1. THE DIGESTIVE SYSTEM

2. Evolution of digestion
Amoeba – engulfs food, lysomes Hydra – digestive sack with single opening Earthworm/bird – pharynx, esophagus, crop, gizzard, intestine, anus Chordata – more complex digestive system

3. The Four Stages of Digestion
1. ingestion – taking in nutrients (i.e. eating)
2. digestion – breaking down complex molecules into smaller ones
3. absorption – taking up of digested molecules
4. egestion – removal of wastes

6. 1. INGESTION and DIGESTION: the mouth
- food enters mouth
- it is broken up by teeth:
incisors cut
canines and bicuspids pierce and tear
molars crush and grind
- action of chewing stimulates salivary glands which secrete saliva
saliva: a mixture of water, mucus and enzymes (ex. amylase: breaks down starches into simpler carbohydrates)

8. food and saliva form a bolus that is pushed to the pharynx (dual purpose: air or food)
- flap-like epiglottis directs food down esophagus
- bolus moves down esophagus through rhythmic muscle contractions (peristalsis) until it reaches the cardiac sphincter of the stomach

10. 2. DIGESTION a) DIGESTION: the stomach
- the stomach is the site of food storage (1.5 L capacity) and initial protein digestion
- to enter and to exit the stomach the food must pass through sphincters: constrictor muscles that surround a tubelike structure
- cardiac sphincter relaxes and lets bolus fall in

11. - stomach is J-shaped and contains gastric juice (secreted by stomach lining): a mixture of hydrochloric acid, enzymes that work well at low pH (ex. pepsin: begins to break down proteins into polypeptides), and mucus
- HCl breaks down fibres, including bacteria, mucus protects stomach lining from HCl (if mucus layer is destroyed, ex. Heliobacter pylori we get stomach ulcers as the lining is digested)
- some absorption starts here: water, alcohol, some medications (ex. aspirin)

13. - mixture of partially digested food, water, and gastric juice is called chyme
- chyme passes out of stomach through pyloric sphincter and into small intestine

14. b) DIGESTION: the small intestine and pancreas
- measures up to 7 m in length, only 2.5 cm in diameter
- pyloric sphincter empties chyme into duodenum, the first 25 cm of the small intestine
- this is where the majority of digestion takes place
- secretions from the pancreas and liver enter the duodenum through a duct

16. liver
four functions: synthesis and breakdown, detoxification, storage
1. synthesis: produces bile (made of bile salts), which breaks down fats, bile is stored and concentrated in gallbladder until needed
- bile functions as an emulsifier, to allow the fats to travel through the digestive system and be further broken-down
- if too much bile /too concentrated bile is produced the bile stored in the gallbladder can start to crystallize forming gallstones which then block the bile duct

17. breakdown: removes the highly toxic nitrogen group from amino acids to form urea (component of urine)
breaks down blood cells (stores products in gallbladder for removal – feces is brown), and any obstruction will turn skin yellow (jaundice)

18. 3. detoxification: works to remove toxins from the body
- removes alcohol and other chemicals through detoxification
4. storage: stores carbohydrates and vitamins

19. pancreas
- produces sodium bicarbonate, which raises the pH of chyme from 2.5 to 9.0
- secretes digestive enzymes, ex. lipase (breaks down fat) and trypsin (breaks down proteins)

20. small intestine
- secretes maltase (maltose to glucose) and peptidase (complete protein breakdown)

21. SO:
Carbohydrates  glucose (amylase, maltase)
Proteins  amino acids (pepsin, peptidase)
Fats (triglycerides)  fatty acids and glycerol (bile, lipase)

22. 3. ABSORPTION a) ABSORPTION: small intestine
- after the duodenum, the following two sections of the small intestine are the jejunum and the ileum
- fingerlike villi increase the surface area of the small intestine, and absorb the glucose, amino acids, fatty acids and glycerol
- once absorbed the nutrients enter the bloodstream and are distributed throughout the body

24. b) ABSORPTION: large intestine (colon)
- water is absorbed
- harmless bacteria live in large intestine and produce vit K and B which are also absorbed

25. 4. EGESTION: rectum and anus
- cellulose helps to void bowels, removing potentially toxic wastes
- ppl who don’t eat enough cellulose are at risk of colon cancer
entire process: hours

26. Interesting….
- the nervous and hormonal systems act on digestion before we even eat: seeing, smelling and tasting food stimulates gastric secretions
- swallowing stimulates production of gastric juices, ex. the hormone gastrin stimulate gastric juice release before food gets to stomach

28. - speed of digestion also varies with food type
- large meal activates 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)