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)

http://upload.wikimedia.org/wikipedia/commons/5/57/Azawakh_K9.jpg

• 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

http://upload.wikimedia.org/wikipedia/commons/0/0f/Peristalsis.gif

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)