If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Oxygen movement from alveoli to capillaries

Watch as a molecule of oxygen makes its way from the alveoli (gas layer) through various liquid layers in order to end up in the blood. Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Rishi Desai.

Want to join the conversation?

Video transcript

So imagine you have a molecule of oxygen. It has to first get into your mouth. Or I guess, it could also go through your nose. And it's going to join up, either way, and go down into your trachea. And from there, it can split off to your left lung or your right lung. Let's say that we're facing this person. On the left, you've got one big lung over here, with a little cardiac notch for the heart. And the right side, you've got the second lung, of course. And this one does not have any spot for the heart, because it sits on the other side. And what I want to do is actually zoom in and focus on this little aveolus right here, because we know we have millions of these alveoli in the lungs. And that's where all the gas exchange is happening. But exactly what happens needs to be clarified. We need to kind of zoom in and get some details. So let's focus in on what happens here between the alveolus, which is the last part of that bronchial tree and the blood vessel. I'm actually going to speed this up for you. [MUSIC PLAYING] So there you have all the layers between the alveolus and the capillary, pretty impressive, huh. And we have this molecule of oxygen. I'm drawing a circle around it. It's going to make its way from this alveolus out of the gas. And first it's going to have to go into the liquid phase. That's kind of a big deal. It's going to enter this thin layer of fluid, which coats the inside of the alveolus. Then the molecule of oxygen is going to go through the epithelial cells. Those are the cells that kind of make the walls of the alveolus look the way it does. Those are kind of the flat, pancake-shaped cells. And it's going to go to the base membrane. This base membrane, remember, is kind of a foundation. It offers a lot of structural support to the lungs. And below the base membrane, it has this layer of connective tissue that this molecule of oxygen has to get through, enters another layer of base membrane. And then it goes down into the endothelial cells. These are the cells that are also kind of pancake shaped. And these are going to make the walls of the capillary. From there, the oxygen molecule goes into the plasma and then finally gets into the red blood cell. And of course, the red blood cells are packed full of hemoglobin. So this is a little hemoglobin protein here. And this hemoglobin has four spots on it. It's going to allow four molecules of oxygen to bind it. And so once our oxygen gets there, it's going to hope to find some hemoglobin that it's got a little free spot. And once it binds to the hemoglobin, the red blood cell is going to now carry that oxygen out to the rest of the body, wherever it's needed. So that's kind of how oxygen gets from the alveolus out to the body. Now, let me make a little bit of space. I'm going to show you what I want to do. I want to do kind of an interesting thing here. Hopefully, it'll help you understand this journey that the oxygen molecule is taking a little bit better. So let's imagine something like this, where you've got a nice little rectangle. I'm going to try to draw this rectangle out on the side for you, in kind of the same way I'm drawing it here. So just keep your eye on the colors, because I'm not going to relabel anything, just to kind of keep it nice and easy. What I'm going to do is just imagine that the oxygen is starting at the top of this rectangular three-dimensional square-like object I'm drawing, I guess, a three-dimensional cube, rectangular cubed. And then it's got to get to the bottom of this rectangular cube. So at the bottom, we've got the red blood cell and the hemoglobin. That's the last layer down here. And the top layer was the alveolus or the gas. So I actually sketched that in as well. And so that would be the very top layer. And it has to get through all these layers. This blue layer, for example, this is that liquid that's lining the inside of the alveolus. And let me draw a molecule of oxygen starting its journey up here. That's the gas phase, right. So it has to actually get from the gas stage through the liquid layer, into the next layer, which is the epithelial cell. That's this guy right here. That's the second layer. Third layer, we said was the base membrane. I'm just kind of going through them one by one. And this is also kind of a nice way of a review, I suppose, as well. Then you have all that connective tissue, a nice, thick layer of connective tissue. That's the green. And remember, the base membrane and the connective tissue, they're both chock full of proteins, different types of proteins. But both are there for structural support. Got some more base membrane here on this side, and this is going to be right before you get to the endothelial cells. That was the endothelial layer. This is the cell that kind of offers the capillary walls. And then we've got some plasma, we said. The oxygen has to get through some plasma and finally is going to get into the red blood cell. So this whole bit, the reason I'm even drawing it like this or taking the time to draw it like this is that this entire layer right here-- this is all liquid. This is all liquid and predominately water. So remember, our bodies are heavily water-based. So our molecule literally is going from gas, which is at the top of our rectangular cube, all the way down through many, many different layers of liquid. So it kind of makes it easy, if you can divide it into these two categories, gas and liquid. In fact, this is now, hopefully, going to help connect with these equations that we've been learning. So now, let me throw up a couple of equations that we've talked about before. And let's see if we can figure out how they relate to what we're going through now and whether there's any clear relationship as to how to use these pictures that we've drawn up. So this first equation, this is the alveolar gas equation. We've talked about this before. There's a video on this as well, if you want to refresh yourself. The first part of this alveolar gas equation tells us how much oxygen is going into the alveolus. Remember, this top layer right here. This is our alveolus right here. So it says how much oxygen is going into that alveolus. And this is actually the second bit is how much is going out. And if you, of course, subtract what's going in from what's going out, you're left with, what is the partial pressure of oxygen in that gas space. What is this blue PO2 equal? And this is actually kind of a nice segue for our second equation. We have this second equation, which helps us figure out how much oxygen is going to defuse, or any molecule really, according to this formula. This is fixed law. And we can actually figured it out, by taking a few parameters. We can say, well, if you know that the gradient P1 minus P2 is a certain amount. And if you know the area and the diffusion coefficient and the thickness, then you can figure out this V. And this V really the amount of oxygen, in this case. And we're going to focus on oxygen right now. Amount of oxygen defusing over time-- so this is actually very helpful, because if you start noticing that the amount of oxygen defusing over time and the oxygen delivery that's coming into the red blood cells is low, then you might show wondering why that could be. And remember, the red blood cell layer, that's down here. This is our red blood cell layer. So you start wondering, how is oxygen getting from that alveolus down to the red blood cells. And we can call the partial pressure of oxygen, the alveolus. We can call that P1. And we can call the partial pressure of oxygen down here into the red blood cells. We call that P2. And so then when we figure out from the alveolar gas equation what this is, that is basically telling us this. So the two equations are basically very related. So if I notice that the amount of oxygen diffusing from the alveolus to the red blood cell layer is off, if it's less or more than what I expect, I have to go through a mental checklist. I have to think, well, is the Fi O2 what I thought it was. Usually, room air is 21%. But maybe, this person is on 40% or 50%, because they're getting a face mask. And they're getting a lot more oxygen than what is in the environment. So that could be one reason for getting a higher value. You might also get a higher or lower value, because maybe you're not at sea level. Maybe we're working with a patient at a mountain level or maybe below sea level. So that could also explain an abnormal amount of oxygen defusing over time. And these two things that I've drawn in orange box, they're both going to affect P1. This is the initial partial pressure of oxygen in the alveolus. Some of these things are probably less likely to be changing. I wouldn't expect that the respiratory quotient is changing. If the person has a steady diet, then that shouldn't be any different. The partial pressure of water probably also isn't changing, especially if we're at body temperature. And the partial pressure of carbon dioxide, there, that could actually change. But just to keep things simple, and if I'm only thinking about oxygenation, I'm just going to assume that's going to be probably not the reason either. So going through my mental checklist. I know P1 is going to be something I want to think very carefully about. I also want to think really carefully about area. What if it's because the person I'm dealing with has had many alveoli that are no longer working. Let's say, only half of their alveoli are working. That means that half of their surface areas is gone. So they're really not getting as effective gas exchange, because half their surface area is gone. And effectively, only half of their alveoli are able to get oxygen to diffuse across. So surface area is very, very important to think about and as is thickness. And when I say, thickness, remember, the oxygen has to get all the way from this gas layer down into the red blood cell layer. So that's a very big way to go. And if you add a bunch of liquid to this layer right here, maybe to the connective tissue, if there's more fluid in those particular layers-- those are usually the ones affected-- then that's going to increase the thickness. So there's one more reason for why my amount of oxygen defusing over time may be off from what I had expected. And again, down here, I wouldn't expect my diffusion coefficient-- I wouldn't expect this to be different than what I had expected, because the diffusion coefficient is pretty stable. If we know that we're talking about oxygen within water, at a certain body temperature, that's not going to change a whole lot. And finally, this P2, this is the partial pressure of oxygen that was returning from the body. So if the body is using up a bunch of oxygen and returning it, what is the oxygen level in that blood that's coming back? And I wouldn't expect that to change much, because the body is probably using a fairly consistent amount of oxygen. So I'm not going to assume that's the reason. So again, if you ever come across an abnormal amount of oxygen defusing over time from the alveolus down into the blood, you've got to go through this checklist and think about these formulas and how they help us be very systematic in going through each of these variables and thinking, what could be the reason that the amount of oxygen diffusing over time is more or less than what we expect.