Although seals, whales, and dolphins have had cause to adapt to oxygen deprivation on dives, some humans can outdo them – at least on some measures. Take that, millions of years of evolution as marine mammals. The discovery may lead to better treatments for people who suffer a loss of blood flow to the brain.
Even when oxygen deprivation is not severe enough to kill, it can do considerable damage to brain cells and induce unconsciousness. The fact that some people are able to train themselves to operate underwater in conditions where most humans cannot has inspired Professor Erika Schagatay of Mid-Sweden University for 30 years of study.
In Philosophical Transactions of the Royal Society B, researchers present a major advance in that work, thanks to new technology. "Before now, understanding the effects on these exceptional divers' brains and cardiovascular systems during such deep dives, and just how far these humans push their bodies, was not possible, as all research was done during simulated dives in the lab,” Schagatay said in a statement.
The team equipped divers in the ocean with devices that measure heart rate, blood volume, and brain oxygen levels, that were specifically adapted to withstand the pressures at impressive depths. This not only allowed them to study the extremes the human body is capable of, but also to provide a warning when a diver has gone too far and is in danger of losing consciousness. Unless rescued, a blackout would be fatal under these conditions.
Freediving is one of the most extreme of extreme sports, with divers sometimes reaching depths below 100 meters (328 feet) on a single breath. The authors had five leading freedivers engage in 17 dives to depths of between 21 and 107 meters (70-350 feet) for periods of up to 249 seconds. Although much longer dives have been achieved, the exceptional pressures the divers faced made these far more challenging. The same equipment was used to track juvenile grey seals on their own dives.
Project leader Dr Chris McKnight of the University of St Andrews said; "We measured heart rates as low as 11 beats per minute and blood oxygenation levels, which are normally 98 percent oxygenated, drop to 25 percent, which is far beyond the point at 50 percent at which we expect people to lose consciousness and equivalent to some of the lowest values measured at the top of Mount Everest."
McKnight told IFLScience; "Seals can absolutely tolerate much lower cerebral oxygenation levels than the 'normal' human before becoming unconscious," however, the divers are going far beyond the range of normal human capacity. Despite the enormous amount of time seals spend underwater, more than 80 percent of their dives last less than two minutes, McKnight added. They're about catching fish in the most economical fashion, rather than setting records. They do this best with dives that don't deplete them too badly, enabling brief recovery times at the surface – although he noted elephant seals take a different approach.
These observations, McKnight added, may; “Offer a unique way of understanding how the body responds to low blood oxygen, low brain oxygenation and severe cardiovascular suppression.”
Schagatay has previously shown nitrate-rich foods such as beet juice can help the body cope with oxygen deprivation, in that case at high altitudes. We don't recommend trying to replicate the freedivers' achievements with nothing more than a beet juice diet.
McKnight, on the other hand, is a marine mammal specialist who has previously shown that seals go into oxygen conservation mode even before they actually dive, No doubt this is just one of the reasons they can dive for so much longer than even the most accomplished humans, despite being outmatched in the oxygen deprivation stakes.