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?