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prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
23
The Digestive
System: Part A
© 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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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
• 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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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 orifice 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
Figure 23.13 Deglutition (swallowing).
Slide 2
Bolus of food
Tongue
Pharynx
Epiglottis
Glottis
Trachea
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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.
Figure 23.13 Deglutition (swallowing).
Slide 3
Uvula
Bolus
Epiglottis
Esophagus
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.
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Figure 23.13 Deglutition (swallowing).
Upper
esophageal
sphincter
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Slide 4
Bolus
3 The constrictor muscles of the
pharynx contract, forcing food into
the esophagus inferiorly. The upper
esophageal sphincter contracts
(closes) after food enters.
Figure 23.13 Deglutition (swallowing).
Slide 5
4 Peristalsis moves
Relaxed muscles
food through the
esophagus to the
stomach.
Circular muscles
contract
Bolus of food
Longitudinal muscles
contract
Gastroesophageal
sphincter closed
Stomach
© 2013 Pearson Education, Inc.
Figure 23.13 Deglutition (swallowing).
Slide 5
4 Peristalsis moves
Relaxed muscles
food through the
esophagus to the
stomach.
Circular muscles
contract
Bolus of food
Longitudinal muscles
contract
Gastroesophageal
sphincter closed
Stomach
© 2013 Pearson Education, Inc.
Figure 23.13 Deglutition (swallowing).
Slide 6
Relaxed
muscles
5 The gastroesophageal
sphincter surrounding the
cardial orifice opens, and
food enters the stomach.
Circular muscles contract
Gastroesophageal
sphincter opens
© 2013 Pearson Education, Inc.
Figure 23.13 Deglutition (swallowing).
Slide 7
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 orifice 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
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
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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
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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