 Does this look familiar? I hope so. You've been here before. Guess what? I'm not going to redraw this picture for you because I already drew the diaphragm and we're going to talk a little bit in a little bit more detail about ventilation. But I want you to remember that we're talking about the pleural cavity which is inside the thoracic cavity. And the diaphragm, by contracting, the diaphragm changes the volume of the pleural cavity. Now, somewhere back in the day, there was an old boy named Boyle. And Boyle, old boy Boyle, he figured out that... Tell me if you agree with this. That if you have a closed container of gas like in a lung, if the volume of the gas increases, the volume of the space holding the gas increases, can you visualize this? The pressure of that gas is going to decrease. And the same goes the other direction. If the volume of a container of gas decreases, do you agree that one's actually a little more intuitive, then the pressure of that gas increases. Think about if I had all of you guys in a classroom and you guys were little gas molecules, and pressure is nothing more than gas molecules bouncing off of a wall. And so whatever the wall is, is it your lung, is it your alveolus, where is the wall? When a gas molecule bangs up against a wall or your head or something, that's air pressure. So if you take a room and you guys are all bouncing off the walls and then I cut the room in half, are you going to bounce off the walls more or less? You're going to bounce off the walls more and that's basically pressure just increased. So watch what happens when you breathe. First of all, so we're going to start, we want to inhale. So the first thing that happens is the diaphragm contracts. When the diaphragm contracts, this automatically increases the volume of the plural cavity. Do you agree with that? If the volume of the plural cavity increases, what happens to the pressure? When, thanks Boyle, oh boy Boyle, he told us that, dude, the pressure is going to decrease. If the pressure of the gas in your lungs just decreased because you increased the volume, that pressure, lung pressure, is now less than atmospheric pressure, right? Does that make perfect sense? If the pressure in your lung is less than atmospheric pressure, what direction is air going to flow? Dude, air flows from high pressure to low pressure. That's what wind is. It's air moving from high pressure to low pressure. So if you lower the pressure, the air pressure in your lungs, air from the atmosphere will go in as long as there's an open route to get there. 100% of the time, unless you block it off, 100% of the time this is going to happen. Now air from the atmosphere is going to come into your lungs just because you changed the volume. And now if you want to exhale, we're going to relax the diaphragm. This is a relaxed diaphragm. And if we relax it, what's going to happen to the volume? The volume's going to decrease. If the volume decreases, what happens to the pressure? The pressure increases. So where does the air go? It's going to go back the other direction from the lung into the atmosphere. Perfectly clear. It's that easy. That is a relaxed exhalation. If you are inhaling and exhaling with more force, like if you're running and you're doing... or you're trying to hyperventilate or whatever, if you're breathing forcibly, inhaling and exhaling with force, then you're going to use muscles to help you expand and contract the thoracic cavity to change the volume of the thoracic cavity. And there are muscles. The internal and external intercostals are found between your ribs. So those guys are going to contract and relax to change the volume of your thoracic cavity. That's how you breathe. It's that simple. Shall we look at what gas exchange is? Because that was the whole point.