Breathing 101

A letter from a reader (thank you, Mr. “Smith”) got me thinking—could the fight against improbable medical claims be aided by a better knowledge of science? In another attempt to bring complicated science to the masses, today we will learn a bit about how we breathe. The first thing we need to understand is what we breathe.

Let us speak of air. We know we need it. Most of us know that the oxygen that makes up about twenty percent of it is necessary for life. If you think a little bit more, you probably realize that in addition to the oxygen content, there is another variable that is critical in making air breathable. When we climb a mountain or get in a plane, the air is less breathable. Why is that? The air up there is still 21% oxygen, so what’s the deal?

The breathability of air depends not only on the concentration of oxygen in the air, but also on the air pressure. The combination of these factors is known as the “partial pressure of oxygen” (ppO2, often simplified to pO2). When looking at the “breathability” of air, we use something called the “alveolar gas equation” which tells us partial pressure of oxygen is at the alveolus (the pAO2) (the part of the lung where gas exchange takes place).

Alveolar gas equation:

PAO2 = (FiO2 x [Patm – PH2O]) – (PaCO2 ÷ R)


FiO2 is the fraction of inspired oxygen (0.21 at room air)
Patm is the atmospheric pressure (760 mmHg at sea level)
PH2O is the partial pressure of water (47 mmHg at 37 degrees C),
PaCO2 is the arterial carbon dioxide tension
R is the “respiratory quotient” which for our purposes can be considered a constant of 0.8.

(PaCO2/R) can, for our purposes, be estimated to be about 50.
PH2O can also be called 50 for simplicity.

Without needing a formal mathematics background, you learn a bit just from playing with the variables. For example, changing the FiO2 will change the amount of oxygen you breathe. A normal sea-leval PAO2 is about 100 torr. If you give someone 100% oxygen by face mask, you can increase the PAO2 to about 660 torr (more or less).

Additionally, changing the atmospheric pressure, for instance by going up a mountain, will change your PAO2. For example, if you visit the foothills above Denver, the atmospheric pressure is only about 605 torr, making the PAO2 about 66 torr. With the addition of 100% oxygen by face mask, you can only achieve a PAO2 of 505 torr, significantly less than the 660 achievable at sea level. (This also lets you calculate the supplemental oxygen necessary to bring you to your accustomed PAO2. Since I live at sea level, I’m accustomed to a PAO2 of 100 torr. To feel “comfortable” in the foothills above Denver, I would need to breathe a 27% oxygen mix rather than my usual 21%. I’ll probably skip the oxygen tank.)

Now, let’s say I drive up to Pike’s Peak, an altitude of about 4500 meters. To feel at home, I’d have to take in about 40% oxygen, which is quite a bit.

One of the consequences of this equation is that there is a maximum altitude at which, no matter how high the oxygen concentration, you just can’t breathe. For example, at 9,000m (just above the height of Mt. Everest), the maximum PAO2 you can achieve, while breathing 100% oxygen, is 150 torr. This is survivable—for a time, until oxygen toxicity begins to kill you. At about 11,000 meters, where commercial jets fly, breathing 100% oxygen will give you a PAO2 of about 87 torr—that’s uncomfortable. At 15,000 meters (almost 50,000 ft), even with 100% oxygen, the PAO2 is only about 50 torr. If you get much higher, it won’t matter how much supplemental oxygen you take, there just won’t be enough atmospheric pressure for you to live. This is the primary reason astronauts wear pressure suits.

So now we understand everything there is to know about what we breathe. We know that we need a combination of an adequate concentration of oxygen, and adequate air pressure. But how does all that yummy oxygen get where it needs to be?

Stay tuned…


  1. OK, I’m sorry, I can’t help it–but the verb is BREATHE.

    Grammar nazi aside–Can you explain oxygen toxicity?

  2. Dizzlski

    While stationed on a US Navy submarine, it was suggested to the Captain that lowering the O2 onboard would limit fires breaking out. He decided (wisely) that he would like his crew to actually be able to fight a fire if one did spark up (not to mention any number of casualties which occur).

  3. Even more insidious than hypoxia on a sub or other closed environment is CO2 poisoning.

  4. Punk Rock Medicine

    Solid breakdown of O2 acquisition.
    Just remember, nobody said you have to use 100% on top of Everest…
    O2 toxicity is avoidable.


  5. Fascinating! I must admit that I bought into the notion that there was “less oxygen” the higher you went, mostly because I didn’t bother to think about it much. As someone with asthma, I should have realized that the amount of oxygen doesn’t matter as much as how much your lungs can actually absorb.

    So is the altitude sickness I get when I visit Breckenridge (headaches, nausea) caused by the lowered air pressure or the lowered O2 in my blood?

  6. Even more insidious than hypoxia on a sub or other closed environment is CO2 poisoning.

    I’m not sure I agree with this. As I understand it, the breathing reflex is triggered by CO2 sensors, so that high CO2 concentration will be very noticeable, in that everyone will feel short of breath. Hypoxia, OTOH, is painless and more or less unnoticeable. (I have direct evidence of that from a helium-breathing experiment done by a friend. Woke up on the floor going “whaaaa?”, he did.) So I’d go with the latter for insidiousness.

    Or did you mean CO2 poisoning is easier to come by because it requires such low concentrations?

  7. insidious was a poor choice of words.

    Increasing pCO2 does cause hyperpnea. I suppose the point I was trying to make was that even with an adequate PO2, increasing PCO2 can kill you. You can have adequate O2 supplies, but if your CO2 scrubbers go out, you’re f’d.

  8. Learn something new everyday. I thought that the reason that astronauts wear pressure suits is so that their blood doesn’t boil.

  9. Okay, thanks, that makes sense then. And yeah, it’s relatively easy to keep enough O2 around and a bit harder to scrub out the CO2, as they found on Apollo 13, e.g. (Hooray for duct tape!)

    Also, thank you for the term “hyperpnea”. I had to look it up (and teach Firefox how to spell it), but it’s a great word! Right up there with “cnidarian” in the list of words that make my vocal tract glad I’m not in the biological sciences.

    Also also, why does PACO2 come into the equation? Does it bind to the same sites on the hemoglobin as the O2? Never mind, I just looked that up too, and see that it’s modulated via the blood acidity. From the Wikipedia article on the Bohr effect, it seems the acidity of the blood is raised by the CO2 coming out of tissues, thus facilitating the transfer of O2 to those tissues. Sweet!

  10. And part II is coming…

  11. Hedberg, you’ve been reading too much bad science fiction. Or anyway, scientifically incorrect science fiction. Though you’d experience some evolution of gas in the blood, your blood would not boil, any more than it boils when you ascend too quickly while SCUBA diving.

    Geoffrey Landis has a very complete explanation of what happens if you are exposed to vacuum. Let’s just say that the fact that your blood doesn’t technically boil is in the same category as “You didn’t see Lefors out there, did you?” at the end of Butch Cassidy and the Sundance Kid.

  12. Learn something new twice a day.

    From Landis:
    If the external pressure drops to zero, at a blood pressure of 75 Torr the boiling point of water is 46 degrees Celsius (115 F). This is well above body temperature of 37 C (98.6 F). Blood won’t boil, because the elastic pressure of the blood vessels keeps it it a pressure high enough that the body temperature is below the boiling point

    My error was not from being misinformed by reading science fiction, bad or otherwise, it was from being misinformed in a grade school science class about fifty years ago. It’s not important, anyway, as the point of the comment was political and not about physics or physiology. A joke’s not funny if you have to explain it.

  13. Sorry, I shouldn’t have assumed it was from science fiction, since many teachers and websites and books, who really ought to know better, still claim that your blood will boil (or worse, your whole body will explode) in vacuum.

    Sorry about your joke, too!

    Not to mention that run-on sentence at the start of this comment.

    Well. My work here is done. ***slinks away***

  14. Anonymous

    Lots of things you learn in grade school turn out to be not true. You want kids to learn about changes in atmospheric pressure with altitude and how boiling temperatures change with pressure and about the ill effects caused to animals as a result of extremely low air pressure so you end up telling them that if they get to a sufficiently high altitude their blood will boil even though it’s real cold up there. Then you go to English class and learn to write 100 word sentences with no punctuation just like Cormac McCarthy when what you really want to do is be able to write songs like Cormac McCarthy.

  15. Calli Arcale

    Some interesting factoids about oxygen in space:

    * The Space Shuttle maintains an atmosphere of about 14 PSI (yeah, it uses imperial measures — sorry), with about 20% oxygen and 80% nitrogen. It depresses to 10 PSI prior to EVA to make it easier for the spacewalkers to adjust to the suit environment.

    * ISS maintains 14 PSI as well, but the Quest airlock can go down to 10 PSI for EVA-prep purposes.

    * Spacesuits for EVA (the American EMU and the Russian Orlan) go down to 5 PSI with a 100% oxygen environment — and even at 5 PSI, grasping objects in the inflated suit is described as being like squeezing a tennis ball constantly for eight hours straight.

    * There is one reported instance of people dying of “sucking space”. The crew of Soyuz 11 were killed when a pressure equalization valve opened during module separation (just after retrofire, and just before reentry) and vented the cabin atmosphere. The vehicle performed flawlessly from that point on, bringing them to a perfect touchdown, but the crew were already dead, and were given a state funeral. They remain the only people ever to die in space. (The Columbia crew had descended to a lower altitude by the time of their deaths.) All subsequent Soyuz crew have been required to wear pressure suits, in case something like this happens again.

  16. (yeah, it uses imperial measures — sorry)
    No reason to apologize when there are lots of non-SI pressure units commonly used. PSI is no more nonstandard than bar or atm or torr or mm of Hg. The SI unit of pressure is Pa (Pascal). I don’t like that unit as I always have to look up the factor to convert to N/m^2 so that I can use the value.

  17. Also, thank you for the term “hyperpnea”. I had to look it up (and teach Firefox how to spell it), but it’s a great word! Right up there with “cnidarian” in the list of words that make my vocal tract glad I’m not in the biological sciences.

Leave a Reply

Your email address will not be published. Required fields are marked *