Diffusion of Gases

by Jason Amores Sumpter
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gas diffusion is described by fix Law of diffusion, which basically says that gas is diffuse due to five factors. But really, there are three important ones that we're gonna look at. Those are surface area, uh, surface area. The diffusion will occur over distance across which the diffusion will occur and the partial pressures of the gas defusing now increasing surface area for gas exchange will increase the rate of diffusion or surface area, more diffusion. Hopefully, that's not a surprise. That's a concept that comes up again and again. Biology now decreasing the distance that the gas has to travel will actually increase the rate of diffusion. Uh, so think about this. Like the thickness of a membrane. The gas has to get across the membrane, the thinner, the membrane, the, uh, the you know, higher the rate of diffusion, you know, less distance to travel. And lastly, partial pressure. We said that partial pressure will drive. The diffusion of gas is so surface area and distance is great. But if you don't have a difference in partial pressures, you're not gonna have diffusion of gasses. And by increasing the difference in partial pressure between the two environments you will increase the rate of diffusion. So, uh, the greater the difference. The partial pressures of the gas in those two environments, the higher the rate of diffusion you'll have now. Partial pressure is, of course, what's going to drive oxygen and carbon dioxide diffusion in the lungs, the blood and the tissues. Now the partial pressure of oxygen in the lungs is going to be higher, then the partial pressure of oxygen in the blood that's going to drive oxygen from the lungs into the blood. And of course, it would make sense, then, that the partial pressure of oxygen in the blood is higher than that in the tissues. And that's what's going to allow oxygen to unload from the blood to you tissues. Now with carbon dioxide, we kind of have the reverse scenario. The, uh, partial pressure of carbon dioxide in the lungs is lower, then the partial pressure of carbon dioxide in the blood. And that's what drives CO two into the lungs to be exhaled. And likewise, the partial pressure of CO two in the blood is going to be lower than the partial pressure of CO two in the tissues. So that's what's going to drive co two from the tissues into the blood. So you know, basically partial pressure is what drives the diffusion of gasses. And these gasses that we're focusing on, which we breathe in and out are no exception. Now it's worth noting that muscles tend to have particularly low partial pressure of oxygen, especially during during exercise when their energy demands increase. And this is why, um, you know the muscles. They're gonna be super greedy with oxygen, its's that they have. They tend to have a, you know, a lower partial pressure of oxygen. So they're going to suck up a lot of the oxygen out of the blood, which is good because they need it now. In mammals, a zay said before each breath of fresh air is going to mix with some oxygen depleted air. Right. That air that was that stale air that was sitting in the dead space is going to mix with the fresh air. And that's what's going Thio go into your Alvey lie and you know what? What's gonna be performing gas exchange? So point is that the partial pressure of oxygen and l've Eli is going to be less than the partial pressure of oxygen that's in the atmosphere. And you know, it's not ideal, But clearly the system still works. So, uh, you know I'm here. I'm alive. You guys where you're alive so clearly it's good enough. Now hemoglobin is gonna be that magic little protein that will bind oxygen and transported in the blood and also unload oxygen at the tissues. Ah, hemoglobin is a protein with Quaternary structure. It has four sub units and it actually has this really cool property we call cooperative binding, which is basically a property of a binding system. It's not exclusive to hemoglobin, where the binding of one thing alters the binding of subsequent things. That's kind of a very vague general way to describe cooperative binding in the case of hemoglobin, what's actually happening is that when he hemoglobin binds one oxygen, it actually goes, undergoes a confirmation. I'll change. So it's it physically changes shape, and this shape change actually makes it easier to buy another oxygen. So binding oxygen makes it easier to bind mawr oxygen and uhh! You know that's super cool because it leads to, uh, you know, this this interesting pattern of loading and unloading oxygen when hemoglobin doesn't have oxygen. Right When it gets to the lungs, for example, and it picks up oxygen and it picks up that one oxygen, it's gonna make it way easier for it to bind the remaining three oxygen that it can carry right. Conversely, when it gets to the tissues and it offloads an oxygen because you know the tissues are demanding that oxygen by offloading that one oxygen, it will actually undergo confirmation I'll change. That makes it easier to offload the rest of its oxygen's. So, uh, this cooperative binding just makes hemoglobin mawr efficient at doing its job. Basically, And you can see, uh, the confirmation I'll change between the D Oxy and the Oxy form. You know, here we have the D Oxy form. Here's the oxy form. Uh, you know, I don't expect you to look at this and say, Oh, of course I see how this shape change would drive oxygen binding. I just want you thio notice that it's a different shape, that's all. Now, uh, the cooperative binding that hemoglobin experiences is going to lead Teoh a graph of oxygen saturation that looks like this. It's going to have a shape like that, which we call Sig model. It's a sigmoid aled graph. Ah, and the significance of this is going to come into play on the next page, so why don't we go ahead and flip the page?