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The Gas t roin t e s t i nal Trac t

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Overv iew o f GI t r ac t

ReceivingConducting, storingDigestion, absorption

absorbing water, defecating

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T he Ph ary nx - r ec e iv in g

Mechanical breakdown of food.Lubrication of food.Initial digestion of polysaccharides(complex sugars).Salivary glands:

parotid (par= ear, otid=ear)submandibularsubmaxillary

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In i t i a l c a rbohyd ra t e d iges t i on

Parotid gland -large exocrine gland,releases water, mucous, and -amylase.Esophagus- conducting and storage.Muscular, skeletal muscle in anterior part,smooth muscle as near stomach.

Crop is a pouch off of the esophagus, actsas a storage organ in birds.

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Parietal cells:HCl andintrinsicfactor

Chief cells:pepsinogen

(converted toactiveenzyme,pepsin)

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Sm al l in t es t i ne - d ig es t i on and abso rp t ion

Receives secretory products from pancreas, liver(via gallbladder)most absorption of nutrients and water takesplace heremostly alkaline digestion of proteins, fats,carbos, because bicarbonate (HCO- 3 frompancreas) neutralizes stomach acid

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Sm al l in t es t i ne - d ig es t i on and abso rp t ion

duodenum- receives liver and pancreassecretions, secretes digestive enzymes jejunum and ileum-digestion and absorption

secretion- transport away from body;absorption, transport toward bloodstream

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Small intestine

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Brush border

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Glycoca lyx

Protein rich coat that covers microvilli.Contains dipeptidases, enzymes thatbreakdown disaccharides, mucous.

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Intestinal surface area

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Large in t e s t i ne

Most water reabsorbed in small intestine,but-the principal role of the large intestine isreabsorption of remaining water- 8 L of water may be secreted by GI tract in thecourse of a day.Consolidation of waste.

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3 t y pes o f m o vem ent

Peristalsis- traveling wave of constrictionsegmentation-asynchronous constrictionof gut tube- breaks down large foodparticlesregurgitation- reverse peristalsis

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Co nt r ol o f m o t i l i t y

PSNS stimulatesSNS inhibits. Exception- irritable bowelsyndrome- SNS stimulates movement of colonhormonal control

gastrin(stomach)- stimulates motilitysecretin (duodenum) inhibits motility

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Cel lu lar basi s for m o t i l i t y Smooth muscle throughout GI tract

GI smooth muscle is myogenic- contracts 12-3times per minuteGI smooth muscle acts as a functional

syncytium- nexus or gap junctions connectmuscle cellsneuronal and hormonal cues are not requiredfor contraction, but are important for changingrate of contraction

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Nut r i t i ona l Requi r em en t s

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Figure 4 .1 The c om pos i t ion o f t he adu l t human body

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Figure 4 .2 Am ino ac id

c hem is t ry (Pa r t 1 )

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Figure 4 .2 Am ino ac id c hem is t ry (Pa r t 2 )

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Glycine

Aminoacids

Alanine Serine Threonine Cysteine Proline Valine

Histidine Methionine AsparagineAspartateIsoleucineLeucine

Glutamine

Glutamate

Lysine TyrosineArginine Phenylalanine Tryptophan

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Fig u re 4 .3 Fa t t y a c id s a n d t r i a c y lg ly c e ro l s (Pa r t 1 )

Fig u re 4 3 Fa t t y a c id s a n d t r i a c y lg ly c e ro l s (Pa r t 2 )

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Fig u re 4 .3 Fa t t y a c id s a n d t r i a c y lg ly c e ro l s (Pa r t 2 )

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Figure 4 .4 Carbohydra t e c hem is t ry

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Ex oc r ine Sec re t ions

Salivary glandsStomachPancreasLiver and gall bladderSmall intestine

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S t o m a c h

Chief cellsPepsinogen (inactive) converted to pepsin(active) form of a proteolytic enzyme.

Parietal cellsIntrinsic factor- required for vitamin B12absorption

Hydrochloric acidMucous cells

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Di sac c h ar id e b reak d ow n

Starches broken down into disaccharides bysalivary amylase (30%) and pancreatic amylase(70%)Maltase- breaks down maltose to two glucose

molecules.Lactase- breaks down lactose into galactose andglucose

Sucrase- breaks down sucrose into fructose andglucose.

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Peptides and carbohydrates are broken down by enzymatichydrolysis

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Panc r ea t i c s ec r et i on s

EndoproteasesCleave internal peptide bondsTrypsin- trypsinogen converted to trypsin byenterokinase. Attacks on carboxyl side of arginine or lysine.Chymotrypsin- chymotrypsinogen converted tochymotrypsin in small intestine by trypsin.

Attacks on carboxyl side of tyrosine,phenylalanine, tryptophan, leucine ormethionine.

Panc r ea t i c s ec r et i on s

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Panc r ea t i c s ec r et i on s

(cont )Carboxy- and aminopeptidases- cleave outsideamino acids.Lipases- break down triglycerides into fattyacids and monoglycerides

Trypsin inhibitor- stored in pancreatic secretorygranules. Prevents early intracellular activationof trypsin.

Also lots of water and bicarbonate ions toneutralize and dilute acids in S.I.

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Liver and b i l e

Bile is produced in the liver, stored in gallbladder.Bile salts- one of the most importantcomponents of bile, required for fat digestion.Bile is very alkaline, neutralizes acid in S.I.Bile transports waste products from liver,

cholesterol, bilirubin, drugs, steroids, andvitamins.

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Bi le s al t s

Bile salts are amphipathic, help solubilizefatty acids for uptake.Fatty acids and monoglycerides have a

strong tendency to re-associate afterlipolysis- bile salts prevent fatty acids andmonoglycerides from reassociating.

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Cholic acid

Diges t ion absorp t ion and

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Diges t ion , absorp t ion , and

a s s i m i l a t i o n

Carbohydrate digestion and absorption.Fat digestion and absorption.Protein digestion and absorption.

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SGLT- sodium

dependentglucosetransporter.

Glut- facilitateddiffusiontransporter

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Fructose transport jejunumGlut 5

Insulin dependentMuscle,

adipose

Glut 4

High affinityglucose transport

BrainGlut 3

Low affinityglucose transport

Small intestineGlut 2

High affinity

glucose transport

Brain, RBCGlut 1

GLUT family of proteins

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Fa t d igest ion

Emulsification- breaking up large lipiddroplets so that enzymes can digest them.Enzymatic digestion (hydrolysis) of triglycerides- pancreatic lipaseMicelle formation-hydrolysis products formwith bile salts to make micelles .

FAs and glycerol and monogyceridesabsorbed.

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A p o A

Ap o A

A p o

A

A p o C

A p o C

HDL

LDL A p o

B

A p o

A

A p o

B

A p o B

A p o B

A p o B A p o B

A p o

BA p o B

ApoC

A p o C

VLDL

CHYLO A p o

A

A p oA

A p o

B

A p o C

A p o C

A p o

B

A p o B

A p o

C A p

o B

Ap o A

Albumin

The Fate of Dietary Lipids

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Pro t e in d iges t ion

Pepsin responsible for about 20% of allprotein digestion.Pancreatic proteases and peptidases-

chymotrypsin, trypsin, carboxy- andaminopeptidases. Amino acids and di- or tripeptides uptakeby specific transporters in the enterocyte.

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Virtually all of the watersecreted from exocrineglands is reabsorbed in theGI tract.

H or m o na l c o nt r ol o f

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d iges t i on

Phases of digestion.Cephalic phase-sensory input from food.Gastric phase-food enters stomach.

Intestinal phase-chyme enters duodenum.

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Cepha l ic phase

Increase acid secretion in stomachIncrease enzyme secretion, but low fluidvolume from pancreas.

Small increase in bile secretion andcontraction of gall bladder.

Hormonal and neuronal control of stomach secretions

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Parietalcell

ECLcell

G cell

D cell

HCl

Histamine

Gastrin

Somatostatin(SS)

+

++

-

ACh

+

+

-

+

Vagusnerve

Sensory stimuli

ECL, enterochromaffin-like cell

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Gas t r ic ho rm ones

Gastrin-stimulates HCl secretion,stimulates stomach motilitySomatostatin- inhibits HCl secretion;

inhibits gastrin and histamine secretion; and,inhibits gastric emptying.

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Gas t r i c phase

Identical to cephalic phase, but stimulus isstomach distention

Cerebral

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cortex

Brain stem

Vagusnerve

Acidsecretion

Stomach gastrin

Cephalic phase

Stomachdistention

gastric phase

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In t e s t i nal phase Chyme enters the duodenum-2 subphasesEarly intestinal phase

pH > 3.0, gastrin from duodenum stimulatesHCl secretion

Late intestinal phasepH < 2, acid stimulates secretin releasefrom the duodenumFatty acids, amino acids, stimulatecholecystokinin secretion from duodenum

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In t e s t i nal phase

Late intestinal phaseFatty acids and sugars stimulate gastricinhibitory peptide (GIP)

Fatty acids, amino acids, stimulatecholecystokinin secretion from duodenum Vagus nerve releases the neuropeptide

vasoactive intestinal polypeptide (VIP)

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Summary

CCK, Secretin, GIP, and VIP all inhibit acidsecretion: They stimulate SS release.

CCK and Secretin integrate the function of the stomach, pancreas, and gall bladder.

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