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prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
23
The Digestive
System
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Digestive System
• Two groups of organs
1. Alimentary canal (gastrointestinal or GI
tract)
• Mouth to anus
• Digests food and absorbs fragments
• Mouth, pharynx, esophagus, stomach,
small intestine, and large intestine
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Digestive System
2. Accessory digestive organs
• Teeth, tongue, gallbladder
• Digestive glands
– Salivary glands
– Liver
– Pancreas
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Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)
Tongue*
Parotid gland
Sublingual gland
Submandibular gland
Salivary
glands*
Pharynx
Esophagus
Stomach
Pancreas*
(Spleen)
Liver*
Gallbladder*
Transverse colon
Small
intestine
Anus
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Duodenum
Jejunum
Ileum
Descending colon
Ascending colon
Cecum
Sigmoid colon
Rectum
Appendix
Anal canal
Large
intestine
Digestive Processes
• Six essential activities
1.
2.
3.
4.
5.
6.
Ingestion
Propulsion
Mechanical breakdown
Digestion
Absorption
Defecation
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Figure 23.2 Gastrointestinal tract activities.
Ingestion
Mechanical
breakdown
• Chewing (mouth)
• Churning (stomach)
• Segmentation
(small intestine)
Digestion
Food
Pharynx
Esophagus
Propulsion
• Swallowing
(oropharynx)
• Peristalsis
(esophagus,
stomach,
small intestine,
large intestine)
Stomach
Absorption
Lymph
vessel
Small
intestine
Large
intestine
Blood
vessel
Mainly H2O
Feces
Defecation
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Anus
Figure 23.3 Peristalsis and segmentation.
From
mouth
Peristalsis: Adjacent segments of alimentary
tract organs alternately contract and relax,
moving food along the tract distally.
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Segmentation: Nonadjacent segments of
alimentary tract organs alternately contract and
relax, moving food forward then backward.
Food mixing and slow food propulsion occur.
GI Tract Regulatory Mechanisms
1. Mechanoreceptors and chemoreceptors
– Respond to stretch, changes in osmolarity
and pH, and presence of substrate and end
products of digestion
– Initiate reflexes that
•
•
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Activate or inhibit digestive glands
Stimulate smooth muscle to mix and move lumen
contents
GI Tract Regulatory Mechanisms
2. Intrinsic and extrinsic controls
– Short reflexes - enteric nerve plexuses (gut
brain) respond to stimuli in GI tract
– Long reflexes respond to stimuli inside or
outside GI tract; involve CNS centers and
autonomic nerves
– Hormones from cells in stomach and small
intestine stimulate target cells in same or
different organs to secrete or contract
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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
Long reflexes
Visceral afferents
Internal
(GI tract)
stimuli
Extrinsic visceral (autonomic)
efferents
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
Peritoneum and Peritoneal Cavity
• Peritoneum - serous membrane of
abdominal cavity
– Visceral peritoneum on external surface of
most digestive organs
– Parietal peritoneum lines body wall
• Peritoneal cavity
– Between two peritoneums
– Fluid lubricates mobile organs
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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 Liver
canal organ
Two schematic cross sections of abdominal cavity illustrate
the peritoneums and mesenteries.
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Peritoneum and Peritoneal Cavity
• Mesentery - double layer of peritoneum
– Routes for blood vessels, lymphatics, and
nerves
– Holds organs in place; stores fat
• Retroperitoneal organs posterior to
peritoneum
• Intraperitoneal (peritoneal) organs
surrounded by peritoneum
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Figure 23.5b The peritoneum and the peritoneal cavity.
Mesentery
resorbed
and lost
Alimentary
canal organ
Alimentary canal organ in
a retroperitoneal position
Some organs lose their mesentery and move,
becoming retroperitoneal, during development.
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Homeostatic Imbalance
• Peritonitis
– Inflammation of peritoneum
– Causes by e.g., piercing abdominal wound,
perforating ulcer, ruptured appendix
– Peritoneal coverings stick together, localizing
infection
– Dangerous and lethal if widespread
– Treated with debris removal and antibiotics
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Blood Supply: Splanchnic Circulation
• Branches of aorta serving digestive organs
– Hepatic, splenic, and left gastric arteries
– Inferior and superior mesenteric arteries
• Hepatic portal circulation
– Drains nutrient-rich blood from digestive
organs
– Delivers it to the liver for processing
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Histology of the Alimentary Canal
• Four basic layers (tunics)
– Mucosa
– Submucosa
– Muscularis externa
– Serosa
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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
• Longitudinal muscle
• Circular muscle
Mesentery
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Nerve
Artery
Gland in mucosa
Vein
Duct of gland outside
Lymphatic vessel
alimentary canal
Serosa
• Epithelium
(mesothelium)
• Connective tissue
Lumen
Mucosa-associated
lymphoid tissue
Mucosa
• Lines lumen
• Functions – different layers perform 1 or
all 3
– Secretes mucus, digestive enzymes, and
hormones
– Absorbs end products of digestion
– Protects against infectious disease
• Three sublayers: epithelium, lamina
propria, and muscularis mucosae
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Mucosa
• Epithelium
– Simple columnar epithelium and mucussecreting cells (most of tract)
• Mucus
– Protects digestive organs from enzymes
– Eases food passage
– May secrete enzymes and hormones (e.g., in
stomach and small intestine)
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Mucosa
• Lamina propria
– Loose areolar connective tissue
– Capillaries for nourishment and absorption
– Lymphoid follicles (part of MALT)
• Defend against microorganisms
• Muscularis mucosae: smooth muscle 
local movements of mucosa
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Submucosa
• Submucosa
– Areolar connective tissue
– Blood and lymphatic vessels, lymphoid
follicles, and submucosal nerve plexus
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Muscularis Externa
• Muscularis externa
– Responsible for segmentation and
peristalsis
– Inner circular and outer longitudinal layers
• Circular layer thickens in some areas 
sphincters
• Myenteric nerve plexus between two muscle layers
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Serosa
• Visceral peritoneum
– Areolar connective tissue covered with
mesothelium in most organs
– Replaced by fibrous adventitia in esophagus
– Retroperitoneal organs have both an
adventitia and serosa
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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
• Longitudinal muscle
• Circular muscle
Mesentery
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Nerve
Artery
Gland in mucosa
Vein
Duct of gland outside
Lymphatic vessel
alimentary canal
Serosa
• Epithelium
(mesothelium)
• Connective tissue
Lumen
Mucosa-associated
lymphoid tissue
Enteric Nervous System
• Intrinsic nerve supply of alimentary canal –
enteric neurons (more than spinal cord)
• Major nerve supply to GI tract wall; control
motility
– Submucosal nerve plexus
• Regulates glands and smooth muscle in the
mucosa
– Myenteric nerve plexus
• Controls GI tract motility
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Enteric Nervous System
• Linked to CNS via afferent visceral fibers
• Long ANS fibers synapse with enteric
plexuses
– Sympathetic impulses inhibit digestive
activities
– Parasympathetic impulses stimulate digestive
activities
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Functional Anatomy: Mouth
• Oral (buccal) cavity
– Bounded by lips, cheeks, palate, and tongue
– Oral orifice is anterior opening
– Lined with stratified squamous epithelium
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Figure 23.7a Anatomy of the oral cavity (mouth).
Soft
palate
Palatoglossal
arch
Uvula
Hard
palate
Oral cavity
Palatine
tonsil
Tongue
Oropharynx
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
Esophagus
Trachea
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Sagittal section of the oral cavity and pharynx
Lips and Cheeks
• Contain orbicularis oris and buccinator
muscles
• Oral vestibule - recess internal to lips
(labia) and cheeks, external to teeth and
gums
• Oral cavity proper lies within teeth and
gums
• Labial frenulum - median attachment of
each lip to gum
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Figure 23.7b Anatomy of the oral cavity (mouth).
Upper lip
Gingivae
(gums)
Palatine
raphe
Hard
palate
Soft
palate
Uvula
Palatine
tonsil
Superior
labial
frenulum
Palatoglossal
arch
Palatopharyngeal
arch
Posterior wall
of oropharynx
Tongue
Sublingual
fold with
openings of
sublingual
ducts
Lingual frenulum
Opening of
Submandibular
duct
Gingivae (gums)
Oral vestibule
Lower lip
Anterior view
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Inferior labial
frenulum
Palate
• Hard palate - palatine bones and palatine
processes of maxillae
– Slightly corrugated to help create friction
against tongue
• Soft palate - fold formed mostly of skeletal
muscle
– Closes off nasopharynx during swallowing
– Uvula projects downward from its free edge
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Tongue
• Skeletal muscle
• Functions include
– Repositioning and mixing food during chewing
– Formation of bolus
– Initiation of swallowing, speech, and taste
• Intrinsic muscles change shape of tongue
• Extrinsic muscles alter tongue's position
• Lingual frenulum: attachment to floor of
mouth
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Tongue
• Surface bears papillae
– Filiform—whitish, give the tongue roughness
and provide friction; do not contain taste buds
– Fungiform—reddish, scattered over tongue;
contain taste buds
– Vallate (circumvallate)—V-shaped row in
back of tongue; contain taste buds
– Foliate—on lateral aspects of posterior
tongue; contain taste buds that function
primarily in infants and children
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Tongue
• Lingual lipase
– Secreted by serous cells beneath foliate and
vallate papillae secrete
– Fat-digesting enzyme functional in stomach
• Terminal sulcus marks division between
– Body - anterior 2/3 residing in oral cavity
– Root - posterior third residing in oropharynx
– Just posterior to vallate papillae
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Figure 23.8 Dorsal surface of the tongue, and the tonsils.
Epiglottis
Palatopharyngeal
arch
Palatine tonsil
Lingual tonsil
Palatoglossal
arch
Terminal sulcus
Foliate papillae
Vallate papilla
Medial sulcus
of the tongue
Dorsum of tongue
Fungiform papilla
Filiform papilla
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Salivary Glands
• Major salivary glands
– Produce most saliva; lie outside oral cavity
– Parotid
– Submandibular
– Sublingual
• Minor salivary glands
– Scattered throughout oral cavity; augment
slightly
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Salivary Glands
• Function of saliva
– Cleanses mouth
– Dissolves food chemicals for taste
– Moistens food; compacts into bolus
– Begins breakdown of starch with enzymes
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Salivary Glands
• Parotid gland
– Anterior to ear; external to masseter muscle
– Parotid duct opens into oral vestibule next to
second upper molar
– Mumps is inflammation of parotid glands
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Salivary Glands
• Submandibular gland
– Medial to body of mandible
– Duct opens at base of lingual frenulum
• Sublingual gland
– Anterior to submandibular gland under tongue
– Opens via 10–12 ducts into floor of mouth
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Figure 23.9 The salivary glands.
Tongue
Teeth
Ducts of
sublingual
gland
Frenulum
of tongue
Sublingual
gland
Parotid
gland
Parotid duct
Masseter muscle
Body of mandible
(cut)
Posterior belly of
digastric muscle
Mylohyoid
muscle (cut)
Submandibular
duct
Anterior belly of
digastric muscle
Submandibular
gland
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Mucous cells Serous cells
forming demilunes
Salivary Glands
• Two types of secretory cells
– Serous cells
• Watery, enzymes, ions, bit of mucin
– Mucous cells
• Mucus
• Parotid, submandibular glands mostly
serous; sublingual mostly mucous
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Composition of Saliva
• 97–99.5% water, slightly acidic
– Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3–
– Salivary amylase and lingual lipase
– Mucin
– Metabolic wastes—urea and uric acid
– Lysozyme, IgA, defensins, and a cyanide
compound protect against microorganisms
PLAY
Animation: Rotating head
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Control of Salivation
• 1500 ml/day
• Intrinsic glands continuously keep mouth moist
• Major salivary glands activated by
parasympathetic nervous system when
– Ingested food stimulates chemoreceptors and
mechanoreceptors in mouth 
– Salivatory nuclei in brain stem send impulses along
parasympathetic fibers in cranial nerves VII and IX
• Strong sympathetic stimulation inhibits salivation
and results in dry mouth (xerostomia)
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Teeth
• Tear and grind food for digestion
• Primary and permanent dentitions formed
by age 21
• 20 deciduous teeth erupt (6–24 months of
age)
– Roots resorbed, teeth fall out (6–12 years of
age) as permanent teeth develop
• 32 permanent teeth
– All but third molars in by end of adolescence
• Third molars at 17–25, or may not erupt
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Classes of Teeth
• Incisors
– Chisel shaped for cutting
• Canines
– Fanglike teeth that tear or pierce
• Premolars (bicuspids)
– Broad crowns, rounded cusps – grind/crush
• Molars
– Broad crowns, rounded cusps – best grinders
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Figure 23.10 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)
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)
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Permanent
teeth
Dental Formulas
• Shorthand indicator of number/position of
teeth
– Ratio of upper to lower teeth for 1/2 of mouth
– Primary:
– Permanent:
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Tooth Structure
• Crown - exposed part above gingiva
(gum)
– Covered by enamel—hardest substance in
body (calcium salts and hydroxyapatite
crystals)
• Enamel-producing cells degenerate when tooth
erupts  no healing if decay or crack
• Root - portion embedded in jawbone
– Connected to crown by neck
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Tooth Structure
• Canine, incisor, and premolar  one root
– First upper premolar often has two
• First two upper molars  three roots
• First two lower molars  two roots
• Third molar roots vary; often single fused
root
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Tooth Structure
• Cement - calcified connective tissue
– Covers root; attaches it to periodontal
ligament
• Periodontal ligament
– Forms fibrous joint called gomphosis
– Anchors tooth in bony socket
• Gingival sulcus - groove where gingiva
borders tooth
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Tooth Structure
• Dentin - bonelike material under enamel
– Maintained by odontoblasts of pulp cavity
• Pulp cavity - surrounded by dentin
– Pulp - connective tissue, blood vessels, and
nerves
• Root canal - as pulp cavity extends to root
• Apical foramen at proximal end of root
– Entry for blood vessels, nerves, etc.
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Figure 23.11 Longitudinal section of a canine tooth within its bony socket (alveolus).
Enamel
Dentin
Crown
Neck
Dentinal
tubules
Pulp cavity
(contains
blood vessels
and nerves)
Gingival
sulcus
Gingiva
(gum)
Cement
Root
Root canal
Periodontal
ligament
Apical
foramen
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Bone
Tooth and Gum Disease
• Dental caries (cavities) - demineralization
of enamel and dentin from bacterial action
– Dental plaque (film of sugar, bacteria, and
debris) adheres to teeth
– Acid from bacteria dissolves calcium salts
– Proteolytic enzymes digest organic matter
– Prevention: daily flossing and brushing
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Tooth and Gum Disease
• Gingivitis
– Plaque calcifies to form calculus (tartar)
– Calculus disrupts seal between gingivae and
teeth
– Anaerobic bacteria infect gums
– Infection reversible if calculus removed
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Tooth and Gum Disease
• Periodontitis (from neglected gingivitis)
– Immune cells attack intruders and body
tissues
• Destroy periodontal ligament
• Activate osteoclasts  dissolve bone
– Possible tooth loss; may promote
atherosclerosis and clot formation in coronary
and cerebral arteries
– Risk factors - smoking, diabetes mellitus, oral
piercing
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Pharynx
• Food passes from mouth  oropharynx 
laryngopharynx
– Allows passage of food, fluids, and air
– Stratified squamous epithelium lining; mucusproducing glands
– Skeletal muscle layers: inner longitudinal,
outer pharyngeal constrictors
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Esophagus
• Flat muscular tube from laryngopharynx to
stomach
• Pierces diaphragm at esophageal hiatus
• Joins stomach at cardial orifice
• Gastroesophageal (cardiac) sphincter
• Surrounds cardial orifice
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Homeostatic Imbalance
• Heartburn
– Stomach acid regurgitates into esophagus
– Likely with excess food/drink, extreme
obesity, pregnancy, running
– Also with hiatal hernia - structural
abnormality
• Part of stomach above diaphragm
• Can  esophagitis, esophageal ulcers,
esophageal cancer
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Esophagus
• Esophageal mucosa contains stratified
squamous epithelium
– Changes to simple columnar at stomach
• Esophageal glands in submucosa secrete
mucus to aid in bolus movement
• Muscularis externa - skeletal superiorly;
mixed in middle; smooth inferiorly
• Adventitia instead of serosa
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Figure 23.12a Microscopic structure of the esophagus.
Mucosa
(stratified
squamous
epithelium)
Submucosa
(areolar
connective
tissue)
Lumen
Muscularis
externa
• Circular layer
• Longitudinal
layer
Adventitia
(fibrous
connective
tissue)
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Figure 23.12b Microscopic structure of the esophagus.
Mucosa
(stratified
squamous
epithelium)
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Esophagus-stomach
junction
Simple columnar
epithelium of stomach
Digestive Processes: Mouth
• Ingestion
• Mechanical breakdown
– Chewing
• Propulsion
– Deglutition (swallowing)
• Digestion (salivary amylase and lingual
lipase)
• ~ No absorption, except for few drugs
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Mastication
• Cheeks and closed lips hold food between
teeth
• Tongue mixes food with saliva; compacts
food into bolus
• Teeth cut and grind
• Partly voluntary
• Partly reflexive
– Stretch reflexes; pressure receptors in
cheeks, gums, tongue
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Deglutition
• Involves tongue, soft palate, pharynx,
esophagus
• Requires coordination of 22 muscle
groups
• Buccal phase
– Voluntary contraction of tongue
• Pharyngeal-esophageal phase
– Involuntary – primarily vagus nerve
– Control center in the medulla and lower pons
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Figure 23.13 Deglutition (swallowing).
Slide 1
Bolus of food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Trachea
Esophagus
1 During the buccal phase, the upper
esophageal sphincter is contracted.
The tongue presses against the hard
palate, forcing the food bolus into the
oropharynx.
2 The pharyngeal-esophageal phase
begins as 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.
4 Peristalsis moves
food through the
esophagus to the
stomach.
Relaxed muscles
Circular muscles
contract
Upper
esophageal
sphincter
3 The constrictor muscles of the
pharynx contract, forcing food into
the esophagus inferiorly. The upper
esophageal sphincter contracts
(closes) after food enters.
Relaxed
muscles
5 The gastroesophageal
sphincter surrounding the
cardial oriface opens, and
food enters the stomach.
Bolus of food
Longitudinal muscles
contract
Circular muscles contract
Gastroesophageal
sphincter closed
Gastroesophageal
sphincter opens
Stomach
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Bolus
Stomach: Gross Anatomy
• In upper left quadrant; temporary storage;
digestion of bolus to chyme
• Cardial part (cardia)
– Surrounds cardial orifice
• Fundus
– Dome-shaped region beneath diaphragm
• Body
– Midportion
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Stomach: Gross Anatomy
• Pyloric part
– Antrum (superior portion)  pyloric canal 
pylorus
– Pylorus continuous with duodenum through
pyloric valve (sphincter controlling stomach
emptying)
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Figure 23.14a Anatomy of the stomach.
Cardia
Fundus
Esophagus
Muscularis
externa
• Longitudinal layer
• Circular layer
• Oblique layer
Serosa
Body
Lumen
Lesser
curvature
Rugae of
mucosa
Greater
curvature
Duodenum
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Pyloric sphincter
(valve) at pylorus
Pyloric
canal
Pyloric
antrum
Figure 23.14b Anatomy of the stomach.
Liver
(cut)
Fundus
Body
Spleen
Lesser
curvature
Greater
curvature
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Stomach: Gross Anatomy
• Greater curvature - convex lateral
surface
• Lesser curvature - concave medial
surface
• Mesenteries tether stomach
– Lesser omentum
• From liver to lesser curvature
– Greater omentum – contains fat deposits &
lymph nodes
• Greater curvature  over small intestine  spleen
& transverse colon  mesocolon
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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
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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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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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Stomach: Gross Anatomy
• ANS nerve supply
– Sympathetic from thoracic splanchnic nerves
via celiac plexus
– Parasympathetic via vagus nerve
• Blood supply
– Celiac trunk (gastric and splenic branches)
– Veins of hepatic portal system
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Stomach: Microscopic Anatomy
• Four tunics
• Muscularis and mucosa modified
– Muscularis externa
• Three layers of smooth muscle
• Inner oblique layer allows stomach to churn, mix,
move, and physically break down food
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Figure 23.15a Microscopic anatomy of the stomach.
Surface
epithelium
Mucosa
Lamina
propria
Muscularis
mucosae
Submucosa
(contains
submucosal
Oblique
plexus)
layer
Muscularis
Circular
externa
layer
(contains
Longitudinal
myenteric
layer
plexus)
Stomach wall
Serosa
Layers of the stomach wall
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Stomach: Microscopic Anatomy
• Mucosa
– Simple columnar epithelium composed of
mucous cells
• Secrete two-layer coat of alkaline mucus
– Surface layer traps bicarbonate-rich fluid beneath it
– Dotted with gastric pits  gastric glands
• Gastric glands produce gastric juice
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Figure 23.15b Microscopic anatomy of the stomach.
Gastric pits
Surface epithelium
(mucous cells)
Gastric
pit
Mucous neck cells
Parietal cell
Gastric
gland
Chief cell
Enteroendocrine cell
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Enlarged view of gastric pits and
gastric glands
Gastric Glands
• Cell types
– Mucous neck cells (secrete thin, acidic
mucus of unknown function)
– Parietal cells
– Chief cells
– Enteroendocrine cells
© 2013 Pearson Education, Inc.
Figure 23.15c Microscopic anatomy of the stomach.
Pepsinogen
Pepsin
HCI
Mitochondria
Parietal cell
Chief cell
Enteroendocrine
cell
© 2013 Pearson Education, Inc.
Location of the HCl-producing parietal cells
and pepsin-secreting chief cells in a gastric
gland
Gastric Gland Secretions
• Glands in fundus and body produce most
gastric juice
• Parietal cell secretions
– Hydrochloric acid (HCl)
•  pH 1.5–3.5 denatures protein, activates pepsin,
breaks down plant cell walls, kills many bacteria
– Intrinsic factor
• Glycoprotein required for absorption of vitamin B12
in small intestine
© 2013 Pearson Education, Inc.
Gastric Gland Secretions
• Chief cell secretions
– Pepsinogen - inactive enzyme
• Activated to pepsin by HCl and by pepsin itself (a
positive feedback mechanism)
– Lipases
• Digest ~15% of lipids
© 2013 Pearson Education, Inc.
Gastric Gland Secretions
• Enteroendocrine cells
– Secrete chemical messengers into lamina
propria
• Act as paracrines
– Serotonin and histamine
• Hormones
– Somatostatin (also acts as paracrine) and gastrin
© 2013 Pearson Education, Inc.
Mucosal Barrier
• Harsh digestive conditions in stomach
• Has mucosal barrier to protect
– Thick layer of bicarbonate-rich mucus
– Tight junctions between epithelial cells
• Prevent juice seeping underneath tissue
– Damaged epithelial cells quickly replaced by
division of stem cells
• Surface cells replaced every 3–6 days
© 2013 Pearson Education, Inc.
Homeostatic Imbalance
• Gastritis
– Inflammation caused by anything that
breaches mucosal barrier
• Peptic or gastric ulcers
– Erosions of stomach wall
• Can perforate  peritonitis; hemorrhage
– Most caused by Helicobacter pylori bacteria
– Some by NSAIDs
© 2013 Pearson Education, Inc.
Figure 23.16 Photographs of a gastric ulcer and the H. pylori bacteria that most commonly cause it.
Bacteria
Mucosa
layer of
stomach
A gastric ulcer lesion
© 2013 Pearson Education, Inc.
H. pylori bacteria
Digestive Processes in the Stomach
• Mechanical breakdown
• Denaturation of proteins by HCl
• Enzymatic digestion of proteins by pepsin
(and milk protein by rennin in infants)
• Delivers chyme to small intestine
© 2013 Pearson Education, Inc.
Digestive Processes in the Stomach
• Lipid-soluble alcohol and aspirin absorbed
into blood
• Only stomach function essential to life
– Secretes intrinsic factor for vitamin B12
absorption
• B12 needed  mature red blood cells
• Lack of intrinsic factor causes pernicious anemia
• Treated with B12 injections
© 2013 Pearson Education, Inc.
Regulation of Gastric Secretion
• Neural and hormonal mechanisms
• Gastric mucosa  up to 3 L gastric
juice/day
• Vagus nerve stimulation  secretion 
• Sympathetic stimulation  secretion 
• Hormonal control largely gastrin
–  Enzyme and HCl secretion
– Most small intestine secretions - gastrin
antagonists
© 2013 Pearson Education, Inc.
Regulation of Gastric Secretion
• Three phases of gastric secretion
– Cephalic (reflex) phase – conditioned reflex
triggered by aroma, taste, sight, thought
– Gastric phase – lasts 3–4 hours; ⅔ gastric
juice released
• Stimulated by distension, peptides, low acidity,
gastrin (major stimulus)
• Enteroendocrine G cells stimulated by caffeine,
peptides, rising pH  gastrin
© 2013 Pearson Education, Inc.
Stimuli of Gastric Phase
• Gastrin  enzyme and HCl release
– Low pH inhibits gastrin secretion (as between meals)
• Buffering action of ingested proteins  rising pH
 gastrin secretion
• Three chemicals - ACh, histamine, and gastrin stimulate parietal cells through secondmessenger systems
– All three are necessary for maximum HCl secretion
© 2013 Pearson Education, Inc.
HCl Formation
• Parietal cells pump H+ (from carbonic acid
breakdown) into stomach lumen
– K+ goes into cells to balance charge
– HCO3– from carbonic acid breakdown
•  blood (via Cl– and HCO3– antiporter)
•  blood leaving stomach more alkaline 
Alkaline tide
– Cl– (from blood plasma via antiporter) follows
H+ HCl
© 2013 Pearson Education, Inc.
Figure 23.18 Mechanism of HCl secretion by parietal cells.
Gastric gland
Blood
capillary
Chief cell
CO2
CO2 + H2O
H2CO3
Stomach lumen
Carbonic
anhydrase
H+
K+
HCO3−
Alkaline
tide
Parietal cell
H+-K+
ATPase
H+
K+
HCI
HCO3−
Cl−
HCO3−- Cl−
Interstitial antiporter
fluid
© 2013 Pearson Education, Inc.
Cl−
Cl−
Regulation of Gastric Secretion
• Intestinal phase
– Stimulatory component
• Partially digested food enters small intestine 
brief intestinal gastrin release
– Inhibitory effects (enterogastric reflex and
enterogastrones)
• Chyme with H+, fats, peptides, irritating substances
 inhibition
© 2013 Pearson Education, Inc.
Enterogastric Reflex
• Three reflexes act to
– Inhibit vagal nuclei in medulla
– Inhibit local reflexes
– Activate sympathetic fibers  tightening of
pyloric sphincter  no more food entry to
small intestine
•  Decreased gastric activity  protects
small intestine from excessive acidity
© 2013 Pearson Education, Inc.
Intestinal Phase
• Enterogastrones released
– Secretin, cholecystokinin (CCK),
vasoactive intestinal peptide (VIP)
• All inhibit gastric secretion
• If small intestine pushed to accept more
chyme  dumping syndrome
– Nausea and vomiting
– Common in gastric reduction for weight loss
© 2013 Pearson Education, Inc.
Figure 23.17 Neural and hormonal mechanisms that regulate release of gastric juice.
Inhibitory events
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
Intestinal
phase
Stimulate
Inhibit
© 2013 Pearson Education, Inc.
Vagus
nerve
Local
reflexes
2 Food chemicals
G cells
(especially peptides and
caffeine) and rising pH
activate chemoreceptors
1 Presence of
partially digested
foods in duodenum
or distension of the
duodenum when
stomach begins to
empty
Medulla
Vagus
nerve
Lack of
stimulatory
impulses to
parasympathetic
center
Cerebral
cortex
Gastrin
secretion
declines
G cells
Overrides
parasympathetic
controls
Sympathetic
nervous
system
activation
1 Loss of
appetite,
depression
1 Excessive
acidity
(pH < 2)
in stomach
2 Emotional
stress
Gastrin
release
to blood
Intestinal
(enteric)
gastrin
release
to blood
Stomach
secretory
activity
Enterogastric
reflex
Brief
effect
Local
reflexes
Vagal
nuclei
in medulla
Pyloric
sphincter
Release of
enterogastrones
(secretin, cholecystokinin,
vasoactive intestinal
peptide)
1 Distension
of duodenum;
presence of
fatty, acidic, or
hypertonic
chyme; and/or
irritants in
the duodenum
2 Distension;
presence of
fatty, acidic,
partially
digested food
in the
duodenum
Response of the Stomach to Filling
• Stretches to accommodate incoming food
– Pressure constant until 1.5 L food ingested
• Reflex-mediated receptive relaxation
– Coordinated by swallowing center of brain stem
– Gastric accommodation
• Plasticity (stress-relaxation response) of smooth
muscle (see Chapter 9)
© 2013 Pearson Education, Inc.
Gastric Contractile Activity
• Peristaltic waves move toward pylorus at
rate of 3 per minute
– Basic electrical rhythm (BER) set by enteric
pacemaker cells (formerly interstitial cells of
Cajal)
– Pacemaker cells linked by gap junctions 
entire muscularis contracts
• Distension and gastrin increase force of
contraction
© 2013 Pearson Education, Inc.
Gastric Contractile Activity
• Most vigorous near pylorus
• Chyme is either
– Delivered in ~3 ml spurts to duodenum, or
– Forced backward into stomach
© 2013 Pearson Education, Inc.
Figure 23.19 Deglutition (swallowing).
Pyloric
valve
closed
1 Propulsion: Peristaltic
waves move from the fundus
toward the pylorus.
© 2013 Pearson Education, Inc.
Slide 1
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.
Regulation of Gastric Emptying
• As chyme enters duodenum
– Receptors respond to stretch and chemical
signals
– Enterogastric reflex and enterogastrones
inhibit gastric secretion and duodenal filling
• Carbohydrate-rich chyme moves quickly
through duodenum
• Fatty chyme remains in duodenum 6 hours
or more
© 2013 Pearson Education, Inc.
Figure 23.20 Neural and hormonal factors that inhibit gastric emptying.
Presence of fatty, hypertonic,
acidic chyme in duodenum
Duodenal enteroendocrine cells
Chemoreceptors and
stretch receptors
Secrete
Enterogastrones
(secretin,
cholecystokinin,
vasoactive intestinal
peptide)
Target
Via short
reflexes
Enteric
neurons
Duodenal
stimuli
decline
Via long
reflexes
CNS centers
sympathetic
activity;
parasympathetic
activity
Contractile force and
rate of stomach
emptying decline
© 2013 Pearson Education, Inc.
Initial stimulus
Stimulate
Physiological response
Inhibit
Result
Homeostatic Imbalance
• Vomiting (emesis) caused by
• Extreme stretching
• Intestinal irritants, e.g., bacterial toxins, excessive
alcohol, spicy food, certain drugs
• Chemicals/sensory impulses  emetic
center of medulla
• Excessive vomiting  dehydration,
electrolyte and acid-base imbalances
(alkalosis)
© 2013 Pearson Education, Inc.
Small Intestine: Gross Anatomy
• Major organ of digestion and absorption
• 2-4 m long; from pyloric sphincter to
ileocecal valve
• Subdivisions
– Duodenum (retroperitoneal)
– Jejunum (attached posteriorly by mesentery)
– Ileum (attached posteriorly by mesentery)
© 2013 Pearson Education, Inc.
Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)
Tongue*
Parotid gland
Sublingual gland
Submandibular gland
Salivary
glands*
Pharynx
Esophagus
Stomach
Pancreas*
(Spleen)
Liver*
Gallbladder*
Transverse colon
Small
intestine
Anus
© 2013 Pearson Education, Inc.
Duodenum
Jejunum
Ileum
Descending colon
Ascending colon
Cecum
Sigmoid colon
Rectum
Appendix
Anal canal
Large
intestine
Duodenum
• Curves around head of pancreas; shortest
part – 25 cm
• Bile duct (from liver) and main pancreatic
duct (from pancreas)
– Join at hepatopancreatic ampulla
– Enter duodenum at major duodenal papilla
– Entry controlled by hepatopancreatic
sphincter
© 2013 Pearson Education, Inc.
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
Tail of pancreas
Pancreas
Jejunum
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
© 2013 Pearson Education, Inc.
Main pancreatic duct and sphincter
Duodenum
Head of pancreas
Jejunum and Ileum
• Jejunum
– Extends from duodenum to ileum
– About 2.5 m long
• Ileum
– Joins large intestine at ileocecal valve
– About 3.6 m long
© 2013 Pearson Education, Inc.
Gross Anatomy of Small Intestine
• Vagus nerve (parasympathetic) and
sympathetics from thoracic splanchnic
nerves serve small intestine
• Superior mesenteric artery brings blood
supply
• Veins (carrying nutrient-rich blood) drain
into superior mesenteric veins  hepatic
portal vein  liver
© 2013 Pearson Education, Inc.
Structural Modifications
• Increase surface area of proximal part for
nutrient absorption
– Circular folds (plicae circulares)
– Villi
– Microvilli
© 2013 Pearson Education, Inc.
Structural Modifications
• Circular folds
– Permanent folds (~1 cm deep) that force
chyme to slowly spiral through lumen  more
nutrient absorption
• Villi
– Extensions (~1 mm high) of mucosa with
capillary bed and lacteal for absorption
• Microvilli (brush border) – contain
enzymes for carbohydrate and protein
digestion
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
Mucosaassociated
lymphoid
tissue
Intestinal
crypt
Muscularis
mucosae
Duodenal
gland
© 2013 Pearson Education, Inc.
Villus
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
© 2013 Pearson Education, Inc.
Intestinal crypt
Figure 23.23 Microvilli of the small intestine.
Mucus
granules
Microvilli
forming the
brush border
Absorptive cell
© 2013 Pearson Education, Inc.
Intestinal Crypts
• Intestinal crypt epithelium renewed every
2-4 days
– Most - secretory cells that produce intestinal
juice
– Enteroendocrine cells  enterogastrones
– Intraepithelial lymphocytes (IELs)
• Release cytokines that kill infected cells
– Paneth cells
• Secrete antimicrobial agents (defensins and
lysozyme)
– Stem cells divide to produce crypt cells
© 2013 Pearson Education, Inc.
Homeostatic Imbalance
• Chemotherapy targets rapidly dividing
cells
– Kills cancer cells
– Kills rapidly dividing GI tract epithelium 
nausea, vomiting, diarrhea
© 2013 Pearson Education, Inc.
Mucosa
• Peyer's patches protect especially distal
part against bacteria
– May protrude into submucosa
• B lymphocytes leave intestine, enter
blood, protect intestinal lamina propria with
their IgA
• Duodenal (Brunner's) glands of the
duodenum secrete alkaline mucus to
neutralize acidic chyme
© 2013 Pearson Education, Inc.
Intestinal Juice
• 1-2 L secreted daily in response to
distension or irritation of mucosa
• Slightly alkaline; isotonic with blood
plasma
• Largely water; enzyme-poor (enzymes of
small intestine only in brush border);
contains mucus
• Facilitates transport and absorption of
nutrients
© 2013 Pearson Education, Inc.
The Liver and Gallbladder
• Accessory organs
• Liver
– Many functions; only digestive function  bile
production
• Bile – fat emulsifier
• Gallbladder
– Chief function  bile storage
© 2013 Pearson Education, Inc.
Liver
• Largest gland in body
• Four lobes—right, left, caudate, and
quadrate
© 2013 Pearson Education, Inc.
Liver
• Falciform ligament
– Separates larger right and smaller left lobes
– Suspends liver from diaphragm and anterior
abdominal wall
• Round ligament (ligamentum teres)
– Remnant of fetal umbilical vein along free
edge of falciform ligament
© 2013 Pearson Education, Inc.
Figure 23.24a Gross anatomy of the human liver.
Sternum
Nipple
Bare area
Liver
Falciform
ligament
Left lobe of
liver
Right lobe of liver
Gallbladder
© 2013 Pearson Education, Inc.
Round ligament
(ligamentum
teres)
Figure 23.24b Gross anatomy of the human 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
Bare area
Caudate lobe
of liver
Sulcus for
inferior
vena cava
Hepatic vein
(cut)
Bile duct
(cut)
Right lobe
of liver
Gallbladder
© 2013 Pearson Education, Inc.
Liver: Associated Structures
• Lesser omentum anchors liver to stomach
• Hepatic artery and vein enter at porta
hepatis
• Bile ducts
– Common hepatic duct leaves liver
– Cystic duct connects to gallbladder
– Bile duct formed by union of common hepatic
and cystic ducts
© 2013 Pearson Education, Inc.
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
Tail of pancreas
Pancreas
Jejunum
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
© 2013 Pearson Education, Inc.
Main pancreatic duct and sphincter
Duodenum
Head of pancreas
Liver: Microscopic Anatomy
• Liver lobules
– Hexagonal structural and functional units
– Composed of plates of hepatocytes (liver
cells)
• Filter and process nutrient-rich blood
– Central vein in longitudinal axis
© 2013 Pearson Education, Inc.
Figure 23.25a–b Microscopic anatomy of the liver.
Lobule
© 2013 Pearson Education, Inc.
Central Connective
vein tissue septum
Liver: Microscopic Anatomy
• Portal triad at each corner of lobule
– Branch of hepatic artery supplies oxygen
– Branch of hepatic portal vein brings nutrient-rich
blood
– Bile duct receives bile from bile canaliculi
• Liver sinusoids - leaky capillaries between
hepatic plates
• Stellate macrophages (hepatic
macrophages or Kupffer cells) in liver
sinusoids remove debris & old RBCs
© 2013 Pearson Education, Inc.
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
Stellate macrophages
in sinusoid walls
Portal vein
© 2013 Pearson Education, Inc.
Bile duct
Portal venule
Portal arteriole
Portal triad
Liver: Microscopic Anatomy
• Hepatocytes – increased rough & smooth
ER, Golgi, peroxisomes, mitochondria
• Hepatocyte functions
– Process bloodborne nutrients
– Store fat-soluble vitamins
– Perform detoxification
– Produce ~900 ml bile per day
© 2013 Pearson Education, Inc.
Liver
• Regenerative capacity
– Restores full size in 6-12 months after 80%
removal
– Injury  hepatocytes  growth factors 
endothelial cell proliferation
© 2013 Pearson Education, Inc.
Homeostatic Imbalance
• Hepatitis
– Usually viral infection, drug toxicity, wild
mushroom poisoning
• Cirrhosis
– Progressive, chronic inflammation from
chronic hepatitis or alcoholism
– Liver  fatty, fibrous  portal hypertension
• Liver transplants successful, but livers
scarce
© 2013 Pearson Education, Inc.
Bile
• Yellow-green, alkaline solution containing
– Bile salts - cholesterol derivatives that
function in fat emulsification and absorption
– Bilirubin - pigment formed from heme
• Bacteria break down in intestine to stercobilin 
brown color of feces
– Cholesterol, triglycerides, phospholipids, and
electrolytes
© 2013 Pearson Education, Inc.
Bile
• Enterohepatic circulation
– Recycles bile salts
– Bile salts  duodenum  reabsorbed from
ileum  hepatic portal blood  liver 
secreted into bile
© 2013 Pearson Education, Inc.
The Gallbladder
• Thin-walled muscular sac on ventral
surface of liver
• Stores and concentrates bile by absorbing
water and ions
• Muscular contractions release bile via
cystic duct, which flows into bile duct
© 2013 Pearson Education, Inc.
The Gallbladder
• High cholesterol; too few bile salts 
gallstones (biliary calculi)
– Obstruct flow of bile from gallbladder
• May cause obstructive jaundice
– Gallbladder contracts against sharp crystals
 pain
– Treated with drugs, ultrasound vibrations
(lithotripsy), laser vaporization, surgery
© 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
© 2013 Pearson Education, Inc.
Pancreatic Juice
• Protease activation in duodenum
– Trypsinogen activated to trypsin by brush
border enzyme enteropeptidase
– Procarboxypeptidase and chymotrypsinogen
activated by trypsin
© 2013 Pearson Education, Inc.
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
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Figure 23.23 Microvilli of the small intestine.
Mucus
granules
Microvilli
forming the
brush border
Absorptive cell
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Figure 23.3a Peristalsis and segmentation.
From
mouth
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Large Intestine
• Regions
– Cecum
– Appendix
– Colon
– Rectum
– Anal canal
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Figure 23.30c Mesenteries of the abdominal digestive organs.
Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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.
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 23.32 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 23.32 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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Absorption of Nucleic Acids
• Absorption
– Active transport across epithelium 
bloodstream
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Absorption of Vitamins
• In large intestine
– Vitamin K and B vitamins from bacterial
metabolism are absorbed
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Malabsorption of Nutrients
• Causes
– Anything that interferes with delivery of bile or
pancreatic juice
– Damaged intestinal mucosa (e.g., bacterial
infection; some antibiotics)
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.