الأسبوع 3 · الفصل 2 «التنفس»: 2-1 كيف يعمل التنفس، 2-2 كيف تعمل الرئتان · 3 دروس
1
What changes when we breathe
Science P6: Term 1, Chapter 2, pp. 18–23
GoalCompare the air we breathe in with the air we breathe out.
New words
breathe in · to take air into the lungs (inhale)breathe out · to push air out of the lungs (exhale)water vapour · water as an invisible gas
Explanation
Every few seconds you breathe in and breathe out. The air going in and the air coming out are not the same. Breathed-in air has more oxygen and very little carbon dioxide. Breathed-out air has less oxygen and more carbon dioxide, because the body used some oxygen and made carbon dioxide. But be careful: we do not breathe out only carbon dioxide. Breathed-out air still has plenty of oxygen, which is why rescue breathing can help someone who has stopped breathing. Breathed-out air also holds water vapour. You can see this when you breathe onto a cold mirror or glass: a mist of tiny drops appears. Scientists compare the two airs by collecting them in plastic bags and using limewater or gas detector tubes.
Worked examples
Gas
Breathed in (about, out of 100)
Breathed out (about, out of 100)
oxygen
21
16
carbon dioxide
less than 1
4
The exact amounts are about 21 out of 100 for oxygen in breathed-in air and about 16 out of 100 in breathed-out air. So the body took about 5 out of every 100 parts of oxygen.
Omar breathes onto a cold glass window in winter and sees a mist. The mist is tiny drops of water that came from his breath as water vapour.
Mina counts her breaths for one minute while sitting: 16 breaths. After she steps up and down gently, she counts 24 in the next minute. Moving makes the body need more oxygen, so we breathe faster.
Class activity
Cold glass test. With a grown-up, hold a cold glass or mirror near your mouth and breathe out gently once. What do you see? Wipe it clear. Now count your breaths for one minute while sitting. Do not hold your breath and do not blow into any drink.
Practice
1
Which air has more carbon dioxide: breathed-in or breathed-out air?
Breathed-out air.
2
Does breathed-out air still contain oxygen?
Yes, plenty of oxygen is still in it.
3
What is the mist on a cold mirror when you breathe out?
Tiny drops of water from water vapour in your breath.
4
Look at the table. How much more oxygen is in the air breathed in?
