Holding Your Breath

In the short term—say, two or three minutes—you have complete control over your ability to hold your breath. But try to hold it for much longer and you’ll run into your body’s stubborn survival instinct.

As you hold your breath, the level of oxygen in your blood begins to sink. At the same time, the level of carbon dioxide—the waste left over from your body’s energy generating operations—starts to rise. Interestingly, it’s the change in the carbon-dioxide level that your body really notices. Your brain detects this increase and eventually triggers an overwhelming urge to resume breathing.

Reckless divers sometimes take advantage of biology to increase their breath-holding time by hyperventilating before they dive. (To hyperventilate is to suck in more air than you actually need, usually by taking quick, deep breaths.) After hyperventilating, the diver feels a reduced urge to breathe and can stay underwater longer. This isn’t because he’s loaded up on oxygen (he isn’t). Rather, it’s because he’s lowered the amount of carbon dioxide in his body. As a result, it takes longer to trigger the brain’s safety mechanism that forces the diver to start breathing again. This practice is dangerous on several counts. The most obvious risk is that both oxygen deprivation and carbon dioxide overload lead to dizziness and can cause a blackout, which will leave the diver lying quietly unconscious at the bottom of a body of water.

Incidentally, the current record for breath-holding is a staggering 17 minutes, set by the endurance artist David Blaine. To pass the 5-minute mark, Blaine employs several specialized techniques. Most important is the mammalian diving reflex—an evolutionary throwback that causes the human body to lower its heart rate and blood pressure quickly when submerged in cold water, thereby conserving oxygen. Also important is inhaling pure oxygen rather than air (which allows practiced breath-holders to double their breath-free time) and using a trick called lung packing (swallowing hard to force more air into the lungs). All the rest is practice and masochism.

Source of Information : Oreilly - Your Body Missing Manual