Oxygen content (video) | Khan Academy
A normal PaO2 and alveolar-arterial PO2 difference (A-a gradient) do NOT rule PaO2 is a major determinant of SaO2, and the relationship is the familiar .. one of two ways: 1) anemia, or 2) altered affinity of hemoglobin for binding oxygen. Guide for Adjusting and Assessing Therapy and Routine Nursing Care Means for Interpreting . the partial pressure of oxygen (PO2) and oxygen saturation. 2 relationship between oxygen content and saturation is linear for any given. Oxygen is transported in the blood in two ways: oxygen dissolved in SaO2 and pO2 have direct relationships, if one is decreased so is the.
I would say, actually probably the two had the same oxygen content. That would be my guess based on this set up. So that's one important thing to remember that it's per ml. So let's just keep that in mind. And now let me actually just jot down for you the exact equation, kind of the formula. If want to mathematically calculate oxygen content, how would that look? Well, CaO2 is quicker to write. So let me just jot that down.
And the units on this are milliliters of oxygen per, I said, milliliters of blood. So these are the units here. And this is going to equal-- to figure this out, I need to know the hemoglobin concentration. And there it's the grams of hemoglobin per milliliters of blood. And then, I have to multiply this by a constant.
And the constant is 1. And what that number is, is it's telling me the milliliters of oxygen that I can expect to bind for each gram of hemoglobin. So that's actually quite a nice little number to have handy because now you can see that the units are about to cancel.
This will cancel with this. And I end up with our correct units. But there's one more thing I have to add in here which is the oxygen saturation. Remember, this O2 saturation. And if I know the O2 saturation, remember, there's this nice little curve.
This is O2 saturation. And if I'm looking at just the arterial side, I could write, S little a O2. And I could compare to the partial pressure in the arterial side of oxygen. And remember, we have these little S-shaped curves. And all I want to point out is that, for any increase in my PaO2, in the partial pressure of oxygen, I'm going to have an increase in the O2 saturation.
So there's an actual relationship there. And we usually measure this in percentage. Percentage of oxygen that is bound to hemoglobin. And so this is the same thing here, as a certain percentage. So this whole top part of the formula, then, this whole bit in my brackets really is telling me about hemoglobin bound to oxygen. Now remember, that's not the only way that oxygen actually travels in the blood.
Let me write out this second way that oxygen likes to get around. And the second way is when it dissolves in the blood. So this is all going to be plus.
And the second part of the equation is the partial pressure of oxygen. And this is measured in millimeters of mercury. So that's the unit. And this is times, now this is another constant, 0. And then, keep track of the units here because we have to end up with these units.
What’s The Difference Between Oxygen Saturation And PaO2?
So you know everything has to cancel out to end up with that. So I have milliliters of oxygen on top. And I'm going to want to cancel my millimeters of mercury. So take that times milliliters of blood. So these are the units on the bottom. And they end up the same as we just worked through. We've got this crosses out with that. And my units are going to end up perfect. And this bottom bit, that I'm going to put in purple brackets. This bit tells me about dissolved oxygen.
So I have my oxygen bound to hemoglobin. And I have my dissolved oxygen. These are the two parts of my formula. So let me actually just quickly, before I move on, circle in blue, then, the important parts that I want you keep your eyeballs on.
There is the total O2 content, hemoglobin, oxygen saturation, and partial pressure of oxygen. And remember, this guy influences this guy. And we saw that on the O2 curve that I just drew. Let me just bring it up again, so I can remind you what I'm talking about.
5c. Saturation du sang veineux mélangé en oxygène (SvO2)
In this graph, you can see how the two are related. There's a very nice relationship between the two. So this is my formula for calculating the total oxygen content. So let's actually use this formula. Let's think through this. And when I think through it, I always go through all of my four variables.
Let me just jot them down here. So we keep track of them. Let's do PaO2, SaO2, and then hemoglobin and the total oxygen content. These are my four variables. Now, let's do a little problem together. Let me make a little bit of space. And let's say I have two little containers. And the first container, this first one is full of blood. Here's a B for blood. And here's a second container full of plasma. Remember, plasma is a part of the blood. But it's not all of the blood. Plasma specifically does not have any red blood cells or any hemoglobin.
So let me just write that down. No hemoglobin in the plasma side. Neither the PaO2 nor the SaO2 provide information on the number of oxygen molecules, i. Note that neither PaO2 nor SaO2 have units that denote any quantity.
This is because CaO2 is the only value that incorporates the hemoglobin content.
Oxygen Saturation (SaO2)
Oxygen content can be measured directly or calculated by the oxygen content equation introduced in Chapter 2: I have shown the 3 short paragraphs above to dozens of students, interns, residents; almost all will say they understand the differences, no problem. But, when given questions to test their understanding, they don't show much understanding. So more instruction is needed and, yes, a few problems along the way.
Understanding will come from closely reviewing this material AND working on all the problems; do that, and you should be able to teach the subject! PaO2, the partial pressure of oxygen in the plasma phase of arterial blood, is registered by an electrode that senses randomly-moving, dissolved oxygen molecules.
The amount of dissolved oxygen in the plasma phase -- and hence the PaO2 -- is determined by alveolar PO2 and lung architecture only, and is unrelated to anything about hemoglobin. In this situation a sufficient amount of blood with low venous O2 content can enter the arterial circulation and lead to a reduced PaO2. However, given a normal amount of shunting, neither anemia nor abnormal hemoglobin binding will affect PaO2. Oxygen molecules that pass through the thin alveolar-capillary membrane enter the plasma phase as dissolved free molecules; most of these molecules quickly enter the red blood cell and bind with hemoglobin Figure There is a dynamic equilibrium between the freely dissolved and the hemoglobin-bound oxygen molecules.
However, the more dissolved molecules there are i. Oxygen pressure, saturation and content. Schematic shows cross section of lungs and pulmonary circulation. CO2, nitrogen and other gas molecules are omitted for clarity. PaO2 is always slightly lower than PAO2 because of normal venous admixture, here represented by a connection between the venous and pulmonary circulations. See text for discussion.
Thus hemoglobin is like an efficient sponge that soaks up oxygen so more can enter the blood. Hemoglobin continues to soak up oxygen molecules until it becomes saturated with the maximum amount it can hold - an amount that is largely determined by the PaO2.
Of course this whole process is near instantaneous and dynamic; at any given moment a given O2 molecule could be bound or dissolved. However, depending on the PaO2 and other factors, a certain percentage of all O2 molecules will be dissolved and a certain percentage will be bound Figure If there is no interference as from carbon monoxide, for examplethe free O2 molecules bind to these sites with great avidity.
The total percentage of sites actually bound with O2 is constant for a given set of conditions, and is the 'saturation of blood with oxygen'. In summary, PaO2 is determined by alveolar PO2 and the state of the alveolar-capillary interface, not by the amount of hemoglobin available to soak them up.
PaO2, in turn, determines the oxygen saturation of hemoglobin along with other factors that affect the position of the O2-dissociation curve, discussed below. Neither the amount of hemoglobin, nor the binding characteristics of hemoglobin, should affect the amount of dissolved oxygen, and hence should not affect the PaO2. Stated another way, the number of dissolved oxygen molecules is independent of the amount of hemoglobin or what is bound to it. To repeat one more time because it is so importantPaO2 is not a function of hemoglobin content or of its characteristics, but only of the alveolar PO2 and the lung architecture alveolar-capillary interface.
This explains why, for example, patients with severe anemia or carbon monoxide poisoning or methemoglobinemia can and often do have a normal PaO2. The most common physiologic disturbance of lung architecture, and hence of a reduced PaO2, is ventilation-perfusion V-Q imbalance. Less common causes are reduced alveolar ventilation, diffusion block, and anatomic right to left shunting of blood. SaO2 is determined mainly by PaO2.
The relationship between the two variables is the familiar oxygen dissociation curve Figure The dissociation curve is experimentally determined from in vitro titration of blood with increasing partial pressures of oxygen. At low oxygen pressures there is relatively little increase in SaO2 for a given change in PaO2.
PaO2 is the most important but not the only determinant of SaO2. Shifts of the O2-dissociation curve are discussed in Chapter 6. For now, consolidate your understanding of the difference between PaO2 and SaO2. Think of PaO2 as the driving pressure for oxygen molecules entering the red blood cell and chemically binding to hemoglobin; the higher the PaO2, the higher the SaO2. The oxygen dissociation curve, showing PaO2 vs. SaO2 and PaO2 vs. X represents blood gas values of a case presented in Chapter 6.