Deep: Freediving, Renegade Science, and What the Ocean Tells Us About Ourselves

by James Nestor

I ask about the convulsions. She explains that the body responds to extreme breath-holding in three stages. Convulsions are the first-stage response. “You start reacting not from the lack of oxygen, but from the buildup of carbon dioxide,” she says. “When that starts, it’s just a caution that you’ve only got a few minutes to go before you really need to breathe.” The second-stage response occurs when the spleen releases up to 15 percent more fresh, oxygen-rich blood into the bloodstream. This usually occurs only when the body goes into shock, an extreme state whose symptoms include low blood pressure, rapid heartbeat, and organ shutdown. But it also happens during extreme breath-holding. A freediver anticipates the spleen’s delivery of fresh blood, feels it happen, and uses it as a turbo-charge to dive even deeper.