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Chapter 6Chapter 6DIGESTION & DIGESTION & ABSORPTIONABSORPTION
§ 1. Introduction§ 1. Introduction
Concept ofConcept of digestiondigestion && absorptionabsorption
** Mechanical & chemical digestionMechanical & chemical digestion
Characteristics of Characteristics of digestive smooth musclesdigestive smooth muscles
** General characteristicsGeneral characteristics
** Electrophysiological characteristicsElectrophysiological characteristics
Resting potential (RP)Resting potential (RP)
Slow wave or basic electrical rhythm Slow wave or basic electrical rhythm (BER)(BER)
Action potential (AP)Action potential (AP)
Electrical activities & contraction of Electrical activities & contraction of digestive smooth muscles digestive smooth muscles
Secretion ofSecretion of digestive glandsdigestive glands
** Main functions of digestive fluidsMain functions of digestive fluids
Gastrointestinal innervationGastrointestinal innervation
** Intrinsic (or enteric) nervous systemIntrinsic (or enteric) nervous system
SubmucosalSubmucosal & myenteric plexus& myenteric plexus
** Extrinsic nervous systemExtrinsic nervous system
Parasympathetic & sympathetic nervesParasympathetic & sympathetic nerves
Gastrointestinal hormonesGastrointestinal hormones
** General functionsGeneral functions
** Brain-gut peptidesBrain-gut peptides
§ 2. Digestion in the mouth & esophagus§ 2. Digestion in the mouth & esophagus
SalivaSaliva & its function & its function
** Nature & Composition of salivaNature & Composition of saliva
pH 6.6~7.1, 99% waterpH 6.6~7.1, 99% water
Salivary amylase, lysozyme, salt, etc.Salivary amylase, lysozyme, salt, etc.
** FunctionsFunctions of salivaof saliva
Moisten the cavity & solve food; keep the Moisten the cavity & solve food; keep the
mouth clean; anti-bacteria; digest starchmouth clean; anti-bacteria; digest starch
** Regulation of salivary secretionRegulation of salivary secretion
Pure neuroregulationPure neuroregulation
Conditioned & unconditioned reflexConditioned & unconditioned reflex
Mastication or chewing & its effectsMastication or chewing & its effects
** DefinitionDefinition
** EffectsEffects
Break up large food particles; mixes theBreak up large food particles; mixes the
food with saliva; aid swallowing food with saliva; aid swallowing
Deglutition or swallowingDeglutition or swallowing
** DefinitionDefinition
** ProcessProcess
1st, 2nd & 3rd phase1st, 2nd & 3rd phase
Peristalsis & lower esophageal sphincterPeristalsis & lower esophageal sphincter
§ 3. Digestion in the stomach§ 3. Digestion in the stomach
Gastric juiceGastric juice & its secretion & its secretion
** Nature, Compositions & functionsNature, Compositions & functions
pH 0.9~1.5, colorless, 1.5~2.5 L/daypH 0.9~1.5, colorless, 1.5~2.5 L/day
Hydrochloric acidHydrochloric acid (HCl)(HCl)
Secreted by Secreted by parietal cellsparietal cells
Basic secretion: 0~5 mmol/hBasic secretion: 0~5 mmol/h
Max secretion: 20~25 mmol/hMax secretion: 20~25 mmol/h
Mechanism of HCl secretionMechanism of HCl secretion
Functions: see next pageFunctions: see next page
Relationship between max volume of HCl Relationship between max volume of HCl secretion & the number of parietal cellssecretion & the number of parietal cells
Functions of HCl:Functions of HCl:
TTurn pepsinogen into pepsin & provide a urn pepsinogen into pepsin & provide a
medium of low pH favoring pepsin action; medium of low pH favoring pepsin action;
KKill many swallowed virulent organisms; ill many swallowed virulent organisms;
PPromote the secretion of pacreas & bile;romote the secretion of pacreas & bile;
PPromote the absorption of calcium & ferrumromote the absorption of calcium & ferrum
PepsinogenPepsinogen
Secreted bySecreted by chief cells chief cells & activate by & activate by HClHCl
Function: Function: digest proteins into proteoses, peptones,digest proteins into proteoses, peptones,
polypeptides, with little free amino acidspolypeptides, with little free amino acids
MucusMucus
Secreted by Secreted by neck neck && surface mucous cells, surface mucous cells,
cardiac & pyloric glandscardiac & pyloric glands
Function: form Function: form mucus-bicarbonate barriermucus-bicarbonate barrier
Intrinsic factorIntrinsic factor
Secreted by Secreted by parietal cellsparietal cells
Function:Function: Promote the absorption of Vit BPromote the absorption of Vit B1212
** Regulation of gastric secretionRegulation of gastric secretion
Endogenous releasing substancesEndogenous releasing substances
Acetylcholine (ACh), gastrin & histamineAcetylcholine (ACh), gastrin & histamine
Somatostatin (SST) (release-inhibiting)Somatostatin (SST) (release-inhibiting)
Action of acetAction of acetylcholine, gastylcholine, gastrin & histamirin & histamine on the perine on the perietal cellsetal cells
Gastric secretion in the digestive phaseGastric secretion in the digestive phase
Cephalic phaseCephalic phase
Mechanisms:Mechanisms:
CConditioned & onditioned & UUnconditioned reflexnconditioned reflex
VVagal efferent & with agal efferent & with GGastrin secretionastrin secretion
through through gastrin-releasing peptide (GRgastrin-releasing peptide (GRP)P)
Experiment: Experiment: Sham feeding Sham feeding by Pavlovby Pavlov
Characteristics:Characteristics:
LLarge quantity (30%)arge quantity (30%)
HHigh acidity & digestive powerigh acidity & digestive power
Gastric phaseGastric phase Mechanisms:Mechanisms: DDistensionistension of gastric fundus & body initiatingof gastric fundus & body initiating vagovagal & local plexus reflexesvagovagal & local plexus reflexes DDistension of pylorus initiating a release of istension of pylorus initiating a release of gastrin through intrinsic plexusgastrin through intrinsic plexus CChemical stimulation of G cells initiaying ahemical stimulation of G cells initiaying a release of gastrinrelease of gastrin Experiment:Experiment: Pavlov pouchPavlov pouch Characteristics:Characteristics: LLarge quantity (60%)arge quantity (60%) HHigh acidity & digestive powerigh acidity & digestive power
Intestinal phaseIntestinal phase
Mechanisms: Mechanisms:
Mainly humoral regulationMainly humoral regulation
CChemical & hemical & MMechanical stimulationechanical stimulation
initiating releases of initiating releases of GGastrin, astrin, EEntero-ntero-
oxyntin & oxyntin & OOther humoral factorsther humoral factors
Experiment: Experiment: Pavlov pouchPavlov pouch
Characteristics:Characteristics:
SSmall quantity (10%)mall quantity (10%)
LLower acidity & digestive powerower acidity & digestive power
Cephalic, Cephalic, gastric, & gastric, & intestinal intestinal phase of phase of gastric gastric juice juice secretionsecretion
Summary of cephalic, gastric, & intestinal Summary of cephalic, gastric, & intestinal phase of gastric juice secretionphase of gastric juice secretion
Inhibitory regulationInhibitory regulation
Hydrochloric acid (HCl)Hydrochloric acid (HCl)
A typical example of negative feedbackA typical example of negative feedback
Conditions & Mechanisms:Conditions & Mechanisms:
pH ≤ 1.2~1.5 in the gastric antrumpH ≤ 1.2~1.5 in the gastric antrum
Inhibition of G cells, Release of SSTInhibition of G cells, Release of SST
pH ≤ 2.5 in the duodenumpH ≤ 2.5 in the duodenum
Release of secretin, bulbogastroneRelease of secretin, bulbogastrone
Fat: Fat: Initiating release of Initiating release of enterogastroneenterogastrone
Hypertonic solution:Hypertonic solution: Entero-gastric reflexEntero-gastric reflex
Gastric motility & emptyingGastric motility & emptying
** Main types of Main types of gastric motilitygastric motility
Tonic contractionTonic contraction
Maintaining the shape & place of stomach, Maintaining the shape & place of stomach,
as well as the intragastric pressure as well as the intragastric pressure
Receptive relaxationReceptive relaxation
Definition & its innervationDefinition & its innervation
Peptide or NO –ergic vagal fibersPeptide or NO –ergic vagal fibers
Gastric peristalsisGastric peristalsis
Initiate from middle stomach, 3 times/minInitiate from middle stomach, 3 times/min
Mix & grind food into chyme & help empty Mix & grind food into chyme & help empty
** Gastric emptyingGastric emptying & its control & its control
Definition & some rulesDefinition & some rules
Carbohydrate > proteins > lipids, Mix 4~6 hCarbohydrate > proteins > lipids, Mix 4~6 h
Discontinuous emptyingDiscontinuous emptying
Adapting for digestion & absorptionAdapting for digestion & absorption
Control of emptyingControl of emptying
Promotion in stomach:Promotion in stomach: vagovagal & plexus vagovagal & plexus
reflex, gastrin release reflex, gastrin release
Inhibition in duodenum:Inhibition in duodenum: entero-gastric entero-gastric
reflex & enterogastronereflex & enterogastrone
** Gastric motilityGastric motility in the digestive interphase in the digestive interphase
Migrating motor complex (MMC)Migrating motor complex (MMC)
Phase I:Phase I: resting phase, 90~120 min resting phase, 90~120 min
Phase II:Phase II: irregular spike & sporadic irregular spike & sporadic
peristalsis, 30~45 min peristalsis, 30~45 min
Phase III:Phase III: large amount of spikes & regular large amount of spikes & regular
strong contraction, 5~10 minstrong contraction, 5~10 min
Phase IV:Phase IV: transition period, about 5 min transition period, about 5 min
Significance:Significance: act as a street cleaner act as a street cleaner
VomitingVomiting
Definition: Definition: a reflex, benefit & harma reflex, benefit & harm
§ 4. Digestion in the intestine§ 4. Digestion in the intestine Pancreatic juicePancreatic juice & i& its secretionts secretion ** Nature, Compositions & functionsNature, Compositions & functions pH 7.8~8.4, colorless & odourless, 1~2 L/daypH 7.8~8.4, colorless & odourless, 1~2 L/day Bicarbonate (HCOBicarbonate (HCO33
)) Neutralize HCl & provide a weak basic Neutralize HCl & provide a weak basic medium favoring digestive enzyme actionmedium favoring digestive enzyme action Pancreatic enzymes: Pancreatic enzymes: amylase, lipase, colipase,amylase, lipase, colipase, trypsinogen & chymtrypsinogen, etc.trypsinogen & chymtrypsinogen, etc. Turn trypsinogen into trypsin by entero-Turn trypsinogen into trypsin by entero- kinase, turn chymtrypsinogen into chym-kinase, turn chymtrypsinogen into chym- trypsin by trypsintrypsin by trypsin Trypsin inhibitor: Trypsin inhibitor: a polypeptidea polypeptide
** Regulation of pancreatic secretionRegulation of pancreatic secretion
Nervous regulationNervous regulation
Vagus nerve: Vagus nerve: ACh, gastrinACh, gastrin
Characteristics: HCharacteristics: H22O & HCOO & HCO33, enzymes, enzymes
Sympathetic nerve: Sympathetic nerve: ACh, NAACh, NA
Characteristics: weak effectCharacteristics: weak effect
Humoral reulationHumoral reulation
Secretin:Secretin: H H22O & HCOO & HCO33, enzymes, enzymes
Cholecystokinin (CCK):Cholecystokinin (CCK):
Characteristics: HCharacteristics: H22O & HCOO & HCO33, enzymes, enzymes
Feedback:Feedback: CCK-releasing peptideCCK-releasing peptide
BileBile secretionsecretion && gallbladder emptyinggallbladder emptying
** Nature, Compositions & functionsNature, Compositions & functions
Hepatic bile: Hepatic bile: pH 7.4, golden yellowpH 7.4, golden yellow
Bladder bile: Bladder bile: pH 6.8, color become darkerpH 6.8, color become darker
Compositions:Compositions: H H22O, ions, bile acid, bile O, ions, bile acid, bile
pigment, fatty acid, cholesterol, lecithin, pigment, fatty acid, cholesterol, lecithin,
mucoprotein, etc., mucoprotein, etc., but no enzymebut no enzyme
Functions of bile (mainly by bile salt): Functions of bile (mainly by bile salt):
Fat emulsification; lipid absorption;Fat emulsification; lipid absorption;
Promote the absorption of fat-soluble VitsPromote the absorption of fat-soluble Vits
** Control of bile secretion & gallbladder emptyingControl of bile secretion & gallbladder emptying Nervous regulationNervous regulation Vagus nerve: Vagus nerve: ACh, gastrinACh, gastrin Hepatic bile secretionHepatic bile secretion (small amounts) (small amounts) Gallbladder contractionGallbladder contraction (slightly) (slightly) Humoral reulationHumoral reulation Gastrin:Gastrin: direct to hepatic cells & gallbladder;direct to hepatic cells & gallbladder; indirect to stomachindirect to stomachHClHClsecretin secretin Secretin:Secretin: act to bile duct & not to hepatic cells, act to bile duct & not to hepatic cells, so: Hso: H22O & HCOO & HCO33
, bile salt (, bile salt ()) Cholecystokinin (CCK):Cholecystokinin (CCK): gallbladder gallbladder contraction & Oddi’s sphincter dialationcontraction & Oddi’s sphincter dialation Bile salt:Bile salt: enterohepatic circulation of bile salt enterohepatic circulation of bile salt
SecretionSecretion ofof the small intestinethe small intestine
** Intestinal glands Intestinal glands
BBrunner’s glandrunner’s gland (basic mucus) (basic mucus)
LLieberkuhn Cryptieberkuhn Crypt (main part) (main part)
** Nature, Compositions & functionsNature, Compositions & functions
pH 7.6, iso-osmotic solution, 1~3 L/daypH 7.6, iso-osmotic solution, 1~3 L/day
EEnterokinase,nterokinase, the only one secreted the only one secreted
OOligopeptidase,ligopeptidase, ooligosaccharidaseligosaccharidase those those
contained in the epithelial cellscontained in the epithelial cells
** Regulation of secretion of small intestine Regulation of secretion of small intestine
Mainly through Mainly through plexus reflexplexus reflex initiated by initiated by
inflation of the intestineinflation of the intestine
Movements ofMovements of the small intestinethe small intestine
** Movement in the digestive interphase Movement in the digestive interphase
MMCMMC similar to that in the stomach similar to that in the stomach
** Main movement forms in digestive phase Main movement forms in digestive phase
TTonic contraction:onic contraction: the same as in stomach the same as in stomach
SSegmentation:egmentation: peculiar to the small intestine peculiar to the small intestine
PPeristalsis: 0.5~2.0 cm/seristalsis: 0.5~2.0 cm/s
PPeristaltic rush:eristaltic rush: 2~25 cm/s 2~25 cm/s
** Regulation of small intestinal movement Regulation of small intestinal movement
** Regulation of small intestinal movement Regulation of small intestinal movement
Intrinsic plexus: Intrinsic plexus: receivereceive mechanical &mechanical &
chemical stimuli and then cause peristalsischemical stimuli and then cause peristalsis
Extrinsic plexus: Extrinsic plexus: parasympathetic nerveparasympathetic nerve intestinal activitiesintestinal activities; sympathetic never; sympathetic never intestinal activitiesintestinal activities Humoral factors: Humoral factors: gastrin, CCK, Enk, 5-HT, etc.gastrin, CCK, Enk, 5-HT, etc.
Functions ofFunctions of the the ileocecal sphincterileocecal sphincter
** Permit some of chyme to squirt into the cecum Permit some of chyme to squirt into the cecum
each time when a peristalsis wave reaches iteach time when a peristalsis wave reaches it
** Prevent reflux of colonic contents into the ileum Prevent reflux of colonic contents into the ileum
§ 5. Digestion in the colon§ 5. Digestion in the colon
Main functionMain function of the colonof the colon
AAbsorb waterbsorb water
AAbsorb Vit B & K synthesized by bacteriabsorb Vit B & K synthesized by bacteria
FFormation & temporarily storage fecesormation & temporarily storage feces
SecretionSecretion of the colonof the colon
ppH 8.3~8.4H 8.3~8.4, , MMucus & HCOucus & HCO33
PProtect mucosa against injuries & rotect mucosa against injuries & llubricate fecesubricate feces
MovementsMovements of the colonof the colon
HHaustral to and fro movement, austral to and fro movement, SSegmentation oregmentation or
PPolyhaustral pushing movement, olyhaustral pushing movement, PPeristalsis, eristalsis,
MMass peristalsisass peristalsis
Feces &Feces & DefecationDefecation ** Feces:Feces: contain inorganic material, undigested contain inorganic material, undigested plant fibers, bacteria, water, etc.plant fibers, bacteria, water, etc. ** Defecation:Defecation: a reflex initiated by distension of a reflex initiated by distension of the rectum with fecesthe rectum with feces RReflex process, eflex process, PPrimary & rimary & hhigher centersigher centers Action of colonic bacteriaAction of colonic bacteria ** Benefit:Benefit: synthesis Vit. Bsynthesis Vit. Bcoco & K & K ** Harm:Harm: produce some poisonous substancesproduce some poisonous substances Action of dietary fibers on the intestinal functionAction of dietary fibers on the intestinal function RReduce absorption of water; educe absorption of water; EEnhance colonicnhance colonic movement; movement; IIncrease the volume of feces; ncrease the volume of feces; LLowow the caloric ratio of the foodthe caloric ratio of the food
§ 6. Absorption in the small intestine§ 6. Absorption in the small intestine
Sites of absorptionSites of absorption
OOral cavity & ral cavity & SStomach: littletomach: little
DDuodenum & uodenum & UUpper jejunum: most nutrientspper jejunum: most nutrients
IIleum: bile salts & Vit. Bleum: bile salts & Vit. B1212
CColon: water & electrolytesolon: water & electrolytes
Proofs as the main absorptive regionProofs as the main absorptive region
HHuge absorptive surface (200 muge absorptive surface (200 m22) )
PPlenty of capillaries & lymph capillarieslenty of capillaries & lymph capillaries
LLarge quantity of digestive fluid (6~8 L/day)arge quantity of digestive fluid (6~8 L/day)
FFood has almost completely been digestedood has almost completely been digested
Absorption of main nutrientsAbsorption of main nutrients
** WaterWater
88 L/day, L/day, ppassive & assive & iiso-osmotic absorbedso-osmotic absorbed
DDifferent absorbability in different partsifferent absorbability in different parts
** Inorganic slatsInorganic slats
SSodium:odium: 95%~99%, jejunum>ileum>colon 95%~99%, jejunum>ileum>colon
active transportactive transport
FFerrum:errum: 1/10, mainly in duodenum & jejunum, 1/10, mainly in duodenum & jejunum,
transferrintransferrin dependent, active transport dependent, active transport
CCalcium:alcium: promote by promote by Vit. DVit. D, active transport, active transport
AAnions:nions: mainly mainly Cl Cl & & HCOHCO33, passive transport , passive transport
** CarbohydrateCarbohydrate
AAbsorptive form:bsorptive form: monosarccharide monosarccharide
MMechanism:echanism: secondary active transport secondary active transport
** ProteinProtein
AAbsorptive form:bsorptive form: amino acid amino acid
MMechanism:echanism: secondary active transport secondary active transport
** FatsFats
AAbsorptive form:bsorptive form: glycerol, monoglyceride, glycerol, monoglyceride,
fatty acid, cholesterolfatty acid, cholesterol
MMechanism:echanism: passive diffusion passive diffusion
PPathway:athway: blood & lymph blood & lymph