1. Introduction to the Digestive System

2. Digestion
Animals cannot ingest macromolecules made by other animals and use them directly
2 kinds:
Mechanical digestion
Chemical digestion

3. Vertebrate Digestive System
Food moves through digestive tract by peristalsis:
Mouth  pharynx esophagus  stomach 
small intestine  large intestine  anus
Accessory organs:
Salivary glands
Liver
Gall bladder
Pancreas

4. Layers of digestive tract

5. Mouth Start of food processing Mechanical digestion – teeth Incisors
Canines
Premolars
Molars

6. Chemical digestion: Salivary glands secrete 1 L saliva/day
Salivary amylase

7. Pharynx and Esophagus Bolus moves through pharynx
Epiglottis covers trachea during swallowing
Esophagus leads to stomach
Peristalsis moves food along

9. Stomach
Ring of sphincter muscle relaxes and allows bolus to enter stomach
As stomach fills, rugae of stomach smooth out

10. Chemical Digestion in Stomach
Gastric glands in mucosa secrete:
HCl
Intrinsic factor
Pepsinogen (converts to pepsin in presence of acidic gastric juice)
Pepsin, main digestive enzyme of stomach, breaks down large proteins into short polypeptides

11. Food spends 3-4 hours in stomach
Muscle contraction churns food
Salivary amylase works until stomach becomes too acidic
Partly digested food becomes chyme
Peristalsis releases chyme in spurts through pylorus into small intestine

12. Protection of Stomach
Lining of stomach secretes large amounts of alkaline mucus
Epithelial cells are tightly joined to prevent leaking

13. Small Intestine In human, approx. 17 feet long
Divided into 3 regions: duodenum, jejunum, and ileum
Lining is folded and composed of villi and microvilli

14. Digestion in Small Intestine
Bile, secreted by liver, mechanically digests fats
Enzymes secreted by small intestine and pancreas:
Polypeptides and peptides
Trypsin, chymotrypsin, peptidases
Completion of carbohydrate digestion
Pancreatic amylase, maltase, sucrase, lactase
Digestion of fat droplets
lipase

15. Absorption of Nutrients
Nutrients must pass through epithelium to reach blood or lymph
Simple diffusion, facilitated diffusion, active transport

16. Large Intestine Approx. 4 feet long
Includes cecum and appendix, a blind pouch
Sections include ascending, transverse, descending & sigmoid colon
Ends at rectum & anus

17. Functions of Large Intestine
Undigested material and unabsorbed chyme passes slowly through large intestine
Water and sodium are absorbed
Bacteria in large intestine feed and produce vitamin K and some B vitamins for use by host