Download - Chapter 21 The Digestive System
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Ch 21: DigestionCh 21: Digestion
Anatomy of digestive system (review!)
4 major processes of digestive system:
1.1. MotilityMotility
2.2. SecretionSecretion
3.3. DigestionDigestion
4.4. AbsorptionAbsorption
Fort MackinacMackinac Is, MI
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GI AnatomyGI Anatomy
Fig 21-1
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MotilityMotility
2 purposes:2 purposes:
Forward movement of foodForward movement of foodMechanical mixingMechanical mixing
GI smooth muscles contract GI smooth muscles contract spontaneouslyspontaneously
Pacemaker cells, (Interstitial Cells of Pacemaker cells, (Interstitial Cells of Cajal), connected by gap junctions, Cajal), connected by gap junctions, generate slow wave potentialsgenerate slow wave potentials
APs spread throughout longitudinal APs spread throughout longitudinal muscles (gap junctions) muscles (gap junctions) wave of wave of contractioncontraction
Like cardiac muscle, CaLike cardiac muscle, Ca2+ 2+ can regulate can regulate contraction strengthcontraction strength
1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption
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Different Patterns of ContractionDifferent Patterns of Contraction
TonicTonic Contractions Contractions Sustained contraction, usually in the Sustained contraction, usually in the stomachstomach
PhasicPhasic Contractions ContractionsPeristaltic contractionsPeristaltic contractions
progressive waves moving along segments of progressive waves moving along segments of longitudinal layer longitudinal layer forward propulsion forward propulsion
circular layer contracts proximal to circular layer contracts proximal to bolusbolusEspecially esophagusEspecially esophagus
Segmental contractionsSegmental contractions alternate contraction & relaxation lead to mixingalternate contraction & relaxation lead to mixing A side effect of narcoticsA side effect of narcotics
Fig 21-4
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SecretionSecretion
9 L of fluid pass through the GIT 9 L of fluid pass through the GIT (only 2 L from food & drink) (only 2 L from food & drink) Secretion and Reabsorption Secretion and Reabsorption importantimportant
Ions and water; similar to renal Ions and water; similar to renal mechanismsmechanisms
SalivaSaliva Hydrochloric Acid (Parietal Hydrochloric Acid (Parietal
Cells)Cells) Bicarbonate Bicarbonate (enzyme necessary ?) (enzyme necessary ?) Enzymes Enzymes (zymogens)(zymogens) Mucus Mucus (Goblet cells)(Goblet cells) Bile Bile (bile salts – function?) (bile salts – function?)
Net Fluid Balance in GI systemNet Fluid Balance in GI system
Fig 21-5
1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption
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Regulation of GIT Regulation of GIT p 689p 689
ANSANS Parasympathetic (R & R)Parasympathetic (R & R) SympatheticSympathetic
Emotional (cephalic reflexes)Emotional (cephalic reflexes) E.g., smell of foodE.g., smell of food
ENS (Enteric Nervous System)ENS (Enteric Nervous System) Self-contained (intrinsic)Self-contained (intrinsic)
GI peptides can have regulatory GI peptides can have regulatory role as hormones or paracrinesrole as hormones or paracrines E.g., Gastrin, CCKE.g., Gastrin, CCK
Table 21-1Table 21-1
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Digestion OverviewDigestion Overview
Mechanical breakdown and mixing aid Mechanical breakdown and mixing aid enzymatic breakdownenzymatic breakdown
ChewingChewing
Tonic contractions, esp. stomachTonic contractions, esp. stomach
Enzymatic breakdown converts Enzymatic breakdown converts macromolecules into absorbable unitsmacromolecules into absorbable units
Bile emulsifies fatsBile emulsifies fats
Optimal pH of enzymes indicates location of Optimal pH of enzymes indicates location of activityactivity
1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption
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Absorption OverviewAbsorption Overview
Most nutrient absorption takes place in ?Most nutrient absorption takes place in ? Fats absorbed into lactealsFats absorbed into lacteals Everything else absorbed into portal veinEverything else absorbed into portal vein
Alcohol & aspirin across gastric epitheliumAlcohol & aspirin across gastric epithelium
Additional: HAdditional: H22O, ions & some vitamins O, ions & some vitamins
absorbed in ________________absorbed in ________________
Mechanisms analogous to renal absorptionMechanisms analogous to renal absorption
1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption
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CHO CHO Digestion & AbsorptionDigestion & Absorption
~50% of calories in average American ~50% of calories in average American dietdiet
Starch (polysaccharide) and sucrose Starch (polysaccharide) and sucrose (disaccharide)(disaccharide)
Cellulose (roughage) not digestibleCellulose (roughage) not digestible
Enzymes: Enzymes: amylases, disaccharidasesamylases, disaccharidases (maltase, sucrase, lactase)(maltase, sucrase, lactase)
Absorbed only as monosaccharides Absorbed only as monosaccharides (glucose, fructose)(glucose, fructose)
Small intestineSmall intestine
Fig 21-14
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Protein Digestion and AbsorptionProtein Digestion and Absorption
Variable digestibilityVariable digestibility30-60% of protein not from diet30-60% of protein not from dietFirst digestion in Stomach by HClFirst digestion in Stomach by HClProteases secreted as proenzymesProteases secreted as proenzymes
Pepsin(-ogen), trypsin, etc.Pepsin(-ogen), trypsin, etc.
Absorption of single a.a. and di- and Absorption of single a.a. and di- and tripeptidestripeptides
Specific receptors required for larger Specific receptors required for larger chainschains
May serve as May serve as allergens allergens (Ch 24)(Ch 24)DDAVPDDAVP
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Lipid (fat) DigestionLipid (fat) Digestion
Mostly triglycerides in dietMostly triglycerides in diet Cholesterol, Fat-soluble Cholesterol, Fat-soluble
vitamins, others.vitamins, others.
Combination with bile salts Combination with bile salts creates an emulsioncreates an emulsion
Colipase and lipase allow Colipase and lipase allow formation of small micellesformation of small micelles
Absorption of fat via Absorption of fat via diffusion across apical CMdiffusion across apical CM
Chylomicrons in the cell are Chylomicrons in the cell are absorbed into lactealsabsorbed into lacteals
See fig 21-19
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Other StuffOther Stuff
Water soluble vitamins—Water soluble vitamins—mediated transportmediated transport
Fat soluble vits. via absorptionFat soluble vits. via absorption
Water,Ions and MineralsWater,Ions and Minerals Various locations and methods, Various locations and methods,
e.g, diffusion, carrier proteinse.g, diffusion, carrier proteins
Nucleic AcidsNucleic Acids
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Colon CancerColon Cancer
22ndnd largest cause of cancer deaths largest cause of cancer deaths
Cellulose (indigestible) = fiber, Cellulose (indigestible) = fiber, roughageroughage
Significance of “roughage” in diet??Significance of “roughage” in diet??
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Phases of Digestion/AbsorptionPhases of Digestion/Absorption
1.1. CephalicCephalic
2.2. GastricGastric
3.3. IntestinalIntestinal
4.4. DefecationDefecation
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1.1. CephalicCephalic
2.2. GastricGastric
3.3. IntestinalIntestinal
4.4. DefecationDefecation
•Anticipation
•Salivation
•Mastication
•Mechanical digestion
•Deglutition
•Peristalsis in esophagus
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Reflux EsophagitisReflux Esophagitis = = Heartburn = GERDHeartburn = GERD
Lower esophageal Lower esophageal sphincter dysfunctionsphincter dysfunction
Why reflux against gravity?Why reflux against gravity?
See p.700
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1.1. Intrapleural pressure Intrapleural pressure during inspiration during inspiration
Wall of esophagus Wall of esophagus expands expands
Subatmospheric Subatmospheric pressure in pressure in esophageal lumen sucks esophageal lumen sucks acidic stomach contents into acidic stomach contents into esophagusesophagus
GERD, cont’d
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1.1. CephalicCephalic2.2. GastricGastric3.3. IntestinalIntestinal4.4. DefecationDefecation
•Storage
•Relaxation or contraction as needed.
•Digestion
•HCl (Parietal cells)
•Pepsin (Chief cells)
•Mechanical
•Immune Protection
•Ingested pathogens
•Respiratory mucus
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1.1. CephalicCephalic2.2. GastricGastric3.3. Intestinal – Intestinal – Chyme (ingesta) enters small intestineChyme (ingesta) enters small intestine4.4. DefecationDefecation
o Neutralization of HCl
o NaHCO3 from pancreas
o Running Problem: Peptic Ulcer and antacids, including H2 antagonists and proton pump inhibitors
o Pancreatic enzymes
o Digest protein, CHO
o Bile acids
o Emulsion of Fat
o Absorption of H2O
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Bacterial fermentation of CHO and proteins Bacterial fermentation of CHO and proteins
Lactate, some vitamins are digested and/or absorbedLactate, some vitamins are digested and/or absorbed
More HMore H22O absorptionO absorption
Osmotic diarrhea vs. secretory diarrheaOsmotic diarrhea vs. secretory diarrhea Osmotic-solutes prevent HOsmotic-solutes prevent H22O reabsorptionO reabsorption
Lactose intoleranceLactose intolerance
Secretory- bacterial toxins ("flush out' pathogens)Secretory- bacterial toxins ("flush out' pathogens) Defecation ReflexDefecation Reflex
Fig 21-21
1.1. CephalicCephalic2.2. GastricGastric3.3. IntestinalIntestinal4.4. DefecationDefecation
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Lactose IntoleranceLactose Intolerance
Lactose = glucose + galactoseLactose = glucose + galactose
Lactase only found in juvenile Lactase only found in juvenile mammalsmammals
Adaptive (dominant) mutation in Adaptive (dominant) mutation in populations with dairy-based populations with dairy-based cultures cultures
Lactose intolerance in Lactose intolerance in 95% of Native Americans,95% of Native Americans, 90% of Asian Americans90% of Asian Americans 70% of African Americans70% of African Americans 50% of Mexican Americans50% of Mexican Americans
Clinical focus p. 707
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