In the normal person, 97% of oxygen is transported to the tissues by haemoglobin and only 3% is transported in solution.
Haemoglobin is composed of four globin subunits. Binding of oxygen to these subunits is co-operative resulting in the characteristic sigmoidal oxygen dissociation curve. The shape of the curve permits haemoglobin to be highly saturated with oxygen when leaving the lungs, usually more than 97% saturated, and yet be able to rapidly unload large amounts of oxygen at the lower oxygen concentrations found in the tissues (around 20-40 mmHg).
One hundred millilitres of blood can carry about 20 ml of oxygen when the haemoglobin is 100% saturated. At rest only 25% of the transported oxygen is utilised, however during strenuous exercise up to 85% may be used.
The affinity of haemoglobin for oxygen may altered according to the globin subunits which make up the haemoglobin molecule and according to the chemical environment. An increased oxygen affinity shift the oxygen dissociation curve to the left while a decreased affinity shifts the oxygen dissociation curve to the right.
Factors which decrease the oxygen affinity include:
The expression of foetal haemoglobin results in an increased oxygen affinity.
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