Stephen G. Saupe - Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; (320) 363-2782;

Gink & Go Climb a Mountain

SettingGink and Go are in the mountains of Colorado on a Spring Break trip.  They are hiking at an altitude of about 10,000 (meters = 3,000).

Gink:   Phew, this is exhausting.  I am really out of breath (huff and puff loudly).
Go:   Me too.  But it’s not surprising.
Gink:   What do you mean?
Go:   Don’t you remember Saupe’s class on gas exchange when we discussed partial pressures?
Gink:   Oh no (groan loudly)!  Why do you always have to bring up that AIRhead? 
Go:   Nice pun.  Anyway, the table he showed in class nicely explained why we’re having a little trouble catching our breath.
Gink:   It did? I don’t understand those PO2 calculations.
Go:   It’s easy.  You know that partial pressure refers to the pressure of a specific gas in a mixture.  Since air is a mixture of nitrogen, oxygen and traces of water vapor, carbon dioxide and other gases, each exerts its influence on the total pressure. 
Gink:   So that must mean that partial pressure is a function of both gas concentration and the total air pressure. 
Go:    You’ve got it.
Gink:   So to calculate the PO2 at sea level, you just multiply 0.21 times the air pressure which is 760 mm Hg?
Go:   That’s right, because the concentration of oxygen in the air is 21% no matter where on earth you are.  Let’s try it (calculate the PO2 values now & complete the table):


Elevation (m)

Air Pressure
(mm Hg)

PO2 (mm Hg)

Mt. Everest




Mt. Kilimanjaro




Colorado mountain




Sea level




Gink:   Ok.  But - my main question is, ‘Like, so what, dude?”
Go:   It’s important because the driving force for oxygen uptake or carbon dioxide release is the PO2 or PCO2 gradient between alveolar air and blood.
Gink:   Is that because Fick’s law predicts the direction and rate of diffusion based on the concentration gradient, or in this case, the partial pressure gradient?
Go:   Exactly.  The diffusion rate is directly proportional to the partial pressure gradient. 
Gink:   Since the partial pressure of oxygen in blood that enters the lung is about 40 mm Hg, it means that the PO2 in the alveoli air must be even greater. 
Go:   Right.  The partial pressure of oxygen in the alveolar air is about 100 mm Hg which means that oxygen readily diffuses in to the lung. 
Gink:   But why isn’t the partial pressure of oxygen in the alveolar air closer to 160 mm Hg which is the same as the inspired atmospheric air?
Go:   Because when the air enters the lung it gets saturated with water which contributes to the overall pressure.  In addition, the fresh air mixes with old air that remained in the lung after the previous exhalation.  The main point is that as long as a PO2 gradient exists between alveolar air and blood, oxygen will enter the blood. 
Gink:   And the greater the difference between PO2 in the alveolar air and blood plasma, the greater the rate of oxygen uptake and the easier it is to breathe.  No wonder climbers on climbers on Everest need supplemental oxygen
Go:   And that’s why we’re having some difficulty breathing.  Releasing carbon dioxide works by a similar manner.
Gink:   Let’s change the subject.  This is giving me a headache, just like one of Saupe’s lectures.


  1. What are the main gases in air and what is their concentration?

  2. What is partial pressure? How is it calculated?

  3. What is Fick's Law?

  4. Calculate the PCO2 at sea level.

  5. If the PCO2  in the alveolar air is about 40 mm Hg, then the PCO2 (mm Hg) in blood is predicted to be:
        a.  about 40     b.  greater than 40    c.  less than 40

  6. Considering your answer above, the PCO2  (mm Hg) in a tissue must be:
        a.  about 0.03    b.  greater than 40    c.  less than 40

  7. The PO2  in alveolar air at sea level is approximately 100 mm Hg.  Why is this value less than in the inspired air?

  8. If the   in the alveolar air is 100 mm Hg, then the  PO2 (mm Hg) in blood is:
        a.  about 100     b.  greater than 100    c.  less than 100

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Last updated: January 30, 2008    � Copyright by SG Saupe