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
Current time:0:00Total duration:11:46

Oxygen movement from alveoli to capillaries

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 right 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 on the right side you've got the 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 wanted to do is actually kind of zoom in and focus on this little alveolus 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 managed you're going to speed this up for you so there you have all the layers between the alveolus and the capillary pretty impressive huh and we have this molecule of oxygen 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 right 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 to look the way it does this is a kind of the flat pancake shelbs shaped cells and it's going to go into the basement membrane this basement membrane remember it's kind of a foundation 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 basement membrane and then it goes down into the endothelial cells these are the cells that 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 right 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 then 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 will 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 re label any 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 cube 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 right that's the last layer down here and the top layer was the alveolus or the gas so they actually just sketched that in as well and so that would be the very top layer has to go through all these layers right this blue layer for example this is that that liquid that's lining the inside of the alveolus and let me draw molecule of oxygen starting its journey up here that's the gas phase right so it has to actually guess 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 basement 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 nice thick layer of connective tissue that's the green and remember the base membrane of the connective tissue they're both chock-full of proteins different types of proteins but both they're for structural support got some more basement membrane here on this side and this is going to be right before you get to the endothelial cells right 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 we're taking all the time to draw like this is that this entire layer right here this is all liquid right this is all liquid and predominantly water so remember our bodies are heavily water based so a 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 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 relate to what we're kind of 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 right 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 right we have this second equation which helps us figure out how much is going to how much oxygen is going to diffuse or any molecule really according to this formulas is ficks law and we can actually figure 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 V and this V is really the amount the amount of oxygen in this case we're going to focus on oxygen right now amount of oxygen diffusing over time so this is actually a very helpful because if you start noticing that the amount of oxygen diffusing over time of the oxygen delivery that's coming into the red blood cells is low then you might start 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 figured out from the alveolar gas equation what this is that is basically telling us this right so the two equations are basically very related so if I notice that the amount of oxygen diffusing from the alveolus to the red CEL layer is off if it's less or more than what I expect I have to go through a mental checklist afrinic well you know is the fio2 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 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 diffusing over time and these two things that I've drawn in Orange Box they're both going to affect p1 right this is the initial partial pressure of oxygen in the alveolus some of these things are probably less likely to be changing right I wouldn't expect the respiratory quotient is changing you know if the person has kind of 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 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 you know what if it's because what if the person I'm dealing with has had a mini alveoli that are no longer working let's say only half of their alveoli are working that means that half of their surface area is gone right so they're really not getting as effective gas exchange because half the surface area isn't 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 right 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 you the ones affected then that's going to increase the thickness so there's one more reason for why my amount of oxygen diffusing 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 right if we know that we're talking about oxygen within water at a certain body temperature that's not going to change a 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 that's the reason so again if you ever kind of come across an abnormal amount of oxygen diffusing over time from the alveolus down into the the blood you got to go through this kind of checklist and think about these formulas and how they help us be very systematically 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