Hello, everyone. We're here to continue our discussion of respiratory physiology. And in this mini-lecture, we're going to jump into a major concept associated with respiratory physiology, lung compliance.
Lung compliance. This lung compliance is, here in this general kind of explanation, is the ability of the lungs and the tissues around the lungs. to be able to stretch and expand, to go along with that, to contract as well. So in talking about the lungs, we're talking about the walls of the alveoli, all of the alveoli, in some cases the respiratory bronchioles, they have a little bit of stretch and give associated with them, and connective tissue that will be surrounding the lungs and be a part of the chest wall. Now, we could also think of tissues around the lungs as being the ribs and the musculature associated with the chest wall.
All of these have to have pliability, the ability to stretch and contract. Let's jump into a little more detail associated with this. I gave you a general definition of lung compliance.
The one that I have up here at the top is a little more specific, kind of dealing with what we're going to be jumping into here. That lung compliance is the extent the lungs can expand for every unit of what is called transpulmonary pressure. Transpulmonary pressure. And that transpulmonary pressure is the difference in pressure from inside of the opioid compared to outside of the alveoli in the tissues themselves. To get started on this discussion, I have to give you an idea of the lungs and how they're sitting in our body.
Our lungs, well, in their development, the chest wall was actually there. The chest wall and the chest cavity were actually there before the lungs developed. The lungs actually as they develop, kind of push their way into the chest cavity itself. How does that happen? Well, in this image here, if you look at the top of this image, I don't know if any of you remember playing with balloons, either as a kid or at parties recently or what have you.
And if you've ever pushed your hand into the balloon, don't pop the balloon, but just kind of push your hand into it, you can, in some cases, depending on how thin the... the plastic is that's creating the balloon, you can see your hand kind of pushing into it. Part of the balloon is sitting right on top of your hand, just like in this image here.
We're not popping it, so you still have the air spaced component in the middle, and then you have a part of the balloon that is not touching the hand and away from it. And as you push further and further into the balloon, you have this kind of formation that is occurring. You have your actual hand, again, the part of the balloon that is actually sitting on your hand, and then part of the balloon that's sitting away with a space in between, in this case an air space component. Our lungs have developed in very much the same way. I mentioned the chest wall actually was developed or partially developed way before the lungs actually developed.
And as they develop, they bud. away from the bronchi, the developing bronchi and trachea, and kind of burgeon out, kind of flower out, expanding into the chest cavities. As they do this, the inside of the chest cavity already had a connective tissue coating on the inside.
As the lungs push their way into the chest cavity, they don't break that connective tissue coating. In fact, part of the connective tissue coating sits right on top of the lung tissue itself. There is a space in between that coating that sits right on the lungs and the remainder of that coating that is part of the chest wall itself.
Each one of these pieces of tissue and spaces has a name. So if you look over here to your right, you can see sitting right on top of the lung tissue . This part of the connective tissue that was part of the chest wall is called the visceral pleura.
The visceral pleura. The part of that connective tissue that is remaining on the outer wall, here you can see here, of the chest wall, is called the parietal pleura. The parietal pleura.
In between, there's an intrapleural space that has intrapleural. fluid. Now it's not a lot of fluid. It's mainly a water kind of cirrus-based fluid, but it's a very important element of this space that is sitting here between the lungs and the actual chest wall. Everybody have that?
Now that we have this kind of basic anatomy here, we need to kind of put some of the terms together that I mentioned above, like the transpulmonary pressure, alveolar pressure, and pleural pressure. So here on our left, we have another image looking at a diagram of the lungs. We have the lung tissue here on the inside in kind of a purplish color. Sitting right on the surface of the lung, we will have the visceral pleura, the remaining connective tissue sitting against the wall of the chest, the parietal pleura, and of course the pleural cavity in this light blue color. Now, what's sitting in that area?
Remember the intrapleural fluid? There is a small amount of intrapleural fluid sitting in that pleural cavity. Now, that's very important, as I mentioned prior. It's important because that fluid, that water, is generating surface tension.
And we're going to spend a little more time with this here in a few moments. Surface tension, again, is the attraction of water molecules to itself. And because we have this wall here with this connective tissue and water molecules kind of attached to it, and we have the... Visceral pleura, the connective tissue here on the lungs, it has water associated with it.
The water molecules from both sets of pleura are attracting each other, are attracting each other. To go along with it, because this pleural cavity is a closed area, no air or extra fluid can get into or out of this area. So it's a trapped area, trapped spot. That's pretty important to us because you'll see pressures inside of that pleural cavity will play a role on the pressures inside of the lung.
In fact, let's look at that right now. If you look over on the other side of this diagram, it's trying to show you intraalveolar pressure. The pressure from gases inside of the alveoli that make up the lung tissue can go...
anywhere from 760 millimeters of mercury pressure to zero to zero. If it's zero, you can imagine air would come rushing into this area here because the gas pressure outside of the body is much higher. Now we're going to look over here. The pressure in the pleural cavity, the intrapleural pressure sits very consistently.
at 756 millimeters of mercury in the chest's relaxed state. So 756 millimeters of mercury here. Here, if we have air, gases sitting in the lungs, we're going to have a pressure sitting of about 760 millimeters of mercury. So you can see there's a slight difference between the two.
That slight difference is what's called the transpulmonary pressure. Here, 760 inside of the lungs, 756 outside in the pleural cavity. The difference between the two, about four millimeters of mercury of pressure. About four millimeters of mercury of pressure.
Little bit of difference there. Now that difference between the two, the pleural cavity has less pressure than what's sitting here. inside of the alveoli.
Inside of the alveoli. That's an important concept for us. In the image down at the bottom here, Here in purple, lung tissue or the alveoli. Here in kind of this turquoise type color sitting out here is the pleural cavity with its intrapleural fluid.
And as you can see labeled here on the outer component here is the chest wall and the parietal pleura. The parietal pleura. Remember, we talked about inside.
compared to inside of the alveoli, inside of the alveoli compared to the intrapleural area, that there's a 4 millimeter of mercury difference. That 4 millimeters of mercury difference, well, remember, out in the pleural cavity, it's less 4 millimeters of mercury than inside, inside the pleural cavity. That's going to help attract the visceral pleura, the wall of the alveoli, outward. It's kind of a negative pressure.
It's sucking the alveoli, pulling it out, pulling it towards the chest wall. Pulling it towards the chest wall. So there's kind of a negative pressure sitting in that pleural cavity space. If the chest wall gets pulled outward further, that's going to make this pleural cavity or this intrapleural pressure become even more negative. Why?
Because we're creating more space. No air can move in there. So that means we're literally kind of creating a vacuum, which means if this gets lower in pressure, even to a negative six, that's going to pull on the alveolar wall even more, pull it out, creating a larger space inside of the alveoli, meaning it's going to have a negative space. That's going to attract air or gases into it. That's how we breathe.
We are literally a human vacuum cleaner. We create negative pressures, sucking air into our body. We do that by expanding the thoracic cage, the chest wall, and using the diaphragm to lower the bottom of the actual chest cavity. Does that make sense, folks?
In this image, showing you a lot of numbers here very quickly, this first image in the middle that I want you to look at was the one we were just looking at prior. Under normal situations and rest, we have a normal negative pressure in the pleural cavity that's pulling on the alveoli and creating a negative pressure inside, allowing air to fill, allowing air to fill at a normal rate, at a normal rate. If we expand the chest wall, we pull the chest wall out using those muscles that we mentioned in our last mini lecture, creates a greater negative pressure in the cavity, in the pleural cavity, pulling on the alveolar wall, pulling out the alveoli, stretching it out, creating an even greater negative pressure inside, sucking more air in. If the muscles... of the chest wall relax, they will recoil, push back in.
That will make this less negative, actually forcing, pushing on the alveolar wall, squeezing it, pushing air out, pushing air out. Everybody got that? Let's look at one more image here, down below.
Let's look at this on a more macro level. Again, in A resting state, our lung tissue, which would be filled with alveoli. You can see the black line that's forming the outline of the lungs. That would be your visceral pleura. The black line that's forming the wall of the chest itself, that would be the parietal pleura.
And the space in between, the pleural space with its fluid, helping to create this negative pressure. an attraction between the parietal pleura and the visceral pleura. If the chest wall gets pulled outward by way of the musculature, again, that we've talked about before, and we have the diaphragm contracting, which means it will flatten out, pulling the chest wall down towards it, that will create a greater negative pressure in that pleural space. Attracting or pulling the lung tissue with it, we expand and make the lungs larger.
That will create a greater negative pressure inside, pulling air into our body, pulling the gases that we need, oxygen, nitrogen, into our body, into the lungs. We relax those muscles so that there is no more contraction of the chest wall. The chest wall itself, because of the connective tissue and the way the joints are made up from the ribs itself, has a natural recoil. It will start to squeeze on its own without any muscle contraction having to take place. The diaphragm, relaxing, pushes automatically back to its original spot, its original space.
It gets a little help from the liver sitting underneath it. That will squeeze the lungs itself. That will create a positive pressure.
Pushing air out. Pushing air out. Now, this is all very important, folks. You have heard of what folks have called a deflated lung.
You've heard of that before, and there are many different terms for a deflated lung. What that means is that somehow, some way, the individual has been injured, and that means the outer parietal pleura, this outer component of the chest wall, has been punctured. As it punctures, that means air and the pressure that is sitting in the pleural space can equalize with pressure in the outside world. That means no negative pressure.
No negative pressure, no pulling on the alveoli. No pulling on the alveoli, they will shrivel up. They will shrivel. They literally will contract in on themselves.
You can't take air into the lungs. You can't take air into the lungs. So you've heard of a punctured lung.
That's what we're worried about. If we break... the seal on that pleural space, that cavity, and it has a chance to equalize with pressure outside of the body, we're in trouble. We are not going to be able to pull air into our lungs itself. Well, you will hear in future mini lectures about what are called compliance curves, how flexible our tissue is associated with the lungs.
And so that elasticity of the lungs is really vital to us. If we damage the chest wall and the connective tissue associated with the chest wall, or we damage the lung tissue to a point that it has difficulty repairing itself, and so It will put band-aids on some of the holes that have been created, usually with connective tissue. Connective tissue is not very stretchy, which means the more connective tissue on the actual lung tissue, the less elasticity.
This is what happens in smoking. That as you take in those very, very abrasive carbon particles into the lung tissue, it will damage the alveoli. During early smoking, your lungs may be able to handle some damage and repair itself easily.
But as you continue the smoking and the damage occurs over and over again, the lung tissue will not be able to keep up repairing itself. And so it has to go to drastic measures. It seals itself with connective tissue.
Literally. It's like putting paper mache on the top of a balloon. Once that paper mache material dries, the balloon can't expand or contract anymore.
In fact, you can pop the balloon and you'll keep the shape because of the paper mache keeping that constant shape that the balloon was in prior. With smoking, if that's the case, then that means you can't expand or contract. the lungs to be able to draw air in.
So the elasticity is really important here folks, the elasticity of the lung tissue. Now this elasticity also is going to be affected by how much water is actually sitting in the lung tissue because those fluids will create surface tension, surface tension. And again this surface tension has to do with water.
and how water molecules attract themselves to each other. It's the hydrogen bonds. And so literally, they will pull from every direction.
Water molecules love to kind of link to each other, and they do. And they'll pull the walls of the alveoli together. If we don't reduce this surface tension or reduce the amount of fluid that is sitting in that tissue, the alveoli will literally slap together.
closed down on themselves and there's no way to pull them apart. We have to be really careful with that. So how does our body deal with this?
Because we do need fluids there for both that pleural cavity and because the air is being humidified, remember, from our nasal cavity. So how do we deal with the water that may be sitting in the alveoli? We deal with it by secreting a particular substance called surfactant. Surfactant is secreted from those type 2 cells, and as surfactant is secreted, it will break up. the water molecules, it will inhibit them being attracted to each other.
As an alveoli gathers more air or more air comes into an alveoli and it stretches out, the surfactant molecules get pulled apart and that means in between, water molecules can kind of accumulate and attract each other. That will put pressure on the alveolar wall to kind of keep its shape. It won't be able to be pulled out much further. So this will prevent the alveoli from expanding too far and exploding.
If we push air out of the lungs, this is where the real danger comes. We don't want water molecules to gather too tightly because they will pull the walls in ever closely and then it'll cause the actual alveoli to collapse. As the surfactant molecules get closer and closer together, they prevent water molecules from linking to each other and attaching to the walls.
It prevents the actual alveoli from collapsing on itself. So, surfactant is really very, very important to us. Now, this surfactant, this surfactant, again, is secreted by what are called type 2 cells in the alveoli, in the alveoli.
Surfactant is made up mainly of phospholipids, fats, proteins, and particular ions. In fact, the main component of surfactant is something called phospholipid dipalmitoyle lecithin. I know, funny name you've probably never heard of, except this term here in the middle, dipalmitoyle. Do any of you... Know of a dishwashing soap called Palmolive dishwashing soap?
Palmolive dishwashing soap is made of phospholipid dipalmitoyl lecithin. Its job? To break down water molecules so that they don't attract each other. That's what keeps the food or materials stuck to your plate.
So... This substance is used in soaps to help break down the surface tension. That way, the material on the plates or the glasses or whatever that you're watching come right off.
Pretty amazing kind of stuff. Surfactant is also the substance that during childbirth we wait for the child to develop. That's what's actually kind of predicting our child coming to term, is the development of surfactant. This is what's going to allow the child to start breathing, because it breaks down the surface tension of all the fluids that are sitting in the lungs for when the child is inside of the womb itself. And so once they come out to the open air, surfactant is necessary so that the actual alveolar tissue can do the job that they do.
Pretty cool, huh? Let's review very, very quickly here, folks. Diaphragm and inspiratory intercostal muscles contract.
When they contract, that's going to expand the thoracic cage, your chest cavity. When that happens, that's going to cause the interpleural Pressure to go sub-atmospheric, get more negative, get more negative. If that causes it to get more negative, the transpulmonary pressure gets larger.
That's going to pull on the lung tissue causing it to expand. As they expand, the alveoli, the pressure inside of the alveoli, become negative. Becomes negative, I should say, sub-atmospheric.
That's going to suck air into the alveoli from the outside world. We're breathing. When the diaphragm and the inspiratory intercostal stop contracting, the chest wall recoils, pushing back.
causing a change in the interpleural pressure, changing the transpulmonary pressure, pushing on the lungs themselves, causing the pressure on the actual alveolar tissue to rise as a positive pressure, compressing the alveoli, pushing air out, out of the lungs, and out of the body. This is a difficult... concept to kind of understand folks.
Go back and watch this video again, take notes, and ask questions. We'll talk to you later.