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Current time:0:00Total duration:7:59

Video transcript

let's say this person is lying here in front of me and I'm thinking about how the air is kind of passing through their nose and their mouth and kind of entering their their lungs and specifically I'm kind of interested this time in how much oxygen is actually getting to their alveolar sacs so kind of deep inside their lungs they have these branches or conducting and rest Troi bronchioles but at the end of course they have these alveolar sacs that we've talked about and I'm interested in kind of thinking about how much oxygen is really down there right at the very ends and you have to excuse this alveolar sac it really is that it's it looks a little bit like a three leaf clover I guess but that's the issue right how much oxygen is deep down in here where the X is so how do we figure this out I want to first think about the air this gentleman is breathing in right he's breathing in air from the atmosphere so this is atmospheric pressure air and we say ATM for short and we know that atmospheric pressure at sea level is 760 millimeters of mercury it's going to be lower at higher altitudes right so if you're at the top of a mountain of course it would be less than that and this pressure is made up of many many different molecules kind of bouncing around right so I've got some molecules of oxygen let's say this is about 21% this is my oxygen and before I move on I should mention fio2 you might come across this and fio2 stands for the fraction which in this case was 21% or 0.2 one fraction of inspired meaning how much oxygen you took in or air you took in fraction of inspired oxygen and the fraction happens to be 21% which is of course much much lower than the nitrogen now nitrogen when I draw it this way it's pretty impressive right all the purple is nitrogen this is about 78% of what you're breathing in and the last little tiny little bit I'm going to draw the green line this is mostly argon and argon is a in Greek actually comes from the term lazy but it basically kind of reminds me when I think of that that our gun is not going to do much it's not going to react with anything that you know is in our body and of course you have other we have less than 1% and this would be things like carbon dioxide so this is a breakdown of the air that you know my my friend is breathing in this is my friend breathing and if I want to now think about how much oxygen they're taking in all I have to do is a little tiny bit of math I can say okay well po2 this is the partial pressure of oxygen is just 0.2 1 or 21% times 760 millimeters of mercury and this turns out to be 160 millimeters of mercury now that oxygen kind of goes down in his lungs and it goes through his trachea into his you know all the little bronchioles and down into the alveolar sac and when it gets there on the way over there an interesting thing happens the body temperature here is 37 degrees Celsius right he's got a normal body temperature and what that does is you remember the air is kind of going through these bronchioles and trachea and as it does there's a lot of moisture in the respiratory tree right there's moisture there and that moisture when it starts heating up and of course 37 degrees is pretty warm it's going to start leaving the liquid phase and going into the gas phase so all of a sudden you have now little molecules and you're drawing them as little dots of water that's here and it's kind of start entering and mingling with the gas that's kind of going through so the gas that got taken in that he inhaled is now mingling and what happens as a result is that water has what we call a vapor pressure a vapor pressure and that vapor pressure is going to change depending on the temperature but a 37 degrees that vapor pressure ends up being 47 millimeters of mercury in other words if the temperature is 37 degrees then we can expect that some of those water molecules will leave the liquid and enter the gas phase and it turns out that the amount of molecules or the number of molecules that leave are going to generate a pressure that is 47 millimeters of mercury and this is pretty standard this is known off of a table and in fact if you think about it if you just generated lots of heat let's say you actually were you know boiling water well that would be a hundred degrees Celsius and the vapor pressure there would be very high right because it's boiling and it would be 760 so boiling is actually 760 so just keep that in mind boiling water boiling water has a vapor pressure vapor pressure of and what do you think 760 reminds you of well that is atmospheric pressure so this is kind of interesting vapor pressure is going to equal atmospheric pressure when you are boiling water and that's actually exactly what's happening as you boil but I don't want to get too distracted where we're not boiling water inside of our bodies or our lungs we're actually much cooler than that but we are warm or 37 degrees and so you do have some of these little water molecules that have entered the gas phase and so if overall it's got to be this whole thing has got to be 760 right so on average our lung pressures are going to be the same as atmospheric pressure but now you've got water taking up 47 so if water is taking up 47 the rest of those little gas molecules have got to be 700 third 713 right so this is the rest now what was in that rest well it's going to be the same as before right it's going to be I'm going to try to sketch it as best as possible this is going to be my oxygen right here this is 21% of 713 and then we have lots and lots of nitrogen still right same kind of breakdown as before and remember this is all air that is being inhaled so we're not talking about breathing out we're just talking about breathing in and this purple right right here and this is 78% again this is 78% of 713 and we still have a little bit of that argon and those other gases I won't write it all out but you get the idea that basically now because water is taking up some of the overall pressure all the other gases are going to have of course a lower partial pressure so what is the partial pressure of the air that's entering into that alveolar sac well it's going to be basically fio2 fio2 which is 21% I'll write that here and then we have the atmospheric pressure right this is atmospheric pressure over here and we said that was 760 let me draw a little arrow so we know what's pointing to what 760 millimeters of mercury and then from that to account for the partial pressure of water because now we have some water vapor in there we have to subtract out 47 47 so so far if you've kept up with this math you see that we have what does that work out to be about 150 millimeters of mercury now this is the partial pressure of oxygen at this spot let me just make it very clear with my arrow not at this orange X so far we figured out that we have a partial pressure that's a little bit lower than when we started and that was because of the partial pressure of water let's pick up there in our next video