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Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)
Tongue
Esophagus
Liver
Gallbladder
Duodenum
Jejunum
Small
intestine Ileum
Anus
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Parotid gland
Sublingual gland Salivary
Submandibular
glands
gland
Pharynx
Stomach
Pancreas
(Spleen)
Transverse colon
Descending colon
Ascending colon
Large
Cecum
intestine
Sigmoid colon
Rectum
Vermiform appendix
Anal canal
Figure 23.2 Gastrointestinal tract activities.
Ingestion
Mechanical
digestion
• Chewing (mouth)
• Churning (stomach)
• Segmentation
(small intestine)
Chemical
digestion
Food
Pharynx
Esophagus
Propulsion
• Swallowing
(oropharynx)
• Peristalsis
Stomach
(esophagus,
stomach,
small intestine,
large intestine)
Absorption
Lymph
vessel
Small
intestine
Large
intestine
Defecation
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Blood
vessel
Mainly H2O
Feces
Anus
Figure 23.3 Peristalsis and segmentation.
From
mouth
(a) Peristalsis: Adjacent segments of
alimentary tract organs alternately contract
and relax, which moves food along the tract
distally.
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(b) Segmentation: Nonadjacent segments
of alimentary tract organs alternately
contract and relax, moving the food
forward then backward. Food mixing and
slow food propulsion occurs.
Figure 23.3a Peristalsis and segmentation.
From mouth
(a) Peristalsis: Adjacent segments of alimentary
tract organs alternately contract and relax,
which moves food along the tract distally.
Copyright © 2010 Pearson Education, Inc.
Figure 23.3b Peristalsis and segmentation.
(b) Segmentation: Nonadjacent segments of
alimentary tract organs alternately contract and
relax, moving the food forward then backward.
Food mixing and slow food propulsion occurs.
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Figure 23.4 Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract.
External stimuli
(sight, smell, taste,
thought of food)
Central nervous system
and extrinsic autonomic nerves
Long reflexes
Afferent impulses
Internal
(GI tract)
stimuli
Efferent impulses
Chemoreceptors,
osmoreceptors, or
mechanoreceptors
Local (intrinsic)
nerve plexus
(“gut brain”)
Effectors:
Smooth muscle
or glands
Short reflexes
Gastrointestinal
wall (site of short
reflexes)
Lumen of the
alimentary canal
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Response:
Change in
contractile or
secretory activity
Figure 23.5 The peritoneum and the peritoneal cavity.
Abdominopelvic
cavity
Vertebra
Dorsal
mesentery
Parietal
peritoneum
Ventral
mesentery
Visceral
peritoneum
Peritoneal
cavity
Alimentary
canal organ
Mesentery
resorbed
and lost
Liver
(a) Schematic cross sections of
abdominal cavity illustrate the
peritoneums and mesenteries.
Alimentary
Alimentary canal organ in
canal organ
a retroperitoneal position
(b) Some organs lose their mesentery
and become retroperitoneal during
development.
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Figure 23.5a The peritoneum and the peritoneal cavity.
Abdominopelvic
cavity
Vertebra
Dorsal
mesentery
Parietal
peritoneum
Ventral
mesentery
Visceral
peritoneum
Peritoneal
cavity
Alimentary
canal organ
Liver
(a) Schematic cross sections of abdominal cavity
illustrate the peritoneums and mesenteries.
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Figure 23.5b The peritoneum and the peritoneal cavity.
Abdominopelvic
cavity
Mesentery
resorbed
and lost
Alimentary
Alimentary canal organ in
canal organ
a retroperitoneal position
(b) Some organs lose their mesentery and
become retroperitoneal during development.
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Figure 23.6 Basic structure of the alimentary canal.
Intrinsic nerve plexuses
• Myenteric nerve plexus
• Submucosal nerve plexus
Glands in submucosa
Mucosa
• Epithelium
• Lamina propria
• Muscularis
mucosae
Submucosa
Muscularis
externa
Nerve
Artery
Vein
Mesentery
Gland in mucosa
Lymphatic
Duct of gland outside
vessel
alimentary canal
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• Longitudinal
muscle
• Circular muscle
Serosa
• Epithelium
• Connective
tissue
Lumen
Mucosa-associated
lymphoid tissue
Figure 23.7 Anatomy of the oral cavity (mouth).
Soft palate
Palatoglossal arch
Hard palate
Uvula
Upper lip
Oral cavity
Palatine
tonsil
Tongue
Oropharynx
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
Gingivae
(gums)
Palatine
raphe
Hard palate
Soft palate
Uvula
Palatine
tonsil
Trachea
(a) Sagittal section of the oral cavity and pharynx
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Palatoglossal
arch
Palatopharyngeal
arch
Posterior wall
of oropharynx
Tongue
Sublingual
fold with
openings of
sublingual
ducts
Esophagus
Superior labial
frenulum
Lingual frenulum
Opening of
submandibular
duct
Gingivae (gums)
Vestibule
Lower lip
(b) Anterior view
Inferior labial
frenulum
Figure 23.7a Anatomy of the oral cavity (mouth).
Soft palate
Palatoglossal arch
Hard palate
Uvula
Oral cavity
Palatine tonsil
Tongue
Oropharynx
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
Esophagus
Trachea
(a) Sagittal section of the oral cavity and pharynx
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Figure 23.7b Anatomy of the oral cavity (mouth).
Gingivae (gums)
Palatine raphe
Hard palate
Soft palate
Uvula
Palatine tonsil
Sublingual fold
with openings of
sublingual ducts
Vestibule
Lower lip
(b) Anterior view
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Upper lip
Superior labial
frenulum
Palatoglossal arch
Palatopharyngeal
arch
Posterior wall
of oropharynx
Tongue
Lingual frenulum
Opening of
submandibular duct
Gingivae (gums)
Inferior labial
frenulum
Figure 23.8 Dorsal surface of the tongue, and the tonsils.
Epiglottis
Palatopharyngeal
arch
Palatine tonsil
Lingual tonsil
Palatoglossal
arch
Terminal sulcus
Foliate papillae
Circumvallate
papilla
Midline groove
of tongue
Dorsum of tongue
Fungiform papilla
Filiform papilla
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Figure 23.9 The salivary glands.
Ducts of
sublingual
gland
Tongue
Teeth
Lingual
frenulum
Sublingual
gland
Mylohyoid
muscle (cut)
Anterior belly of
digastric muscle
(a)
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Parotid
gland
Parotid duct
Masseter muscle
Body of
mandible (cut)
Posterior belly of
digastric muscle
Submandibular
duct
Submandibular
gland
Mucous
cells
(b)
Serous cells
forming
demilunes
Figure 23.9a The salivary glands.
Ducts of
sublingual
gland
Tongue
Teeth
Lingual
frenulum
Sublingual
gland
Mylohyoid
muscle (cut)
Anterior belly of
digastric muscle
(a)
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Parotid
gland
Parotid duct
Masseter muscle
Body of
mandible (cut)
Posterior belly of
digastric muscle
Submandibular
duct
Submandibular
gland
Figure 23.9b The salivary glands.
(b)
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Mucous
cells
Serous cells
forming
demilunes
Figure 23.10a Human dentition.
Incisors
Central (6–8 mo)
Lateral (8–10 mo)
Canine (eyetooth)
(16–20 mo)
Molars
First molar
(10–15 mo)
Second molar
(about 2 yr)
(a)
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Deciduous
(milk) teeth
Incisors
Central (7 yr)
Lateral (8 yr)
Canine (eyetooth)
(11 yr)
Premolars
(bicuspids)
First premolar
(11 yr)
Second premolar
(12–13 yr)
Molars
First molar (6–7 yr)
Second molar
(12–13 yr)
Third molar
(wisdom tooth)
(17–25 yr)
Permanent
teeth
Figure 23.10b Human dentition.
(b)
Deciduous teeth
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Permanent teeth
Figure 23.11 Longitudinal section of a canine tooth within its bony alveolus.
Crown
Neck
Enamel
Dentin
Dentinal tubules
Pulp cavity (contains
blood vessels and
nerves)
Gingiva (gum)
Cementum
Root
Root canal
Periodontal
ligament
Apical foramen
Bone
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Figure 23.12 Microscopic structure of the esophagus.
Mucosa
(contains a stratified
squamous epithelium)
Submucosa (areolar
connective tissue)
Lumen
Muscularis externa
• Circular layer
• Longitudinal layer
Adventitia (fibrous
connective tissue)
(a)
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(b)
Figure 23.12a Microscopic structure of the esophagus.
(a)
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Mucosa
(contains a stratified
squamous epithelium)
Submucosa (areolar
connective tissue)
Lumen
Muscularis externa
• Longitudinal layer
• Circular layer
Adventitia (fibrous
connective tissue)
Figure 23.12b Microscopic structure of the esophagus.
Mucosa
(contains a stratified
squamous epithelium)
(b)
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Figure 23.13 Deglutition (swallowing) (1 of 5).
Bolus of food
Tongue
Pharynx
Epiglottis
Glottis
Trachea
1 Upper esophageal sphincter is contracted. During
the buccal phase, the tongue presses against the hard
palate, forcing the food bolus into the oropharynx
where the involuntary phase begins.
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Figure 23.13 Deglutition (swallowing) (2 of 5).
Uvula
Bolus
Epiglottis
Esophagus
2 The uvula and larynx rise to prevent food from
entering respiratory passageways. The tongue blocks
off the mouth. The upper esophageal sphincter
relaxes, allowing food to enter the esophagus.
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Figure 23.13 Deglutition (swallowing) (3 of 5).
Bolus
3 The constrictor muscles of the pharynx contract,
forcing food into the esophagus inferiorly. The upper
esophageal sphincter contracts (closes) after entry.
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Figure 23.13 Deglutition (swallowing) (4 of 5).
Relaxed muscles
Circular muscles
contract
4 Food is moved through
the esophagus to the
stomach by peristalsis.
Bolus of food
Longitudinal muscles
contract
Gastroesophageal
sphincter closed
Stomach
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Figure 23.13 Deglutition (swallowing) (5 of 5).
Relaxed
muscles
5 The gastroesophageal
sphincter opens, and food
enters the stomach.
Gastroesophageal
sphincter opens
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Figure 23.14a Anatomy of the stomach.
Cardia
Esophagus
Muscularis
externa
• Longitudinal layer
• Circular layer
• Oblique layer
Lesser
curvature
Fundus
Serosa
Body
Lumen
Rugae of
mucosa
Greater
curvature
Duodenum
(a)
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Pyloric
Pyloric
canal
antrum
Pyloric sphincter
(valve) at pylorus
Figure 23.14b Anatomy of the stomach.
Fundus
Liver
(cut)
Body
Spleen
Lesser
curvature
Greater
curvature
(b)
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Figure 23.15a Microscopic anatomy of the stomach.
Surface
epithelium
Mucosa
Lamina propria
Submucosa
(contains submucosal
plexus)
Muscularis externa
(contains myenteric
plexus)
Serosa
Muscularis
mucosae
Oblique layer
Circular layer
Longitudinal
layer
(a) Layers of the stomach wall (l.s.)
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Stomach wall
Figure 23.15b Microscopic anatomy of the stomach.
Gastric pits
Surface epithelium
(mucous cells)
Gastric
pit
Mucous neck cells
Parietal cell
Chief cell
Gastric
gland
Enteroendocrine cell
(b) Enlarged view of gastric pits and gastric glands
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Figure 23.15c Microscopic anatomy of the stomach.
Pepsinogen
HCl
Pepsin
Mitochondria
Parietal cell
Chief cell
Enteroendocrine
cell
(c) Location of the HCl-producing parietal cells and
pepsin-secreting chief cells in a gastric gland
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Figure 23.16 Photographs of a gastric ulcer lesion and of the bacteria that most commonly cause it.
Bacteria
Mucosa
layer of
stomach
(a) A gastric ulcer lesion
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(b) H. pylori bacteria
Figure 23.16a Photographs of a gastric ulcer lesion and of the bacteria that most commonly cause it.
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Figure 23.16b Photographs of a gastric ulcer lesion and of the bacteria that most commonly cause it.
Bacteria
Mucosa
layer of
stomach
(b) H. pylori bacteria
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Figure 23.17 Neural and hormonal mechanisms that regulate release of gastric juice.
Stimulatory events
Cephalic
phase
Gastric
phase
1 Sight and thought
of food
Cerebral cortex
Conditioned reflex
2 Stimulation of
taste and smell
receptors
Hypothalamus
and medulla
oblongata
1 Stomach
distension
activates
stretch
receptors
Vagovagal
reflexes
1 Presence of low
pH, partially digested
foods, fats, or
hypertonic solution
in duodenum when
stomach begins to
empty
Stimulate
Inhibit
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Medulla
Vagus
nerve
Vagus
nerve
Local
reflexes
2 Food chemicals
G cells
(especially peptides and
caffeine) and rising pH
activate chemoreceptors
Intestinal
phase
Inhibitory events
Gastrin
release
to blood
Intestinal
(enteric)
gastrin
release
to blood
Lack of
stimulatory
impulses to
parasympathetic
center
Cerebral
cortex
Gastrin
secretion
declines
G cells
Overrides
parasympathetic
controls
Sympathetic
nervous
system
activation
1 Excessive
acidity
(pH <2)
in stomach
2 Emotional
upset
Stomach
secretory
activity
Enterogastric
reflex
Brief
effect
1 Loss of
appetite,
depression
Local
reflexes
Vagal
nuclei
in medulla
Pyloric
sphincter
1 Distension
of duodenum;
presence of
fatty, acidic,
hypertonic
chyme, and/or
irritants in
the duodenum
2 Distension;
Release of intestinal
presence of
hormones (secretin,
cholecystokinin, vasoactive fatty, acidic,
partially
intestinal peptide)
digested food
in the
duodenum
Figure 23.18 Mechanism of HCl secretion by parietal cells.
Blood
capillary
Chief cell
CO2
CO2 + H2O
Carbonic
H2CO3 anhydrase
H+
K+
Stomach lumen
H+-K+
ATPase
H+
K+
HCO3–
Alkaline
tide
HCI
Parietal cell
HCO3–
Cl–
Interstitial
fluid
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Cl–
HCO3–- Cl–
antiporter
Cll–
Figure 23.19 Peristaltic waves in the stomach.
Pyloric
valve
closed
1 Propulsion: Peristaltic
waves move from the
fundus toward the
pylorus.
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Pyloric
valve
closed
2 Grinding: The most
vigorous peristalsis and
mixing action occur
close to the pylorus.
Pyloric
valve
slightly
opened
3 Retropulsion: The pyloric
end of the stomach acts as a
pump that delivers small
amounts of chyme into the
duodenum, simultaneously
forcing most of its contained
material backward into the
stomach.
Figure 23.20 Neural and hormonal factors inhibiting gastric emptying.
Presence of fatty, hypertonic,
acidic chyme in duodenum
Duodenal enteroendocrine cells
Chemoreceptors and
stretch receptors
Secrete
Enterogastrones
(secretin,
cholecystokinin,
vasoactive intestinal
peptide)
Duodenal
stimuli
decline
Initial stimulus
Physiological response
Result
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Target
Via short
reflexes
Enteric
neurons
Contractile force and
rate of stomach
emptying decline
Via long
reflexes
CNS centers
sympathetic
activity;
parasympathetic
activity
Stimulate
Inhibit
Figure 23.21 The duodenum of the small intestine, and related organs.
Right and left
hepatic ducts
of liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Mucosa
with folds
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
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Tail of pancreas
Pancreas
Jejunum
Duodenum
Main pancreatic duct
and sphincter
Head of pancreas
Figure 23.22a Structural modifications of the small intestine that increase its surface area for digestion and absorption.
Vein carrying blood to
hepatic portal vessel
Muscle
layers
Circular
folds
Villi
(a)
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Lumen
Figure 23.22b Structural modifications of the small intestine that increase its surface area for digestion and absorption.
Microvilli
(brush border)
Absorptive cells
Lacteal
Goblet cell
Blood
capillaries
Mucosa
associated
lymphoid tissue
Intestinal crypt
Muscularis
mucosae
Duodenal gland
(b)
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Vilus
Enteroendocrine
cells
Venule
Lymphatic vessel
Submucosa
Figure 23.22c Structural modifications of the small intestine that increase its surface area for digestion and absorption.
Absorptive cells
Goblet
cells
Villi
(c) Intestinal crypt
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Figure 23.23 Villi and microvilli of the small intestine.
Villi
Microvilli
Desquamating
cells
(a)
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(b)
Absorptive
cell
Figure 23.23a Villi and microvilli of the small intestine.
Villi
Desquamating
cells
(a)
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Figure 23.23b Villi and microvilli of the small intestine.
Microvilli
(b)
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Absorptive
cell
Figure 23.24a Gross anatomy of the human liver.
Sternum
Nipple
Liver
Bare area
Falciform
ligament
Left lobe of liver
Right lobe
of liver
Gallbladder
(a)
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Round ligament
(ligamentum
teres)
Figure 23.24b Gross anatomy of the human liver.
Sternum
Nipple
Liver
Lesser omentum
(in fissure)
Left lobe of liver
Porta hepatis
containing hepatic
artery (left) and
hepatic portal vein
(right)
Quadrate lobe
of liver
Ligamentum teres
(b)
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Bare area
Caudate lobe
of liver
Sulcus for
inferior
vena cava
Hepatic vein
(cut)
Bile duct (cut)
Right lobe of
liver
Gallbladder
Figure 23.25a-b Microscopic anatomy of the liver.
(a)
Lobule
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(b)
Central vein
Connective
tissue septum
Figure 23.25c Microscopic anatomy of the liver.
Interlobular veins
(to hepatic vein)
Central vein
Sinusoids
Bile canaliculi
Plates of
hepatocytes
Bile duct (receives
bile from bile
canaliculi)
Fenestrated
lining (endothelial
cells) of sinusoids
Hepatic
macrophages
in sinusoid walls
Portal vein
(c)
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Bile duct
Portal venule
Portal arteriole
Portal triad
Figure 23.26 Structure of the enzyme-producing tissue of the pancreas.
Small duct
Acinar cells
Basement membrane
Zymogen granules
(a)
Rough endoplasmic
reticulum
Acinar cells
(b)
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Figure 23.26a Structure of the enzyme-producing tissue of the pancreas.
Small
duct
Acinar cells
Basement
membrane
Zymogen
granules
Rough
endoplasmic
reticulum
(a)
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Figure 23.26b Structure of the enzyme-producing tissue of the pancreas.
Acinar
cells
(b)
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Figure 23.27 Activation of pancreatic proteases in the small intestine.
Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
Trypsin
(inactive)
Chymotrypsinogen
Chymotrypsin
(inactive)
Procarboxypeptidase
Carboxypeptidase
(inactive)
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Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice.
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
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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4 Bile salts and,
to a lesser extent,
secretin
transported via
bloodstream
stimulate liver to
produce bile
more rapidly.
5
CCK (via
bloodstream)
causes
gallbladder to
contract and
hepatopancreatic
sphincter to
relax; bile enters
duodenum.
6
During
cephalic and
gastric phases,
vagal nerve
stimulation
causes weak
contractions of
gallbladder.
Figure 23.29a Gross anatomy of the large intestine.
Left colic
(splenic) flexure
Transverse
mesocolon
Epiploic
appendages
Right colic
(hepatic)
flexure
Transverse
colon
Superior
mesenteric
artery
Haustrum
Descending
colon
Ascending
colon
IIeum
Cut edge of
mesentery
Teniae coli
IIeocecal
valve
Cecum
Vermiform appendix
Sigmoid
colon
Rectum
Anal canal
(a)
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External anal sphincter
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
(b)
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Figure 23.30a Mesenteries of the abdominal digestive organs.
Falciform ligament
Liver
Gallbladder
Spleen
Stomach
Ligamentum teres
Greater omentum
Small intestine
Cecum
(a)
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Figure 23.30b Mesenteries of the abdominal digestive organs.
Liver
Gallbladder
Lesser omentum
Stomach
Duodenum
Transverse colon
Small intestine
Cecum
Urinary bladder
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(b)
Figure 23.30c Mesenteries of the abdominal digestive organs.
Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
(c)
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Figure 23.30d Mesenteries of the abdominal digestive organs.
Liver
Lesser omentum
Pancreas
Stomach
Transverse
mesocolon
Duodenum
Transverse colon
Mesentery
Greater omentum
Jejunum
Ileum
Visceral peritoneum
Parietal peritoneum
(d)
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Urinary bladder
Rectum
Figure 23.31 Defecation reflex.
Impulses from
cerebral cortex
(conscious
control)
1
Sensory
nerve fibers
Distension, or stretch, of the
rectal walls due to movement
of feces into the rectum
stimulates stretch receptors
there. The receptors transmit
signals along afferent fibers to
spinal cord neurons.
2
Voluntary motor
nerve to external
anal sphincter
Sigmoid
colon
A spinal reflex is initiated in
which parasympathetic motor
(efferent) fibers stimulate
contraction of the rectal walls
and relaxation of the internal
anal sphincter.
Stretch receptors in wall
Rectum
External anal
sphincter
(skeletal muscle)
3
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 that feces may pass.
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Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (1 of 4).
Carbohydrate digestion
Foodstuff
Enzyme(s)
and source
Site of
action
Starch and disaccharides
Oligosaccharides
and disaccharides
Lactose Maltose Sucrose
Galactose Glucose Fructose
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Salivary
amylase
Pancreatic
amylase
Brush border
enzymes in
small intestine
(dextrinase, glucoamylase, lactase,
maltase, and sucrase)
Mouth
Small
intestine
Small
intestine
Path of absorption
• 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.
Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (2 of 4).
Protein digestion
Foodstuff
Protein
Large polypeptides
Small polypeptides,
small peptides
Amino acids
(some dipeptides
and tripeptides)
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Enzyme(s)
and source
Pepsin
(stomach glands)
in presence
of HCl
Pancreatic
enzymes
(trypsin, chymotrypsin,
carboxypeptidase)
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Site of
action
Path of absorption
• Amino acids are absorbed
by cotransport with
Stomach
sodium ions.
• Some dipeptides and
tripeptides are absorbed
via cotransport with H++
Small
and hydrolyzed to amino
intestine
acids within the cells.
• Amino acids leave the
epithelial cells by
Small
facilitated diffusion, enter
intestine
the capillary blood in the
villi, and are transported
to the liver via the hepatic
portal vein.
Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (3 of 4).
Fat digestion
Foodstuff
Enzyme(s)
and source
Unemulsified
fats
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Pancreatic
lipases
Monoglycerides Glycerol
and fatty acids
and
fatty acids
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Site of
action
Path of absorption
• Fatty acids and monoglycerides
enter the intestinal cells via
diffusion.
Small
intestine • Fatty acids and monoglycerides
are recombined to form
triglycerides and then
combined with other lipids and
proteins within the cells, and
the resulting chylomicrons are
Small
extruded by exocytosis.
intestine
• 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.
Figure 23.32 Flowchart of chemical digestion and absorption of foodstuffs (4 of 4).
Nucleic acid digestion
Foodstuff
Enzyme(s)
and source
Nucleic acids
Pentose sugars,
N-containing bases,
phosphate ions
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Pancreatic ribonuclease and
deoxyribonuclease
Brush border
enzymes
(nucleosidases
and phosphatases)
Site of
action
Path of absorption
• Units enter intestinal cells
by active transport via
Small
intestine membrane carriers.
• Units are absorbed into
capillary blood in the villi
Small
and transported to the
intestine
liver via the hepatic portal
vein.
Figure 23.33 Protein digestion and absorption in the small intestine.
Amino acids of protein fragments
Brush border enzymes
Apical membrane (microvilli)
Lumen of
intestine
Pancreatic
proteases
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.
Na+
Na+
Absorptive
epithelial
cell
2 The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side (transcytosis).
Amino
acid
carrier
3 The amino acids leave the
Active transport
Passive transport
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Capillary
villus epithelial cell by facilitated
diffusion and enter the capillary
via intercellular clefts.
Figure 23.34 Emulsification, digestion, and absorption of fats.
Fat globule
1 Large fat globules are emulsified
(physically broken up into smaller fat
droplets) by bile salts in the duodenum.
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 lipoid substances and proteins
to form chylomicrons.
4 Chylomicrons are extruded from the
Epithelial
cells of
small
intestine
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Lacteal
epithelial cells by exocytosis. The
chylomicrons enter lacteals. They are
carried away from the intestine by lymph.
Figure 23.35 Embryonic development of the digestive system.
Lung bud
Brain
Oral
membrane
Heart
Yolk sac
Cloacal
membrane
Body
stalk
Stomodeum
Foregut
Site of
liver
development
Midgut
Spinal cord
Bile
duct
Gallbladder
Hindgut
Cystic duct
Ventral pancreatic bud
Proctodeum
Endoderm
(a)
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Stomach
Liver
(b)
Dorsal
pancreatic
bud
Duodenum
Figure 23.35a Embryonic development of the digestive system.
Brain
Oral
membrane
Heart
Yolk sac
Cloacal
membrane
Body
stalk
(a)
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Stomodeum
Foregut
Site of
liver
development
Midgut
Spinal cord
Hindgut
Proctodeum
Endoderm
Figure 23.35b Embryonic development of the digestive system.
Lung bud
Liver
Stomach
Bile
duct
Stomodeum
Gallbladder
Cystic duct
Ventral pancreatic bud
(b)
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Dorsal
pancreatic
bud
Duodenum
Table 23.1 Hormones and Paracrines that Act in Digestion (1 of 2)
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Table 23.1 Hormones and Paracrines that Act in Digestion (2 of 2)
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Table 23.2 Overview of the Functions of the Gastrointestinal Organs (1 of 2)
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Table 23.2 Overview of the Functions of the Gastrointestinal Organs (2 of 2)
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Table 23.3 Control of Small Intestinal Motility
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Making Connections 23.1 Homeostatic Interrelationships Between the Digestive System and Other Body Systems
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