Treatment of food.docx

Treatment of food.docx

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Treatmentoffood

Chapter6

Treatmentoffoodinthealimentarycanal

Thehumandigestivesystem（消化系統）

1.Thehumandigestivesystemconsistsofthealimentarycanal（消化道）andtheassociateddigestiveglands（消化腺）.

2.Thealimentarycanal（gut）isalongtuberunningfromthemouthtotheanus.Themainfunctionofthealimentarycanalisforthedigestionandabsorptionoffood.

3.Thedigestiveglandsproducedigestivejuiceswhichusuallycontaindigestiveenzymes（exceptbile）.Theseenzymesspeedupthedigestionofcomplexfoodintosmaller,simplerandsolublemolecules.

4.Foodistakenintothemouthbytheprocessofingestion.Insidethealimentarycanal,thefoodwillbedigestedandtheusefulproductsofdigestionareabsorbedintotheblood.Thesenutrientsarethenusedbythebodyforgrowthandotherfunctions,thisiscalledassimilation.Undigestedfoodwillberemovedoutofthebodybyegestion.

Functionsofdifferentpartsofthealimentarycanal

Partsofthealimentarycanal

Functions

1.Buccalcavity（mouth）

a.Ingestion

b.Mechanicaldigestion（chewing）

c.Chemicaldigestionofstarch

2.Pharynx

Swallowing

3.Oesophagus

Passageoffood

4.Stomach

a.Mechanicaldigestion（churning）

b.Chemicaldigestionofprotein

5.Smallintestine:

duodenumandileum

a.Chemicaldigestionofallkindsoffood.

b.Absorptionofdigestedfood

6.Colon

Absorptionofwaterandmineralsfrom

undigestedfood

.

7.Rectum

Storageoffaeces

8.Anus

Removaloffaeces（egestion）

Importantpoints:

1.Thedigestiveglandsare:

salivaryglands,gastricglands（inthestomach）,theliver,thepancreas,and

theintestinalgland（inthesmallintestine）.

2.Usually,thesedigestiveglandssecretedigestivejuicesthroughaductintothealimentarycanal.

3.Theinnerwallofthewholealimentarycanaliscoveredbyalayerofmucusforprotection.Themucusalsohelps

tolubricatethefood,makingiteasiertopassdownwardsbyperistalsis.

A.Ingestion（攝食）

Thisistheprocessbywhichfoodintakenintothealimentarycanalthroughthemouth.

Movementoffoodalongthealimentarycanal

I.Swallowing（吞嚥）

1.Inthemouth,thefoodismixedwithsalivaandischewed（咀嚼）bytheteethintosmallerpieces.Thetonguerollsthechewedfoodintoaballcalledbolus（食團）.

2.Thetonguepushesthebolustowardsthepharynx（咽）,whereitisswallowedintotheoesophagus（食道）.

Note:

Swallowingistheprocessofmovingfoodfromthepharynxintotheoesophagus..Itisareflexaction（反射動作）whichoccursautomatically（i.e.,withoutthinking）

3.Duringswallowing,

（a）thesoftpalate（軟顎）israisedupwardstopreventthefoodfromgoingupthenasalcavity;

（b）theepiglottis（會厭軟骨）islowereddowntoclosetheentranceoftrachea.Thispreventsfoodfromenteringthetrachea.

Examinationtips:

Duringswallowing,breathistemporarilyheld.Thusitisimpossibletoswallowfoodandbreatheatthesametime.

II.Peristalsis（蠕動）

1.Thewallofthealimentarycanalismadeupoftwolayersofmuscles:

circularmusclesandlongitudinalmuscles.

2.Thepresenceoffoodinthealimentarycanalstimulatesthecircularmusclesbehindittocontractandthoseinfrontofittorelax.Asaresult,thefoodispushedforwardsbyperistalsis.

3.Peristalsisisarhythmicwaveofcontractionandrelaxationofthemusclesofthealimentarycanaltopushthefoodforwards.Thisprocessbeginsfromtheoesophagusandcontinuesdowntotherectum.

4.Peristalsisalsohelpstomixthefoodwithdigestivejuicestospeedupenzymaticdigestionoffood.

Examinationtips:

Theimportanceofperistalsis（andthemusclelayersinthewallofthealimentarycanal）

1.Peristalsisinthestomach（churning）isanexampleofmechanicaldigestion.（Nochemicalchangeoccurs）

2.Mucushelpstomoistenthefoodandthisspeedsupperistalsis.

3.Peristalsisinthesmallintestinealsohelpstobringdigestednutrientsclosertothevilli,thisincreasestheefficiencyof

absorptionoffood.（Asthemusclesareresponsibleforperistalsis,thesemusclesarethusimportantinthe

digestionandabsorptionoffoodinthesmallintestine!

）

4.Peristalsisoccursinthewholealimentarycanal,fromtheoesophagustotherectum.However,itdoesnotoccurin

thedigestiveglands.Therefore,peristalsisdoesnotoccurineverypartsofthedigestivesystem!

（MC）

5.Thepresenceofbulkyfoodmassstimulatesthemusclesofthealimentarycanaltocontractandspeedsupperistalsis.

Thisexplainswhyroughageisimportantinthedietasitcannotbedigestedanditabsorbswatertoformalargebulky

mass.（Finefoods,e.g.,refinedriceandwheat,cannotstimulateperistalsis）

B.Digestion（消化作用）

1.Thisistheprocessofbreakingdownfoodintosimpleandsolublemoleculeswhichcanbeabsorbed.

2.Therearetwoformsofdigestion:

（a）Mechanicaldigestion（機械式消化）

Largepiecesoffoodarebrokendownphysicallyintosmallerpieces.Thisactionincreasesthesurfaceareaofthefoodfortheenzymestoacton.

e.g.,chewingoffoodinthemouthandchurningoffoodinthestomach.

（b）Chemicaldigestion（化學式消化）

Large,complexfoodmoleculesarebrokendowntosmaller,simplerandsolublemolecules,thereactionisspeededupbydigestiveenzymes.

Digestioninthemouth

1.Thefoodischewedbytheteethintosmallerpieces.

2.Thesalivaryglands（唾液腺）secretesaliva（唾液）whichcontains

（a）waterandmucus（黏液）tomoistenandlubricatethefoodsothatitcanbeswallowedeasily.

（b）salivaryamylase（澱粉酶）whichspeedsupthedigestionofstarchintomaltose.

Digestioninthestomach

1.Asthefoodboluspassesthroughtheoesophagus,thecardiacsphincter（賁門括約肌）relaxestoletthebolusenterthestomach.

2.Theglandsinthestomachwallsecretegastricjuice（胃液）whichcontains

（a）proteases（蛋白酶）whichspeedupthedigestionofproteinstopolypeptides.

（b）hydrochloricacid（鹽酸）whichkillsthebacteriainfoodandstopstheactionofsalivaryamylase.Italsoprovidesanacidicmediumfortheproteasestowork.

3.Peristalticmovementofthestomachwallchurns（攪動）thefoodandmixesitwithgastricjuice.

4.Thestomachwallsecretesathicklayerofmucuswhichprotectsthestomachwallfromdamageby

（a）acid.

（b）thedigestiveactionofproteases.

Note:

Thewholealimentarycanalislinedwithmucus,butthestomachwallhasthethickestlayer.

5.Whendigestioninthestomachiscomplete,foodischangedintoasemi-liquidcalledchyme（食糜）.Graduallythepyrolicsphincter（幽門括約肌）relaxes,allowingchymetoentertheduodenuminsmallquantities.

Digestionintheduodenum

Theduodenumreceivesthreedigestivejuices.Allcontainsodiumhydrogencarbonate（重碳酸鈉）whichneutralizestheacidicchymeandprovidesanalkalinemediumfortheenzymestowork.

I.Pancreaticjuice（胰液）

1.Itissecretedbythepancreas,andisdischargedthroughthepancreaticduct（胰管）intotheduodenum.

2.Itcontainsthreedigestiveenzymeswhichcatalyse（speedup）thedigestionofdifferentfoodsubstances:

（a）amylase:

starchintomaltose.

（b）proteases:

proteinsintopolypeptides.

（c）lipase:

fatsintofattyacidsandglycerol.

II.bile（膽汁）

1.Itissecretedbytheliverandisstoredtemporarilyinthegallbladder（膽囊）.Whentheacidicchymeenterstheduodenum,thegallbladdercontractsandforcesbilethroughthebileduct（膽管）intotheduodenum.

2.Biledoesnotcontainenzyme.Itcontainsbilesalts（膽鹽）toemulsifyfatsintotinydroplets,therebyincreasingthesurfaceareaforlipasetoacton.

3.Bilecontainsbilepigments,whicharemetabolicwasteproducedfromthebreakdownofredbloodcellsintheliver.Bilepigmentshavenofunctionindigestion.

Examinationtips:

Blockageofbileductandpancreaticduct

1.Ifthebileductisblocked,bilecannotbedischargedintotheduodenum,digestionoffatwillbelessefficient.

（Itwillnotstopaslipasescanwork）.Notethatthelivercanstillsecretebile!

Thesufferershouldavoideatingfoodsrichinfats.

Whenthebileductisblocked,bileisforcedintoblood,makingtheskinyellow（jaundice黃疸）

2.Whenthegallbladderofapersonisremoved,thedigestionoffatswillalsobecomeinefficient.Withoutthegallbladder,

bilecannotbestored.Whenfattyfoodsreachtheduodenum,thelivercannotproducesufficientbiletoemulsify

fatsintodroplets.Thiswilldecreasethesurfaceareaforlipasestowork.（lessfatisdigested）

3.Heatinghasnoeffectonbileasitdoesnotcontainenzyme.Heatingdenaturesenzymesofotherdigestivejuices.

4.Ifthepancreasisremovedorthepancreaticductisblocked,lipasescannotbereleasedintotheduodenum.Fatcannot

bedigestedbyenzymesandwillpassouttogetherwiththefaeces.（oilyfaeces）

Atlipasesandproteasescannotbereleasedandtheyaccumulateinthepancreas,theydigestthetissuesofthe

pancreas（e.g.,digestcellmembraneswhicharemadeupoflipidsandproteins）.Thisresultsininflammation.（發炎）

5.Somedrugsactbyinhibitingtheactionoflipases.Takingthesedrugswillresultinless/nofatdigestionandleadingto

oilyfaeces.

6.Thecolourofbileisduetobilepigments.Thesepigmentsaremetabolicwastesproducedfromdestructionofredblood

cells.Bilepigmentsareresponsibleforthe"yellowish-brown"colourofnormalfaeces.Whenthebileductisblocked,

thefaecesbecomegreyishwhiteincolour.

7.Completeequationforfatdigestion:

bilesaltslipase

Fatsfatdropletsfattyacids+glycerol

III.Intestinaljuice（腸液）

1.Itissecretedbytheintestinalglandsinthewallofthesmallintestine.

2.Itcontains