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The Human Digestive System
The Digestive System
The digestive system: is a group of organs that take in
food and change it into a form the body can use.
Digestion: is the changing large molecules into smaller
ones capable of being used by the cells in the body.
Cells need food for energy, growth and repair.
Can be compared to a factory – materials are
transported to different locations and are changed at
every stop.
The final product is quite different than the original.
Nutritional Requirements
Humans need to take in food for:
fuel = chemical energy
raw materials = carbon source
essential nutrients = animals cannot
make
elements (N, P, K, Fe, Na, K, Ca...),
NAD, FAD, etc.
Getting & Using Food
Ingest
taking in food
Digest
Physical digestion
breaking up food into smaller pieces
chemical digestion
breaking down food into molecules small enough to be absorbed into
cells
enzymes
Absorb
absorb across cell membrane
diffusion
active transport
Eliminate
undigested extracellular material passes out of digestive system
Physical vs. Chemical Change
Food gets broken down as it passes through the
digestive system in 2 ways
1. Physical Change:
Large pieces are broken down into smaller ones.
The food is still in the same form (only the size and shape
of the food particles change).
Occurs through chewing, grinding and mixing.
2. Chemical Change:
This occurs when food changes form so the body may use
it.
Body chemicals are added to food to make this happen.
These chemicals are called enzymes.
Digestion
Digestion
Physical Change
Food is broken down
into small pieces
Food is not in final form
Chemical
Change
Food is changed to a
new form
Food is in final form for
cells use
Digestive System
Water, vitamins, and minerals can be used by cells so do
not need to be changed
Fats, proteins, and carbohydrates cannot be used so they
need to be broken down by enzymes
Enzymes: are chemicals that speed up the rate of
change
There are different kinds of enzymes for different
nutrients.
Human digestive system
Alimentary Canal
Digestive System
Main organ passageway:
1. Mouth (includes salivary glands)
2. Esophagus
3. Stomach
4. Small intestine
5. Large intestine
6. Anus
Additional organs necessary:
Pancreas, liver, and gallbladder all secrete
enzymes into the small intestine.
Common processes & structures
Movement & Control
peristalsis
push food along by rhythmic waves of smooth
muscle contraction in walls of digestive system
sphincters
muscular ring-like valves, regulate the passage of
material between sections of digestive system
Accessory glands
salivary glands, pancreas, liver & gall bladder
secrete digestive juices (enzymes & fluid)
Swallowing (& not choking)
Epiglottis
problem: breathe & swallow through same orifice
flap of cartilage
closes trachea (windpipe) when swallowing
food travels down esophagus
Esophagus
move food along to stomach by peristalsis (pushing of food along by
waves of smooth muscle contractions)
Ingestion
Mouth
physical digestion
teeth
breaking up food
chemical digestion
saliva
amylase
o enzyme digests starch
mucin
o slippery protein (mucus)
o protects soft lining of digestive system
o lubricates food for easier swallowing
buffers
o neutralizes acid to prevent tooth decay
anti-bacterial chemicals
o kill bacteria that enter mouth with food
mouth
break up food
moisten food
digest starch
kill germs
Stomach
Functions
food storage
can stretch to fit ~2L food
disinfect food
HCl = pH 2 so kills bacteria
Has a mucus layer to protect itself
from enzymes and HCl
chemical digestion
pepsin
enzyme breaks down proteins
After the stomach completes its
part in digestion the food is now
in a liquid form called chyme.
But the stomach is made out of protein!
What stops the stomach from digesting itself?
mucus secreted by stomach cells protects
stomach lining
Stomach
A large muscular bag that has gastric juice (juices given
off by the lining of the stomach) that mixes and digests
protein. The gastric juice is made of an emzyme
(pepsin) and HCl that digests protein.
The walls are muscular and thick that churns and
grinds up the food. Once the food is mixed with the
digestive juices, it starts to look like a milk shake.
The partially digested food is squeezed out of the
stomach in spurts and moves to the small intestine.
Food usually stays in the stomach for 4 – 7 hours.
mouth
break up food
moisten food
digest starch
kill germs
stomach
kills germs
store food
break up food
digest proteins
cardiac
sphincter
pyloric
sphincter
Small Intestine
Function
major organ of digestion & absorption
chemical digestion
digestive enzymes
absorption through lining by diffusion
over 6 meters in length!
small intestine has huge surface area = 300m2 (~size of
tennis court)
Structure
A hollow tube with finger-like projections called villi.
Each villus contains blood vessels that carry digested food.
From the blood vessels the food is carried to body cells.
Duodenum
1st section of small intestines
acid food from stomach mixes with digestive
juices from accessory glands:
pancreas
liver
gall
bladder
Pancreas
Makes enzymes to digest:
Fats
Proteins
Carbohydrates
These enzymes pass from
the pancreas to the small
intestine through a small
tube.
Pancreas also produces
insulin to regulate sugars.
mouth
break up food
moisten food
digest starch
kill germs
pancreas
produces enzymes to
digest proteins, starch &
fat
stomach
kills germs
break up food
digest proteins
store food
Liver
Digestive System Functions
produces bile
stored in gallbladder until needed
breaks up fats
Stores unused sugar
Circulatory System
Connection
bile contains
colors from old
red blood cells
collected in liver =
iron in RBC rusts &
makes feces brown
mouth
break up food
moisten food
digest starch
kill germs
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes to
digest proteins, starch &
fats
stomach
kills germs
break up food
digest proteins
store food
Digestive Enzymes
Carbohydrate Digestion:
Mouth
Salivary glands
Small Intestine
Pancreas
Protein Digestion:
Stomach
Enzymes and HCl
Small Intestine
Pancreas
Fat Digestion:
Small Intestine
Pancreas
Liver
Bile
Digestive enzymes
Absorption by Small Intestines
Absorption through villi & microvilli
finger-like projections
increase surface area for absorption
Absorption of Nutrients
Occurs through diffusion.
Final forms of nutrients:
Carbohydrates (starches and sugars) =
glucose
Protein = amino acids
Fats = glycerine and fatty acids
mouth
break up food
moisten food
digest starch
kill germs
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes to
digest proteins & starch
stomach
kills germs
break up food
digest proteins
store food
small intestines
breakdown all foods
- proteins
- starch
- fats
- nucleic acids
absorb nutrients
Large intestines (colon)
Function
re-absorb water
use ~9 liters of water every
day in digestive juices
> 90% of water reabsorbed
not enough water absorbed
back to body
o diarrhea
too much water absorbed back to body
o Constipation
No nutrients are absorbed here
Rectum
Last section of colon (large intestines)
eliminate feces
undigested materials
extracellular waste
omainly cellulose from plants
oroughage or fiber
salts
masses of bacteria
mouth
break up food
moisten food
digest starch
kill germs
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes to
digest proteins & carbs
stomach
kills germs
break up food
digest proteins
store food
small intestines
breakdown food
- proteins
- starch
- fats
absorb nutrients
large intestines
absorb water
Appendix
Vestigial organ
Problems with the Digestive
System
Ulcers
A sore or hole inside either the stomach or small intestine.
Caused by the stomach lining being digested or “eaten away” by
enzymes and stomach acids.
The mucus layer helps prevent this but this can be damaged by a
bacterial infection called Helicobacter pylori thus resulting in an ulcer.
Heartburn
Caused by stomach acids moving into the esophagus.
This causes a burning feeling.
The esophagus is lies behind the heart so it feels like the heart is
burning.
Eating too much at once or too much acidic or rich foods can cause acid
to back up.
Digestive System - Glands
Salivary glands – secrete amylase for preliminary starch
digestion.
Glands in the stomach lining – secrete mucus and gastric
juice (contains pepsin and hydrochloric acid) for initial
protein digestion.
Liver – secretes bile to emulsify fats.
Pancreas – secretes sodium bicarbonate to neutralize
stomach acids and enzymes to act on proteins (proteases),
carbohydrates (amylase) and fats (lipases).
Walls of the small intestine – secrete juices that complete the
digestion of carbohydrates, fats and proteins.
Lab Activity
Stations
Activity 1: How long is the Digestive
System?
Materials: 4 different balls of yarn
Description: Students are to cut pieces of different
coloured yarn for each of the following
measurements. Then they are to tie them together to
show how long each piece of the digestive system is
in reference to each other.
Esophagus
25 cm
Stomach
20 cm
Small Intestine 700 cm
Large Intestine 150 cm
Activity 2: Digestion
Materials:
Sugar Cubes
Granulated Sugar
Clear cups filled with water
Description:
Place a sugar cube in a cup of water. Place
about a teaspoon full of granulated sugar in the
other cup of water. Observe.
Activity 3: Carbohydrate
Digestion
Materials: Unsalted crackers
Description: Students chew 2 unsalted
crackers for two minutes without
swallowing.
Activity 4: Hands on Digestion
Materials:
Meatball size of hamburger
One plastic bag
1 M HCL
Digestive juice A (pepsin, trypsin, and water)
Digestive juice B (bile salts, pancreatin enzyme and
water)
Description: Place hamburger, 3 eyedroppers full of
HCL, 1 tbsp of Digestive juice A and 2 Tbsp of Digestive
juice B in the plastic bag. Knead with your hands for 10
minutes (stimulates the stomach) and it will have been
reduced to liquid with a definite odor.
Activity 5: How do villi aid the Small
Intestine in absorption?
Materials:
Paper towels
4 cups of equal amounts of water
Graduated cylinder
Description: Compare how 1,2,3, then 4 paper towels
absorb. Dip each paper towel into a cup of water
(same amount of water in each cup). Record the
volume of water left in each cup (using the graduated
cylinder).
Activity 6: The Digestive System
Story
Description: Assign each student as a
part of the digestive system and one
student as food. As the food student
moves past each digestive system part,
the student labelled as that part must
describe what they are doing to the
food.
Questions:
What system in your body is the same length as the
completed piece of yarn?
In Activity 2, what can you conclude must be done to food
before digestion occurs?
What physical and chemical changes occurred to the soda
crackers?
What caused the physical and chemical changes to the
crackers?
Did you notice a taste change in the crackers?
How is physical digestion simulated in Activity 4?
Questions continued
What evidence was there that chemical digestion occurred in
the hamburger?
Which paper towel had the largest surface area?
What relationship can you see between the largest surface
area and how villi aid in absorption?
Using Activity 6, create a diagram to demonstrate the passing
of food through the digestive system.