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.