1. The Digestive System: Part A23
The Digestive System: Part A

2. Digestive System Two groups of organs
Alimentary canal (gastrointestinal or GI tract)
Digests and absorbs food
Mouth, pharynx, esophagus, stomach, small intestine, and large intestine

3. Digestive System Accessory digestive organs Teeth, tongue, gallbladder
Digestive glands
Salivary glands
Liver
pancreas

4. Parotid gland Mouth (oral cavity) Sublingual gland Salivary glands
Tongue
Submandibular
gland
Pharynx
Esophagus
Stomach
Pancreas
(Spleen)
Liver
Gallbladder
Transverse colon
Duodenum
Descending colon
Small
intestine
Jejunum
Ascending colon
Ileum
Cecum
Large
intestine
Sigmoid colon
Rectum
Vermiform appendix
Anus
Anal canal
Figure 23.1

5. Six essential activities
Digestive Processes
Six essential activities
Ingestion
Propulsion
Mechanical digestion
Chemical digestion
Absorption
Defecation

6. Ingestion Food Mechanical digestion Pharynx • Chewing (mouth)
Esophagus
• Churning (stomach)
Propulsion
• Segmentation
(small intestine)
• Swallowing
(oropharynx)
Chemical
digestion
• Peristalsis
(esophagus,
stomach,
small intestine,
large intestine)
Stomach
Absorption
Lymph
vessel
Small
intestine
Blood
vessel
Large
intestine
Mainly H2O
Feces
Defecation
Anus
Figure 23.2

7. (a) Peristalsis: Adjacent segments of
From
mouth
(a) Peristalsis: Adjacent segments of
alimentary tract organs alternately contract
and relax, which moves food along the tract
distally.
(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

8. GI tract regulatory mechanisms
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

9. GI tract regulatory mechanisms
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

10. Central nervous system and extrinsic autonomic nerves
External stimuli
(sight, smell, taste,
thought of food)
Central nervous system
and extrinsic autonomic nerves
Long reflexes
Afferent impulses
Efferent impulses
Effectors:
Smooth muscle
or glands
Internal
(GI tract)
stimuli
Chemoreceptors,
osmoreceptors, or
mechanoreceptors
Local (intrinsic)
nerve plexus
(“gut brain”)
Short reflexes
Response:
Change in
contractile or
secretory activity
Gastrointestinal
wall (site of short
reflexes)
Lumen of the
alimentary canal
Figure 23.4

11. 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

12. (a) Schematic cross sections of abdominal cavity
Abdominopelvic
cavity
Vertebra
Dorsal
mesentery
Parietal
peritoneum
Ventral
mesentery
Visceral
peritoneum
Peritoneal
cavity
Alimentary
canal organ
Liver
(a) Schematic cross sections of abdominal cavity
illustrate the peritoneums and mesenteries.
Figure 23.5a

13. 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

14. Alimentary canal organ in a retroperitoneal position
Mesentery
resorbed
and lost
Abdominopelvic
cavity
Alimentary
canal organ
Alimentary canal organ in
a retroperitoneal position
(b) Some organs lose their mesentery and
become retroperitoneal during development.
Figure 23.5b

15. 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

16. Histology of the Alimentary Canal
Four basic layers (tunics)
Mucosa
Submucosa
Muscularis externa
Serosa

17. 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
Nerve
• Epithelium
Artery
• Connective
tissue
Vein
Gland in mucosa
Lumen
Lymphatic
vessel
Duct of gland outside
alimentary canal
Mucosa-associated
lymphoid tissue
Mesentery
Figure 23.6

18. Three sublayers: epithelium, lamina propria, and muscularis mucosae
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

19. Mucosa Epithelium Simple columnar epithelium and mucus-secreting cells
Protects digestive organs from enzymes
Eases food passage
May secrete enzymes and hormones (e.g., in stomach and small intestine)

20. 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

21. Submucosa and Muscularis Externa
Dense connective tissue
Blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus

22. Submucosa and Muscularis Externa
Responsible for segmentation and peristalsis
Inner circular and outer longitudinal layers
Myenteric nerve plexus
Sphincters in some regions

23. Serosa Visceral peritoneum
Replaced by the fibrous adventitia in the esophagus
Retroperitoneal organs have both an adventitia and serosa

24. 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
Nerve
• Epithelium
Artery
• Connective
tissue
Vein
Gland in mucosa
Lumen
Lymphatic
vessel
Duct of gland outside
alimentary canal
Mucosa-associated
lymphoid tissue
Mesentery
Figure 23.6

25. 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

26. 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

27. Mouth Oral (buccal) cavity Bounded by lips, cheeks, palate, and tongue
Oral orifice is the anterior opening
Lined with stratified squamous epithelium

28. (a) Sagittal section of the oral cavity and pharynx
Soft palate
Palatoglossal arch
Uvula
Hard palate
Oral cavity
Palatine tonsil
Tongue
Oropharynx
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
Esophagus
Trachea
(a) Sagittal section of the oral cavity and pharynx
Figure 23.7a

29. 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

30. Upper lip Gingivae (gums) Superior labial frenulum Palatine raphe
Palatoglossal arch
Hard palate
Palatopharyngeal
arch
Soft palate
Uvula
Posterior wall
of oropharynx
Palatine tonsil
Tongue
Sublingual fold
with openings of
sublingual ducts
Lingual frenulum
Opening of
submandibular duct
Gingivae (gums)
Vestibule
Inferior labial
frenulum
Lower lip
(b) Anterior view
Figure 23.7b

31. 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

32. 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

33. Tongue Surface bears papillae
Filiform—whitish, give the tongue roughness and provide friction
Fungiform—reddish, scattered over the tongue
Circumvallate (vallate)—V-shaped row in back of tongue
These three house taste buds
Foliate—on the lateral aspects of the posterior tongue

34. Terminal sulcus marks the division between
Tongue
Terminal sulcus marks the division between
Body: anterior 2/3 residing in the oral cavity
Root: posterior third residing in the oropharynx

35. 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
Figure 23.8

36. Salivary Glands
Extrinsic salivary glands (parotid, submandibular, and sublingual)

37. Intrinsic (buccal) salivary glands are scattered in the oral mucosa
Secretion (saliva)
Cleanses the mouth
Moistens and dissolves food chemicals
Aids in bolus formation
Contains enzymes that begin the breakdown of starch

38. Salivary Glands Parotid gland Submandibular gland
Anterior to the ear external to the masseter muscle
Parotid duct opens into the vestibule next to second upper molar
Submandibular gland
Medial to the body of the mandible
Duct opens at the base of the lingual frenulum

39. Salivary Glands Sublingual gland
Anterior to the submandibular gland under the tongue
Opens via 10–12 ducts into the floor of the mouth

40. Tongue Teeth Parotid gland Ducts of sublingual gland Parotid duct
Masseter muscle
Frenulum
of tongue
Body of
mandible (cut)
Sublingual
gland
Posterior belly
of digastric
muscle
Mylohyoid
muscle (cut)
Submandibular
duct
Anterior belly of
digastric muscle
Submandibular
gland
Mucous
cells
Serous cells
forming
demilunes
(a)
(b)
Figure 23.9

41. Composition of Saliva Secreted by serous and mucous cells
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
PLAY
Animation: Rotatable head

42. Control of Salivation
Intrinsic glands continuously keep the mouth moist
Extrinsic salivary glands produce secretions when
Ingested food stimulates chemoreceptors and mechanoreceptors in the mouth
Salivatory nuclei in the brain stem send impulses along parasympathetic fibers in cranial nerves VII and IX
Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia)

43. Primary and permanent dentitions are formed by age 21
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

44. (b)
Deciduous teeth
Permanent teeth
Figure 23.10b

45. Premolars (bicuspids) and molars
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

46. Incisors Incisors Central (7 yr) Central (6–8 mo) Lateral (8 yr)
Lateral (8–10 mo)
Canine (eyetooth)
(11 yr)
Canine (eyetooth)
(16–20 mo)
Premolars
(bicuspids)
Molars
First molar
(10–15 mo)
First premolar
(11 yr)
Deciduous
(milk) teeth
Second molar
(about 2 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
(a)
Figure 23.10a

47. 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

48. Crown: the exposed part above the gingiva (gum)
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

49. Cementum: calcified connective tissue
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

50. Dentin: bonelike material under enamel
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

51. Enamel Dentin Crown Dentinal tubules Pulp cavity (contains
blood vessels and
nerves)
Neck
Gingiva (gum)
Cementum
Root
Root canal
Periodontal
ligament
Apical foramen
Bone
Figure 23.11

52. 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

53. 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

54. 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