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Alveolar gas equation - part 1

Find out how to calculate exactly how much oxygen is deep down inside your lungs! 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.

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  • winston baby style avatar for user Emre Kar, MD
    There was a 160mmHg O2 first. Then vapor join to the gas mix and O2 partial pressure went down to 150mmHg, right? But I couldn't understand why did it go down? Because there are same amount of O2 as first, their partial pressure has to be go down because of vapor I understand that too, but why the exact pressure is going down? The multipication of volume and pressure has to be constant if you're not adding gas in it. And I don't think we add gas. So I'm a little confused. Thank you for your attention :)
    (9 votes)
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    • winston default style avatar for user David
      It's because the vapour takes up some of the 760 mmhg (760 stays the same) of pressure yet O2 has the same percentage of the bar's total pressure. The bar's total pressure is 760 mmhg subtract the amount of pressure the vapour is creating so therefore since the total pressure of the bar goes down, any percentage of the bar will also go down (which also includes the O2).

      In simpler words, a little bit of the 760 is taken up because of the vapour so the bar has less total pressure (713 mmHg). 760 mmHg * 21% > 713 mmHg * 21%. Therefore, the pressure of O2 goes down.

      Oh, and yes you do add gas when the air comes in your mouth, you're adding water vapour.

      Dr. Rishi describes this around , Great question! :D
      (7 votes)
  • leaf green style avatar for user Neil
    I have a question regarding alveoli and surfactant.
    I understand how a smaller alveolus has a higher pressure than bigger alveoli therefore it tends to collapse more, and how surfactant prevents it from doing so by reducing surface tension.
    Now my lecturers notes say that small alveoli have a higher surface tension whereas some internet searching said the exact opposite. I am a bit weary of my lecturers notes since he tends to have mistakes like this.
    I basically want to know which have a higher surface tension, large or small alveoli? (why would also help)

    Thanks
    (4 votes)
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  • leaf grey style avatar for user Romesnowboards
    Where is the alveolar sac in the body?
    (4 votes)
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  • starky ultimate style avatar for user Andrew
    Why doesn't the introduction of H20 molecules just increase the total pressure above 770mmHg?
    (2 votes)
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    • blobby green style avatar for user Romanisagoodguy
      because this 760 mm of Hg is the column of mercury that provides you the pressure of entire atmosphere. The atmosphere is quite long, decades of kilometres. the atmospheric pressure would increase indeed if you put or distribute quite a lot of molecules of water all along the decades of km of atmosphere. When were are speaking about a human then we are limited by the size of the body, which is very small comparing to atmosphere. So nearly any change of composition at that small scale would not change atmospheric pressure. it will be the same, but since the concentration locally (in your lungs) can be changed then the partial pressure would change and certain gas molecules would want to migrate to the fluid locally. But average atmospheric pressure of the earth would still be the same and would not be affected by any local change in human body
      (2 votes)
  • male robot donald style avatar for user Isaacskilling
    around it says somthing that boiling water vapour pressure is 760mmhg, the same pressure as the normal air around us, is that why the water vapour rises into the air becase it is equalized with the air and the steam floats into the air?
    (2 votes)
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    • blobby green style avatar for user Muhammad Azkar
      Water boils when it's vapour pressure becomes equal ro atmospheric pressure an elementry level defination as you know. So if atmospheric pressure equal to 760 mmhg then obviously vapour pressure of water at it's boiling point will be equal to 760 mmhg..
      (2 votes)
  • leaf blue style avatar for user Gagan Prakash
    @2.45 you say that the partial pressure of oxygyen is 0.21 times the total pressure that is 760, but isnt partial pressure dependent on the number of moles(ie number of particles) of oxygen in air and not the volume of oxygen( which is what you are considering by taking 21% of air being oxygen by volume)
    this link is a video by sal about how to calculate partial pressure and in that he first calculates the number of moles of each gas and then takes then from that derives the partial pressure.
    https://www.khanacademy.org/science/chemistry/gases-and-kinetic-molecular-theory/ideal-gas-laws/v/partial-pressure
    (2 votes)
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    • blobby green style avatar for user Romanisagoodguy
      at room temperature the same amount of molecules of any gas consumes the same volume, it is called molar volume. This molar volume actually was measured at T=0 C, P=1 atm, at it states 22.4 litre/mol but at room temperature this will be true also sine dT is very small from 0 to 25C.
      (2 votes)
  • blobby green style avatar for user Hidalgotj2000
    So is oxygen, nitrogen, argon and CO2 mix the same in all levels on planet earth. If I breath air I am always breathing these same percentages no matter where I am ?
    (1 vote)
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    • leaf blue style avatar for user dysmnemonic
      For ambient air, the percentages will usually be close enough. The most important changes to think about for understanding the aveolar gas equation are things like high altitude (where H2O will displace bigger percentages of the O2) and supplemental oxygen (where the pO2 will be significantly increased).
      (2 votes)
  • male robot hal style avatar for user Clemens Kang
    Would this be Sal talking in this video?
    (1 vote)
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  • blobby green style avatar for user Aakash Kumar
    where is the alveolor sac in the body
    (1 vote)
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  • mr pink red style avatar for user Deborah David
    Why is it that sometimes the nitrogen percentage in exhaled air higher than the percentage in inhaled air?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

Let's say this person is lying here in front of me. And I'm thinking about how the air is passing through their nose and their mouth and entering their lungs. And specifically I'm interested this time in how much oxygen is actually getting to their alveolar sacs. So, deep inside their lungs they have these branches, they're conducting in respiratory bronchials. But at the end, of course, they have these alveolar sacs that we've talked about. And I'm interested in thinking about how much oxygen is really down there at the very ends. And you have to excuse this alveolar sac. It really is that. It looks a little bit like a three-leaf clover, I guess. But that's the issue. 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. 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. So, if you're at the top of a mountain, it would be less than that. And this pressure is made up of many, many different molecules bouncing around. 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 0.21-- 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. 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-- in Greek, it actually comes from the term lazy. But it basically reminds me when I think of that, that argon is not going to do much. It's not going to react with anything that is in our body. And of course, you have other. You have less than 1%. And this would be things like carbon dioxide. So, this is a breakdown of the air that 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 OK, pO2-- this is the partial pressure of oxygen-- is just 0.21, 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 and into his-- all the little bronchials 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. He's got a normal body temperature. And what that does is-- the air is going through these bronchials and trachea. And as it does, there's a lot of moisture in the respiratory tree. 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. I'm going to draw them as little dots of water. That's here. And it's going to start entering and mingling with the gas that's 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. And that vapor pressure is going to change depending on the temperature. But at 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 boiling water-- that would be 100 degrees Celsius. And the vapor pressure there would be very high, because it's boiling. And it would be 760. So boiling is actually 760. So just keep that in mind. Boiling water has a vapor pressure of-- And what do you think 760 reminds you of? That is atmospheric pressure. So it's 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. We're not boiling water inside of our bodies or our lungs. We're actually much cooler than that. But we are warm. We're at 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. 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's taking up 47, the rest of those little gas molecules have got to be 713. So this is the rest. What was in that rest? It's going to be the same as before. It's going to be-- and 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. Same kind of break down 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 here-- and this is 78%. Again, this is 78% percent 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 of the other gases are going to have a lower partial pressure. So what is the partial pressure of the air that's entering into that alveolar sac? It's going to be basically FiO2, which is 21%. I'll write that here. And then we have the atmospheric pressure. This is atmospheric pressure over here. And we said that was 760. We can 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. 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've 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.