Can you explain the oxygen dissociation curve.

What is the graph telling us?
On the graph, we can see two axes. We have partial pressure of oxygen on the X axis and Hb saturation on the Y axis. Partial pressure is another way of saying how much oxygen there is in the blood - the more oxygen, the higher the partial pressure. Hb saturation tells us out of all the millions of Hb molecules we have in our blood, how many of those are bound to oxygen. Therefore, this is a graph showing us the relationship between the amount of oxygen in someone's blood and how that can affect the proportion of Hb that is bound to an oxygen molecule.
Why is the graph shaped like it is?
The amount of oxygen bound to Hb is determined by the partial pressure. At different partial pressures of oxygen, Hb does not bind evenly to oxygen. This is due to the structure of Hb. Hb has a quaternary structure. It has 4 polypeptide chains, each of which have a haem group (iron Fe2+) - hence the name. This hame group is what the oxygen binds to. It is hard for that first molecule to bind to one of the polypeptide chains due to its structure, however, when that molecule of oxygen binds it causes a change in the quaternary structure. This makes it easier for the next oxygen molecule to join and so forth. This is known as positive co-operative binding (KEY WORD). This creates the initial steep gradient until around 8kPa. The gradient begins to level off here. This represents the 4th polypeptide chain - due to probability, it is less likely that an oxygen molecule will bind to that molecule of Hb now (it used to have 4 places to bind, now it only has one). However, you can see that even at 8kPa there is a high saturation of Hb. This is an evolutionary advantage because if you were breathing less than you should, say in a mild respiratory disease, you can still maintain a high saturation of oxygen.
When this is understood - the next question may be, what moves the curve left and right.