The Oxygen Dissociation Curve
- The oxygen dissociation curve shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO2)
- Partial pressure of oxygen refers to the pressure exerted by oxygen within a mixture of gases; it is a measure of oxygen concentration
- Haemoglobin is referred to as being saturated when all of its oxygen binding sites are taken up with oxygen; so when it contains four oxygen molecules
- The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen
- When haemoglobin has a high affinity it binds easily and dissociates slowly
- When haemoglobin has a low affinity for oxygen it binds slowly and dissociates easily
- In other liquids, such as water, we would expect oxygen to becomes associated with water, or to dissolve, at a constant rate, providing a straight line on a graph, but with haemoglobin oxygen binds at different rates as the pO2 changes; hence the resulting curve
- It can be said that haemoglobin's affinity for oxygen changes at different partial pressures of oxygen
Oxygen dissociation curve diagram
The oxygen dissociation curve is a sigmoidal shape
Interpreting the curve
- When the curve is read from left to right, it provides information about the rate at which haemoglobin binds to oxygen at different partial pressures of oxygen
- At low pO2, in the bottom left corner of the graph, oxygen binds slowly to haemoglobin; this means that haemoglobin cannot pick up oxygen and become saturated as blood passes through the body's oxygen-depleted tissues
- Haemoglobin has a low affinity for oxygen at low pO2, so saturation percentage is low
- At medium pO2, in the central region of the graph, oxygen binds more easily to haemoglobin and saturation increases quickly; at this point on the graph a small increase in pO2 causes a large increase in haemoglobin saturation
- At high pO2, in the top right corner of the graph, oxygen binds easily to haemoglobin; this means that haemoglobin can pick up oxygen and become saturated as blood passes through the lungs
- Haemoglobin has a high affinity for oxygen at high pO2, so saturation percentage is high
- Note that at this point on the graph increasing the pO2 by a large amount only has a small effect on the percentage saturation of haemoglobin; this is because most oxygen binding sites on haemoglobin are already occupied
- At low pO2, in the bottom left corner of the graph, oxygen binds slowly to haemoglobin; this means that haemoglobin cannot pick up oxygen and become saturated as blood passes through the body's oxygen-depleted tissues
- When read from right to left, the curve provides information about the rate at which haemoglobin dissociates with oxygen at different partial pressures of oxygen
- In the lungs, where pO2 is high, there is very little dissociation of oxygen from haemoglobin
- At medium pO2, oxygen dissociates readily from haemoglobin, as shown by the steep region of the curve; this region corresponds with the partial pressures of oxygen present in the respiring tissues of the body, so ready release of oxygen is important for cellular respiration
- At this point on the graph a small decrease in pO2 causes a large decrease in percentage saturation of haemoglobin, leading to easy release of plenty of oxygen to the cells
- At low pO2 dissociation slows again; there are few oxygen molecules left on the binding sites, and the release of the final oxygen molecule becomes more difficult, in a similar way to the slow binding of the first oxygen molecule