Gas Exchange - Video Tutorials & Practice Problems
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1
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Gas Exchange and Breathing
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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 electron acceptor in the electron can perform gas exchange across their body surfaces, 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, Plathilmenthys, you know, flatworms, that sort of thing. Now for larger animals, we're gonna need respiratory organs that are specialized, to allow us to perform gas exchange. And the lungs are going to be the example we'll be looking at. And, essentially, these organs provide the surface area necessary for gas exchange. And here you can see, mammalian, 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 bronchioles all through the lungs, and of course those will end in alveoli. And, in the alveoli, that's where the magic happens. That's where gas exchange occurs. Oxygen that is inhaled is going to move into the bloodstream in those capillaries that surround the alveoli. And carbon dioxide is going to move from the bloodstream into the alveoli to, of course, be exhaled. Now where these molecules are going to and from is cells, and specifically, mitochondria in cells. So, the oxygen 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 electron acceptor in the electron transport chain. And of course, c02 from the mitochondria is going to be transported into the blood, and out into the alveoli to be exhaled. And that c o two is coming as a byproduct of, metabolism. You know, specifically like the citric acid cycle, where those carbon compounds are broken down, and, the carbons are oxidized and given off as CO 2. So respiratory organs provide that surface area for gas exchange, but we need a way of getting air in and out of the diaphragm, comes into play. The diaphragm is what we use, of course not everyone does it like humans or mammals do, and we actually will see that there are, you know, 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. 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 onto you. So with positive pressure ventilation we have air pushed into the lungs. This is like what frogs do, that's why they, they, you know, 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 let me 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, 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, relax our ribs, and they move down and that, 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. Now with that, let's flip the page.
2
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Air Movement and Partial Pressure
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When we inhale and exhale, not all of the air we bring in actually participates in gas exchange. Only the air that makes it to the alveoli will participate in gas exchange. Some of the air we inhale will actually just sit in our trachea, our bronchi, and our bronchioles. And we call this the dead space. Really great name. You know, I often rag on scientists for how bad their naming can be. 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 tidal volume, you know, like the, like the, ebb and flow of waves. Right? It's like that. Tidal in and out. That's that's where that name comes from. So again, very nice kind of poetic name there. So, as I'm sure you're aware, you know, you can breathe in and out past that tidal volume, past that point where it's comfortable. Right? You can force more air into your lungs, and you can also force more air out on your exhalations. And we call that maximum volume of air where you force as much air in 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 some 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 gonna go into the dead space, and what will fill the alveoli. And all I really want you to know is that what fills the than what fills the dead space. Additionally, 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, the air that goes into the alveoli. 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 going to have totally fresh air filling your alveoli every time. You're gonna have a mix of some fresh air, this stuff here, and some, I guess we'll call it stale air, that was left over in the dead dead space when you exhaled your last breath. And, you know, the point here is just that what's in your alveoli 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 real, 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 gases except for 1. 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, room, you know, keep it at the same temperature, but I remove all of the gases except for nitrogen. And you can see behind my head I have some pie charts that show the composition of gases 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, you know, some pressure. I'm just gonna write, 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'd multiply my total pressure by 0.78, and this would give me my partial pressure of nitrogen. Right? So the total pressure times the amount that 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, just as long as I make sure I change this number to reflect that gas's percentage in the composition. So, for example, oxygen, you can see, here it's like 21% roughly, you know, so we could, 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, you know, 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, you know, mixed up about how the composition of gases, is affected by altitude. Now, you know, you hear people say oh, you know, there's less, you know, there's less oxygen at higher altitude. So, here's what's actually going on. The atmosphere at higher altitude has the same composition of gases as it does at sea level. The composition of gases 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 gases stacked on top of on top of you. So at sea level there's more gas stacked on top of you. Right? Whereas if you're at higher altitude, there's a thinner layer of gas sitting on top of you. So there's less pressure pushing down on you. Now, you know, what what this means is that what's changing with altitude is the partial pressure of gases. The partial pressure of gases, those partial pressures are going to be lower at higher altitudes. The composition, the percent composition, is unchanged. Now, the reason for, you know, caring at all about partial pressures is because gases actually diffuse based on partial pressures. And, you know hopefully you could have guessed this from everything we've learned about diffusion, and gases 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 gradient 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 our gases are going to diffuse from that higher partial pressure to the lower partial pressure. With that, let's flip the page.
3
concept
Diffusion of Gases
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Gas diffusion is described by Fick's law of diffusion, which basically says that gases diffuse due to five factors, but really there are 3 important ones that we're gonna look at, and those are surface area, surface area that diffusion will occur over, distance across which the diffusion will occur, and the partial pressures of the gas diffusing. Now increasing surface area for gas exchange will increase the rate of diffusion. More surface area, more diffusion. Hopefully that's not a surprise, that's a concept that comes up again and again in biology. Now decreasing the distance that the gas has to travel will actually increase the rate of diffusion. So think about this like the thickness of a membrane. The gas has to get across the membrane. The thinner the membrane, the, 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 gases. So surface area and distance is great, but if you don't have a difference in partial pressures, you're not going to have diffusion of gases. And by increasing the difference in partial pressure between the two environments, you will increase the rate of diffusion. So, 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 than 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 tissues. Now with carbon dioxide, we kind of have the reverse scenario. The, partial pressure of carbon dioxide in the lungs is lower than the partial pressure of carbon dioxide in the blood, and that's what drives c02 into the lungs to be exhaled. And likewise, the partial pressure of c02 in the blood is going to be lower than the partial pressure of c02 in the tissues. So that's what's going to drive c o two from the tissues into the blood. So, you know, basically partial pressure is what drives the diffusion of gases, and these gases 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 exercise when their energy demands increase, and this is why, you know, the muscles are gonna be super greedy with oxygen. It's it's, that they have they tend to have, 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, as I 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 gonna mix with the fresh air and that's what's going to, go into your alveoli and, you know, what what's gonna be performing gas exchange. So point is that the partial pressure of oxygen in alveoli 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, you know, I'm here, I'm alive, you guys are here alive, so, clearly it's good enough. Now, hemoglobin is going to be that magic little protein that will bind oxygen and transport it in the blood and also unload oxygen at the tissues. Hemoglobin is a protein with quaternary structure. It has 4 subunits, 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 hemoglobin binds one oxygen, it actually goes, undergoes a conformational change, so it's it physically changes shape, and this shape change actually makes it easier to bind another oxygen. So binding oxygen makes it easier to bind more oxygen. And, you know, that's super cool because it leads to, 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 3 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 conformational change that makes it easier to offload the rest of its oxygens. So, this cooperative binding just makes hemoglobin more efficient at doing its job basically. And you can see, the, you know, conformational change between the deoxy and the oxy form, you know, here we have the deoxy form, here's the oxy form. 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 to notice that it's a different shape. That's all. Now, the cooperative binding that hemoglobin experiences is going to lead to a graph of oxygen saturation that looks like this. It's going to have a shape like that which we call sigmoidal, it's a sigmoidal graph. And the significance of this is going to come into play on the the next page, so why don't we go ahead and flip the page.
4
concept
Oxygen Dissociation Curve and the Bohr Shift
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11m
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This sigmoidal graph of oxygen saturation has a bunch of different names, and they're all correct. So use whichever one you like. It can be called the oxygen dissociation curve, sometimes it's the oxygen hemoglobin equilibrium curve, or sometimes people call it the oxyhemoglobin saturation curve. These are not the best names, you know. But I get the job done and either way it's just a sigmoidal curve that is just trying to illustrate the oxygen saturation of hemoglobin at different partial pressures of oxygen. So here on the y axis we have our, 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 oxygen saturation or ox the rate of oxygen saturation in hemoglobin is not, you know, doesn't correspond linearly to the partial pressure of oxygen. That's why it's that, you know, that that's that's why we say that this has a sigmoidal shape, right? It's, it has that curve to it, and the reason for that is because of the cooperative binding. Right? When, oxyhemoglobin, you know, only has a little bit of oxygen bound, it's or I'm sorry, when it doesn't have any oxygen bound, it's going to, you know, take a little bit for it to bind 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, at the rate, you know, at, low partial pressure and really high partial pressure is less than in the middle because, essentially that is, you know, as, you know, let's say like ox one oxygen is bound here, it's gonna make it really easy to bind those next oxygens and then, you know, as one oxygen is released here it's gonna make it really easy to release those other oxygens here. So that's that's, that cooperative binding is what gives the sigmoidal shape of this graph. So with all that, let's make it even more confusing, that curve can actually move to the right and to 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, the opposite reasons of a right shift. So that right shift we call the Bohr shift. Sometimes it's the Bohr effect, named after the guy who, you know, theorized it. And it's essentially a shift of the curve to the right, so, you know, that that direction, And, it's going to be due to a number of factors, we're only gonna look at 2 really. And those two factors are decreasing pH, so, lower pH, things getting more acidic, acidic, and also increasing the partial pressure of c02. So hemoglobin also can can also bind c o 2. However, all you really need to worry about is the fact that increasing partial pressure of c o two will lower hemoglobin's affinity for oxygen. So more, higher c o two concentration makes hemoglobin have a lower affinity for oxygen, which is going to, cause it to unload oxygen. Right? Which makes sense because tissues that are consuming a lot of oxygen are going to generate a lot of c02, which is going to cause an increase in the partial pressure of c02. So, essentially, the idea there is that tissues that are, you know, performing a lot of cellular respiration, they're consuming lots of oxygen, are going to have a higher, a higher partial pressure of c o 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. I'm not an artist. But, essentially, the idea is that now hemoglobin, has a lower affinity for oxygen. Right? Because at, it it will take higher partial pressures to achieve the same level of saturation. That's essentially all, all that boils down to. And it and it's just a mechanism that makes hemoglobin more efficient at unloading oxygen, in in tissues that really need it. Now, the other thing we're gonna look at as I said, was pH, how pH affects it. So, lowering pH, which could also be thought of as increasing the acid concentration. So lowering pH or increasing acid, however you want to think of it, will result in lowering hemoglobin's affinity for oxygen. Now, the way I like to think about this is that c o 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 c o 2, the more carbonic acid, which means the 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 to, allow hemoglobin to unload its oxygen more easily. So this is just another way of sort of detecting those c o two concentrations. Right? Not only is it affected by the partial pressure of c o 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 due to carbonic acid which comes from c o two. So these are 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. 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 as lowering the pH. And of course, increasing the partial pressure of c o 2. And that causes a right shift or a reduced affinity of Hemoglobin for oxygen, which makes it better at unloading formation of anhydrase that catalyzes the formation of carbonic acid from c o two and water. And essentially that is going to, help ensure that c o two that makes it into the blood will, you know, basically, you know, very rapidly be converted into carbonic acid. This has a few effects. 1st, it's going to lower the partial pressure of CO 2 in the blood. If you're taking CO 2 and converting it into a different model molecule, that means that you don't have the CO 2 anymore. So this is going to ensure that the partial pressure of CO2 is lower in the blood than, for example, tissues, which will ensure that, the diffusion of c o 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 gases diffuse in the right direction. Now, in addition pH. And again, this will, the pH. And again, this will, induce the Bohr shift and make hemoglobin better at unloading oxygen, which, will be important, you know, in terms of hemoglobin being able to, unload oxygen most effectively in the tissues that have the highest demand which are gonna be the tissues that are giving off the most c02. Now, lastly, it's worth noting that in terms of, homeostasis of gases our body isn't actually detecting those gases directly. Right? It actually uses pH detectors in the respiratory center, which is part of the medulla oblongata, brain region in the brain stem, those pH detectors are what are going to regulate ventilation. So, kind of cool. Now you, you know, hopefully can understand why, using pH detectors is, you know, an effective method of monitoring the gases in our blood and monitoring ventilation. Right? Because of how c o two will, 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.