physiology

1. Physiologyof digestion
Digestion:
 Digestionisthe processof gradual breakdownof foodsthat we eat ina soluble formsuitable
for absorption.Forexample,meat,evenwhencooked,ischemicallytoocomplextobe
absorbedfromthe alimentarycanal.so,itfirstdigestedbefore absorption.
 Digestionreleasesitsconstituents:glucose,aminoacids,mineral salts,fatandvitamins,which
are readyfor absorption.
The activitiesof the digestive systemcanbe groupedunderfive mainheadings.
1. Ingestion:Thisisthe takingof foodintothe alimentarytract,i.e.eatinganddrinking.
2. Propulsion:Thismixes andmovesthe contentsalongthe alimentarytract.
3. Digestion.Thisconsistsof:
• mechanical digestion: breakdownof foodby,e.g.mastication(chewing)
• chemical digestion:breakdownof foodintosmall moleculesbyenzymesproducedbydigestiveglands.
4. Absorption: This isthe processby whichdigestedfoodsubstancespassthroughthe walls
of alimentarycanal intothe bloodandlymphcapillariesto use bybodycells.
5. Elimination: undigestedandunabsorbedsubstancesare excretedfromthe alimentarycanal as
faecesbythe processof defaecation.
Mechanismof digestion
1. Mechanical digestion
i. Mastigation:
 The teethare admirablydesignedforchewing.The anteriorteeth(incisors) provide astrong
cuttingaction,and the posteriorteeth(molars)provideagrindingaction.
 Chewingisimportantfordigestionof all foods,butitisespeciallyimportantformostfruits
and raw vegetablesbecause theyhave indigestiblecellulose membranesaroundtheir
nutrientportionsthatmustbe brokenbefore the foodcanbe digested.
 Furthermore,chewingaidsthe digestionof foodforanothersimplereason:Digestiveenzymes
act onlyon the surfacesof foodparticles;therefore,the rate of digestionisdependentonthe
total surface area exposedtothe digestive secretions.
 In addition,grindingthe foodtoa veryfine particulate consistencypreventsexcoriationof the
gastrointestinal tractandincreasesthe ease withwhichfoodisemptiedfromthe stomachinto
the small intestine,thenintoall succeedingsegmentsof the gut.
ii.Swallowing(deglutition):
 Swallowingisacomplicatedmechanism,principallybecause the pharynxservesrespirationand
swallowingboth.

2.  The pharynx isconvertedforonlya few secondsat a time intoa tract for swallowingof food.
 Tongue helpsinmixingof salivawiththe food.
 Salivamoistensandlubricatesthe food,whichchangesintosemisolidformcalledbolus.The
bolusisthenswallowedthroughOesophagustothe stomach.
 Peristalsismovementof alimentarycanal alsohelpsinswallowing.
iii.Churningin stomach:
 The wall of stomachundergoesperiodicmovementaswell ascontractionproducingchurning
movementcalledperistalsis,whichresultsinbreakdownof complexfoodintosimplerform.
 The bolusaftermixingwithgastricjuice,turnintofine soluble formknownaschime.
2. Chemical digestion
 It involvesthe breakingof covalentchemical bondsinorganicmoleculesbydigestive
enzymes.
 Carbohydratesare brokendownintomonosaccharides,proteinsare brokendownintoamino
acids,and fatsare brokendownintofattyacidsand glycerol.
i. Digestionof Carbohydrates
 Ingestedcarbohydratesconsistprimarilyof polysaccharides,suchasstarches(rice,bread),
disaccharides,suchassucrose (table sugar) andlactose (milksugar); andmonosaccharides,such
as glucose andfructose (foundinmanyfruits).
 Duringthe processofdigestion,polysaccharidesare brokendownintosmallerchainsandfinally
intodisaccharidesandmonosaccharides.Disaccharidesare brokendownintomonosaccharides.
a) digestionofcarbohydrates inmouth;
 Carbohydrate digestionbeginsinthe oral cavitywiththe partial digestionof starchesby
salivaryamylase.
 About30 percentof starch is hydrolysedherebythisenzyme amylase (optimumpH6.8) intoa
disaccharide –maltose.
 Lysozyme presentinsalivaactsasan antibacterialagentthatpreventsinfections.
b) digestionofcarbohydrates in stomach and intestine;
 A minoramountof digestionoccursinthe stomach throughthe actionof gastricamylase
and gelatinase.
 Carbohydrate digestioniscontinuedinthe intestine bypancreaticamylase.
 A seriesof disaccharidasesenzymesthatare releasedbyintestinalepitheliumdigest
disaccharidesintomonosaccharides.
ii.Digestionof Proteins

3.  Proteinsare takenintothe bodyfroma numberof dietarysources.
 Pepsinsecretedbythe stomachcatalyzesthe cleavage of covalentbondsinproteinstoproduce
smallerpolypeptide chains.
a) digestionofprotein instomach and intestine;
 Gastric pepsindigestsasmuchas 10%–20% of the total ingestedprotein.
 The mucosa of stomach has gastricglands.
 Gastric glandshave three majortypesof cellsnamely –
(i) mucuscells: whichsecrete mucus;
(ii) pepticorchief cells;whichsecretethe proenzyme pepsinogen;and
(iii) parietaloroxynticcells;whichsecrete HCl andintrinsicfactor(factoressential for
absorptionof vitaminB12).
 The stomach storesthe foodfor 4-5 hours.
 The foodmixesthoroughlywiththe acidicgastricjuice of the stomachbythe churning
movementsof itsmuscularwall andiscalledthe chyme.
 The proenzyme pepsinogen,onexposure tohydrochloricacidgetsconvertedintopepsin.
 Pepsinthenconvertsproteinsintoproteosesandpeptones(peptides).
 The mucus and bicarbonatespresentinthe gastricjuice playan importantrole inlubricationand
protectionof the mucosal epitheliumfromexcoriationbythe highlyconcentratedhydrochloric
acid.
 HCl providesthe acidicpH(pH 1.8) optimal forpepsins.
 Renninisa proteolyticenzyme foundingastricjuice of infants whichhelpsinthe digestionof
milkproteins.
b) digestionofproteinin intestine;
 The bile,pancreaticjuice andthe intestinaljuice are the secretionsreleasedintothe small
intestine.
 Pancreaticjuice andbile are releasedthroughthe hepato-pancreaticduct.
 The pancreaticjuice containsinactive enzymes –trypsinogen,chymotrypsinogen,
procarboxypeptidases.
 Trypsinogenisactivatedbyanenzyme,enterokinase,secretedbythe intestinal mucosainto
active trypsin,whichinturnactivatesthe otherenzymesinthe pancreaticjuice.
 Pancreaticproteinases(allsecretedintheirinactiveforms) digestpeptidesintoamino
acids:
 Trypsinogenisactivatedbyenterokinase(secretedbyduodenum) intotrypsin,whichinturn
activatesthe other3 enzymes –chymo-trypsinogenbecomeschymotrypisn,proaminopeptidase

4. becomesaminopeptidase,andprocarboxypeptidase becomes
carboxypeptidase.
iii.Digestionof Lipids
 Lipidsare moleculesthatare insoluble oronlyslightlysolubleinwater.
 Lipidsinclude triglycerides, phospholipids,cholesterol,steroids,andfat-solublevitamins.
 The firststepin lipiddigestionisemulsification,whichisthe transformationof large lipid
dropletsintomuchsmallerdroplets.
 The emulsificationprocessincreasesthe surface areaof the lipidexposedtothe digestive
enzymesbydecreasingthe dropletsize.
 Emulsificationisaccomplishedbybilesaltssecretedbythe liverandstoredinthe
gallbladder.
 Lipase digestslipidmolecules.
 The vast majorityof lipase issecretedbythe pancreas.A minoramountof lingual lipase is
secretedinthe oral cavity,isswallowedwiththe food,anddigestsasmall amount(<10%) of
lipidinthe stomach.
 The stomach alsoproducesverysmall amountsof gastriclipase.
 The primaryproducts of lipase digestionare free fattyacidsandglycerol andfew cholesterol
and phospholipids.
3. Absorption
 Absorptionisthe processbywhichthe endproductsof digestionpassthroughthe intestinal
mucosainto the bloodor lymph.
 It iscarried outby passive,active orfacilitatedtransportmechanisms.
 Water movesbyosmosis;small fatsolublesubstances,e.g.fattyacidsandglycerol,are able to
diffuse throughcell membranes;while othersare generallytransportedinsidethe villi byother
mechanisms.
i. Passive transport:
 Small amountsof monosaccharideslikeglucose,aminoacidsandsome electrolyteslikechloride
ionsare generallyabsorbedbysimplediffusion.
 The passage of these substancesintothe blooddependsuponthe concentrationgradients.
ii.Active transport:
 Active transportoccurs againstthe concentrationgradientandhence requiresenergy.
 Variousnutrientslikeaminoacids,monosaccharideslikeglucose,electrolyteslike Na+are
absorbedintothe bloodbythismechanism.

5.  Some substanceslike glucoseandaminoacidsare absorbedwiththe helpof carrierproteins.
Thismechanismiscalledthe facilitatedtransport.
 Fatty acidsand glycerol beinginsoluble,cannotbe absorbedintothe blood.Theyare first
incorporatedintosmall dropletscalledmicelleswhichmove intothe intestinal mucosa.Theyare
re-formedintoverysmall proteincoatedfatglobulescalledthe chylomicrons
whichare transportedintothe lymphvessels(lacteals) inthe villi.Theselymphvessels
ultimatelyreleasethe absorbedsubstancesintothe bloodstream.The absorbedsubstances
finallyreachthe tissueswhichutilise themfortheiractivities.Thisprocessiscalledassimilation.
4. Defaecation
 The digestive wastes,solidifiedintocoherentfaecesinthe rectuminitiate aneural reflex
causingan urge or desire foritsremoval.
 The egestionof faecestothe outside throughthe anal opening(defaecation) isavoluntary
processand iscarriedout by a massperistalticmovement.