Chapter 26
*Lecture Outline
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Chapter 26 Outline
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General Structure and Functions of the Digestive System
Oral Cavity
Pharynx
General Arrangement of Abdominal GI Organs
Esophagus
The Swallowing Process
Stomach
Small Intestine
Large Intestine
Accessory Digestive Organs
Aging and the Digestive System
Development of the Digestive System
Introduction
The digestive system includes organs that:
• ingest the food
• transport the ingested material
• digest the material into smaller usable
components
• absorb the necessary digested nutrients
into the bloodstream
• expel waste products from the body
Introduction
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The digestive system is composed of two
separate categories of organs:
1. Digestive organs
– collectively make up the gastrointestinal (GI) tract, also
called the digestive tract or the alimentary canal
2. Accessory digestive organs
Digestive System
Figure 26.1
GI Tract Organs
The GI tract organs are as follows:
• oral cavity
• pharynx
• esophagus
• stomach
• small intestine
• large intestine
Accessory Digestive Organs
The accessory digestive organs are not part of
the long GI tube, but often develop as outgrowths
of that tube. They are as follows:
•……….. teeth
•……….. tongue
•……….. salivary glands
• ………. liver
•……….. gall bladder
•……….. pancreas
Digestive System Functions
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Ingestion
Digestion
Propulsion
Secretion
Absorption
Elimination of wastes
Peristalsis and Segmentation
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Propulsion of food along the GI tube involves
two types of movement:
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peristalsis
segmentation
Peristalsis is the ripple-like wave of muscular
contraction that forces material to move further
along the GI tract.
Segmentation is the churning and mixing of
material helping to disperse the material and
mix it and combine it with digestive organ
secretions.
Peristalsis and Segmentation
Figure 26.2
Oral Cavity
Contains the following structural features:
• cheeks, lips, and palate
• tongue
• salivary glands
• teeth
Cheeks, Lips, and Palate
• Cheeks form the lateral wall of the oral cavity
and are comprised mainly of the buccinator
muscles.
• The cheeks end anteriorly as the lips.
• The gingivae (gums) cover the alveolar
processes of the teeth.
• The internal surface of the upper and lower lips
are attached to the gingivae by a thin, midline
mucosa fold called the labial frenulum.
Cheeks, Lips, and Palate
• The palate forms the roof of the oral cavity.
• The anterior two-thirds of the palate is
called the hard palate because it is
comprised of bone. The posterior one-third
of the palate is soft and muscular and is
called the soft palate.
• Extending from the soft palate posteriorly is
the uvula, which elevates during
swallowing and closes off the posterior
entrance to the nasopharynx.
Cheeks, Lips, and Palate
Figure 26.3
Cheeks, Lips, and Palate
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The fauces represent the opening from
the oral cavity to the oropharynx.
The fauces are bounded laterally by
paired muscular folds:
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palatoglossal arch
palatopharyngeal arch
• The palatine tonsils (see Chapter
24) are housed between the two
arches.
Tongue
• The tongue manipulates and mixes ingested
materials during chewing and helps compress
the partially digested materials into a bolus.
• A bolus is a globular mass of ingested
materials that can be more easily swallowed.
• The inferior surface of the tongue attaches to the
floor of the oral cavity by a thin, midline mucous
membrane called the lingual frenulum.
Tongue
Figure 26.3
Salivary Glands
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Salivary glands produce and secrete saliva
into the oral cavity.
Saliva serves the following functions:
– moistens ingested materials to become a slick
bolus
– moistens, cleanses, and lubricates the
structures of the oral cavity
– chemical digestion of ingested materials
– antibacterial action
– dissolves materials so that taste receptors on
the tongue can be stimulated
Salivary Glands
Three pairs of salivary glands are
located external to the oral cavity:
• parotid glands
• submandibular glands
• sublingual glands
Salivary Glands
Figure 26.4
Parotid Salivary Glands
• Largest of the three salivary glands
• Located anterior and inferior to the
ear. Lateral side of face, under ears
• Secrete 25–30% of total saliva
• Parotid duct runs parallel to the
zygomatic arch and pierces the
buccinator muscle just opposite the
second upper molar
• Also secrete amylase
Parotid Salivary Glands
Figure 26.4
Submandibular Salivary
Glands
• Reside inferior/lateral to the body
of the mandible
• Produce the majority of the saliva
(60–70%)
• A submandibular duct transports
saliva from each gland through a
papilla in the floor of the mouth on the
lateral sides of the lingual frenulum
Submandibular Salivary
Glands
Sublingual Salivary Glands
• Inferior/anterior to the tongue
• Each gland extends multiple tiny
sublingual ducts that open onto the
inferior surface of the oral cavity just
posterior to the submandibular duct
papilla
• Contribute only 3–5% of total saliva
Sublingual Salivary Glands
Figure 26.4
Salivary Gland Secretion
Two types of secretory cells are found
in salivary glands:
• Mucous cells—secrete mucin,
which forms mucus upon hydration
• Serous cells—secrete a watery fluid
containing ions, lysozyme, and
salivary amylase
Salivary Gland Secretion
Figure 26.4
Salivary Gland Secretion
Teeth
• The teeth are collectively known as the
dentition.
• A tooth has an exposed crown, a
constricted neck, and one or more roots
that fit into dental alveoli.
• Dentin forms the primary mass of the
tooth. It is harder than bone.
• Each root is covered with cementum.
• The external surface of the dentin is
covered with a layer of enamel that forms
the crown of the tooth.
Teeth
• The center of the tooth is a pulp
cavity that contains connective tissue
called pulp.
• A root canal opens into the
connective tissue through an opening
called the apical foramen. Blood
vessels and nerves pass through this
opening and are housed in the pulp.
Teeth
Figure 26.5
Surfaces of the Teeth
Teeth have several surfaces:
• mesial surface
• distal surface
• buccal surface
• labial surface
• lingual surface
• occlusal surface
Teeth
Two sets of teeth develop and erupt in
a normal lifetime:
• deciduous teeth—erupt between 6–
30 months, 20 in number, and are often
called milk teeth
• permanent teeth—replace the
deciduous teeth and are 32 in
number
Teeth
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Central incisor (7–9 mos)
Lateral incisor (9–11 mos.)
Canine (18–20 mos.)
1st molar (14–16 mos.)
Upper
teeth
2nd molar (24–30 mos.)
Permanent teeth
2nd molar (20–22 mos.)
Lower
teeth
Deciduous teeth
1st molar (12–14 mos.)
Canine (16–18 mos.)
Lateral incisor (7–9 mos.)
Central incisor (6–8 mos.)
(a) Child’s skull
(b) Deciduous teeth
Right Upper (Maxillary) Quadrant
Left Upper (Maxillary) Quadrant
Central incisor (7–8 yrs.)
Lateral incisor (8–9 yrs.)
Canine (11–12 yrs.)
1st premolar (10–11 yrs.)
2nd premolar (10–12 yrs.)
Upper teeth
7
1st molar (6–7 yrs.)
6
Hard palate
2
1
16
17
32
18
31
19
30
20
29
21
28
22
27
26 25 24 23
3rd molar (17–25 yrs.)
2nd molar (11–13 yrs.)
1st molar (6–7 yrs.)
9 10
11
12
13
14
15
5
4
3
2nd molar (12–13 yrs.)
3rd molar (17–25 yrs.)
8
Lower teeth
2nd premolar (11–12 yrs.)
1st premolar (10–12 yrs.)
(d) Teeth numbering
Canine (9–10 yrs.)
Lateral incisor (7–8 yrs.)
Central incisor (6–7 yrs.)
Figure 26.6
Right Lower (Mandibular) Quadrant
Left Lower (Mandibular) Quadrant
(c) Permanent teeth
a: © The McGraw-Hill Companies, Inc./Photo by Christine Eckel
Permanent Teeth
• Incisors—most anteriorly placed, shaped
like chisels, and have a single root
• Canines—posterolateral to the incisors,
pointed tips for puncturing and tearing
• Premolars—posterolateral to canines,
have flat crowns with prominent ridges
called cusps for crushing and grinding
• Molars—thickest and most posterior teeth,
also adapted for crushing and grinding of
ingested materials
Oral Cavity Structures
Pharynx
• Shared by the respiratory and digestive
systems
• Three skeletal muscle pairs of pharyngeal
constrictors (superior, middle, and
inferior) form the wall of the pharynx and
participate in swallowing
• CN X innervates most pharyngeal muscles
• Branches of external carotid arteries
supply the pharynx
• Internal jugular veins drain the pharynx
Peritoneum
Abdominopelvic cavity is covered with
moist serous membranes:
• parietal peritoneum—lines the
inside
surface of the body wall
• visceral peritoneum—covers the
surface
of internal organs within the cavity
Peritoneum
• Organs that are completely surrounded by
visceral peritoneum are called
intraperitoneal organs. They include the
stomach and most of the small intestines.
• Organs that lie in direct contact with the
posterior abdominal and pelvic walls and
are only covered on their anterolateral
surfaces with visceral peritoneum are
called retroperitoneal organs. Examples
are the pancreas, ascending and
descending colon of the large intestines,
and the rectum.
Peritoneum
Figure 26.7
Mesenteries
• Folds of peritoneum that support and
stabilize intraperitoneal GI tract organs
• Blood vessels, lymphatic vessels, and
nerves are sandwiched between the two
folds and supply the digestive organs
Mesenteries
Figure 26.7
Mesenteries
• The greater omentum extends inferiorly like an
apron from the greater curvature of the stomach
and covers most of the abdominal organs.
• The lesser omentum connects the lesser
curvature of the stomach and the proximal end
of the duodenum to the liver.
• The mesentery proper suspends most of the
small intestines from the posterior abdominal
wall.
• The mesocolon is a peritoneal fold that
attaches parts of the large intestine to the
posterior abdominal wall.
Mesenteries
Figure 26.8
Mesenteries
Figure 26.8
The Wall of the Abdominal GI
Tract
The wall of the GI tract from the esophagus to the
large intestine is composed of four concentric
layers called tunics. From deep (in contact with
ingested materials) to superficial (the external
covering) they are:
• mucosa
• submucosa
• muscularis
• adventitia or serosa