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.