Transcript 23 - Humble ISD

PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
23
The Digestive
System: Part C
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Pancreas
• Endocrine 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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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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Figure 23.26b Structure of the enzyme-producing tissue of the pancreas.
Acinar cells
Pancreatic
duct
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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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Figure 23.27 Activation of pancreatic proteases in the small intestine.
Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
(inactive)
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Trypsin
Chymotrypsinogen
(inactive)
Chymotrypsin
Procarboxypeptidase
(inactive)
Carboxypeptidase
Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice.
1 Chyme enter
-ing duodenum
causes duodenal
enteroendocrine
cells to release
cholecystokinin
(CCK) and
secretin.
2 CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3 CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3− -rich
pancreatic juice.
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Slide 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.
6 During cephalic
and gastric phases,
vagal Nerve stimulates gallbladder to
contract weakly.
CCK secretion
Secretin secretion
Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice.
1 Chyme enter
-ing duodenum
causes duodenal
enteroendocrine
cells to release
cholecystokinin
(CCK) and
secretin.
2 CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3 CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3− -rich
pancreatic juice.
© 2013 Pearson Education, Inc.
Slide 6
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 secretion
Secretin secretion
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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Figure 23.23 Microvilli of the small intestine.
Mucus
granules
Microvilli
forming the
brush border
Absorptive cell
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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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Figure 23.3a Peristalsis and segmentation.
From
mouth
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Peristalsis: Adjacent segments of alimentary
tract organs alternately contract and relax,
moving food along the tract distally.
Large Intestine
• Regions
– Cecum
– Appendix
– Colon
– Rectum
– Anal canal
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Figure 23.29a Gross anatomy of the large intestine.
Left colic
(splenic) flexure
Right colic
(hepatic) flexure
Transverse
mesocolon
Transverse colon
Epiploic
appendages
Superior
mesenteric artery
Descending colon
Haustrum
Ascending colon
IIeum
Cut edge of
mesentery
IIeocecal valve
Tenia coli
Sigmoid colon
Cecum
Appendix
Rectum
Anal canal
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External anal sphincter
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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Colon
•
•
•
•
Ascending colon (right side
Transverse colon
Descending colon (left side)
Sigmoid colon in pelvis  rectum
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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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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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Rectum and Anus
• Rectum
• Anal canal
– 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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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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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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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 antibodymediated response  restricts microbes
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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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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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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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Figure 23.31 Defecation reflex.
Slide 1
Impulses from
cerebral cortex
(conscious
control)
Sensory
nerve fibers
Voluntary motor
nerve to external
anal sphincter
Sigmoid
colon
External anal
sphincter
(skeletal muscle)
Rectum
Stretch receptors in wall
2 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.
Involuntary motor nerve
(parasympathetic division)
Internal anal sphincter (smooth muscle)
3 If it is convenient to defecate,
voluntary motor neurons are inhibited,
allowing the external anal sphincter to
relax so feces may pass.
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1 Feces move into and
distend the rectum,
stimulating stretch receptors
there. The receptors transmit
signals along afferent fibers
to spinal cord neurons.
Digestion
• Digestion
– 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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Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (2 of 4)
Foodstuff
Enzyme(s) and source
Site of action Path of absorption
Proteins
Pepsin
(stomach glands)
in presence
of HCl
Stomach
Small
intestine
Small polypeptides,
small peptides
Pancreatic
enzymes (trypsin,
chymotrypsin,
carboxypeptidase)
Small
intestine
Amino acids
(some dipeptides
and tripeptides)
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Large polypeptides
Protein
digestion
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• 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.
Digestion of Lipids
• Pre-treatment—emulsification by bile salts
– Does not break bonds
• Enzymes—pancreatic lipases
–  Fatty acids and monoglycerides
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Figure 23.34 Emulsification, digestion, and absorption of fats.
Fat globule
1 Bile salts in the duodenum emulsify
large fat globules (physically break them
up into smaller fat droplets).
Bile salts
Fat droplets
coated with
bile salts
2 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.
Micelles made up of fatty acids,
monoglycerides, and bile salts
3 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.
Epithelial
cells of
small
intestine
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4 Chylomicrons are extruded from
the epithelial cells by exocytosis. The
chylomicrons enter lacteals and are
carried away from the intestine in
lymph.
Lacteal
Slide 1
Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (3 of 4)
Foodstuff
Enzyme(s) and source
Site of action Path of absorption
Unemulsified triglycerides
Fat
digestion
Monoglycerides (or diglycerides
with gastric lipase) and fatty acids
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Lingual lipase
Mouth
Gastric lipase
Stomach
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Small
intestine
Pancreatic
lipases
Small
intestine
• 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.
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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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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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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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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Absorption of Vitamins
• In large intestine
– Vitamin K and B vitamins from bacterial
metabolism are absorbed
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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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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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Cancer
• 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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