1.9—Haemoglobin, oxygen dissociation and Bohr effect

Haemoglobin is a globular protein with four subunits, each able to bind oxygen reversibly. The percentage saturation curve is sigmoid because binding at one site affects the affinity of the remaining sites.

In the lungs, high oxygen partial pressure loads haemoglobin. In respiring tissues, lower oxygen pressure and higher carbon dioxide promote unloading, so oxygen is delivered where demand is greatest.

A working muscle produces more carbon dioxide and heat. The dissociation curve shifts right, meaning haemoglobin reaches a lower saturation at the same oxygen pressure and releases more oxygen.

The Bohr effect links carbon dioxide to oxygen delivery: carbon dioxide forms carbonic acid, lowering pH and reducing haemoglobin affinity. Foetal haemoglobin has a different affinity so oxygen can move from maternal blood to the foetus.

A right shift does not mean the blood contains no oxygen; it means affinity is lower at a given pressure. Read the curve and state the condition before inferring loading or unloading.