1. The Digestive System Part 2 Objectives:
1. Explain the anatomy of the digestive system
2. Explain the difference between mechanical and chemical digestion and where each occurs
3. State the organ that begins the chemical digestion of each of the macromolecules.
4. Follow the path of a spaghetti and meat ball dinner through the digestive system.

2. The Stomach
Mechanical digestion occurs here – physical mixing and grinding of food particles
Chemical digestion occurs here – acids, hormones, and enzymes released to break food down further

3. The Pathway of Food
1. The bolus enters the cardia section of the stomach – just under the diaphragm and heart (hence the name)
Stomach grinds down bolus and mixes it with acids and enzymes – bolus becomes chyme
2. Chyme moves to the upper round section of the stomach called the fundus
Chyme is further ground, mixed, and held for minutes
Enzymatic digestion occurs (acid here is only at 30% strength so it won’t interfere with enzymes)

4. The Pathway of Food
3. Chyme moves to the large middle section called the body
Peristalsis mixes and churns chyme
Area where digestion begins
HCl and pepsin at work – stops enzymatic action
4. Antrum is the last part of the stomach before the pylorus
The majority of stomach digestion is done here
Food held a long time and released to the duodenum in a slow and methodical manner

6. Parts of the Stomach

7. Small Intestine - Functions
1. Chemical Digestion
neutralize acid from stomach
add digestive enzymes and bile
break proteins, carbohydrates and lipids into absorbable materials
2. Absorption - 95% of food absorbed here

8. Small Intestine - Structure
Regions: (stomach connects via pyloric sphincter)
Duodenum – cm long
Jejunum – 3 m long
Ileum – 4 m long
Mucosa adaptations: villi containing blood and lacteal capillaries (absorbs digested lipids) = maximizes absorption

9. Small Intestine

10. Accessory Organs Pancreas: digestive role
Secretes digestive enzymes into small intestine:
lipases break down lipids,
carbohydrases (like amylase and lactase) break down carbohydrates
proteases (like trypsin and pepsin) break down proteins
produces hormones to regulate blood sugar (insulin and glucagon)
produces bicarbonate ions to neutralize HCl after digestion
Liver
Produces bile (acts as emulsifer – begins fat breakdown)
Metabolic functions: storage of glycogen (tightly packed glucose); protein synthesis; chemical processing – detoxification (alcohol, drugs)
Gallbladder: stores bile

11. Break down of the Macromolecules
(AMYLASE)
CHO’s digested to------ MONOSACCHARIDES
(starch) (glucose)
(PEPTIDASE)
PROTEIN digested to  AMINO ACIDS
(BILE) (LIPASE)
FATS ---emulsified----digested to --- GLYCEROL &
FATTY ACIDS

12. Accessory Organs
pancreas

13. Absorption of Proteins and Carbs
(circuatory system)

14. Absorption of Fats
This mechanism of absorption is a little different because lipids cannot go directly into the blood since they are not soluble.
mechanism of entry into circulation!

15. Large Intestine

16. Large Intestine (Colon)
Functions: absorbs nutrients and water, and produces/eliminates waste
Structure:
cecum: cecum receives chyme from the small intestine (via ileocecal valve)
appendix = VESTIGIAL STRUCTURE - the shrunken remainder of the cecum
ceca in herbivores are large and contain mutualistic bacteria that help digest cellulose
Why still there? – protect beneficial bacteria … and not a huge selective pressure for elimination
Colon: ascending, transverse, descending, sigmoid

17. Colon … continued Rectum: Last 20 cm of colon End of digestive tract
Passes feces from large intestine to the anus
Anus:
Pair of circular muscles called anal sphincters
Controls defecation

18. Control of Digestive System
The nervous and hormonal systems act on digestion before we even eat: seeing, smelling and tasting food stimulates gastric secretions
Regulation dependent on volume and content of food
Nervous system: stretch receptors in stomach

19. These gastric juices start the digestion of protein Hormones:
Swallowing stimulates production of gastric juices, ex. the hormone gastrin stimulate gastric juices to be released before food gets to stomach.
These gastric juices start the digestion of protein
Hormones:
Gastrin: stimulates release of gastric juice
Secretin: stimulates pancreas to secrete water and bicarbonate
Cholecystokinin (CCK): signals pancreas to secrete digestive enzymes

20. Speed of Digestion
The speed of digestion is under homeostatic control. It also varies with food type
- A larger meal activates more receptors which stimulates stronger stomach muscle contractions and faster emptying
- Fatty meal: digestion slows down in response to a hormone from the small intestine (enterogasterone) to allow more time for fat digestion (we feel full longer after a fatty meal)

21. What do the parts do? Mouth Gullet Stomach Small intestine
Large intestine
Rectum
Anus
Teeth grind food, saliva added.
Muscles squeeze food down.
Food mixed with strong acid.
Absorbs all nutrients.
Water is absorbed.
Faeces are formed and stored.
Faeces are egested.

22. Plenary
How is the small intestine structured to allow maximum absorption?
Why should we ensure that we eat fiber?
What is the function of the liver, pancreas and gall bladder?
Why are they technically not considered to be digestive organs.
Name three enzymes involved in chemical digestion, state which macromolecule they breakdown.

23. Your Task Read Complete
Trace the path of a spaghetti and meat ball dinner from ingestion to absorption.