Exchange of gases in the Lungs


Breathing is a gas exchange mechanism.  Your Lungs help to do just that!  They contain millions of  tiny air pockets called ALVEOLI (air sacs)  which are adapted to maximise the diffusion of carbon dioxide and oxygen (see diagram below).  Alveoli are good at gas exchange because they have:


  • a large surface area.  If all the air sacs were flattened out, they would cover an area of 100m2.
  • moist surface which speed up diffusion.
  • very thin walls (just one cell thick) which speeds up diffusion.
  • lots of blood capillaries to carry the gasses.
  • many blood capillaries which help the diffusion O2 and CO2 as their walls are only one cell thick.



In the alveolus (single air sac) oxygen passes from a high concentration (loads of oxygen) through the thin wall into the blood capillary where there is a low concentration (very little oxygen) .  This is called diffusion, because oxygen is moving from high concentration to low concentration.  When blood returns to lungs it has high concentration of carbon dioxide, so CO2 diffuses out of the blood into the alveolus.

Lung Alveoli

This image is a diagram illustrating the process of gas exchange in the human lungs, specifically in an alveolus. It shows how oxygen (O2) is taken into the blood and carbon dioxide (CO2) is expelled.

Note: Deoxygenated blood appears blue in this diagram for explanation purposes only.

Inhaled & exhaled air


Atmospheric air

Exhaled air


Oxygen, O2




Carbon dioxide, CO2




Nitrogen, N2









When compared to atmospheric air, exhaled air contains:

  • More CO2 (Carbon dioxide)
  • Less O2 (Oxygen)
  • Slightly more Nitrogen


Tags:Diffusion, Lung diffusion, Exchange of gases, Alveoli, Lung cells



© 2012 science-resources.co.uk. All rights reserved | Design by W3layouts