Gas Exchange

Hi. In this video, we'll be talking about gas exchange and respiratory physiology. Now, gas exchange allows animals to get the oxygen that they need for cellular respiration, as it's the final Elektronik sector in the electron transport chain. And it also allows them to get rid of Waste Co two from their metabolism. Now small animals can perform gas exchange across their body surfaces due to their high surface area to volume ratio. And you'll see this in organisms like Platt hell mentees, you know, flatworms, that sort of thing. Now, for larger animals, we're going to need respiratory organs that air specialized to allow us to perform gas exchange with lungs are going to be the example will be looking at. And, uh, essentially these organs provide the surface area and necessary for gas exchange. And here you can see, um, the Malian, specifically human respiratory anatomy. We have air that will enter through Well, I guess the mouth or the nose. Really? Either way, it's going to go down the trachea and split into the bronchi, which will diverge into bronchial Z all through the lungs. And of course, those will end in Alvi, Eli and, uh, in the Alvey lie. That's where the magic happens. That's where gas exchange occurs. Oxygen that is inhaled is going to move into the bloodstream. Uh, in those capital Aries that's surround the Alvey lie, and carbon dioxide is going to move from the blood stream into the Alvey. Lie to, of course, be exhaled. Now, where these molecules air going to and from is cells and specifically mitochondria in cells. So theocracy gin is going to be transported through the blood and delivered to tissues, where it will make its way to the mitochondria to act as the final electronic sector in the electron transport chain. And of course, CO two from the mitochondria is going to be transported into the blood and out into the valve. Eli to be exhaled. And that CO two is coming as a byproduct of metabolism. Um, you know, specifically like the citric acid cycle, where those, uh, carbon compounds are broken down and the carbons are oxidized and given office CO two. So respiratory organs provide that surface area for gas exchange. But we need a way of getting air in and out of the respiratory organs. That's where this lovely muscle the diaphragm comes into play. Uh, the diaphragm is what we use. Of course, not everyone does it like humans or mammals Dio And, uh, we actually will see that there are, you know, uh, different types of ventilation and we're gonna look at positive pressure and negative pressure. Ventilation real quick. So essentially positive pressure, not just ventilation, but in general, positive pressure is like a form of pushing. It's like a squeezing, pushing force. Negative pressure is more like a pulling force. Um, you know, think about like, for example, you know, taking some container and and sucking the air out of it Like, I don't know if you've ever done this, but like you take a glass and you stick it up to your mouth and just suck all the air out of it and it just stays stuck to your face. That's because of that pulling force, that negative pressure that is pulling it on to you. So, with positive pressure, ventilation, we have air pushed into the lungs. This is like what frogs do. That's why they inflate that big pouch in their mouth. And then they actually squeeze that air through positive pressure down into their lungs. We, on the other hand, we use our diaphragms, and what we do is basically we jump out of the way Here, here's our diaphragm. That muscle, we pull it down and at the same time we draw out our ribs. And by doing this, um, we create negative pressure in this area known as the thoracic cavity. So, basically, by expanding the volume of that cavity, we create negative pressure, and then all you really have to do is just open your mouth and the air is gonna come shooting into your lungs, right? It's gonna get pulled in. And when we exhale, we just relax. That diaphragm, uh, relax our ribs and they moved down And that, uh, you know, changes the volume of this cavity and causes the air to be exhaled. So we breathe. Using that system, it's called negative pressure ventilation. It involves negative pressure, sort of pulling force, and we rely on our diaphragm and the movement of our ribs to generate that negative pressure. With that, let's flip the page

When we inhale and exhale Not all of the air we bring in actually participates in gas exchange on Lee the air that makes it to the Al Ville I will participate in gas exchange Some of the air we inhale will actually just sit in our trachea or bronchi and our bronchi als And we call this the dead space Really great name. You know, I often rag on scientists for how bad they're naming congee. This is a good one. This is a really good name. Now when you breathe in and out that volume of air that you inhale and exhale also has an excellent name. We call that the title volume, you know, like the like the ebb and flow of waves, right? It's like that title in and out. That's that's where that name comes from. So again, very nice kind of poetic name there. Ah, So as I'm sure you're where you know, you can breathe in and out past that title volume past that point where it's comfortable, right? You can force mawr air into your lungs. You could also force more out on your exhalations and we call that maximum volume of air where you force as much area as you can when you inhale. And as much as you can out when you exhale, that total volume is the vital capacity. Now, there's actually still Cem air that's going to remain in your in your lungs after you've forced an exhalation. And we call that remaining air the residual volume. Now, looking at our diagram here, you can see that our title volume is made up of what's going to go into the dead space and what will fill the Alvey lie. And all I really want you to know is that what fills the Alvey lie is a lot or is greater volume, uh, than what fills the dead space. Additionally, what I want you to take note of in this figure is that the air from the dead space, you know from your last breath is actually going to mix in with Theo Air that goes into the Alvey ally, and it's actually gonna be some fresh air that fills your dead space. So essentially, what I'm trying to point out here is that you're not gonna have a totally fresh air filling your Alvey lie every time you're gonna have a mix of some fresh air, this stuff here and e guess we'll call it stale air that was left over in the dead space when you exhale your last breath. And, uh, you know the point here is just that what's in your Alvey ally is not totally fresh air, and you'll see why I'm stressing this point, why it becomes important to gas exchange in a little bit. But before we get there, I also want to talk about partial pressure, which is kind of a confusing, weird idea. But it's pretty essential to understanding how gas exchange works. So the first thing to know about partial pressure is it's it's not really it's hypothetical pressure, and it's the hypothetical pressure of, you know, Let's say you take a container of air, you know from our atmosphere, and you remove all the gasses except for one. The pressure left over from that one gas that's still taking up that same volume, You know that you captured in the container. That is what we call the partial pressure, and it's the partial pressure of that particular gas. So let me give you an example. Let's say that I take a container of air from the atmosphere, I seal it off, and I remember, you know, keep it at the same temperature. But I remove all of the gas is except for nitrogen. And you can see behind my head have some pie charts that show the composition of gasses in the atmosphere. And you can see that nitrogen is the biggest part of the pie, weighing in at 78% of atmosphere composition. So if we wanted to know the partial pressure of nitrogen, what I would do is I would say, Okay, my total pressure is, you know, uhh. You know, some pressure. I'm just gonna right, like total pressure tp times the percent composition of that gas in the mixture. So in the case of nitrogen, it's 78%. So I would multiply my total pressure bye 0.78 And this would give me my partial pressure of nitrogen. Right? So the total pressure times the amount, the the percent of the composition that nitrogen takes up, which is 78%. And that gives me my partial pressure for nitrogen. I could do the same for any other gas, you know, Justus, Long as they make sure I change this number to reflect that gasses percentage in the composition. So, for example, oxygen could see here. It's like 21%. Roughly eso we could, uh, you know, find our partial pressure of oxygen by taking the total pressure and multiplying it by 0.21 So I don't actually care if you can calculate partial pressures. That's not what I want you to be able to do. I just want you to understand conceptually what the partial pressure is telling us. And the reason I want you to understand this is because people often get, um, you know, mixed up about how, uh, the composition of gasses eyes affected by altitude. Now you know, you hear people say, Oh, you know, there's less. You know there's less oxygen, higher altitude. So here's what's actually going on. The atmosphere at higher altitude has the same composition of gasses as it doesn't sea level. The composition of gas is of the atmosphere is the same regardless of altitude. What changes is the total pressure. The total pressure is higher at sea level than it is at high altitudes. And you know, you can think about it as being like gas is stacked on top of on top of you. So at sea level, there's Mawr gas stacked on top of you, right? Whereas if you're a higher altitude, there's a thinner layer of gas sitting on top of you, so there's less pressure pushing down on you. Now, uh, you know, what this means is that what's changing with altitude is the partial pressure of gas is the partial pressure of gas is, um, those partial pressures are gonna be lower at higher altitudes. The composition, the percent composition, is unchanged. Now, the reason for you know, carrying it all about partial pressures is because gas is actually defused based on partial pressures. And, uh, you know, hopefully you could have guessed this from. You know, everything we learned about diffusion and gasses will move from an area of higher partial pressure to an area of lower partial pressure. So you know that, you know, think of it as like a concentration Grady and almost right at higher partial pressure. It's like we have a higher concentration of gas there and at lower partial pressure. It's like we have a lower concentration of gas there. So are gas. Is air going to defuse from that higher partial pressure to the lower partial pressure, but that let's flip the page.

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?

This sigmoid aled graph of oxygen saturation has a bunch of different names, and they're all correct. So use whichever one you like. It could be called the oxygen dissociation curve. Sometimes it's the oxygen, hemoglobin, equilibrium curve. Or sometimes people call it the Oxy hemoglobin saturation curve. Ah, these air. Not the best names, you know. But they get the job done. And either way, it's just a sigmoid curve that is just trying. Thio illustrate the oxygen saturation of hemoglobin, a different partial pressures of oxygen. So here on the Y axis, we have our, uh, saturation of oxygen. And on the X axis we have our partial pressures of oxygen. So what you hopefully will notice is that as partial pressure of oxygen increases, oxygen saturation increases. Hopefully, you also notice that this is not a straight line, meaning that the rate of oxygen saturation or ox, the rate of oxygen saturation in hemoglobin is not, you know, doesn't correspond linear Lee to the partial pressure of oxygen. That's why it's that you know, the that's that's why we say that this has a sigmoid. It'll shape right. It has that curve to it, and the reason for that is because of the cooperative binding, right? Uh, when oxy hemoglobin, you know, on Lee has ah little bit of oxygen bound. It's or I'm sorry. When it doesn't have any oxygen bound, it's going thio, uh, you know, take a little bit for it. Thio find some oxygen. But once it has some oxygen bound, that's why the rate of saturation kind of shoots up in this middle region. Right? That's that's the important thing to take note of is that the rate here is a two rate at low partial pressure and really high partial pressure is less then in the middle. Because essentially that is, you know as, uh, you know, let's say like one oxygen is bound here, it's going to make it really easy to bind those next oxygen's. And then, you know, as one Oxygen is released here, it's gonna make it really easy to release those other oxygen's here. So that Z, that cooperative binding is what gives Thesiger boy, it'll shape of this graph. So with all that, let's make it even more confusing. That curve can actually move to the right into the left. Now we're only going to really talk about the right shift. But the left shift is basically just gonna be due to, uh, the opposite reasons of a right shift. So that right shift we call the boar shift. Sometimes it's the boar effect named after Thebe guy who theorized it, and it's essentially a shift of the curve to the right. So, you know, that drew that direction. And, uh, it's going to be due to a number of factors we're only gonna look at to really, And those two factors are decreasing pH. So lower ph things getting mawr acidic and also increasing the partial pressure of CO two. So hemoglobin also, uh, can you, uh, can also bind co two? However, all you really need to worry about is the fact that increasing partial pressure of co two will lower hemoglobin affinity for oxygen mawr Higher Co two concentration makes hemoglobin have a lower affinity for oxygen, which is going thio cause it thio unload oxygen right, which makes sense because tissues that air consuming a lot of oxygen are going to generate ah lot of CO two, which is going to cause an increase in the partial pressure of co two So essentially, the idea there is that tissues that are performing a lot of cellular respiration, they're consuming lots of oxygen are going to have a higher Ah, a higher partial pressure of co two. And this is going to cause hemoglobin to unload its oxygen more efficiently. And we can visualize that on our graph by having our curve, you know, shift over to the right. So here's my new curve. Sorry, it's so ugly. Not an artist. Um, but essentially, the idea is that now hemoglobin has a lower affinity for oxygen, right? Because at, uh, it will take higher partial pressures to achieve the same level of saturation. That's essentially all, uh, all that boils down to. And and it's just a mechanism that makes hemoglobin mawr efficient at unloading oxygen, uh, in in tissues that really need it. Now, the other thing we're gonna look at, as I said was ph how pH effects it so lowering ph, which could also be thought of as increasing the acid concentration. So lowering pH or increasing asset however you want to think of it will result in lowering hemoglobin affinity for oxygen? No, the way I like to think about this is that CO two when it gets into the blood is going to combine with water and form carbonic acid. Acid means lower pH. So basically, the more co two the Mawr carbonic acid, which means thelancet lower the pH and that lower pH is going to cause a right shift in the curve. And remember that a right shift in the curve is going. Thio allow hemoglobin to unload its oxygen more easily. So this is just another way of sort of detecting those co two concentrations, right? Uh, not only is it affected by the partial pressure of CO two, but it's also affected by Ph. And that pH is in a large or that pH fluctuation isn't going to be in a large part, do you two carbonic acid, which comes from CO. Two. So these air just ways of making hemoglobin better at doing its job. Basically. And you know, as I said, there's other stuff that can affect this curve. Uh, I don't want you to worry about any of that. I just want you to worry about increasing acid, right? That's the same Azaz lowering the pH and, of course, increasing the partial pressure of CO two, and that causes a right shift or a reduced affinity of hemoglobin for oxygen, which makes it better at unloading its oxygen. Now there's actually an enzyme called carbonic and hydrates that catalyze is the formation of carbonic acid from CO two and water. And essentially, that is going to help ensure that co two that makes it into the blood will, you know, basically, uh, very rapidly be converted into carbonic acid. This has, ah few effects. First, it's going to lower the partial pressure of CO two in the blood. If you're taking CO two and converting it into a different model molecule, that means that you don't have the CO two anymore. So this is going to ensure that the partial pressure of CO two is lower in the blood than, for example, tissues, which will ensure that the diffusion of CO two goes from the tissues into the blood. So it's just another way in which, you know the body has mechanisms to ensure that gas is diffused in the right direction. Now, in addition to lowering the partial pressure of CO two in the blood, you know, by turning the CO two into something else. It's turning that co two into acid, which lowers the pH. And again. This will induce the board shift and make hemoglobin better at unloading oxygen, which will be important. You know, in terms of hemoglobin being able, thio unload oxygen most effectively in the tissues that have the highest demand, which are gonna be the tissues that air giving off the most co two Now, lastly, it's worth noting that in terms of home, yo Stasis of gas is our body isn't actually detecting those gasses directly. Right? It actually uses ph detectors in the respiratory center, which is part of the medulla oblong gotta brain region in the brain stem. Uh, those PH detectors air what are going to regulate ventilation. So, uh, kind of cool. Now, you, you know, hopefully can understand why, uh, using pH detectors is, you know, a NIF effective method of monitoring the gas is in our blood and monitoring ventilation. Right? Because of how co two will, uh, you know, turn into acid in the blood. So kind of a cool way that our body indirectly monitors this system. Now, that's all I have for this lesson. I'll see you guys next time