Freediving on a single breath
Free-divers face low oxygen and high pressure enviroments as they dive to great depths on one lungful of air. We spoke to Peter Lindholm from the Karolinska Institute in Sweden to find out more about the physiology of achieving this.
Chris - So, when people do professional free-diving, how deep do they go and how long do they have to hold their breath for?
Peter - The competitive divers, they can hold their breath and swim for something like 4 to 5 minutes which gives them an opportunity to swim down to about 100 meters using their swimming trunks basically. No fins, no nothing. But if they use a monofin, the record today is about 25 meters deeper. If you compare that, that's 250 meters which is about 5 lengths in the big Olympic swimming pool. So, it's quite significant.
Chris - Stupendous depth, isn't it? What will they do before they go down to that depth to prepare themselves physiologically?
Peter - First, they train for a few years. You must take that into account; they are good swimmers, and they have adapted their body to breath holding in some extent and they also have a good stamina, good physical shape. But if you're looking at what they're actually doing on the day, they would prepare most likely with not eating a lot. You don't want to be too full. Then they prepare by warming up with a few breath holds, shorter breath holds to get into the warm up phase then they hyperventilate. They usually hyperventilate for a few minutes, reducing the CO2, the carbon dioxide stores in their body, and also, topping up a little more oxygen in their lungs and in their venous blood. So, they actually maximize the amount of oxygen stores they could carry for this dive and they severely reduce the carbon dioxide stores which has a few other implications. And then before they go down, they use something they call lung packing which I call glossopharyngeal insufflation or glossopharyngeal breathing where they actually swallow air into the lungs. You can learn this manoeuvre and you swallow air into your lungs and you can now actually pack this air down, and the best divers pack 50% extra air into their lungs compared to what their full lung volume would be without this technique.
Chris - Does that not have a consequence for, it makes them a lot more buoyant?
Peter - Yes, so depending on what depth you're looking at. If you're looking at a shallow dive, you know, snorkelling around in 5 to 10 meters, you don't want to be too buoyant. But if you're going for 100 meters or deeper, they use a lead weight. So, they usually buoyant themselves so that somewhere half down, they actually become negatively buoyant, so they actually can sink down to the bottom. And then they have to swim up and they're very heavy because the air is compressed at depth, so they actually become very heavy. And when you swim up, it's tough swim in the beginning but air will then expand, and it will give them great buoyancy in the end. The buoyancy at the end is a safety factor. It's good to be buoyant if you're tired or if something happens, you want to pop up.
Chris - Can we just return to the point you made about carbon dioxide first because you said there were some other manifestations in the body - when a person hyperventilates, they feel quite giddy. Is that sensible when you're going to be diving to more than 300 feet underwater?
Peter - If you hyperventilate regularly like the divers do when they prepare for these dives, the brain adapts somehow, so they're not so giddy. But there are some other problems and actually, they can get giddy at depth because of nitronarcosis, but that's another issue. If you had anything more about the carbon dioxide, it comes with a certain risk too because if you do this, you might not have the problem of being giddy if you are used to it, but it takes out the respiratory drive, so you might not feel the same urge to breathe as you would, holding your breath normally.
Chris - And this of course means you may not notice that you're becoming extremely low in oxygen and you may not know when is the right time to surface.
Peter - Exactly. If you swim to 100 meters, you need to swim another 100 meters up. So, if you would just continue, continue down and not look to get up, you would probably reach 200 meters, right? So, the divers have to decide to turn, not depending on the carbon dioxide signal or their respiratory drive, or their urge to breathe. They have to decide that, yesterday or last week, I made 99 meters, I came up, I felt good, I feel strong today. I'll try 100 meters.
Chris - And at those stupendous depths and therefore, pressures because every 10 meters of water is 10 times atmospheric pressure, isn't it? So, that's a huge amount of pressure. This will significantly squeeze the volume of air in the lungs down. Does this mean more oxygen is going to dissolve in the blood?
Peter - Yes. You actually can squeeze out more oxygen from the little there is in the lungs, but you'll have to sort of survive coming up because you have to pay that back when you go up. So, the last 10 meters before you surface, the pressure will reduce very quickly and if you have squeezed out too much of your oxygen, there is not enough left in the lungs to make you to the surface. So, you could actually have a situation where you have put out too much oxygen into your body. You still have oxygen in that blood in their muscle, but the brain doesn't get enough oxygen during those last seconds of the surfacing.
Chris - If we look around the world, we haven't always had competitive Olympic free-divers, but we have had people who have in certain cultures and civilisations become very good at diving very deep for very long times. I'm thinking of pearl divers and things like that. Are some people very good at doing this genetically?
Peter - There's still research to be done if we're going to decide how much is genetics and how much is training, but there are a few traits that makes you a better diver. The first I would say is big lungs and people are different. Some people have actually bigger lungs to their body size than others, so that's one.
Another trait is something we called a diving response. It's an ability to, when you hold your breath, reduce the heart rate and that also depends on your ability to vasoconstrict to limit the blood flow going to your limbs. What this means is that the body has an ability to reduce or limit the blood that flows to your, not so important parts. Depends what you mean by important, but in terms of oxygen supply, your muscles or bones, or other skin structure, they can survive for at least half an hour without oxygen, the brain cannot. So the body will save some or conserve some of the oxygen in the lungs to be used by the brain during this dive. This diving response, the ability to vasoconstrict the vessels and reduce the heart rate, varies a lot between individuals. So, that would be two traits which both have some potential to be trained, but I think personally from the data I've collected and seen, I think a bigger component is genetic than the training component in this.