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prepared by
Janice Meeking,
Mount Royal College
CHAPTER
23
The Digestive
System: Part A
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Digestive System
•
Two groups of organs
1. Alimentary canal (gastrointestinal or GI tract)
•
Digests and absorbs food
•
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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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 23.1
Digestive Processes
•
Six essential activities
1. Ingestion
2. Propulsion
3. Mechanical digestion
4. Chemical digestion
5. Absorption
6. Defecation
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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 23.2
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 23.3
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
•
Activate or inhibit digestive glands
•
Stimulate smooth muscle to mix and
move lumen contents
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GI tract regulatory mechanisms
2. Intrinsic and extrinsic controls
•
Enteric nerve plexuses (gut brain) initiate
short reflexes in response to stimuli in the GI
tract
•
Long reflexes in response to stimuli inside or
outside the GI tract involve CNS centers and
autonomic nerves
•
Hormones from cells in the stomach and
small intestine stimulate target cells in the
same or different organs
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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 23.4
Peritoneum and Peritoneal Cavity
• Peritoneum: serous membrane of the
abdominal cavity
• Visceral peritoneum on external surface of
most digestive organs
• Parietal peritoneum lines the body wall
• Peritoneal cavity
• Between the two peritoneums
• Fluid lubricates mobile organs
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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 23.5a
Peritoneum and Peritoneal Cavity
• Mesentery is a double layer of peritoneum
• Routes for blood vessels, lymphatics, and
nerves
• Holds organs in place and stores fat
• Retroperitoneal organs lie posterior to the
peritoneum
• Intraperitoneal (peritoneal) organs are
surrounded by the peritoneum
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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 23.5b
Blood Supply: Splanchnic Circulation
• Arteries
• Hepatic, splenic, and left gastric
• Inferior and superior mesenteric
• 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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Nerve
Artery
Vein
Mesentery
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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
Serosa
• Epithelium
• Connective
tissue
Lumen
Gland in mucosa
Lymphatic
Mucosa-associated
Duct of gland outside
vessel
lymphoid tissue
alimentary canal
Figure 23.6
Mucosa
• Lines the lumen
• Functions
• 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
• 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)
• Muscularis mucosae: smooth muscle that
produces local movements of mucosa
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Submucosa and Muscularis Externa
• Submucosa
• Dense connective tissue
• Blood and lymphatic vessels, lymphoid
follicles, and submucosal nerve plexus
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Submucosa and Muscularis Externa
• Muscularis externa
• Responsible for segmentation and peristalsis
• Inner circular and outer longitudinal layers
• Myenteric nerve plexus
• Sphincters in some regions
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Serosa
• Visceral peritoneum
• Replaced by the fibrous adventitia in the
esophagus
• Retroperitoneal organs have both an adventitia
and serosa
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Nerve
Artery
Vein
Mesentery
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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
Serosa
• Epithelium
• Connective
tissue
Lumen
Gland in mucosa
Lymphatic
Mucosa-associated
Duct of gland outside
vessel
lymphoid tissue
alimentary canal
Figure 23.6
Enteric Nervous System
• Intrinsic nerve supply of the alimentary canal
• 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 the CNS via afferent visceral fibers
• Long ANS fibers synapse with enteric
plexuses
• Sympathetic impulses inhibit secretion and
motility
• Parasympathetic impulses stimulate
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Mouth
• Oral (buccal) cavity
• Bounded by lips, cheeks, palate, and tongue
• Oral orifice is the anterior opening
• Lined with stratified squamous epithelium
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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 23.7a
Lips and Cheeks
• Contain orbicularis oris and buccinator
muscles
• Vestibule: recess internal to lips and cheeks,
external to teeth and gums
• Oral cavity proper lies within the teeth and
gums
• Labial frenulum: median attachment of each
lip to the gum
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Gingivae (gums)
Palatine raphe
Hard palate
Soft palate
Uvula
Palatine tonsil
Sublingual fold
with openings of
sublingual ducts
Vestibule
Lower lip
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
(b) Anterior view
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Figure 23.7b
Palate
• Hard palate: palatine bones and palatine
processes of the maxillae
• Slightly corrugated to help create friction
against the tongue
• Soft palate: fold formed mostly of skeletal
muscle
• Closes off the nasopharynx during swallowing
• Uvula projects downward from its free edge
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Tongue
• Functions include
• Repositioning and mixing food during chewing
• Formation of the bolus
• Initiation of swallowing, speech, and taste
• Intrinsic muscles change the shape of the tongue
• Extrinsic muscles alter the tongue’s position
• Lingual frenulum: attachment to the floor of the
mouth
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Tongue
•
Surface bears papillae
1. Filiform—whitish, give the tongue roughness and
provide friction- have keratin in the cells
2. Fungiform—reddish, scattered over the tongue
3. Circumvallate (vallate)—V-shaped row in back of
tongue
4. Foliate—on the lateral aspects of the posterior
tongue
Note: The Fungiform, Circumvallate and Foliate house
taste buds
Go To Taste PowerPoint
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Tongue
• Terminal sulcus marks the division between
• Body: anterior 2/3 residing in the oral cavity
• Root: posterior third residing in the oropharynx
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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 23.8
SALIVA
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Salivary Glands (Two groups)
• Extrinsic salivary glands - (parotid,
submandibular, and sublingual)
• Intrinsic (buccal ) salivary glands - scattered in
the oral mucosa
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Saliva Functions
• Cleanses the mouth
• Moistens and dissolves food chemicals
• Aids in bolus formation
• Contains enzymes that begin the breakdown
of starch
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Extrinsic Salivary Glands
• Parotid gland (Otic Ganglion – CN IX)
• Anterior to the ear external to the masseter
muscle
• Parotid duct opens into the vestibule next to
second upper molar
• Submandibular gland (Submandibular
Ganglion CN VII)
• Medial to the body of the mandible
• Duct opens at the base of the lingual frenulum
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Salivary Glands
• Sublingual gland (Submandibular Ganglion
CN VII)
• Anterior to the submandibular gland under the
tongue
• Opens via 10–12 ducts into the floor of the
mouth
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Tongue
Teeth
Parotid
gland
Ducts of
sublingual
gland
Frenulum
of tongue
Sublingual
gland
Mylohyoid
muscle (cut)
Anterior belly of
digastric muscle
(a)
Submandibular
gland
Parotid duct
Masseter muscle
Body of
mandible (cut)
Posterior belly
of digastric
muscle
Submandibular
duct
Mucous
cells
(b)
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Serous cells
forming
demilunes
Figure 23.9
Gingivae (gums)
Palatine raphe
Hard palate
Soft palate
Uvula
Palatine tonsil
Sublingual fold
with openings of
sublingual ducts
Vestibule
Lower lip
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
(b) Anterior view
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Figure 23.7b
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Figure 14.4
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Figure 14.5
Saliva is secreted by serous and mucous
cells
• Serous cells produce a watery secretion containing
enzymes, ions, and tiny bit of mucin.
• Mucous cells produce mucin – a glycoprotein substance
that when mixed with water produces mucous.
• The parotid gland only has serous cells.
• Submandibular and buccal glands produce equal
proportions of serous and mucous
• Sublingual glands only produce mucous
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Composition of Saliva
• 97–99.5% water, slightly acidic solution containing
• 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
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Additional Data on Saliva
• Normal flora in the mouth convert food-derived
nitrates into nitrites which in-turn convert to nitric
oxide in an acidic environment. This transformation
occurs around the gums, where acid producing
bacteria tend to cluster, and in the HCl rich
secretions of the stomach. This activity acts as
bacteriocidal agents in these locations.
• Saliva tests for HIV antibodies, oral cancer and
diabetes are available
• Quick assessments of body hormones can be
performed on saliva
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Control of Salivation (1)
• Intrinsic glands continuously keep the mouth moist
• Extrinsic salivary glands produce secretions when
• Ingested food stimulates chemoreceptors and
mechanoreceptors in the mouth – chemoreceptors are
activated more strongly by acids like vinegar and citrus
juice – mechanoreceptors are stimulated by any object in
the mouth
• Sight or smell of food is enough to get juices flowing
• Irritations of lower GI by bacterial toxins, spicy foods or
hyperacidity especially when associated feeling of nausea
– also increases salivation
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Control of Salivation (2)
• Salivatory nuclei in the brain stem (Pons and Medulla)
send impulses along parasympathetic fibers in cranial
nerves VII and IX
• Strong sympathetic stimulation inhibits salivation and
results in dry mouth (xerostomia)
• Average output of saliva is 1 to 1 ½ liter per day
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Teeth
• Primary and permanent dentitions are formed
by age 21
• 20 deciduous teeth erupt (6–24 months of
age)
• Roots are resorbed, teeth fall out (6–12 years
of age) as permanent teeth develop
• 32 permanent teeth
• All except third molars erupt by the end of
adolescence
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(b)
Deciduous teeth
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Permanent teeth
Figure 23.10b
Classes of Teeth
• Incisors
• Chisel shaped for cutting
• Canines
• Fanglike teeth that tear or pierce
• Premolars (bicuspids) and molars
• Have broad crowns with rounded cusps for
grinding or crushing
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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 23.10a
Dental Formulas
• A shorthand way of indicating the number and
relative position of teeth
• Ratio of upper to lower teeth for one-half of the
mouth
• Primary: 2I,1C, 2M
• Permanent: 2I,1C, 2PM, 3M
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Tooth Structure
• Crown: the exposed part above the gingiva
(gum)
• Covered by enamel—the hardest substance in
the body (calcium salts and hydroxyapatite
crystals)
• Root: portion embedded in the jawbone
• Connected to crown by neck
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Tooth Structure
• Cementum: calcified connective tissue
• Covers root and attaches it to the periodontal
ligament
• Periodontal ligament
• Forms fibrous joint called a gomphosis
• Gingival sulcus: groove where gingiva borders
the tooth
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Tooth Structure
• Dentin: bonelike material under enamel
• Maintained by odontoblasts of pulp cavity
• Pulp cavity: cavity surrounded by dentin
• Pulp: connective tissue, blood vessels, and
nerves
• Root canal: extends from pulp cavity to the
apical foramen of the root
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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 23.11
Tooth and Gum Disease
• Dental caries (cavities): gradual
demineralization of enamel and dentin
• Dental plaque (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 the seal between the
gingivae and the teeth
• Anaerobic bacteria infect gums
• Infection reversible if calculus removed
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Tooth and Gum Disease
• Periodontitis
• Immune cells attack intruders and body tissues
• Destroy periodontal ligament
• Activate osteoclasts
• Consequences
• Possible tooth loss, promotion of
atherosclerosis and clot formation in
coronary and cerebral arteries
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