Gas Exchange: Study with Video Lessons, Practice Problems & Examples

Gas exchange is vital for cellular respiration, primarily occurring in the alveoli of the lungs, where oxygen enters the bloodstream and carbon dioxide is expelled. The efficiency of this process is influenced by factors such as surface area, distance, and partial pressures of gases. Hemoglobin plays a crucial role in oxygen transport, exhibiting cooperative binding that enhances oxygen uptake and release. The Bohr shift illustrates how increased carbon dioxide and decreased pH lower hemoglobin's affinity for oxygen, facilitating oxygen delivery to metabolically active tissues.

Gas Exchange and Breathing

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 transport chain. 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 Platyhelminths, you know, flatworms, that sort of thing. Now, for larger animals, we're going to need respiratory organs that are specialized to allow us to perform gas exchange. The lungs are going to be the example we'll be looking at. 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. 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, CO₂ from the mitochondria is going to be transported into the blood, and out into the alveoli to be exhaled. And that CO₂ 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 CO₂. So, respiratory organs provide that surface area for gas exchange, but we need a way of getting air in and out of the lungs; 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 different types of ventilation, and we're going to 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, for example, taking some container 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 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 going to come shooting into your lungs. Right? It's going to 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 a 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.

Air Movement and Partial Pressure

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 criticize 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 ebb and flow of waves. Right? It's like that. Tidal in and out. That's where that name comes from. So again, a very nice kind of poetic name there.

As I'm sure you're aware, 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 lungs after you've forced an exhalation. And we call that remaining air the residual volume. Looking at our diagram here you can see that our tidal volume is made up of what's going to go into the dead space, and what will fill the alveoli. All I really want you to know is that more fills the alveoli 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 going to 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 going to 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 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 pressures, 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 pressures is 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 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.

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 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 the 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, some pressure. I'm just going to 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 portion of the composition that nitrogen takes up, which is 78%, 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 about 21%, you know, so we could find our partial pressure of oxygen by taking the total pressure and multiplying it by 0.21.

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 confused about how the composition of gases is affected by altitude. Now, you know, you hear people say oh, there's less oxygen at higher altitudes. Here's what's actually going on. The atmosphere at higher altitudes has the same composition of gases as it does at sea level. The composition of gases in 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 you. So, at sea level, there's more gas stacked on top of you. Right? Whereas if you're at 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, what this means is that what's changing with altitude is the partial pressure of gases. The partial pressures of gases are going to be lower at higher altitudes. The proportion, the percent composition, is unchanged.

Now, the reason for caring at all about partial pressures is because gases actually diffuse based on partial pressures. And, hopefully, you could have guessed this from everything we've learned about diffusion: gases will move from an area of higher partial pressure to an area of lower partial pressure. So, 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 area to the lower partial pressure area. With that, let's flip the page.

Diffusion of Gases

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 going to 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 are 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 in 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 CO2 into the lungs to be exhaled. And likewise, the partial pressure of CO2 in the blood is going to be lower than the partial pressure of CO2 in the tissues. So that's what's going to drive CO2 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 going to be super greedy with oxygen. 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 going to mix with the fresh air and that's what's going to, go into your alveoli and, you know, what what's going to 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 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 a 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 next page, so why don't we go ahead and flip the page.

Oxygen Dissociation Curve and the Bohr Shift

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 the 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 in hemoglobin does not correspond linearly to the partial pressure of oxygen. That's why we say that this has a sigmoidal shape, right? It has that curve to it, and the reason for that is because of the cooperative binding. Right? When oxyhemoglobin only has a little bit of oxygen bound, it's going to 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 the important thing to take note of is that the rate here at low partial pressure and really high partial pressure is less than in the middle because, essentially that is, as one oxygen is bound here, it's going to make it really easy to bind those next oxygens and then, as one oxygen is released here it's going to make it really easy to release those other oxygens here. So 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 going to 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 theorized it. And it's essentially a shift of the curve to the right, so, you know, that direction. And, it's going to be due to several factors, we're only gonna look at 2 really. And those two factors are decreasing pH, so, lower pH, things getting more acidic, and also increasing the partial pressure of CO2. So hemoglobin can also bind CO2. However, all you really need to worry about is the fact that increasing partial pressure of CO2 will lower hemoglobin's affinity for oxygen. So more, higher CO2 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 CO2, which is going to cause an increase in the partial pressure of CO2. 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, a higher partial pressure of CO2, 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 will take higher partial pressures to achieve the same level of saturation. That's essentially all that boils down to. And it's just a mechanism that makes hemoglobin more efficient at unloading oxygen 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 about it, will result in lowering hemoglobin's affinity for oxygen. Now, the way I like to think about this is that CO2, when it gets into the blood, is going to combine with water and form carbonic acid. Acid means lower pH, so, basically, the more CO2, 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 CO2 concentrations. Right? Not only is it affected by the partial pressure of CO2, but it's also affected by pH. And that pH fluctuation is going to be in large part due to carbonic acid which comes from CO2. So these are just ways of making hemoglobin better at doing its job, basically. And, you know, as I said, there

Here’s what students ask on this topic:

What is the role of alveoli in gas exchange?

Alveoli are tiny air sacs in the lungs where gas exchange occurs. They provide a large surface area for oxygen to diffuse into the bloodstream and for carbon dioxide to diffuse out. Oxygen from inhaled air passes through the alveolar walls and enters the capillaries, where it binds to hemoglobin in red blood cells. Simultaneously, carbon dioxide from the blood diffuses into the alveoli to be exhaled. This process is driven by differences in partial pressures of oxygen and carbon dioxide between the alveoli and the blood.

How does the diaphragm contribute to breathing?

The diaphragm is a dome-shaped muscle located below the lungs. During inhalation, it contracts and moves downward, expanding the thoracic cavity and creating negative pressure. This negative pressure pulls air into the lungs. During exhalation, the diaphragm relaxes and moves upward, reducing the thoracic cavity's volume and pushing air out of the lungs. This process is known as negative pressure ventilation and is essential for efficient breathing in humans and other mammals.

What is the Bohr effect and how does it influence oxygen delivery?

The Bohr effect describes how increased levels of carbon dioxide (CO₂) and decreased pH (more acidic conditions) reduce hemoglobin's affinity for oxygen. This rightward shift in the oxygen dissociation curve facilitates the release of oxygen from hemoglobin to metabolically active tissues that produce more CO₂ and have lower pH. This mechanism ensures that tissues with higher energy demands receive more oxygen, enhancing cellular respiration and energy production.

What factors affect the rate of gas diffusion in the lungs?

The rate of gas diffusion in the lungs is influenced by three main factors: surface area, distance, and partial pressures. Increasing the surface area, such as the extensive network of alveoli, enhances diffusion. Reducing the distance gases must travel, like the thin alveolar-capillary membrane, also increases diffusion rates. Lastly, a greater difference in partial pressures of gases (oxygen and carbon dioxide) between the alveoli and blood drives more efficient gas exchange.

How does hemoglobin facilitate oxygen transport in the blood?

Hemoglobin is a protein in red blood cells that binds to oxygen in the lungs and transports it to tissues. It exhibits cooperative binding, meaning that the binding of one oxygen molecule increases its affinity for additional oxygen molecules. This property allows hemoglobin to efficiently load oxygen in the lungs and unload it in tissues where it is needed. Hemoglobin also helps transport carbon dioxide back to the lungs for exhalation.