The third-stage response is the blackout, which happens when the brain senses that there’s not enough oxygen for it to support itself and so shuts off, like a light switch, to conserve energy. Though the brain represents only about 2 percent of the body’s weight, it uses 20 percent of the body’s oxygen. The presence of liquid in the mouth or throat triggers another reflexive line of defense: the larynx automatically closes, stopping water from entering the lungs. Freedivers learn to sense the arrival of convulsions and spleen release, and they know exactly when to head back to the surface so
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