Human Respiratory System
Primary 6 · Science and Elementary Technology
Human Respiratory System
Lesson 2 of 5
15 min
Let's begin
Keza lies on the grass and puts one hand on her belly, just below her ribs. When she breathes in, her hand rises. When she breathes out, it sinks back down.
"Something inside me is moving," she says. She is right. Today you will find out what it is, and even build a model that breathes.
Breathing in is also called inhalation or inspiration. This is what happens:
Breathing out is also called exhalation or expiration:
| Breathing in | Breathing out | |
|---|---|---|
| Diaphragm | Contracts, flattens, moves down | Relaxes, domes up |
| Rib cage | Moves up and out | Moves down and in |
| Space in the chest | Gets bigger | Gets smaller |
| Air | Rushes in | Is pushed out |
Scientists show breathing with a bell jar model. You can make one from a clear plastic bottle with the bottom cut off (ask an adult to cut it). Push a Y-shaped tube (or two drinking straws taped together) through a stopper in the bottle's neck. Tie a small balloon to each lower end. Stretch a rubber sheet (a cut-open balloon) over the open bottom.
Pull the rubber sheet down: the balloons blow up. Push it up: the balloons go flat.
| Part of the model | Part of the body |
|---|---|
| Bottle or bell jar | Chest and rib cage |
| Straight part of the tube | Trachea |
| Two branches of the Y-tube | Bronchi |
| Balloons | Lungs |
| Rubber sheet | Diaphragm |
The air you breathe out is not the same as the air you breathe in.
Breathing is moving air in and out of the lungs. Respiration happens inside every cell: the cell uses oxygen to break down food and release energy, making carbon dioxide and water. Breathing brings in the oxygen that respiration needs.
When you are resting, you breathe roughly 15 to 20 times a minute. When you run, your muscles need more oxygen and make more carbon dioxide, so you breathe faster and deeper.
Tip
Down to fill, up to empty
When the diaphragm goes DOWN, air comes IN. When it goes UP, air goes OUT. Put your hand on your belly and feel it happen.
Try each one on your own first, then tap Show answer to check.
What does the diaphragm do when you breathe in?
Answer: It contracts, flattens and moves down, so the chest gets bigger and air rushes in.
In the bell jar model, what do the balloons and the rubber sheet stand for?
Answer: The balloons stand for the lungs; the rubber sheet stands for the diaphragm.
Byiringiro breathes on a cold window and it becomes misty. What does this show?
Answer: The air we breathe out contains water vapour.
Count your breaths for one minute while sitting. Then do 20 star jumps and count again. Which number is bigger, and why?
Answer: The count after exercise is bigger, because your working muscles need more oxygen and make more carbon dioxide.
Are breathing and respiration the same thing?
Answer: No. Breathing moves air in and out of the lungs; respiration releases energy from food inside the cells.
What you learned today
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Quick check
Match each part of the bell jar model to the part of the body it stands for.
Tap one on the left, then its partner on the right.
