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Hemoglobin-Oxygen Dissociation Curve


So we're gonna pick back up here with the Hemoglobin- Oxygen Dissociation Curve. You could see in your lecture companion notebook, there's a part here where we talk about the Bohr effect. And we're gonna do this all in the context of the Hemoglobin- Oxygen Dissociation Curve. You can see the same things we've talked about, but as we move on this curve, at first, hemoglobin does not have a high affinity for oxygen, so we have to have a lot of oxygen to start getting hemoglobin to be saturated with it.

But then, because of cooperativity, picks up significantly and then levels off. And that's the sigmoidal shaped curve due to cooperativity. Well, inside our body, obviously, hemoglobin, there's few ways, two ways oxygen's transported in the blood. So oxygen in the blood can be either bound to hemoglobin, or it can be dissolved directly in the blood.

And there's a very small percentage, 1%, that dissolve in blood. So, the major way that we get oxygen to our tissue is by having it bound to hemoglobin. The big question is how do we get it off the hemoglobin and into the tissues? Or how do we get it from the tissues and get hemoglobin to bind it again? And the answer is that our body will cause this curve to be shifted towards the right, or towards the left, based on the physiologic conditions that are existing within our body.

So here, I will draw the hemoglobin oxygen dissociation curve, that is right shifted. And here, I'll just put an arrow. You can see the original in black. And the right shifted one, on the right. And then I'll put the hemoglobin oxygen dissociation curve that is left shifted, and I'll just add an arrow to the left.

There's a few conditions in the body. We'll start with a right shift, so what causes the hemoglobin oxygen is oxygen dissociation curved shift to the right. A lot of people have memorized it by increases. So, increase in temperature, increase in PO2. Increase in hydrogen ion, which is equal to a decrease in pH.

And another thing is increase in the molecule called 2,3-bisphosphoglyceric, whose purpose in life is to cause there to a right shape in hemoglobin, okay? And I like to think of this as conditions that exist when we exercise. So, when we exercise, our temperature goes up, we get hotter. When we exercise, we make more carbon dioxide in our tissues, and as a response to this, we hyperventilate.

And the whole reason we hyperventilate, and for the rest of your MCAT life, and on into your medical career, you need to think of hyperventilation as blowing off CO2. And increasing. O2, so blowing off CO2, increase in O2. So, the reason we hyperventilate while we exercise is we're trying to get rid of the CO2 we've created.

When we exercise, our blood become more acidic. The production of lactate contributes to that. And then 2,3-BPG can be associated with exercise. It's more commonly associated with changes that happen in altitude, and what happens when we're anemic. But these are four changes that you need to absolutely know.

You will absolutely be tested on this on the MCAT, can't guarantee every single one of you, but more than half of you will require this knowledge to in your MCAT to answer some question. So, the left shifts are all the opposite, decrease temperature, decrease PCO2, decrease hydrogen ion concentration, decrease 2,3-bisphosphoglycerate. And the important thing to understand here is what does this right shift and left shift mean?

So let's take a given concentration of oxygen, in fact, maybe, let's take even slightly less. So here we are at this point, and if we draw the line up, that means for the right shifted curve, hemoglobin, is not very significantly saturated. Okay, this is a pretty low value. So hemoglobin is not really that interested in binding oxygen at this point.

In the normal curve, hemoglobin is binding oxygen moderately well. And in the right shifted curve, we picked a point so that, or in the left shifted curve, we picked a point, that meant that hemoglobin is binding oxygen very well, and hemoglobin has a high oxygen affinity. So you see in the right shifted curve, hemoglobin has low oxygen affinity, and then as we move towards normal and a left shift occur, hemoglobin's affinity for oxygen increases.

And what does this mean? This means that as hemoglobin carrying oxygen flows through the blood in the tissues, in the right shifted curve, it doesn't have a very high affinity for oxygen, so this is gonna facilitate unloading of oxygen to the tissues, versus the left shifted curve, where it's actually gonna facilitate binding of O2, so it's gonna hold onto that oxygen.

And this is good in things like left shifts are good in any physiologic condition, where the temperatures decrease, or the PSO2 is decreased, where hydrogen ions decrease, or where 2,3-BPG is decreased. We also see it in the fetus. Fetal hemoglobin has a left shifted curve, as compared to adult hemoglobin. And what does that mean?

As the fetus' blood contacts the maternal blood, the fetus' hemoglobin oxygen dissociation curve is left shifted, so it's gonna bind O2 more tightly than the mom's, and that facilitates the transfer of oxygen from the mom to the fetus. And then a right shift as hemoglobin flows through the blood, if we're exercising, or if we're septic, or any other physiologic condition that meets these four criteria that we've talked about.

Oxygen is gonna be unloaded from hemoglobin into the tissues. And that's exactly what we want in these conditions. When we're exercising, we want oxygen to be easily unloaded to our tissues. And in this section, the Bohr effect is mentioned. And the Bohr effect Is simply restating what I have already talked about with pH. The Bohr effect is that we get a right shift, and pH goes down, and the reason that happens, and I want you to be familiar with how.

The carbon dioxide that we have in our body leads to an increased pH. So, this reaction that I'm writing out here shows that when our tissues produce carbon dioxide, it combines with the water in our blood plasma, and there's an enzyme called carbonic anhydrase. And it converts this CO2 and H2O into bicarbonate, and that bicarbonate actually dissociate into the components you see here almost significant, which is this H+.

So, you can see that as the amount of CO2 we have increases in the body, this is gonna lead to an increase in hydrogen ion and acidic conditions in the blood. So that's why anytime, as I mentioned, you need to think of carbon dioxide in the blood as acid in the blood. So, when our carbon dioxide goes up in our blood, our pH goes down in the blood. And according to the Bohr effect, when carbon dioxide goes up, therefore, ph goes down.

And pH and the Bohr effect, decreased pH causes a right shift in the curve. And just as a reminder, again, hyperventilation, we blow off CO2, and when we blow off CO2 we are getting rid of CO2, which means that we're able to decrease the amount of hydrogen ion in the body. And by doing that, we are able to increase the pH. Hypoventilation, on the other hand, when we're hypoventilating, we are retaining CO2 and the amount of oxygen we have in our blood decreases.

So, this is a very important, heavily MCAT tested concept. I actually didn't have it on my MCAT, I guess that since I'm one of the lucky ones, but more students than not have had it on theirs, it seems.

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