Why binding affinity of haemoglobin decreases as we move to high altitude?

Oxygen in inhaled air diffuses into the blood in lungs. Blood has a massive capacity to dissolve oxygen – much more oxygen can dissolve in blood than could dissolve in same amount of water. This is because blood contains haemoglobin – a specialized protein that binds to oxygen in lungs so that the oxygen can be transported to rest of the body.

Amount of haemoglobin in blood increases at high altitude. This is one of the best-known features of acclimatization (acclimation) to high altitude. Increasing the amount of haemoglobin in the blood increases 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 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, other subunits become more likely to bind oxygen. This feature of haemoglobin gives the haemoglobin saturation curve its characteristic S shape. The curve shifts a little in some conditions like on ascent to high altitude. This is important because a shift to right indicates that oxygen is bound less tightly, so that less is taken up in lungs but it is easily released in the tissues.