What is the oxygen-haemoglobin dissociation curve?

The oxygen-haemoglobin dissociation curve is a graph that compares the % of haemoglobin saturated with oxygen on the y-axis to the partial pressure of oxygen in the blood on the x-axis (mmHg). It is a useful tool for understanding how oxygen is transported to the tissues of the body from the lungs.

In areas where the partial pressure of oxygen in the blood is high (i.e. in the capillaries of the lungs), almost all of the haemoglobin will become saturated with oxygen. In areas where the partial pressure of oxygen is low (i.e. in the capillaries supplying the tissues of the body), the haemoglobin will release it’s oxygen and thereby supply the cells of the tissue with oxygen for respiration.

Haemoglobin’s affinity for oxygen (i.e. the strength with which oxygen binds to haemoglobin) and hence the position of the curve is affected by physiological factors.

If the curve shifts to the right it means that the haemoglobin has decreased affinity for oxygen. In general, factors that decrease haemoglobin’s oxygen affinity are those physiological states where body tissues require more oxygen e.g. during exercise. While exercising, temperature, carbon dioxide levels and lactic acid levels increase and all these changes cause right shift.

Left shift occurs under opposite conditions to right shift, and is where the haemoglobin has increased affinity for oxygen and won’t release it as readily.

Here is a table showing the effects on the curve caused by changes in several physiological factors:

Factor	Increase	Decrease
Temperature	Right shift	Left shift
PaCO2	Right shift	Left shift
pH	Left shift	Right shift