And, of course, you have to have this slide, which is Laplace 's law , and I know all the medical students remember this, because they learned it in their physiology class. But this tells you why surfactant works. Why does it help a baby with breathing. So, Laplace 's law talks about the pressure that is required to hold open that little alveolus. And in the numerator, you have the surface tension of the wall, and in the denominator, you have the radius. So this figure tells you about the two different variables. On the left side, you see the effect of surface tension. So, this is a little alveolus that has surfactant, so its surface tension is reduced, and you don't need much pressure to hold that alveolus open, according to Laplace 's law. This alveolus doesn't have surfactant, because the mom either didn't get steroids, or the baby didn't get surfactant. The surface tension is very high, so according to this relationship, you have got to put a lot of pressure in to hold that alveolus open. This is your third objective. You don't have to answer this question. This slide talks about the effect of airway diameter or radius, and here is a nice big airway with a large radius, so when the denominator is high, the pressure is low, it doesn't take much pressure. Alternatively, with the smaller ones, it requires more pressure. So it's the smallest alveoli that are the ones that collapse that give you that picture on the x-ray that you see with RDS, which we call ground glass, and I will have a couple of them for you.