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Haemoglobins are protein molecules with a quaternary structure that has
evolved to make it efficient at loading oxygen under one set of conditions
but unloading it in another
...

To be efficient, haemoglobin must:

Their structure:
Primary Structure: sequence of amino acids in the four polypeptide
chains

Quaternary Structure: all four polypeptides are linked together to form
an almost spherical shape
...
Its this ion that can combine
with a single oxygen molecule, making a total of four O2 molecules that
can be carried by a single haemoglobin molecule in humans
...
If they have a low affinity for
oxygen they find it more difficult to take up oxygen than release it
...


Region of body

O2 concentration CO2 concentration Affinity of haemo- Result
globin for oxygen

Gas Exchange Surface

High

Low

High

Oxygen is associated

Respiring Tissues

Low

High

Low

Oxygen is dissociated

Haemoglobin

Haemoglobin binds with oxygen through a process called loading
(taking place in the lungs) and when haemoglobin releases its oxygen
this is unloading (taking place in tissues)
...


Haemoglobin changes its affinity (chemical attraction) for oxygen under different conditions
...
In the presence of CO2, the shape of the haemoglobin molecule binds more
loosely to oxygen, making it release it
...


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Each species produces a haemoglobin with a slightly different amino acid sequence
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