Week 3 · Chapter 2 Respiration: 2-1 How Breathing Works; 2-2 How Lungs Work · 3 lessons
2
Inside the lungs: where gases swap places
Science P6: Term 1, Chapter 2, pp. 18–23
GoalTrace the path of air from the nose to the air sacs and explain gas exchange.
New words
windpipe · the tube that carries air from the throat to the lungslungs · the two organs that take oxygen from the air into the bloodair sacs (alveoli) · tiny bags in the lungs where gases pass into and out of the bloodgas exchange · oxygen going into the blood and carbon dioxide leaving it
Explanation
Air enters through the nose or mouth and goes down the windpipe. The windpipe splits into two smaller tubes, one for each lung. Inside each lung the tubes divide again and again, and each ends in tiny air sacs. The air sacs are wrapped in very thin blood vessels. Oxygen passes from the air in the sacs into the blood, and carbon dioxide passes from the blood into the air in the sacs. This swap is called gas exchange. The blood then carries oxygen to every part of the body, where it is used. The carbon dioxide in the sacs leaves when we breathe out. Boxes and arrows are the best way to show the path, because real organs look very different from drawings.
nose or mouth→windpipe→two tubes to the lungs→air sacs next to blood vessels
Worked examples
nose→windpipe→left lung→air sac
Follow one breath: the air goes in through the nose, down the windpipe, along a tube into the left lung and into an air sac. There, oxygen enters the blood.
In the air sacs
Goes into the blood
Leaves the blood
Gas
oxygen
carbon dioxide
At the air sacs the blood takes oxygen and gives away carbon dioxide. This is why blood leaving the lungs has more oxygen.
Farida runs in the playground. Her muscles use more oxygen, so her heart beats faster to carry more blood to the lungs and her breathing is quicker.
Class activity
Path cards. Write each part on a card: nose, windpipe, left lung, right lung, air sac, blood vessel. Put them in the order air meets them. Tell a partner what happens at the air sacs. No breath-holding contests.
Practice
1
Which tube carries air from the throat to the lungs?
The windpipe.
2
Where does oxygen enter the blood?
In the air sacs of the lungs.
3
Which gas leaves the blood at the air sacs?
Carbon dioxide.
4
Put in order: air sac, nose, windpipe, lung. Say the letters.
