When we previously introduced respiration, we said that carbon dioxide and oxygen are always moving down their pressure gradients between the air and the blood and between the blood and the tissues. Well, now we want to look at this in a little bit more detail. We're going to put some values on those gradients, and we're going to look at the nitty gritty. Before we really get into it, though, we need to note that the composition of atmospheric air is actually different than the air in the alveoli. So previously, we've been talking about atmospheric air and you can see those numbers here. You'll note they may be a little different than what we've seen before. It's okay. They're close enough. The one major difference here, we pulled argon out because we don't really care about that physiologically and we put water vapor in instead because that is going to be a little important. Alright. But these numbers, that's not actually part of the gradient that we're talking about. The gradient is between the air and the alveoli in the blood, and the composition of the air in the alveoli well, you can see here you can look at those numbers. They're pretty much different across the board. Now importantly, one number that is not different is going to be that total pressure. In both the alveoli and the atmospheric air, the total pressure is at 760 millimeters of mercury because remember that intrapulmonary pressure always equalizes to atmospheric pressure during ventilation. Alright. But we can go through this, and the first thing that we'll see here is that that water vapor there's way more water vapor in the lungs than in the atmospheric air. And that means that sort of more of that total pressure in the lungs is going to be taken up by that water vapor. And that's just because, right, it's wet inside your body. So in your lungs, that air is just very humid. Alright. But we can go now to carbon dioxide and oxygen. Those are the molecules we really care about, and you can see here look at that percent composition for carbon dioxide. There's over 100 times the percentage of carbon dioxide in your lungs than in the air around you and that converts to over 100 times the partial pressure in that alveoli. Now that's because during ventilation, right, your tidal volume is only 500 milliliters, but the residual capacity, the amount of air that doesn't move out of your lungs with every breath, is something like closer to 2 liters. So you're only replenishing a bit of that air with each breath, and that carbon dioxide is just always moving from the blood into the alveoli, so that partial pressure is much higher than in the atmosphere here. Well, the opposite is going to be true for the oxygen. You can see here there's much less oxygen in the alveoli compared to the atmospheric air, and that converts to significantly lower partial pressure due to oxygen in the alveoli than in the atmospheric air. You'll note the difference is relatively not nearly as great compared to the difference for carbon dioxide, but it is significant. Alright. Finally, there's nitrogen. Nitrogen is not that interesting physiologically. You'll note that it's a little bit different there, but we don't worry about that. The big thing here though is that I'm just going to gray out this atmospheric air, those columns there, because we don't care about them. What's important is those partial pressures in the alveoli and specifically the partial pressures for oxygen and carbon dioxide. These are numbers you might want to remember. Partial pressure for oxygen is going to be 104. Partial pressure for carbon dioxide is going to be 40 millimeters of mercury in the alveoli. Alright. Let's take a look at this now. Alright. We are going to be talking about 2 gradients that determine the movement of the gases in the body, and that's going to