The shape of the haemoglobin dissociation curve can be explained by the behaviour of a haemoglobin molecule as it combines with or loses oxygen molecules.

When an oxygen molecule combines with one haem group, the whole haemoglobin molecule is slightly distorted. The distortion makes it easier for a second oxygen molecule to combine with a second haem group. This in turn makes it easier for a third oxygen molecule to combine with a third haem group. It is then still easier for a fourth and final oxygen molecule to combine.

Up to an oxygen partial pressure of around 2kPa, on average only one oxygen molecule is combined with each haemoglobin molecule. Once this oxygen molecule is combined, however, it becomes succesively easier for the second and third oxygen molecules to combine, so the curve rises very steeply.

Over this part of the curve, a small change in the partial pressure of oxygen causes a very large change in the amount of oxygen which is carried by the haemoglobin.

	Created by Aarushi Pandit about 9 years ago

A revision slide set on "Transport in Mammals"