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enter image description here

I find this graph incredibly confusing. Is it saying that when there is a decreased affinity (of hemoglobin to oxygen, I think?), the partial pressure of oxygen increases with the same percent saturation of hemoglobin? What does that even mean if that's the case?

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A graph is generally meant to be interpreted as y being a response to a change in x. This graph highlights that as the partial pressure of oxygen $P_{O2}$ increases, so does the percentage of hemoglobin that are saturated in oxygen. The relationship is not linear (unlike you seem to suggest) but is logistic.

The graph also highlights how three different level of affinity would affect the relationship between $P_{O2}$ and the fraction of hemoglobin that is saturated and highlights side consequences of such affinity variation.

Finally, with vertical dotted lines, the graph indicates what the $P_{O2}$ is in the lungs and in the other tissues.

why a decreased affinity is a shift to the right and an increased affinity is a shift to the left?

Consider a single point in the x axis such as $P_{O2} = 40$ for example. If affinity is increased you would expect that the percent saturation of Hb would increase, right? This causes a shift of the left.

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  • $\begingroup$ That all makes sense, I guess I should've been more specific. The only thing I'm really confused about is why a decreased affinity is a shift to the right and an increased affinity is a shift to the left? $\endgroup$
    – Patrick
    Nov 4 '18 at 23:45
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    $\begingroup$ @AliceD X and Y are rarely arbitrary choices. In no way do I mean that correlation implies causation but a person makes a graph as to imply a causality of X on Y. Also, when statistical analysis is involved as in any linear model (incl. GLM, ANOVA, ...) one of the variable is considered as a response to the other variable variation. $\endgroup$
    – Remi.b
    Nov 4 '18 at 23:55
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    $\begingroup$ @AliceD Of course, thinking everything as causation can be very misleading but this is still what is portrait through graphic representation. Google what goes in x axis what goes in y axis for more info. Note that following your comment, I corrected my first sentence from "always" to "generally", which is much more correct. Thanks! $\endgroup$
    – Remi.b
    Nov 4 '18 at 23:55
  • $\begingroup$ @PatrickSalas Pease, see edit in my answer $\endgroup$
    – Remi.b
    Nov 4 '18 at 23:58
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    $\begingroup$ At $P_{O2} = 40$, the "baseline" affinity curve indicate a Y value of about 75. The increased affinity curve (the left shifted one) indicates a Y value of about 95. So, yes shifting a curve toward the left here lead to matching a greater perceneage of Hb saturated for a given $P_{O2}$ which is what you would expect from a case where affinity is increased. Btw, your issue has nothing to do with biology. It is about basic of mathematics and how to interpret a graph. $\endgroup$
    – Remi.b
    Nov 5 '18 at 3:11
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You're right- there is a lot going on in this graph and it is hard to understand it at a glance.

Let's take the lines one at a time. The middle line is the curve that represents the relationship between PO2 dissolved in blood and percent hemoglobin saturation under physiological (essentially, healthy) conditions. We can see that percent Hb saturation is very low when the PO2 is between 0 and 20 mmHg, that Hb saturation sharply increases between approximately 20mmHg and 50mmHg, and that Hb saturation reaches near-maximal levels after the PO2 reaches approximately 50 or 60mmHg. From just looking at the middle line, we can learn that coming out of the lung, Hb will be about 100% saturated, while in tissue, at a much lower PO2, the Hb saturation is still around 75%.

Now, let's look at the dashed line, to the left of the middle line. This line represents the shift in hemoglobin saturation that occurs when PCO2 drops, when pH raises (blood becomes more basic, which could also occur as a result of inappropriately low PCO2), or when temperature decreases. This results in an increased affinity of Hb for O2, so Hb saturation stays much higher at a much lower PO2, when compared to the middle line. Therefore, at a PO2 that is very close to what tissue might see, Hb is still very close to maximally saturated. This becomes a problem for O2 delivery to tissue, since O2 is less likely to leave Hb to oxygenate the surrounding tissue.

Last, let's look at the rightmost line. This line represents the converse of the dashed line- here, PCO2 has increased, pH has decreased (blood is now more acidic), or temperature has increased. This causes a decrease in Hb affinity for O2, meaning that even at a high PO2, Hb will, on average, be carrying less oxygen. So, when blood leaves the lung, Hb might be almost maximally saturated, but by the time PO2 reaches tissue levels, Hb is actually closer to minimal saturation under these conditions. This causes issues with O2 delivery since there is just less O2 to be delivered once the PO2 reaches the level it might be in tissue.

Hope this helps- LMK if more questions.

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