The Impossible Physiology of the Free Diver


Photo by Jacques de Vos

The amazing underwater athletes are rewriting the science of the body.

The then-33-year-old, South African, former acting student knew the feat would require rigorous training and commitment. But Prinsloo felt at home in the water. She’d grown up on a dusty, 200-hectare horse farm outside Pretoria, chasing her sister from dam to stream to swimming pool and back and dreaming of becoming a mermaid. So in the spring of 2011, Prinsloo packed a bag and traveled to an ashram in the foothills of the Indian Himalayas. She spent a month meditating and practicing Vinyasa Yoga.

After six weeks, it was off to Dahab, Egypt, an isolated Bedouin Village hemmed in on one side by the Red Sea, and the Sinai Mountains on the others. On dive days, Prinsloo and fellow diver Yaniv Keinan hopped in a 4×4 and bounced past tourists on camels down a rutted desert track until they reached the water. They waded 50 feet through knee-high water out to the edge of a sinkhole, about 400 feet deep, known as the “Blue Hole.”

As Prinsloo floated on her back above the hole and prepared to dive, the mood was every bit as solemn as at that Ashram back in India. Quiet and still, Prinsloo focused on the oxygen moving in and out of her lungs, becoming aware of her heartbeat, trying not to think. When she moved, she did so in exaggerated slow motion, as if in a trance.

At a depth of 200 feet, she opened her eyes in the translucent water. “It was like a blue glow,” she said. “The way light would look to a moth.”

Prinsloo’s slow, deep breaths ensured the maximum oxygenation of her blood, and the opening up of any constriction in the airways of her lungs. She needed her lungs loose and relaxed, able to expand to store the maximum amount of oxygen when she dived. After five minutes, Prinsloo reached a deeply meditative state. “As you drop down, the perfect dive is where you don’t have a single thought,” she said.

Prinsloo dove with her eyes closed, alternatively focusing on kicking and equalizing the pressure between her ears by pinching her nose and blowing. As she descended, her body underwent an amazing transformation. Her heart rate and metabolism slowed dramatically. The arteries in her limbs constricted, pushing oxygen-rich blood back to her body’s vital organs. The walls of her lungs shrunk in volume, while at the same time filling with extra blood and hardening to counteract the increasing pressure at depth.

About 60 feet down, Prinsloo reached “negative buoyancy.” Her now dense body gave gravity the upper hand over buoyancy. She surrendered and let gravity pull her down, falling as the water pressure closed in on her with a soft embrace. It was almost as if Prinsloo had found a way to achieve her childhood dream. She had transformed, if not exactly into a mermaid, than into a centered, aquamarine version of herself. At a depth of about 200 feet, she opened her eyes in the translucent water. “It was like a blue glow,” she said. “The way light would look to a moth.”

What free divers are capable of “is amazing,” said Peter Lindholm, a group leader for baromedicine and human physiology in extreme environments at Sweden’s Karolinska Institute. “But there’s nothing magical about it. You can explain it if you know physiology.”

In recent years, the feats of free divers have forced Lindholm and other scientists who study physiology to explain not only what humans are capable of underwater, but how much control we have over the speed of normal metabolic functions. Free divers, they say, may push evolutionary buttons honed in a simpler epoch. A time, perhaps, before obstetricians, when it was paramount to slow down to survive a perilous passage through a birth canal that restricted blood flow. It was a time when natural selection, for whatever reason, favored traits that allowed us to downshift our biological systems into a slow gear we are only beginning to rediscover. It can be a dangerous process, physiologists say, but available to us all…



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