Oxygen-haemoglobin dissociation curve

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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. It is usually a sigmoid plot. Haemoglobin molecules can bind up to four oxygen molecules in a reversible way. Many factors influence the affinity of this binding and alter the shape of the curve including:

pH
the concentration of 2,3-Diphosphoglycerate
the type of heamoglobin molecules (adult vs fetal types)
the presence of poisons especially carbon monoxide

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.

Factors altering the curve

Left shift of the curve is a sign of haemoglobins increased affinity for oxygen (eg. at the lungs). Similaryly, right shift shows decreased affinity, as would appear on an increase in body temperature, hydrogen ion, 2,3-bisphosphoglycerate or carbon dioxide concentration (the Bohr efect)

The 'plateau' portion of the oxyhemoglobin 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 oxyhemoglobin 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).

Under normal resting conditions in a healthy individual, the normal position of the curve is at a pH of 7.4. A shift in the position of the curve with a change in pH is called the Bohr effect.

The curve shifts to the right (representing a decrease in affinity for O₂) - in acute acidosis, increase in p(CO₂), increase in temperature, and increase in 2,3-diphosphoglycerate (DPG). The curve shifts to the left (representing an increase in affinity for O₂) - in acute alkalosis, decrease in p(CO₂), decrease in temperature, and decrease in 2,3-diphosphoglycerate (DPG).

The oxygen dissociation curve for fetal hemoglobin is to the left of that of adult hemoglobin, in order to facilitate diffusion of oxygen across the placenta. The oxygen dissociation curve for myoglobin exists even further to the left.

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

[Example of an Oxygen-haemoglobin dissociation curve]

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