We previously looked at Dalton's law of partial pressure. Well, now we need to add another law to the mix here. We're going to be looking at Dalton's law and Henry's law, and that's because these two laws together explain the movement of molecules by respiration, and that's what we really care about. We're really interested in how these oxygen molecules and carbon dioxide molecules move in and out of the blood. Alright. So let's look at these 2, Dalton's law. We're actually going to restate it here just slightly to be a little closer to how we've been using it. So we're going to say that Dalton's law says that the partial pressure of a gas is equal to its percent composition multiplied by the total pressure. Remember, that's how we calculated those partial pressures of the gases that make up atmospheric air. We looked at their percent composition, we multiplied it by total atmospheric pressure, and that gave us the individual partial pressures. Alright. Well, now to this, we're going to add Henry's law. And Henry's law says that the amount of a gas that dissolves in a liquid is proportional to the partial pressure. Alright. Well, hopefully for respiration, you see why that's important. We're interested in getting these gas molecules, oxygen and carbon dioxide, in and out of the blood. We're worried about how do they dissolve into the blood. Henry's law tells us that to know how that works, we have to understand the partial pressure. Now, two quick notes about Henry's law here: two other things play a factor in how easily things go into a liquid. First off is just, well, let's say here the specific values depend on the solubility. Some gas molecules are just more soluble than others are, and also temperature. Now while temperature is important for Henry's law, it's actually not important physiologically, because we have homeostasis and our body's temperatures are constant. So at a constant temperature, we don't need to worry about that variable for figuring out how all this works. Okay. So let's look at these two laws and see how they work together. So first off, Dalton's law. Well, if we're at sea level, the total pressure is going to be 760 millimeters of mercury. Now to represent that, we have a chamber here, and we have gas molecules represented by sort of balls in that chamber, and we have oxygen molecules represented by those red balls in the chamber there. And oxygen is about 21% of the atmospheric air. So to figure out the partial pressure of oxygen, we take that 21% and multiply it by 760 millimeters mercury, that atmospheric air. So it comes out to 160 millimeters of mercury. Now previously when we looked at that, we had a number that was a little bit more specific. For our purposes here, that's that's close enough. Alright. So 160 millimeters of mercury is the partial pressure of oxygen at sea level, but what if you're not at sea level? What if you're on a mountain in Colorado, for example? Alright. Well, on a mountain in Colorado, the atmospheric pressure is closer to 500 millimeters of mercury. So here to represent that, we have this same chamber, but we have proportionally fewer molecules in that chamber because the pressure is less. Now, importantly, though, we have the same percentage of those molecules are oxygen. The percent composition on that mountain is still, I'm sorry, is still 21% oxygen. So to figure out the partial pressure of oxygen, we take that 21%. We multiply it now by 500 millimeters of mercury. So it gives us, in this case, a 105 millimeters of mercury. Right? So the percent composition in both cases is the same, but the partial pressures are very different. Right. Well, let's see how this works with Henry's law. So we're going to take the same chamber, but now we're going to add some water on the bottom of it. So we have the same number of molecules in there. It's still at 760 millimeters of mercury. It's still 21% oxygen, and that partial pressure of oxygen is still 160 millimeters of mercury. Alright. We have one more line there on the bottom. We're going to come back to that in just a second. But we do the same thing for that mountain in Colorado. Well, now we have fewer total molecules up in that chamber, but it's still over this water. Well, the part the I'm sorry. The percent composition of oxygen, that's still 21%. The, partial pressure of oxygen is still 105 millimeters of mercury. But if you look at these, what does Henry's law say about which one of these chambers is going to have more oxygen dissolved in the water? Well, Henry's law says that how much of that oxygen actually dissolves in the water is proportional to that partial pressure. So at sea level, more oxygen dissolves. Up on that mountain in Colorado, well, the partial pressure is less, less dissolves. Again, even though the percent composition is the same, we care about the partial pressures. And maybe now that makes sense, even though it's still 21% of the air is oxygen on that mountain in Colorado. Well, now you know why you might feel a little lightheaded when you're at that altitude. Alright. Dalton's law and Henry's law. I understand remembering the names of laws can be a little confusing sometimes. We have a quick memory tool here for you. Though the way I say it, I say Dalton divides the pressure, Henry hydrates it. Remember, Dalton's law says that we can take that total pressure and divide it up into those individual partial pressures. Henry hydrates it. Henry's law tells us that knowing those partial pressures tells us how much of those gases go into solution. Alright. We'll practice this more like always. I'll see you there.
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22. The Respiratory System
Law of Partial Pressure
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