1. Chapter 21c
The Digestive System

2. Chemical and mechanical digestion begins in the mouth
The Cephalic Phase
Chemical and mechanical digestion begins in the mouth
Salivary secretion is under autonomic control
Softens and lubricates food
Chemical digestion: Salivary amylase and some lipase
Saliva also has a protective function
Chewing: mastication

3. Swallowing Reflex Figure 21-24, step 1 1
Tongue pushes bolus against soft palate and back of mouth, triggering swallowing reflex.
Soft palate
Hard palate
Tongue
Bolus
Epiglottis
Glottis
Larynx
Tonically contracted upper esophageal sphincter
Figure 21-24, step 1

4. Swallowing Reflex Figure 21-24, step 2 2
Upper esophageal sphincter relaxes while epiglottis closes to keep swallowed material out of the airways.
Epiglottis
Figure 21-24, step 2

5. Swallowing Reflex Figure 21-24, step 3 3
Food moves downward into the esophagus, propelled by peristaltic waves and aided by gravity.
Figure 21-24, step 3

6. Storage Digestion Protection The Gastric Phase Stomach
Acid, enzymes, and signal molecules
Protection

7. Enterochromaffin- like cell
The Gastric Phase
Activity of secretory cells of the gastric mucosa
Gastric mucosa
Cell Types
Substance Secreted
Stimulus for Release
Function of Secretion
Mucus
Tonic secretion; with irritation of mucosa
Physical barrier between lumen and epithelium
Opening of gastric gland
Mucous neck cell
Bicarbonate
Secreted with mucus
Buffers gastric acid to prevent damage to epithelium
Gastric acid (HCl)
Activates pepsin; kills bacteria
Parietal cells
Acetylcholine, gastrin, histamine
Intrinsic factor
Complexes with vitamin B12 to permit absorption
Enterochromaffin- like cell
Acetylcholine, gastrin
Stimulates gastric acid secretion
Histamine
Pepsin(ogen)
Acetylcholine; acid, secretin
Digests proteins
Chief cells
Gastric lipase
Digests fats
Somatostatin
Acid in the stomach
Inhibits gastric acid secretion
D cells
Acetylcholine, peptides and amino acids
G cells
Gastrin
Stimulates gastric acid secretion
Figure 21-25

8. Integration of cephalic and gastric phase secretion in the stomach
The Gastric Phase
Integration of cephalic and gastric phase secretion in the stomach 1
Food 1
Input via vagus nerve
Lumen of stomach
Gastric mucosa 1
Food or cephalic reflexes initiate gastric secretion.
Enteric sensory neuron
Amino acids or peptides 2
Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine.
G cell
Gastrin 2 3
Acid stimulates short reflex secretion of pepsinogen.
D cell
Somatostatin + 4
Negative feedback pathway 4
Somatostatin release by H+ is the negative feedback signal that modulates acid and pepsin release.
ECL cell
Histamine H+
Parietal cell 3
Enteric plexus
Enteric sensory neuron
Pepsin
Pepsinogen
Chief cell
Figure 21-26

9. Enteric sensory neuron Amino acids or peptides
The Gastric Phase 1 1
Input via vagus nerve
Food
Lumen of stomach
Gastric mucosa 1
Food or cephalic reflexes initiate gastric secretion.
Enteric sensory neuron
Amino acids or peptides
G cell
Gastrin
Enteric plexus
Figure 21-26, step 1

10. Enteric sensory neuron Amino acids or peptides
The Gastric Phase 1 1
Input via vagus nerve
Food
Lumen of stomach
Gastric mucosa 1
Food or cephalic reflexes initiate gastric secretion.
Enteric sensory neuron
Amino acids or peptides 2
Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine.
G cell
Gastrin 2
ECL cell
Histamine H+
Parietal cell
Enteric plexus
Figure 21-26, steps 1–2

11. Enteric sensory neuron Amino acids or peptides Enteric sensory neuron
The Gastric Phase 1 1
Input via vagus nerve
Food
Lumen of stomach
Gastric mucosa 1
Food or cephalic reflexes initiate gastric secretion.
Enteric sensory neuron
Amino acids or peptides 2
Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine.
G cell
Gastrin 2 3
Acid stimulates short reflex secretion of pepsinogen.
ECL cell
Histamine H+
Parietal cell
Enteric plexus 3
Enteric sensory neuron
Pepsin
Pepsinogen
Chief cell
Figure 21-26, steps 1–3

12. The Gastric Phase Figure 21-26, steps 1–4 H+ 1 1 Input via vagus nerve
Food
Lumen of stomach
Gastric mucosa 1
Food or cephalic reflexes initiate gastric secretion.
Enteric sensory neuron
Amino acids or peptides 2
Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine.
G cell
Gastrin 2 3
Acid stimulates short reflex secretion of pepsinogen.
D cell
Somatostatin + 4
Negative feedback pathway 4
Somatostatin release by H+ is the negative feedback signal that modulates acid and pepsin release.
ECL cell
Histamine H+
Parietal cell
Enteric plexus 3
Enteric sensory neuron
Pepsin
Pepsinogen
Chief cell
Figure 21-26, steps 1–4

13. Bicarbonate is a chemical barrier that neutralizes acid.
The Gastric Phase
The mucus-bicarbonate barrier of the gastric mucosa
Stomach lumen
Gastric juice pH ~ 2
The mucus layer is a physical barrier.
Mucus layer
HCO3–
Bicarbonate is a chemical barrier that neutralizes acid.
HCO3–
pH ~ 7 at cell surface
Mucus droplets
Gastric mucous cell
Capillary
Figure 21-27

14. Reflexes of Cephalic and Gastric Phases
Long and short reflexes of the cephalic and gastric phases of digestion
Figure 12-23

15. Chyme in the small intestine inhibits gastric motility and secretion
The Intestinal Phase
Chyme in the small intestine inhibits gastric motility and secretion
Food into stomach
Acid secretion
STOMACH
Pepsin and lipase secretion
Gastric motility
Chyme into small intestine
Enteric nervous system
SMALL INTESTINE
Hyper- osmotic solution
Fats, proteins
Carbohydrates
Acid
? Endocrine cell
GIP
GLP-1
CCK
Secretin
Insulin secretion
Pancreatic enzyme secretion
Pancreatic bicarbonate secretion
PANCREAS
Figure 21-28

16. Bicarbonate neutralizes gastric acid
The Intestinal Phase
Bicarbonate neutralizes gastric acid
Goblet cells secrete mucus for protection and lubrication
Bile
Fat digestion
Digestive enzymes
Enteropeptidase

17. Activation of pancreatic zymogens
The Intestinal Phase
Activation of pancreatic zymogens
Lumen of small intestine
Pancreatic duct
Pancreatic secretions (include inactive zymogens)
ZYMOGENS
Trypsinogen
• Chymotrypsinogen • Procarboxypeptidase • Procolipase • Prophospholipase
Enteropeptidase in brush border activates trypsin.
Trypsin
activates
ACTIVATED ENZYMES
Intestinal mucosa
• Chymotrypsin • Carboxypeptidase • Colipase • Phospholipase
Figure 21-29

18. Digestive tract arteries
Hepatic Portal System
Most fluid is absorbed in the small intestine
Aorta
Capillaries of liver
Hepatic vein
Inferior vena cava
Liver
Hepatic artery
Capillaries of digestive tract: stomach, intestines, pancreas, and spleen
Hepatic portal vein
Digestive tract arteries
Figure 21-30

19. Most digestion occurs in small intestine
The Intestinal Phase
Most digestion occurs in small intestine
Large intestine concentrates waste for excretion
Motility in large intestine
Mass movement triggers defecation
Defecation reflex
Digestion and absorption in large intestine
Diarrhea can cause dehydration

20. The Intestinal Phase: Anatomy of the Large Intestine
Lymphoid nodule
Intestinal glands
Hepatic portal vein
Aorta
Tenia coli
Inferior vena cava
Transverse colon
Muscularis mucosae
Submucosa
Ascending colon
Longitudinal layer (tenia coli)
Descending colon
Muscularis externa
Circular muscle
Ileocecal valve
Ileum
Haustra
Cecum
Appendix
Sigmoid colon
Rectum
Rectum
Internal anal sphincter
External anal sphincter
Anus
Figure 21-31

21. The Intestinal Phase: Anatomy of the Large Intestine
Figure (1 of 3)

22. The Intestinal Phase: Anatomy of the Large Intestine
Figure (2 of 3)

23. The Intestinal Phase: Anatomy of the Large Intestine
Figure (3 of 3)

24. M cells sample the contents of the gut
Immune Function
M cells sample the contents of the gut
Immune cells secrete cytokines
Cytokines trigger inflammatory response
Increase in Cl–, fluid, and mucus secretion
Vomiting is a protective reflex

25. Function and processes
Summary
Function and processes
Digestion, absorption, motility, secretion, and GALT
Anatomy
Motility
Tonic and phasic contractions
Slow wave potentials, interstitial cells of Cajal, migrating motor complex, peristaltic contractions, and segmental contractions

26. Secretion Regulation Summary
Parietal cells, CFTR chloride channel, mucous cells, goblet cells, saliva, and bile salts
Regulation
Short reflexes, ENS, long reflexes, intrinsic neurons, gastrin family, secretin family, and other hormones

27. Digestion and absorption
Summary
Digestion and absorption
Amylase, disaccharidases, endopeptidases, exopeptidases, lipase, colipase, micelles, chylomicrons, and intrinsic factor
The cephalic phase
The gastric phase
Mucous cells, chief cells, D cells, ECL cells, and G cells

28. The intestinal phase Immune functions Summary
Brush border, mass movement, and defecation reflex
Immune functions
M cells and vomiting