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During exercise more respiration occurs, and hence more oxygen is required.

So the oxygen dissocation curve is shifted to the right.

So at a given Oxygen partial pressure, the haemoglobin is less saturated with oxygen, i.e. haemoglobin has less affinity for oxygen.

What is confusing me is that the role of haemoglobin is to move oxygen around the body, so if it has a lower affinity for oxygen, won’t it compromise this function? Even though more oxygen is released, doesn't the haemoglobin have to carry it to all the tissues in the body? So how is this change in affinity of benefit if it reduces the oxygen subsequently available for other tissues?

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    $\begingroup$ Search this list for h(a)emoglobin. This sort of question has been asked before. And while you are about it, tell us what effort you made to answer this question yourself. It is well described in numerous places on the web. $\endgroup$
    – David
    Dec 4, 2017 at 23:19
  • $\begingroup$ I have done a google search but could only find information stating that when we need oxygen, the curve shifts to the right and Hb % Sat decreased. What I want to know (and could not find on the web or here) is that if the point of Hb is to move oxygen around the body, wouldn't a decreased Hb affinity for oxygen reduce the oxygen parts of our body get? So wouldn't a right shift in one way 1) make it easier for the body to absorb oxygen but in another way 2) stop Hb transporting oxygen to parts of the body that need oxygen. I.e. wouldn't these two things work against each other. Thanks. $\endgroup$
    – K-Feldspar
    Dec 4, 2017 at 23:24
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    $\begingroup$ I have tightened up your question, spelling haemoglobin in full (avoid abbreviations for clarity and indexing, especially in titles). I have also tried to answer your question. I think it is useful to have the curve in front of you, but I could have just said that it is not a zero-sum game, but even if it were, one can change the distribution of resources without starving those with lower priority. $\endgroup$
    – David
    Dec 6, 2017 at 0:00
  • $\begingroup$ The only part of the body with a major effect on O2 Sat/affinity is in the muscles, where the ph is decreased. The rest of the body will be relatively unaffected, and compensatory mechanisms kick in. $\endgroup$
    – anongoodnurse
    Dec 6, 2017 at 2:45

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It may help to consider the diagram below which shows the percentage saturation of haemoglobin with oxygen (and hence how much it takes up or releases) at the different partial pressures in different tissues. (The actual pattern — and values — depends, of course, on the blood flow to different tissues.)

enter image description here

In general, the curves show that at the high oxygen pressure of the lungs haemoglobin is near saturated with oxygen — Hb takes O2 up —, whereas at the lower oxygen pressure in the peripheral tissues the saturation is less — Hb releases some O2. So in normal circumstances oxygen will be gradually released to tissues as it circulates through the body. The oxygen pressure in the tissues is acting as the control valve for release.

In cases where there is a vastly increased demand for oxygen, this normal control is insufficient. The hydrogen ions in the acids produced by respiration (carbon dioxide and perhaps lactate) act as an additional control as they cause the oxygen dissociation curve of haemoglobin to change e.g. from 1 to 2 in the diagram. (The molecular mechanism of this is understood, but that is not the question.) Hence at the same oxygen pressure, more oxygen is released (cf. the percentage saturation at 1 and 2).

Does this harm tissues other than the ones producing the acid? They obviously have lower priority, but you can see there is still oxygen left at 2, and physiological mechanisms (faster heart beat etc.) will increase the blood flow in general. So it is wrong to think of this as ‘all or nothing’, but as part as a general response which alters the balance towards the tissues in greatest need.

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