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## Health and medicine

### Course: Health and medicineÂ >Â Unit 4

Lesson 2: Gas exchange

# 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.

## Want to join the conversation?

• 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 :)
• 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
• 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
• The tissues of the alveoli experience more tension when it is large due to higher pressure within. When it is small and relaxed due to low pressure, the structural cells experience much less tension.
• Where is the alveolar sac in the body?
• It relates to the air sacs in the lung's alveoli.
• Why doesn't the introduction of H20 molecules just increase the total pressure above 770mmHg?
• 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
• 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?
• 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..
• @2.45 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.
• 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.
• 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)
• 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).
• If we have all that mercury in our bodies, then why is mercury so bad for us. Like if a person has mercury poisoning?