Grade by Grade
B. Life Systems · Human Organ Systems

The Digestive System: From Food to Fuel

B2.1 B2.2 A1.5 A3.2About 45 minutes
Learning goals
  • I can name the organs of the digestive system and describe what each one does.
  • I can trace the path of food through my body.
  • I can explain the difference between mechanical and chemical digestion.
  • I can use measurement to model how long my intestines are.

Words to know

digestion
Breaking food down into nutrients small enough for the body to use.
nutrients
The useful parts of food, such as proteins, carbohydrates, fats, vitamins and minerals.
mechanical digestion
Breaking food into smaller pieces by chewing, squeezing and churning.
chemical digestion
Breaking food down with digestive juices, such as saliva and stomach juices.
esophagus
The muscular tube that carries food from your throat to your stomach.
small intestine
A long, coiled tube where most digestion finishes and nutrients go into the blood.
large intestine
A wider tube where water is taken back into the body and solid waste forms.
absorb
To take in. The small intestine absorbs nutrients into the blood.

Where does your lunch go?

You eat a sandwich at noon. A few hours later, its nutrients are feeding your muscles, your brain, even your toenails. How? Your digestive system breaks the food down into tiny pieces called nutrients, small enough to pass into your blood.

In Grade 4 Health you learned which nutrients foods give you. Now you will see how your body gets them out of the food.

Two kinds of digestion

  • Mechanical digestion breaks food into smaller pieces: your teeth chew, your stomach churns.
  • Chemical digestion uses digestive juices to break food into new, tiny substances (nutrients). Saliva, stomach juices and juices from the pancreas and liver all help.
A whole grain turkey sandwich cut in half on a plate, with a red apple and a glass of water.
Your teeth start mechanical digestion of a lunch like this. Then digestive juices break it down into nutrients your blood can carry. AI-generated image

The trip through the digestive system

  1. Mouth. Teeth cut and grind the food (mechanical). Saliva wets the food and starts breaking down starches (chemical). Your tongue rolls the food into a soft ball.
  2. Esophagus. When you swallow, muscles in this tube squeeze in waves to push food down to your stomach. They are so strong that astronauts can swallow in space!
  3. Stomach. A stretchy, J-shaped bag of muscle. It churns food and mixes it with strong acid and juices. Food stays here a few hours and becomes a thick, soupy mix.
  4. Small intestine. A long, coiled tube, about 6 metres long in an adult. Juices from the pancreas, and bile from the liver, finish digestion. The inside is covered with tiny finger-like bumps called villi. Nutrients pass through the villi into the blood.
  5. Large intestine. Wider but shorter, about 1.5 metres in an adult. It takes water back into the body. What is left is solid waste called feces (poop).
  6. Rectum and anus. The rectum stores the waste until it leaves the body through the anus when you go to the bathroom.

The liver and pancreas are helper organs: food does not go through them, but their juices flow into the small intestine.

mouthesophagusliverlarge intestinestomachsmall intestine
The digestive system. Front view: the person is facing you, so their left side is on your right. The stomach is on the person's left; the liver is on the person's right. The large intestine frames the coiled small intestine.
OrganMechanical, chemical or both?Main job
mouthbothchew; saliva starts breaking down starch
esophagusneither (it moves food)squeezes food down to the stomach
stomachbothchurns food with acid and juices
small intestinechemicalfinishes digestion; absorbs nutrients into the blood
large intestineneitherabsorbs water; forms solid waste
liver and pancreaschemical (helpers)make bile and digestive juices for the small intestine
The digestive organs and their jobs.
Worked example

Let's follow a bite of apple

Trace a bite of apple through the digestive system.

  1. Mouth: teeth crunch it into small pieces; saliva wets it and starts chemical digestion.
  2. Esophagus: you swallow; muscles squeeze the food down.
  3. Stomach: churning and acid turn it into a soupy mix.
  4. Small intestine: juices from the pancreas finish digesting it; the sugars and other nutrients pass through the villi into the blood.
  5. Large intestine: water is taken back; the tough fibre that could not be digested forms part of the solid waste.
  6. Rectum and anus: the waste leaves the body.

Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus.

Math connection

6 metres is 600 cm. 1.5 metres is 150 cm. The small intestine is 4 times as long as the large intestine, because 150 × 4 = 600. It fits inside you because it is folded and coiled up.

small intestine: about 6 m = 600 cmlarge intestine: about 1.5 m = 150 cm12340 cm150 cm300 cm450 cm600 cm
The small intestine (600 cm) is 4 times as long as the large intestine (150 cm): four large-intestine lengths fit along it. It is "small" because it is narrower, not shorter.
Just for fun

Why did the sandwich hang around in the stomach for hours?

It was waiting for its churn! (Food really can stay in the stomach for a few hours while it gets churned into soup.)

Hands-on activity

Model the digestive system

Part A: stomach in a bag. You need: a zip-top freezer bag, a few crackers, about 125 mL of water, a grown-up.

  1. Put the crackers in the bag. This is the food after chewing.
  2. Add the water (a model of the stomach juices). Squeeze out the air and seal the bag tightly.
  3. Squish and squeeze the bag for 2 minutes, like your stomach churning. What happens to the crackers?
  4. Over a sink, a grown-up snips a tiny corner and squeezes the mush out slowly, like food leaving the stomach for the small intestine.

Part B: how long? You need: string or yarn, a measuring tape.

  1. Measure and cut 6 m of string (small intestine) and 1.5 m of a different string or yarn (large intestine).
  2. Lay them out straight in a hallway. Then coil them up to fit in a shoebox lid. How does your body fit it all in?

Grown-up notes: do not eat the model food. Use a plastic zip bag, not glass. Supervise scissors. Wash hands after.

Your turn

1

List the path of food from start to finish.

2

What is the difference between mechanical and chemical digestion?

3

Where do most nutrients pass into the blood?

4

What is the main job of the large intestine?

5

Name two helper organs that make juices for digestion.

6

Is chewing mechanical or chemical digestion?

More practice: warm-up

1

Which tube carries food from your throat to your stomach?

2

What liquid in your mouth starts digestion?

3

Which is longer: the small intestine or the large intestine?

4

Which organ churns food with acid?

5

What are the useful parts of food called?

More practice: on your own

1

Why do you think it is called the small intestine if it is longer?

2

Why is it important to chew your food well?

3

How many times longer is a 6 m small intestine than a 1.5 m large intestine? Show your work.

4

Why do the villi help the small intestine do its job?

5

In the stomach-in-a-bag model, what stood for the stomach juices and what stood for the churning?

Challenge

1

Food does not pass through the liver. Explain how the liver still helps digestion.

2

Name one way the stomach-in-a-bag model is like a real stomach and one way it is different.

3

A person had part of their large intestine removed. Predict one change in their body and explain.

Check your understanding

Quiz

1.

Which is the correct order?

2.

Chewing is an example of...

3.

Most nutrients pass into the blood in the...

4.

The main job of the large intestine is to...

5.

Which organ makes bile?

6.

In an adult, about how long is the small intestine?

For parents and teachers

Big idea: digestion breaks food into nutrients (mechanically and chemically) so they can be absorbed into the blood in the small intestine. The large intestine takes back water.

How to help at home:

  • At a meal, ask your child to narrate where the food is going next.
  • Use the measuring tape together; converting 6 m to 600 cm connects to Grade 5 math.

Common difficulties:

  • Thinking food passes through the liver or pancreas. They are helpers that send juices.
  • Thinking digestion happens only in the stomach. Most chemical digestion and absorption happen in the small intestine.
  • The words feces, rectum and anus are correct science names. Use them matter-of-factly.

Safety: do not eat the model food; supervise scissors.