Pressure in the Lungs and Pleural Cavity: Study with Video Lessons, Practice Problems & Examples

The lungs are elastic organs that rely on a balance of inward and outward forces to remain open. Inward forces include elastic recoil and surface tension, which can lead to lung collapse, a condition known as atelectasis. Surfactant reduces surface tension, preventing this collapse. Outward forces are primarily due to intrapleural pressure, which is negative and created by the pleural cavity's fluid. This pressure keeps the lungs inflated as long as it remains less than intrapulmonary pressure, which equals atmospheric pressure during normal breathing.

Forces That Shape the Lungs

As we introduced ventilation, we said that lungs are these elastic passive organs that change size in response to the changing size of the thoracic cavity. But that leaves us with this question: if the lungs are passive and elastic, how do they stay open? Why don't they just collapse? And we're going to say here that there are inward and outward forces that balance to keep the lungs open, because the lungs' natural tendency is going to be to collapse. You've almost certainly heard of somebody having a collapsed lung. That's something that can happen if these forces are not balanced. So, we're going to say, first, there are two types of forces. There are inward and outward forces that we want to balance here. So, we'll start with these inward forces. So, again, the lungs' natural tendency is to collapse. This elastic thing is going to try to get smaller, and that's for two reasons. First, well, elasticity and recoil. So in the lungs, there's a lot of collagen. There's a lot of connective tissue. Collagen is that protein that's strong like a rope, but also importantly, this protein elastin. Remember, elastin is this protein that's like a rubber band, and so a rubber band is always going to be trying to shrink down to its smallest size. Now, there's also going to be surface tension. Now, inside the alveoli, the surface of those alveoli is going to be covered in fluid, and so there's surface tension in there. Surface tension is just sort of that natural tendency of liquid water to stick to itself. And so all the edges of those alveoli are sort of naturally going to want to stick to each other because of that liquid. That will lead to the lungs collapsing. So, we're going to say here that the surface tension pulls the alveoli together or closed, you can say. Now we just do want to note that this is reduced by something called surfactant. Surfactant is some proteins that are put into that fluid by the body, and it sort of breaks up the surface tension a little bit sort of like soap in water. Now, it doesn't break up all the way though, importantly, but it does reduce it a little bit. Okay. So that's our inward forces. Our lungs are naturally trying to collapse, but we also have these outward forces, and this keeps the lungs from collapsing. And the main thing that we're going to talk about here is what we call intrapleural pressure. Alright. Intrapleural pressure all has to do with the pleural cavity. So, we're going way back to remember when we talked about the serosa, the linings of the organs, the lining of our body cavities? A pleural cavity is this sort of wet vacuum-sealed bag that surrounds the lungs. So, we can see here in orange, we're sort of representing the size of the thoracic cavity. And then in purple here, we have what are the pleural cavities, the pleura, the membranes that surround these lungs. So, remember, in these pleural cavities, the pleura, there's sort of two sides to them. There's the visceral pleura, which is attached to the lung. Remember that word viscera? That sort of means the body's organs. So that visceral side is sort of right up there, sort of superglued to the lung, and the parietal pleura, we're going to say here, is going to be attached to the chest wall. So, we have these two linings of the membranes, both sort of super glued, one to the lung and one to the chest wall, and between them, there's going to be a fluid. Just a little bit of fluid that creates adhesion. So much like that surface tension. Right? This water is sticking together, and it's going to stick this between this visceral and parietal pleura. Alright. So, to model this, I'm going to I'm going to do a little model here. I'm going to put a glove on this hand, and you can see this glove says chest on it. So, this represents my chest wall. I'm going to put a glove on my other hand,

Pressure in the Lungs and Pleural Cavity Example 1

This example asks for each factor listed below. Write a c if it would contribute to the collapse of the lung. Write an r if it would contribute to resisting the lungs' tendency to collapse. So remember, we said the lung's natural tendency is to get smaller and to collapse. There are some forces that sort of add to that, and there are other forces that resist that. And we had this list here, and we've got to figure out which is which. So let's go through them one by one. Our first one says surface tension in the alveoli. So what do you think? Does that lead to the lung's tendency to collapse, or does it resist that tendency? Remember, surface tension in the alveoli inside the lungs, it's wet. It's covered with this fluid, mostly water, and water has this property that it naturally sort of just sticks to itself. So inside the alveoli, those walls of the alveoli are going to be attracted to each other by that water, and that water is going to, kind of, want to stick to itself, and that would lead to the collapse of the lung. So that surface tension in the alveoli is contributing to that lung's tendency to collapse. So I'm going to put a big old c on that line. Next, we have here intrapleural pressure. Way to think about that one. Remember, intrapleural pressure, that's the pressure inside the pleural cavity. And we have this diagram here. In purple here, we see the pleura, and between those pleural membranes, there is that pleural cavity surrounding the lungs. And remember, in there, we said that there is a negative pressure, and that's because we have those two sides, the pleura, the visceral pleura, and the parietal pleura, and they're right up next again right up against each other. And the lungs are sort of trying to pull them apart, but they can't get away from each other because there's really nothing in there to fill up that space. That creates that negative pressure which keeps the lungs from collapsing. So I'm going to put out here a big old r. That negative intrapleural pressure resists the lungs' tendency to collapse. Next up, we have the pleural fluid. What do you think about that one? Well, remember the pleural fluid, that's what's in the pleural cavity. That's the liquid that's in there, and there's just a little bit, and that works to keep those two layers of the pleural membrane, the pleura, stuck together. Right? Kind of like that first one, the surface tension. Right? The liquid sort of naturally sticks to itself, and so that's another thing that keeps those 2 those that membrane stuck to itself and keeps the lungs from pulling apart and pulling that membrane away from the chest wall. So that is going to resist the lungs tendency to collapse. Next up, we have elasticity recoil of the lungs. What do you think about that one? Well, we said the lungs, they have a lot of elastin protein in them. Elastin is that protein, kind of like a rubber band, and the lungs are sort of naturally stretched bigger than they want to be. And so that rubber band protein in there, that elastin, is sort of always recoiling, trying to get smaller again. And so that is going to be the major factor adding into this, the collapse of this lung, the tendency for the lungs to collapse. Finally, we have pulmonary surfactant. What do you think about that? Well, remember, pulmonary surfactant is put into the pulmonary fluid because there is that surface tension in the alveoli. Our first one here, that surface tension in the alveoli, is reduced by pulmonary surfactant. Surfactant kind of works like soap and it breaks up that surface tension, not completely but a little bit, and so it just helps resist that tendency to collapse. So put a big old r on that line. With that, remember we have these balancing forces, and that's why your lungs don't collapse even though that is their natural tendency.

More practice after this. We'll see you there.

What factor is most important in keeping the lungs from collapsing?

• A. a)   Collagen.
• B. Intrapleural pressure.
• C. Elastin.
• D. Fluid lining the alveoli.

Individuals with COPD (chronic obstructive pulmonary disorder) have degraded elastin protein, causing their lungs to have less recoil. How would this affect intrapleural pressure and why?

• A. Intrapleural pressure would be more negative because less recoil would lead to less inward pull on the pleura.
• B. Intrapleural pressure would be less negative because less recoil would lead to less inward pull on the pleura.
• C. Intrapleural pressure would become positive because less recoil would lead to an outwards push on the pleura.
• D. Intrapleural pressure would become positive because less recoil would lead to less inward pull on the pleura.

Formalizing Pressure Relationships in the Lungs

We previously introduced the idea of these pressures in the lungs, and specifically, we talked about intrapleural pressure, that negative pressure that keeps the lungs from collapsing. Well, now we want to talk about a couple more pressures, and we want to get into some nitty gritties. We want to add some vocabulary, some notation, and add some numbers to this. So we're going to start off just by saying that there are 3 distinct pressures that we want to look at when we're talking about the lung. Those are atmospheric pressure, intrapulmonary pressure, and intrapleural pressure. So we're going to go through these one by one, see how they relate to each other. Let's start with atmospheric. So atmospheric pressure and for all of these, we're going to have this notation where we can write it in shorthand. We're going to write a capital P for pressure, and then we have something in subscript that we can write. And for atmospheric pressure, it's a little ATM. So this PATM, that means atmospheric pressure. This is just the amount of pressure in the air. Now you're probably familiar here on Earth, we live in a pressurized environment. There's a lot of air pressure just pushing in on us at all times. Well, we're going to say that that PATM, that's going to be equal to 760 millimeters of mercury at sea level. Now that 760, that can vary some, and it varies some with elevation, especially. But we're always going to use this 760 millimeters of mercury, the air pressure at sea level, as, sort of, our standard physiological measure. So that's going to be our default. That also means, though, that the other pressures that we're going to talk about, they're all going to be compared to that atmospheric pressure. So as we go through these other pressures, our intrapulmonary pressure and our intrapleural pressure, we're not going to use numbers in the 100. We're just going to use numbers either plus or minus compared to 760. So if it's equal to atmospheric pressure, we'll write a 0. And if it's more than, we'll just write plus something. And if it's less than, we'll write minus how much it is. Alright. So to see how that works, let's talk about these other pressures. So the first one that we're talking about here is intrapulmonary pressure. Alright. Intrapulmonary intra means sort of within, and pulmonary means the lungs. So this is the pressure in the lungs, and our subscript for this is this capital P, and we have a subscript PPUL there for that pulmonary. Alright. So we have this diagram here reminding us of our lungs, and we also see the pleural cavities around there. This arrow, you can see, is going to the inside of the lungs. We're talking about in the lungs, the pressure in the alveoli there. So we're going to say here that this intrapulmonary pressure is going to be equal to atmospheric pressure between inspiration and expiration. So at the end of a breath out, at the end of a breath in, your for the pressure in the alveoli should be equal to atmospheric pressure, and that is because it's in, what we said, open system. Right? So in this drawing here, right, we have the trachea. This just goes up into your respiratory tract. That's by default open. So as the lungs change size, air just moves in and out, and it equalizes to atmospheric pressure. So the difference here from atmospheric pressure, how we write this out, we're going to say sort of by default, it's 0. So this should be equal to atmospheric pressure. Now as the lungs change size, we're going to go into a lot of detail coming up, that pressure does change a little bit. It goes up or down, so plus or minus, by about 2 millimeters of mercury. Some text say plus or minus 1.5, somewhere in that range. Alright. So intrapulmonary pressure, by default, equal to atmospheric pressure, but it does go up and down just a little bit around that zero one. Alright. Our next pressure here is intrapleural pressure, and intrapleural intra within, plural within that plural cavity. And our, notation for this is the capital P, and then in subscript, lowercase PIP. This is that pressure in the plural cavity. And we're going to say here that that pressure is going to be less than intrapulmonary pressure. So intrapleural pressure is always less than intrapulmonary pressure. And in our diagram here, we can remember that intrapleural pressure is in this pleural cavity here, in that purple part here surrounding each lung. And that is less than intrapulmonary pressure because of that negative pressure that we talked about that's created by resisting the recoil of the lungs. So remember, those lungs are elastic. They're naturally recoiling, sort of shrinking down smaller, but you have that sort of like vacuum sealed bag stuck between the lung and the chest wall. And as that gets pulled apart, it creates that little bit of suction, which creates that negative pressure and resists that recoil. So for the values here, we're going to say the difference from atmospheric pressure here, it's going to be in the range of negative 4 to negative 6 millimeters of mercury. K? Again, always negative. Now importantly, we're going to say here as long as that intrapulmonary pressure is greater than the intrapleural pressure, the lungs are going to stay inflated with air, and that's important. You don't want your lungs to collapse. If these two numbers are ever equal to each other, that means that the recoil can sort of win out in that relationship, and the lungs will start to shrink down, and the space in that intrapleural cavity will become greater. And as that happens, that creates less space in your lungs, and, well, you can't ventilate your lungs properly, and that's bad. We're going to look at an example of how that happens. Come on up. You should check it out. See you there.

Pressure in the Lungs and Pleural Cavity Example 2

This example tells us that pneumothorax is a pathology caused by air entering the pleural cavity. For example, if air entered a hole in the chest wall through a stab wound. The air in the pleural cavity results in the collapse of a lung. For a patient experiencing pneumothorax, how would you expect the pressure in each location to change? For each pressure, write increase, decrease, or no change on the lines below. And here we want to analyze for atmospheric pressure, intrapulmonary pressure, and intrapleural pressure. Alright. So we have a quick little diagram here. We see what looks like sort of your normal lungs, but, uh-oh, we have a stab wound coming in here. It looks like a shard of glass or something coming in through the chest wall, and importantly, breaking that pleural membrane, the pleura there, stabbing into that. Well, then in our next picture, we see here that the lung is collapsed. We can see this sort of hole in the pleura there, and this pleural cavity, the space in

True or false: if false choose the answer that best corrects the statement.

In healthy lungs, the intrapulmonary pressure is always lower than the intrapleural pressure.

• A. True.
• B. False, the intrapulmonary pressure is only lower during some parts of ventilation.
• C. False, the intrapulmonary pressure is always greater than the intrapleural pressure.
• D. False, the intrapulmonary and intrapleural must be the same for ventilation to occur.

Pleural effusion is a condition where fluid accumulates in the pleural cavity. Which pressure or pressures would you expect to be most directly impacted by this fluid accumulation?

• A. Intrapleural pressure.
• B. Intrapulmonary pressure.
• C. Both A & B are correct.
• D. Neither pressure would be impacted.

Here’s what students ask on this topic:

What is intrapleural pressure and why is it important for lung function?

Intrapleural pressure is the pressure within the pleural cavity, the space between the visceral and parietal pleura surrounding the lungs. This pressure is typically negative, meaning it is lower than atmospheric pressure. The negative intrapleural pressure is crucial because it counteracts the lungs' natural tendency to collapse due to elastic recoil and surface tension. By maintaining a negative pressure, the pleural cavity creates a suction effect that keeps the lungs inflated and allows for proper ventilation. If intrapleural pressure becomes equal to or greater than intrapulmonary pressure, the lungs can collapse, leading to conditions like pneumothorax.

How does surfactant reduce surface tension in the alveoli?

Surfactant is a substance composed of lipids and proteins that is secreted by the cells in the alveoli. It reduces surface tension by disrupting the cohesive forces between water molecules on the alveolar surface. This action prevents the alveoli from collapsing and makes it easier for them to expand during inhalation. By reducing surface tension, surfactant decreases the work of breathing and helps maintain stable alveolar structures, which is essential for efficient gas exchange in the lungs.

What are the differences between intrapulmonary pressure and intrapleural pressure?

Intrapulmonary pressure, also known as alveolar pressure, is the pressure within the alveoli of the lungs. It fluctuates with breathing, being equal to atmospheric pressure (0 mmHg) at the end of inspiration and expiration, and slightly positive or negative during the breathing cycle. In contrast, intrapleural pressure is the pressure within the pleural cavity and is always negative, typically ranging from -4 to -6 mmHg. This negative pressure is essential for keeping the lungs inflated by counteracting the inward forces of elastic recoil and surface tension.

What happens during a pneumothorax and how does it affect lung pressure?

A pneumothorax occurs when air enters the pleural cavity, disrupting the negative intrapleural pressure. This can happen due to trauma, lung disease, or spontaneously. When air enters the pleural space, the pressure becomes equal to or greater than the intrapulmonary pressure, eliminating the suction effect that keeps the lungs inflated. As a result, the affected lung collapses, leading to impaired breathing and reduced oxygenation. Treatment often involves removing the air from the pleural space to restore negative pressure and re-expand the lung.

How do atmospheric pressure and intrapulmonary pressure relate during normal breathing?

During normal breathing, intrapulmonary pressure (pressure within the alveoli) fluctuates relative to atmospheric pressure. At the end of inspiration and expiration, intrapulmonary pressure equals atmospheric pressure (0 mmHg). During inhalation, the thoracic cavity expands, causing intrapulmonary pressure to drop below atmospheric pressure, allowing air to flow into the lungs. During exhalation, the thoracic cavity contracts, increasing intrapulmonary pressure above atmospheric pressure, pushing air out of the lungs. These pressure changes facilitate the movement of air in and out of the lungs, enabling gas exchange.