Chapter 21

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THIRD EDITION
HUMAN PHYSIOLOGY
AN INTEGRATED APPROACH
Dee Unglaub Silverthorn, Ph.D.
Chapter 21
Digestion
PowerPoint® Lecture Slide Presentation by
Dr. Howard D. Booth, Professor of Biology, Eastern Michigan University
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Digestive Anatomy: Overview
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Figure 21-2a: ANATOMY SUMMARY: The Digestive System
Processes of the Digestive System: Overview
• Motility
• Secretion
• Absorption
• Elimination
Figure 21-1: Processes of the digestive system
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Digestive Secretions:
(7 L / Day From Tissues into Lumen)
• Salivary glands
• Pancreas
• Water
• Enzymes
• Mucus
• Ions: H+, K+, Na+
•
HCO3-, Cl-
• Mass Balance (H2O)
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Figure 21-5: Daily mass balance in the digestive system
Digestive Anatomy: Histological Overview
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Figure 21-2c: ANATOMY SUMMARY: The Digestive System
The GI tract has four layers
• Mucosa
• Epithelial cells
• Lamina propria
• Muscularis mucosae
• Submucosa
• muscularis externa
• Serosa (connective tissue
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Digestive Anatomy: Histological Overview
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Figure 21-2e: ANATOMY SUMMARY: The Digestive System
Motility: Smooth Muscle Contractions
• Parastalsis –
moves 
• Segmentation –
mixes
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Figure 21-4: Contractions in the GI tract
Chemistry of Digestion: Carbohydrates
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Figure 21-6: Carbohydrate digestion
Chemistry of Digestion: Proteins
• Proteins  enzyme
hydrolysis  amino acids
• Enzymes: endopeptidases
& exopeptidases
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Figure 21-7: Endopeptidases and exopeptidases
Chemistry of Digestion: Fats
Figure 21-8: Fat digestion
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Regulating Digestion: CNS and Enteric Nervous
System (ENS)
Figure 21-9: The enteric nervous system
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Phases of Digestion: Overview
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Figure 21-11: Overview of functions in different regions of the digestive system
Chemical and mechanical digestion in the mouth
• Saliva
• Lubricates and softens food
• Salivary amylase
• Breaks down starches into smaller carbs
• Mechanical digestion; mastication
• Protection
• Lysosomes and immunoglobulins kill viruses
and bacteria.
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Swallowing reflex: Soft Palate & Esophagus
Figure 21-13: The swallowing reflex
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Gastric Phase: The Stomach
• Storage – sends chyme to the small intestine at
a comfortable pace that the duodenum can
handle.
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Hot Dogs
53 1/2 Nathan's Hot Dogs and Buns
12 minutes
Takeru Kobayashi
Digestion in the Stomach
• HCl or gastric acid – parietal C.
• Kills bacteria
• Denatures proteins
• Pepsin – chief C.
• Protein digestion; collagen breakdown
• Lipase – chief C. 10% of fat digestion
• Salivary amylase from the mouth upon exposure
to low pH is inactivated.
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Secretion in the Stomach
• Parietal cells – pH-1
• Chief cells – inactive pepsinogen  active pepsin
• D cells – somatostatin
• Enterochromaffin-like (ECL) cells – histamine
• G cells – the hormone gastrin.
• Mucuos cells – mucus and bicarbonate
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Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
The Stomach Secretions
Figure 21-17: Integration of secretion in the stomach
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Gastric Phase: The Stomach’s chemical barrier
Figure 21-15: The mucus-bicarbonate barrier of the gastric mucosa
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Figure 21-17: Integration of secretion in the stomach
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Intestinal Phase: ENDOCRINE RESPONSE
• The pyloric sphincter
sends chyme into the
duodenum in spurts.
• Acidic chyme in the
duodenum stimulates
hormone release into the
blood of:
• Secretin,
cholecystokinin, GIP
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Intestinal Phase: Reflexes Direct Digestive Action
Figure 21-18: The intestinal phase of gastric function
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Intestinal Phase: NEURAL RESPONSE
• High OsM of the chyme
• Neural osmoreceptors
• Sitmulates neural
secretion of an
unknown bloodborne substance
• Inhibits
gastric
motility
• Inhibits acid
secretion.
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Intestinal Phase: EXOCRINE RESPONSE
• Pancreatic bicarbonate secretions
• Secretin stimulates bicarbonate release.
• Neutralize HCl, add bile & enzymes
• Pancreatic exocrine enzyme secretions
• CCK stimulates pancreatic release of inactive enzymes
• Figure 21-20
• Liver adds bile via gall bladder
• CCK stimulates gall bladder contraction
• Bile; a non-enzyme
• Bile salts
• Act as detergents
• Bilirubin
• cholesterol
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Intestinal phase: intestinal secretions
• Bicarbonate
• Brush border enzymes
• Peptidases
• Disaccharidases
• Enteropeptidase
• Activates inactive pancreatic enzymes
• Stationary enymes
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Disaccharidases
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Carbohydrate absorption in the intestine
Figure 21-24: Carbohydrate absorption
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Hydrolysis to simple sugars
Absorption: transport
Na+/ glucose symport
Fructose GLUT
To ECF  capillary
Bile salts and fat digestion
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Figure 21-23: Bile salts
Let’s remember what “fat” is. . .
• Phospholipids
• Triglycerides
• cholesterol
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Diet
Content
Pritikin
U.S. Average
10-15%
33%
<3%
11%
Protein*
15-20%
15%
Carbohydrates*
65-75%
52%
Fiber
>40g/d
15.6g/d
High-fiber, wholegrains
≥ 5 serv./d
1.0 serv./d
Fruits & Vegetables
≥ 7 serv./d
4.8 serv./d
<100mg
265mg
Fat*
Saturated fat*
Cholesterol
* = % of calories
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NHANES IV (1999-2000), NHANES III (1988-94), CDC/NCHS.
U.S.D.A.’s Continuing Survey of Food Intake. (1994-1996).
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Figure 21-26: Fat digestion and absorption
Lipoproteins: the transport form of cholesterol
Liver makes VLDL
In the plasma: HDL and LDL
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Intestinal Phase: Fat Digestion & Absorption
• Hydrolyzed to small peptides, dipeptides, &
amino acids
• Membrane transport: H+, Na+ cotransports &
transcytosis
• To ECF then into capillary
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Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
The hepatic portal system
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Figure 21-19: The hepatic portal system
Intestinal Phase:
Large Intestine, H2O Absorption & Defecation
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Figure 21-27: Anatomy of the large intestine
Intestinal Phase:
Large Intestine Digestion & Absorption
• Bacterial digest significant amounts of complex
carbs and proteins through fermentation.
• Most water is reabsorbed
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Figure 21-28: NaCl reabsorption by colonocytes
Digestive Health: Protection & Problems
• Immune defense: M-cells, Peyer's patches,
lymphocytes
• Irritable bowel disease – chronic inflammation
• Diarrhea: leads to dehydration (4 million
deaths/yr)
• Osmotic-solutes prevent H2O reabsorption
• Secretory- bacterial toxins ("flush out'
pathogens)
• Vomiting (emesis) can lead to alkalosis
• Ulcers- H. pylori
• "heart-burn"  acid reflux disease
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