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Chapter 6 Chapter 6 DIGESTION & DIGESTION & ABSORPTION ABSORPTION § 1. Introduction § 1. Introduction Concept of Concept of digestion digestion & & absorption absorption * * Mechanical & chemical Mechanical & chemical digestion digestion Characteristics of Characteristics of digestive digestive smooth muscles smooth muscles * * General characteristics General characteristics * * Electrophysiological Electrophysiological characteristics characteristics Resting potential (RP) Resting potential (RP) Slow wave or basic Slow wave or basic electrical rhythm (BER) electrical rhythm (BER)

Chapter 6 DIGESTION & ABSORPTION § 1. Introduction Concept of digestion & absorption Concept of digestion & absorption * Mechanical & chemical digestion

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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

IntrinsicIntrinsic (or enteric) (or enteric) nervous nervous systemsystem

Extrinsic Extrinsic nervous nervous systemsystem

§ 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

Peristalsis of the esophagusPeristalsis of the esophagus

§ 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)

Mucus-bicarbonate barrierMucus-bicarbonate barrier

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

Experiment of Sham feeding by PavlovExperiment of Sham feeding by Pavlov

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

Heidenhain (R) & Pavlov (L) pouchHeidenhain (R) & Pavlov (L) pouch

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

Migrating motor complex (MMC)Migrating motor complex (MMC)

§ 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

Enterohepatic Enterohepatic circulation of circulation of bile saltbile 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

Segmentation of the small intestineSegmentation of the small intestine

** 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

Sites of absorptionSites of absorption

Enlargement Enlargement of the surface of the surface area of the area of the intestineintestine

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

Digestion & absorption of fats in the intestineDigestion & absorption of fats in the intestine