Hold your breath Part 2 | Freedive Earth

Hold your breath Part 2

In this section we're going to look at how the amount of air we take in with the final breath affects the amount of oxygen we take down, and discuss some ways of measuring the relevant lung volumes.


The air we breathe is 21% oxygen. If we measure the total pressure of the air we find that it amounts to roughly 100 Kilo Pascals (KPa) which is pretty much the same as 1atm, 1bar or 14.7 PSI (for all you Yanks out there). We're going to stick with KPa because it makes the maths easier: 21% of 100 is 21 so the Partial Pressure of Oxygen in the lungs (PO2) is 21KPa. Because of the inefficiency in the transfer of oxygen to the blood by diffusion, the maximum Partial Pressure of blood in the Arteries (PaO2) is around 11KPa when breathing air.


If we breathe 100% Oxygen before a static instead of air, the situation becomes quite different. The PO2 (that's the one in the lungs just to remind you) is now 100KPa (i.e. the whole 1atm), roughly 5 times higher than when breathing air alone. The result of this is that the PaO2 (in the arteries) is going to be much higher as well - at least 20KPa in fact. So there's a benefit, not only in terms of the percentage of oxygen you can get into your lungs, but also what you can carry in your blood. That's why it's not allowed in competition.

Measuring Lung Volumes

When we're restricted to just breathing air, the only way we can increase our initial oxygen load before a hold is to take a bigger breath, or to pack more. A spirometer will help you to measure the size of your final breath, if you've never tried one, you can have a look and buy one online here. Alternatively you can make one by filling a water cooler bottle (one of the big 19l ones or something) with a known volume of water and carefully turn it upside down underwater so that no air gets in. Take a full inhale, pack as much as you normally would, then exhale through a hose into the bottle. An amount of water equal to the volume of air you've exhaled will be displaced from the bottle.

The problem with, a spirometer is that it only measures the maximum amount of air you can inhale and then forcefully exhale, a value known as the Forced Vital Capacity (FVC) or just Vital Capacity (VC). The amount of air that you can't access by exhaling, the Residual Volume (RV), remains hidden. Measuring RV accurately is usually done using a technique called helium dilution: the subject sits in an airtight box or is attached to a sealed spirometer and a known (small) amount of helium is pumped in to the air they're breathing. Critically, this helium doesn't cross the membrane of the lungs into the blood because it's so insoluable at atmospheric pressures. So the poor critter in the box keeps on breathing this extra gas for a few minutes, by which time the helium has been diluted by the air remaining in the lungs and a technician can measure the new concentration of helium. Knowing the volume of the box and the FVC from spirometry they're able to work out the RV using the formula:

P1V1 = P2V2

Where P is Pressure and V is Volume 

Sadly, helium dilution chambers aren't widely available on ebay, and cost a bit more even than a trip to Vertical Blue, but here's a nifty little trick you can try to measure your own RV using a spirometer, a shot line and 10m of water:

Take a breath at the surface. Descend to 10m along the line and exhale absolutely everything you can without reverse packing. This leaves you with just your RV remaining in your lungs. Ascend to the surface and exhale into a spirometer. The 'RV' you had at 10m will have expanded to twice its size in accordance with Boyle's law so the amount you exhale will be, more or less, your RV. Caution!! Exhaling underwater is dangerous for all sorts of reasons!! Don't do this unless you're comfortable with it, and you're under the supervision of someone more experienced than you.

If the spirometer trick isn't an option for you, you can always guess. Most people have an RV of around 25% of TLC. If you do lots of exhale stretching as part of your training, you can probably get it down as low as 20%, but not much less.

Calculating Lung and Oxygen Volumes

If you have a measured volume for RV, calculating TLC is easy, just add RV to VC. To work out your Total Lung Capacity (TLC) using VC (from spirometry - we'll use a value of 6l in this example) and a percentage RV of 20% of TLC:

First, calculate the percentage of TLC represented by VC that's: 

100%-%RV = 100%-20% = 80%

 Next, work out 1% of TLC:


In this example:

VC/80 = 6/80 = 0.075

Multiply by your %RV, in this example:

0.075 x 20 = 1.5

That's your RV in litres

Then add RV to VC

 1.5 + 6 = 7.5l

As we've previously discussed, 21% of that air is oxygen, so the total amount of oxygen carried in the lungs here is:

 0.075 x 20 = 1.5

If you'd breathed pure oxygen of course, the amount of oxygen would be 7.5l (7.5x100%).

Let's say you pack another 10 times on top of this. Depending on your technique , each pack will give you around 100ml of air so that's 10x100ml=1l more air, taking your TLC to 8.5L. Using the same formula , that's now 8.5x0.21=1.785 of O2.

 Broadly speaking, it doesn't really matter how much air you put into your lungs, the PaO2 is driven by the difference in Partial Pressure between the lungs and blood and will stay at about 11KPa regardless. One possible exception to this is if you pack enough air to raise the pressure in the lungs above 100KPa, this will probably drive more oxygen into the blood, but the difference will be small (and that's another story!). The main benefit from packing is more that the total amount of O2 in the lungs is higher.

In the next section we'll look at the role of blood physiology in the breath-hold. Do not adjust your set.


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