1. 23
The Digestive
System: Part C

2. Pancreas Endocrine function Exocrine function
Pancreatic islets secrete insulin and glucagon
Exocrine function
Acini (clusters of secretory cells) secrete pancreatic juice
To duodenum via main pancreatic duct
Zymogen granules of acini cells contain proenzymes
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3. Small duct Acinar cell Basement membrane Zymogen granules Rough
Figure 23.26a Structure of the enzyme-producing tissue of the pancreas.
Small
duct
Acinar cell
Basement
membrane
Zymogen
granules
Rough
endoplasmic
reticulum
Duct cell
One acinus
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4. Acinar cells Pancreatic duct
Figure 23.26b Structure of the enzyme-producing tissue of the pancreas.
Acinar cells
Pancreatic
duct
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5. Watery alkaline solution (pH 8) neutralizes chyme
Pancreatic Juice
1200 – 1500 ml/day
Watery alkaline solution (pH 8) neutralizes chyme
Electrolytes (primarily HCO3–)
Enzymes
Amylase, lipases, nucleases secreted in active form but require ions or bile for optimal activity
Proteases secreted in inactive form
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6. Figure 23.27 Activation of pancreatic proteases in the small intestine.
Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
(inactive)
Chymotrypsinogen
Procarboxypeptidase
Trypsin
Chymotrypsin
Carboxypeptidase
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7. Chyme enter -ing duodenum causes duodenal
Figure Mechanisms promoting secretion and release of bile and pancreatic juice.
Slide 1
Chyme enter -ing duodenum causes duodenal
enteroendocrine cells to release cholecystokinin
(CCK) and secretin. 1 4
Bile salts and, to a lesser extent, secretin
transported via
bloodstream stimulate Liver to produce bile more rapidly. 5
CCK (via blood stream) causes gallbladder to contract and Hepatopancreatic
Sphincter to relax. Bile Enters duodenum.
CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2
CCK induces
secretion of enzyme-rich
pancreatic juice. Secretin causes secretion of HCO3− -rich pancreatic juice. 3
During cephalic and gastric phases, vagal Nerve stimu- lates gallbladder to
contract weakly. 6
CCK secretion
Secretin secretion
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8. Chyme enter -ing duodenum causes duodenal
Figure Mechanisms promoting secretion and release of bile and pancreatic juice.
Slide 6
Chyme enter -ing duodenum causes duodenal
enteroendocrine cells to release cholecystokinin
(CCK) and secretin. 1 4
Bile salts and, to a lesser extent, secretin
transported via
bloodstream stimulate Liver to produce bile more rapidly. 5
CCK (via blood stream) causes gallbladder to contract and Hepatopancreatic
Sphincter to relax. Bile Enters duodenum.
CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2
CCK induces
secretion of enzyme-rich
pancreatic juice. Secretin causes secretion of HCO3− -rich pancreatic juice. 3
CCK secretion
Secretin secretion
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9. Digestion in the Small Intestine
Chyme from stomach contains
Partially digested carbohydrates and proteins
Undigested fats
3–6 hours in small intestine
Most water absorbed
~ All nutrients absorbed
Small intestine, like stomach, no role in ingestion or defecation
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10. Mucus granules Microvilli forming the brush border Absorptive cell
Figure Microvilli of the small intestine.
Mucus
granules
Microvilli
forming the
brush border
Absorptive cell
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11. Motility of the Small Intestine
Peristalsis
Initiated by rise in hormone motilin in late intestinal phase; every 90–120 minutes
Each wave starts distal to previous
Migrating motor complex
Meal remnants, bacteria, and debris moved to large intestine
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12. Figure 23.3a Peristalsis and segmentation.
From
mouth
Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally.
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13. Large Intestine Regions Cecum Appendix Colon Rectum Anal canal
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14. External anal sphincter
Figure 23.29a Gross anatomy of the large intestine.
Left colic
(splenic) flexure
Transverse
mesocolon
Right colic
(hepatic) flexure
Epiploic
appendages
Transverse colon
Superior
mesenteric artery
Descending colon
Haustrum
Ascending colon
IIeum
Cut edge of
mesentery
IIeocecal valve
Tenia coli
Sigmoid colon
Cecum
Appendix
Rectum
Anal canal
External anal sphincter
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15. Subdivisions of the Large Intestine
Cecum – first part of large intestine
Appendix – masses of lymphoid tissue
Part of MALT of immune system
Bacterial storehouse  recolonizes gut when necessary
Twisted  enteric bacteria accumulate and multiply
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16. Ascending colon (right side Transverse colon
Descending colon (left side)
Sigmoid colon in pelvis  rectum
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17. Greater omentum Transverse colon Transverse mesocolon Descending colon
Figure 23.30c Mesenteries of the abdominal digestive organs.
Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
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18. Liver Lesser omentum Pancreas Stomach Duodenum Transverse mesocolon
Figure 23.30d Mesenteries of the abdominal digestive organs.
Liver
Lesser omentum
Pancreas
Stomach
Duodenum
Transverse mesocolon
Transverse colon
Mesentery
Greater omentum
Jejunum
Ileum
Visceral peritoneum
Parietal peritoneum
Urinary bladder
Rectum
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19. Rectum and Anus Rectum Anal canal Sphincters
Last segment of large intestine
Opens to body exterior at anus
Sphincters
Internal anal sphincter—smooth muscle
External anal sphincter—skeletal muscle
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20. Anal columns Pectinate line Anal sinuses Anus
Figure 23.29b Gross anatomy of the large intestine.
Rectal valve
Rectum
Hemorrhoidal
veins
Levator ani muscle
Anal canal
External anal
sphincter
Internal anal
sphincter
Anal columns
Pectinate line
Anal sinuses
Anus
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21. Enter from small intestine or anus
Bacterial Flora
Enter from small intestine or anus
Colonize colon
Synthesize B complex vitamins and vitamin K
Metabolize some host-derived molecules (mucin, heparin, hyaluronic acid)
Ferment indigestible carbohydrates
Release irritating acids and gases (~500 ml/day)
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22. Viruses and protozoans
Intestinal Flora
Viruses and protozoans
Bacteria prevented from breaching mucosal barrier
Epithelial cells recruit dendritic cells to mucosa  sample microbial antigens  present to T cells of MALT  IgA antibody-mediated response  restricts microbes
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23. Digestive Processes in the Large Intestine
Residue remains in large intestine 12–24 hours
No food breakdown except by enteric bacteria
Vitamins (made by bacterial flora), water, and electrolytes (especially Na+ and Cl–) reclaimed
Major functions - propulsion of feces to anus; defecation
Colon not essential for life
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24. Homeostatic Imbalance
Low fiber diet  narrowed colon  strong contractions  increased pressure on walls  diverticula (herniations of mucosa)
Diverticulosis commonly in sigmoid colon
Affects ½ people > 70 years
Diverticulitis
Inflamed diverticula; may rupture and leak into peritoneal cavity; may be life threatening
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25. Homeostatic Imbalance
Irritable bowel syndrome
Functional GI disorder
Recurring abdominal pain, stool changes, bloating, flatulence, nausea, depression
Stress common precipitating factor
Stress management important in treatment
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26. stimulating stretch receptors there. The receptors transmit
Figure Defecation reflex.
Slide 1
Impulses from
cerebral cortex
(conscious
control)
Sensory
nerve fibers
Feces move into and
distend the rectum,
stimulating stretch receptors
there. The receptors transmit
signals along afferent fibers
to spinal cord neurons. 1
Voluntary motor
nerve to external
anal sphincter
Sigmoid
colon
Stretch receptors in wall
A spinal reflex is initiated in which
parasympathetic motor (efferent) fibers
stimulate contraction of the rectum and
sigmoid colon, and relaxation of the
internal anal sphincter. 2
Rectum
External anal
sphincter
(skeletal muscle)
Involuntary motor nerve
(parasympathetic division)
Internal anal sphincter (smooth muscle)
If it is convenient to defecate,
voluntary motor neurons are inhibited,
allowing the external anal sphincter to
relax so feces may pass. 3
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27. Digestion Digestion Enzymes Hydrolysis
Catabolic; macromolecules  monomers small enough for absorption
Enzymes
Intrinsic and accessory gland enzymes break down food
Hydrolysis
Water is added to break bonds
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28. Figure 23. 32 Flowchart of digestion and absorption of foodstuffs
Figure Flowchart of digestion and absorption of foodstuffs. (2 of 4)
Foodstuff
Enzyme(s) and source
Site of action
Path of absorption
Proteins
Amino acids are absorbed via
cotransport with sodium ions.
Some dipeptides and tripeptides
are absorbed via cotransport with
H+ and hydrolyzed to amino acids
within the cells.
Infrequently, transcytosis of small
peptides occurs.
Amino acids leave the epithelial
cells by facilitated diffusion, enter
the capillary blood in the villi, and
are transported to the liver via the
hepatic portal vein.
Pepsin
(stomach glands)
in presence
of HCl
Stomach
Large polypeptides
Pancreatic
enzymes (trypsin,
chymotrypsin,
carboxypeptidase)
Small
intestine
Protein
digestion
Small polypeptides,
small peptides
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Small
intestine
Amino acids
(some dipeptides
and tripeptides)
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29. Pre-treatment—emulsification by bile salts Enzymes—pancreatic lipases
Digestion of Lipids
Pre-treatment—emulsification by bile salts
Does not break bonds
Enzymes—pancreatic lipases
 Fatty acids and monoglycerides
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30. with bile salts to form micelles which
Figure Emulsification, digestion, and absorption of fats.
Slide 1
Fat globule
Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). 1
Bile salts
Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate
with bile salts to form micelles which
“ferry” them to the intestinal mucosa. 2
Fat droplets
coated with
bile salts
Micelles made up of fatty acids,
monoglycerides, and bile salts
Fatty acids and monoglycerides
leave micelles and diffuse into
epithelial cells. There they are
recombined and packaged with other
fatty substances and proteins to form
chylomicrons. 3
Chylomicrons are extruded from
the epithelial cells by exocytosis. The chylomicrons enter lacteals and are
carried away from the intestine in
lymph. 4
Epithelial
cells of
small
intestine
Lacteal
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31. Figure 23. 32 Flowchart of digestion and absorption of foodstuffs
Figure Flowchart of digestion and absorption of foodstuffs. (3 of 4)
Foodstuff
Enzyme(s) and source
Site of action
Path of absorption
Unemulsified triglycerides
Fatty acids and monoglycerides
enter the intestinal cells via
diffusion.
Fatty acids and monoglycerides are
recombined to form triglycerides
and then combined with other lipids
and proteins within the cells. The
resulting chylomicrons are
extruded by exocytosis.
The chylomicrons enter the lacteals
of the villi and are transported to
the systemic circulation via the
lymph in the thoracic duct.
Some short-chain fatty acids are
absorbed, move into the capillary
blood in the villi by diffusion, and
are transported to the liver via the
hepatic portal vein.
Lingual lipase
Mouth
Gastric lipase
Stomach
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Small
intestine
Fat
digestion
Pancreatic
lipases
Small
intestine
Monoglycerides (or diglycerides
with gastric lipase) and fatty acids
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32. ~ All food; 80% electrolytes; most water absorbed in small intestine
Absorption
~ All food; 80% electrolytes; most water absorbed in small intestine
Most prior to ileum
Ileum reclaims bile salts
Most absorbed by active transport  blood
Exception - lipids
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33. Homeostatic Imbalance
Whole proteins not usually absorbed
Can be taken up by endocytosis/exocytosis
Most common in newborns  food allergies
Usually disappear with mucosa maturation
Allows IgA antibodies in breast milk to reach infant's bloodstream  passive immunity
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34. Absorption of Lipids Absorption of monoglycerides and fatty acids
Enter lacteals; transported to systemic circulation
Hydrolyzed to free fatty acids and glycerol by lipoprotein lipase of capillary endothelium
Cells can use for energy or stored fat
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35. Absorption of Vitamins
In small intestine
Fat-soluble vitamins (A, D, E, and K) carried by micelles; diffuse into absorptive cells
Water-soluble vitamins (vitamin C and B vitamins) absorbed by diffusion or by passive or active transporters.
Vitamin B12 (large, charged molecule) binds with intrinsic factor, and is absorbed by endocytosis
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36. Absorption of Vitamins
In large intestine
Vitamin K and B vitamins from bacterial metabolism are absorbed
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37. Malabsorption of Nutrients
Causes
Anything that interferes with delivery of bile or pancreatic juice
Damaged intestinal mucosa (e.g., bacterial infection; some antibiotics)
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38. Malabsorption of Nutrients
Gluten-sensitive enteropathy (celiac disease)
Immune reaction to gluten
Gluten causes immune cell damage to intestinal villi and brush border
Treated by eliminating gluten from diet (all grains but rice and corn)
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39. Stomach and colon cancers rarely have early signs or symptoms
Metastasized colon cancers frequently cause secondary liver cancer
Prevention
Regular dental and medical examination
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