How Long Can a Piglet Hold its Breath?
With a particular interest in freediving, she’s one of the few people in the world currently studying human responses to apnea in the aquatic environment. She’s published papers on everything from the effect of fasting on Static Apnea performance to the length of time spent underwater by traditional or tribal freedivers each day. She’s also trained some pigs to freedive… but more on that later. Here’s another pretty amazing character in the Freediving hall of fame:
Sperm whales can dive to an eye-watering 2000m and are thought to remain submerged for as much as 90 minutes. What’s more, they do so on a partial exhale, to limit their absorption of nitrogen and escape the bends. How is this possible?
Physiological Adaptations of Diving Mammals
Many diving mammals, and sperm whales in particular have a lot of extra diving-related advantages not available to humans. These include a collapsible ribcage (useful in avoiding lung-squeeze) and special oxygen-hungry myoglobin which is packed extra-tightly into their cells. Sperm whales also have a unique diving organ called the Spermaceti which allows them to echolocate at depth, stun fish(!) and may help with buoyancy control. Far more important than this, however, is the so-called ‘Mammalian Dive Reflex’ which produces characteristic changes in (among other things) heart rate and blood distribution during, and even before a dive. Although it’s called the mammalian dive reflex, it actually applies to all these types of animals too:
Put simply, the dive reflex results in:
- Reduced heart rate (Bradycardia) 2.
- Redistribution of blood to the central organs (Peripheral Vasoconstriction and Blood Shift)
- Contraction of the spleen All of which are aimed at increasing the amount of oxygen available, either by reducing the rate it’s consumed (1 and 2) or its availability in the blood (3).
How Does the Mammalian Dive Reflex Work?
In its simplest form, the dive reflex works in two ways By prioritising three organs: Heart, lungs and brain. By triggering the release of red cells from the spleen. As we’ve discussed before, every cell in your body requires oxygen to function. By far the biggest user of oxygen in the body is the brain, which, despite accounting for only about 2% of the body’s weight, uses around 25% of its total oxygen. The brain, therefore, is very sensitive to breath-holding. If it doesn’t get enough oxygen, even for a few seconds, you’ll black out, no question. Oxygen is carried by haemoglobin in the blood, which is pumped around the body by the heart. The heart is a muscle and so requires oxygen to contract. Without the heart beating, no other organs, including the brain, can get any oxygen at all, so the heart itself must also have a constant supply of oxygen. Deprive it, and a heart attack is the result. Clearly, oxygen gets into the body and into the blood via the lungs. Without blood moving through the vessels of the lungs, it doesn’t matter how much air you pack in there, none of it will get through to the organs that need it. A situation where there is air in the lungs, but no blood to carry it is called a ventilation-perfusion (VQ) mismatch. Taking all this into account we can see that the critical organs of the body that must have a constant supply of oxygen and blood to ensure our survival are the heart, lungs and brain. All the other organs, as well as your limbs, fingers and toes will obviously suffer if they’re deprived of oxygen but, for short periods at least, they can do without it. The dive reflex exploits this fact by allowing blood (and therefore oxygen) to be diverted away from areas that need it less, by vasoconstriction (narrowing) of the arteries that supply them, and by slowing down the heart to reduce the amount of oxygen it has to use in pumping blood around.
Freediving and the Spleen, fact or fiction?
The spleen is a mysterious little organ located on the upper left side of the abdomen:
Its function is not completely understood, but we know it has a role in storing blood cells, and is an important part of the immune system. Nevertheless, it’s not ‘essential’ to life, and is often surgically removed when damaged or dysfunctional. There is good evidence that the spleen is involved, even for humans, in the dive reflex, where it’s been shown to help in prolonging breath-hold. It does this by contracting, and releasing stored red cells during breath-holding and even, in some cases, during the breathe-up period. The effects are prolonged: In one study, subjects spleens didn’t return to their normal size until about 8 or 9 minutes after their last breath-hold. Erika’s team have also shown that body:spleen size ratio, as well as their total lung volume, is important in predicting an athlete’s ability as a freediver.
The Dive Reflex in Humans - Controversy in the Scientific Community
In animals like those mentioned above, and especially aquatic species like whales, dolphins and seals, the mammalian dive reflex is huge, with Bradycardia down to, for example, just 10 beats per minute. The interesting thing, and the finding that’s caused controversy about the dive reflex in humans, is that other animals, like pigs for example, which never go near the water also have bradycardia when they get their faces wet: This one was trained by Erica Schagatay’s team to voluntarily immerse its little snout for up to 50 seconds. Bless.
Lots of people like to talk about the aquatic past of humans but studies like this can be taken to suggest that the ‘dive reflex’ in land-animals is really just a protective response to hypoxia, and has nothing to do with diving. So what’s really going on?
Triggering the Mammalian Dive Reflex in Humans
Traditionally, we’re taught that there are 3 main triggers for the dive reflex in humans: 1. Facial Immersion 2. Rising CO2 3. Increasing Pressure at Depth The evidence for facial immersion is pretty good, and has even isolated the nerves involved:
The Cutaneous Distribution of the Branches of the Trigeminal Nerve. V1, the Ophthalmic Division, is the most important in triggering the dive reflex in humans. Bradycardia does happen when humans hold their breath on dry land too, presumably because of rising CO2. In many cases, though, there is debate about whether this represents part of a ‘dive reflex’ or purely a survival mechanism. The effect of pressure is less well understood but it looks as though Bradycardia is greater when diving to depth compared to breath-holding at the surface and also happens to scuba divers who aren’t holding their breath at all. In reality, the factors controlling the dive reflex (or whatever it might be) are likely to be complex:
Triggers of the Mammalian Dive Reflex - Breathtakingly Complex
What’s definitely been shown to be the case, though is that trained divers show a greatly increased effect of the dive reflex than untrained people during breath-hold and even, amazingly, before diving begins. Although it’s possible to argue that all of this just relates to some basic survival mechanism, to me that doesn’t explain why the reflex is triggered by facial immersion, well in advance of a decrease in arterial oxygen, or in Scuba-Divers who are breathing normally. Nor does it take into account the fact that psychological stress also plays a role in its effect: In one study published in the journal “Aviation Space and Environmental Medicine” showed that psychological training of helicopter pilots helped them to improve their breath-hold time during a simulated crash into cold water. In untrained individuals the ‘dive reflex’ was over-ridden by the ‘cold-shock’ response which resulted in an increase in heart rate and respiratory rate, despite the presence of peripheral vasoconstriction. The trained individuals held their breath on average 80% longer than those who were untrained. If all that’s at work is a survival response to a threatening situation, or low arterial oxygen, why should that be? Dr Otter is the resident physiology expert at Freedive-Earth. If you’ve got a question for her, put it in the comments box below, or contact us!