Chapter 5 of 8

All 8 chapters
  1. 1Organs and systems, readunderstood
  2. 2Digestion, readunderstood
  3. 3Nutrients and diet, readunderstood
  4. 4Breathing, readunderstood
  5. 5Gas exchange in the lungs, readunderstood
  6. 6The heart and blood, readunderstood
  7. 7Pulse, breathing and exercise, readunderstood
  8. 8Drugs, alcohol and health, readunderstood

S3.3.5 Chapter 5

Gas exchange in the lungs

How does the oxygen in the air get into my blood?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how oxygen enters the blood and carbon dioxide leaves it
  • Explain how smoke from tobacco and cooking fires harms the lungs

Breathing brings air into your chest. But the air is not the part your blood needs. It needs one gas in the air. Draw the place where it passes in.

Draw it first

Draw an air sac and a capillary

  1. One tiny air sac and one tiny blood tube beside it, drawn in two steps. First, a round air sac with a thin wall, and next to it a capillary, a narrow blood tube, with blood cells inside. Second, an arrow for oxygen goes from the air sac into the blood, and an arrow for carbon dioxide goes from the blood into the air sac. Labels: air sac, capillary, blood, oxygen, carbon dioxide.air saccapillaryblood

    1 Draw a tiny air sac with a very thin wall. Draw a capillary, a narrow tube of blood, right against it.

  2. One tiny air sac and one tiny blood tube beside it, drawn in two steps. First, a round air sac with a thin wall, and next to it a capillary, a narrow blood tube, with blood cells inside. Second, an arrow for oxygen goes from the air sac into the blood, and an arrow for carbon dioxide goes from the blood into the air sac. Labels: air sac, capillary, blood, oxygen, carbon dioxide.oxygencarbondioxideair saccapillaryblood

    2 Draw an arrow for oxygen into the blood, and an arrow for carbon dioxide out of the blood.

Your lungs hold millions of tiny air sacs. Each one is like a very small balloon, wrapped in a net of capillaries. A capillary is a blood tube so thin that blood cells pass through it in single file.

Three boxes joined by arrows, each arrow marked with the word air. Windpipe, then small tubes, then air sacs. Labels: windpipe, small tubes, air sacs, air.windpipesmall tubesair sacsairair
The windpipe splits into smaller and smaller tubes. Each ends in a cluster of air sacs.

The wall of an air sac and the wall of a capillary are both very thin. The blood is very close to the air.

The blood that comes to the lungs has used up some of its oxygen. It holds a lot of carbon dioxide, a waste gas. The air in the sacs has plenty of oxygen and little carbon dioxide.

Gases spread from where there is more of them to where there is less. So oxygen passes from the air sac into the blood. Carbon dioxide passes from the blood into the air sac. This swap is gas exchange. You breathe the carbon dioxide out.

Air holds about 21 parts in 100 of oxygen. Breathed-out air has less oxygen and more carbon dioxide. It still holds about 16 parts in 100 of oxygen.

See it three ways

With things

Put a hand on your chest and breathe in deeply. The air goes towards your air sacs. The blood then carries its oxygen to every cell, even in your fingers.

As a picture

Three boxes joined by arrows, each arrow marked with the word oxygen. Air sac, then blood, then cells. Labels: air sac, blood, cells, oxygen.air sacbloodcellsoxygenoxygen

In words

Oxygen passes from an air sac into the blood. The blood carries it to every cell. Carbon dioxide makes the trip the other way.

Why does the body want oxygen? Every cell uses it. In respiration, a cell uses oxygen to release energy from sugar. It makes carbon dioxide and water as waste.

In words: sugar + oxygen → carbon dioxide + water, and energy is released. Respiration happens in every cell. Breathing is only how the air gets in and out.

A table with two columns, breathing and respiration. Breathing happens in the chest and respiration in every cell. Breathing moves air, and respiration frees energy. Labels: breathing, respiration, in the chest, in every cell, moves air, frees energy.breathingrespirationin the chestin every cellmoves airfrees energy
Breathing and respiration are two different things.

Smoke harms the lungs. Tobacco smoke holds a sticky tar, poisons, and a drug called nicotine that is hard to give up. It damages the lining of the tubes. It breaks down the walls between air sacs. Then there are fewer, bigger sacs with less surface, so less oxygen can pass in. It also holds a gas that takes the place of oxygen in the blood. Then the blood carries less. It harms people who breathe it near a smoker.

Smoke from a cooking fire indoors harms the lungs too. Wood, charcoal or dung can all give smoke that causes coughing and lung disease. Children and the person who cooks are at most risk. Never burn charcoal in a closed room, even when you see no smoke. It makes a poison gas that you cannot see or smell.

Two rows of circles that stand for air sacs. The top row has six small circles, labelled air sacs of healthy lungs. The bottom row has two big circles, labelled air sacs harmed by smoke. Smoke breaks down the walls between sacs, so there are fewer, bigger sacs with less surface for oxygen to pass through. Labels: air sacs of healthy lungs, air sacs harmed by smoke.air sacs of healthy lungsair sacs harmed by smoke
Many small sacs give a large surface. After smoke damage there are fewer, bigger sacs, so less oxygen can pass into the blood.
A table with two columns, smoke from and what helps. Tobacco: stay away. Cooking fire: let smoke out, and cook outside. Labels: smoke from, what helps, tobacco, stay away, cooking fire, let smoke out, cook outside.smoke fromwhat helpstobaccostay awaycooking firelet smoke outcooking firecook outside
A window, a door or a chimney lets the smoke out. Keep small children away from the smoke. Never burn charcoal in a closed room. A health worker can advise.

Try it

Mist from your breath

You need: 2 spoons, a cup of cold water, a dry cloth.

Be safe: Use only cold water from the tap or a jar. Do not blow on anything hot. Do not put a spoon in your mouth.

  1. Put both spoons in the cold water for one minute. Dry them with the cloth.
  2. Hold one cold spoon close to your mouth. Breathe out slowly on to it.
  3. Hold the other cold spoon the same way, in the room air, for the same time. Do not breathe on it.

What you will see: A thick mist forms on the spoon you breathed on. The other spoon stays clear, or mists only a little. Breath is warm and carries much water.

If you cannot try it, this is what happens.
A table with two columns, spoon and you see. The spoon you breathed on gets a thick mist. The spoon in room air gets little or none. Labels: spoon, you see, breathed on, thick mist, room air, little or none.spoonyou seebreathed onthick mistroom airlittle or none

A wrong idea many people have

We breathe in oxygen and breathe out only carbon dioxide.

Why it feels right: People say that oxygen goes in and carbon dioxide comes out.

The test that breaks it: The air sacs take only some of the oxygen from each breath. The rest is breathed out again.

What is true: Breathed-out air has less oxygen and more carbon dioxide than breathed-in air. It still holds some oxygen.

Key words

air sacs
the tiny bags at the ends of the lung tubes. Gases pass in and out of the blood here.
capillaries
the thinnest blood tubes. They wrap round the air sacs.
gas exchange
oxygen passing into the blood while carbon dioxide passes out.
respiration
what every cell does to release energy from sugar, using oxygen.

Back to your guess

The answer is B: Oxygen passes through the very thin walls of tiny air sacs into the blood.

The lungs hold millions of tiny air sacs. Oxygen passes through their thin walls into the blood, and carbon dioxide passes the other way.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. In the picture, which way does oxygen go?

    One air sac and one capillary against it. An arrow for oxygen and an arrow for carbon dioxide cross between them. Labels: air sac, capillary, blood, oxygen, carbon dioxide.oxygencarbondioxideair saccapillaryblood
    Tap your answer.

    1. Yes, that is right. The air in the sac has more oxygen than the blood, so oxygen passes into the blood.

    2. Not this one. That is the way carbon dioxide goes. Oxygen goes into the blood. Go to S3.3.5. Try another.

    3. Not this one. The walls are very thin, so gases do pass through them. Go to S3.3.5. Try another.

    See the answer

    From the air sac into the blood

    The air in the sac has more oxygen than the blood, so oxygen passes into the blood.

    • If you chose B: That is the way carbon dioxide goes. Oxygen goes into the blood. Go to S3.3.5.
    • If you chose C: The walls are very thin, so gases do pass through them. Go to S3.3.5.

  2. Which gas leaves the blood in the lungs, and is breathed out?

    Tap your answer.

    1. Not this one. Oxygen goes the other way, into the blood. The blood needs it for the cells. Go to S3.3.5. Try another.

    2. Yes, that is right. The blood brings carbon dioxide from the cells. It passes into the air sacs and is breathed out.

    3. Not this one. Air is a mix of gases, and it does not go into the blood. One gas in it does. Go to S3.3.5. Try another.

    See the answer

    Carbon dioxide

    The blood brings carbon dioxide from the cells. It passes into the air sacs and is breathed out.

    • If you chose A: Oxygen goes the other way, into the blood. The blood needs it for the cells. Go to S3.3.5.
    • If you chose C: Air is a mix of gases, and it does not go into the blood. One gas in it does. Go to S3.3.5.

  3. Why are air sacs good at passing gases into the blood?

    Tap your answer.

    1. Yes, that is right. A thin wall and a close blood tube let gases pass easily. Millions of sacs give a very large surface.

    2. Not this one. Air sacs have no muscles. The gases spread through the wall by themselves. Go to S3.3.5. Try another.

    3. Not this one. Air sacs are tiny. There are millions of them, and that gives a large surface. Go to S3.3.5. Try another.

    See the answer

    Their walls are very thin and there are millions of them, each close to a capillary

    A thin wall and a close blood tube let gases pass easily. Millions of sacs give a very large surface.

    • If you chose B: Air sacs have no muscles. The gases spread through the wall by themselves. Go to S3.3.5.
    • If you chose C: Air sacs are tiny. There are millions of them, and that gives a large surface. Go to S3.3.5.

  4. Where does respiration happen?

    Tap your answer.

    1. Not this one. The lungs are where gases swap. Respiration is what each cell does with the oxygen. Go to S3.3.5. Try another.

    2. Yes, that is right. Every living cell uses oxygen to release energy from sugar. That is respiration.

    3. Not this one. The windpipe only carries air. Respiration happens inside cells. Go to S3.3.5. Try another.

    See the answer

    In every cell of the body

    Every living cell uses oxygen to release energy from sugar. That is respiration.

    • If you chose A: The lungs are where gases swap. Respiration is what each cell does with the oxygen. Go to S3.3.5.
    • If you chose C: The windpipe only carries air. Respiration happens inside cells. Go to S3.3.5.

  5. How does smoke harm the lungs?

    Tap your answer.

    1. Not this one. A cough is only a sign. Smoke damages the tubes and the air sacs, and that harm can last. Go to S3.3.5. Try another.

    2. Yes, that is right. Smoke breaks down the walls between air sacs. There are fewer, bigger sacs with less surface, so less oxygen can pass into the blood.

    3. Not this one. Exercise makes muscles stronger. Smoke only harms the lungs, and the lungs have no muscles to train. Go to S3.3.5. Try another.

    See the answer

    It damages the tubes and breaks down air sac walls, so less oxygen can pass in

    Smoke breaks down the walls between air sacs. There are fewer, bigger sacs with less surface, so less oxygen can pass into the blood.

    • If you chose A: A cough is only a sign. Smoke damages the tubes and the air sacs, and that harm can last. Go to S3.3.5.
    • If you chose C: Exercise makes muscles stronger. Smoke only harms the lungs, and the lungs have no muscles to train. Go to S3.3.5.

  6. Smoke from a cooking fire indoors can harm the lungs. What helps?

    Tap your answer.

    1. Yes, that is right. A window, door or chimney lets smoke out, so you breathe less of it. Children are at the most risk.

    2. Not this one. Smoke still goes down the windpipe into the lungs, whichever way you breathe. Go to S3.3.5. Try another.

    3. Not this one. All smoke harms the lungs. Smoke from a cooking fire indoors does too. Go to S3.3.5. Try another.

    See the answer

    Let the smoke out, and keep small children away from it

    A window, door or chimney lets smoke out, so you breathe less of it. Children are at the most risk.

    • If you chose B: Smoke still goes down the windpipe into the lungs, whichever way you breathe. Go to S3.3.5.
    • If you chose C: All smoke harms the lungs. Smoke from a cooking fire indoors does too. Go to S3.3.5.

  7. From an earlier chapter

    When you breathe in, what does your diaphragm do?

    Tap your answer.

    1. Not this one. That is what it does when you breathe out. Go to S3.3.4. Try another.

    2. Yes, that is right. The diaphragm tightens, flattens and moves down. The chest gets bigger and air flows in.

    3. Not this one. The diaphragm moves each time you breathe. It is the main muscle for breathing. Go to S3.3.4. Try another.

    See the answer

    It moves down and flattens

    The diaphragm tightens, flattens and moves down. The chest gets bigger and air flows in.

    • If you chose A: That is what it does when you breathe out. Go to S3.3.4.
    • If you chose C: The diaphragm moves each time you breathe. It is the main muscle for breathing. Go to S3.3.4.

Keep it

Copy this into your exercise book.

Oxygen passes from the air sacs into the blood. Carbon dioxide passes out. Respiration happens in every cell. Smoke harms the lungs.

Draw: Draw one air sac with a blood tube beside it. Add an arrow for oxygen in and an arrow for carbon dioxide out.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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