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34|ANIMAL NUTRITION

AND THE DIGESTIVE

SYSTEM

Figure34.1Forhumans,fruits andvegetables areimportantinmaintainingabalanceddiet .(credit :modificationof

work by Julie Rybarczyk)

Chapter Outline

34.1:Digestive Systems

34.2:Nutrition and Energy Production

34.3:Digestive System Processes

34.4:Digestive System Regulation

Introduction

Allliving organismsneednutrie ntstosurvive.Whileplants canobtaint hemoleculesrequiredforce llularfunc tionthrough

theprocessof photosynthesis,mostani mal sobtaintheirnutrients bytheconsumpt ionofotherorganisms.Atthecel lular

level,thebiologicalmol eculesne cessaryforanimalfunctionarea minoacids,lipidmolecules,nucl eotides,andsimple

sugars.However ,thefoodconsumedconsists ofprotein,fa t,a ndcomplexcarbohydrate s.Animalsmustconvertt hese

macromoleculesintothesimplemoleculesrequire dformaintaini ngcellul arfunctions,suchasa ssemblingnewmolecules,

cells,andtissues.Thec onversionofthe foodconsumedtothenutrie ntsrequire disamulti-stepproc essi nvolvingdigest ion

andabsorpti on.Duringdigestion,foodparti clesare brokendowntosmallercompone nts,a ndlater,theyareabsorbedbythe

body. Chapter 34 | Animal Nutrition and the Digestive System955

Oneoft hec hallenges inhumannutritionismaintaining abalance betweenfoodinta ke,storage,andenergyexpe nditure.

Imbalancescanhaveserioushealthc onsequences.Forexa mple,ea tingtoomuchfood whilenotexpendingmuchener gy

leadstoobesity,whi chinturn willincreasetheriskofde velopingillnesse ssuchast ype-2diabe tesandcardiova scular

disease.Therecentrisei nobesitya ndrelateddiseases makesunde rstandingthe roleofdietandnutriti oninma intaining

good health all the more important.

34.1|Digestive Systems

By the end of this section, you will be able to:

•Explain the processes of digestion and absorption •Compare and contrast different types of digestive systems •Explain the specialized functions of the organs involved in processing food in the body •Describe the ways in which organs work together to digest food and absorb nutrients

Animalsobtaintheirnut ritionfromtheconsumption ofotherorganisms.Depe ndingontheirdiet,anima lsc anbecl assified

intothefollowing categori es:planteaters(herbi vores),meateaters(carnivores),andthose thateatbothpl antsandanimals

(omnivores).Thenutrientsa ndmacromol eculespresentinfoodarenot immedia telyaccessibletothecells.Thereare

anumbe rofprocessesthat modify foodwithintheanima lbodyinordertom akethenutrients and organicmolecules

accessibleforcellularfunction.Asa nima lsevolvedincomplexityofformandfunc tion,theirdigesti vesyste mshavealso

evolved to accommodate their various dietary needs.

Herbivores, Omnivores, and Carnivores

Herbivoresareanima lswhoseprimaryfoodsourceisplant-based. Exa mplesofherbivores,asshown inFigure34.2

includevertebratesl ikedeer,koalas,andsomebirdspecie s,aswellasi nvertebratessuchasc ricketsandcate rpillars.T hese

animalshaveevolveddigestivesyst emscapa bleofhandlinglarge amountsofplantmaterial .Herbivorescanbefurthe r

classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).

Figure34.2Herbivores,liket his(a)mule deerand(b)monarchcaterpillar ,eat primarilyplantmat erial.(credita:

modification of work by Bill Ebbesen; credit b: modification of work by Doug Bowman)

Carnivoresareanim alsthateatotheranimals. Thewordcarnivoreis derivedfromLat inandlitera llymeans"meateater ."

Wildcatssuchas lions,showninFigure34.3aandtigers areexamplesofverte brateca rnivores,asaresnakesandsharks,

whileinvertebra tecarnivoresincludeseastars,spiders,and ladybugs,showninFigure34.3b.Obli gatecarnivoresarethose

956Chapter 34 | Animal Nutrition and the Digestive System

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thatrelyent irelyonanima lfleshtoobtaintheirnutrient s;e xamplesofobligatecarnivore saremembersof thecat family,

suchasl ionsandche etahs.Faculta tivecarni voresarethosethatalsoeatnon-ani malfoodinadditiontoanimalfood.Note

thatthereisno clearline thatdiffe renti atesfacultativeca rnivoresfromomnivores;dogs wouldbeconsideredfa cultative

carnivores.

Figure34.3Carnivoresliket he(a)lion eatprimarilymeat.The (b)ladybug isalsoa carnivorethat consumessmall

insects called aphids. (credit a: modification of work by Kevin Pluck; credit b: modification of work by Jon Sullivan)

Omnivoresareanima lsthateatbothplant-andani mal-derivedfood. InLatin,omnivore meansto eateverything.

Humans,bears(shownin Figure34.4a),and chickens areexampleofvertebrateom nivores;invert ebrateomnivoresinclude

cockroaches and crayfish (shown inFigure 34.4b).

Figure34.4Omnivoreslikethe (a)bearand (b)crayfisheatboth plantandanimal basedf ood.(credita: modification

of work by Dave Menke; credit b: modification of work by Jon Sullivan)

Invertebrate Digestive Systems

Animalshaveevolveddiffe renttypesof digestivesystemstoa idinthedigestionofthe differentfoodsthey consume .

Thesimpl estexampleisthatofagastrovascularcavityandisfoundi norganisms with onlyoneopeni ngfordigestion.

Platyhelminthes(flatworms),Ctenophora(combjellies),a ndCnidaria(coral,jell yfish,andseaanemones) usethistypeof

digestion.Gastrovascularcavi ties,asshowninFigure34.5a,are typical lyablindtubeorcavitywithonlyoneope ning,t he

"mouth",whichalsoservesa san"anus".Ingest edmateria lentersthemout handpasses throughaholl ow,tubularcavity.

Cellswithinthecavi tysecretedigestive enzymest hatbreakdownthefood.Thefoodpart iclesareengulfedbythec ells

lining the gastrovascular cavity. Thealimentarycanal,showni nFigure34.5b,i samoreadva nced system:it consistsofonetubewithamout hatone

endandan anusatt heother.E arthwormsare anexampl eofana nimalwithanali mentarycanal.Oncethe foodisingest ed

throughthemout h,itpasse sthroughtheesophagusa ndisstored inanorganc alledt hecrop;then itpassesinto thegizza rd

whereitis churnedanddi gested.Fromt hegizzard,the foodpassesthrough theintestine,thenutrient sa reabsorbed,and the

waste is eliminated as feces, called castings, through the anus. Chapter 34 | Animal Nutrition and the Digestive System957

Figure34.5(a)Agast rovascularcavity hasasingleopeningthrough whichfoodisingest edandwast eisexcret ed,as

shownint hishydra andinthisjellyfish medusa.(b)A nalimentary canalhast woopenings:amouth foringestingfood,

and an anus for eliminating waste, as shown in this nematode.

Vertebrate Digestive Systems

Vertebrateshaveevolvedmorecomplexdigest ivesystem stoadapttotheir dietaryne eds.Someanimalshaveasi ngle

stomach,whileothershavemul ti-chamberedst omachs.Birdsha vedeveloped adigestivesystemadaptedtoeating unmasticated food.

Monogastric: Single-chambered Stomach

Asthe wordmonogastricsuggests,thistypeof digestivesystemc onsistsofone ("mono" )stomachchamber("gastri c").

Humansandmanya nimals haveamonogastricdi gestivesystemasillustrated inFigure34.6ab.T heprocessofdi gestion

beginswiththe mouthandtheint akeoffood.The teethplay animporta ntroleinm asticating(chewi ng)orphysi cal ly

breakingdownfoodintosmal ler parti cles.Theenz ymespresentinsalivaalsobegin tochemicallybreakdownfood.The

esophagusisalongt ubethatc onnect sthemouth tothestom ach.Usingperist alsis,orwave-likesmoothmusc lecontrac tions,

themuscle softheesophaguspushthefoodtowards thest omach.Inorde rtospeedupt heac tionsofenz ymes inthestomac h,

thestomac hisanextremelyaci dicenvironme nt,with apHbetween1.5and2.5.Thegastricjuice s,whichincludeenzymes

inthestom ach,ac tonthefoodparticlesandc ontinuetheprocessof dige stion.Further breakdown offoodtakesplaceinthe

smallintestinewhere enzymesproducedbytheliver,t hesmalli ntestine,andthepancreas continuetheproce ssofdi gestion.

Thenutrient sareabsorbedintothebloodst reamacross theepitheli alcellsli ningthewal lsofthesm allintestines.T hewaste

materialtravelsontothelar geintestinewherewateri sabsorbe dandthedri erwastem aterialiscompa ctedintofeces;itis

stored until it is excreted through the rectum.

958Chapter 34 | Animal Nutrition and the Digestive System

This OpenStax book is available for free at http://cnx.org/content/col11448/1.10 Figure34.6(a)Humansand herbivores,suchas the(b) rabbit,have amonogast ricdigestive system.However,in

therabbitthesmall intestine andcecumareenlargedt oallowmore timetodigestplantmaterial.T heenlargedorgan

providesmoresurf aceareaf orabsorptionofnut rients.Rabbit sdigesttheirfood twice:thef irsttime foodpasses

throughthedigestive system,itcollects inthececum,andt henitpassesassoft fecescalled cecotrophes.Therabbit

re-ingests these cecotrophes to further digest them. Avian

Birdsfacespeci alchallenge swhenitcomestoobtaini ngnutritionfromfood.Theydonothaveteeth andsothei rdigestive

system,showninFigure34.7,must beableto process un-masticate dfood.Birdsha veevolvedavarie tyofbeaktypesthat

reflectthevastvarietyi ntheirdi et,rangi ngfromseedsandinsectstofrui tsandnuts. Becausemostbirdsfly ,theirmetabolic

ratesarehighinorder toeffi ciently processfoodand keepthei rbodyweight low.Thestomachofbirdshastwochambers:

theproventriculus,where gastricjui cesareproducedtodi gestthefoodbeforeitentersthe stomach,a ndthegizzard,

wherethefoodi sstored,soa ked,a ndmechani callyground.Theundi gestedmate rialformsfood pelletsthataresometimes

regurgitated.Mostofthechemicaldigestion anda bsorpti onhappensin theintestineandthewasteisexcret edthroughthe

cloaca. Chapter 34 | Animal Nutrition and the Digestive System959 Figure34.7Theavianesophagushasa pouch,calleda crop,whichst oresf ood.Food passesfrom thecrop to

thefirstoft wostomachs,calledt heproventriculus, whichcontainsdigestivejuices thatbreakdownfood. Fromt he

proventriculus,thefood entersthesecond stomach,calledthegizzard, whichgrindsfood.Some birdsswallowstones

orgrit, whicharestoredint hegizzard, toaid thegrindingprocess.Birds donothave separateopeningstoexcret e

urineandf eces.I nstead,uricacidfrom thekidneysissecretedintot helargeint estine andcombinedwithwaste from

the digestive process. This waste is excreted through an opening called the cloaca.

Avian Adaptations

Birdshaveahighly efficient ,simplified digestivesyst em.Recentfossilevidencehasshownthatthe evolutionarydivergenceofbirdsf romother landanimalswas characterized bystreamliningandsimplifying thedigestivesystem. Unlikemanyotheranimals,birds donothaveteet hto chewtheir food.Inplaceof lips,they havesharppointybeaks.T hehornybeak, lackofjaws, andthesmallert ongueoft hebirdscan betraced backtotheir dinosaurancestors. Theemergenceofthese changesseemstocoincidewit ht he inclusionofseeds inthe birddiet. Seed-eating birdshavebeaksthat areshapedf orgrabbingseedsand thetwo-compartmentst omachallowsfordelegationoft asks.Sincebirdsneedt oremainlight inordertof ly, theirmetabolicrates areveryhigh,whichmeans theydigesttheirf oodveryquickly andneedt oeat often. Contrast this with the ruminants, where the digestion of plant matter takes a very long time.

Ruminants

Ruminantsaremai nlyherbivoreslikecows,sheep,a ndgoats,whoseentiredie tconsistsofeat inglarge amountsof

roughageorfibe r.Theyhaveevolveddi gestivesystemst hathelp themdigestvastamounts ofcellulose.Aninterest ing

featureoftheruminant s'mout histhat theydonothaveupperincisort eeth.Theyuset heirlower teeth,tonguea ndlipsto

tear and chew their food. From the mouth, the food travels to the esophagus and on to the stomach.

960Chapter 34 | Animal Nutrition and the Digestive System

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Tohelp digestthelarge amountofplantma terial, thestomachoftherum inantsisamul ti-c hamberedorgan, asillustrated

inFigure34.8.The fourcompartm ents ofthestomacharecalledtherumen,ret icul um,omasum, andabomasum.These

chamberscontainmanym icrobesthatbreakdowncell uloseandfermenti ngestedfood.Theabomasumist he"true"stomac h

andist heequivale ntofthemonoga stricstomachchamberwheregast ricj uicesare secreted.Thefour-compartm entgastric

chamberprovideslargerspac eandthemic robialsupportnecessarytodigestpl antmaterial inrumina nts.Theferm entation

processproducesla rgeamountsofgas inthestomachcham ber,whichmustbeel imi nated.Asi nothera nimals,t hesmall

intestine plays an important role in nutrient absorption, and the large intestine helps in the elimination of waste.

Figure34.8Ruminantanimals,such asgoats andcows,have four stomachs.Thefirst twost omachs,the rumenand

thereticulum,contain prokaryotesandprotist sthatareablet odigestcellulosefiber. Theruminant regurgitatescud

fromthereticulum, chewsit,andswallows itintoathird stomach,t heomasum,whichremoveswater .T hecudt hen

passes onto the fourth stomach, the abomasum, where it is digested by enzymes produced by the ruminant.

Pseudo-ruminants

Someanima ls,suchascamelsandalpa cas,arepse udo-ruminants.T heyeatalotofplantmateri alandroughage .Digesting

plantmateria lisnoteasybecauseplantcellwal lscontai nthe polymericsugarmol eculecellulose. Thedigestiveenz ymesof

theseanimals cannotbreakdowncellulose,butmi croorganismspresent inthedigestive system can.Therefore,thedigestive

systemmustbeable tohandlel arge amountsofroughagea ndbreakdownt hecellulose.Pse udo-ruminantshaveathree -

chamberstomachinthe digestivesystem.However,the ircec um - apouchedor ganatthebeginningofthela rgeintestine

containingmanymicroorganismst hatarenecessa ryforthedigestionofplantmaterials - isla rgeandisthe sitewheret he

roughage is fermented and digested. These animals do not have a rumen but have an omasum, abomasum, and reticulum.

Parts of the Digestive System

Thevertebra tedigestivesystemisdesignedt ofacilitatethetransform ationoffoodmat terint othenutrientcomponents that

sustain organisms.

Oral Cavity

Theoralca vity,orm outh,isthepointofentryoffoodinto thedi gestivesystem,i llustrate dinFigure34.9.T hefood

consumedisbrokenintosm allerpa rticles bymasticat ion,thechewingactionofthete eth.Allmammalshaveteetha ndca n

chew their food.

Theextensi vechemicalprocessofdigest ionbeginsinthemouth.Asfoodisbeingc hewed,saliva,produc ed bythesalivary

glands,mixeswit hthefood.Salivaisa waterysubstanceproduc edinthe mouths ofmanyanimals.T herearethre ema jor

glandsthatsec retesaliva - the parotid,thesubmandibular,and thesublingual.Salivacontainsm ucusthatmoistensfood and

buffersthepHofthe food.Salivaa lsoc onta insimmunogl obulins andlysozymes,whichhave antibacteri alactiontoreduce

toothdecaybyi nhibitinggrowthofsomebac te ria.Salivaalsocontainsane nzymecal le dsalivaryamylasethatbeginsthe

Chapter 34 | Animal Nutrition and the Digestive System961

processofconverti ngsta rchesinthefoodintoadisac charidecalledmalt ose.Anothere nzymecal led lipaseisproduced

bythe cellsi nthetongue.Lipasesareac lassofe nzymestha tcanbreakdownt riglycerides.Thelinguall ipasebegins the

breakdownoffatcom ponents inthefood.T hechewingandwettingac tionprovided bythetee tha ndsaliva preparethefood

intoamassca lle dthebolusforswal lowing.Thetonguehelpsinswall owing - movingt hebolusfromthe mouthintothe

pharynx.Thepha rynxopenstot wopassageways: thetrac hea,which leadsto thelungs,andtheesophagus,which leadsto

thestoma ch.Thetracheahasanopeni ngcalle dtheglottis,whichisc overedbyac artilagi nousflapcall edtheepiglottis.

Whenswallowing,t heepiglottisclosesthegl ottisa ndfoodpassesintotheesophagusandnotthetrac hea.Thisarrangeme nt

allows food to be kept out of the trachea. Figure34.9Digestionoffoodbegins inthe (a)oralcavity.F oodis masticated byteethandmoist enedbysaliva

secretedfromt he(b)salivaryglands.Enzymes inthesalivabegint odigestst archesand fats. Withthehelp ofthe

tongue,theresulting bolusismovedintot heesophagusbyswallowing.(credit :modificat ionofwork bythe National

Cancer Institute)

Esophagus

Theesophagusisatubul aror ganthatconnectsthe mouthtothest omach.Thechewe dandsoftenedfoodpassesthrought he

esophagusafterbei ngswallowed.Thesmooth musclesofthee sophagusundergoaseriesofwavel ikemoveme ntscalled

peristalsisthatpushthefoodtowa rdthe stomach,a si llustratedin Figure34.10.The peristal siswaveisunidirectional - it

movesfoodfromthemout ht othe stomach,a ndreversemovement isnotpossibl e.Theperi stalticmovementofthe esophagus is an involuntary reflex; it takes place in response to the act of swallowing.

Figure 34.10The esophagus transfers food from the mouth to the stomach through peristaltic movements.

Aring-l ikemusclecalle dasphincterformsvalves inthedigestivesystem .Thegast ro-esophagealsphi ncterislocatedat

thestomac hendoftheesophagus.Inresponsetoswa llowinga ndthe pressureexerte dbythebolusoffood,t his sphincter

opens,andt hebolusente rsthestoma ch.Whent hereisnoswallowinga ction,thi ssphincterisshutandprevents thecontents

ofthe stomachfrom travelinguptheesophagus.Many anim alshaveatruesphincter;however,i nhuma ns,therei snotrue

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sphincter,buttheesophagusremains close dwhenthere isnoswallowing action.Acidrefluxor"hea rtburn"occurswhen

the acidic digestive juices escape into the esophagus.

Stomach

Ala rgepartofdigestionocc ursinthe stoma ch,showninFigure34.11.The stomachisasacl ike organthatsecretesgastri c

digestivejuices.ThepHi nthestomachisbetween 1.5and2.5.T hishi ghlyacidi cenvironment isrequiredforthechemic al

breakdownoffoodandthe ext ract ionofnutrie nts.Whene mpty,thestomachisarathersma llor gan;however,itcanexpand

toupto20 tim esitsre sting sizewhenfilledwithfood. Thischaracte risticisparticul arlyusefulforanim alsthat needtoeat

when food is available. Figure34.11Thehumanstomachhas anextremely acidicenvironmentwheremostoft heprot eingets digested. (credit: modification of work by Mariana Ruiz Villareal) Which of the following statements about the digestive system is false? a.Chyme is a mixture of food and digestive juices that is produced in the stomach. b.Food enters the large intestine before the small intestine. c.In the small intestine, chyme mixes with bile, which emulsifies fats. d.The stomach is separated from the small intestine by the pyloric sphincter.

Thestomac hisalsothemajorsit eforprotein digestioninani malsothe rthanruminants.Prote indigestionis medi atedby

anenzyme calledpepsinint hestomachchamber.Pepsinissecre tedbythechiefcellsint hestom achin aninactiveform

calledpepsinogen.Pepsi nbreakspeptide bondsandcleaves proteinsintosmall erpol ypeptides;i talsohelpsactivatemore

pepsinogen,startingaposit ivefeedbackmechanismt hatge neratesmorepepsin.Anothe rcelltype - parietalc ells - secrete

hydrogenandchl orideions,whic hcombineinthelum entoformhydroc hloricacid,the primaryacidiccom ponentofthe

stomachjuices.Hydrochloric acidhelpstoconvertthei nactivepepsinogento pepsin.Thehighlyacidi cenvironmental so

killsmanymicroorga nismsinthefooda nd,combinedwiththeactionoftheenzyme pepsin,result sinthe hydrolysis of

proteininthefood.Chem icaldige stionis faci litatedbythechurningactionofthe stomach.Contractionand relaxati onof

smoothmusclesm ixesthestomachcontents aboutevery20minutes. Thepartia llydigestedfoodandgastri cjuicemixture

iscalledchyme.Chym epassesfromthest omachtothesm all intestine .Furtherproteindigestiont akesplac einthesm all

intestine.Gastricemptyingoccurswi thintwotosixhoursaft erameal.Onlyasmal lamountofchymei sreleasedintothe

smallintestineat atime.Themovementofchyme fromthestoma chi ntothesmallintestine isregulat edbythepyloric

sphincter.

Whendigestingprot einandsomefats,thestoma chliningmustbeprote ctedfromge ttingdige stedbypepsin.T hereare

twopointst oconsiderwhendesc ribinghowthe stomachliningis protected. First,aspreviouslymentione d,the enzyme

pepsinissynthesiz edinthe inactiveform.Thisprotectst hechiefc ells,bec ausepepsinogendoesnothavethesamee nzyme

Chapter 34 | Animal Nutrition and the Digestive System963

functionalityofpepsin.Second,thestomachha sat hickmucusli ningthatprotect stheunderlyi ngtissuefromtheactionof

thedigesti vejuices.Whenthismucus liningisruptured,ulcersc anforminthestom ach.Ulcersa reopenwoundsinoron

an organ caused by bacteria (Helicobacter pylori) when the mucus lining is ruptured and fails to reform.

Small Intestine

Chymemovesfromt hestomachtot hesma llintest ine.Thesmallintestine istheorga nwherethedi gestionofprotein, fats,

andcarbohydrat esiscompleted.Thesmall intestine isalongtube-likeorganwithahighlyfol dedsurface containingfinger-

likeprojecti onscalledthevilli.The apicalsurfa ceofeachvillushasm anymicroscopicproj ect ionscalledmicrovi lli.These

structures,illustratedin Figure34.12,are linedwit hepithelialcel lsonthelumi nalsideandallowforthenutrientstobe

absorbedfromthedigest edfood andabsorbedi ntothebloodstreamontheot hersi de.Thevil liandmicrovi lli,withthe ir

manyfolds,increa sethesurfac eareaoftheintestineand increasea bsorptioneffici encyofthenutrie nts.Absorbe dnutrients

inthebl oodarecarri edintot hehepatic portalvein,whichleadst otheliver.T here,thel iverregulatesthedistri butionof

nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens.

Figure34.12Villiarefoldson thesmall intestineliningthat increasethe surfaceareato facilitat etheabsorption

of nutrients. Which of the following statements about the small intestine is false?

a.Absorpt ivecellsthatlinethesmallint estinehave microvilli,smallprojectionst hatincreasesurf acearea

and aid in the absorption of food. b.The inside of the small intestine has many folds, called villi. c.Microvilli are lined with blood vessels as well as lymphatic vessels. d.The inside of the small intestine is called the lumen.

Thehumansm allintestine isover6mlongandis dividedintothreeparts:theduodenum, thejejunum ,andthe ile um.

The"C-shape d,"fixedpartofthesmalli ntestineisca lledthe duodenumandisshowni nFigure34.11.The duodenum

isseparat edfromthestomachbythepyloricsphinc te rwhichopens toallowchymetomovefromt hest omachto the

duodenum.Intheduodenum, chyme ismixed withpancreat icjuicesin analkal inesolutionrichin bicarbonatethat

neutralizestheacidityofchymeandac tsasa buffer.Pancreaticjuicesal socont ainseveraldi gestiveenz ymes.Digestive

juicesfromthepancreas,li ver, andgall bladder,aswellasfromgla ndcellsoftheintesti nalwalli tself,entertheduodenum.

Bileisproducedi ntheliverand storedandc oncentrat edinthegallbladde r.Bile containsbile saltswhic hemulsifylipids

whilethepancrea sproducesenz ymesthatcatabolize starches,disacc harides,proteins,andfats.The sedigestivejui cesbreak

downthe foodpartic les inthechymeintoglucose,triglyceride s,andam inoacids. Somechemical digestionoffoodta kes

place in the duodenum. Absorption of fatty acids also takes place in the duodenum.

Thesecondpa rtofthesmall intestine isc alledt hejejunum,showni nFigure34.11.Here ,hydrolysisofnutrient sis

continuedwhilemostofthec arbohydratesandam inoacids are absorbedthrought heintestinallining.Thebulkof chemica l

digestion and nutrient absorption occurs in the jejunum.

964Chapter 34 | Animal Nutrition and the Digestive System

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Theileum,al soillustrate dinFigure34.11isthela stpartoft hesmallintest ineandhere thebi lesalt sandvitaminsare

absorbedintobloodst ream.Theundigest edfoodis senttothecolonfromtheile umviaperist alticmovementsofthe muscl e.

Theile umendsandthelargei ntestinebe ginsatthe ileoce calvalve.Thevermi form,"worm-like,"appendixisloca tedatthe

ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity.

Large Intestine

Thelargeintestine,il lustratedinFigure34.13,rea bsorbsthewaterfrom theundigeste dfoodmaterialandproc esse sthe

wastemateria l.Thehumanlargeintestineismuchsm alleri nlengthcompare dtothesmallint estinebutlargerindiamete r.

Ithas threeparts: thececum,the colon,andthere ctum.The cecumjoinstheileumto thecolon andistherece ivingpouch

forthe wastemat ter.Thecolon ishometomanybacteriaor"inte stinalflora "thataidi nthedigest iveprocesses. Thecolon

canbedividedi ntofour regions,theascendingc olon,thetransversec olon,t hedescendingcolon andthesi gmoidcolon.

Themain functionsofthecolonare toextractthewat era ndmineral saltsfromundigeste dfood,andt ostorewaste material.

Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet.

Figure34.13Thelargeintestine reabsorbswat erfromundigestedfoodandst oreswaste materialuntilitis eliminated.

Rectum and Anus

Therectumisthete rminalend ofthelargeintestine,asshown inFigure34.13.The primaryrole oftherectumisto store

thefeces untildefecation.T hefecesarepropel ledusingperistalticmovementsduri ngelimi nation.Theanusisanope ning

atthefar -endofthedi gestivetractandi sthe exitpointfor thewastematerial.T wosphinctersbetweent herectuma ndanus

control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary.

Accessory Organs

Theorgansdi scussedabovearet heorgansofthedigestive tractthrough whichfoodpa sses.Accessory organsareorgans

thataddsecret ions(enzym es)thatcatabolizefoodintonut rients.Accessoryorgansincl udesalivaryglands, theliver,the

pancreas,andthegallbladde r.Theli ver,panc reas,andgallbladderareregulatedbyhormone sinresponset othefood

consumed.

Theliveristhela rgestinte rnalorganinhumansandit playsaveryimportantrolein digestionoffat sanddetoxi fying

blood.Theli verproducesbi le,adigestivejui cethatisrequi redforthebreakdownoffatt ycom ponentsofthefoodinthe

duodenum. The liver also processes the vitamins and fats and synthesizes many plasma proteins.

Thepancreasisanother importantglandthat secretesdigestivejuice s.Thechymeproduce dfromthestomach ishighly

acidicinnature;thepanc reaticj uicescontainhigh levelsof bicarbonate,analkalithatneutra lizestheacidicchyme .

Additionally,thepancreaticjuicescont ainal argevarietyofenzymesthatarerequi redforthedigestionofprotei nand

carbohydrates.

Thegallbladderisasmal lorga nthataidsthelive rbystoringbile andconcentrati ngbilesalts.W henchyme containing

fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum. Chapter 34 | Animal Nutrition and the Digestive System965

34.2|Nutrition and Energy Production

By the end of this section, you will be able to:

•Explain why an animal's diet should be balanced and meet the needs of the body •Define the primary components of food

•Describe the essential nutrients required for cellular function that cannot be synthesized by the animal body

•Explain how energy is produced through diet and digestion •Describe how excess carbohydrates and energy are stored in the body

Giventhediversi tyofanim allifeonourplanet,it isnot surprisingthattheanimaldie twouldalsovary substant ial ly.

Theanima ldietisthesourceofmat erialsneeded forbuildingDNA andother complexm olec ulesneededforgrowth,

maintenance,andreproduction;collectively theseprocesse sarecalledbiosynthesis. Thedietisalsothesourceofmaterials

forAT Pproductioninthe cells.Thedietm ustbebalanc edtoprovide themineralsandvitam insthata rerequiredforcel lular

function.

Food Requirements

Whatarethefundam entalrequi rementsof theanimaldiet?Theanima ldietshouldbewellbalancedandprovide nutrie nts

requiredforbodilyfuncti onand themineralsa ndvitaminsrequiredfor mainta iningstruct ureandregulationnecessaryfor

good health and reproductive capability. These requirements for a human are illustrated graphically inFigure 34.14

Figure 34.14For humans, a balanced diet includes fruits, vegetables, grains, and protein. (credit: USDA)

Thefirstste pinensuring thatyouaremee tingthe foodrequirementsof yourbodyisanaware ness ofthefoodgroups

andthenut rientsthe yprovide.Tolearnmoreaboute achfoodgroupandt herecommendedda ilyamount s,explorethis

interactive site (http://openstaxcollege.org/l/food_groups)by the United States Department of Agriculture.

966Chapter 34 | Animal Nutrition and the Digestive System

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Let's Move! Campaign

Obesityisagrowingepidemicandt herate ofobesity amongchildren israpidlyrising inthe UnitedStates. Tocombatchildhoodobesity andensuret hatchildrenget ahealthystart inlife, firstlady MichelleObama haslaunched theLet 'sMove!campaign.The goalofthiscampaignistoeducat eparent sandcaregivers onprovidinghealt hynutrit ionandencouragingactive lifestylestofuturegenerat ions.This programaims to involvethe entirecommunity,including parents,teachers,andhealt hcareproviderstoensurethatchildren haveaccesst ohealthy foods - morefruits, vegetables,andwholegrains - andconsume fewercaloriesfrom

processedfoods. Anothergoalis toensurethatchildrengetphysicalact ivity. With theincreaseint elevision

viewingandst ationarypursuit ssuchasvideogames,sedentarylif estyleshavebecomethe norm.Learn more at www.letsmove.gov.

Organic Precursors

Theorganic moleculesrequired forbuildingcellularmaterialand tissuesmustcomefromfood. Carbohydra tesorsugars

aretheprim arysourceof organiccarbonsinthea nimalbody. During digestion,digestiblec arbohydratesareult imate ly

brokendownint ogluc oseandusedtoprovide energythroughmet abolicpa thways.Compl excarbohydrat es,includi ng

polysaccharides,canbebrokendownintoglucosethroughbi ochemicalm odific ation;howeve r,hum ansdonotproduce

theenzyme cellulaseandlac ktheabilitytoderiveglucosefromthe polysacchari decellulose.Inhuma ns,thesemol ecules

providethefi berrequired formovingwastethrough thelargeintesti neand ahealthycolon. Theintestinal floraint hehuman

gutare abletoe xtractsomenutrit ionfromthe seplantfibers.Theexcesssugars inthebodya reconvertedintoglycogen

andstoredi ntheliverand musclesfor later use.Glycogenstoresareuse dtofuelprol ongedexertions,suchaslong-dist ance

running,andto provideener gyduringfoodshorta ge.Excess glycogencanbeconvert edtofat s,whic harestoredinthel ower

layeroftheskinofm amm alsforinsul ationand ener gystorage.Excessdigestiblecarbohydrate sare storedbymammalsin

order to survive famine and aid in mobility.

Anotherimportantre quirementisthatofnitrogen. Proteincatabolismprovidesasource oforganicni trogen.Aminoac ids

arethebui ldingblocks ofproteinsandproteinbre akdownprovidesami noac idsthatareusedforce llularfunc tion.The

carbonandnitroge nderivedfrom thesebecomethebuildi ngblockfornucleotide s,nucl eicacids,prote ins,cells,andti ssues.

Excessnitrogenmustbee xcretedasitist oxic.Fat sadd flavortofoodandpromoteasenseofsati etyorful lness.Fatt yfoods

arealso significantsources ofenergybecauseonegramoffatcont ainsninecalori es.Fa tsarerequiredint hediettoaidthe

absorption of fat-soluble vitamins and the production of fat-soluble hormones.

Essential Nutrients

Whiletheanimalbody cansynthesi zemanyofthemolecule srequi redforfunctionfromt heorganicprecursors,thereare

somenutrient sthatneedtobeconsumedfrom food.Thesenutrient saretermedessentialnutrients,me aningtheymustbe

eaten, and the body cannot produce them.

Theomega-3 alpha-linolenicac idandtheomega-6linoleicacidareessenti alfattyacidsnee dedtoma kesomeme mbrane

phospholipids.Vitaminsareanothe rclassofessential organicmoleculest hat arerequiredinsmallquantitie sformany

enzymestofunctionand,fort hisreason,a reconsideredtobeco-e nzymes.Absenc eorl owlevelsofvi taminscanha ve

adram aticeffectonhealth,asout linedinTable34.1andTable34.2.Bot hfat-solubleandwa ter-solublevitaminsmust

beobta inedfromfood.Minerals,li stedinTable34.3,are inorganice ssentialnutrientsthatm ustbeobtainedfromfood.

Amongtheir manyfunctions,mineral shelpinstruc tureandregulationand areconsideredco-factors.Certai naminoacids

alsomustbeprocure dfromfoodandc annotbe synthesizedbythe body. These aminoac idsarethe"esse ntia l"aminoacids.

Thehumanbody cansynthesizeonl y11 ofthe20required aminoacids;therestmustbe obtainedfrom food.Theesse ntia l

amino acids are listed inTable 34.4. Chapter 34 | Animal Nutrition and the Digestive System967

Water-soluble Essential Vitamins

VitaminFunctionDeficiencies Can Lead ToSources

Vitamin B

1 (Thiamine)

Needed by the body to process lipids,

proteins, and carbohydrates Coenzyme removes CO 2 from organic compounds

Muscle weakness, Beriberi: reduced

heart function, CNS problems

Milk, meat,

dried beans, whole grains

Vitamin B

2 (Riboflavin)

Takes an active role in metabolism,

aiding in the conversion of food to energy (FAD and FMN)

Cracks or sores on the outer surface

of the lips (cheliosis); inflammation and redness of the tongue; moist, scaly skin inflammation (seborrheic dermatitis)

Meat, eggs,

enriched grains, vegetables

Vitamin B

3 (Niacin)

Used by the body to release energy from

carbohydrates and to process alcohol; required for the synthesis of sex hormones; component of coenzyme NAD + and NADP +

Pellagra, which can result in

dermatitis, diarrhea, dementia, and death

Meat, eggs,

grains, nuts, potatoes

Vitamin B

5 (Pantothenic acid)

Assists in producing energy from foods

(lipids, in particular); component of coenzyme A

Fatigue, poor coordination, retarded

growth, numbness, tingling of hands and feet

Meat, whole

grains, milk, fruits, vegetables

Vitamin B

6 (Pyridoxine)

The principal vitamin for processing

amino acids and lipids; also helps convert nutrients into energy

Irritability, depression, confusion,

mouth sores or ulcers, anemia, muscular twitching

Meat, dairy

products, whole grains, orange juice

Vitamin B

7 (Biotin)

Used in energy and amino acid

metabolism, fat synthesis, and fat breakdown; helps the body use blood sugar

Hair loss, dermatitis, depression,

numbness and tingling in the extremities; neuromuscular disorders

Meat, eggs,

legumes and other vegetables

Vitamin B

9 (Folic acid)

Assists the normal development of cells,

especially during fetal development; helps metabolize nucleic and amino acids

Deficiency during pregnancy is

associated with birth defects, such as neural tube defects and anemia

Leafy green

vegetables, whole wheat, fruits, nuts, legumes

Vitamin B

12 (Cobalamin)

Maintains healthy nervous system and

assists with blood cell formation; coenzyme in nucleic acid metabolism

Anemia, neurological disorders,

numbness, loss of balance

Meat, eggs,

animal products

Vitamin C

(Ascorbic acid)

Helps maintain connective tissue: bone,

cartilage, and dentin; boosts the immune system

Scurvy, which results in bleeding, hair

and tooth loss; joint pain and swelling; delayed wound healing

Citrus fruits,

broccoli, tomatoes, red sweet bell peppers

Table 34.1

968Chapter 34 | Animal Nutrition and the Digestive System

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Fat-soluble Essential Vitamins

VitaminFunction

Deficiencies

Can Lead

To

Sources

Vitamin A

(Retinol)

Critical to the development of bones, teeth, and

skin; helps maintain eyesight, enhances the immune system, fetal development, gene expression

Night-blindness,

skin disorders, impaired immunity

Dark green leafy

vegetables, yellow- orange vegetables fruits, milk, butter

Vitamin D

Critical for calcium absorption for bone

development and strength; maintains a stable nervous system; maintains a normal and strong heartbeat; helps in blood clotting

Rickets,

osteomalacia, immunity

Cod liver oil, milk,

egg yolk

Vitamin E

(Tocopherol)

Lessens oxidative damage of cells,and prevents

lung damage from pollutants; vital to the immune system

Deficiency is

rare; anemia, nervous system degeneration

Wheat germ oil,

unrefined vegetable oils, nuts, seeds, grains

Vitamin K

(Phylloquinone)

Essential to blood clotting

Bleeding and

easy bruising

Leafy green

vegetables, tea

Table 34.2

Figure34.15Ahealthy dietshouldincludeavariety offoods toensure that needsfor essentialnutrientsare met.

(credit: Keith Weller, USDA ARS)

Minerals and Their Function in the Human Body

MineralFunction

Deficiencies Can

Lead To

Sources

*Calcium

Needed for muscle and neuron function;

heart health; builds bone and supports synthesis and function of blood cells; nerve function

Osteoporosis, rickets,

muscle spasms, impaired growth

Milk, yogurt, fish, green leafy

vegetables, legumes

Table 34.3

Chapter 34 | Animal Nutrition and the Digestive System969

Minerals and Their Function in the Human Body

MineralFunction

Deficiencies Can

Lead To

Sources

*Chlorine

Needed for production of hydrochloric

acid (HCl) in the stomach and nerve function; osmotic balance

Muscle cramps, mood

disturbances, reduced appetite

Table salt

Copper

(trace amounts)

Required component of many redox

enzymes, including cytochrome c oxidase; cofactor for hemoglobin synthesis

Copper deficiency is

rare

Liver, oysters, cocoa,

chocolate, sesame, nuts

Iodine

Required for the synthesis of thyroid

hormones

Goiter

Seafood, iodized salt, dairy

products Iron

Required for many proteins and

enzymes, notably hemoglobin, to prevent anemia

Anemia, which causes

poor concentration, fatigue, and poor immune function

Red meat, leafy green

vegetables, fish (tuna, salmon), eggs, dried fruits, beans, whole grains *Magnesium

Required co-factor for ATP formation;

bone formation; normal membrane functions; muscle function

Mood disturbances,

muscle spasms

Whole grains, leafy green

vegetables

Manganese

(trace amounts)

A cofactor in enzyme functions; trace

amounts are required

Manganese deficiency

is rare

Common in most foods

Molybdenum

(trace amounts)

Acts as a cofactor for three essential

enzymes in humans: sulfite oxidase, xanthine oxidase, and aldehyde oxidase

Molybdenum deficiency

is rare *Phosphorus

A component of bones and teeth; helps

regulate acid-base balance; nucleotide synthesis

Weakness, bone

abnormalities, calcium loss

Milk, hard cheese, whole

grains, meats *Potassium

Vital for muscles, heart, and nerve

function

Cardiac rhythm

disturbance, muscle weakness

Legumes, potato skin,

tomatoes, bananas

Selenium

(trace amounts)

A cofactor essential to activity of

antioxidant enzymes like glutathione peroxidase; trace amounts are required

Selenium deficiency is

rare

Common in most foods

*Sodium

Systemic electrolyte required for many

functions; acid-base balance; water balance; nerve function

Muscle cramps,

fatigue, reduced appetite

Table salt

Zinc (trace

amounts)

Required for several enzymes such as

carboxypeptidase, liver alcohol dehydrogenase, and carbonic anhydrase

Anemia, poor wound

healing, can lead to short stature

Common in most foods

*Greater than 200mg/day required

Table 34.3

970Chapter 34 | Animal Nutrition and the Digestive System

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Essential Amino Acids

Amino acids that must be consumedAmino acids anabolized by the body isoleucinealanine leucineselenocysteine lysineaspartate methioninecysteine phenylalanineglutamate tryptophanglycine valineproline histidine*serine threoninetyrosine arginine*asparagine

*The human body can synthesize histidine and arginine, but not in the quantities required, especially for

growing children.

Table 34.4

Food Energy and ATP

Animalsneedfoodtoobtain energyandmai nta inhomeosta sis.Homeostasisisthe abilityofasystem tomaint ainastable

internalenvironmentevenin thefaceofexternalchange stotheenvironme nt.Forexam ple,thenorm albodytempera ture

ofhuma nsis37°C(98.6°F).Huma nsmainta inthis temperat ureevenwhenthee xternaltemperatureishotorcold.Itt akes

energy to maintain this body temperature, and animals obtain this energy from food.

Theprimary sourceofenergyforanim alsisc arbohydrat es,mainlyglucose. Glucose iscalledthebody'sfuel.The digestible

carbohydrates in an animal's diet are converted to glucose molecules through a series of catabolic chemical reactions.

Adenosinetriphosphate, orATP,istheprimaryenergy currenc yincells;ATPst oresener gyinphosphate esterbonds.ATP

releasesenergywhenthephosphodiest erbondsarebrokenandATP isconvert edtoADPa ndaphosphategroup.ATPis

producedbytheoxidat ive reactions inthecytoplasmandmitoc hondrionofthecell,wherecarbohydrate s,proteins,a nd

fatsundergoaseri esofmetaboli cre actionscollecti vely calledcellularrespiration.For example,glycolysisisaseries of

reactionsinwhichglucoseisc onvertedto pyruvicacidandsom eofitschemi calpote ntialenergyistransferred toNADH

and ATP.

ATPisrequired forallce llularfunctions.Iti sused tobuildtheorga nicmoleculesthatarerequiredforc ellsandtissues;

itprovidese nergyformusclec ontractionandforthe transm issionofelectrica lsignalsinthenervoussystem.Whenthe

amountofATPis avai lableinexc essofthebody' srequirements,theliverusestheexc essATPa ndexcessglucoseto

producemole culescalledglycogen.Glycogenisa polymericformofglucoseandisstored intheliverandskel etalmuscle

cells.Whenbloodsugardrops,the liverrelease sglucosefrom storesofglyc ogen.Ske letalm uscleconvertsglycogen to

glucoseduringintense exercise. Theprocessofconverting glucoseandexcessATPtoglycogen andthest orageofexcess

energy is an evolutionarily important step in helping animals deal with mobility, food shortages, and famine.

Chapter 34 | Animal Nutrition and the Digestive System971

Obesity

Obesityisamajorhealthconcern int heUnited Stat es,andthereisa growingfocus onreducing obesityand thediseasesitmay leadto, suchast ype-2diabetes, cancersoft hecolonandbreast, andcardiovascular disease. How does the food consumed contribute to obesity? Fattyfoodsarecalorie-dense,meaningthat theyhavemorecaloriesper unitmasst hancarbohydrates or proteins.Onegram ofcarbohydrateshasfour calories,onegram ofprotein hasfourcalories,and onegram of fat has nine calories. Animals tend to seek lipid-rich food for their higher energy content. Thesignalsofhunger ("timet oeat") andsatiet y("timetost opeating")arecont rolledinthehypothalamus regionof thebrain. Foodsthatarerich infat tyacidstendt opromotesatiet ymoret hanfoods thatare rich only in carbohydrates. ExcesscarbohydrateandA TPareusedbyt helivert osynthesizeglycogen.Thepyruvate producedduring glycolysisisused to synthesizef attyacids.When thereismoreglucosein thebodythanrequired,the resultingexcesspyruvateis converted intomoleculesthatevent uallyresultin thesynthesisoff attyacids withinthebody. Thesefatty acidsarestoredinadipose cells - thefatcells inthemammalianbodywhose primary role is to store fat for later use. Itisimportantt onote thatsomeanimalsbenefit fromobesity. Polarbearsandsealsneed bodyfatfor insulationandtokeep themf romlosingbodyheatduringA rctic winters.Whenfood isscarce,storedbody fatprovidesenergyfor maintaininghomeost asis.Fat spreventfamineinmammals, allowingthemto access energywhenf oodisnot availableonadailybasis; fats arestored whenalarge killismade orlot soff ood is available.

34.3|Digestive System Processes

By the end of this section, you will be able to:

•Describe the process of digestion •Detail the steps involved in digestion and absorption •Define elimination •Explain the role of both the small and large intestines in absorption

Obtainingnutritionandene rgyfromfoodisamulti-stepproc ess.Fortrue animals,thefirstste pis ingest ion,thea ctof

takinginfood.Thisis followedbydi gesti on,absorption,ande liminat ion.Inthefollowingsect ions, eachofthesestepswill

be discussed in detail.

Ingestion

Thelar gemoleculesfoundini ntactfoodcannotpassthroughthecellme mbrane s.Foodneedst obebrokenintosmaller

particlessothatanimalscanha rnesst henutrientsand organicmolecules.Thefirststep inthisprocess isingestion.Ingest ion

istheproce ssoftaking infoodthroughthemout h.In vertebrates,thetee th,saliva ,and tongueplay importantrol esin

mastication(preparingthefoodintobolus).Whilethe foodisbeingmechani callybroke ndown,t heenzymesin salivabegi n

tochemi callyprocessthefoodaswell.Thecombinedac tionoftheseproce ssesmodifi est hefoodfromlar geparticlestoa

soft mass that can be swallowed and can travel the length of the esophagus.

Digestion and Absorption

Digestionistheme chanical andchemicalbreakdownoffoodintosmallor ganicfragme nts.Itisimportanttobreakdown

macromoleculesintosmallerfragmentsthatareof suitablesize forabsorptionacrossthedigestiveepit helium.Large,

complexmoleculesofprote ins,polysaccharides,andlipidsmustbere duced tosimplerpartic lessuchassimplesugarbefore

theycanbeabsorbe dbythedigestive epit helialcel ls.Di fferentorga nsplayspecificrolesint hedigestiveprocess.The

972Chapter 34 | Animal Nutrition and the Digestive System

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animaldietneedscarbohydrat es,protein,andfa t,aswella svitaminsandinorganiccomponent sfornutritionalbala nce.

How each of these components is digested is discussed in the following sections.

Carbohydrates

Thedigesti onofcarbohydratesbeginsinthe mouth. Thesalivaryenzymeamylasebe ginsthebreakdownoffood starches

intomaltose ,adisaccharide.Asthebolusoffoodt ravel sthroughtheesophagustothestomach,nosignifica ntdige stion

ofca rbohydratestakesplace.Theesopha gusproducesnodigestiveenz ymesbutdoesproducemucousforlubri cation.T he

acidic environment in the stomach stops the action of the amylase enzyme.

Thenextste pofcarbohydrat edigestiontake spl aceintheduodenum.Recal lthatthechymefromthestom achentersthe

duodenumandmi xeswith thedigestivesecreti onfromthepancreas, liver, andgallbladder.Pancreat icjuicesalsocontain

amylase,whichcontinuesthebrea kdownofstarchand glycogenintomaltose,adi saccharide .Thedisaccha rides arebroken

downint omonosaccharide sbyenzymescalledmaltases,sucrases,and lactases,whic harealsopresent inthebrushborde r

ofthe smallint estinalwall.Malt asebreaksdownmaltoseintoglucose.Othe rdisaccharides,such assucroseandlactoseare

brokendownbysucra seand lac tase,respecti vely.Suc rasebreaksdownsucrose(or"tablesugar")intogl ucoseandfructose,

andlac tasebreaksdownlactose(or"milk sugar")intogluc oseand galactose.Themonosacchari des(gl ucose)thusproduced

areabsorbeda ndthencanbeuse dinme tabolicpathways toharnessenergy .The monosaccha ridesaret ransportedacross

theintest inalepitheliumintothebloodstream tobetransportedtothedifferentcel lsinthebody.T hestepsin carbohydrate

digestion are summarized inFigure 34.16andTable 34.5.

Figure34.16Digestionofcarbohydratesis performedby severalenzymes.Starch andglycogenare brokendowninto

glucosebyamylase andmaltase. Sucrose(t ablesugar)and lactose(milksugar)are brokendownbysucraseand lactase, respectively.

Digestion of Carbohydrates

EnzymeProduced By

Site of

Action

Substrate

Acting On

End Products

Salivary amylaseSalivary glandsMouth

Polysaccharides

(Starch)

Disaccharides (maltose),

oligosaccharides

Pancreatic amylasePancreas

Small intestine

Polysaccharides

(starch)

Disaccharides (maltose),

monosaccharides

Oligosaccharidases

Lining of the intestine;

brush border membrane Small intestine

Disaccharides

Monosaccharides (e.g.,

glucose, fructose, galactose)

Table 34.5

Protein

Ala rgepartofproteindigest iontake spla ceinthestomach.T heenzymepepsinplaysani mportantroleinthedige stion

ofprote insbybreakingdownthei ntac tproteintopeptide s,whic hareshortcha insoffourtonineaminoaci ds.Inthe

duodenum,otherenzym es - trypsin,elastase,and chymotrypsin - actonthepept ides reducingthe mtosmallerpeptides.

Trypsinelasta se,carboxypeptidase,andchymotrypsinareproduce dbythepancreasandreleasedintotheduode numwhere

theyactonthe chyme.Furtherbrea kdownofpe ptidest osingleaminoacids isaidedbyenz ymescalle dpept idases(those

thatbreakdownpe ptides).Specific all y,carboxypeptidase,dipeptidase,and aminopeptidaseplayimportant rolesin

reducingthepeptides tofreeam inoacids.Theaminoaci dsareabsorbe dintothe bloodstreamthroughthesmal lintestines.

The steps in protein digestion are summarized inFigure 34.17andTable 34.6. Chapter 34 | Animal Nutrition and the Digestive System973

Figure34.17Proteindigestionisamultist epprocessthatbegins inthe stomachand continuesthroughtheintestines.

Digestion of Protein

Enzyme

Produced

By

Site of

Action

Substrate Acting

On

End Products

Pepsin

Stomach chief

cells

StomachProteinsPeptides

Trypsin

Elastase

Chymotrypsin

PancreasSmall intest ineProteinsPeptides

CarboxypeptidasePancreasSmall intestinePept ides

Amino acids and

peptides

Aminopeptidase

Dipeptidase

Lining of intestineSmall intestinePeptidesAmino acids

Table 34.6

974Chapter 34 | Animal Nutrition and the Digestive System

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Lipids

Lipiddigestionbegins inthestomachwiththe aidoflinguall ipaseandga striclipa se.However,thebulkoflipi ddigestion

occursinthesm allinte stinedueto pancreaticlipase .Whenchymeenterstheduodenum,the hormonal responsestri gger

therele aseofbile,whichisproduced int heliverandstoredi nthegallbladde r.Bile aidsinthedi gestionofl ipids,prima rily

triglyceridesbyemulsification.Emulsifi ca tionisaprocessinwhichlargelipidglobulesarebroke ndownintoseveralsmall

lipidglobules.These smallglobulesaremore widelydistribute dinthechymerathe rthanforminglargea ggregates.L ipids

arehydrophobicsubsta nces:int hepresenceofwater,they willaggrega tetoformgl obulestominimiz eexposure towater.

Bilecontainsbile salts,whichareamphipat hic,meaningt heycontainhydrophobicandhydrophi licparts.Thus, thebile

saltshydrophilicsideca ninterfacewithwa terononesi deandthehydrophobicside interfaceswithlipidsonthe other.By

doing so, bile salts emulsify large lipid globules into small lipid globules.

Whyisem ulsificat ionimportantfordigestionoflipids?Pancreaticjuice scontainenzymescal led lipases(enzymesthat

breakdownlipids).If thel ipidinthechymea ggregat esintol argeglobules,verylit tlesurfac eareaoft helipidsisava ilable

forthe lipasest oacton,leavinglipid digestioni ncomplete.By forming anemulsion,bilesalts increasetheavaila blesurface

areaofthelipi dsma nyfold.Thepa ncreaticlipases canthenact onthelipi dsmoreef ficientlyanddigestthem,asdeta iledin

Figure34.18.Li pasesbreakdownthelipidsi ntofattyaci dsand glyceride s.Thesemoleculesc anpassthroughthepl asma

membraneofthecelland ente rtheepit helialcellsoft heintest inallining.Thebile saltssurroundl ong-chainfattyacids

andmonoglyce ridesformingtinyspherescalledmi celles.Them icellesm oveintothebrushborder ofthesmallintestine

absorptivecellswheret helong-chainfattyacids andmonoglyce ridesdiffuse outofthemicellesint otheabsorptivecells

leavingthemicelle sbehindin thechyme.Thelong-chainfattyacidsand monoglyceridesrec ombineint heabsorptive

cellstoformtriglyceri des,whicha ggregat eintoglobulesandbecomecoatedwithprot eins.Theselar gespheresarecalled

chylomicrons.Chyl omicronscontaintriglycerides, cholesterol,andotherli pidsandhaveproteinsontheirsurface.The

surfaceisalsoc omposedofthe hydrophilicphosphate"he ads"ofphospholipids.Together ,the yenablet hechylomicron

tomovein anaqueouse nvironmentwi thoutexposingt helipidstowat er.Chylomicronsleavet heabsorptivecell svia

exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein.

Figure 34.18Lipids are digested and absorbed in the small intestine. Chapter 34 | Animal Nutrition and the Digestive System975

Vitamins

Vitaminscanbeeitherwater -solubleorlipid-solubl e.Fa tsolublevitaminsareabsorbe dinthesamemanneraslipids.Itis

importanttoconsumesomeamount ofdietary lipidtoai dtheabsorpti onoflipid-solublevi tam ins.Water-solublevitam ins

can be directly absorbed into the bloodstream from the intestine.

Thiswebsite(http://openstaxcol lege.org/l/digest_enzymes)hasanove rviewofthe digestionofprotein,fat ,and

carbohydrates.

976Chapter 34 | Animal Nutrition and the Digestive System

This OpenStax book is available for free at http://cnx.org/content/col11448/1.10 Figure34.19Mechanicalandchemical digestionof foodt akesplaceinmanyst eps,beginningin themouthand ending in the rectum. Which of the following statements about digestive processes is true? a.Amylase, maltase, and lactase in the mouth digest carbohydrates. b.Trypsin and lipase in the stomach digest protein. c.Bile emulsifies lipids in the small intestine. d.No f ood is absorbed until the small intestine.

Elimination

Thefinalst epindigestion istheelim inationof undigestedfoodcontentandwaste products.Theundi gestedfood material

entersthecolon,wherem ostofthewat erisreabsorbed. Recall thatthec oloni salsohom etothemicrofloraca lled"i ntestinal

flora"thataid inthedigestion process.Thesemi -solidwaste ismovedt hroughthecolonbyperistalti cmovementsoft he

muscleandisstoredi ntherec tum.Asthe rectumexpandsin responsetostora geoffeca lmatter,ittriggerst heneura lsignals

requiredtosetupthe urgetoel iminate .The solidwaste iseli minated throughtheanususingperistalticmovementsofthe

rectum. Chapter 34 | Animal Nutrition and the Digestive System977

Common Problems with Elimination

Diarrheaandconstipati onaresomeof themostcommonhealthconcernsthat affectdi gestion.C onstipa tionisa condition

wherethefec esareha rdenedbecauseofexcess waterrem ovalinthecolon.Inc ontrast,ifenoughwater isnot removedfrom

thefeces, itresultsindiarrhea .Manybact eria,includingthe onesthatc ausecholera,affectthe proteinsinvol vedinwater

reabsorption in the colon and result in excessive diarrhea.

Emesis

Emesis,orvomiting,ise lim inationoffoodbyforcefulexpulsion throughthem outh.Itisofteninresponsetoanirritantthat

affectsthedigestivetract, includingbut notlimitedtoviruses,ba cteria,emotions,sights,a ndfoodpoisoning.Thisforceful

expulsionofthefoodisdue tot hestrongcontra cti onsproducedbyt hestom achmusc les.Theproc essofeme sisisregulated

by the medulla.

34.4|Digestive System Regulation

By the end of this section, you will be able to:

•Discuss the role of neural regulation in digestive processes •Explain how hormones regulate digestion

Thebraini sthecontrolcent erforthese nsationof hungerandsatiet y.Thefunctionsofthedigesti vesyst emareregul ated

through neural and hormonal responses.

Neural Responses to Food

Inre actiontothesmell,sight,or thoughtoffood,li kethatshown inFigure34.20,the firstresponseis thatofsal ivation.

Thesali varyglandssecretemoresali vainresponsetost imulationbytheautonomic nervoussystemt riggeredbyfoodin

preparationfordigestion.Simulta neously ,thestomachbeginstoproduc ehydrochloricacidtodigestthefood.Recallthat

theperista lticmovementsoftheesophagusandotherorga nsofthedigestivetractareundert hecont rolofthebrai n.The

brainpreparesthe semusclesformovement aswell.Whe nthestomachisfull,the partof thebrainthatdetects satiety signals

fullness.Therearethree overlappingphasesof gastricc ontrol - thecephali cphase,thegastricphase,a ndtheintestina l

phase - each requires many enzymes and is under neural control as well. Figure34.20Seeingaplateof foodt riggersthesecretionof salivaint hemouthandt heproductionofHCLin the stomach. (credit: Kelly Bailey)

Digestive Phases

Theresponseto foodbeginse venbeforefoode nters themouth.Thefirst phaseofingestion, called thecephalicphase,is

controlledbytheneuralresponsetot hesti mulusprovided byfood.Allaspect s - suchassight ,sense,and smell - tri gger

theneuralre sponsesresultingi nsalivationa ndsecretionofgastricjui ces.Thega stricandsalivarysecretioninthec ephalic

978Chapter 34 | Animal Nutrition and the Digestive System

This OpenStax book is available for free at http://cnx.org/content/col11448/1.10

phasecan alsotakeplac eduetothethoughtof food.Rightnow,if youthinkaboutapieceof chocol ateorac rispypotat o

chip,theincrea seinsali vationisacephalicpha seresponsetot hethought.Thecentralnervoussystemprepares thestomac h

to receive food.

Thegastricphasebeginsoncethefooda rrivesint hestomach. Itbui ldsonthestimulati onprovidedduring thecephal ic

phase.Gastricac idsandenzymesprocess theingestedmate rials.The gastricphaseisstimulat edby(1)distension ofthe

stomach,(2)adecreasein thepH ofthe gastriccontents,and(3)the presenceofundi gestedma teri al. Thisphase consistsof

local, hormonal, and neural responses. These responses stimulate secretions and powerful contractions.

Theintestinalphasebeginswhenchymee ntersthesm allintesti netriggeringdigestivese cretions.T hisphasecontrolsthe

rateofgastrice mptyi ng.Inadditiontogast rinemptying,whenchymeente rsthe smallintestine,itt riggersother hormonal

and neural events that coordinate the activities of the intestinal tract, pancreas, liver, and gallbladder.

Hormonal Responses to Food

Theendocrinesystemcontrolstheresponseoftheva riousglandsi nthebodya ndthe releaseof hormonesattheappropri ate

times.

Oneoft heim portantfact orsunderhormonalcontrolisthest omachacidenvironme nt.Duringthe gastricphase,t hehormone

gastrinissecret edbyGcellsinthestomach in responset othepresenceofproteins.Gastri nstimula testhe releaseof

stomachacid,orhydrochlorica cid(HCl)whichai dsin thedigestionoftheprote ins.However,whenthest omachisempt ied,

theacidi cenvironmentneednotbemai ntainedandahormonecalle dsomatostatinstopstherel easeofhydrochl oricacid.

This is controlled by a negative feedback mechanism.

Int heduodenum,dige stivesecretions fromtheliver,pancreas,a ndgallbladderplayanimportant rolei ndigesti ngchyme

duringtheint estinalpha se.Inordertoneutralizetheacidi cchyme,ahormonec all edsecretinstimulatesthepancreas

toproduceal kalinebic arbonatesolutionanddeliverit totheduodenum.Secretinactsi ntandemwithanothe rhormone

calledcholecystokinin(CCK).Notonlydoes CCK stimulat ethepancre astoproducethe requisitepancreaticjuices,ital so

stimulates the gallbladder to release bile into the duodenum.

Visitthiswebsite(http:/ /openstaxcollege.or g/l/enteric_endo)tolea rnmoreabouttheendocrinesyst em.Revi ewthe

text and watch the animation of how control is implemented in the endocrine system.

Anotherlevelof hormonalcontroloccursinre sponseto thecomposition offood.Foodshighinli pidstakealongt ime

todigest. Ahormonecalledgastricinhibitory peptideissecret edbythesmallintestineto slowdown theperista ltic

movements of the intestine to allow fatty foods more time to be digested and absorbed.

Understandingthehormonalcontroloft hedigestive systemisani mportantareaofongoi ngrese arch.Sci entistsare

exploringtheroleofea chhormonein thedigesti veproce ssanddevelopingwayst otarget thesehormones. Advancesc ould

lead to knowledge that may help to battle the obesity epidemic. Chapter 34 | Animal Nutrition and the Digestive System979 alimentary canal aminopeptidase anus bile bolus carboxypeptidase carnivore cephalic phase cholecystokinin chylomicron chyme chymotrypsin digestion dipeptidase duodenum elastase endocrine system esophagus essential nutrient gallbladder gastric inhibitory peptide gastric phase gastrin gastrovascular cavity gizzard herbivore ileum ingestion

KEY TERMS

tubular digestive system with a mouth and anus proteasethatbreaksdownpe ptidestosingle aminoacids; secre tedbythebrushborderofsmalli ntestine exit point for waste material digestive juice produced by the liver; important for digestion of lipids mass of food resulting from chewing action and wetting by saliva proteasethatbreaksdownpe ptidestosingle aminoacids; secre tedbythebrushborderofthesmal l intestine animal that consumes animal flesh first phase of digestion, controlled by the neural response to the stimulus provided by food hormone that stimulates the contraction of the gallbladder to release bile small lipid globule mixture of partially digested food and stomach juices pancreatic protease mechanical and chemical break down of food into small organic fragments

protease that breaks down peptides to single amino acids; secreted by the brush border of small intestine

first part of the small intestine where a large part of digestion of carbohydrates and fats occurs pancreatic protease systemthatcontrol stheresponseoftheva riousglandsinthebodya ndtherele aseofhormones att he appropriate times tubular organ that connects the mouth to the stomach nutrient that cannot be synthesized by the body; it must be obtained from food organ that stores and concentrates bile hormonesecret edbythesmallintestineint hepre senceoffat tyacidsandsugars;it also

inhibits acid production and peristalsis in order to slow down the rate at which food enters the small intestine

digestivephasebeginningoncefoode ntersthest omach;gastricaci dsandenzym esproc esstheingested materials hormone which stimulates hydrochloric acid secretion in the stomach digestive system consisting of a single opening muscular organ that grinds food animal that consumes strictly plant diet

last part of the small intestine; connects the small intestine to the large intestine; important for absorption of B-12

act of taking in food

980Chapter 34 | Animal Nutrition and the Digestive System

This OpenStax book is available for free at http://cnx.org/content/col11448/1.10 intestinal phase jejunum lactase large intestine lipase liver maltase mineral monogastric omnivore pancreas pepsin pepsinogen peristalsis proventriculus rectum roughage ruminant salivary amylase secretin small intestine somatostatin sphincter stomach sucrase trypsin villi vitamin thirddigestive phase;beginswhenchymeenters thesmallinte stinetriggering digestivese cretionsand controlling the rate of gastric emptying second part of the small intestine enzyme that breaks down lactose into glucose and galactose digestive system organ that reabsorbs water from undigested material and processes waste matter enzyme that chemically breaks down lipids organ that produces bile for digestion and processes vitamins and lipids enzyme that breaks down maltose into glucose inorganic, elemental molecule that carries out important roles in the body digestive system that consists of a single-chambered stomach animal that consumes both plants and animals gland that secretes digestive juices enzyme found in the stomach whose main role is protein digestion inactive form of pepsin wave-like movements of muscle tissue glandular part of a bird's stomach area of the body where feces is stored until elimination component of food that i