1. The Digestive System

2. Overview of the Digestive System
Organs are divided into two groups
Alimentary canal
Mouth, pharynx, and esophagus
Stomach, small intestine, and large intestine
Accessory digestive organs
Teeth and tongue
Gallbladder, salivary glands, liver, and pancreas

3. Digestive Processes Ingestion – occurs in the mouth
Propulsion – movement of food
Peristalsis – major means of propulsion
Mechanical digestion – prepares food for chemical digestion
Chewing, churning of food in stomach, segmentation
Chemical digestion – complex molecules broken down to chemical components
Mouth
Stomach
Small intestine
Absorption – transport of digested nutrients
Defecation – elimination of indigestible substances as feces

4. Digestive Tract Smooth Muscle Movement
Peristalsis
Major means of propulsion
Adjacent segments of the alimentary canal relax and contract
Segmentation
Rhythmic local contractions of the intestine
Mixes food with digestive juices
Figure 22.3a

5. Histological Organization of Digestive Tract
Mucosa lines digestive tract (mucous epithelium)
Epithelium lines the lumen, moistened by secretions of mucosal glands
Lamina propria – loose areolar tissue
Muscularis mucosa – layer of smooth that creates folds called rugae
Submucosa - layer of dense irregular connective tissue, vascularized (blood and lymphatic), innervated – nerve plexus, submucosal glands
Muscularis externa - smooth muscle arranged in circular and longitudinal layers
Adventitia or Serosa - serous membrane made of areolar connective tissue with collagen and elastic fibers, covers most of the muscularis externa

6. Digestive Tract Membranes
Peritoneum – a serous membrane
Parietal peritoneum – lines the body wall
Visceral peritoneum – surrounds digestive organs
Peritoneal cavity – a fluid filled space

7. Digestive Tract Membranes
Mesentery – a double layer of peritoneum
Holds organs in place
Sites of fat storage
Provides a route for circulatory vessels and nerves

8. Mesenteries Greater omentum – a “fatty apron” of peritoneum
Attaches the greater curvature of the stomach to the dorsal body wall
Covers the transverse colon and a large part of the small intestine
Figure 22.10c

9. Mesenteries Lesser omentum attaches to lesser curvature of stomach
Figure 22.10b

10. Oral cavity Lined by oral mucosa = stratified squamous but no serosa
Roof of cavity = hard and soft palates
Uvula guards opening to pharynx
Floor of cavity = tongue
Mechanical processing
Assistance in chewing and swallowing
Sensory analysis by touch, temperature, and taste receptors

11. The Oral Cavity Its functions include:
Analysis of material before swallowing
Mechanical processing by the teeth, tongue, and palatal surfaces
Lubrication
Limited digestion
Figure 24.6a, b

12. Salivary glands (three pairs)
Parotid, sublingual, and submandibular
Produce saliva - watery solution includes electrolytes, buffers, glycoproteins, antibodies, enzymes
Functions include: lubrication, moistening, and dissolving, Initiation of digestion of complex carbohydrates

13. Teeth Function in mastication of bolus Contain three layers
Enamel covering crown
Dentin forms basic structure
Root - periodontal ligaments hold teeth in alveoli
20 primary teeth AKA deciduous teeth
32 teeth of secondary dentition

14. The Pharynx Oropharynx and laryngopharynx
Common passageway for food, liquids, and air
Lined with stratified squamous epithelium, no serosa
External muscle layer
Consists of superior, middle, and inferior pharyngeal constrictors
Pharyngeal constrictor muscles assist in swallowing

15. The Esophagus Muscular tube - begins as a continuation of the pharynx
Carries solids and liquids from the pharynx to the stomach
Passes through esophageal hiatus in diaphragm
Joins the stomach inferior to the diaphragm
Cardiac sphincter – closes lumen to prevent stomach acid from entering esophagus
The wall of the esophagus contains mucosal (stratified squamous), submucosal, and muscularis layers

16. The Esophagus Epithelium is stratified squamous epithelium
When empty – mucosa and submucosa in longitudinal folds
Mucous glands – primarily compound tubuloalveolar glands
Muscularis externa
Skeletal muscle first third of length
Adventitia – most external layer

17. The Stomach
Site where food is churned into chyme, Mechanical breakdown of food
Breaking of chemical bonds via acids and enzymes, decretion of pepsin and HCl begins protein digestion
Bulk storage of undigested food, food remains in stomach approximately 4 hours

18. Anatomy Of The Stomach Cardia – superior, medial portion
Fundus – portion superior to stomach-esophageal junction
Body – area between the fundus and the curve of the J
Pylorus – antrum and pyloric canal adjacent to the duodenum
Rugae - ridges and folds in relaxed stomach

19. Microscopic Anatomy of the Stomach
Muscularis has three layers
Circular and longitudinal layers and oblique layer
Epithelium is simple columnar epithelium
Mucosa dotted with gastric pits holds gastric glands

20. Microscopic Anatomy of the Stomach
Gastric glands of fundus and body
Mucous neck cells - secrete a special mucus
Parietal (oxyntic) cells - secrete hydrochloric acid and gastric intrinsic factor
Chief (zymogenic) cells - secrete pepsinogen
Glands produce 1500ml juice per day

21. Small intestine Longest portion of the alimentary canal
Site of most enzymatic digestion and absorption
Secretions and buffers provided by pancreas, liver, gall bladder
Three subdivisions:
Duodenum
Jejunum
Ileum
Ileocecal sphincter - transition between small and large intestine

22. The Duodenum Receives digestive enzymes and bile
Main pancreatic duct and common bile duct enter duodenum
Sphincters control entry of bile and pancreatic juices

23. The Small Intestine – Microscopic Anatomy
Modifications for absorption
Circular folds (plicae circulares) - transverse ridges of mucosa and submucosa
Villi - finger-like projections of the mucosa, covered with simple columnar epithelium
Microvilli - further increase surface area for absorption
Lacteals - terminal lymphatic in villus

24. Histology of the Intestinal Wall
Absorptive cells - uptake digested nutrients
Goblet cells - secrete mucus that lubricates chyme
Enteroendocrine cells - secrete hormones
Intestinal crypts - epithelial cells secrete intestinal juice

25. Large Intestine Digested residue contains few nutrients
Small amount of digestion by bacteria
Main functions
absorb water and electrolytes
compact material into feces
Absorb vitamins produced by bacteria
Store fecal matter prior to defecation

26. Gross Anatomy of Large Intestine
Four areas of the colon
Ascending
Transverse
Descending
Sigmoid
Special features of large intestine
Teniae coli - thickening of longitudinal muscularis
Haustra - puckering created by teniae coli
Cecum - blind pouch, beginning of large intestine
Vermiform appendix - Contains lymphoid tissue

27. The Rectum Rectum - descends along the inferior half of the sacrum
Anal Canal - the last subdivision of the large intestine
Lined with stratified squamous epithelium
Last portion of the digestive tract
Terminates at the anal canal
Internal and external anal sphincters
Defecation reflex triggered by distention of rectal walls

28. Microscopic Anatomy of Large Intestine
Villi are absent
Contains numerous goblet cells
Intestinal crypts – simple tubular glands
Lined with simple columnar epithelial tissue
Epithelium changes at anal canal, becomes stratified squamous epithelium

29. The pancreas Pancreatic duct penetrates duodenal wall
Endocrine functions - insulin and glucagons
Exocrine functions - majority of pancreatic secretions, pancreatic juice secreted into small intestine
Carbohydrases
Lipases
Nucleases
Proteolytic enzymes

30. The Pancreas Exocrine function Endocrine function
Acinar cells make, store, and secrete pancreatic enzymes
Enzymes are activated in the duodenum
Endocrine function
Produces insulin and glucagon
Regulates blood sugar

31. The liver
Performs metabolic and hematological regulation and produces bile
Four Lobes: left, right, caudate, quadrate
Histological organization
Lobules containing single-cell thick plates of hepatocytes
Lobules empty into bile ducts, bile ducts merge to left/right hepatic ducts
L/R hepatic ducts merge to form common hepatic duct
Common hepatic meets cystic duct to form common bile duct

32. Liver lobule - basic functional unit
Hepatocytes form irregular plates arranged in spoke-like fashion
Bile canaliculi carry bile to bile ductules
Bile ductules lead to bile ducts in portal areas

33. The Gallbladder Hollow, pear-shaped organ
Stores, modifies and concentrates bile
Bile – bile salts help buffer acids and acts as emulsifier for lipid digestion
Figure 24.21a, b