1. Chapter 18
The Digestive
System
18-1

2. 18-4

3. Functions of GI Tract
18-5

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. Absorption Is passage of digested end products into blood or lymph
18-8

6. 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

7. Regulation of GI Tract
Parasympathetic effects, arising from vagus and spinal nerves, stimulate motility and secretions of GI tract
Sympathetic activity reduces peristalsis and secretory activity
GI tract contains an intrinsic system that controls its movements--the enteric nervous system
GI motility is influenced by paracrine and hormonal signals
18-18

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

16. 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

17. 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

18. Intestinal Enzymes
Attached to microvilli are brush border enzymes that are not secreted into lumen
Enzyme active sites are exposed to chyme
18-43

19. 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
= 400 different species of commensal bacteria
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

20. 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 AT of NaCl into intestinal lumen
18-51

21. 18-60

22. Detoxification 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

23. Secretion of Glucose, Triglycerides and Ketones
Liver helps regulate blood glucose by removing it from blood or releasing it to blood
Removes it via glycogenesis and lipogenesis
Or produces it via glycogenolysis and gluconeogenesis
Can convert free fatty acids into ketone bodies (ketogenesis) that can be used for energy during fasting
18-66

24. Neural and Endocrine Regulation
Neural and endocrine mechanisms modify activity of GI system
Vagus nerve is heavily involved in regulating and coordinating digestive activities
GI tract is both an endocrine gland and target for action of hormones
Hormones include secretin, gastrin, CCK, and GIP
18-77

25. Enteric Nervous System
Submucosal and myenteric plexuses contain as many neurons as spinal cord
Includes preganglionic Parasymp axons, ganglion cell bodies, postganglionic Symp axons; and afferent intrinsic and extrinsic sensory neurons; interneurons, and glia
Peristalsis is controlled by enteric NS
18-84

26. Digestion and Absorption of Protein
Begins in stomach when pepsin digests proteins to form polypeptides
In SI, endopeptidases (trypsin, chymotrypsin, elastase) cleave peptide bonds in interior of polypeptides
SI exopeptidases (carboxypeptidase, aminopeptidase) cleave peptide bonds from ends of polypeptides
18-91

27. Digestion and Absorption of Lipids
Occurs in SI
Arrival of lipids in duodenum causes secretion of bile
Fat is emulsified by bile salt micelles
Forms tiny droplets of fat dissolved in bile salt micelles
Greatly increases surface area for fat digestion
18-93

28. Digestion and Absorption of Lipids continued
Pancreatic lipase hydrolyzes triglycerides to free fatty acids and monglycerides
Phospholipase A breaks down phospholipids into fatty acids and lysolecithin
18-94

29. Transport of Lipids continued
Cholesterol and triglycerides from liver form VLDLs which are secreted and take triglycerides to cells
Once triglycerides are removed, VLDLs become LDLs
LDLs transport cholesterol to organs and blood vessels
HDLs transport excess cholesterol back to liver
High ratio of HDL-cholesterol to total cholesterol is believed to confer protection against atherosclerosis
18-99