Question..Why didn\'t they feel the need to breathe? . Deep sea \"free divers\"

Question

Question..Why didn't they feel the need to breathe?  

. Deep sea "free divers" test their respiratory systems by trying to dive as far as possible underwater with just one breath of air. The divers hyperventilate, take a deep breath, then start swimming to the greatest depth possible. Pressure gauges register the depth they attain, and this is how they compete with other divers. This is a dangerous practice, however, as they must decide when to begin their ascent with enough "reserve" to make it to the surface. Occasionally, they will pass out just before they surface -this is called "shallow water blackout." Interestingly, when asked about these incidents, divers say they really didn't feel the need to breathe; they simply lost consciousness. Speculate as to why shallow water blackout occurs, (HINT: the divers were at great depths, and are now returning to the surface).

Explanation / Answer

In normal cases, that is outside water, when we hold breath, the feel of need to breathe is triggered by the rising level of CO2 (And not lower oxygen levels). The divers do not feel the need to breathe because, during deep sea diving, the CO2 levels remain low in the body and hence the brain does not get the "signal" to take a breath.

Do note that it is given that the divers 'hyperventilate" before doing the dive. When hyperventilation is done, it eliminates more CO2 from the body and causes Hypocapnia (low CO2 in the blood). i.e. hyperventilation artificially depletes the resting concentration of carbon dioxide causing a low blood carbon dioxide So, when the diver dives his CO2 level is already down and oxygen keeps getting low. When the divers are underwater and are holding the breath, the level of oxygen keeps decreasing for two reasons. 1) Due to the activity of diving to great depths and then swimming back, the brain consumes more oxygen and 2) Rapid drop in the partial pressure of oxygen in the lungs on the ascent as the ambient pressure drops and the gas in the lungs expands to surface volume. However, the CO2 level remains low and hence they do not feel the urge to breathe even though the brain is critically getting short of oxygen (as the trigger for breathing in increased CO2 levels).The low oxygen levels then cause cerebral hypoxia which can be life-threatening.

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