 Hello, my doggies, are you ready? I'm coming back to this picture that we've already done. And I'm coming back to this one. I erased some labels. I don't want you to forget that we have the visceral pleura and the parietal pleura surrounding our pleural cavity. But we're going to talk about how can we get air from the atmosphere into the alveoli. So if you're thinking about, oh boy, boil, you should be thinking that, dude, we have to somehow be changing, really we have to be changing volumes in order to change pressures. Now, here's the deal. Atmospheric pressure is 760 millimeters of mercury at sea level. Take a deep breath. We are not going to calculate anything based on atmospheric pressure. Atmospheric pressure is critical, but it changes. If you're at sea level, it's going to be 760 on a certain kind of day. If you're on Mount Everest, it's going to be much less than that. And if you're anywhere in between, it's going to be somewhere between that. So atmospheric pressure is going to change. Thank you, physiologists, who said, dude, let's just set atmospheric pressure at zero. And do you remember how we set membrane potential? We were talking about membrane potentials. We set the extracellular fluid at a charge of zero millivolts. And then we determined membrane potential by comparing the intracellular fluid, the charge of the intracellular fluid to the extracellular fluid and found the difference between them. And that was my membrane potential. We're doing exactly the same thing here. We're going to start out at the base. We're going to start out at the moment before inhalation. So at the beginning of an inhale, can you visualize this? So we're at the place where, go ahead and just mess with this a little bit. You exhale and you pause. There's a moment where air is not moving. And then you inhale and now air is moving in. And then there's a moment at the end of an inhale where you pause again and air isn't moving anymore. And then you begin and exhale and then the air moves out. And then you end your exhale and air isn't moving anymore. And then you begin your inhale. Do you see how, like, write that down. I don't have room to write this down because I'm going to show you what's happening to the pressures. At the beginning of an inhale, we don't have any movement. Right before the beginning of an inhale, let's say there's no movement of air. If this is true, then pressure outside, atmospheric pressure, has to equal pressure in your alveoli. Can you read that at all? Here's atmospheric pressure. We're just calling it zero, which means before a breath happens, we have to have pressure of the alveoli. It has to be equal to zero. Right? That's the only way we can have no movement. So our challenge is at the beginning of an inhale, we have to somehow motivate atmosphere to move into the alveolis. So what is the thing that's going to have to happen? We're going to have to have a pressure gradient. We're going to have to have a lower pressure inside the alveolis so that the air can move from high pressure to low pressure. So somehow we have to decrease pressure in the alveoli. And those of you who've had anatomy, how does that happen? What do we do? If we make it happen, we contract who? The diaphragm. There are other muscles involved, but the contraction of the diaphragm, look at what happens. I told you that the diaphragm was this kind of this shaped structure and when the skeletal muscle myofilaments shorten, folds down, not folds, but it shortens down and it, what just happened to the volume of this space in here? Dude, the contracting diaphragm increases the volume of the pleural cavity. If you increase the volume, you're going to decrease the pressure. When the pressure decreases, air rushes in. Now, if this was not a mouth and was not open, if we closed it off and yet contracted our diaphragms, we would create a low pressure zone and air would want to rush in, but it couldn't. I'll try it. You can actually inhale without allowing air in to rather unpleasant sensation, but go ahead and do it. And it's because you created a low pressure zone in your alveoli. Air rushes in. How long does air rush until? Until the pressure, until the pressure in the alveoli is equal to atmospheric pressure again. It has to go back to zero. Now, let's start our exhale. We're going to begin, that says begin, exhale. If we are going to actually begin an exhale, all we have to do is relax the diaphragm. That's going to decrease the volume with the consequences of increasing the pressure right inside this space. So now my pressure is greater than zero. My pressure is three compared to the outside. And now that's going to push the air back out. The end of the exhale, air rushes out until the pressure inside your lungs is zero. Now, think about this. You can actually take a breath, you know, just a normal little breath. Even an exhale, breathe out all the way. It's hard to talk and demonstrate at the same time, but I can breathe out all the way. And then I can, like, breathe out even more. And that's just by contracting more things. I can contract my internal intercostals and my diaphragm even more. I mean, relax my diaphragm even more and use some of my ab muscles to, like, squeeze out more air because I decreased the volume even more. Think about all I'm doing is changing the volume of my pleural cavity and that causes the air to move. Now, there's all sorts of graphs that we can do to mess with this, which we definitely will do in class. But let's take a look at, you've got the pressure changes and the mechanics of how we move air in and out. Now I want to talk about what's going on inside this intraplural space because there is a pressure in there as well and it's important, intraplural pressure. Be right back.