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Chapter 18
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I. Introduction to the Digestive
System
A. Introduction
1. From food, humans must get basic organic
molecules to make ATP, build tissues, and serve
as cofactors and coenzymes.
a. Digestion breaks polymers (carbohydrates,
fats, and proteins) into monomer building
blocks via hydrolysis reactions
b. Absorption takes these monomers into the
bloodstream to be used by the cells
Digestion of food through hydrolysis reactions
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Carbohydrate
O H
H
O H
H
+
O
HO
Carbohydratedigesting enzymes
H2O
OH
Maltose
Disaccharide
+
Water
+
Water
O H
H
O H
H
+
HO
OH
HO
Glucose
OH
Glucose
+
Monosaccharides
Protein
H
H
R
O
N
C
C
H
H
O
N
C
C
H
R
OH +
H2O
ProteinH
digesting enzyme
H
R
O
N
C
C
OH
+
H
+
Water
O
N
C
C
H
Amino acid
Lipid
+
Amino acid
H
O
C17H35C
H
C17H35COO
C
H
C17H35COO
C
H
C
R
H
Peptide
(portion of protein molecule)
C17H35COO
H
+
3H2O
Triglyceridedigesting enzyme
OH
HO
C
H
HO
C
H
HO
C
H
O
C17H35C
H
H
+
OH
O
C17H35C
OH
H
Fat
+
Water
Fatty acids
+
Glycerol
OH
Introduction, cont
2. The digestive tract is open at both ends and is
continuous with the environment
a. Considered “outside” the body
b. Materials that cannot be digested (cellulose)
never actually “enter” the body.
3. One-way transport allows for specialization of
function along the tract
B. Digestive Tract Functions
1. Motility – movement of food through the tract
a. Ingestion: taking food into the mouth
b. ________: chewing and mixing food with saliva
c. Deglutination: swallowing
d. Peristalsis: wave-like, one-way movement
through tract
e. ____________: churning and mixing while
moving forward
Digestive Tract Functions, cont
2. Secretion
a. ________: digestive enzymes, hydrochloric
acid, mucus, water, and bicarbonate
b. Endocrine: hormones to regulate digestion
3. Digestion - breaking food down into smaller units,
both physically and chemically
4. __________ - passing broken-down food into
blood or lymph
Digestive Tract Functions, cont
5. Storage and elimination - temporary storage and
subsequent elimination of undigested food
molecules
6. Immune barrier
a. Simple columnar epithelium with tight junctions
prevents swallowed pathogens from entering
body.
b. Immune cells in connective tissue of the tract
promote immune responses
C. Digestive System Divisions
1. Gastrointestinal tract (GI tract, alimentary canal):
____ feet long, from mouth to anus
Oral cavity 
Pharynx 
Esophagus 
Stomach 
Small intestines 
Large intestines 
Anus
2. Accessory organs: teeth, tongue, salivary glands,
liver, gallbladder, pancreas
Organs of the Digestive System
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Parotid
gland
Oral cavity
Teeth
Pharynx
Tongue
Submandibular
gland
Sublingual
gland
Esophagus
Diaphragm
Stomach
Liver
Common
bile duct
Gallbladder
Pancreas
Duodenum
Ascending
colon
Small
intestine
Transverse
colon
Descending
colon
Sigmoid colon
Cecum
Rectum
Appendix
Anal canal
Anus
C. GI Tract Layers
1. Also called tunics
2. There are four tunics:
a. Mucosa: inner secretory and absorptive layer;
may be folded to increase surface area
b. __________: very vascular, to pick up nutrients;
also has some glands and nerve plexuses
c. Muscularis: smooth muscle; responsible for
peristalsis and segmentation; myenteric plexus
for control by the ANS
d. ________: outer binding and protective layer
Layers of the Digestive Tract
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Artery
Vein
Mesentery
Serosa
Plica circularis
Mucosa
Submucosa
Longitudinal muscle
(a)
Circular muscle
Serosa
Muscularis
externa
Outer
longitudinal
Myenteric
plexus
Inner circular
Submucosal
plexus
Gland in submucosa
Muscularis mucosae
Lymph nodule
Intestinal crypt
(of Lieberkühn)
Lamina propria
Lacteal
(b)
Villi
Epithelium
E. Regulation of the GI Tract
1. Parasympathetic division (extrinsic regulation)
a. Stimulates esophagus, stomach, small
intestine, pancreas, gallbladder, and first part
of large intestine via vagus nerve
b. Spinal nerves in sacral region stimulate lower
large intestine.
c. Preganglionic neurons synapse on
__________ and _________ plexuses.
Regulation of the GI Tract, cont
2. Sympathetic division (extrinsic regulation)
a. Inhibits peristalsis and secretion
b. Stimulates contraction of sphincters
3. Hormones - from brain or other digestive organs
Regulation of the GI Tract, cont
4. Intrinsic regulation
a. Intrinsic sensory neurons in gut wall help in
intrinsic regulation via separate enteric nervous
system
b. Paracrine signals
II. From Mouth to Stomach
A. Mouth
1. Mastication: Chewing breaks food down into
smaller pieces for deglutition and mixes it with
saliva.
2. Saliva: contains mucus, an antimicrobial agent, and
salivary amylase to start digestion of starch.
3. Deglutition
a. Involves coordinated contraction of 25 pairs of
muscles
b. Three parts:
1) Oral: voluntary; muscles of mouth and tongue
mix food with saliva to form a bolus.
2) Pharyngeal: involuntary; initiated by receptors in
the posterior oral cavity and oropharynx
a) Uvula (soft palate) lifts to cover nasopharynx,
and the epiglottis covers vocal cords.
b) Upper esophageal sphincter relaxes.
Deglutition, cont
3) __________: automatic; controlled by the
swallowing center of brain stem; bolus is moved
down esophagus to stomach via peristalsis
Peristalsis in the esophagus
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Peristaltic
wave
Esophagus
Bolus
Stomach
B. Esophagus
1.
2.
3.
4.
About 10 inches long
Passes through the diaphragm via the esophageal hiatus
Lined with nonkeratinized stratified squamous epithelium
Mouth, pharynx, and upper esophagus lined with skeletal
muscles innervated by somatic motor neurons
5. Lower esophagus lined with smooth muscle controlled by
autonomic nervous system
6. Lower esophageal sphincter opens to allow food to pass
into stomach. It stays closed to prevent regurgitation.
C. Stomach
1. Functions
a. Stores food
b. Churns food to mix with gastric secretions
c. Begins protein digestion
d. Kills bacteria in the food (acid)
e. Moves food into small intestine in the form of
chyme
2. Stomach Structure
a. Regions
1) Food is delivered from the esophagus to the
cardiac region.
2) Upper region = fundus
3) Lower region = body
4) Distal region = pyloric region; ends at pyloric
sphincter
b. Lining has folds called _______.
Regions and Structure of the Stomach
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Cardia
Esophagus
Fundus
Adventitia
Longitudinal
muscle
Pyloric
sphincter
Duodenum
Body
Circular
muscle
Pyloric
antrum
Oblique
muscle
Submucosa
Pylorus
Mucosa
Gastric rugae
Stomach Structure, cont
c. Gastric pits at base of folds lead to gastric glands
that contain several types of secretory cells:
1) Mucus neck cells secrete mucus to help protect
stomach lining from acid.
2) ________ cells secrete HCl acid and intrinsic
factor (helps small intestine absorb vitamin B12).
3) _____ (zygomatic) cells secrete pepsinogen, the
inactive form of a protein digesting enzyme
Stomach Structure, cont
4) Enterochromaffin-like (ECL) cells secrete
histamine and serotonin (paracrine signals).
5) G cells secrete _______ (hormone).
6) D cells secrete somatostatin (hormone).
7) Also secretes the hormone ______ that
signals the brain to regulate hunger
Gastric Pits & Gastric Glands
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Gastric pits
Gastric
gland
Mucous
cell
Mucosa
Parietal
cell
Submucosa
Chief
cell
(b)
(a)
D. Pepsin and HCl Secretion
1. Formation of HCl
a. Primary active transport of H+ via H+/K+ATPase
pumps
b. Facilitated diffusion of Cl− coupled to downhill
movement of bicarbonate
c. Parietal cells secrete Cl- as well as H+ into
gastric juice while secreting bicarbonate into the
blood
Secretion of gastric acid by parietal cells
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Lumen of stomach
H+
Facilitative
diffusion
Cl–
K+
Cl–
K+
Primary active transport
(ATPase carrier)
Parietal cell
H+
CO2 + H2O
CA
H+
H2CO3
HCO3–
Cl–
Secondary
active transport
HCO3–
Cl–
Blood capillaries
2. Stimulation of HCl Secretion
a. Gastrin: made in G cells; carried to parietal cells in
blood; Also stimulates ECL cells to make histamine
b. Histamine: also stimulates parietal cells via H2
histamine receptors
1) Examples: Tagamet and Zantac block H2
receptors.
c. Parasympathetic neurons and ACh: stimulate
parietal and ECL cells
3. Functions of HCl
a. Drops pH to 2
b. Ingested proteins are denatured (allows
enzymes access).
c. Pepsinogen is converted to active pepsin
(digests proteins).
d. Serves as the optimal pH for pepsin activity
4. Function of ______ – catalyzes the hydrolysis of
peptide bonds in the ingested proteins
Activation of pepsin
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Lumen of
stomach
Ingested
protein
Pepsinogen
Pepsin
HCI
Short
peptides
Pepsin
Gastric
mucosa
5. Stomach Defenses
a. Acid and pepsin could damage the stomach
lining.
b. Defenses that help prevent this:
1) Adherent layer of mucus with alkaline
bicarbonate
2) Tight junctions between epithelial cells
3) Rapid epithelial mitosis that replaces
epithelium every three days
6. Digestion and Absorption in the Stomach
a. Proteins begin digestion in the stomach.
b. Starches begin digestion in the _____, but salivary
amylase is not active at pH 2, so this activity stops
in the stomach.
c. Alcohol and NSAIDs (aspirin) are the only common
substances absorbed in the stomach (due to high
lipid solubility).
7. Peptic Ulcers
a. Peptic ulcers: erosions of the mucosa of the
stomach or duodenum produce by HCl
b. Helicobacter pylori: bacterium that reduces
mucosal barriers to acid
c. Treatment for ulcers combines K+/H+ pump
inhibitors (Prilosec) and antibiotics.
8. Acute Gastritis
a. Inflammation of the submucosa caused by acid
b. Histamine released as part of the inflammatory
response can stimulate more acid release.
c. Prostaglandins are needed to stimulate protective
alkaline mucus production.
d. NSAIDs inhibit prostaglandin activity and can lead
to gastritis.
e. Tagamet and Zantac inhibit H2receptors.
9. Duodenal ulcers
a. Protected by adherent layer of mucus with
bicarbonate
b. Bicarbonate secreted by Brunner’s glands
c. Acidic chyme neutralized by bicarbonate from the
pancreas
d. Zollinger-Ellison syndrome – ulcers due to high
amounts of gastrin produced by a tumor
III. Small Intestine
A. Small Intestine Structure
1. Starts at the pyloric sphincter and ends at the
ileocecal valve
2. Three sections:
a. ________ – first 10 inches
b. ________ – middle 2/5
c. ______ – last 3/5
3. Mucosa and submucosa folded into plicae
circulares; mucosa further folded into villi; and
epithelial plasma membranes folded into microvilli;
these greatly increase the surface area for
absorption of nutrients
Small Intestine Structure
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Plica circulares
Villi
Duodenum
Ascending
colon
Stomach
Jejunum
Mucosa
Submucosa
Mesentery
Ileum
Cecum
Muscularis
externa
Appendix
Plica circulares
Circular
muscle
(a)
(b)
Longitudinal
muscle
Serosa
4. Small Intestine Functions
a. Complete digestion of carbohydrates, proteins, and
fats
b. Absorption of nutrients
1) Sugars, lipids, amino acids, calcium, and iron
absorbed in duodenum and jejunum
2) Bile salts, vitamin B12, water, and electrolytes in
ileum
3) Very rapid due to villi and microvilli
B. Villi and Microvilli
1. Villi
a. Columnar epithelium with goblet cells (mucus)
b. Capillaries absorb monosaccharides and amino
acids, and lacteals absorb fats.
c. Intestinal crypts (Crypts of Lieberkuhn) with
Paneth cells (secrete antibacterial molecules of
lysozyme and defensin) and mitotic stem cells
(divide by mitosis to replenish intestinal cells
every 4 to 5 days)
2. __________ (brush border) – folding of the apical
surface of each epithelial cell
Structure of an intestinal villus
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Simple columnar
epithelium
Lacteal
Villus
Capillary network
Goblet cells
Intestinal crypt
Lymph vessel
Arteriole
Venule
C. Intestinal Enzymes
1. Called brush border enzymes
2. Not released into lumen, but stay attached to
plasma membrane with active site exposed to
chyme
3. Hydrolyze disaccharides, polypeptides and
other substrates to simple nutrient molecules
Location of Brush Border Enzymes
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Lumen of small intestine
Enz
Microvilli
Epithelial cell of duodenum
Brush Border Enzymes
D. Intestinal Contractions and Motility
1. Peristalsis is weak. Movement of food is much
slower due to pressure at pyloric end.
2. Segmentation is stronger and serves to mix the
chyme.
3. Smooth muscle contractions occur automatically
due to endogenous pacemaker activity
a. Graded depolarizations called slow waves
produced by pacemaker cells called interstitial
cells of Cajal produce action potentials in
muscle cells.
Segmentation of the small intestine
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Intestinal Contractions/Motility, cont
b. Depolarization from the interstitial cells of Cajal
opens voltage-gated Ca2+ channels in muscle
cells  action potential and contraction.
c. Stimulation travels short distances through gap
junctions but must be regenerated in
neighboring pacemaker regions.
d. Produces contractions needed for
segmentation
Slow waves in the intestines
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Slow waves
–28
mV
–62
5 sec
4. Regulation of Contraction
a. Autonomic nerves influence enteric nervous
system to stimulate or inhibit cells of Cajal.
b. Acetylcholine from parasympathetic system
interacts with muscarinic ACh receptors to
increase amplitude and duration of slow waves.
Regulation of Contraction
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Structures
Interstitial
cells of
Cajal
Functions
Production
of slow waves
Conduction of
slow waves to
smooth muscle
Smooth
muscle
cells
Autonomic
axon
Depolarization
and opening of
Ca2+ channels,
production of
action potentials
Neural input to ICC
and smooth muscle
IV. Large Intestine
A. Large Intestine Structure
1. Regions
a. Through the ileocecal valve into the
b. Cecum
c. Ascending colon 
d. Transverse colon 
e. Descending colon 
f. Sigmoid colon 
g. Rectum 
h. Anal canal 
i. Anus
The large intestine
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Transverse colon
Ascending
colon
Ileocecal
valve
Cecum
Appendix
Rectum
Descending
colon
Ileum
Haustrum
Sigmoid colon
Anal canal
Large intestine structure, cont
2. Mucosa – columnar epithelial cells with goblet
cells, crypts, lymphatic nodules, but no villi
3. Outer surface forms pouches called haustra
4. Large Intestine Function
a. Absorption of water, electrolytes, vitamin K, and
some B vitamins
b. Production of vitamin _______ vitamins via
microbial organisms
c. Storage of feces
B. Intestinal Microbiota
1. Several hundred different species of bacteria live in
the large intestine; also called microflora
a. Some are commensal. The bacteria benefit, and
we aren’t harmed or benefitted.
b. Others are mutualistic. Both bacteria and us
benefit.
c. Bacteria are mostly anaerobic.
d. An infant receives its initial colonization of
bacteria from its mother during birth.
2. Benefits from Microbes
a. Microbes make vitamin K and some B vitamins.
b. They also make fatty acids from cellulose. Some of
these are used for energy by large intestine
epithelial cells. We can’t absorb the fatty acids, but
they help absorb electrolytes such as sodium,
calcium, bicarbonate, magnesium, and iron.
c. They outcompete harmful species of bacteria.
d. Disruption of normal microflora can lead to
inflammatory bowel disease.
3. Protection from intestinal bacteria
a. Mucus from goblet cells prevents contact with
epithelial cells.
b. Secretion of _______ by Paneth cells in the crypts.
c. Secretion of IgA antibodies by plasma cells
Intestinal Microbiota, cont
4. Normal intestinal microbiota helps maintain a
healthy epithelial barrier, limits inflammation, and
promotes repair of damaged epithelium.
5. Commensal bacteria promote production of
regulatory T lymphocytes rather than killer T cells
a. Intestinal dendritic cells are antigen-presenting
cells to activate T cells.
C. Fluid and electrolyte absorption
1. Most absorption occurs in small intestine, but some
is left for large intestine.
2. Not all water is absorbed; about 200 ml is left per
day to be excreted with feces.
3. Water is absorbed passively following an osmotic
gradient set up by active Na+/K+pumps.
a. ________ stimulates greater salt and water
absorption here.
4. Some minor secretion of water occurs by osmosis.
D. Defecation
1. As material passes to the rectum, pressure there
increases, the internal anal sphincter relaxes, and
the need to defecate rises.
2. The external anal sphincter controls defecation
voluntarily.
3. During defecation, longitudinal rectal muscles
contract to increase pressure as the anal
sphincters relax.
4. Aided by contraction of abdominal and pelvic
skeletal muscles – Valsalva’s maneuver
V. Liver, Gallbladder, and
Pancreas
A. Liver
1. Structure
a. Largest abdominal organ; immediately beneath
the diaphragm, mostly on the right side
b. Has amazing regenerative abilities due to
mitosis of hepatocytes
c. Composed of hepatocytes that form hepatic
plates separated by capillaries called sinusoids
1) Capillaries have fenestrae with no diaphragm
or basement membrane
2) Very permeable, allowing passage of blood
proteins, fat, and cholesterol
Structure, cont
d. Kupffer cells are also in the sinusoids.
e. Hepatic damage due to alcohol or viral hepatitis
causes liver fibrosis that can lead to cirrhosis of the
liver.
Microscopic structure of the liver
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Hepatic
sinusoids
Central
vein
Hepatic
plate
Bile
ductule
Branch of hepatic
portal vein
Branch of
hepatic artery
Bile canaliculi
Bile ductule
Portal
triad
2. Hepatic Portal System
a. Products of digestion absorbed in intestines are
delivered to the liver via the hepatic portal vein.
b. Veins from the pancreas, gallbladder, stomach,
omentum, and spleen also join with the hepatic
portal vein.
c. After circulating through liver capillaries, the blood
leaves via the hepatic vein to join regular venous
circulation
Hepatic Portal System, cont
d. _____________– a unique pattern of capillaries 
veins  capillaries  veins
e. Total hepatic blood flow is about 25% of cardiac
output; needed to maintain hepatic clearance.
3. Liver Lobules
a. Hepatic plates are arranged as liver lobules with
hepatic arteries, hepatic portal veins, and a central
vein.
b. Hepatic artery and hepatic portal vein open into
the sinusoids between the plates
c. Bile secreted by the hepatocytes is released into
bile canaliculi, which drain into bile ducts, then to
hepatic ducts away from the liver.
Flow of blood and bile in a liver lobule
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Branch of
portal vein
Bile
ductule
Branch of hepatic artery
Bile
canaliculus
Hepatic plate
Sinusoids
Central vein
4. Enterohepatic circulation
a. Aside from bile, the liver secretes other substances
into the bile ducts to clear them from the blood.
b. Some of the molecules released into the bile are
absorbed again in the small intestine and returned
to the liver.
c. These molecules are part of enterohepatic
circulation; analogous to renal clearance.
d. Eventually molecules are excreted in feces.
Enterohepatic Circulation
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Liver
Hepatic
portal vein
Gallbladder
Small
intestine
Common
bile duct
Compounds excreted by the liver into bile
B. Liver Functions
1. Bile production and secretion
a. The liver makes __________ ml of bile per day.
b. Bile is composed of:
1) Bile pigment (_______)
2) Bile salts
3) Phospholipids (lecithin)
4) ____________
5) Inorganic ions
c. Bilirubin
1) Produced in ______, liver, and bone marrow
a) Derived from _____ (minus iron) from
hemoglobin
b) Not water-soluble
1) Carried attached to _______ in the blood
2) Not directly filtered by kidneys or secreted
into bile
3) Some free bilirubin is conjugated with
glucuronic acid to make it water- soluble
Bilirubin, cont
2) Conjugated bilirubin is secreted into the bile,
where it is taken to the small intestine.
a) ________ there turn it into urobilinogen,
which makes feces brown.
b) 30−50% is absorbed by the intestines and
taken back to the liver.
c) Some is used to make bile, and some
remains in blood to be filtered by the
kidneys where it gives an amber color to the
urine.
Pathway for the metabolism of heme & bilirubin
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Red Blood Cells
Hemolysis in spleen,
liver, bone marrow
Hemoglobin
Heme
Fe2+
Recycled to
bone marrow
Carbon
monoxide (CO)
Biliverdin
NADPH
NADP
Bilirubin
Hemoglobin
Carboxyhemoglobin
O2
Oxyhemoglobin
Liver
Carbon monoxide
eliminated in
exhaled breath
Conjugated bilirubin
(bilirubin glucuronide)
Bilirubin eliminated
in bile excreted in feces
Enterohepatic circulation of urobilinogen
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General circulation
Kidney
Vena cava
Liver
Hepatic
portal vein
Gallbladder
Small
intestine
Common
bile duct
Urobilinogen
in urine
Bilirubin Bacteria Urobilinogen
Urobilinogen
in feces
d. Bile Salts
1) Made from bile acids conjugated with glycine or
taurine
2) Bile acids: derived from __________
a) Four polar groups on each molecule
b) Cholic acid and chenodeoxycholic acid
c) Most is recycled in enterohepatic circulation.
d) ½ gram of cholesterol is broken down and lost in
the feces through this pathway.
Bile Salts, cont
3) Form ________ with polar groups toward water
and nonpolar groups inward toward the fat
a) Fats enter the micelle and are emulsified.
b) Provides a greater surface area for fat digestion
by lipase
Bile acids form micelles
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OH
CH3
CH CH2 CH2
HO
OH
(a)
Cholic acid (a bile acid)
Nonpolar
Polar
(b)
Simplified representation of bile acid
(c)
Micelle of bile acids
COOH
2. Detoxification of Blood
a. The liver can remove hormones, drugs, and other
substances in three ways:
1) Excreted into _____
2) Phagocytized by ________ lining sinusoids
3) Chemically altered by hepatocytes
a) ________ is converted into urea, porphyrins
into bilirubin, and purines into uric acid
b) Urea is returned to the blood to be filtered by
the kidneys.
c) Steroids and xenobiotics are altered and then
secreted into bile.
3. Secretion of Glucose, triglycerides, and ketone
bodies
a. The liver helps balance blood glucose levels by
removing glucose and storing it as ________
(glycogenesis) and triglycerides (________) or by
breaking down glycogen (glycogenolysis) and
releasing it into the blood.
b. The liver can also make glucose from amino acids
(_____________) and convert fatty acids into
ketones (ketogenesis).
4. Production of plasma proteins
a. Plasma albumin and most plasma globulins,
clotting factors and angiotensinogen are produced
by the liver.
b. Albumin provides colloid osmotic pressure and
transport
c. Globulins provide transport and blood clotting
d. Clotting factors I, II, III, V, VII, IX, XI
B. Gallbladder
1. Sac-like organ attached to the inferior surface of
the liver
2. Stores and concentrates bile from the liver:
Liver 
Bile ducts 
Hepatic duct 
Cystic duct 
Gallbladder 
Cystic duct 
Common bile duct 
Sphincter of Ampulla or Oddi into the duodenum
Pancreatic juice and bile secretion into the
duodenum
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Hepatic ducts
Pancreatic acinus
Cystic duct
Common bile
duct
Gallbladder
Pancreatic
juice
Pancreatic islet
(of Langerhans)
Pancreas
Pancreatic duct
Duodenal
papilla
Duodenum
Gallstones
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(a)
(b)
a: © Dr. Sheril Burton; b: © Martin/Custom Medical Stock Photo
C. Pancreas
1. Has endocrine and exocrine functions
a. Endocrine: Islets of Langerhans cells
(Pancreatic Islets) make insulin and glucagon.
b. Exocrine: Acinar cells make pancreatic juice,
which is delivered to the duodenum via the
pancreatic duct.
Pancreas Structure
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Endocrine
portion
Pancreatic islet
(of Langerhans)
Zymogen granules
Exocrine
portion
Acinar cells
Pancreatic
acini
To pancreatic duct
and duodenum
(b)
(a)
a: © Ed Reschke
Duct
Acinus
2. Pancreatic Juice
a. Made up of bicarbonate + 20 digestive enzymes
b. Enzymes for all three classes of macromolecules
c. Bicarbonate formation
1) Made by cells lining the ductules
2) Made from CO2 from the blood
a) First, carbonic acid is made.
b) This dissociates to form H+ and bicarbonate.
c) The bicarbonate is secreted into pancreatic
juice, and H+ goes back into the blood.
d) Bicarbonate is countertransported with Cl−.
3) People with cystic fibrosis have trouble secreting
bicarbonate because of defective chloride carriers,
which can lead to destruction of the pancreas.
Formation and Secretion of Bicarbonate
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Lumen
Blood
K+
HCO3–
Na+
K+
ATP
HCO3–
CI–
CI–
K+
CI–
Na+
Na+
HCO3–
H2CO3
CO2
Na+
H+
H+
d. Pancreatic Enzymes
1) Most are inactive (zymogens) until they reach the
small intestine.
a) Enterokinase activates trypsinogen  trypsin (to
digest protein).
b) Trypsin activates other enzymes.
Activation of Pancreatic Enzymes
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Pancreatic
juice
Other inactive
enzymes
(zymogens)
Active
enzymes
Trypsin
En
Microvilli
Epithelial cell of duodenum
VI. Regulation of the Digestive
System
A. Introduction
1. Neural and endocrine control mechanisms
a. Modifies GI tract functioning
b. Sight, smell, taste, or thought of food can
stimulate salivation and gastric secretions to
“prime” the digestive tract for food.
c. Stimulation goes from brain to organ via vagus
nerve; a conditioned reflex
d. Short reflexes – do not involve the brain
e. GI tract produces some hormones and is the
target for action
Effects of GI hormones
B. Regulation of gastric function
1. Motility and secretion are somewhat automatic.
a. Contractions are stimulated spontaneously by
pacesetter cells in greater curvature of stomach
(intrinsic regulation).
b. Secretion of HCl and pepsinogen occurs when
proteins enter the stomach.
1) Initiated and regulated by G cells (gastrin), D
cells (somatostatin), and ECL cells
(histamine)
2. Three phases of extrinsic gastric regulation
a. Cephalic phase: control by brain via vagus nerves
1) Stimulates ECL (major response), chief cells,
and parietal cells
2) Lasts for the first 30 minutes of a meal
Three phases of extrinsic gastric regulation, cont
b. Gastric phase: triggered by arrival of food into
stomach
1) Gastric secretion is stimulated by stomach
distension (amount of food that enters) and the
chemical nature of the chyme.
2) Positive feedback occurs; as more proteins are
broken down, more secretions are released to
break them down (glucose has no effect and fat
inhibits gastric secretion).
Gastric phase, cont
3) There is also a negative-feedback system. As pH
drops (due to more HCl), somatostatin is released.
This inhibits gastrin secretion.
4) Large amounts of proteins and polypeptides buffer
pH, so secretion matches protein concentration.
Regulation of gastric secretion
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Gastric
epithelium
Amino
acids
Stomach
lumen
+
Vagus nerve
+
Gastrin
–
G cell
ECL
cell
Systemic
circulation
+
+
Histamine
HCl
+
Parietal cell
Three phases of extrinsic gastric regulation, cont
c. ________________: inhibition of gastric activity
when chyme enters the small intestine
1) Slows the movement of chyme into the
duodenum to allow enough time for digestion
and absorption
2) Stretch when food enters the duodenum
stimulates a neural reflex that inhibits gastric
stimulation via the vagus nerve.
3) The presence of fats stimulates the duodenum
to make enterogastrone which inhibits gastric
function.
4) Enterogastric hormones
a) GIP – glucose-dependent insulinotropic peptide
– stimulates insulin secretion and inhibits gastric
function
b) Somatostatin
c) CCK – cholecystokinin – secreted by the
duodenum in response to chyme
d) GLP-1 – glucagon-like peptide-1 – secreted by
the ileum – insulin stimulator
Three Phases of Gastric Secretion
C. Regulation of Intestinal Function
1. Enteric nervous system (ENS): neurons and glial
cells that innervate the intestines
a. Includes myenteric plexus and submucosal
plexus
b. Acts independently from CNS but with some
feedback to CNS via vagus nerve (extrinsic
afferents)
c. Involved in regulation by the ANS
d. Innervates interstitial cells of Cajal and smooth
muscle to intrinsically regulate peristalsis
Enteric Nervous System Coordinates Peristalsis
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2.
Motor neurons
stimulate contraction
behind bolus
Excitation
Smooth
muscle
+
Interneurons activate
motor neurons
ACh
Contraction
Substance P
ACh
1.
Bolus of food
stimulates sensory
neurons
Distention
Interneurons activate
motor neurons
Mucosal
stimulus
Inhibition
Bolus
2.
Motor neurons
produce relaxation
in front of bolus
Relaxation
–
Motor neurons
Interneurons
Sensory
Lumen
Mucosa and
submucosa
Circular
muscle
Longitudinal
muscle
2. Paracrine Regulation
a. Enterochromaffin-like cells in intestinal mucosa
secrete serotonin and motilin in response to
pressure (filling) and chemicals in the food. This
stimulates muscle contractions.
b. ________: made in ileum and colon; stimulates the
secretion of water and Cl− and inhibits absorption of
Na+. More water and salt are lost in feces. Acts by
activating guanylate cyclase to produce second
messenger cGMP
3. Intestinal reflexes
a. Gastroileal reflex: increased gastric activity,
increased ileum activity and movement of food
through ileocecal valve
b. ___________ reflex: distension of ileum causes a
decrease in gastric motility
c. Intestino-intestinal reflex: Over-distension of one
portion of the intestine causes relaxation of other
portions.
D. Regulation of pancreatic juice and bile secretion
1. When chyme enters the duodenum, two hormones
are produced:
a. Secretin is produced in response to a drop in
pH. (Production stops with a rise in pH.)
b. Cholecystokinin (CCK) is produced in response
to the presence of partially digested proteins
and fats in chyme. (Production stops when food
leaves small intestine.)
2. Secretion of Pancreatic Juice
a. Pancreatic enzyme production of trypsin, lipase,
and amylase is stimulated by ACh from vagus
nerve, CCK, and secretin.
1) ACh and CCK use Ca2+ as a second
messenger.
2) Secretin uses cAMP as a second messenger.
b. Bicarbonate production is stimulated by secretin.
3. Secretion of Bile
a. The liver produces bile continuously, but the arrival
of food into the duodenum stimulates increased
production of bile.
b. Happens when:
1) Bile acids are returned to the liver after intestinal
absorption via enterohepatic circulation.
2) Secretin and CCK stimulate increased
bicarbonate secretion into bile.
3) CCK (in response to the presence of fat in
chyme) stimulates gallbladder contraction.
E. Trophic effects of GI hormones
1. GI hormones have a trophic effect for maintenance
of their target organs
2. Structure of the gastric mucosa is dependent on
gastrin
3. Structure of the acinar cells of the pancreas
depends on CCK
VII. Digestion and Absorption
of Food
A. Digestion & Absorption of Carbohydrates
1. Most carbohydrates are ingested as starch or
sugars such as sucrose and lactose
a. Starch digestion begins in mouth with salivary
amylase; polysaccharides  shorter chains
b. No digestion in the stomach – too acidic
c. Continues in intestines with pancreatic
amylase; short chains  disaccharides &
maltriose
d. Brush border enzymes finish breaking down
disaccharides (maltose, sucrose, lactose) to
simple sugars (mainly glucose)
Action of pancreatic amylase
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Short
oligosaccharide
Glucose
Amylase
Maltriose
Starch
Maltose
2. Absorption of Carbohydrates
a. Monosaccharides are absorbed across the
epithelium via:
1) Secondary active transport with sodium (Clfollows)
2) Facilitated diffusion through GLUT carriers into
interstitial fluid and then to capillary blood of the
villus
3) Water follows the NaCl through a paracellular
route and is absorbed with glucose and NaCl
B. Digestion & Absorption of Proteins
1. Digestion of proteins
a. Begins in ______ with pepsin and hydrochloric
acid to produce short-chain polypeptides
b. Finishes in duodenum and jejunum with
pancreatic trypsin, chymotrypsin, elastase, and
carboxypeptidase, and the brush border enzyme
aminopeptidase.
c. Final products are amino acids, some dipeptides
and tripeptides
2. Absorption of Proteins
a. Free amino acids cotransported with Na+
b. Dipeptides and tripeptides cross via secondary
active transport using a H+ gradient.
1) Hydrolyzed into free amino acids within the
cytoplasm of the epithelial cells
c. Free amino acids move by facilitated diffusion into
interstitial fluid, then to the blood capillaries of the
villi
Digestions & Absorption of Proteins
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Lumen
Polypeptide chain
Exopeptidase
Endopeptidase
Amino acids
Brush border
enzymes
(microvilli)
Tripeptidase
Dipeptidase
Epithelial cell
Capillary blood
C. Digestion & Absorption of Fats
1. Digestion of fats
a. Fat digestion begins in ________ when bile
emulsifies the fat and the pancreatic enzyme
lipase, aided by colipase, breaks it down into
fatty acids and glycerol
b. Phospholipase A (from pancreas) digests
phospholipids into fatty acids and lysolecithin.
Digestion of Fats
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H
O
H
C
O
C
O
H
C
O
C
O
H
C
O
C
Triglyceride
H
Glycerol
Fatty acids
Lipase
+
2 HOH
H
H
H
C
OH
C
O
C
OH
O
C
+
H
Monoglyceride
Free fatty acids
Fat Emulsification and Digestion
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Step 1: Emulsification of fat droplets by bile salts
From liver and
gallbladder
Step 2: Hydrolysis of triglycerides in emulsified fat
droplets into fatty acid and monoglycerides
Step 3: Dissolving of fatty acids and monoglycerides
into micelles to produce “mixed micelles”
Bile
duct
Micelles of bile salts,
cholesterol, and lecithin
1
2
From
stomach
+
Free fatty
acids
Lipase
Monoglycerides
Fat
droplets
(triglycerides)
Emulsified
fat droplets
(triglycerides)
3
Into
micelles
2. Absorption of Fats
a. Fatty acids, monoglycerides, and lysolecithin move
into bile ________ and are transported to brush
border.
b. The fat molecules then leave the micelles and
diffuse into the epithelial cells of the villi.
c. Inside the epithelial cells, they are regenerated into
triglycerides, cholesterol, and phospholipids and
combined with proteins to form ____________.
d. Chylomicrons are secreted by exocytosis into the
central lacteal of the villus
Fat Absorption
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Mixed Micelles
with
Triglycerides
Bile salts,
lipase
Fatty acids
Lumen of
small intestine
Monoglycerides
Fatty acids Monoglycerides
Triglycerides
+ Protein
Chylomicrons
To thoracic duct
Lacteal
3. Transport of Lipids in Blood
a. The lymphatic system drops chylomicrons into the
bloodstream at the thoracic duct.
b. They pick up an apolipoprotein (ApoE), which
allows them to bind to receptors on the capillary
endothelium within muscles and adipose tissue.
1) Here they are digested by lipoprotein lipase,
which releases free fatty acids for use by muscle
cells or for storage by fat cells.
2) The remaining remnant particle containing
cholesterol is released and travels in the blood
to the liver.
Transport of Lipids in Blood, cont
c. Cholesterol and triglycerides made in the liver are
combined with other apolipoproteins to form verylow-density lipoproteins (VLDLs) to deliver
triglycerides to organs.
d. Once triglycerides are removed, they are lowdensity lipoproteins (LDLs), which transport
cholesterol to organs.
e. Excess cholesterol is returned to the liver on highdensity lipids (HDL).
Transport of Lipids in Blood
f. HDL particles bind to receptors in the blood vessel
walls and capture phospholipids and free
cholesterol, reducing cholesterol amounts.
g. Once the HDL is full, it travels to the liver and
unloads the cholesterol.
Lipid Carrier Proteins
Characteristics of Major Digestive Enzymes