The hypoxia/suffocation instinct, the irresistible urge to breathe, is caused by carbon dioxide levels in the blood. The nervous system has no means of directl

The hypoxia/suffocation instinct, the irresistible urge to breathe, is caused by carbon dioxide levels in the blood. The nervous system has no means of directly detecting oxygen, or its absence. If you walk into a room of pure nitrogen there is almost no warning. The CO2 levels remain low, everything is fine while all your oxygen is rapidly dumped to the oxygen-free air - two deep breaths gets rid of most of it. Another 15 seconds after that you are dizzy, another 15 to 45 after that you are unconscious. Terrible design, IMHO, but presumably not a common situation in the ancient environment. If you hyperventilate, it drives out the carbonic acid and CO2 in the body. But the blood is normally already near saturation for oxygen. This unusually low level of CO2 allows you to hold your breath much longer by suppressing the suffocation response, sometimes to the point of unconsciousness. You don't realize you should breathe. You feel fine, just a little lighthea... This is why you should never hyperventilate before diving under water.