gi tract-2009
TRANSCRIPT
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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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