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.