No announcement yet.

400061 - can not understand explanation about equivalent altitudes

  • Filter
  • Time
  • Show
Clear All
new posts

  • 400061 - can not understand explanation about equivalent altitudes

    Can anyone help me to understand what this explanation means, it just confuses me the way it's written!

  • #2

    Hi Flybi

    This is quite a lengthy explanation so please forgive me.

    At sea level the partial pressure of Oxygen at the alveoli is 103 mmHg. Imagine we are in a balloon ascending so we are unable to pressurise! As we go up the pressure is dropping, such that at 10,000 the alveoli pressure has reduced to 55 mmHg, generally the limit for gaseous exchange to occur. Above this we will start to suffer the effects of hypoxia.

    So at 10,000 feet we would go onto an oxy/air mix which keeps us at 103 mmHg (i.e. sea level). This pressure will remain as we carry on climbing up to a maximum of 33,700 feet by which time our oxy/air mix would be at 100% Oxygen. So at 33,700 feet we are on 100 oxygen and have an aveoli pressure of 103 mmHg, the same as being at sea level.

    We want to keep ascending in our balloon and by definition I can't increase the oxygen from my regulator as it is already on 100%. Thus the pressure will start to reduce from 103 mmHg. As stated before, 55 mmHg is the limit for us humans for diffusion to take place and we reach this limit at 40,000 ft. So at 40,000 ft, on 100% oxygen, we are effectively the same as being at 10,000 ft breathing ambient air, so this is on the limit and ok.

    If however we want to go higher, then we would need to use pressure breathing, a system that literally forces the air into the lungs ensuring diffusion can take place.

    I hope this explanation helps to clear things up but if not please do ask for any further clarification.

    Best of luck


    1. MSL - Alveoli partial pressure of O2 = 103 mmHg (Total of all the partial pressures = 760 mmHg)
    2. At 10,000 ft - O2 partial pressure = 55 mmHg (Limit for gaseous exchange to effectively take place)
    3. Go onto Oxy/Air mix - keeps us at 103 mmHg up to 33,700 ft (100% O2 at 33,700 ft)
    4. 33,700 ft - 40,000 ft - Alveoli pressure reduces from 103 mmHg down to 55 mmHg (Still on 100% O2. The same as being at 10,000 ft breathing ambient air)
    5. 40,000+ ft - 100% O2 + pressure breathing.


    • #3
      Thanks Tony, what you have written makes perfect sense and helps my understanding a lot.

      Regarding the LO:

      "State the equivalent altitudes when breathing ambient air and 100 % oxygen for MSL and approximately 10 000, 30 000 and 40 000 ft. "

      How would you interpret this? The only 'equivalent' that I can come up with is the following:

      - Breathing 100% oxygen at 33,700ft to 40,000ft is the equivalent of breathing ambient air at 10,000ft
      - Breathing 100% oxygen up to 33,700ft is the equivalent of breathing ambient air at MSL (whatever altitude we are at above MSL the oxygen is adjusted to give us 103mmHg)

      What are the other equivalent altitudes that the LO is referring to?

      Many thanks


      • #4
        I think that covers it to be honest. MSL is MSL! 10,000 ft you are only breathing ambient air but with the pressure reduction mentioned before. 33,700 ft and 100% oxygen is the equivalent of MSL with the partial pressure reducing as you go go up to 40,000 ft on 100% oxygen. At 40,000 ft it's equivalent to 10,000 ft. So that covers every base.

        No problem at all. :-)