Oxygen-haemoglobin dissociation curve

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The O₂-Hb dissociation curve is a sigmoidal curve that represents the relationship between O₂ concentration and the percentage saturation of Hb. As the concentration increases from about 90% there is a significant plateau in the curve, which has several important biological repercussions.

The oxygen-haemoglobin dissociation curve plots the proportion of haemoglobin in its saturated form on the vertical axis against the prevailing oxygen tension on the horizontal axis.

[edit] Sigmoidal shape

It is usually a sigmoid plot. Haemoglobin molecules can bind up to four oxygen molecules in a reversible way.

The shape of the curve results from the interaction of bound oxygen molecules with incoming molecules. The binding of the first molecule is difficult. However, this facilitates the binding of the second and third molecules, and it is only when the fourth molecule is to be bound that the difficulty increases, partly as a result of crowding of the haemoglobin molecule, partly as a natural tendency of oxygen to dissociate.

The 'plateau' portion of the oxyhaemoglobin dissociation curve is the range that exists at the pulmonary capillaries (minimal reduction of oxygen transported until the p(O₂) falls below 60 mmHg).

The 'steep' portion of the oxyhaemoglobin dissociation curve is the range that exists at the systemic capillaries (a small drop in systemic capillary p(O₂) can result in the release of large amounts of oxygen for the metabolically active cells).

[edit] Factors shifting curve

Many factors influence the affinity of this binding and alter the shape of the curve:

	right shift	left shift
temperature	high	low
DPG	high	low
p(CO₂)	high	low
p(CO)	low	high
pH (Bohr effect)	low (acidosis)	high (alkalosis)
type of hemoglobin	adult hemoglobin	fetal hemoglobin

Left shift of the curve is a sign of haemoglobin's increased affinity for oxygen (eg. at the lungs). Similarly, right shift shows decreased affinity, as would appear with an increase in body temperature, hydrogen ion, 2,3-diphosphoglycerate or carbon dioxide concentration (the Bohr effect)

Carbon monoxide has a much higher affinity for haemoglobin than oxygen does. In carbon monoxide poisoning, oxygen cannot be transported and released to body tissues thus resulting in hypoxia.

With fetal hemoglobin, the shift facilitates diffusion of oxygen across the placenta. The oxygen dissociation curve for myoglobin exists even further to the left.