Breath hold divers are faced with two main physiological challenges: pressure induced compression and extended time without breathing, exposing them to extremes of hypoxia/hypercapnia. Current world records are 214 m for depth and 11:35 min for duration. Hypoxic loss of consciousness is frequently observed during competitions. The major physiological components of the diving response that occurs during breath holding are peripheral vasoconstriction, bradycardia, decreased cardiac output, increased cerebral and myocardial blood flow, increased blood pressure, splenic contraction and preserved O(2) delivery to the brain and the heart. Sympathetic nervous activity is exceptionally engaged at the end of voluntary breath holds. We hypothesize that these adaptations to extended cessation of breathing ending with extreme hypoxia can be used as a model of brain survival response during conditions involving profound brain deoxygenation and in some instances reduced brain perfusion.
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