Transcript the digestive system

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. 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
• - 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, 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
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)