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Life Processes in Animals

Gases swap at the alveoli

Air reaches the alveoli, the small sacs of air in the lungs. Then two gases swap places.

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The idea

An alveolus has a very thin wall. Because the wall is thin, gases pass across it easily. Tiny blood tubes press against the wall.

Swap means each gas goes the other way.

One alveolus, with a ring of blood round it.
Step 1 of 4
  1. One alveolus, with a ring of blood round it.
  2. Oxygen passes from the air into the blood.
  3. Carbon dioxide passes from the blood into the air.
  4. The blood carries the oxygen to every part of the body.

Oxygen passes from the air into the blood. Carbon dioxide passes from the blood into the air.

About one fifth of the air we breathe in is oxygen. That is about 21 parts in every 100.

So breathed-out air has less oxygen than breathed-in air. Breathed-out air also has more carbon dioxide.

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This track

  1. 0 Start here
  2. 1 Digestion, absorption and egestion
  3. 2 One path, and a job at every stop
  4. 3 Saliva and starch
  5. 4 Ruminants and birds
  6. 5 Checkpoint: what a body does with food
  7. 6 Breathing is not respiration
  8. 7 Lungs, gills and moist skin
  9. 8 Getting food and oxygen around
  10. 9 What a body cannot use has to leave
  11. 10 The path of air
  12. 11 How the chest moves
  13. 12 Gases swap at the alveoli
  14. 13 Villi give food a way in
  15. 14 Checkpoint: the breathing path, the chest and villi
  16. 15 The big picture
  17. 16 Track quiz

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