The oxygen you breathe is transferred to your blood and delivered to the various tissues of your body, where it is converted into energy. The waste product of this process is CO2, which is carried back to the lungs and released from the body upon exhalation. When you hold your breath, O2 is still converted to CO2, but the latter has nowhere to go. It recirculates in your veins, acidifying your blood and signaling your body to breathe, first with a burning sensation in your lungs, and eventually in the form of strong, painful spasms of your diaphragm.
The blood of a seasoned free diver has been shown to acidify more slowly than those of us who spend our lives inhaling and exhaling reflexively. Activation of the sympathetic nervous system causes their peripheral blood vessels to contract soon after they stop breathing, thereby conserving oxygen-rich blood by redirecting it from the extremities to the vital organs, especially the brain and heart.
ANALYSIS: Extreme Breath-Holding: How It's Possible
Many freedivers also practice meditation to literally calm their hearts. Reducing the body's metabolic rate attenuates the conversion of oxygen to CO2). Meditation has a calming effect on the mind, as well; much of the battle, when holding one's breath, is mental. To know, logically, that your body can persist on the oxygen already available to it. To ignore outright the mind and body's compulsion to breathe.
There are other tricks to holding one's breath that rely less on extended training and more on increasing what divers refer to as one's "total gas storage." Take "buccal pumping," for instance, which was developed by spear-fishing breath-holders long ago and introduced to sport diving by U.S. Navy diver Robert Croft in the 1960s.
Also known as "lung packing," buccal packing involves taking the deepest breath possible, then using oral and pharyngeal muscles, along with the glottis, to hold the throat shut while shunting air, cheekfulls at a time, from the mouth down into the lungs. It's been said that by repeating this pumping movement up to 50 times, a diver can increase his total lung capacity by as much as three liters.
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A 2003 study that measured the lung capacity of a breath-hold diver gives a more conservative figure, noting an increase following buccal pumping from 9.28 liters to 11.02. Lung capacity can also vary considerably from person-to-person: The average lung capacity is 4 liters for women and six for men, though acclaimed free diver Herbert Nitsch has a reported lung capacity of 14 liters.
Then there's hyperventilating, which divers often do to flush their systems of CO2 and pre-load their bodies, instead, with unconverted oxygen. The most extreme version of this technique involves breathing nothing but pure O2 for as much as 30 minutes before submerging one's head beneath the water. The air we breathe is only about 21 percent oxygen (the rest is mostly nitrogen), which means that a breath held on atmospheric air will last significantly shorter than one held on pure O2.
This technique was how magician David Blaine managed to break the world-record for breath-holding in 2008, with a time of 17 minutes and 4 seconds, and how Stig Severinsen blew that time out of the water in 2012, with a mind-blowing performance of 22 minutes. (It bears mentioning that "static apnea," as discussed earlier, is defined by AIDA, and so is distinguished from the Guinness World Record for "breath holding underwater," which allows for the use of pure oxygen in preparation.)