Editing Apnea (section)

==Complications==
Under normal conditions, [[human]]s cannot store much [[oxygen]] in the body. Prolonged apnea leads to severe lack of oxygen in the [[circulatory system|blood circulation]], leading to dysfunction of [[Organ system|organ systems]]. Permanent [[brain damage]] can occur after as little as three minutes and [[death]] will inevitably ensue after a few more minutes unless ventilation is restored. However, under special circumstances such as [[hypothermia]], [[hyperbaric oxygenation]], [[#Apneic oxygenation|apneic oxygenation]] (see below), or [[extracorporeal membrane oxygenation]], much longer periods of apnea may be tolerated without severe detrimental consequences.

Untrained humans usually cannot sustain voluntary apnea for more than one or two minutes, since the urge to breathe becomes unbearable.{{Citation needed|date=February 2011}} The reason for the time limit of voluntary apnea is that the rate of breathing and the volume of each breath are tightly regulated to maintain constant values of [[carbon dioxide|CO<sub>2</sub>]] tension and [[pH]] of the [[blood]] more than oxygen levels. In apnea, CO<sub>2</sub> is not removed through the lungs and accumulates in the blood. The consequent rise in CO<sub>2</sub> tension and drop in pH result in stimulation of the respiratory centre in the brain which eventually cannot be overcome voluntarily. The accumulation of carbon dioxide in the lungs will eventually irritate and trigger impulses from the respiratory center part of the brain and the [[phrenic nerve]]. Rising levels of carbon dioxide signal the body to breathe and resume unconscious respiration forcibly. The lungs start to feel as if they are burning, and the signals the body receives from the brain when CO<sub>2</sub> levels are too high include strong, painful, and involuntary contractions or spasms of the diaphragm and the muscles in between the ribs. At some point, the spasms become so frequent, intense and unbearable that continued holding of the breath is nearly impossible.{{Citation needed|date=February 2021}}

When a person is immersed in water, physiological changes due to the mammalian [[diving reflex]] enable somewhat longer tolerance of apnea even in untrained persons as breathing is not possible underwater. Tolerance can in addition be trained. The ancient technique of [[free-diving]] requires breath-holding, and world-class free-divers can hold their breath underwater up to depths of {{convert|214|metres}} and for more than four minutes.<ref>{{cite web|url=http://www.divemagazine.co.uk/news/article.asp?uan%3D3745 |title=Where is it |access-date=2008-03-02 |url-status=dead |archive-url=https://web.archive.org/web/20080927221313/http://www.divemagazine.co.uk/news/article.asp?uan=3745 |archive-date=27 September 2008}} for 214-metre diving record</ref> Apneists, in this context, are people who can hold their breath for a long time.