1. Chapter 18
Lecture
Outline

2. Functions of GI Tract
18-5

3. Motility Is movement of food through GI tract by means of:
Ingestion--taking food into mouth
Mastication--chewing food and mixing it with saliva
Deglutition--swallowing food
Peristalsis--rhythmic wave-like contractions that move food through GI tract
18-6

4. Secretion
Includes release of exocrine and endocrine products into GI tract
Exocrine secretions include: HCl, H2O, HCO3-, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine
Endocrine includes hormones secreted into stomach and small intestine to help regulate GI system
e.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and somatostatin
18-7

5. Digestion
Refers to breakdown of food molecules into smaller subunits
Absorption
Is passage of digested end products into blood or lymph
18-8

6. Storage and Elimination
Includes temporary storage and subsequent elimination of indigestible components of food
Immune Barrier
Includes physical barrier formed by tight junctions between cells of small intestine
And cells of the immune system that reside in connective tissue just below epithelium
18-9

7. Structure of Digestive System
18-10

8. Digestive System continued
Organs include oral cavity, pharynx, esophagus, stomach, and small and large intestine
Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas
18-12

9. Layers of GI Tract
18-13

10. Mucosa Is the absorptive and secretory layer lining lumen of GI tract
In places is highly folded with villi to increase absorptive area
Contains lymph nodules, mucus-secreting goblet cells, and thin layer of muscle
18-14

11. Submucosa
Is a thick, highly vascular layer of connective tissue where absorbed molecules enter blood and lymphatic vessels
Contains glands and nerve plexuses (submucosal plexus) that carry ANS activity to muscularis mucosae of small and large int.
18-15

12. Muscularis
Is responsible for segmental contractions and peristaltic movement through GI tract
Has an inner circular and outer longitudinal layer of smooth muscle
Activity of these layers moves food through tract while pulverizing and mixing it
Myenteric plexus between these layers is major nerve supply to GI tract
Includes fibers and ganglia from both Symp and Parasymp systems
18-16

13. Serosa Is outermost layer; serves to bind and protect
Consists of areolar connective tissue covered with layer of simple squamous epithelium
18-17

14. From Mouth to Stomach
18-19

15. From Mouth to Stomach
Mastication (chewing) mixes food with saliva which contains salivary amylase
An enzyme that catalyzes partial digestion of starch
18-20

16. From Mouth to Stomach continued
Deglutition (swallowing) begins as voluntary activity
Oral phase is voluntary and forms a food bolus
Pharyngeal and esophageal phases are involuntary and cannot be stopped
To swallow, larynx is raised so that epiglottis covers entrance to respiratory tract
A swallowing center in medulla orchestrates complex pattern of contractions required for swallowing
18-21

17. From Mouth to Stomach continued
Peristalsis propels food thru GI tract
= wave-like muscular contractions
After food passes into stomach, the gastroesophageal sphincter constricts, preventing reflux
18-23

18. Stomach
18-24

19. Stomach Is most distensible part of GI tract Empties into the duodenum
Functions in: storage of food; initial digestion of proteins; killing bacteria with high acidity; moving soupy food mixture (chyme) into intestine
18-25

21. Stomach continued
Is enclosed by gastroesophageal sphincter on top and pyloric sphincter on bottom
Is divided into 3 regions:
Fundus
Body
Antrum
18-26

22. Stomach continued Inner surface of stomach is highly folded into rugae
Contractions of stomach churn chyme, mixing it with gastric secretions
Eventually these will propel food into small intestine
18-27

23. Stomach continued Gastric mucosa has gastric pits in its folds
Cells that line folds deeper in the mucosa, are exocrine gastric glands
18-28

24. Stomach continued
Gastric glands contain cells that secrete different products that form gastric juice
Goblet cells secrete mucus
Parietal cells secrete HCl and intrinsic factor (necessary for B12 absorption in intestine)
Chief cells secrete pepsinogen (precursor for pepsin)
18-29

25. HCl in Stomach
Is produced by parietal cells which pump H+ into lumen via an H+/ K+ pump (pH ~1)
Cl- is secreted by facilitated diffusion
H+ comes from dissociation of H2CO3
Cl- comes from blood side of cell in exchange for HCO3-
18-31

27. HCl in Stomach continued
Is secreted in response to the hormone gastrin; and ACh from vagus
These are indirect effects since both stimulate release of histamine which causes parietal cells to secrete HCl
18-32

28. HCl in Stomach continued
Makes gastric juice very acidic which denatures proteins to make them more digestible
Converts pepsinogen into pepsin
Pepsin is more active at low pHs
18-33

29. Protection of Stomach Against HCL and Pepsin
Both HCL and pepsin can damage lining and produce a peptic ulcer
1st line of defense is the adherent layer of mucus
= a stable gel of mucus coating the gastric epithelium
Contains bicarbonate for neutralizing HCL
Is a barrier to actions of pepsin
Gastric epithelial cells contain tight junctions to prevent HCL and pepsin from penetrating the surface
Gastric epithelial cells are replaced every 3 days
18-34

30. Digestion and Absorption in Stomach
Proteins are partially digested by pepsin
Carbohydrate digestion by salivary amylase is soon inactivated by acidity
Alcohol and aspirin are the only commonly ingested substances that are absorbed
18-35

31. Gastric and Peptic Ulcers
Peptic ulcers are erosions of mucous membranes of stomach or duodenum caused by action of HCl
In Zollinger-Ellison syndrome, duodenal ulcers result from excessive gastric acid in response to high levels of gastrin
Helicobacter pylori infection is associated with ulcers
Antibiotics are useful in treating ulcers
And also proton pump inhibitors such as Prilosec
Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation
Is why histamine receptor blockers such as Tagamet and Zantac can treat gastritis
18-36

32. Small Intestine
18-37

33. Small Intestine (SI)
Is longest part of GI tract; approximately 3m long
Duodenum is 1st 25cm after pyloric sphincter
Jejunum is next 2/5 of length
Ileum is last 3/5 of length; empties into large intestine
18-38

34. Small Intestine (SI) continued
Absorption of digested food occurs in SI
Facilitated by long length and tremendous surface area
18-39

35. Small Intestine (SI) continued
Surface area increased by foldings and projections
Large folds are plicae circulares
Microscopic finger-like projections are villi
Apical hair-like projections are microvilli
18-40

36. Small Intestine (SI) continued
Each villus is covered with columnar epithelial cells interspersed with goblet cells
Epithelial cells at tips of villi are exfoliated and replaced by mitosis in crypts of Lieberkuhn
Inside each villus are lymphocytes, capillaries, and central lacteal
18-41

37. Small Intestine (SI) continued
A carpet of hair-like microvilli project from apical surface of each epithelial cell
Create a brush border
18-42

38. Intestinal Contractions and Motility
2 major types of contractions occur in SI:
Peristalsis is weak and slow and occurs mostly because pressure at pyloric end is greater than at distal end
Segmentation is major contractile activity of SI
Is contraction of circular smooth muscle to mix chyme (shown in diagram)
18-44

39. Large Intestine
18-48

40. Large Intestine (LI) or Colon
Has no digestive function but absorbs H2O, electrolytes, B and K vitamins, and folic acid
Internal surface has no villi or crypts and is not very elaborate
Contains large population of microflora
= 1013 to 1014 commensal bacteria of 400 species
which produce folic acid and vitamin K and ferment indigestible food to produce fatty acids
And reduce ability of pathogenic bacteria to infect LI
antibiotics can negatively affect commensals
18-49

41. Large Intestine (LI) or Colon continued
Extends from ileocecal valve at end of SI to anus
Outer surface bulges to form pouches (haustra)
18-50

42. Fluid and Electrolyte Absorption in LI
SI absorbs most water but LI absorbs 90% of water it receives
Begins with osmotic gradient set up by Na+/K+ pumps
Water follows by osmosis
Salt and water reabsorption stimulated by aldosterone
LI can also secrete H2O via active transport of NaCl into intestinal lumen
18-51

43. Liver
18-53

44. Structure of Liver Liver is the largest internal organ
Hepatocytes form hepatic plates that are 1–2 cells thick
Plates are separated by sinusoids which are fenestrated and permeable even to proteins
Contain phagocytic Kupffer cells
18-54

45. Structure of Liver continued
A damaged liver can regenerate itself from mitosis of surviving hepatocytes
In some cases, such as alcohol abuse or viral hepatitis, regeneration does not occur
Can lead to liver fibrosis and ultimately cirrhosis
18-55

46. Hepatic Portal System Food absorbed in SI is delivered 1st to liver
Capillaries in digestive tract drain into the hepatic portal vein which carries blood to liver
Hepatic vein drains liver
Liver also receives blood from the hepatic artery
18-56

47. Liver Lobules Are functional units formed by hepatic plates
In middle of each is central vein
At edge of each lobule are branches of hepatic portal vein and artery which open into sinusoids
18-57

48. Liver Lobules continued
Bile is secreted by hepatocytes in bile canaliculi
Empty into bile ducts which flow into hepatic ducts that carry bile away from liver
18-58

49. Enterohepatic Circulation
Is recirculation of compounds between liver and intestine
Many compounds are released in bile, reabsorbed in SI, and returned to liver to be recycled
Liver excretes drug metabolites into bile to pass out in feces
18-59

51. Detoxication of Blood
Liver can remove hormones, drugs, and other biologically active molecules from blood by:
Excretion into bile
Phagocytosis by Kupffer cells
Chemical alteration of molecules
e.g. ammonia is produced by deamination of amino acids in liver
Liver converts it to urea which is excreted in urine
18-64

52. Gallbladder and Pancreas
18-68

53. Gallbladder Is a sac-like organ attached to inferior surface of liver
Stores and concentrates bile continuously produced by liver
When SI is empty, sphincter of Oddi in common bile duct closes and bile is forced into gallbladder
Expands as it fills with bile
When food is in SI, sphincter of Oddi opens, gall bladder contracts, and bile is ejected thru cystic duct into common bile duct then to duodenum
18-69

54. Pancreas Is located behind stomach
Has both endocrine and exocrine functions
Endocrine function performed by islets of Langerhans
Secretes insulin and glucagon
Exocrine secretions include bicarbonate solution and digestive enzymes
These pass in pancreatic duct to SI
Exocrine secretory units are acini
18-70

55. Pancreatic juice and bile are secreted into the duodenum
18-71

56. The Pancreas is Both an Exocrine and Endocrine Gland
A photomicrograph of the endocrine and exocrine portions of the pancreas
An illustration depicting the exocrine acini, where acinar cells prod. Inactive enzymes stored as zymogen granules which are secreted via ducts into duodenum
18-72

57. Pancreatic Juice Contains water, bicarbonate, and digestive enzymes
Digestive enzymes include amylase for starch, trypsin for proteins, and lipase for fats
Brush border enzymes are also required for complete digestion
18-73

58. The Activation of Pancreatic Juice Enzymes
Most pancreatic enzymes are produced in inactive form (zymogens)
Trypsin is activated by brush border enzyme, enterokinase
Trypsin in turn activates other zymogens
18-74