The moment David goes under, his body goes into survival mode. His blood starts to move away from his extremities and flow toward his heart and brain. This ensures that oxygen is available for the two most vital organs in his body.
Dr. Ralph Potkin, the pulmonologist who has been monitoring David's training, says his ability to hold his breath for this length of time is incredible. "He's been training, but most importantly, David is very motivated," he says. "He's got great control of his mind over his body."
As blood concentrates in his chest cavity, David's lungs shrink by almost 30 percent. During this time, the heart also senses that oxygen is in short supply and begins to beat less often. David slows his heart rate even further through mind control. "The slower the heart rate, the less oxygen you use, and so [it's] very beneficial in this oxygen-deprived state to have his heart rate real slow," Dr. Potkin says.
As toxic carbon dioxide builds up in the body, Dr. Potkin says David's mind is able to block out the uncomfortable sensation. "This is mind over body, there's no question about it," he says. "This is a very Zen or meditative sport."