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 which makes diffusion quicker.
	lots of blood capillaries to carry the gasses.
	the blood capillaries 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.