What does an oxygen-haemoglobin dissociation curve show?

An oxygen-haemoglobin dissociation curve shows how saturated haemoglobin gets with oxygen at different partial pressures. The beginning of the curve starts off shallow, this is due to the tight quarternary structure of haemoglobin which makes it difficult for an oxygen molecule to bind to a binding site. However, once the first oxygen molecule has been bound the structure of the haemoglobin is slightly altered which subsequently makes it easier for two other oxygen molecules to bind. This explains the sudden sharp rise in the centre of the graph. The end of the curve is less steep as there is only one binding site left, so the final oxygen molecule can only bind to this one specific site making it more difficult.