Plunging 335 feet below the surface of the ocean and making it back on a single breath, or simply holding your breath for 11 minutes and 35 seconds, clearly requires a very special set of skills and traits.
But until a recent conference talk, I’d never considered whether those same characteristics might be useful in other settings where oxygen is scarce — such as the thin air of high-altitude trekking and mountaineering.
[...]
Schagatay’s initial research interest was in what she calls “professional” freedivers, as opposed to recreational or competitive freedivers.
[...]
These diving populations, Schagatay and others have found, share three distinctive characteristics with successful competitive freedivers, who take part in contests around the world sanctioned by AIDA, the international freediving authority:
Big lungs: In one study of 14 world championship freedivers, vital capacity — the maximal amount of air you can expel from your lungs — was correlated with their competition scores. The three best divers in the group had an average vital capacity of 7.9 liters, while the three worst averaged just 6.7 liters. And it’s not just genetic: Schagatay found that an 11-week program of stretching increased lung volume by nearly half a liter.
Lots of red blood cells: Divers do tend to have higher levels of hemoglobin, the component of red blood cells that carries oxygen. That’s probably a direct result of their diving. Even if you just do a series of 15 breath holds, you’ll have a surge of natural EPO an hour later, which triggers red blood cell formation.
But there’s a more direct and immediate way of boosting your red blood cell count: squeezing your spleen, which can store about 300 milliliters of concentrated red blood cells. Seals, who are among the animal kingdom’s most impressive divers, actually store about half their red blood cells in their spleens, so they don’t waste energy pumping all that extra blood around when it’s not needed. When you hold your breath (or even just do a hard workout), your spleen contracts and sends extra oxygen-rich blood into circulation. Not surprisingly, spleen size is correlated with freediving performance.
A robust “mammalian diving response”: When you hold your breath, your heart rate drops by about 10 percent, on average. Submerge your face in water, and it will drop by about 20 percent. Your peripheral blood vessels will also constrict, shunting precious oxygen to the brain and heart. Together, these oxygen-conserving reflexes are known as the mammalian diving response — and once again, the strength of this response is correlated with competitive diving performance.
[...]
In a study published last year, they followed 18 trekkers to Everest Base Camp at 17,500 feet (5,360 meters). Sure enough, the trekkers with the biggest lungs, the biggest spleens, and the biggest reduction in heart rate during a breath-hold were the least likely to develop symptoms of acute mountain sickness.
The size of the spleen isn’t the only thing that matters — its benefits depend on a strong squeezing response to get all the red blood cells out. In a 2014 study of eight Everest summiters, they found that three repeated breath holds prior to the ascent caused spleen volume to squeeze, on average, from 213 milliliters to 184 milliliters. After the ascent, the same three breath holds caused the spleen to squeeze down to 132 milliliters. Prolonged exposure to altitude had strengthened the spleen’s diving response. In fact, there’s also evidence that simply arriving at moderate altitude will cause a sustained mild spleen contraction, as your body struggles to cope with the oxygen-poor air.