Treatment of food.docx
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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
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