Download The Gastrointestinal System

PRINCIPLES OF
HUMAN PHYSIOLOGY
20
THIRD EDITION
Cindy L. Stanfield | William J. Germann
The Gastrointestinal
System
Part A
PowerPoint® Lecture Slides prepared by W.H. Preston, College of the Sequoias
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
Four Basic Digestive Processes
Figure 20.1
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Major Structures of the GI System
Figure 20.2
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Layers
Figure 20.3
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Mouth, Pharynx, Esophagus, and Stomach
Figure 20.4
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Esophagus
•
•
•
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Muscular tube from pharynx to
stomach
•
Upper 1/3—skeletal muscle
•
Lower 2/3—smooth muscle
Upper esophageal sphincter
•
Skeletal muscle
•
Between pharynx and
esophagus
Lower esophageal sphincter
•
Smooth muscle
•
Between esophagus and
stomach
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Figure 20.4 (1 of 3)
Gastric Pits
Figure 20.4 (2–3 of 3)
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Acidic Environment of Stomach
• pH = 2
• Necessary for activating pepsinogen
• Denatures proteins
• Kills bacteria
• Gastric mucosal barrier
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•
Protective layer of mucus and bicarbonate
•
Secreted from neck cells and goblet cells
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Pyloric Sphincter
Regulates passage of chyme between
stomach and small intestine
Figure 20.4
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Small Intestine
Figure 20.5 (1 of 3)
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Small Intestine Wall
• Villi increase surface
area of epithelium
•
Contain blood
vessels and
lacteal for
absorption of
nutrients
• Microvilli increase
surface area of
epithelial cells
•
Form brush border
Figure 20.5
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Crypts of Lieberkuhn
• Epithelial cells in
crypts secrete
bicarbonate-rich fluid
•
Secreted in
proximal small
intestine
•
Absorbed in distal
small intestine
Figure 20.5
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Circulatory Route: Absorbed Material
Figure 20.6
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Features of the Colon
Figure 20.7
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Accessory Structures
Figure 20.2
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Structure of an Accessory Gland
Figure 20.8
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Characteristics of Saliva
• Rich in bicarbonate ions
• Contains mucus
• Enzymes
•
Salivary amylase
•
Lysozyme
Figure 20.9
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The Pancreas
• Has exocrine and
endocrine portions
• Exocrine portion
produces pancreatic
juice
•
Rich in bicarbonate
•
Enzymes
• Pancreatic
amylase and
lipases
• Proteases
• Nucleases
Figure 20.10
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GI Organs and Functions
Table 20.1
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Carbohydrates
• Typical diet: 250–800 grams carbohydrates
• Most consumed as disaccharides or
polysaccharides
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Sucrose
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Lactose
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Maltose
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Starch
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Glycogen
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Cellulose (fiber, cannot be digested)
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Carbohydrate Digestion
• Only monosaccharides absorbed
• Disaccharides and polysaccharides must
be digested to monosaccharides
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Digestion of Starch
• Enzyme of digestion
•
Salivary amylase
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Pancreatic
amylase
• End product
•
Disaccharides
(maltose)
•
Limit dextrins
Figure 20.12
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Further Digestion
• Enzymes of digestion
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Dextrinase – limit dextrins  glucose
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Glucoamylase – polysaccharides  glucose
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Sucrase – sucrose  fructose + glucose
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Lactase – lactose  galactose + glucose
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Maltase – maltose  2 glucose
• Location of enzymes—brush border of small
intestine
•
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“Brush border enzymes”
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Absorption of Monosaccharides
• Absorption = transport from lumen to blood
• Glucose and galactose absorbed by
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Secondary active transport across apical membrane
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Facilitated diffusion across basolateral membrane
• Fructose absorbed by
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Facilitated diffusion across both membranes
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Digestion / Absorption of Maltose
Figure 20.13
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Proteins
• Typical diet: 125 grams/day protein
•
Only require 40–50 grams
• Proteins to be digested and absorbed include
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Consumed in diet
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Secreted into lumen of intestinal tract
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Sloughed off with cells lining intestinal tract
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Protein Digestion in Stomach
• Protein digestion begins in stomach
• Enzyme = pepsin
• Inactive (secreted form) = pepsinogen
• Activated by acid
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Pepsin: Activation and Activity
• Chief cells secrete
pepsinogen
•
Pepsinogen is the
inactive form
•
HCl cleaves
pepsinogen to
pepsin
•
Pepsin = active
form
• Parietal cells secrete
HCl
Figure 20.14
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Protein Digestion in Small Intestine
• Pancreatic proteases
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Trypsin
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Chymotrypsin
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Carboxypeptidase
• Brush border proteases
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Aminopeptidase
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Enterokinase
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Lipids
• Typical diet: 25–160 grams lipids
•
90% triglycerides
• Lipids face special problem in digestion
and absorption
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•
Not water soluble
•
Do not mix with stomach, intestinal contents
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Form fat droplets
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Lipid Digestion
• Enzymes of digestion = lipases
• Secreted from pancreas
• Lipases can only act on molecules near edge
of fat droplet
• Bile salts increase surface area of droplets by
breaking large droplet into several small droplets
= emulsification
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Bile Salts
• Synthesized in liver
from cholesterol
• Secreted in bile
to duodenum
• Amphipathic
molecule
• Emulsify fat
Figure 20.16
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Emulsification of a Fat Globule
• Before: little surface
exposed to lipases
• After: much more
surface exposed to
lipases
Figure 20.17
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Digestion by Lipases
Triglycerides  Monoglyceride + 2 Fatty acids
• Some fatty acids and
monoglycerides absorbed
• Others form micelles
• Equilibrium between
micelles and fatty acids and
monoglycerides formed by
action of lipases
Figure 2.4b
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Absorption of Monoglycerides and Fatty Acids
• Equilibrium between free fatty acids and
monoglycerides and those in micelles
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Free form can be absorbed by simple diffusion
across epithelium
• Inside epithelial cell
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•
Enter smooth ER and reform triglycerides
and other lipids
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Lipids enter Golgi apparatus to be packaged
into chylomicrons
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Lipase Activity
Figure 20.18
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Enterohepatic Circulation
Figure 20.19
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Absorption of Fat
Lumen
Monoglyceride
Fatty acid
Smooth
endoplasmic
reticulum
Other lipids
and proteins
Golgi
apparatus
Triglyceride
Chylomicron
Enterocyte
Blood
Interstitial fluid
Lacteal
To general circulation
Lymph
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Figure 20.20
Absorption of Vitamins
• Fat-soluble vitamins (A, D, E, and K)
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Absorbed with lipids
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Dissolve in lipid droplets, micelles, chylomicrons
• Water-soluble vitamins
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Require special transport proteins
• Vitamin B12
•
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Absorbed only when bound to intrinsic factor
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Absorption of Minerals: Na+, Cl-, and K+
• Sodium
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Solvent drag with water reabsorption
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Actively absorbed in jejunum, ileum, and colon
• Chloride
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Passively follows sodium absorption
• Potassium
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Passively absorbed
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Secreted when lumenal concentrations very low
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Absorption of Minerals: Calcium
• Actively absorbed in duodenum and jejunum
• Binds to brush border protein = calcium-binding
protein
•
Transported into epithelial cell
• Transported out of cell across basolateral
membrane by Ca2+ pump
• 1,25-(OH)2D3 increases calcium absorption by
increasing concentration of calcium-binding
protein
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Absorption of Minerals: Iron
• Transferrin secreted by enterocytes into lumen of
small intestine
• Transferrin binds iron
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Transferrin-iron complex binds receptor
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Taken into cell by receptor-mediated endocytosis
• Some iron stored in enterocyte as ferritin
• Some iron transported into blood
•
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Transported in blood bound to transferrin
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Absorption and Secretion of Bicarbonate
• Jejunum
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Bicarbonate ions passively absorbed
• Ileum and colon
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Bicarbonate secreted in exchange for chloride ions
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Absorption of Water
• Water in GI tract
•
7 liters secretions
•
2 liters intake
• Water absorption is passive
• Water follows absorption of solutes by osmosis
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GI Regulation
• Regulation of GI function is not based on the
concept of homeostasis
• Regulate GI function to maximize absorption,
regardless of whether nutrients are needed
• Regulate conditions in lumen of GI tract
to maximize absorption
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GI Hormones and Actions
Table 20.2
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Neural and Endocrine Control
Figure 20.21
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Phases of Gastrointestinal Control
• Cephalic phase—stimuli originate in head
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Thoughts, taste, and smell
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Requires input from CNS (long reflexes)
• Gastric phase—stimuli originate in stomach
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Long and short reflex and GI hormones
• Intestinal phase—stimuli originate in small
intestine
•
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Long and short reflex and GI hormones
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Short-Term Regulation of Food Intake
• Short-term regulation: hunger versus satiety
• Satiety factors
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Insulin
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CCK
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Neural input from mechano- and chemo-receptors
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Long-Term Regulation of Food Intake
• Leptin = hormone released from adipose cells
when calories exceeds demands
• Leptin suppresses hunger and increases
metabolism
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Saliva Secretion
Taste and texture of food
Mechanoreceptors
and taste receptors in mouth
Salivary center of medulla
Autonomic nervous system
Stimulate salivation
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Gastric Acid Secretion
•
Parietal cells produce
hydrochloric acid
•
Carbonic anhydrase
catelyzes production
of bicarbonate and H+
•
H+ are actively
secreted into lumen of
stomach
•
Bicarbonate is
transported into
intersitital fluid for Cl-
•
Cl- diffuses into lumen
of stomach through
channels
Figure 20.22
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Gastric Secretion: Cephalic Phase
Figure 20.23a
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Gastric Secretion: Gastric Phase
Figure 20.23b
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Pancreatic Juice Secretion
• Acinar cells
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Small volume of
primary secretion
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Water,
electrolytes, and
digestive enzymes
• Duct cells
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Large volume
•
Bicarbonate rich
secretion
Figure 20.8
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Bicarbonate-Rich Fluid Secretion
Figure 20.24
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Regulation of Bile Entry
Figure 20.25
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Rates of Fluid Movement
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Fluid entering lumen
daily
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Fluid leaving lumen daily
•
Intestinal absorption
= 8500 mL
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Ingested = 2000 mL
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Saliva = 1500 mL
•
•
Gastric juice = 2000
mL
Colon absorption =
400 mL
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Total = 8900 mL
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Bile = 500 mL
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Pancreatic juice =
1500 mL
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Intestinal secretions
= 1500 mL
•
Total = 9000 mL
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Amount excreted
in feces =
9000 mL –
8900 mL =
100 mL
Fluid Flows in the GI System
Figure 20.26
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VII. Gastrointestinal Motility and Its Regulation
• Electrical activity in gastrointestinal
smooth muscle
• Peristalsis and segmentation
• Chewing and swallowing
• Gastric motility
• Motility of the small intestine
• Motility of the colon
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GI Motility
• GI motility
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Movements of the wall of the GI tract
•
Due primarily to the contractions of the muscularis
externa (outer muscle layers)
•
Function: to mix and propel
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Figure 20.3
GI Smooth Muscle Activity
Figure 20.27
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Peristalsis
Figure 20.28a
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Segmentation
Figure 20.28b
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Phases of Regulation
• Cephalic phase, excitations
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Anger, aggression
• Cephalic phase, inhibitions
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Pain, fear, depression
• Gastric phase, excitatory stimulus
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Distension of stomach
• Intestinal phase, inhibitory stimuli
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Distension of duodenum
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Contents: osmolarity, acidity, fat
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