Both myoglobin and haemoglobin use an iron-haem prosthetic group to bind oxygen.

When haemoglobin has two oxygen molecules bound, it effectively has a lower affinity for oxygen than when it has no oxygen molecules bound.

The ability of haemoglobin to efficiently transport oxygen from the lungs to the tissues is due to its structural transformation from a monomeric to a tetrameric state in reposnse to oxygen binding.

Haemoglobin can alter its oxygen affinity due to its quaternary structure and its ability to change between two different conformational states.

At the partial pressure of oxygen typical in the lung, haemoglobin will effectively have low affinity for oxygen.