1. Chapter 21b
The Digestive System

2. Regulation of GI Function
Long reflexes integrated in CNS
Short reflexes integrated in ENS
Reflexes involving GI peptides / Hormones

3. Regulation of GI Function
Integration of long and short reflexes in the digestive system
External stimulus
KEY
Stimulus
Receptor
(sight, smell, etc.)
Integrating center
Efferent path
Sensory receptors
Short reflexes
Tissue response
Long reflexes
The cephalic brain
Sympathetic and parasympathetic neurons
Neurons of myenteric and submucosal plexuses
Muscle contraction/relaxation or Exocrine secretion (enzymes, mucus, acid, bicarbonate), paracrines or Endocrine secretions
Sensory receptors and neurons
Smooth muscles or secretory cells
Local stimulus
Inter- neurons
(pH, stretch, osmolarity, products of digestion)
Enteric nervous system “The little brain”
Effectors
Digestive system responses
Figure 21-11

4. Regulation of GI Function
KEY
Stimulus
Receptor
Integrating center
Efferent path
Tissue response
Short reflexes
Neurons of myenteric and submucosal plexuses
Muscle contraction/relaxation or Exocrine secretion (enzymes, mucus, acid, bicarbonate), paracrines or Endocrine secretions
Sensory receptors and neurons
Smooth muscles or secretory cells
Local stimulus
Inter- neurons
(pH, stretch, osmolarity, products of digestion)
Enteric nervous system “The little brain”
Effectors
Digestive system responses
Figure (1 of 3)

5. Regulation of GI Function
External stimulus
KEY
Stimulus
Receptor
(sight, smell, etc.)
Integrating center
Efferent path
Sensory receptors
Tissue response
Long reflexes
The cephalic brain
Sympathetic and parasympathetic neurons
Neurons of myenteric and submucosal plexuses
Muscle contraction/relaxation or Exocrine secretion (enzymes, mucus, acid, bicarbonate), paracrines or Endocrine secretions
Sensory receptors and neurons
Smooth muscles or secretory cells
Local stimulus
(pH, stretch, osmolarity, products of digestion)
Enteric nervous system “The little brain”
Effectors
Digestive system responses
Figure (2 of 3)

6. Regulation of GI Function
External stimulus
KEY
Stimulus
Receptor
(sight, smell, etc.)
Integrating center
Efferent path
Sensory receptors
Tissue response
Short reflexes
Long reflexes
The cephalic brain
Sympathetic and parasympathetic neurons
Neurons of myenteric and submucosal plexuses
Muscle contraction/relaxation or Exocrine secretion (enzymes, mucus, acid, bicarbonate), paracrines or Endocrine secretions
Sensory receptors and neurons
Smooth muscles or secretory cells
Local stimulus
Inter- neurons
(pH, stretch, osmolarity, products of digestion)
Enteric nervous system “The little brain”
Effectors
Digestive system responses
Figure (3 of 3)

7. Regulation of GI Function
Reflexes involving GI Peptides
Overview of GI Reflexes
Cephalic phase of digestion (feedforward) Distension Acid Presence of food
Stimuli
Receptor/ integrating center
Secretory cells of the stomach and small intestine
Efferent pathway
GI peptides
GI smooth muscle Exocrine cells of stomach, pancreas, intestine Other endocrine cells Nervous system
Effectors
Changes in GI motility Release of bile and pancreatic secretions Enzyme, acid, and bicarbonate synthesis/release Hunger/satiety
Responses
Figure 21-12

8. ENS Shares Features with CNS
Intrinsic neurons
Neurotransmitters and neuromodulators
Support cells
Diffusion barrier
Integrating center

9. Gastrin family Secretin family Others Digestive Hormones
Gastrin and CCK
Secretin family
Secretin, VIP, GIP, and GLP-1
Others
Motilin

10. Secretin

11. CCK Fats and Proteins in SI Bile Pancreatic enzymes Trypsin
Chymotrypsin
Procarboxypepsidase
Bicarbonate ions

12. Control of the Digestive System
PLAY
Interactive Physiology® Animation: Digestive System: Control of the Digestive System

13. Digestion and Absorption
A villus and a crypt in the small intestine
Brush border
Microvilli
Enterocyte
Enterocytes
Capillaries
Goblet cells
Crypt lumen
Lacteals
Lamina propria
Endocrine cells
Muscularis mucosae
Figure 21-13

14. Digestion and Absorption
Carbohydrates break down into monosaccharides
Glucose polymers
digest to
Starch, glycogen
Disaccharides
Amylase
Maltose
Sucrose
Lactose
Maltase
Sucrase
Lactase
2 glucose
1 glucose + 1 fructose
1 glucose + 1 galactose
Monosaccharides
Figure 21-14

15. Digestion and Absorption
Carbohydrate absorption in the small intestine
Glucose or galactose
Lumen of intestine
Na+
Fructose
Na+ K+
Intestinal mucosa
Glucose enters the cell with Na+ on the SGLT symporter and exits on GLUT2. Fructose enters on GLUT5 and exits on GLUT2.
KEY
SGLT
GLUT2
GLUT5
Figure 21-15

16. Digestion and Absorption
Amino- terminal end
Amino acids
Peptide bonds
Carboxy- terminal end
Enzymes for protein digestion
H2N
COOH
(a) Peptide structure
Endopeptidase digests internal peptide bonds.
+H2O
H2N
COOH
H2N
COOH
H2N
COOH
(b)
2 smaller peptides
Exopeptidase digests terminal peptide bonds to release amino acids.
Aminopeptidase
Carboxypeptidase
+H2O
+H2O
H2N
COOH
H2N
COOH
H2N
COOH
Amino acid
Amino acid
H2N
COOH
(c)
Peptide
Figure 21-16

17. Digestion and Absorption
Peptide absorption
Proteins
Peptides
Di- and tripeptides cotransport with H+.
Amino acids cotransport with Na+.
Small peptides are carried intact across the cell by transcytosis. H+ H+
Na+
Na+
Peptidases K+
ATP H+
Na+
Na+
Blood
To the liver
Figure 21-17

18. Digestion and Absorption
Triglycerides digest into monoglycerides and free fatty acids
Triglyceride
Lipase, colipase +
Monoglyceride
Free fatty acids
Figure 21-18

19. Digestion and Absorption
Bile salts emulsify fats
Bile salt-coated lipid droplet
Water
Hydrophobic side associates with lipids.
Polar side chains (hydrophilic side associates with water)
(a)
Bile salt
Free fatty acids
Cholesterol
Bile salt
Monoglyceride
Phospholipids
Diglyceride
(b)
Figure 21-19

20. Digestion and Absorption of Fats
Bile salts from liver 1 1
Bile salts from liver coat fat droplets.
Large fat droplets from stomach 2
Pancreatic lipase and coli- pase break down fats into monoglycerides and fatty acids stored in micelles.
Emulsion
Bile salts recycle
Lumen of small intestine
Lipase and colipase 2 2 3a
Monoglycerides and fatty acids move out of micelles and enter cells by diffusion.
Micelles 3b 3b
Cholesterol is transported into cells by a membrane transporter. 3a 4
Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons.
Smooth ER 4
Triglycerides + cholesterol + protein
Cells of small intestine 5
Chylomicrons are released into the lymphatic system.
Chylomicron
Golgi apparatus 5
Interstitial fluid
Capillary
Lacteal
Lymph to vena cava
Figure 21-20

21. Digestion and Absorption of Fats
Bile salts from liver 1 1
Bile salts from liver coat fat droplets.
Large fat droplets from stomach
Emulsion
Lumen of small intestine
Cells of small intestine
Interstitial fluid
Capillary
Lacteal
Figure 21-20, step 1

22. Digestion and Absorption of Fats
Bile salts from liver 1 1
Bile salts from liver coat fat droplets.
Large fat droplets from stomach 2
Pancreatic lipase and coli- pase break down fats into monoglycerides and fatty acids stored in micelles.
Emulsion
Lumen of small intestine
Lipase and colipase 2 2
Micelles
Cells of small intestine
Interstitial fluid
Capillary
Lacteal
Figure 21-20, steps 1–2

23. Digestion and Absorption of Fats
Bile salts from liver 1 1
Bile salts from liver coat fat droplets.
Large fat droplets from stomach 2
Pancreatic lipase and coli- pase break down fats into monoglycerides and fatty acids stored in micelles.
Emulsion
Bile salts recycle
Lumen of small intestine
Lipase and colipase 2 2 3a
Monoglycerides and fatty acids move out of micelles and enter cells by diffusion.
Micelles 3a
Cells of small intestine
Interstitial fluid
Capillary
Lacteal
Figure 21-20, steps 1–3a

24. Digestion and Absorption of Fats
Bile salts from liver 1 1
Bile salts from liver coat fat droplets.
Large fat droplets from stomach 2
Pancreatic lipase and coli- pase break down fats into monoglycerides and fatty acids stored in micelles.
Emulsion
Bile salts recycle
Lumen of small intestine
Lipase and colipase 2 2 3a
Monoglycerides and fatty acids move out of micelles and enter cells by diffusion.
Micelles 3b 3b
Cholesterol is transported into cells by a membrane transporter. 3a
Cells of small intestine
Interstitial fluid
Capillary
Lacteal
Figure 21-20, steps 1–3b

25. Digestion and Absorption of Fats
Bile salts from liver 1 1
Bile salts from liver coat fat droplets.
Large fat droplets from stomach 2
Pancreatic lipase and coli- pase break down fats into monoglycerides and fatty acids stored in micelles.
Emulsion
Bile salts recycle
Lumen of small intestine
Lipase and colipase 2 2 3a
Monoglycerides and fatty acids move out of micelles and enter cells by diffusion.
Micelles 3b 3b
Cholesterol is transported into cells by a membrane transporter. 3a 4
Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons.
Smooth ER 4
Triglycerides + cholesterol + protein
Cells of small intestine
Chylomicron
Interstitial fluid
Capillary
Lacteal
Figure 21-20, steps 1–4

26. Digestion and Absorption of Fats
Bile salts from liver 1 1
Bile salts from liver coat fat droplets.
Large fat droplets from stomach 2
Pancreatic lipase and coli- pase break down fats into monoglycerides and fatty acids stored in micelles.
Emulsion
Bile salts recycle
Lumen of small intestine
Lipase and colipase 2 2 3a
Monoglycerides and fatty acids move out of micelles and enter cells by diffusion.
Micelles 3b 3b
Cholesterol is transported into cells by a membrane transporter. 3a 4
Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons.
Smooth ER 4
Triglycerides + cholesterol + protein
Cells of small intestine 5
Chylomicrons are released into the lymphatic system.
Chylomicron
Golgi apparatus 5
Interstitial fluid
Capillary
Lacteal
Lymph to vena cava
Figure 21-20, steps 1–5

27. Digestion and Absorption
Nucleic acids are digested into nitrogenous bases and monosaccharides
Intestine absorbs vitamins and minerals
Fat-soluble absorbed with fats
Water-soluble by mediated transport
Vitamin B12 when complexed to intrinsic factor and absorbed in ileum
Mineral by active transport

28. Digestion and Absorption
NaCl reabsorption in the small intestine and colon
Lumen of small intestine or colon
Intestinal cell
Interstitial fluid K+
Na+ reabsorbed.
Na+ 1 2
Na+
Na+
Cl–
ATP K+
Na+ H+
HCO3–
Cl–
Cl– 1
Na+ enters cells by multiple pathways. 2
The Na+-K+-ATPase pumps Na+ into the ECF.
Figure 21-21

29. Digestion and Absorption
Summary of motility, secretion, digestion, and absorption in different regions of the digestive system
KEY
M: motility S:secretion D:digestion A:absorption
ORAL CAVITY AND ESOPHAGUS
M: swallowing, chewing
S:saliva (salivary glands), lipase
D:carbohydrates, fats (minimal)
A:none
Salivary gland
Upper esophageal sphincter
STOMACH
M:peristaltic mixing and propulsion
S:HCl (parietal cells); pepsinogen and gastric lipase
(chief cells); mucus and HCO3– (surface mucous cells);
gastrin (G cells); histamine (EC cells) D:proteins, fats A:lipid-soluble substances such as alcohol and aspirin
Esophagus
Lower esophageal sphincter
Liver
SMALL INTESTINE
Gallbladder
M:mixing and propulsion primarily by segmentation
S:enzymes; HCO3– and enzymes (pancreas); bile (liver); mucus (goblet cells); hormones: CCK, secretin, GIP, and other hormones D:carbohydrates, fats, polypeptides, nucleic acids
A:peptides by active transport; amino acids, glucose, and fructose by secondary active transport; fats by simple diffusion; water by osmosis ions, minerals, and vitamins by active transport
Pylorus
Pancreas
Ileocecal valve
LARGE INTESTINE
M:segmental mixing; mass movement for propulsion
S:mucus (goblet cells)
D:none (except by bacteria)
A:ions, water, minerals, vitamins, and small organic molecles produced by bacteria
Rectum
Anal sphincters
Figure 21-22

30. Digestion and Absorption
PLAY
Interactive Physiology® Animation: Digestive System: Digestion and Absorption