Transcript 23 - Dr. Jerry Cronin

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
• Location
– Mostly retroperitoneal, deep to greater
curvature of stomach
– Head encircled by duodenum; tail abuts
spleen
© 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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Figure 23.26b Structure of the enzyme-producing tissue of the pancreas.
Acinar cells
Pancreatic
duct
© 2013 Pearson Education, Inc.
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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Pancreatic Juice
• Protease activation in duodenum
– Trypsinogen activated to trypsin by brush
border enzyme enteropeptidase
– Procarboxypeptidase and chymotrypsinogen
activated by trypsin
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Figure 23.27 Activation of pancreatic proteases in the small intestine.
Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
(inactive)
© 2013 Pearson Education, Inc.
Trypsin
Chymotrypsinogen
(inactive)
Chymotrypsin
Procarboxypeptidase
(inactive)
Carboxypeptidase
Regulation of Bile Secretion
• Bile secretion stimulated by
– Bile salts in enterohepatic circulation
– Secretin from intestinal cells exposed to HCl
and fatty chyme
• Hepatopancreatic sphincter closed unless
digestion active  bile stored in
gallbladder
– Released to small intestine ~ only with
contraction
© 2013 Pearson Education, Inc.
Regulation of Bile Secretion
• Gallbladder contraction stimulated by
– Cholecystokinin (CCK) from intestinal cells
exposed to acidic, fatty chyme
– Vagal stimulation (minor stimulus)
• CCK also causes
– Secretion of pancreatic juice
– Hepatopancreatic sphincter to relax
© 2013 Pearson Education, Inc.
Regulation of Pancreatic Secretion
• CCK induces secretion of enzyme-rich
pancreatic juice by acini
• Secretin causes secretion of bicarbonaterich pancreatic juice by duct cells
• Vagal stimulation also causes release of
pancreatic juice (minor stimulus)
© 2013 Pearson Education, Inc.
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 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.
© 2013 Pearson Education, Inc.
Slide 2
Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice.
Slide 3
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.
CCK secretion
Secretin secretion
© 2013 Pearson Education, Inc.
Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice.
Slide 4
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.
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.
Slide 5
4 Bile salts
and, to a lesser
extent, secretin
transported via
bloodstream
stimulate Liver to
produce bile
more rapidly.
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.
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
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 7
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
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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Requirements for Digestion and Absorption
in the Small Intestine
• Slow delivery of acidic, hypertonic chyme
• Delivery of bile, enzymes, and bicarbonate
ions from liver and pancreas
• Mixing
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Motility of the Small Intestine
• Segmentation
– Most common motion of small intestine
– Initiated by intrinsic pacemaker cells
– Mixes/moves contents toward ileocecal valve
– Intensity altered by long & short reflexes;
hormones
• Parasympathetic  ; sympathetic 
– Wanes in late intestinal (fasting) phase
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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
– From duodenum  ileum ~ 2 hours
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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.
Motility of the Small Intestine
• Local enteric neurons coordinate intestinal
motility
• Cholinergic sensory neurons may activate
myenteric plexus
– Causes contraction of circular muscle
proximally and of longitudinal muscle distally
– Forces chyme along tract
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Motility of the Small Intestine
• Ileocecal sphincter relaxes, admits
chyme into large intestine when
– Gastroileal reflex enhances force of
segmentation in ileum
– Gastrin increases motility of ileum
• Ileocecal valve flaps close when chyme
exerts backward pressure
– Prevents regurgitation into ileum
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Large Intestine
• Unique features
– Teniae coli
• Three bands of longitudinal smooth muscle in
muscularis
– Haustra
• Pocketlike sacs caused by tone of teniae coli
– Epiploic appendages
• Fat-filled pouches of visceral peritoneum
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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
© 2013 Pearson Education, Inc.
Colon
• Retroperitoneal except for transverse and
sigmoid regions
• Ascending colon (right side – to level of
right kidney)  right colic (hepatic)
flexure 
• Transverse colon  left colic (splenic)
flexure 
• 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
– Three rectal valves stop feces from being
passed with gas (flatus)
• Anal canal
– Last segment of large intestine
– Opens to body exterior at anus
• Sphincters
– Internal anal sphincter—smooth muscle
– External anal sphincter—skeletal muscle
© 2013 Pearson Education, Inc.
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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Large Intestine: Microscopic Anatomy
• Thicker mucosa of simple columnar
epithelium except in anal canal (stratified
squamous to withstand abrasion)
• No circular folds, villi, digestive secretions
• Abundant deep crypts with goblet cells
• Superficial venous plexuses of anal canal
form hemorrhoids if inflamed
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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)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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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Motility of the Large Intestine
• Most contractions of colon
– Haustral contractions
• Slow segmenting movements
• Haustra sequentially contract in response to
distension
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Motility of the Large Intestine
• Gastrocolic reflex
– Initiated by presence of food in stomach
– Activates three to four slow powerful
peristaltic waves per day in colon (mass
movements)
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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
© 2013 Pearson Education, Inc.
Defecation
• Mass movements force feces toward
rectum
• Distension initiates spinal defecation
reflex
• Parasympathetic signals
– Stimulate contraction of sigmoid colon and
rectum
– Relax internal anal sphincter
• Conscious control allows relaxation of
external anal sphincter
© 2013 Pearson Education, Inc.
Defecation
• Muscles of rectum contract to expel feces
• Assisted by Valsalva's maneuver
– Closing of glottis, contraction of diaphragm
and abdominal wall muscles  increased
intra-abdominal pressure
– Levator ani muscle contracts  anal canal
lifted superiorly  feces leave body
© 2013 Pearson Education, Inc.
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.
© 2013 Pearson Education, Inc.
1 Feces move into and
distend the rectum,
stimulating stretch receptors
there. The receptors transmit
signals along afferent fibers
to spinal cord neurons.
Figure 23.31 Defecation reflex.
Slide 2
Sensory
nerve fibers
Sigmoid
colon
1 Feces move into and
distend the rectum,
stimulating stretch receptors
there. The receptors transmit
signals along afferent fibers
to spinal cord neurons.
Stretch receptors in wall
© 2013 Pearson Education, Inc.
Figure 23.31 Defecation reflex.
Slide 3
Sensory
nerve fibers
Sigmoid
colon
Rectum
© 2013 Pearson Education, Inc.
1 Feces move into and
distend the rectum,
stimulating stretch receptors
there. The receptors transmit
signals along afferent fibers
to spinal cord neurons.
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)
Figure 23.31 Defecation reflex.
Slide 4
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.
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Digestion of Carbohydrates
• Only monosaccharides can be absorbed
• Monosaccharides absorbed as ingested
– Glucose, fructose, galactose
• Digestive enzymes
– Salivary amylase, pancreatic amylase, and
brush border enzymes (dextrinase,
glucoamylase, lactase, maltase, and sucrase)
– Break down disaccharides sucrose, lactose,
maltose; polysaccharides glycogen and starch
© 2013 Pearson Education, Inc.
Digestion of Carbohydrates
• Starch digestion
– Salivary amylase (saliva)  oligosaccharides
at pH 6.75 – 7.00
– Pancreatic amylase (small intestine) 
breaks down any that escaped salivary
amylase  oligosaccharides
– Brush border enzymes (dextrinase,
glucoamylase, lactase, maltase, sucrase) 
oligosaccharides  monosaccharides
© 2013 Pearson Education, Inc.
Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (1 of 4)
Enzyme(s) and source
Foodstuff
Site of action Path of absorption
Starch and disaccharides
Salivary amylase
Oligosaccharides
and disaccharides
Carbohydrate
digestion
Lactose
Maltose
Sucrose
Galactose
Glucose
Fructose
© 2013 Pearson Education, Inc.
Mouth
Pancreatic
amylase
Small
intestine
Brush border
enzymes in
small intestine
(dextrinase, glucoamylase, lactase,
maltase, and sucrase)
Small
intestine
• Glucose and galactose are
absorbed via cotransport with
sodium ions.
• Fructose passes via facilitated
diffusion.
• All monosaccharides leave the
epithelial cells via facilitated
diffusion, enter the capillary blood
in the villi, and are transported to
the liver via the hepatic portal vein.
Digestion of Proteins
• Source is dietary, digestive enzymes, mucosal
cells; digested to amino acid monomers
• Begins with pepsin in stomach at pH 1.5 – 2.5
– Inactive in high pH of duodenum
• Pancreatic proteases
– Trypsin, chymotrypsin, and carboxypeptidase
• Brush border enzymes
– Aminopeptidases, carboxypeptidases, and
dipeptidases
© 2013 Pearson Education, Inc.
Figure 23.33 Protein digestion and absorption in the small intestine.
Lumen
of intestine
Slide 1
Amino acids of protein
fragments
Brush border enzymes
Pancreatic
proteases
Apical membrane (microvilli)
Na+
Na+
Absorptive
epithelial
cell
1 Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxypeptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
2 The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side.
Amino
acid
carrier
Capillary
© 2013 Pearson Education, Inc.
3 The amino acids leave the villus
epithelial cell by facilitated diffusion
and enter the capillary via
intercellular clefts.
Figure 23.33 Protein digestion and absorption in the small intestine.
Lumen
of intestine
Amino acids of protein
fragments
Brush border enzymes
Pancreatic
proteases
Apical membrane (microvilli)
Na+
Na+
Capillary
© 2013 Pearson Education, Inc.
Slide 2
Absorptive
epithelial
cell
1 Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxypeptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
Figure 23.33 Protein digestion and absorption in the small intestine.
Lumen
of intestine
Slide 3
Amino acids of protein
fragments
Brush border enzymes
Pancreatic
proteases
Apical membrane (microvilli)
Na+
Na+
Absorptive
epithelial
cell
1 Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxypeptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
2 The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side.
Amino
acid
carrier
Capillary
© 2013 Pearson Education, Inc.
Figure 23.33 Protein digestion and absorption in the small intestine.
Lumen
of intestine
Slide 4
Amino acids of protein
fragments
Brush border enzymes
Pancreatic
proteases
Apical membrane (microvilli)
Na+
Na+
Absorptive
epithelial
cell
1 Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxypeptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
2 The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side.
Amino
acid
carrier
Capillary
© 2013 Pearson Education, Inc.
3 The amino acids leave the villus
epithelial cell by facilitated diffusion
and enter the capillary via
intercellular clefts.
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
© 2013 Pearson Education, Inc.
• 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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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 23.34 Emulsification, digestion, and absorption of fats.
Fat globule
Bile salts
Fat droplets
coated with
bile salts
© 2013 Pearson Education, Inc.
1 Bile salts in the duodenum emulsify
large fat globules (physically break them
up into smaller fat droplets).
Slide 1
Figure 23.34 Emulsification, digestion, and absorption of fats.
Fat globule
Bile salts
Fat droplets
coated with
bile salts
© 2013 Pearson Education, Inc.
1 Bile salts in the duodenum emulsify
large fat globules (physically break them
up into smaller fat droplets).
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
Slide 3
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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Lacteal
Slide 4
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
© 2013 Pearson Education, Inc.
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 5
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.
Digestion of Nucleic Acids
• Enzymes
– Pancreatic ribonuclease and
deoxyribonuclease  nucleotide monomers
– Brush border enzyme nucleosidases and
phosphatases  free bases, pentose sugars,
phosphate ions
© 2013 Pearson Education, Inc.
Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (4 of 4)
Foodstuff
Enzyme(s) and source
Site of action Path of absorption
Nucleic acids
Nucleic acid
digestion
Pentose sugars,
N-containing bases,
phosphate ions
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Pancreatic ribonuclease and
deoxyribonuclease
Small
intestine
Brush border
enzymes
(nucleosidases
and phosphatases)
Small
intestine
• Units enter intestinal cells by active
transport via membrane carriers.
• Units are absorbed into capillary
blood in the villi and 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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Absorption of Carbohydrates
• Glucose and galactose
– Secondary active transport (cotransport) with
Na+  epithelial cells
– Move out of epithelial cells by facilitated
diffusion  capillary beds in villi
• Fructose
– Facilitated diffusion to enter and exit cells
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Absorption of Carbohydrates
• Glucose and galactose
– Secondary active transport (cotransport) with
Na+  epithelial cells
– Move out of epithelial cells by facilitated
diffusion  capillary beds in villi
• Fructose
– Facilitated diffusion to enter and exit cells
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Absorption of Protein
• Amino acids transported by several types
of carriers
– Most coupled to active transport of Na+
• Dipeptides and tripeptides actively
absorbed by H+-dependent cotransport;
digested to amino acids within epithelial
cells
• Enter capillary blood by diffusion
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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
– Cluster with bile salts and lecithin to form micelles
– Released by micelles to diffuse into epithelial cells
– Combined with lecithin, phospholipids, cholesterol, &
coated with proteins to form chylomicrons
– 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
• Absorption of short chain fatty acids
– Diffuse into portal blood for distribution
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Absorption of Nucleic Acids
• Absorption
– Active transport across epithelium 
bloodstream
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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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Absorption of Electrolytes
• Most ions actively along length of small intestine
• Iron and calcium are absorbed in duodenum
• Na+ coupled with active absorption of glucose
and amino acids
• Cl– transported actively
• K+ diffuses in response to osmotic gradients; lost
if poor water absorption
• Usually amount in intestine is amount absorbed
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Absorption of Electrolytes
• Iron and calcium absorption related to
need
– Ionic iron stored in mucosal cells with ferritin
– When needed, transported in blood by
transferrin
• Ca2+ absorption regulated by vitamin D
and parathyroid hormone (PTH)
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Absorption of Water
• 9 L water, most from GI tract secretions,
enter small intestine
– 95% absorbed in the small intestine by
osmosis
– Most of rest absorbed in large intestine
• Net osmosis occurs if concentration
gradient established by active transport of
solutes
• Water uptake coupled with solute uptake
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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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Developmental Aspects
• Oral membrane  mouth opening
• Cloacal membrane  anus
• By week 5 alimentary canal continuous
tube from mouth to anus
• Shortly after, accessory organs bud from
mucosa
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Figure 23.36 Embryonic development of the digestive system.
Lung bud
Brain
Oral membrane
Heart
Yolk sac
Stomodeum
Foregut
Liver
Site of liver
development
Body stalk
Gallbladder
Hindgut
Cystic duct
Ventral pancreatic bud
Proctodeum
Endoderm
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Bile
duct
Midgut
Spinal cord
Cloacal
membrane
Stomach
Dorsal
pancreatic
bud
Duodenum
Homeostatic Imbalance
• Cleft palate and cleft lip
• Tracheoesophageal fistula
– Opening between esophagus and trachea
• Cystic fibrosis
– Genetic disease  thick mucus  can block
pancreatic duct
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Developmental Aspects
• Fetal nutrition via placenta, but GI tract
stimulated to mature by amniotic fluid
swallowed in utero
• Newborn's rooting reflex helps infant find
nipple; sucking reflex aids in swallowing
• Newborns double birth weight in six
months; adult diet by 2 years
• Cholecystitis, ulcers – problems of middle
age
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Developmental Aspects
• During old age
– GI tract activity declines, less digestive juice,
absorption less efficient, peristalsis slows 
less frequent bowel movements
– Taste/smell less acute; periodontal disease
often develops
– Diverticulosis, fecal incontinence, and cancer
of GI tract fairly common
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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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