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As seen in the graph above, why do we say that increasing pH increases haemoglobins affinity for O2, but decreasing pH reduces its affinity? If the pH increases above or decreases below haemoglobins optimum pH level, wont haemoglobins affinity for oxygen decrease either way? Same goes for temperature. Why do we say that decreasing the temperature increases haemoglobins affinity for oxygen and vice versa? If the temperature goes above or below the optimum temperature of haemoglobin, wont its affinity for oxygen decrease anyway? So how does this make sense?

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    $\begingroup$ I think you have the wrong idea about what "optimum" means - what's hemoglobin's job? What would happen if it bound oxygen strongly and never let go? $\endgroup$
    – Bryan Krause ♦
    Commented Dec 27, 2022 at 15:21
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    $\begingroup$ Does this answer your question? If during exercise the affinity of oxygen for haemoglobin is decreased, what are the general consequences for tissues? $\endgroup$
    – David
    Commented Dec 28, 2022 at 16:25
  • $\begingroup$ Further to the comment from @BryanKrause, the idea that haemoglobin (Hb) has a pH optimum is nonsense. There is a normal tissue pH, which can act as a signal for lack of oxygen, to which Hb responds. I'd forget about temperature. Body temperature is normally maintained in a narrow range, but pH change is of the essence. The molecular nature of this response is dealt with in my answer to another question: biology.stackexchange.com/questions/55683/…. $\endgroup$
    – David
    Commented Dec 28, 2022 at 16:31

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I think you might be misunderstanding the term affinity, which is a measure of how strongly the two molecules (for example, haemoglobin and oxygen) bind to each other. High affinity means there is a strong bonding, and low affinity means the bonding is weak. Affinity can also be more formally defined via affinity (or binding) constant, $K_{\mathrm{a}}$.

Now take a look at you graph and choose some partial pressure of oxygen, say $P_\ce{O_2} = 30 \; \mathrm{mmHg}$. In the intermediate regime under physiological conditions (solid line), approximately $60\;\%$ of haemoglobin molecules have oxygen bound. However, when pH is increased (upper dotted line), the percentage of haemoglobin with oxygen raises to approximately $75\;\%$ – this is an indication of a stronger bonding (higher affinity). Conversely, when pH is decreased (lower dotted line), the percentage drops to only $35\;\%$ because the bonding is weaker (and we say the affinity is lower).

Binding

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