe
andthesmallcaudatelobe.Inferiortothecaudatelobeliesthequadratelobe,sandwichedbetweentheleftlobeandthe
gallbladder.Afferentbloodvesselsandotherstructuresreachtheliverbytravelingwithintheconnectivetissueofthelesser
omentum.Theyconvergeataregioncalledtheportahepatis(“doorwaytotheliver”).

41. The Liver
Nearlyone-thirdofthebloodsupplytotheliverisarterialbloodfromthehepaticarteryproper.Therestisvenousblood
fromthehepaticportalvein,whichbeginsinthecapillariesoftheesophagus,stomach,smallintestine,andmostofthelarge
intestine.livercells,calledhepatocytes(HEP-a-to.-sı.ts),adjustcirculatinglevelsofnutrientsthroughselectiveabsorption
andsecretion.Thebloodleavingtheliverreturnstothesystemiccircuitthroughthehepaticveins.Theseveinsopenintothe
inferiorvenacava.

42. Histology
Connectivetissuedivideseachlobeoftheliverintoapproximately100,000liverlobules,thebasicfunctionalunitsofthe
liver.Eachlobuleisroughly1mmindiameter.Adjacentlobulesareseparatedbyaninterlobularseptum.Thehepatocytesin
aliverlobuleformaseriesofirregularplatesarrangedlikethespokesofawheel.Theplatesareonlyonecellthick.
Exposedhepatocytesurfacesarecoveredwithshortmicrovilli.
Withinalobule,sinusoidsbetweenadjacentplatesemptyintothecentralvein.Theliversinusoidslackabasement
membrane,solargeopeningsbetweentheendothelialcellsallowsolutes—eventhoseaslargeasplasmaproteins—topass
outofthebloodstreamandintothespacessurroundingthehepatocytes.
TheliningofthesinusoidscontainstypicalendothelialcellsandalargenumberofKupffercells,alsoknownasstellate
reticuloendothelialcells.Thesephagocyticcellsarepartofthemonocyte–macrophagesystem.Theyengulfpathogens,cell
debris,anddamagedbloodcells.Kupffercellsalsostoreiron,somelipids,andheavymetals(suchastinormercury)that
areabsorbedbythedigestivetract.

43. Histology
Bloodenterstheliversinusoidsfromsmallbranchesofthehepaticportalveinandhepaticarteryproper.Atypicalliver
lobulehasahexagonalshapeincrosssection.Therearesixportalareas,orportaltriads,oneateachcornerofthelobule.A
portalareacontainsthreestructures:(1)abranchofthehepaticportalvein,(2)abranchofthehepaticarteryproper,
and(3)asmallbranchofthebileduct.
Branchesfromthearteriesandveinsdeliverbloodtothesinusoidsofadjacentliverlobules.Asbloodflowsthroughthe
sinusoids,hepatocytesabsorbsolutesfromtheplasmaandsecretematerialssuchasplasmaproteins.Bloodthenleavesthe
sinusoidsandentersthecentralveinofthelobule.Thecentralveinsultimatelymergetoformthehepaticveins,whichthen
emptyintotheinferiorvenacava.Liverdiseases,suchasthevariousformsofhepatitis,andconditionssuchasalcoholism,
canleadtodegenerativechangesinthelivertissueandconstrictionofthecirculatorysupply.

45. The Bile Duct System
Theliversecretesafluidcalledbileintoanetworkofnarrowchannelsbetweentheopposingmembranesofadjacentliver
cells.Thesepassageways,calledbilecanaliculi,extendoutward,awayfromthecentralvein.Eventually,theyconnectwith
finebileductules,whichcarrybiletothebileductsinthenearestportalarea.
Therightandlefthepaticductscollectbilefromallthebileductsoftheliverlobes.Theseductsunitetoformthecommon
hepaticduct,whichleavestheliver.Thebileinthecommonhepaticducteitherflowsintothecommonbileduct,which
emptiesintotheduodenalampulla,orentersthecysticduct,whichleadstothegallbladder.Thecommonbileductis
formedbytheunionofthecysticductandthecommonhepaticduct.Thecommonbileductpasseswithinthelesser
omentumtowardthestomach,turnsandpenetratesthewalloftheduodenumtomeetthepancreaticductattheduodenal
ampulla.

47. The Gallbladder
Thegallbladderisahollow,pear-shapedorganthatstoresandconcentratesbilepriortoitsexcretionintothesmallintestine.
Thismuscularsacislocatedinafossa,orrecess,intheposteriorsurfaceoftheliver’srightlobe.Thegallbladderisdivided
intothreeregions:(1)thefundus,(2)thebody,and(3)theneck.Thecysticductextendsfromthegallbladdertothepoint
whereituniteswiththecommonhepaticducttoformthecommonbileduct.Attheduodenum,thecommonbileduct
meetsthepancreaticductbeforeemptyingintoachambercalledtheduodenalampulla,whichreceivesbuffersandenzymes
fromthepancreasandbilefromtheliverandgallbladder.Theduodenalampullaopensintotheduodenumattheduodenal
papilla,asmallmound.Themuscularhepatopancreaticsphincter(sphincterofOddi)encirclesthelumenofthecommon
bileductand,generally,thepancreaticductandduodenalampullaaswell.

48. The Pancreas
Thepancreasliesposteriortothestomach.Itextendslaterallyfromtheduodenumtowardthespleen.Thepancreasisan
elongate,pinkish-grayorganabout15cm(6in.)longandweighingabout80g(3oz).Thebroadheadofthepancreaslies
withintheloopformedbytheduodenumasitleavesthepylorus.Theslenderbodyofthepancreasextendstowardthe
spleen,andthetailisshortandbluntlyrounded.Thepancreasisretroperitonealandisfirmlyboundtotheposteriorwallof
theabdominalcavity.Thesurfaceofthepancreashasalumpy,lobulartexture.Athin,transparentcapsuleofconnective
tissuewrapstheentireorgan.Thepancreaticlobules,associatedbloodvessels,andexcretoryductsarevisiblethroughthe
anteriorcapsuleandtheoverlyinglayerofperitoneum.Arterialbloodreachesthepancreasbywayofbranchesofthe
splenic,superiormesenteric,andcommonhepaticarteries.Thepancreaticarteriesandpancreaticoduodenalarteries
arethemajorbranchesfromthesevessels.Thesplenicveinanditsbranchesdrainthepancreas.Thepancreasisprimarily
anexocrineorgan.Itproducesdigestiveenzymesandbuffers.Thelargepancreaticduct(ductofWirsung)deliversthese
secretionstotheduodenum.(In3–10percentofthepopulation,asmallaccessorypancreaticduct[Santoriniduct]branches
fromthepancreaticduct.)Thepancreaticductextendswithintheattachedmesenterytoreachtheduodenum,whereitmeets
thecommonbileductfromtheliverandgallbladder.Thetwoductsthenemptyintotheduodenalampulla(ampullaof
Vater),achamberlocatedroughlyhalfwayalongthelengthoftheduodenum.

49. The Pancreas
Whenpresent,theaccessorypancreaticductusuallyemptiesintotheduodenumindependently,outsidetheduodenal
ampulla.
Histology
Partitionsofconnectivetissuedividetheinteriorofthepancreasintodistinctlobules.Thebloodvesselsandtributariesof
thepancreaticductsaresituatedwithintheseconnectivetissuesepta.Thepancreasisanexampleofacompound
tubuloalveolargland.
Ineachlobule,theductsbranchrepeatedlybeforeendinginblindpocketscalledpancreaticacini.Eachpancreaticacinusis
linedwithsimplecuboidalepithelium.Pancreaticislets,theendocrinetissuesofthepancreas,arescatteredamongtheacini.
Theisletsaccountforonlyabout1percentofthecellpopulationofthepancreas.
Thepancreashastwodistinctfunctions,oneendocrineandtheotherexocrine.Theendocrinecellsofthepancreaticislets
secreteinsulinandglucagonintothebloodstreamtocontrolbloodsugar.Theexocrinecellsincludetheacinarcellsandthe
epithelialcellsthatlinetheductsystem.Together,theseexocrinecellssecretepancreaticjuice—analkalinemixtureof
digestiveenzymes,water,andions—intothesmallintestine.Acinarcellssecretepancreaticenzymes,whichdomostofthe
digestiveworkinthesmallintestine.