The instinct not to breathe underwater is so strong that it overcomes the agony of running out of air. No matter how desperate the drowning person is, he doesn’t inhale until he’s on the verge of losing consciousness. At that point there’s so much carbon dioxide in the blood, and so little oxygen, that chemical sensors in the brain trigger an involuntary breath whether he’s underwater or not. That is called the “break point”; laboratory experiments have shown the break point to come after 87 seconds. It’s a sort of neurological optimism, as if the body were saying, Holding our breath is
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