Haemoglobins are molecules with a structure - they are made of polypeptide chains. Each is associated with a group containing a Fe2+ . Each Fe2+ can combine with a single molecule, making a total of oxygen molecules carried.
In the , haemoglobin binds with oxygen, this is called loading or .
In the , haemoglobin releases oxygen, this is calld unloading or .
An oxygen dissociation shows the relationship between the of haemoglobin with oxygen and the pressure of oxygen.
A curve which is further to the left shows that the haemoglobin has a high for oxygen, it will take it up easily but release it easily. An example of this is the haemoglobin of the , an animal which lives under sand on the seashore. There is a concentration of oxygen available in the environment of the lugworm. Llamas, which live at high , also have a dissociation curve that is shifted to the of human haemoglobin.
A curve which is further to the right shows that the haemoglobin has a low for oxygen, it will take it up easily but release it easily. Animals which use a lot of energy, such as fast swimming or birds in flight, will have a curve to the of human haemoglobin. A mouse will also have a curve to the , as it has a large to volume ratio, and therefore a high requirement to maintain their body .
Haemoglobin has a affinity for oxygen in the presence of , this is called the effect. This is because dissolved carbon dioxide is , it the pH and causes haemoglobin to change shape. This means that oxygen is more easily from haemoglobin in respiring tissues such as cells.