HI

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