An oxygen dissociation curve shows the percentage saturation of haemoglobin with oxygen (y-axis) against the partial pressure of oxygen, pO2, in kPa (x-axis). For adult haemoglobin the curve is sigmoid (S-shaped).
• Binding of the first O2 changes the shape (conformation) of the haemoglobin, uncovering the other haem groups. The second and third O2 bind much more easily, so a small rise in pO2 gives a large rise in saturation. The curve is steep.
• At high pO2, it is harder for the fourth O2 to bind because most sites are full, so the curve levels off near 100%.The shape is important. At the pO2 in the lungs (about 13 kPa), haemoglobin is about 98% saturated, and small changes in pO2 barely affect loading. The steep part of the curve covers the range of pO2 found in respiring tissues, so a small fall in pO2 causes a large release of oxygen exactly where it is needed.Worked example (reading the curve): at 13 kPa haemoglobin is 98% saturated; at 5 kPa it is about 75% saturated; at 2 kPa it is about 25% saturated. Moving from 5 kPa to 2 kPa (a fall of only 3 kPa) releases a further 50% of the maximum oxygen.
• Binding of the first O2 changes the shape (conformation) of the haemoglobin, uncovering the other haem groups. The second and third O2 bind much more easily, so a small rise in pO2 gives a large rise in saturation. The curve is steep.
• At high pO2, it is harder for the fourth O2 to bind because most sites are full, so the curve levels off near 100%.The shape is important. At the pO2 in the lungs (about 13 kPa), haemoglobin is about 98% saturated, and small changes in pO2 barely affect loading. The steep part of the curve covers the range of pO2 found in respiring tissues, so a small fall in pO2 causes a large release of oxygen exactly where it is needed.Worked example (reading the curve): at 13 kPa haemoglobin is 98% saturated; at 5 kPa it is about 75% saturated; at 2 kPa it is about 25% saturated. Moving from 5 kPa to 2 kPa (a fall of only 3 kPa) releases a further 50% of the maximum oxygen.