1. Misericordia University
Digestion
General Physiology
Tony Serino, Ph.D.
Biology 346
Misericordia University

2. Digestion
The reduction through mechanical and chemical means (hydrolysis) of complex food substances into simple monomers and their absorption into the internal environment.

3. Functions of Digestive System
Motility(Propulsion)
Ingestion –food enters tract
Mastication -chewing
Deglutition -swallowing
Transportation through tract (peristalsis)
Mixing
Egestion (Defecation)
Secretion
Endocrine and Exocrine secretions
Digestion
mechanical and chemical breakdown of food
Absorption
Passage of food particles from external to the internal environment

4. Major Organs of System

5. Accessory Organs
Teeth

6. Basic Histology of Digestive Tract
(Auerbach’s)
(Meissner’s)
(LOCI)

7. Tube Movements Peristalsis Segmentation*
*majority of contractions of SI

8. Control of Overall GI Tract Activity

9. Mouth –ingestion, mastication, mechanical dig.
Salivary glands secrete saliva –mixture of water, mucus, electrolytes, antibodies and enzymes. Enzymes are salivary amylase (pytalin) which breaks down starches and maltase.
Tongue –mixes food and tastes
Teeth –mechanical digestion

10. Pharynx –junctional tube
Pharyngeal Constrictors
Nasopharynx
Oropharynx
Laryngopharynx

11. Deglutition (Swallowing)

12. Deglutition (cont’d)

13. Stomach J-shaped muscular pouch Receives bolus and produces chyme
Liquefies food by mixing it with HCl and vigorous churning
Produces gastric juice: mucous, water, HCl, pepsin, rennin (in infant), GIF (binds to vit. B12 in diet)
Low pH stops amylase activity, but secretes pepsinogen (pepsin) that begins break down of proteins
Absorbs little except imbibed water, electrolytes, and some drugs (ie. alcohol and aspirin)

14. Stomach Acid
HCl acid is produced by the parietal cells of the gastric glands

15. Regulation of Gastric Juice

16. Small Intestine Largest amount of digestion and absorption of gut
Several structures to increse surface area: plica, villli, length, microvilli
Secrete or have bound many enzymes that complete digestion process
Intestinal juice also contains mucous and antibacterial compounds
Absoprtion directly into capillary bed of villi or into villus lacteal

18. Secretin’s receptors are found in the pancreas, which responds with additional bicarbonate delivery: gastric motility and secretion are inhibited.

19. Cholecystokinin’s receptors are located:
in the pancreas, which
responds with additional
enzyme delivery
in the gallbladder, which
contracts to deliver more
bile
in the sphincter
of Oddi, which relaxes to
facilitate delivery of the enzymes and bile salts

20. Large Intestine (Colon)
Massive re-absorption of water and electrolytes by active absorption of Na+
Microbe action produces Vit. K and is absorbed
Responsible for egestion (defecation reflex)

21. Defecation Reflex

22. Liver Metaboblizes all food groups
Storage of Fe and Cu and other metals
Storage of Vit. A, B12, D, E, K
Produces bile and most plasma proteins
Detoxifies the blood, storage of toxins

23. Bile formation by cells in the liver includes 6 components:
bile salts, lecithin, bicarbonate ions, cholesterol, bile pigments, and trace metals.
The bile is stored in the gallbladder and then delivered
into the duodenum upon stimulation from CCK.

24. Bile Salts
Up to 95% of the cholesterol-based bile salts are “recycled” by reabsorption along
the intestine.

25. Secretion of Bile

26. Pancreas Heterocrine gland secretes pancreatic juice and hormones
Pancreatic juice is sodium bicarbonate and digestive enzymes including trypsin, chymotrypsin, carboxypeptidase, lipase, etc.
Hormones: insulin, glucagon, somatostatin

27. Secretion of Pancreatic Juice

28. Chemical Digestion -CHO
Ingest grams per day
Digestible sugars are the disaccharides: sucrose, lactose, and maltose and the polysaccharides: glycogen and starch

29. Chemical Digestion -Proteins
Major proportion of our food intake

30. Amino Acid Absorption

31. Chemical Digestion -Lipids
Small intestine is only major site of digestion since pancreas is largest producer of lipase
Dietary fat accumulate as large globule; bile salts break globule into small droplets (emulsify the fats) providing greater access to enzymes

32. Micelles and Fat absorption
Emulsifying Fat
Micelles and Fat absorption

33. Fat Absorption Big Droplets of Fat Small Droplets of Fat Micelles
Fatty Acids and
Monoglycerides
Chylomicron Assembly
Distribution and Processing

34. Chemical Digestion –Nucleic Acids