To help the binding (=saturation) and dissociation of oxygen, the hemoglobin molecule behaves in a very nice and helpful way!
The binding of the first molecule to an empty hemoglobin is a bit difficult. As shown in the graph below at very low pressure (‘a’) it is actually slightly more difficult to bind the first oxygen molecules (the blue line is below the green line).
However, once the first oxygen is bound, then it becomes easier for hemoglobin to bind the second oxygen molecule, and even easier the third and the easiest to bind the fourth oxygen molecule!
So, when binding to the 2nd, 3rd and 4th oxygen molecule, the saturation will be higher for the same partial pressure (‘b’; the blue line is now above the green line).
At the top of the graph, in ‘c’, when the blood is being saturated in the lungs, saturation easily reaches 90-100%, even if the partial pressure decreases to 80 or even 60 mmHg.
In other words, even if for some reason the alveoli don’t have too much oxygen, the blood will still try to saturate as much as possible. We are therefore, to a certain extend, partially independent of the exact partial pressure of oxygen in the atmosphere.
When the blood flows to the tissues however, the blue curve becomes steep, as the partial pressure decreases, as indicated in ‘b’. This is the part that ‘releases’ the oxygen to the tissue.
Because of the steepness of the curve, a small drop in pO2 will cause a large decrease in desaturation. In other words, many oxygen molecules will be delivered to the tissue, as it should!
The blue curve that we discussed above also turns out to be sensitive to several other factors in the blood such as the temperature, the pH, the pCO2 and a compound called DPG (=2,3 diphosphoglycerate).
These things happen especially in tissues and muscles that are working or exercising very hard.
In those exercising muscles, the temperature will be higher, there will be more CO2 released which will also decrease the pH and increase the DPG
(DPG is a byproduct of the glycolysis that provides more energy to the working cells).
All these factors will make the curve shift to the right (from blue to green in the graph). Why is that useful?
Look at the position of ‘b’ in the blue curve. This was the position of the hemoglobin in the peripheral blood with a saturation of 70% at a partial pressure of 40 mmHg.
Now, the curve has shifted to the right (green curve) because of the temperature, the pH etc. But the partial pressure in the blood is still the same; 40 mmHg! (Because it still has the same amount of oxygen bound to Hb).
Therefore, point ‘b’ has to shift to point ‘c’. In other words, more oxygen is released to the blood, which of course is welcome news for the working muscle!
The opposite would happen (a shift to the left, to ‘d’), if the temperature decreased, the pH increased etc (red curve) but this is much less common.
Note by the way, that these shifts in the oxygen-dissociation curve have hardly any effect in the lungs where the partial pressure and the saturation are much higher.
Nice system, don’t you think? This really helps in, first, getting our oxygen into the blood and, second, delivering it to the tissue where it is most needed.
In some countries, the effect of a lower pH that drives the oxygen from the hemoglobin molecule to the working tissues is called the Bohr effect!
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A straight Dissociation Graph between pO2 and oxygen saturation:
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A normal Oxygen Dissociation graph:
a) more difficult to bind the first oxygen molecule b) it is easier to bind the 2nd, 3rd and 4th molecule c) 100% saturation
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a) 100% saturation b) 70% saturation at 40 mmHg c) 40% saturation at 40 mmHg d) 90% saturation at 40 mmHg