Oxygen in inhaled air diffuses into the blood in the lungs. Blood has a massive capacity to dissolve oxygen – much more oxygen can dissolve in blood than could dissolve in the same amount of water. This is because blood contains haemoglobin – a specialised protein that binds to oxygen in the lungs so that the oxygen can be transported to the rest of the body.
The amount of haemoglobin in blood increases at high altitude. This is one of the best-known features of acclimatisation (acclimation) to high altitude. Increasing the amount of haemoglobin in the blood increases the amount of oxygen that can be carried. However, there is a downside: when there is too much haemoglobin, blood becomes sticky and viscous and it is harder for the heart to pump the blood around the body. This happens in chronic mountain sickness.
Haemoglobin consists of four subunits joined together. When an oxygen molecule binds to one subunit, the other subunits become more likely to bind oxygen. This feature of haemoglobin gives the haemoglobin saturation curve (the graph opposite) its characteristic S shape. Interestingly, the curve shifts a little in some conditions, such as on ascent to high altitude. This is important because a shift to the right indicates that oxygen is bound less tightly, so that less is taken up in the lungs, but it is more easily released in the tissues.
The following physiological variables decrease the affinity of haemoglobin for oxygen, so they cause the curve to shift to the right: H+, temperature, CO2, and a substance called 2,3 DPG. A decrease in any of these variables has the opposite effect - the curve shifts to the left.
For example, in the graph opposite, the green line shows a normal curve. The blue line shows the curve for a patient with very acidic blood (H+=80nM, pH 7.1). In contrast, the red line shows a patient who is hypothermic, with a body temperature of only 30ºC.
When you ascend to high altitude, the curve initially shifts to the right at moderate altitudes, under the influence of 2,3 DPG. At extreme altitude, it shifts to the left because there is much less CO2 in the blood (see acclimatisation to find out why). You can explore these changes in any combination using the interactive oxygen saturation graph.