1. Physiology of the Digestive System
Chapter 26
Anatomy & Physiology

2. Mechanical Digestion Consists of all motility that brings about:
Change in physical state of food from large to small
Churning
Propelling food forward

3. Mastication (chewing)
Begins in mouth
Tongue, cheeks, lips keep food between the surfaces of teeth
Reduce particle size & mix with saliva

4. Deglutition
Oral stage: mouth to oropharynx (voluntary control); under control of cerebral cortex
Pharyngeal stage: oropharynx to esophagus (involuntary); deglutition center in medulla
Esophageal stage: esophagus to stomach (involuntary); deglutition center in medulla

5. Oral stage
Bolus in the middle of tongue pressed against the palate & moved back into the oropharynx
Soft palate & uvula prevent food from entering nasopharynx

6. Pharyngeal stage Involuntary reflexes push bolus toward esophagus
Upward movement of larynx & downward movement of food closes epiglottis

7. Motility of smooth muscle in GI tract
Peristalsis
Segmentation

8. Peristalsis Wavelike ripple; always in a forward direction
Bolus stretching GI wall triggers a reflex contraction of circular muscle

9. Segmentation
Mixing movement; forward & backward movement

10. Gastric motility Takes 2-6 hours to empty stomach
Food churned with gastric juices: chyme
Hormonal control: gastric inhibitory peptide secreted by intestinal mucosa
Nervous mechanism: triggered by presence of acid & distention-enterogastric reflex

11. Intestinal motility Both segmentation & peristalsis
Segmentation in duodenum & jejunum mixes chyme w/digestive juices
Takes 5 hours in small intestine
Peristalsis also stimulated by cholecystokinin-pancreozymin (CCK) secreted by intestinal mucosa in presence of chyme

12. Digestive enzymes Extracellular, protein catalysts Enzyme principles
Specific in action
Function best a specific pH
Catalyze reaction in both directions
Continually being destroyed or eliminated so continually being synthesized
Most digestive enzymes are inactive proenzymes

13. Carbohydrate digestion
Polysaccharides hydrolyzed by amylases (in saliva & pancreatic juices) to disaccharides
Sucrase, lactase, maltase found in cell membrane of villi cells hydrolyze into monosaccharides, mostly glucose

14. Protein digestion
Proteases catalyze hydrolysis into intermediate compounds then finally into amino acids
Main proteases
Pepsin-stomach
Trypsin-pancreas
Chymotrypsin- “
Peptidase-intestines

15. Fat digestion
Lecithin & bile acids in bile emulsify (become small droplets soluble in water) fats
Main fat digesting enzyme: pancreatic lipases

16. Residues of digestion Cellulose: dietary fiber
Undigested connective tissue from meat

17. Saliva Secretion of salivary glands Water (mostly)
Mucus: lubricates food
Salivary amylase: begins digestion of starches
Sodium bicarbonate: increases pH for optimal amylase activity

18. Gastric juice Secreted by gastric glands
Pepsin (by chief cells as pepsinogen)-begins digestion of proteins
HCl (by parietal cells) decreases pH for activation & function of pepsin
Intrinsic factor (by parietal cells) protect vitamin B12
Mucus & water

19. Pancreatic juice Secreted by acinar cells & duct cells
Proteases (trypsin & chymotrypsin)
Lipases
Nucleases: digest nucleic acids
Amylase
Sodium bicarbonate: increase pH
All pancreatic enzymes are secreted as inactive proenzymes

20. Bile Secreted by liver, stored & concentrated in gall bladder
Lecithin & bile salts
Sodium bicarbonate for optimum pH
Cholesterol, detoxification products, bile pigments all eliminated in feces

21. Intestinal juice Secreted by cells of intestinal exocrine cells
Mucus & water: lubricate & aid in mixing
Sodium bicarbonate

22. Control of salivary secretions
Only by reflex mechanisms
Chemical, mechanical, olfactory, visual stimuli

23. Control of gastric secretion
Three phases:
Cephalic phase
Gastric phase
Intestinal phase

24. Cephalic phase Activated by mental factors
Vagus nerve stimulates production of gastrin which stimulates gastric secretions

25. Gastric phase
Products of protein digestion stimulate release of gastrin
Distension of stomach also stimulates release of gastrin

26. Intestinal phase
Fats, carbohydrates & acid in chyme stimulate release of gastric inhibitory peptide (GIP), CCK, secretin which decrease gastric secretion

27. Control of pancreatic secretion
Stimulated by hormones secreted by intestinal mucosa
Secretin: production of pancreatic fluid low in enzyme but high in bicarbonate
CCK
Increased exocrine secretion by pancreas
Opposes gastrin, thus inhibiting gastric secretion
Stimulates contraction of gallbladder

28. Absorption
Passage of substances through the intestinal mucosa into blood
Most occurs in small intestines
Water: osmosis
Secondary active transport: sodium
Sodium cotransport: glucose
With the aid of bile salts in lacteals

29. Elimination Expulsion of the residues of digestion
Act of expelling feces is defecation
Defecation results as a reflex stimulated by receptors in rectal mucosa when rectum is distended