So I understand why the initial portion is flat - The Hb molecule is in the Tense (T) state and has a low affinity for O2. It becomes much steeper as O2 begins to bind Hb because the initial binding of O2 causes a shift to the Relaxed (R) state, which increases the affinity of Hb for O2. What I don't undertand is when you hit about 60 mmHg for PO2 the curve flattens out even before o2 saturation reaches 100%. Isn't O2 binding suppossed to keep getting "easier" until the Hb is completely saturated? Why does it take larger increases in PAO2 to cause slight increases in O2 saturation at the right part of the curve?
Conufsed about the shape of the OxyHb curve...
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from my understanding you have to look at it in a relative perspective. Any curve is supposed to flatten out gradually before it hits saturation. The reason that there is such a large increase in affinity towards the lower end is because at lower pO2 there is a larger of proportion of hb molecules that have empty binding sites. Once you hit a certain threshold of pO2, you have a greater number of hb molecules that have bound sites, therefore creating a greater number of relaxed state of molecules.
The key thing to remember is that even though it gets easier to stick an O2 molecule onto the second, third, and fourth site in the hb molecules, the equilibrium is 2-part: tensed and relaxed, as you mentioned. Therefore, it doesn't really matter that you have 3 sites occupied versus 2. What really matters is that once you have the relaxed state, once 1 or 2 Oxygen molecules bind, that creates higher affinity. The increase in affinity beyond that is not as significant, which explains the smaller slope as you are hitting saturation conditions.
Looking at it in a relative perspective is key.
Sorry for such a long-winded response.
The key thing to remember is that even though it gets easier to stick an O2 molecule onto the second, third, and fourth site in the hb molecules, the equilibrium is 2-part: tensed and relaxed, as you mentioned. Therefore, it doesn't really matter that you have 3 sites occupied versus 2. What really matters is that once you have the relaxed state, once 1 or 2 Oxygen molecules bind, that creates higher affinity. The increase in affinity beyond that is not as significant, which explains the smaller slope as you are hitting saturation conditions.
Looking at it in a relative perspective is key.
Sorry for such a long-winded response.
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Well, when it's flat that's because it's already fully saturated. The O2 level (pressure) in the blood tends to be a lot higher than the minimal pressure needed to fully saturate your Hb....and it never gets fully unsaturated even when passing through tissues. 60 mm sounds bit low for full saturation but ~80 seems about right...
Clinical aside- even though it's easy to saturate Hb, the saturation drops quickly with respiratory problems that allow CO2 to build up in the blood. Imagine a guy in distress with 70 mm O2 and 60 mm Co2. His saturation would be fine and dandy but the high Co2 shifts the curve down to dangerous levels. That's why evolution gave us that confusing and seemingly useless buffer zone on the Hb curve.
Clinical aside- even though it's easy to saturate Hb, the saturation drops quickly with respiratory problems that allow CO2 to build up in the blood. Imagine a guy in distress with 70 mm O2 and 60 mm Co2. His saturation would be fine and dandy but the high Co2 shifts the curve down to dangerous levels. That's why evolution gave us that confusing and seemingly useless buffer zone on the Hb curve.
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