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Uncontrolled hyperventilation is caused by overexertion, panic, and/or fright. A person breathes in and out rapidly but the breaths are shallow. Little oxygen gets into the lungs. The person feels they are out of air. The remedy is to relax and "catch your breath." Underwater the key words are: "Stay calm!" If the diver starts hyperventilating they must stop what is being done, take deeper breaths, and relax!
Controlled hyperventilation is done to increase the time one may hold their breath underwater. If it is done to excess it can be very dangerous. In the case of controlled hyperventilation, every bit of air that can be exhaled is released from the lungs. This lowers the carbon dioxide (CO2) in the blood stream. Then a deep breath is taken. This raises the oxygen in the blood. If this is done enough a person will be able to hold their breath for a much longer time.
There is a sensor in the carotid artery going into the brain. This monitors the level of CO2 in the blood. If it rises too high the sensor sends a signal to a part of the brain that sets in motion all those things a person goes through when they are not getting fresh air into the lungs. So, CO2 controls the breathing rate. There is also an oxygen level sensor in the body, but it is not nearly as effective in stimulating one to breathe when the oxygen level gets low.
Every time a person consciously hyperventilates they lower the CO2, but only raise the oxygen a small amount because there is only 21% in the air. If one hyperventilates 4 or more times there is the chance the CO2 level gets so low that a person can hold their breath to the point of blacking out. What happens is, the CO2 level never climbs back to the point to tell the breath-holder they must breathe before the oxygen level drops to a point the brain causes a blackout. Swimmers trying to go long distances underwater while holding their breath after excessive hyperventilation have blacked out and continued to swim, only to crash into the end of the pool. The part of the brain causing the blackout is in the cerebrum. The swimming coordination is controlled by the cerebellum which continues to function after the blackout.
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