Why You'll Die if You Hold Your Breath for Too Long
Those of you who’ve been following my series of introductory lectures to Freediving physiology will hopefully have a solid grasp of thorny concepts like Boyle’s law and partial pressure. Now it’s time to put some of those together in order to understand how the body copes (or doesn’t cope!) with the demands of deep water diving.
As we’ve already seen, oxygen is all around us and makes up about 21% of the air that we breathe. It’s pretty obvious that we’d die without it, but have you ever wondered why that is? Personally, it’s pretty much all I think about, but then I’m not your average otter. Namaste!
Oxygen is used in the body for the production of energy in a form that cells can use to carry out all the processes they need to to keep you alive. This process is called respiration, and goes something like this:
Budding chemists amongst you will notice that this is essentially a combustion or burning reaction, exactly the same as the reaction that allows us to make fire, so it’s really quite accurate to talk about burning calories. Useable energy is released in the process. Starve your cells of oxygen for long enough and they’ll stop working, simply because they’ve run out of energy, a bit like when you smother a flame or block up the air intake of your car.
Oxygen in the Body
In the lecture on Partial Pressure, we looked at how gasses can partition into a liquid or aqueous phase depending on the solubility of the gas in the liquid, and the pressure of the gas in the gas phase:
You might also remember that in the blood, just about the highest partial pressure you can get by breathing air at the surface is about 12kPa (91.2mmHg). Oxygen is carried in the blood by haemoglobin, a protein molecule which looks a bit like this:
Haemoglobin is excellent at binding oxygen, way better than blood plasma alone and allows you to carry something like 1000 times as much oxygen as you could if you didn’t have it. Oxygen binds to haemoglobin more strongly when the partial pressure is higher:
As the partial pressure decreases, oxygen is released from the haemoglobin and moves into the tissues where it’s used for respiration.
Oxygen and the Brain
Every cell in your body requires oxygen to survive but some of the most prolific users of oxygen are the cells that make up your brain. Despite accounting for only about 1.4% of the mass of your body, your brain can consume up to 20% of the energy you produce… (which...err... is quite a lot, when you think about it…geddit?). Starve your brain of oxygen and you’ll lose consciousness as the cells begin to shut down and in a static breath-hold that goes on long enough to cause you to black out, that’s exactly what happens.
Oxygen and Deep Diving
As you descend under water, the partial pressure of oxygen increases in accordance with Boyle’s Law. If you’ve understood the last 2 lectures, you’ll hopefully be able to see why that is: Partial pressure is just the contribution that an individual gas makes to the overall pressure. In air, for example, oxygen contributes about 21% (roughly 21kPa at sea-level) to the overall pressure. If you double the pressure in your lungs by diving 10m underwater, the partial pressure of oxygen also doubles to about 42kPa. Dive deeper and the pressure increases even more:
Remember that the lungs aren’t 100% efficient at extracting oxygen from the air, so there’s always going to be a smaller partial pressure of oxygen in the blood than there is in the lungs:
This too is slightly misleading, clearly on a breath-hold dive to 30m and back again, you might expect to use some of the oxygen you’ve taken with you:
Remarkably, we can see that at 10m on the way up, it looks as though we’ve got pretty much the same amount of oxygen in the blood as we did before we’ve started! The pressure of the water is compensating for the amount of oxygen we’ve used during the dive by driving more oxygen from the lungs into the blood than it can possibly hold at the surface. Let’s look at what this means for haemoglobin:
If you’ve ever used a pulse oximeter during a breath-hold, you’ll know that it takes a long time for your oxygen saturations (shown on the vertical axis of the graph above) to drop even below 90%. Blackout occurs at saturations of around 50% which corresponds to a partial pressure of about 4 kPa. This is the level at which haemoglobin can’t supply enough oxygen to the brain to allow it to continue the process of respiration. Notice, then, that our diver in the table above, ascending from 30m and surfacing with a partial pressure of 3 kPa will almost certainly black out before he gets there. Notice too that even if we assume that he uses no oxygen in the last 10m of the dive, the drop in partial pressure that occurs in the last 10m will bring him perilously close to blackout too, just by the simple fact of ascending. This is shallow water blackout. So, that’s it for this week, tune in again soon for more deep-down physiology that you really Otter know… Got a question for Dr Otter? Drop us an email, write to us on facebook or comment below. Till next time!