Download THE DIGESTIVE SYSTEM

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, aka 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 destoroyed, 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
• duodenum 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
-
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
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
http://www.youtube.com/watch?v=bo2Ape8J
HqA&feature=related
• 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
mouth
salivary glands
pharynx
epiglottis
esophagus
liver
stomach
cardiac sphincter
pancreas
pyloric sphincter
gall bladder
large intestine
duodenum
small intenstine
colon
anus
• homeostasis
• body’s ability to adjust to the
fluctuating internal and external
environment
DIGESTION AND HOMEOSTASIS
• 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)
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