1Diffusion
Gas particles move about at random, so they spread from where there are lots of them (a high concentration) to where there are fewer. This is diffusion.
diffusionconcentration
Lesson 7.3 notes
Gas exchange in the alveoli
Watch oxygen diffuse into the blood and carbon dioxide diffuse out, compare the air you breathe in and out, and test it with limewater.
2Across the alveolus wall
In the alveoli, oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air. Red blood cells carry the oxygen away.
gas exchange
3Keeping it going
Breathing keeps bringing fresh air, and flowing blood keeps carrying oxygen away. So there's always more oxygen in the alveoli than in the blood, and oxygen keeps diffusing in.
blood flow
4Air in and air out
Your body takes some oxygen out of the air and adds carbon dioxide and water vapour. Nitrogen isn't used, so the amount stays the same.
nitrogen
5Testing for carbon dioxide
Limewater is clear, but it turns milky (cloudy) when carbon dioxide bubbles through it. This is the test for carbon dioxide.
limewatermilky
6Comparing air in and out
Breathed-in air bubbles through tube A and breathed-out air through tube B. Tube B turns milky first, so breathed-out air has more carbon dioxide in it.
carbon dioxide
7Where the gases go
Blood carries oxygen from the lungs to every cell, where respiration uses it. The carbon dioxide that respiration makes is carried back to the lungs and breathed out.
respiration
Explore: Gas exchange lab
Change the breathing, the blood flow and the alveolus wall, and see how fast oxygen gets into the blood.
Common mistake: You don't breathe out only carbon dioxide
Breathed-out air is still mostly nitrogen. It has less oxygen and more carbon dioxide than the air you breathed in, but still about 16% oxygen: that's why rescue breaths work.
Remember
- Oxygen diffuses from the alveoli into the blood; carbon dioxide diffuses from the blood into the alveoli.
- Breathed-out air has less oxygen, more carbon dioxide and more water vapour. Nitrogen stays the same.
- Limewater turns milky with carbon dioxide. The extra carbon dioxide comes from respiration in your cells.
Going further at GCSE