The purpose of the lungs is two-fold:
to transfer oxygen to the blood and to remove carbon dioxide from the blood.
The tiny air sacs give the lungs a large surface area which makes this transfer of gases more efficient.

 images  Air sac

Diagram showing the structure of the lung
The lungs consist of a network of thin tubes that each end up in a tiny air sac that is surrounded by blood capillaries

The capillaries and air sac both have very thin walls only one cell thick so gases can easily pass backwards and forwards by a process called diffusion.

lungs2

When you breath in:

  • Strong muscles pull the diaphragm down and the rib cage expands.
  • This makes air enter the lungs through the mouthWhen you breath out:The diaphragm and rib muscles relax and air is pushed out of the lungs

breathing_inbreathing_out

Animation showing the breathing process

Table showing a comparison between the composition of inhaled with exhaled air.

Gas Air entering lungs – Inhaled air Air leaving lungs – Exhaled air
Oxygen 21% 16%
carbon dioxide 0.03% 3%
Nitrogen 78% 78%
water vapour small amount significantly moist

Notice the following facts
Exhaled air has less oxygen
Exhaled air has more carbon dioxide
Exhaled air has more water vapour
Nitrogen is not needed by our body so stays the same

Useful Links:
The Medical Learning Library: The human respiratory system
(This is written for older students but is still intereresting)