 Hi, darlings. We're back to talk about, we're back to finish our conversation about the respiratory system. In the last lecture, we looked at ventilation. I brought back all the mechanisms of the respiratory system because I want to remind you that we're doing a two-day-long series on the respiratory system and the last lecture was all on ventilation. Today, we're dealing with everything else. So everything, external respiration, hopefully you remember this is gas exchange between alveoli to the blood. And internal respiration was gas exchange between the blood and the body's cells. Cellular respiration we know is happening in the mitochondria, which is the whole reason why gas exchange anywhere is necessary. We wouldn't even need a respiratory system if it wasn't for cellular respiration. So we're not going to spend a great deal of time talking about cellular respiration. We're kind of good on that. But it's really important to remember that's why we're doing this whole thing in the first place. Our main focus today is going to be dealing with these guys. And we're going to start out by reminding ourselves of the concept of partial pressures of gases, knowing that partial pressures motivate gases to move. So a single gas type, like oxygen, will move down its own partial pressure gradient. And that was thanks to old boy Dalton who gave us that law that said gases move down their own partial pressure gradients. So we're going to draw in the next section. We're going to draw a picture of the whole body. And we're going to look at different partial pressures of the two gases that we care about, which are oxygen and carbon dioxide. And we're going to see if we can identify the partial pressure motivation for gases to move during these exchange processes. All right. So we'll be right back to talk about partial pressures.