the digestive system. evolution of digestion amoeba – engulfs food, lysomes hydra – digestive...
TRANSCRIPT
THE DIGESTIVE SYSTEM
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
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
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)
• 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
2. DIGESTIONa) 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
• - 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)
• - mixture of partially digested food, water, and gastric juice is called chyme
• - chyme passes out of stomach through pyloric sphincter and into small intestine
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
liver• four functions: synthesis and breakdown,
detoxification, storage1. 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
2. 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)
3. detoxification: works to remove toxins from the body
• - removes alcohol and other chemicals through detoxification
4. storage: stores carbohydrates and vitamins
• 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)
• small intestine
• - secretes maltase (maltose to glucose) and peptidase (complete protein breakdown)
• SO:
• Carbohydrates glucose (amylase, maltase)
• Proteins amino acids (pepsin, peptidase)
• Fats (triglycerides) fatty acids and glycerol (bile, lipase)
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
• b) ABSORPTION: large intestine (colon)
• - water is absorbed
• - harmless bacteria live in large intestine and produce vit K and B which are also absorbed
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: 24 - 36 hours
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
• - 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)