Alright. Let's take a look at this example here. It says 2 graphs are shown below. The top graph is titled volume of breath, and we can see that graph right there. This shows the volume of air inspired and expired during ventilation. The bottom shows pressure relative to atmospheric pressure in millimeters of mercury and has not been filled in. Alright. So I can guess what we're going to have to do here. But before we do that, let's just look at this top graph. On the y-axis here, we see volume in liters and it goes from 0 to half a liter, so from 0 to 500 milliliters here, and that's our volume of breath. Or you can think of that as the change of the volume inside your lungs. And here we see during inspiration, well, that increases by half a liter or by 500 milliliters. And during expiration, that volume inside your lungs, it goes down by half a liter or 500 milliliters there. Alright. So then we can look at our tasks here. A says to draw a line that represents the approximate change in intrapulmonary pressure during inspiration and expiration, and we want to label that line psubIP.
What is intrapulmonary pressure? That's our first thing we gotta figure out. Remember, intrapulmonary pressure, that's pressure inside lungs in the alveoli. Right. So do you remember what that is and what it varies between? Well, we said the intrapulmonary pressure is always going to be equalizing to atmospheric pressure. So it's going to be going up and down around 0 millimeters of mercury. And we said that it goes up by 1.5 millimeters of mercury, and it goes down by 1.5 millimeters of mercury about. So it's going between that negative and positive 1.5 on this graph. Alright. So to fill this in, the first thing I want to do is I want to try and remember when is it at 0? When is it equal to atmospheric pressure? Well, it's going to be equal to atmospheric pressure sort of at the beginning of inspiration, the end of inspiration, the beginning of expiration, and the end of expiration. So we breathe in and out, and it's at the end of those breaths that that pressure equalizes to atmospheric pressure. So I'm going to go ahead and I'm going to put dots on this graph in those places. Alright. So it's going to be at 0 in those three places, and now I just gotta connect those dots by going up and down. So let's figure out how that's going to work. Well, as the volume goes up, what happens to the pressure? The pressure goes down. Right? So the pressure starts going down as the volume goes up, down about negative 1.5 millimeters of mercury, and then air starts rushing in and equalizes the pressure until it reaches 0 again. Alright. As that volume goes down, well, the pressure goes up. So this pressure is going to start increasing to about positive 1.5, and then air is going to be rushing out to equalize that pressure, and it's going to go down and hit 0 again. Alright. We need to label this, so this is psubPhR.
Next up, we have b. Draw a line that represents the approximate change in intrapleural pressure during inspiration and expiration, and we want to label this line, psubPSVR. Alright. So what is intrapleural pressure? However, intrapleural pressure, that's the pressure in the pleural cavity, and we said that's always negative because it's resisting that sort of inward pull from the elasticity of the lungs. So you remember what it goes between? We said it goes between about negative 4 millimeters of mercury and negative 6 millimeters of mercury, and it's going to be most negative when the lungs are the biggest because when the lungs are stretched to be their biggest, they have their most recoil pulling back in, so that intrapleural pressure has to resist that inward pull the most. When the lungs are the smallest, that intrapleural pressure should be closer to 0. It should be less negative. Alright. So I'm going to sort of put those marks on my graph there. When is the volume of the lungs the smallest? Well, that's when it's going to be at negative 4 here. So it's going to be smallest here. Sort of we can see here the volume is at 0. So that's when my, intrapleural pressure should be at negative 4, and it's going to be at negative 4 again at the end of expiration. Alright. When are the lungs the biggest? Well, the lungs are the biggest when the volume's the greatest, so that's right in the middle here. So right in the middle, that's when this is most negative at negative 6. So I'm going to put a dot there. Now I just gotta connect these. So as you breathe in, right, as you inspire that volume increases and the recoil increases, so the intrapleural pressure responds by becoming more negative. As you breathe out, the volume of the lungs goes down, so there's less recoil, and so that pulls on that intrapleural cavity less I'm sorry, that pleural cavity less. And so that intrapleural cavity goes back up to negative 4. Alright. We got to label this. So this is psubPIPs, and we did it. Alright. A graph like this, you probably see it in your textbook, something like this you may see in lecture. Hopefully, now you see how these things are connected, and you can even fill it in on your own. Alright. More problems after this, and I'll see you there.