B. Life Systems · Human Organ Systems

The Respiratory System: Breathing In and Out

B2.1 B2.2 A1.2 A1.4 A1.5About 45 minutes

Learning goals

I can name the parts of the respiratory system and describe the job of each part.
I can follow the path air takes from my nose to the alveoli.
I can explain how the diaphragm helps me breathe.
I can do a fair test to see how exercise changes my breathing rate.

Words to know

respiratory system
The organ system that brings oxygen into the body and removes carbon dioxide.

oxygen
A gas in the air that every cell in your body needs.

carbon dioxide
A waste gas your cells make. You breathe it out.

trachea
The windpipe: the tube that carries air from your throat toward your lungs.

bronchi
The two tubes that branch off the trachea, one into each lung. (One is a bronchus.)

alveoli
Millions of tiny air sacs in the lungs, where oxygen goes into the blood.

diaphragm
A dome-shaped muscle under the lungs that helps you breathe.

breathing rate
The number of breaths you take in one minute.

Hold your breath... how long can you last?

Don't actually try for long! Most people want to breathe again very quickly. That's because every cell in your body needs a gas called oxygen, all the time. Your respiratory system brings oxygen in and takes a waste gas, carbon dioxide, out. You breathe thousands of times every day without even thinking about it.

The path of air

Follow a breath of air on its trip:

Nose and mouth. Air comes in. Tiny hairs and sticky mucus in your nose trap dust and germs. Your nose also warms the air and makes it moist.
Throat. Air goes down the back of your throat and through your larynx (voice box).
Trachea. The windpipe. It has stiff rings of cartilage that keep it open. You can feel them on the front of your neck.
Bronchi. The trachea splits into two tubes, one for each lung.
Bronchioles. The tubes keep branching into smaller and smaller tubes, like the branches of an upside-down tree.
Alveoli. At the ends are millions of tiny air sacs. Each one is wrapped in tiny blood vessels.

In the alveoli, oxygen passes from the air into the blood. At the same time, carbon dioxide passes from the blood into the air sacs, so you can breathe it out.

When you swallow, a little flap called the epiglottis covers the trachea so food goes down the right tube, toward your stomach.

A homemade lung model: a clear plastic bottle with two balloons on a straw inside and a stretched balloon across the bottom. — In this lung model, the balloons inside act like lungs and the stretched sheet at the bottom acts like the diaphragm. Pull it down and the balloons fill with air. AI-generated image

The respiratory system. Front view: the person is facing you, so their left side is on your right.

How does air get in? The diaphragm!

Your lungs have no muscles of their own. A muscle under them, the diaphragm, does the work, with help from muscles between your ribs.

Breathing in (inhaling): the diaphragm tightens and moves down. Your ribs move up and out. The space in your chest gets bigger, so air rushes in to fill the lungs.
Breathing out (exhaling): the diaphragm relaxes and moves up. The space in your chest gets smaller, so air is pushed out.

Put your hands on your lower ribs and breathe deeply. Can you feel them move out and in?

When you breathe in, the diaphragm tightens and moves down, so the chest space gets bigger and air rushes in. When you breathe out, it relaxes and domes up, pushing air out.

Part	Job
nose and mouth	let air in; the nose cleans, warms and moistens it
trachea	carries air to the bronchi; cartilage rings keep it open
bronchi and bronchioles	carry air into every part of the lungs
alveoli	oxygen goes into the blood; carbon dioxide comes out
diaphragm	moves down to pull air in, moves up to push air out

Parts of the respiratory system and their jobs.

Worked example

Let's do a fair test: does exercise change breathing rate?

Maya wants to know if jumping changes how fast she breathes. How can she do a fair test?

Question: Does 1 minute of jumping jacks change my breathing rate?
Prediction (hypothesis): I think I will breathe faster after jumping, because my muscles need more oxygen.
Variables: she changes only one thing (resting or after jumping). She keeps everything else the same: same person, same way of counting, same timer.
At rest: she sits quietly for 2 minutes. Her dad times 30 seconds while she counts her breaths: 10 breaths. 10 × 2 = 20 breaths per minute.
After exercise: she does jumping jacks for 1 minute, then counts for 30 seconds right away: 18 breaths. 18 × 2 = 36 breaths per minute.
Repeat: she does both tests again on another day and gets similar results, so her results are more reliable.

Conclusion: Maya's breathing rate went from 20 to 36 breaths per minute after exercise. Her prediction was supported: working muscles need more oxygen, so she breathes faster.

Why breathe faster?

When you run or jump, your muscles work hard and need more oxygen. They also make more carbon dioxide. Your brain notices and tells your diaphragm to work faster, so you breathe faster and deeper.

Just for fun

A hiccup is your diaphragm tightening suddenly, all by itself. The "hic!" sound is your vocal cords snapping shut on the air rushing in. It's your diaphragm's version of a surprise party!

Hands-on activity

Breathing investigation (and an optional lung model)

Part A: breathing rate test. You need: a timer, paper and a pencil, a grown-up.

Sit quietly for 2 minutes. Have your grown-up time 30 seconds while you count your breaths (one in plus one out = 1 breath). Multiply by 2.
Walk on the spot for 1 minute. Count again right away.
Do jumping jacks for 1 minute. Count again right away.
Rest for 3 minutes. Count one more time.
Record your results in a table and make a bar graph. What pattern do you see?

Part B (optional): lung model. You need: a clear plastic bottle with the bottom cut off, two balloons, an elastic band, a grown-up.

A grown-up cuts the bottom off the bottle.
Push one balloon into the neck of the bottle and stretch its opening over the bottle's mouth. This is a lung.
Tie a knot in the second balloon, cut off its top half (grown-up) and stretch the bottom part over the open bottom of the bottle. Hold it with the elastic band. This is the diaphragm.
Pull the knot down: the lung balloon fills with air. Push it up: the air goes out. That's how your diaphragm works!

Grown-up notes (safety): Stop if your child feels dizzy, and choose gentle exercise if your child has asthma or a health condition. Do the cutting yourself; cut plastic edges can be sharp. Balloons and broken balloon pieces are a choking hazard: keep them away from young children and throw away scraps right away.

Your turn

Put the parts in the order air travels: bronchi, nose, alveoli, trachea.

What gas does your body need from the air? What waste gas do you breathe out?

Where does oxygen go into the blood?

What does the diaphragm do when you breathe in?

Why does your nose have hairs and mucus?

Leo counts 13 breaths in 30 seconds. What is his breathing rate per minute?

What keeps the trachea from closing?

More practice: warm-up

What is another name for the trachea?

How many lungs do you have?

Which muscle under your lungs helps you breathe?

True or false: you breathe faster when you exercise.

What are the tiny air sacs in the lungs called?

More practice: on your own

Explain what happens to your chest space when you breathe out.

Why is it a good idea to breathe through your nose on a cold winter day?

Sam rests, then counts 9 breaths in 30 seconds. After skipping rope he counts 21 breaths in 30 seconds. How much did his breathing rate per minute go up?

In Maya's test, why did she keep the same timer and the same way of counting?

Why do you sometimes cough when food "goes down the wrong way"?

Challenge

Why is it helpful that the lungs have millions of tiny alveoli instead of one big air bag?

In the bottle model, what does each part stand for? Name one way the model is different from a real body.

Predict what will happen to your breathing rate 5 minutes after you stop running. Explain why.

Check your understanding

Quiz

Which is the correct path of air?

Oxygen moves into the blood in the...

When you breathe in, your diaphragm...

What gas do you breathe out as a waste?

Ana counts 11 breaths in 30 seconds. Her breathing rate is...

In a fair test about exercise and breathing, you should change...

For parents and teachers

Big idea: the respiratory system brings oxygen in and removes carbon dioxide. The diaphragm does the work of breathing. Oxygen enters the blood in the alveoli.

How to help at home:

Do the breathing test together and help your child multiply by 2 to get breaths per minute.
Have your child gently feel the cartilage rings on the front of the neck, and put hands on the ribs while breathing deeply.

Common difficulties:

Thinking the lungs pull air in by themselves. The diaphragm (and rib muscles) do it.
Thinking we breathe out only carbon dioxide. Exhaled air still contains plenty of oxygen; it just has more carbon dioxide than the air we breathe in.

Safety: Stop exercise if your child feels dizzy. If your child has asthma, follow their action plan. You do the bottle cutting; balloons are a choking hazard.