 If we want any kind of gas exchange to happen at all, then the bottom line is we are going to have to set up some kind of partial pressure gradient between the alveoli and the blood and the blood and the body cells. So I'm going to draw you a picture. What? I know, I know. I continually shock you with my strategies for teaching anatomy and physiology. This strategy is going to involve drawing a mouse with a trachea, lop, lop, lop, and an alveolus, because, you know, don't you think they kind of look exactly like this? Not even close. This is a diagrammatic view of the alveolus because I want to fit everything on here. So here's what else I'm going to draw. I need a blood supply. So this right here is where I'm going to do external respiration between the alveolus and the blood. And then I'm also going to do internal respiration between the blood, oops, I need to get it a little bit closer, between the blood. Please, I'm going to show you my arrows in just a second, and the body cells. So as we already know, because we dealt with the concept of capillary exchange, we know that every single cell has to be within diffusion's distance of the capillary or the cell's going to die. You can imagine this little red vessel here is a capillary. And here's my organization. First of all, this is the atmosphere. So this is the air that you're breathing. Our blood flow is going to go this direction. So I would imagine that this is going to be blood, dirty blood, that's coming in contact with an alveolus in the lungs. And it's going to travel to the body. And here's a capillary that's in contact with some body cells. And then the blood, of course, after it drops off its oxygen and picks up its carbon dioxide, it's going to come back to the lungs. And we can throw in all the anatomy. We can go to the correct places in the heart. We can follow it all around. But right now, let's just focus in on what's going on. What are the partial pressure gradients that are going to motivate internal respiration and external respiration? And here's the scoop. I'm going to tell you, we're going to keep track of PO2. I don't think I want it to be green. I think we're going to do PO2 in red and we'll do PCO2 in blue. There is oxygen in our atmosphere. Thank goodness for that. So we can throw a partial pressure. We know that number. It's actually about 160 millimeters of mercury. All my partial pressure units are going to be in millimeters of mercury. I'm going to stop putting my units on here just to save us a little bit of time, but do not forget that we're talking about millimeters of mercury and know that if you ever give me numbers without units, it will be a sad story and probably won't get credit. So don't leave off your units. This is wild. Are you ready for this amazing fact? I mean, we talk about carbon dioxide concentrations in our atmosphere. Carbon dioxide is a greenhouse gas, something we should definitely be concerned about. Do you know how much carbon dioxide is in the atmosphere? What? The partial pressure is 0.25 millimeters of mercury. You got to be kidding me. That isn't very much carbon dioxide in the atmosphere at all. Now, imagine I'm going to take a nice, fresh breath of air and I'm going to divide my alveolus in half just so we can keep track of after external respiration has taken place what our new numbers are. So relax because watch how this is going to work. Obviously, the entire quantity, blob of air from the atmosphere is going to go into the entire alveolus. I'm dividing it in half, so you can imagine the change that's going to happen over time. So this air, this fresh blob of air, whose characteristics I've given you over here, I'm going to throw it into this half of my alveolus and now I'm going to tell you that my partial pressures of my gases is different now. Po2 is actually about 100 millimeters of mercury. Look, I did it. I couldn't help it. And PCO2 is about 40. And I'm telling you which one's which based on the color. Okay, seriously, what happened? That's the fresh air that I just breathed in. Why did my Po2 go down and my PCO2 went up by a whole bunch? Why? What happened? Remember that there was that whole chunk, that blob of air in your lungs that you can't ever breathe out. So you aren't ever going to have an inhale that is purely atmospheric concentrations. You have leftover air in there that is highly concentrated with carbon dioxide and not so much oxygen in that air. So we can accept that when we mix with that residual air that you can never actually breathe out, these are my new partial pressures inside the alveoli at the beginning of a breath. So you take a breath. Fantastic. And now inside my alveolus, these are my partial pressures. It's probably relevant right now to say, okay, what's the partial pressure of the blood that's coming back from the cells? This is the used up blood. The cells have sucked out as much of the gases, the oxygen as they possibly can, and they've had as much of the carbon dioxide as possible, sucked out of them. So watch this. Inside venous blood that has come from the cells, we have a partial pressure of carbon dioxide of 46 millimeters of mercury, and oxygen is at about 40 millimeters of mercury. Okay, why? What did the cells do? It's like thirsty cells sucked all the oxygen out of the blood down to 40 millimeters of mercury. Now, gases move down their partial pressure gradient. Do you agree with that? Absolutely. So what direction is oxygen going to move? We don't have to think very long to go, oh, well, of course, oxygen is going to move from 100 millimeters of mercury down to 40 millimeters of mercury. What's going to happen to the partial pressure in the blood when this happens? Well, the partial pressure of the blood is going to increase. And in fact, because of some amazing mechanisms that we'll talk about later on in this lecture, after external respiration is complete, the blood and the alveoli have effectively changed concentrations. They've switched. So the alveoli now have, now this is after external respiration has occurred, the alveoli have only 40 millimeters of mercury of oxygen, whereas the blood now has 100, which just is more oxygen molecules dissolved in that blood. What do you suppose has happened to the carbon dioxide? Dude, carbon dioxide is going to move down to the partial pressure gradient. It's going to move out of the blood and into the alveolus, and it switches right down. I mean, it makes perfect sense. So we end up with 46 in the alveolus and 40 in the blood. And that travels to the cells. Now, what's going to happen in the cells? Your cells don't ever end up with a partial pressure of oxygen that's high. They're constantly saying, we need more oxygen. They use up the oxygen that they're exposed to. So the PO2 in the cell usually really ends up being about 40. And you can imagine that the more active a cell is, the more it uses up oxygen in its environment, the lower that partial pressure is going to go. So that can vary depending on the metabolic activity of the cell. But check out what's going to happen to if your cells essentially hang out at about 40 because they are using the oxygen that's available to them. Then the oxygen inside, it's going to come around at the beginning of the capillary. We're going to have oxygen at about 100. But by the end, all the oxygen has been sucked out of the blood and into the cells, and they're just using it. That's how we end up with 40 millimeters of mercury when we head back to the alveoli. Carbon dioxide on the other hand, carbon dioxide shows up, the blood shows up with a partial pressure of 40 for carbon dioxide. And your cells, again, they're chronically producing carbon dioxide. So essentially, you can imagine that they're producing it enough to be a partial pressure of about 46. So the carbon dioxide is going to continually diffuse in. In the end game, you end up with about 46 millimeters of mercury in that old used up capillary blood, which travels to the alveolus for external exchange. Internal respiration happens here. Internal respiration happens here. Cellular respiration is happening in here. Does that work for you? You might be like, dude, numbers. The numbers are handy. The numbers help you visualize the push for why this gas exchange happens at all. There are several factors. We're going to look at this diagram and get several factors that can affect gas exchange, internal and external gas exchange.