The oxyhaemoglobin dissociation curve

 

This is a rather rough and ready picture of the oxyhaemoglobin dissociation curve, but hopefully it shows the key points Ok. 

The y-axis is oxygen saturation of the haemoglobin, and the x-axis is partial pressure of oxygen around the haemoglobin.

It starts off from the origin with a gentle upward curve . At this point the haemoglobin is in the tense configuration and not binding oxygen as well as when it's in relaxed.

Then the curve gets steeper. As more oxygen binds to the haemoglobin it starts to move towards the relaxed state, and so binds oxygen more easily. This means that only a small increase in PO₂ is needed for the haemoglobin to bind lots more oxygen - i.e. for the oxygen saturation to increase quite a lot.

Finally it gets flatter as almost all the haemoglobin binding sites have oxygen bound to them (it will obviously never get above 100%)