THE DIGESTIVE SYSTEM

• as heterotrophs we need to take in our food from the outside

• once inside the cell, the nutrients are used as scaffolding for synthesis of chemical compounds or as sources of energy

• the main function of the dig. system is to break down food into nutrients, which are then absorbed and transported by the circulatory system throughout the body

All Cells Require Nutrients for:

1.Growth

2.Maintenance

3. Repair

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

http://upload.wikimedia.org/wikipedia/commons/b/b3/Tractus_intestinalis_duodenum.svg

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 also functions as an emulsifier, to allow the fats to travel through the digestive system and be well 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

http://upload.wikimedia.org/wikipedia/commons/4/43/Gray1224.png

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

Other notes on digestion

• - 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)

• - special receptors located in the body sense blood sugar, body T, oxygen levels, etc.

- when homeostasis is disrupted this sends a message to the brain to correct it

• ex. blood sugar levels:• - constant concentration is crucial to our well-being, if fall

below a certain value our brains shut down (coma)• - when we eat the blood sugar (glucose) goes up• - our body releases insulin (a hormone that activates cell

surface proteins) from the pancreas to bring the glucose inside the cells

• - liver cells collect any extra glucose and convert it to glycogen for storage

• - decrease in blood sugar levels activates the pancreas to release glucagon, a hormone that converts glycogen in the liver back to glucose and releases it into the bloodstream

normal glucose

normal glucose

high glucose

food

low glucose

insulin release

(pancreas)

cells take up excess glucose, liver makes glycogen

glucagon release (pancrea

s)

glycogen release

as glucose (liver)