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Why does the affinity of haemoglobin for oxygen increase if an organism lives at a higher elevation where the oxygen pressure is lower?

Wouldn’t its affinity for oxygen decrease because the acidity of the body would increase with the lack of oxygen? The textbook said otherwise though and I don't understand.

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  • $\begingroup$ There is a subsequent related question with answers here. $\endgroup$ – David Jan 26 '18 at 22:39
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    $\begingroup$ Possible duplicate of Why does the affinity of haemoglobin's for oxygen decrease at high altitudes? $\endgroup$ – tchrist Jan 27 '18 at 6:15
  • $\begingroup$ @tchrist — The problem is that this is the earlier questio, and has an answer with votes. Could it be merged with the later one? I only flagged the link to the later question because I am trying to tidy up the questions on oxygen binding curves (there was another recently) so that anyone asking about this or searching is shown links to them all. (I had to add the haemoglobin tag to some of them.) It seems to me there are three types of question so far — Bohr effect, high altitude/2,3-BPG, and the interaction of the two. (No question about foetal haemoglobin yet.) $\endgroup$ – David Jan 27 '18 at 12:37
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The question seems to be based on the idea that there would be physiological adaptation to low oxygen, possibly through the Bohr effect. In fact the usual mechanism of adaptation is through genetic change.

There are a lot of examples of animals which live (or fly) at high altitude and which have a haemoglobin with a higher oxygen affinity. Examples which spring to mind are the bar-headed goose and the camelids of South America (llamas, alpacas etc.)

Haemoglobin has evolved as an efficient oxygen-delivery protein by having a variable affinity for oxygen: low affinity at low oxygen concentration, high affinity at high oxygen concentration. This is achieved through co-operative binding (allostery), and it ensures that the protein loads up with oxygen in the pulmonary circulation but releases oxygen in the peripheral tissues.

At high altitude, where the partial pressure of oxygen is lower, the haemoglobin of humans, for example, will not be fully saturated, impairing the delivery of oxygen to the body. Haemoglobin variants with an increase in the overall affinity for oxygen can achieve full loading at the expense of slightly less release in the tissues - overall a better oxygen delivery function. This is what happens in the camelids, but there is no such adaptation in humans living at high altitudes.

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    $\begingroup$ Instead humans have more red blood cells when living in higher altitudes. This is why athletes train at high altitudes. $\endgroup$ – Chris May 11 '14 at 18:18
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    $\begingroup$ @Chris, in fact this isn't the case. It is true that if a sea-level dweller goes to high altitude the red blood cell count goes up (ertythrocytosis), and this is exploited by athletes. However this increases the blood's viscosity and can lead to unwanted complications. Tibetans have an adaptation which prevents this increase taking place in the first place. $\endgroup$ – Alan Boyd May 11 '14 at 18:23
  • $\begingroup$ Interesting. There are limits for a maximum hematocrit for athletes, if they go over it (this is mostly due to EPO doping) then they are not allowed to participate. $\endgroup$ – Chris May 11 '14 at 18:28

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