Why did I write this?
For whatever reason, partial pressures are something that just did not click with me the first, second…or third time I learned it. I understood why they are important as well as the impact they have on the necessary skills for freediving, but the way they affect us just didn’t make sense to me.
So I did some re-reading, some extracurricular research, and some soul searching to understand it. What follows are the components of partial pressures as I understand them. Hopefully this helps them make sense for you.
What are Partial Pressures?
Let’s start with some really basic principles about air and gases. Partial pressures are useful to understand near sea-level, high altitudes, and at depth for freediving.
The air you breathe is made of many gases
Generally speaking air is made of the following gases.
| Gas |
% of Air |
| Nitrogen (N2) |
78% |
| Oxygen (O2) |
21% |
| Carbon Dioxied (CO2) |
0.04% |
| Other Trace Gases |
0.96% |
For the purposes of this article, we don’t care about the other trace gases.
These percentages remain the same at different pressures. But the mass of each gas changes for the same volume. This is what Boyle’s Law refers to.
Meaning, at sea-level one liter of air contains the same percentage of oxygen as one liter of air would contain at the top of Mount Everest. In fact, at the top of Everest, there is 1/3 the amount of oxygen in one liter than in the same volume at sea level.
This is why it’s harder to breathe at high altitudes. Your lungs hold the same volume of air, but there is less oxygen (and other gases) within that same volume.
< IMAGE HERE TO SHOW DIFFERENCE IN VOLUME >
Each gas in air has a different measurement of pressure.
This section to be filled out
How does Oxygen get into our blood?
Lungs are filled with little sacks called alveoli. These sacks of air have capilaries in their membranes. When blood passes through these capilaries, O2 is absobed into that blood. This absorption requires pressure. I like to think of the pressure in our lungs pushing O2 into our blood.
The amount of O2 that gets pushed into our blood is determined by the partial pressure of O2 in our lungs.
…Make sure you read that a few times. Because this is the piece of the picture that I couldn’t wrap my head around. This means that if there is more pressure in your lungs, then your blood will absorb O2 at a faster rate. This is referred to as Henry’s Law.
Why does this matter for freediving?
Before I get to the answer, let’s revisit the basic principles:
- The deeper we travel underwater, the more pressure our bodies experience.
- As air pressure increases, three things happen to the air inside our lungs:
- The volume of air decreases (Boyle’s Law).
- The mass of gases in that our lungs remain the same.
- The partial pressures of the gases increase (Dalton’s Law).
- The rate at which gases are absorbed into blood is determined by the partial pressure of oxygen in our lungs (Henry’s Law).
When applied to freediving, this means…
1. The deeper you go, the faster your blood pulls oxygen out of your lungs.
This is probably the easiest to understand. At depth, you’re lungs are pushing more O2 into your blood than on the surface. If too much of the O2 in your lungs was pushed into your blood at depth, then you might not have enough O2 in your lungs as you get closer to the surface.
2. The partial pressure in your lungs decreases as you ascend.
This is a major contributor to shallow water LMC’s and blackouts. As you get within 10 meters of the surface on your ascent, there is less O2 in your lungs, but there is also less pressure to push the remaining O2 into your blood. If you used up too much O2 at depth, the partial-pressure of O2 might be too weak to push that remaining O2 into your blood.
3. Nitrogen Narcosis (aka The Bends)
This is more common in scuba diving, but it also affects freedivers who frequently dive below 25 meters. Nitrogen also gets absorbed from our lungs into our blood. As we ascend, the partial pressure of that gas in our lungs also decreases.
If freedivers continually dive and don’t take long enough surface intervals, the nitrogen in our blood can remain and expand, which causes narcosis when those bubbles travel to our brain.