1. Chapter 24 The Digestive System
BIO 211 Lab
Instructor Dr. Gollwitzer

2. Today in class we will:
Discuss the gastrointestinal tract and identify its components
Identify the accessory organs associated with the digestive tract
The layers that make up the digestive tract wall and each layer’s characteristics
Mucosa
Submucosa
Muscularis externa
Serosa
Begin our discussion of the gastrointestinal tract components in more detail
Oral cavity
Pharynx
Esophagus

3. Digestive System
Gastrointestinal (GI) tract = continuous, muscular tube from mouth to anus
Oral cavity (mouth)
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Anus

4. Digestive System
Accessory digestive organs = any digestive organ attached to the GI tract
Teeth
Tongue
Glandular organs
Salivary glands
Pancreas
Liver
Gall Bladder

5. Figure 24-1

6. Histological Organization
4 Major layers to wall
Mucosa
Submucosa
Muscularis externa
Serosa

7. Figure 24-3

8. Mucosa Inner lining of digestive tract A mucous membrane Consists of
Epithelium - moistened by glandular secretions
Lamina propria = areolar CT “filler”

9. Mucosa: Digestive Epithelia
Depend on location, function, stresses
Simple or stratified
Columnar or squamous

10. Mucosa: Digestive Epithelia
Nonkeratinized stratified squamous epithelium
Where mechanical stress most severe
In oral cavity, pharynx, esophagus, rectum
Life span = 2-3 days
Simple columnar epithelium
Where absorption and secretion occur
(with villi and goblet cells)
Stomach, small intestine, most of large intestine
Life span = 6 days

11. Mucosa: Lamina Propria
Layer of areolar (loose connective) tissue
Contains
Blood vessels, sensory nerve endings, lymphatic vessels, lymphoid tissue
Mucosal glands and glandular secretions
Muscularis mucosae/interna
Band of smooth muscle and elastic fibers
Smooth muscle arranged in 2 concentric layers
Inner, circular layer (around lumen)
Outer, longitudinal layer
Contractions alter shape of lumen and move epithelial folds

12. Mucosa: Glandular Structures
Have secretory function
Associated with simple columnar cells and/or mucous-secreting cells
Goblet cells (exocrine)  mucus
Enteroendocrine cells
 hormones (e.g., G cells  gastrin)
Coordinate activities of digestive tract and accessory glands
e.g., chief cells, G cells, parietal cells

13. Submucosa Layer of dense, irregular connective tissue
Around muscularis mucosae
Contains
Large blood vessels and lymphatic vessels
Exocrine glands  buffers and enzymes into lumen
Submucosal plexus contains nerve fibers and neurons

14. Muscularis Externa Dominated by smooth muscle cells
Inner, circular layer
Outer, longitudinal layer
Important role in mechanical processing and movement of materials along tract
Also contains
Lymphoid nodules (Peyer’s patches)
Masses of lymphoid tissue; have lymphocytes that protect small intestine from bacteria that are normal inhabitants of large intestine
Myenteric plexus/plexus of Auerbach
Network of neurons located between circular and longitudinal muscle layers
Movements coordinated by enteric nervous system (part of ANS)

15. Serosa
A serous membrane = lines sealed, internal subdivisions of ventral body cavity
Covers muscularis externa of most of digestive tract
except oral cavity, pharynx, esophagus, and rectum, which have adventitia (a dense collagen fibrous sheath)

16. Figure 24-3

17. Figure 24-3

18. Oral (Buccal) Cavity Space within mouth
Lined by oral mucosa (nonkeratinized stratified squamous epithelium)
Vestibule = space between cheeks or lips and teeth
Labial frenulum = fold of mucosa; connects lip to gum
Cheeks = lateral walls of oral cavity
Mucosae supported by fat pads and buccinator muscles
Labia (lips)
Gingivae (gums)
Ridges that surround base of each tooth
On alveolar processes of maxillary bones and mandible

19. Oral (Buccal) Cavity Palates Tongue
Hard palate – formed by maxillary bones (anterior) and palatine bones (posterior)
Soft palate – fleshy part posterior to hard palate
Formed from skeletal muscle
Posterior margin supports uvula = dangling process that helps prevent food from entering pharynx prematurely
Tongue
Muscular organ attached to floor of oral cavity
Dorsum (superior) surface covered with lingual papillae (location of taste buds)
Lingual frenulum – thin fold of mucous membrane that attaches tongue to floor of oral cavity

20. Figure 24-6

21. Pharynx Throat area posterior to nasal and oral cavities
Shared by respiratory and digestive systems
Extends between internal nares and entrances to larynx and esophagus
Nasopharynx
Posterior portion or nasal cavity
Separated from oral cavity by soft palate
Contains
Pharyngeal tonsils and opening to auditory (eustachian) tube
Oropharynx
Posterior portion of oral cavity
Between soft palate and base of tongue
Fauces = opening between oral cavity and oropharynx
Laryngopharynx
Inferior part
Between hyoid bone and entrances to esophagus and larynx

22. Esophagus Hollow, muscular tube
From posterior laryngopharynx to stomach
Descends through thoracic cavity posterior to trachea
Enters abdominopelvic cavity through esophageal hiatus in diaphragm

23. Esophageal Histology Muscularis externa transitions from Adventitia
Skeletal muscle fibers (superior third) to
Smooth muscle fibers (inferior third)
Adventitia
CT outside muscularis externa (no serosa)
Anchors esophagus against dorsal body wall

24. Esophageal Sphincters
Circular smooth muscles that control entrance/exit to esophagus
Upper esophageal sphincter
In superior 1 in.
Prevents air from entering
Lower esophageal sphincter
At inferior end
Prevents backflow from stomach

25. Begin our discussion of the 4 accessory digestive organs
Today in class we will:
Complete our more detailed discussion of the gastrointestinal tract components
Stomach
Small intestine
Large intestine
Anus
Begin our discussion of the 4 accessory digestive organs
Teeth
Tongue
Glandular organs
Salivary glands

26. Stomach Muscular tube with extra layers of smooth muscle cells
Strengthen stomach wall
Assist in mixing/churning activities required to form chyme
Muscularis mucosae/interna – has extra outer, circular layer
Muscularis externa – has oblique layer

27. Figure 24-13

28. Stomach Has expanded J shape
Lesser curvature = shorter, inner, medial surface
Greater curvature = longer, outer, lateral surface

29. Figure 24-12b

30. Stomach Has 4 regions Cardia Fundus Body (corpus) Pylorus
Smallest region where esophagus enters (at gastroesophageal/cardiac sphincter)
Fundus
Superior hump to left of cardia that contacts the diaphragm
Body (corpus)
Main (middle) region of the stomach between fundus and curve
Pylorus
Last part of stomach that enters the duodenum through pyloric sphincter

31. Stomach Rugae Folds of gastric mucosa Temporary features
Let gastric lumen expand (almost disappear when stomach full)

32. Stomach Gastric pits Gastric glands
Depressions that open onto gastric surface
Each communicates with several gastric glands
Mucous cells line neck
Gastric glands
In fundus and body of stomach
2 types of secretory cells
Parietal cells
Chief cells

33. Figure 24-13b

34. Small Intestine 3 Regions Duodenum (10 in.) Jejunum (8 feet)
First part; connects to pylorus
“Mixing bowl”
Receives chyme from stomach and secretions from pancreas and liver
Curves in a C that encloses head of the pancreas
Jejunum (8 feet)
Middle part
Site for most of chemical digestion and nutrient absorption
Ileum (12 feet)
Last part
Connects to large intestine via ileocecal valve

35. Small Intestine Plicae circulares Wall
inner folds in intestinal lining
Permanent feature -- do not disappear when small intestine fills
Wall
Mucosa
Intestinal epithelium
Simple columnar epithelium; covers villi (pl; singular = villus)
Villus capillaries/villus capillary network
Lacteals
Intestinal glands (goblet cells, enteroendocrine cells)
Muscularis mucosae/interna
Submucosa, muscularis externa, and serosa as described previously

36. Figure 24-16

37. Figure 24-17

38. Large Intestine (Bowel)
3 Main sections
Cecum
Colon (4.5 ft)
Rectum (6 in.)

39. Large Intestine (Bowel)
Cecum
Pouch-like structure that connects to ileum (at ileocecal valve)
Vermiform appendix = small, worm-like appendage of lymphoid tissue attached to cecum

40. Large Intestine (Bowel)
Colon
Ascending colon – along right side; takes right turn at superior end = right colic/hepatic flexure
Transverse colon – crosses from R to L; takes right turn downward at left end = left colic/splenic flexure
Descending colon – along left side; curves medially at sigmoid flexure
Sigmoid colon – S-shaped; empties into rectum
Haustra
Pouches caused by longitudinal bands of muscularis externa (taeniae coli)
Permit expansion and elongation of colon

41. Large Intestine (Bowel)
Rectum
Anal canal = last portion
Contain small longitudinal folds (anal columns)
Anus/anal orifice = exit of anal canal
Contains 2 sphincters
Internal anal sphincter – smooth muscle, involuntary control
External anal sphincter – skeletal muscle, voluntary control

42. Figure 24-24a

43. Teeth
Located in alveoli (bony sockets) on alveolar processes of maxillary bones and mandible
Number
Primary (deciduous)
5/side/jaw = 20
Secondary
+3 molars/side/jaw = 32
Replace primary teeth

44. Figure 24-9a, b

45. Tooth Type No./ Jaw Location Shape Function No. Roots Incisors 4
Center front
Blade
Clip/cut 1
Cuspids/
Canines 2
Lateral to incisors
Conical w/sharp ridge, pointed tip
Tear/
slash/ shear
Bicuspids/
Premolars
Posterior to cuspids
Flattened crown, prominent ridges
Crush/
mash/
grind
1 or 2
Molars (wisdom teeth) 6
Posterior to bicuspids
Large, flattened crown, prominent ridges
Crush/ grind
3 or more

46. Teeth Parts Crown – above gum line
Neck – boundary between crown and root
Root – below gum line; sits in alveolus

47. Teeth Layers/Regions Outer Inner Enamel Cementum Dentin Pulp cavity
Thin, white layer that covers crown
Hardest biologically manufactured substance
Cementum
Thin layer that covers root; protects and helps anchor tooth
Inner
Dentin
Makes up bulk of tooth
Mineralized matrix (CaPO4 crystals) similar to bone, except acellular
Pulp cavity
Central region
Receives blood vessels and nerves from root canal
Root canal
Narrow tunnel at root/base of tooth
Blood vessels and nerves enter through apical foramen

48. Figure 24-8a, b

49. Salivary Glands Outside oral cavity, but secrete into oral cavity
3 pairs
Parotid glands
Extend from mastoid process of temporal bone across outer surface of masseter muscle
Parotid (Stenson’s) duct – from parotid through buccinator to oral cavity
Sublingual glands
Under the floor of the mouth
Many small sublingual (Rivinus’) ducts open along side of lingual frenulum
Submandibular glands
Along inner surfaces of mandible
Submandibular (Wharton’s) ducts – open into mouth on either side of lingual frenulum immediately posterior to teeth

50. Figure 24-7a, b

51. Today in class we will:
Complete our discussion of the 4 accessory digestive organs
Glandular organs
Pancreas
Liver
Histology
Bile duct system
Gall bladder
Trace the path of food through the digestive system

52. Pancreas
Lies posterior to stomach, extends laterally from duodenum toward spleen
Covered by thin, CT capsule
3 regions
Head – burrowed in loop of duodenum
Tail – tapered end against spleen
Body – slender middle region
Ducts – deliver digestive enzymes and buffers to duodenum
Main pancreatic duct (of Wirsung) – large, main duct  major duodenal papilla with common bile duct  duodenal ampulla (chamber)
Accessory pancreatic duct (of Santorini) – may branch from pancreatic duct  minor duodenal papilla  duodenal ampulla

53. Fig , p. 888

54. Gallbladder Hollow, pear-shaped, muscular sac
Located in recess on posterior surface of liver’s right lobe
3 regions
Fundus – bottom of pouch
Body – main region
Neck – narrow end where cystic duct exits

55. Figure 24-19

56. Liver Largest visceral organ Covered by tough, fibrous capsule
Divided into 4 lobes
Right lobe – largest, in right hypochondriac region
Left lobe – narrow part extending into left hypochondriac region
Caudate lobe – on inferior side; superior, near IVC
Quadrate lobe – inferior to caudate, near gallbladder
Falciform ligament – on anterior surface between R and L lobes

57. Liver Histology Hepatocytes = liver cells; simple cuboidal cells
Each liver lobe divided into lobules
Liver lobule = basic functional unit of liver
Has hexagonal shape (6 corners)
Hepatic triad at each corner contains
Branch of hepatic portal vein
Branch of hepatic artery
Branch of bile duct
Separated by interlobular septa
Each has a central vein
Hepatocytes
Form 1-cell wide plates arranged like spokes around a wheel around central vein
Plates are separated by sinusoids = modified blood vessels for large solutes can pass out of/into blood stream

58. Liver Histology
Branches of hepatic portal vein (from intestine to liver) and hepatic artery (to liver from systemic circulation)  sinusoids
Hepatocytes absorb solutes and secrete materials (plasma proteins) into sinusoids
Blood leaves sinusoids and enters central vein of lobule
Central veins   hepatic veins  IVC  heart
Kupffer cells
Phagocytic cells (lymphatic cells)
Functions
Engulf pathogens, cell debris and damaged blood vessels
Store iron, lipids, heavy metals absorbed by digestive tract

59. Figure 24-20

60. Bile Duct System
Hepatocytes  bile  bile canaliculi  bile ductules  R and L hepatic ducts  common hepatic duct
Common hepatic duct + cystic duct (to/from gallbladder)  common bile duct
Common bile duct meets pancreatic duct at duodenal ampulla  duodenal papilla
Hepatopancreatic sphincter - encircles
Common bile duct, pancreatic duct, and duodenal papilla

61. Figure 24-21a, b