1. The Gastrointestinal System

2. Four Basic Digestive Processes
Figure 20.1
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3. Major Structures of the GI System
Figure 20.2
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4. Layers
Figure 20.3
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5. Mouth, Pharynx, Esophagus, and Stomach
Figure 20.4
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6. Esophagus Muscular tube from pharynx to stomach
Upper 1/3—skeletal muscle
Lower 2/3—smooth muscle
Upper esophageal sphincter
Skeletal muscle
Between pharynx and esophagus
Lower esophageal sphincter
Smooth muscle
Between esophagus and stomach
Figure 20.4 (1 of 3)
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7. Gastric Pits Figure 20.4 (2–3 of 3)
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8. Acidic Environment of Stomach
pH = 2
Necessary for activating pepsinogen
Denatures proteins
Kills bacteria
Gastric mucosal barrier
Protective layer of mucus and bicarbonate
Secreted from neck cells and goblet cells
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9. Regulates passage of chyme between stomach and small intestine
Pyloric Sphincter
Regulates passage of chyme between stomach and small intestine
Figure 20.4
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10. Small Intestine Figure 20.5 (1 of 3)
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11. Small Intestine Wall Villi increase surface area of epithelium
Contain blood vessels and lacteal for absorption of nutrients
Microvilli increase surface area of epithelial cells
Form brush border
Figure 20.5
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12. Crypts of Lieberkuhn
Epithelial cells in crypts secrete bicarbonate-rich fluid
Secreted in proximal small intestine
Absorbed in distal small intestine
Figure 20.5
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13. Circulatory Route: Absorbed Material
Figure 20.6
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14. Features of the Colon Figure 20.7
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15. Accessory Structures Figure 20.2
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16. Structure of an Accessory Gland
Figure 20.8
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17. Characteristics of Saliva
Rich in bicarbonate ions
Contains mucus
Enzymes
Salivary amylase
Lysozyme
Figure 20.9
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18. The Pancreas Has exocrine and endocrine portions
Exocrine portion produces pancreatic juice
Rich in bicarbonate
Enzymes
Pancreatic amylase and lipases
Proteases
Nucleases
Figure 20.10
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19. Liver Functions Secrete bile Processing of nutrients
Glucose  glycogen
Amino acids  fatty acids
Triglycerides and cholesterol synthesis
Lipoprotein synthesis
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20. Liver Functions Removal of old red blood cells
Catabolize hemoglobin
Generates bilirubin
Elimination of wastes and toxins
Bilirubin eliminated as bile pigments
Synthesis of plasma proteins
Secretion and modification of hormones
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21. Liver, gallbladder, and associated ducts
Biliary System
Liver, gallbladder, and associated ducts
Liver synthesizes bile
Gallbladder stores bile
Common bile duct
Transports bile from liver and/or gallbladder to duodenum
Figure 20.11a
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22. Liver, gallbladder, and associated ducts
Biliary System
Liver, gallbladder, and associated ducts
Joins with pancreatic duct to form Ampulla of Vater
Sphincter of Oddi
Regulates flow from pancreas and gallbladder to duodenum
Figure 20.11a
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23. Liver Anatomy
Materials to make bile taken up from blood in sinusoids into hepatocytes
Hepatocytes secrete bile into bile canaliculi, on side opposite of sinusoids
Figure 20.11b
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24. Liver Anatomy Canaliculi drain into bile ducts
Bile ducts drain into common hepatic duct
Figure 20.11b
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25. GI Organs and Functions
Table 20.1
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26. Carbohydrates Typical diet: 250–800 grams carbohydrates
Most consumed as disaccharides or polysaccharides
Sucrose
Lactose
Maltose
Starch
Glycogen
Cellulose (fiber, cannot be digested)
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27. Carbohydrate Digestion
Only monosaccharides absorbed
Disaccharides and polysaccharides must be digested to monosaccharides
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28. Digestion of Starch Enzyme of digestion End product Salivary amylase
Pancreatic amylase
End product
Disaccharides (maltose)
Limit dextrins
Figure 20.12
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29. Further Digestion Enzymes of digestion
Dextrinase – limit dextrins  glucose
Glucoamylase – polysaccharides  glucose
Sucrase – sucrose  fructose + glucose
Lactase – lactose  galactose + glucose
Maltase – maltose  2 glucose
Location of enzymes—brush border of small intestine
“Brush border enzymes”
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30. Absorption of Monosaccharides
Absorption = transport from lumen to blood
Glucose and galactose absorbed by
Secondary active transport across apical membrane
Facilitated diffusion across basolateral membrane
Fructose absorbed by
Facilitated diffusion across both membranes
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31. Digestion / Absorption of Maltose
Figure 20.13
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32. Proteins Typical diet: 125 grams/day protein
Only require 40–50 grams
Proteins to be digested and absorbed include
Consumed in diet
Secreted into lumen of intestinal tract
Sloughed off with cells lining intestinal tract
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33. Protein Digestion in Stomach
Protein digestion begins in stomach
Enzyme = pepsin
Inactive (secreted form) = pepsinogen
Activated by acid
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34. Pepsin: Activation and Activity
Chief cells secrete pepsinogen
Pepsinogen is the inactive form
HCl cleaves pepsinogen to pepsin
Pepsin = active form
Parietal cells secrete HCl
Figure 20.14
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35. Protein Digestion in Small Intestine
Pancreatic proteases
Trypsin
Chymotrypsin
Carboxypeptidase
Brush border proteases
Aminopeptidase
Enterokinase
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36. Lipids Typical diet: 25–160 grams lipids
90% triglycerides
Lipids face special problem in digestion and absorption
Not water soluble
Do not mix with stomach, intestinal contents
Form fat droplets
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37. Lipid Digestion Enzymes of digestion = lipases Secreted from pancreas
Lipases can only act on molecules near edge of fat droplet
Bile salts increase surface area of droplets by breaking large droplet into several small droplets = emulsification
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38. Bile Salts Synthesized in liver from cholesterol
Secreted in bile to duodenum
Amphipathic molecule
Emulsify fat
Figure 20.16
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39. Emulsification of a Fat Globule
Before: little surface exposed to lipases
After: much more surface exposed to lipases
Figure 20.17
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40. Triglycerides  Monoglyceride + 2 Fatty acids
Digestion by Lipases
Some fatty acids and monoglycerides absorbed
Others form micelles
Equilibrium between micelles and fatty acids and monoglycerides formed by action of lipases
Triglycerides  Monoglyceride + 2 Fatty acids
Figure 2.4b
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41. Absorption of Monoglycerides and Fatty Acids
Equilibrium between free fatty acids and monoglycerides and those in micelles
Free form can be absorbed by simple diffusion across epithelium
Inside epithelial cell
Enter smooth ER and reform triglycerides and other lipids
Lipids enter Golgi apparatus to be packaged into chylomicrons
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42. Lipase Activity Figure 20.18
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43. Enterohepatic Circulation
Figure 20.19
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44. Absorption of Fat Figure 20.20
Lumen
Monoglyceride
Triglyceride
Other lipids
and proteins
Golgi
apparatus
Fatty acid
Smooth
endoplasmic
reticulum
Enterocyte
Blood
Interstitial fluid
Chylomicron
Lacteal
To general circulation
Lymph
Figure 20.20
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45. Absorption of Vitamins
Fat-soluble vitamins (A, D, E, and K)
Absorbed with lipids
Dissolve in lipid droplets, micelles, chylomicrons
Water-soluble vitamins
Require special transport proteins
Vitamin B12
Absorbed only when bound to intrinsic factor
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46. Absorption of Minerals: Na+, Cl-, and K+
Sodium
Solvent drag with water reabsorption
Actively absorbed in jejunum, ileum, and colon
Chloride
Passively follows sodium absorption
Potassium
Passively absorbed
Secreted when lumenal concentrations very low
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47. Absorption of Minerals: Calcium
Actively absorbed in duodenum and jejunum
Binds to brush border protein = calcium-binding protein
Transported into epithelial cell
Transported out of cell across basolateral membrane by Ca2+ pump
1,25-(OH)2D3 increases calcium absorption by increasing concentration of calcium-binding protein
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48. Absorption of Minerals: Iron
Transferrin secreted by enterocytes into lumen of small intestine
Transferrin binds iron
Transferrin-iron complex binds receptor
Taken into cell by receptor-mediated endocytosis
Some iron stored in enterocyte as ferritin
Some iron transported into blood
Transported in blood bound to transferrin
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49. Absorption and Secretion of Bicarbonate
Jejunum
Bicarbonate ions passively absorbed
Ileum and colon
Bicarbonate secreted in exchange for chloride ions
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50. Absorption of Water Water in GI tract Water absorption is passive
7 liters secretions
2 liters intake
Water absorption is passive
Water follows absorption of solutes by osmosis
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51. GI Regulation
Regulation of GI function is not based on the concept of homeostasis
Regulate GI function to maximize absorption, regardless of whether nutrients are needed
Regulate conditions in lumen of GI tract to maximize absorption
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52. GI Hormones and Actions
Table 20.2
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53. Neural and Endocrine Control
Figure 20.21
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54. Phases of Gastrointestinal Control
Cephalic phase—stimuli originate in head
Thoughts, taste, and smell
Requires input from CNS (long reflexes)
Gastric phase—stimuli originate in stomach
Long and short reflex and GI hormones
Intestinal phase—stimuli originate in small intestine
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55. Short-Term Regulation of Food Intake
Short-term regulation: hunger versus satiety
Satiety factors
Insulin
CCK
Neural input from mechano- and chemo-receptors
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56. Long-Term Regulation of Food Intake
Leptin = hormone released from adipose cells when calories exceeds demands
Leptin suppresses hunger and increases metabolism
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57. Saliva Secretion Taste and texture of food
Mechanoreceptors and taste receptors in mouth
Salivary center of medulla
Autonomic nervous system
Stimulate salivation
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58. Gastric Acid Secretion
Parietal cells produce hydrochloric acid
Carbonic anhydrase catelyzes production of bicarbonate and H+
H+ are actively secreted into lumen of stomach
Bicarbonate is transported into intersitital fluid for Cl-
Cl- diffuses into lumen of stomach through channels
Figure 20.22
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59. Gastric Secretion: Cephalic Phase
Figure 20.23a
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60. Gastric Secretion: Gastric Phase
Figure 20.23b
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61. Pancreatic Juice Secretion
Acinar cells
Small volume of primary secretion
Water, electrolytes, and digestive enzymes
Duct cells
Large volume
Bicarbonate rich secretion
Figure 20.8
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62. Bicarbonate-Rich Fluid Secretion
Figure 20.24
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63. Regulation of Bile Entry
Figure 20.25
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64. Rates of Fluid Movement
Fluid entering lumen daily
Ingested = 2000 mL
Saliva = 1500 mL
Gastric juice = 2000 mL
Bile = 500 mL
Pancreatic juice = mL
Intestinal secretions = 1500 mL
Total = 9000 mL
Fluid leaving lumen daily
Intestinal absorption = 8500 mL
Colon absorption = 400 mL
Total = 8900 mL
Amount excreted in feces = 9000 mL – 8900 mL = 100 mL
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65. Fluid Flows in the GI System
Figure 20.26
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66. VII. Gastrointestinal Motility and Its Regulation
Electrical activity in gastrointestinal smooth muscle
Peristalsis and segmentation
Chewing and swallowing
Gastric motility
Motility of the small intestine
Motility of the colon
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67. GI Motility GI motility Movements of the wall of the GI tract
Due primarily to the contractions of the muscularis externa (outer muscle layers)
Function: to mix and propel
Figure 20.3
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68. GI Smooth Muscle Activity
Figure 20.27
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69. Peristalsis
Figure 20.28a
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70. Segmentation
Figure 20.28b
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71. Phases of Regulation Cephalic phase, excitations
Anger, aggression
Cephalic phase, inhibitions
Pain, fear, depression
Gastric phase, excitatory stimulus
Distension of stomach
Intestinal phase, inhibitory stimuli
Distension of duodenum
Contents: osmolarity, acidity, fat
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