Ventilation: Study with Video Lessons, Practice Problems & Examples

Understanding ventilation involves Boyle's law, which states that a change in volume leads to a change in pressure, crucial for air movement in and out of the lungs. During inspiration, the diaphragm contracts, increasing thoracic volume and decreasing intrapulmonary pressure, allowing air to flow in. Conversely, during expiration, the diaphragm relaxes, decreasing volume and increasing pressure, forcing air out. Key pressures include atmospheric pressure (760 mmHg), intrapulmonary pressure, and intrapleural pressure, which varies throughout the breathing cycle, influencing respiratory mechanics and efficiency.

Ventilation: Boyle’s Law

Now I'm going to spend some time talking about the physiology of ventilation, but before we really get into it, we need to introduce a physics concept, and that is Boyle's law. Now don't worry. You're not going to have to do any advanced physics calculations, but you should understand this law conceptually and the name of that law is something that some professors like you to know. Alright. So we need to know Boyle's law because ventilation works by altering the pressure gradient. Specifically, we're interested in that gradient between the pressure in the lungs, that intrapulmonary pressure, and the pressure in the atmosphere around us, the atmospheric pressure. Now what Boyle's law says is that pressure 1 times volume 1 equals pressure 2 times volume 2. Now that's the mathematical way to say it, but really in simple terms, all we're saying here is that a change in volume causes a change in pressure. And to illustrate this, we have this syringe here, and I have a syringe here to model as well. So if we have this syringe with some set volume and we have a set amount of air in there, well, in our image there, there's a cap on the syringe. I'm just going to put my finger on the end. Well, what happens if I pull back on the syringe? I'm going to increase the volume. So there's a set number of molecules in this syringe though. No air was able to get in and out. So what would happen to the pressure? Same idea. What happens if I push down on this syringe? I'm compressing the gas in there. What does Boyle's law tell us is going to happen to the pressure? Well, that simple equation should make it pretty easy to understand. Right? If we pull back, that's going to cause an increase in the volume. An increase in volume is going to cause a decrease in the pressure. Alright? If we push down on it, that's going to compress that gases, so a decrease in the volume causes an increase in the pressure. Alright. This is important for ventilation because changing the volume of the thoracic cavity alters the intrapulmonary pressure. That intrapulmonary pressure, the pressure inside the lungs in the alveoli. So that pressure changes, but atmospheric pressure, well, it doesn't change. It's just going to sort of always be at that 760 millimeters of mercury at sea level. So let's see how this works for both inspiration and expiration. So for inspiration, while breathing in, the main thing that's going to happen is the diaphragm, well, it's going to contract. We also have the intercostal muscles working. But here in our illustration, we show this, anatomical model here, and we have an arrow showing that this diaphragm contracts that pulls down. As a diaphragm contracts, that means that the thoracic cavity volume, it goes up. Well, if the volume goes up, what happens to the pressure? The pressure goes down. Well, if the pressure goes down, well, that pressure in the lungs is now less than 760 millimeters of mercury. Alright. But remember, this is an open system. The pressure in the lungs so the area in the lungs, it's connected to the atmospheric air through your respiratory tract. So as that pressure goes down, that means that the pressure there's a gradient. The pressure in the atmosphere is now greater. It's going to push down into your airways and air flows into the lungs. Alright. So that's inspiration. Let's look at expiration. For expiration, the diaphragm relaxes, And when the diaphragm relaxes, what we can see in our anatomical model here, the diaphragm sort of pushes back up. It relaxes and goes back up. That means that the volume in the thoracic cavity, well, the volume goes down. Well, if the volume goes down, Boyle's law says that the pressure goes up. Well, if the pressure goes up, now the pressure in the lungs is greater than 760 millimeters of mercury. And if the pressure is greater, again it's an open system, that means that that pressure in the lungs is going to push up out through the respiratory tract and air flows out of the lungs. Alright. So remember, Boyle's law tells us that it's the change in pressure which causes ventilation to work. We're going to practice this more and look at actual values coming up. I'll see you there.

Ventilation Example 1

This example tells us that during the 1940s and 1950s, iron lungs were a common intervention for patients whose diaphragms were paralyzed due to polio. The patient would be positioned with their body inside a sealed chamber with only their head sticking out. One way iron lungs were designed was to have the foot end of the chamber move in and out, changing the volume inside the chamber. Now we want to use your knowledge of Boyle's law to circle the words in bold that make each statement correct. Alright. So first off, let's remember Boyle's law. So what happens with Boyle's law? Well, Boyle's law says that for a gas, if the volume increases, the pressure is going to decrease. And if the volume decreases, the pressure is going to increase. So a change in volume is going to cause a change in pressure in sort of the opposite direction. Alright. So we're dealing with this now with this thing called an iron lung. This kind of technology is clearly from the last century, it looks like. And in here, you can see in the picture, this person's entirely inside this chamber, except for their head, which is just out in the normal air. So let's go through these statements here. First off, if the volume in the chamber decreases, well, what's going to happen? First off, it says pressure in the chamber, is it going to be greater than, less than, or equal to 760 millimeters of mercury or atmospheric pressure there? So what do you think? Alright. Well, if the volume in the chamber decreases, assuming it started at atmospheric pressure, well, that's going to increase the pressure in the chamber, which is going to mean that it ends up greater than atmospheric pressure. So pressure, volume decreases, the pressure increases. Alright. What do you think about atmospheric pressure though? Well, atmospheric pressure, do you expect it to be greater than, less than, or equal to 760 millimeters of mercury? Well, I don't really expect atmospheric pressure ever to change in response at least to a physiological intervention, so I expect atmospheric pressure here to be equal to 760 millimeters of mercury. Alright. Well, knowing that, then it says pressure in the chamber would force air either into or out of the lungs. Alright. When we looked at this previously, we were talking about changing the pressure in the lungs. Now we're talking about changing the pressure inside the chamber around the whole body for somebody who's paralyzed. So how do you think that would end up working? Well, if you increase the pressure in the chamber, it's gonna push in, sort of, on the entire body, and that's gonna push in on the lungs too. And that will, sort of increase the amount of pressure inside those lungs as that space goes down. The volume in the lungs is gonna go down as that entire body gets pushed down on and that's gonna force that air out of the lungs. Alright. Well, what happens in the other direction? Well, if the volume in the chamber increases. Okay. So, well, if the volume in the chamber increases, what do you expect to happen to the pressure in the chamber? Well, Boyle's Law says that if the volume increases, the pressure goes down. So, therefore, I expect it to go down and be less than 760 millimeters of mercury or less than atmospheric pressure. Alright. Well, what's gonna happen to atmospheric pressure? Again, I don't expect atmospheric pressure ever to change based on what's happening inside an iron lung. So I'm gonna say that it's gonna be equal to 760 millimeters of mercury. And so then the pressure in the chamber would force air. Well, if atmospheric pressure is now greater than the pressure in the chamber, then atmospheric pressure is gonna push into those lungs and push the body open, sort of bigger, fill up more space in that chamber. So that's gonna push air into the lungs. Alright. Now I just want to know this technology is noted, it sort of looks like the last technology of last century, and it definitely is. But, amazingly, in 2024, a man named Paul Alexander passed away after living for 70 years inside an iron lung after he was paralyzed with polio as a child. That left, as of 2024, one person in the United States who is still living in an iron lung since being paralyzed with polio in the 1950s. So technology is old, but it is proven.

Why does air rush into the lungs during an inhale or inspiration?

• A. Increase in atmospheric pressure.
• B. Diaphragm moves upward.
• C. Decrease in air pressure in the lungs.
• D. Contraction of abdominal muscles.

During expiration, which action contributes to the movement of air out of the lungs during eupnea, or quiet breathing?

• A. Contraction of intercostal muscles.
• B. Relaxation of the diaphragm.
• C. Contraction of the abdominal muscles.
• D. Expansion of the ribcage.

Pressure Changes During Ventilation

Now that we understand Boyle's law, we understand how changing the volume of the lungs is going to change the pressure, and that will force air in and out of the lungs as part of ventilation. But we previously defined some very specific pressures. So now we want to go through and look at these and see how they change as part of ventilation and put some values next to those definitions. Alright. So we're going to start off by saying that ventilation alters pressure gradients, but importantly, we're talking about pressure gradients with respect to atmospheric pressure. And remember, we have this sort of notation for writing atmospheric pressure, this capital P, and then in subscript that lowercase ATM there. Alright. So other pressures are going to change, but atmospheric pressure, the pressure in the air around us, is always going to be 760 millimeters of mercury, at least at sea level. At least it's not going to change in response to any physiological changes. It's going to be constant as you ventilate your lungs. Now the pressures that are going to change are going to be the intrapulmonary pressure and the intrapleural pressure. So remember, intrapulmonary pressure, that's that pressure in the lungs in the alveoli, and we have this shorthand for writing that that capital P, subscript, lowercase Pul there. Well, so in the lungs, as we change the volume, that intrapulmonary pressure, we can expect that to change as a result of Boyle's law. But remember, inside the lungs, we said that this is an open system. It's connected to the atmospheric air through our trachea, our respiratory tract, and that's naturally open. So yes, that pressure is going to change, but it's always going to equalize. When I say here, it equalizes to atmospheric pressure as air moves in and out of your lungs. Now our other pressure here is going to be intrapleural pressure. That's that pressure in the pleural cavity, that cavity surrounding the lungs, and we have this shorthand capital(lowercase Pip for writing that. Now remember that cavity is basically, we said, like a wet closed vacuum. So that means that this cannot equalize to atmospheric pressure. Alright. So now we're going to just follow these two pressures around sort of through one cycle of breathing, 1 inspiration and expiration. So we have this graphic here where we're going to do it. So you can see here on the top, we see inspiration. On the bottom, we see expiration, and we have graphics here showing the lungs. You can see during inspiration, we can see those lungs are getting bigger. During expiration, we see those lungs are getting smaller. And then we have 2 other places. We have between expiration and inspiration. So that's over here on the left here. You can see the lungs are really small there. That's after you breathed out, but before you started breathing in again as you're just sorta sitting there with sorta smaller lungs with not a lot of air in them. And then over here, we have between inspiration and expiration on. On the other side, this is sort of after you breathe in but before you started breathing out again as you're just sitting there for, you know, half a second with large lungs that are nice and filled there. Alright. So we're going to follow this around, and we're going to start over here on the left, again, after expiration but before the start of inspiration. So let's fill in some values. So atmospheric pressure is 760 millimeters of mercury. It's always going to be 760 millimeters of mercury, so that's easy. Alright. But what about the intrapulmonary pressure? Well, after expiration but before inspiration, this is when your lungs have time to equalize. So air is going to be in this case, it will have moved out of the lungs until that pressure is equal. So we're gonna say that this is with respect to atmospheric pressure, our intrapleminary pressure is going to be 0. It's gonna be equal to atmospheric pressure. But what about our intrapleural pressure? Remember, that's that pressure in this what we see here in purple, showing that pleural cavity around the lungs. Well, remember that intrapleural pressure we said is always negative, but it's negative because the lungs have recoil. They have that elastin protein pulling on the pleural cavity, and the more it pulls, the more negative that that intrapleural pressure is going to be. But here, we see the lungs are actually they're sorta they're smaller than they they are at other times during the cycle. They've gotten smaller, and so that elastin protein isn't stretched out as much. There's less recoil. So at this point, this value is going to be negative 4. Our intrapleural pressure is gonna be negative 4 millimeters of mercury. Alright. So now that we set that up, let's follow along. Well, now we breathe in. So we go through inspiration here. You can see in the drawing here, the lungs are getting bigger. We have those arrows, sorta, pointing out, showing the lungs are expanding. Well, atmospheric pressure doesn't change, but the volumes of the lungs are increasing, which means the intrapulmonary pressure must be going down. The intrapulmonary pressure is gonna go down as far as negative 2 millimeters of mercury with respect to atmospheric pressure. Now you'll see that sometimes, some texts say negative 1.5, some say negative 2, somewhere in that range there. Alright. What about intrapleural pressure? Well, as the lungs get bigger, they recoil more because you're stretching out that elastin protein more. So at the beginning of inspiration, we were at negative 4 millimeters of mercury, But by the end of inspiration, this is gonna get more negative as that elastin protein recoils more and more, and it's gonna reach as low as negative 6 millimeters of mercury. Alright. Well, let's keep on going. So now we've finished inspiration. That volume has gone up, but because the volume went up, the pressure went down, and this means that air was flowing into the lungs. So now we're over here. The lungs are at the biggest state, and air has flowed into the lungs. Well, the atmospheric pressure, that doesn't change 760. But what about that intrapulmonary pressure? Well, that air flowing into the lungs, that is what equalizes the pressure. So at their largest state here after you breathe in, but that half second before you breathe out again, this is gonna equalize to 0 millimeters of mercury, it's gonna be equal to that atmospheric pressure. Alright. But what about the intrapleural pressure? Well, the intrapleural pressure, remember, that's a function of how big the lungs are. Here, the lungs are large. They're filled with air. They're really stretched out. That elastin protein is really recoiling. It's really pulling on that pleura, the membrane there pulling on that pleural cavity, which means that this becomes the most negative it's gonna be. It's gonna go as low as negative 6 millimeters of mercury. Alright. We followed along again. Well, now we are at expiration. We are breathing out. You can see in our illustration here, expiration. The thoracic cavity is getting smaller. We see that these lungs are getting smaller. We have these arrows there. Oh. Atmospheric pressure hasn't changed, but if the volume of the lungs is going down, what's going to happen to the intrapulmonary pressure? The volume goes down, the pressure goes up, and it will go as high as positive 2. Again, some text will say positive 1.5, 1.5, 2, somewhere in that range there. Alright. The intrapulmonary pressure goes up to positive 2, but what about the intrapleural pressure? Well, as the lungs get smaller, there's less recoil. So it starts at that negative 6 when they're big, but as they get smaller, it becomes less negative, and it ends up at negative 4 there. Alright. We can finish our cycle around. Well, the pressure went up. That means that the air was being forced out of here. So when we make it all the way around, now we're back over here. The air gets forced out. That's when that pressure equalizes. So that's why this intrapulmonary pressure again is at 0 here. It's equalized to atmospheric pressure. And again, the lungs here are at their smallest state, so the intrapleural pressure is less negative. It's at negative 4 millimeters of mercury. Alright. Some values there. Hopefully, that makes sense. You followed along. We are going to practice it some more and an example and practice problems to follow. Give them a try, and I'll see you there.

Ventilation Example 2

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 p_subIP.

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 p_subPhR.

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, p_subPSVR. 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 p_subPIPs, 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.

At the end of inspiration, the intrapulmonary pressure is equal to ____________.

• A. Atmospheric pressure.
• B. Transpulmonary pressure.
• C. Intrapleural pressure.
• D. Both A & C are correct.

When the volume of air in the lungs is the greatest:

• A. The intrapulmonary pressure is equal to the atmospheric pressure.
• B. The intrapulmonary pressure is at its maximum.
• C. The intrapulmonary pressure is at its minimum.
• D. The intrapulmonary pressure is equal to the intrapleural pressure.

Muscles of Ventilation

We've been talking about how by changing the size of the thoracic cavity, we alter pressure gradients, and that forces air in and out of the lungs. But now we need to talk some anatomy and talk about how we actually change the size of that thoracic cavity, and that's going to be by using the muscles of ventilation. And so here, we're going to break those muscles of ventilation into 2 basic groups to start. We're going to talk about the primary respiratory muscles and the accessory respiratory muscles. So the primary respiratory muscles, these are going to be used during quiet breathing or what we call eupnea. And remember, quiet breathing or eupnea, that's that just sort of normal restful breathing where you use muscles for inspiration, but expiration is just the relaxation of muscles. And then accessory respiratory muscles, these are going to be recruited during forced breathing. Now for forced breathing, you're going to use both those primary respiratory muscles and the accessory respiratory muscles. But those accessory respiratory muscles are going to let you take those deeper, harder breaths in where you're using muscles for inspiration and muscular contraction for expiration. Alright. So let's take a look here. So we have 2 anatomical models, one for inspiration on the left, one for expiration on the right. Now for both of these right now, we just have the diaphragm drawn in. But as we go in, we'll be adding more muscles and highlighting them as we go. Alright. So we're going to start with inspiration, and we'll start with those primary respiratory muscles for inspiration. Well, the main one is that diaphragm. So I'm going to highlight that there in pink. And so for inspiration, that diaphragm contracts and it moves down. So you can see that diaphragm as it contracts, it pulls downwards, it sort of flattens out, straightens out, and that makes that space above the diaphragm in that thoracic cavity larger. Alright. Our other muscles that are doing work as part of the primary respiratory muscles that we'll draw in now, these are the external intercostals. The external intercostals, I'll highlight there in blue, and these are muscles that are linking between all your ribs. And you can see on the drawing, the muscle fibers actually connect the ribs and they slant sort of downwards and forwards. So as these contract, we're going to say the ribs move up and out. It's because your ribs are attached almost like hinges. So as they get sort of pulled up by these external intercostals, they sort of just hinge outwards a little bit, and that gives your chest sort of a rounder, more barrel-chested look when you breathe in. It's those intercostal muscles sort of making that rib cage bigger and rounder. And you can even see that on these diagrams, though it's subtle. You can see the one on the left. The rib cage looks a little bit more barrel-chested, a little bit rounder, and those ribs look a little just a little bit more parallel to the ground, whereas on a diagram on the right, they look like they're just pointing downwards just a little bit more. Alright. So that's the primary respiratory muscles, but what if you want to take a deeper breath? Well, we'll start by adding 2 muscles in here that are in the neck. We have the sternocleidomastoid, which I'm highlighting there in blue, and the scalenes, which I'm highlighting there in pink. So both these muscles are in the neck. This is your sternocleidomastoid here. It attaches to your clavicle. Your scalenes are on the side. They attach to the first and second ribs. And if you take a real deep breath, one thing that you'll notice if you're touching your neck is that your neck starts to flex. And what's happening there is that those muscles are pulling up on that rib cage because if you can lift the entire rib cage up while your diaphragm pulls down, that's going to create more room inside that thoracic cavity and it's going to let more air get in. Alright. We're going to have some more muscles that we'll layer on next here. We have the pectoralis minor and the serratus anterior. Pectoralis minor, I've highlighted there in pink. The serratus anterior, I've highlighted there in light blue. Now both these muscles attach to the scapula in your back, and they attach to the ribs in the front. So when these contract, they're going to pull back on that rib cage. And again, if you take a really deep breath, you'll probably feel yourself sort squeezing those shoulders back together. That's pulling that scapula back so that these muscles can lift up and pull that rib cage up and higher, and get more space inside that rib cage, just like that. Alright. Now as just a quick little memory tool for these, sometimes I would say that, for inspiration, we scale up the size of our rib cage. We scale up. And that word 'scale up', well, scale reminds me of the scalenes, but also those other muscles, sternocleidomastoid, scalenes, and serratus anterior. They all start with s's, so I remember my s muscles. And then the other one is our pectoralis minor there, and the p and up reminds me of that. So we scale up our rib cage. We lift it up to get as much space in there as we can. Alright. Well, what about expiration? Well, for expiration, our primary respiratory muscles, remember, these are just going to relax and allow recoil to force the air out. So our diaphragm, which we'll highlight there in pink, is going to relax, and then it is going to move up. Well, it moves up. Switch my pen back to blue here. It moves up, and you can see the shape of it. It's much more sort of rounded there, pushing up into the thoracic cavity, making less space inside that rib cage. Well, we can draw on our external intercostals here. These highlight in light blue. These are going to relax, and as they stop pulling up on those ribs, well, that means that the ribs then move down and they move in. They sort of just hinge backward down, making your rib cage sort of flatter again, making less space inside that thoracic cavity. Okay. Those muscles, though, those were just, remember, relaxing. Now let's talk about if you want to do forced breathing and really force air out with muscular contraction, we're going to use these accessory respiratory muscles. So I'm going to clear out these muscles and start anew so that we can see what we're looking at. And the first ones we're going to add on here are the internal intercostal muscles. Now just like these sound, these are just right underneath those external intercostal muscles, and I'll highlight them there in pink. So these are, again, connecting the ribs. But if you look closely, you'll see that the fibers are oriented in a different direction. They're connecting to the ribs, but they sort of go downwards and back. So when these contract, instead of hinging those ribs up and out, they hinge them down and in and flatten that rib cage even more. Alright. We also have a muscle here called the transversus thoracis, which I've highlighted in blue, and that's right underneath your sternum. And when that contracts, that pulls in and sort of collapses that rib cage, collapses those ribs in again, causing less space inside that thoracic cavity. And the final muscle we'll talk about here, I've drawn now, is that rectus abdominis, that major muscle of the abdomen. So we have this now highlighted in pink, and when that contracts, that squeezes inwards and that pushes on all the internal organs in your abdomen, and it sort of pushes them inwards, and they gotta go somewhere. And they just sort of push inwards and even up into that diaphragm, up into that thoracic cavity a little bit. So here, you're sort of collapsing your rib cage, bringing it downwards, and forcing organs sort of upwards into your thoracic cavity, forcing air out of your lungs. Right? So we can do the whole thing. Right? We'll take a deep breath in. You'll feel that rib cage get really big. You'll feel yourself lift up and pull back on it. And then through expiration, you'll feel yourself pulling down, collapsing inwards, pushing in on those organs so they go up into your thoracic cavity, forcing air out. So breathe in, breathe out. Alright. So those are the muscles of inspiration and the muscles of expiration. We'll practice some more and practice problems to come. I'll see you there.

Ventilation Example 3

As we look at this example here, it says to fill in the table below indicating which muscles you would expect to contract or relax during inspiration and expiration for both eupnea, that quiet breathing, and forced breathing. In each cell, write a "c" if you expect that muscle to be contracting, and write an "r" if you expect that muscle to be relaxed. Alright. As we look here, we have sort of 2 large columns, 1 for eupnea or quiet breathing and one for forced breathing. And in each of these large columns, we have 2 smaller columns, 1 for inspiration and one for expiration.

We look on the left here. We have this big list of muscles that we've been over, so let's see if we can figure out what's going on. Alright. So for eupnea, we'll start there, that quiet breathing, inspiration. Which muscles do you expect to be contracting during inspiration? Alright. We said that that was the role for the primary respiratory muscles, and that was the diaphragm and the external intercostals. The diaphragm pulls down, sort of expanding the space inside the rib cage, and those external intercostals pull kind of up on all of the ribs sort of making you pulling those ribs sort of hinging them up and outwards making the size of that rib cage bigger. So these 2, I expect to be contracting. Now all the rest of these muscles here, these are our accessory respiratory muscles, and we said that they don't do anything during eupnea. So I expect all of these to be relaxed. I'll put an "r" next to all of them, at least in terms of breathing.

Now, what about during expiration? Well, expiration during eupnea, we said that that is just the relaxation of muscles. So, I don't expect anything to be contracting here. So here, I'm just going to put an "r" and I'm just going to put a down arrow indicating that I expect all of these muscles to be relaxed during expiration, during eupnea. Alright. What about forced breathing? Alright. Forced breathing, we said that's when we're going to pull in these accessory muscles.

Let's go through it. We're going to start with inspiration. Which muscles do you expect to be contracting during inspiration? Well, I still expect that diaphragm and those external intercostals to be contracting. Those are always going to be working during breathing. But now we're going to bring in some of those accessory respiratory muscles. Remember, we want to sort of lift the rib cage up, lift it up and back, and those muscles are just sort of trying to bring that rib cage up as much as you can. So when that diaphragm pulls down, you just get as much space in there as you can. So I sort of had a little memory tool for that. I said we scale up the size of the rib cage. And so I remember the scalenes and those other "s" muscles and the up reminds me that one of them starts with a "p."

As I go down here, well, that means that the sternocleidomastoid and the scalenes, those are both sort of lifting up on that rib cage basically from your neck. And then the pectoralis minor and the serratus anterior, those, remember, those are attached to your scapula, and so they're sort of pulling upwards and back on that rib cage, lifting your rib cage up as much as you can. Alright. That means that these other muscles here, transversus thoracis, internal intercostals, and rectus abdominis, well, those aren't going to be doing anything yet. These are going to be relaxed.

Now, what about expiration? Well, for expiration, I still expect that diaphragm and external intercostals to be relaxed. Those muscles are activated during inspiration, so they need to be relaxed during expiration. Now these muscles that were used during forced breathing during inspiration, well, now these are going to have to relax so that we're not pulling up on that rib cage anymore. And now we're going to use these muscles that sort of pull down and in on the rib cage. So, we have the transversus thoracis. Remember, that's that muscle sort of underneath the sternum there that sort of collapses your chest. So that is going to be contracting.

We have the internal intercostals, those muscles between the ribs underneath the external intercostals and running in the opposite direction. So instead of sort of lifting up and hinging those ribs outwards, they pull down and hinge them inwards. Those are going to be contracting. And then finally, the rectus abdominis, that major abdominal muscle is going to be squeezing in, pushing all your organs inwards and sort of pushing up on that diaphragm even more. So that will be contracting as well. Alright. Those are your muscles. That's how we ventilate. More problems after this. I'll see you there.

Which muscle is used for inspiration during both eupnea and forced breathing?

• A. Rectus abdominus.
• B. Sternocleidomastoid.
• C. Serratus anterior.
• D. External intercostal muscles.

Which muscle is likely to be contracting while blowing up a balloon?

• A. Rectus abdominis.
• B. Sternocleidomastoid.
• C. Diaphragm.
• D. Scalenes.

Here’s what students ask on this topic:

What is Boyle's Law and how does it relate to ventilation?

Boyle's Law states that the pressure of a gas is inversely proportional to its volume, given a constant temperature. Mathematically, it is expressed as:

$P_1\times V_1=P_2\times V_2$

In the context of ventilation, this law explains how changes in the volume of the thoracic cavity alter intrapulmonary pressure, facilitating air movement. During inspiration, the diaphragm contracts, increasing thoracic volume and decreasing pressure, allowing air to flow into the lungs. During expiration, the diaphragm relaxes, decreasing thoracic volume and increasing pressure, forcing air out.

What are the primary and accessory respiratory muscles involved in ventilation?

The primary respiratory muscles are used during quiet breathing (eupnea) and include the diaphragm and external intercostal muscles. The diaphragm contracts and moves downward, increasing thoracic volume, while the external intercostals lift the ribs up and out. Accessory respiratory muscles are recruited during forced breathing and include the sternocleidomastoid, scalenes, pectoralis minor, serratus anterior, internal intercostals, transversus thoracis, and rectus abdominis. These muscles help to further expand or contract the thoracic cavity, allowing for deeper or more forceful breaths.

How do intrapulmonary and intrapleural pressures change during the breathing cycle?

Intrapulmonary pressure (P_pul) changes with lung volume. During inspiration, thoracic volume increases, causing P_pul to drop below atmospheric pressure, allowing air to flow in. During expiration, thoracic volume decreases, causing P_pul to rise above atmospheric pressure, forcing air out. Intrapleural pressure (P_ip) is always negative relative to atmospheric pressure due to the elastic recoil of the lungs. It becomes more negative during inspiration (around -6 mmHg) as the lungs expand and less negative during expiration (around -4 mmHg) as the lungs recoil.

What is the role of the diaphragm in ventilation?

The diaphragm is the primary muscle involved in ventilation. During inspiration, it contracts and moves downward, increasing the volume of the thoracic cavity and decreasing intrapulmonary pressure, which allows air to flow into the lungs. During expiration, the diaphragm relaxes and moves upward, decreasing the volume of the thoracic cavity and increasing intrapulmonary pressure, which forces air out of the lungs. This cyclical movement of the diaphragm is essential for effective breathing.

How does atmospheric pressure influence ventilation?

Atmospheric pressure (P_atm) is the pressure exerted by the air surrounding us, typically 760 mmHg at sea level. It serves as a reference point for other pressures involved in ventilation. During inspiration, intrapulmonary pressure drops below P_atm, creating a pressure gradient that allows air to flow into the lungs. During expiration, intrapulmonary pressure rises above P_atm, creating a pressure gradient that forces air out of the lungs. Thus, P_atm is crucial for the pressure gradients that drive ventilation.