question: If the brain can’t go longer than 6 minutes without oxygen before brain damage occurs, how can some free divers hold their breath for 10, 15 and even 20 minutes without getting severe brain damage?
comment: It is the blood that carries oxygen. Divers have their oxygen level highest possible before the dive. Extra breathes minutes before the action can help. Also there is no proof some of them havent got brain damage.
passages: The usual consequence, if the airway is not protected, is drowning. A breath-hold diver who has blacked out and has been promptly returned to the surface, will usually regain consciousness within seconds. While the diver is still unconscious underwater, they are at high risk of drowning. While unconscious the diver has lost voluntary bodily control, but still has protective reflexes that protect the airway. One of these is laryngospasm, which closes the larynx, to preventing water from entering the lungs. After some time a laryngospasm will relax and the airway will open, which may result in water ingress if the diver is still underwater. If the diver has reached the surface, and the divers face is kept above water, when the laryngospasm relaxes spontaneous breathing will often resume. The laryngospasm will eventually relax, and if the diver is still underwater then water will enter the airway and may reach the lungs which will cause complications if resuscitation is successful, and secondary drowning is possible. The time between loss of consciousness and death varies considerably depending on a number of factors but can be as little as two and a half minutes.
   Automatic breathing can be overridden to a limited extent by simple choice, or to facilitate swimming, speech, singing or other vocal training. It is impossible to suppress the urge to breathe to the point of hypoxia but training can increase the ability to breath-hold; for example, in February 2016, a Spanish, professional freediver broke the world record for holding the breath underwater at just over 24 minutes.
   BULLET::::- Breath-Hold Study. The effects of hyperventilation, work, breathing mixture and dive depth on immersed breath-hold duration were investigated to allow an increase in breath-hold time to a maximum safe level without excessive risk of loss of consciousness or functional incapacity due to hypocapnia, hypoxia or hypercapnia.
  
  BULLET::::- Flying After Diving. DAN conducted human trials from 1993 to 1999 to investigate how long to wait after diving before flying for recreational divers with support from the US Navy.
   BULLET::::- Freedivers can only avoid getting into a situation where latent hypoxia occurs. Once oxygen levels reach this point they will probably lose consciousness during the ascent and are at a high risk of drowning. Ensuring that they are positively buoyant in the event of blackout will give them a chance of recovery if they reach the surface.
   Before competition attempts, freedivers perform a preparation sequence, which usually consists of physical stretching, mental exercise and breath exercise. It may include a succession of variable length static apnea and special purging deep breaths. Results of the preparation sequence are slower metabolism, lower heart rate and breath rate, and lower levels of carbon dioxide in the bloodstream and overall mental equilibrium.
  
  Section::::Safety.
  
  Section::::Safety.:Hazards.
   The breathing reflex in the human body is weakly related to the amount of oxygen in the blood but strongly related to the amount of carbon dioxide (see Hypercapnia). During apnea, the oxygen in the body is used by the cells, and excreted as carbon dioxide. Thus, the level of oxygen in the blood decreases, and the level of carbon dioxide increases. Increasing carbon dioxide levels lead to a stronger and stronger breathing reflex, up to the "breath-hold breakpoint", at which the person can no longer voluntarily hold his or her breath. This typically occurs at an arterial partial pressure of carbon dioxide of 55 mm Hg, but may differ significantly between people.
   Trained freedivers are well aware of this and competitions must be held under strict supervision and with competent first-aiders on standby. However this does not eliminate the risk of blackout. Freedivers are recommended to only dive with a 'buddy' who accompanies them, observing from in the water at the surface, and ready to dive to the rescue if the diver loses consciousness during the ascent.
  
  Section::::Hazard control.:Scuba diving.
   Protocols for avoidance of the effects of hyperoxia exist in fields where oxygen is breathed at higher-than-normal partial pressures, including underwater diving using compressed breathing gases. These protocols have resulted in the increasing rarity of seizures due to oxygen toxicity.
   A lack of oxygen or chemical changes in the lungs may cause the heart to stop beating. This cardiac arrest stops the flow of blood and thus stops the transport of oxygen to the brain. Cardiac arrest used to be the traditional point of death but at this point there is still a chance of recovery. The brain cannot survive long without oxygen and the continued lack of oxygen in the blood combined with the cardiac arrest will lead to the deterioration of brain cells causing first brain damage and eventually brain death from which recovery is generally considered impossible. The brain will die after approximately six minutes without oxygen at normal body temperature, but hypothermia of the central nervous system may prolong this.
   The term latent hypoxia strictly refers to the situation while the potential victim is still conscious, but is also loosely applied to the consequential blackout, which is a form of hypoxic blackout also referred to as blackout of ascent or deep water blackout, though deep water blackout is also used to refer to the final stage of nitrogen narcosis.
  
  Section::::Mechanism.
   Experienced free-divers are at particular risk because of their practiced ability to suppress the carbon dioxide induced urge to breathe. Some argue that the highest risk may be to intermediate skilled divers who are training hard and have not recognised their limits.
  
  Where deep breath-hold divers are observed to use hyperventilation timely and informed advice may save their lives but experience suggests that divers are reluctant to change their practice unless they have a very clear understanding of the mechanics of the process.
  
  Section::::Management.
  
  Section::::Management.:Avoidance and prevention.
   The minimum tissue and venous partial pressure of oxygen which will maintain consciousness is about . This is equivalent to approximately in the lungs. Approximately 46 ml/min oxygen is required for brain function. This equates to a minimum arterial ppO of at 868 ml/min cerebral flow.
   A conscious person will hold his or her breath (see Apnea) and will try to access air, often resulting in panic, including rapid body movement. This uses up more oxygen in the blood stream and reduces the time to unconsciousness. The person can voluntarily hold his or her breath for some time, but the breathing reflex will increase until the person tries to breathe, even when submerged.
   assumes that both time and an adequate supply of breathing gas are available, and though this is often true, some situations require immediate learned responses which must be habituated by education, training and repetitive practice to overcome inappropriate instinctive and natural reflexive responses. For example, a diver should exhale whenever ascending to prevent lung overexpansion injuries, and if the diver is subject to a collision or sudden upwelling underwater, the natural reaction may be to tense up and hold his breath, particularly if the breathing gas supply is interrupted at the same time. This reaction could prove fatal if the diver is lifted sufficiently to cause lung overexpansion. Only through education training and practice, and perhaps proper selection, will the diver reflexively exhale as a response to a pressure reduction.
   BULLET::::2. Ischaemic hypoxia is caused by reduced blood flow to the brain arising from cerebral vasoconstriction brought on by low carbon dioxide following hyperventilation, or increased pressure on the heart as a consequence of (lung packing) which can reduce blood circulation in general, or both. If the brain used more oxygen than is available in the blood supply, the cerebral oxygen partial pressure may drop below the level required to sustain consciousness. This type of blackout is likely to occur early in the dive.
   Mechanical trauma is usually obvious when it is the direct cause of death, but it is possible for less obvious injury to cause a short term reduction in the level of consciousness sufficient for the diver to be unable to avoid drowning, or to hinder the diver from taking the necessary action.
  
  Section::::General findings.
   The breath-hold break point can be suppressed or delayed either intentionally or unintentionally. Hyperventilation before any dive, deep or shallow, flushes out carbon dioxide in the blood resulting in a dive commencing with an abnormally low carbon dioxide level; a potentially dangerous condition known as hypocapnia. The level of carbon dioxide in the blood after hyperventilation may then be insufficient to trigger the breathing reflex later in the dive and a blackout may occur without warning and before the diver feels any urgent need to breathe. This can occur at any depth and is common in distance breath-hold divers in swimming pools. Hyperventilation is often used by both deep and distance free-divers to flush out carbon dioxide from the lungs to suppress the breathing reflex for longer. It is important not to mistake this for an attempt to increase the body's oxygen store. The body at rest is fully oxygenated by normal breathing and cannot take on any more. Breath holding in water should always be supervised by a second person, as by hyperventilating, one increases the risk of shallow water blackout because insufficient carbon dioxide levels in the blood fail to trigger the breathing reflex.
   An ascent blackout, or deep water blackout, is a loss of consciousness caused by cerebral hypoxia on ascending from a deep freedive or breath-hold dive, typically of ten metres or more when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it, or from a dive using underwater breathing apparatus using a breathing gas which has too low an oxygen fraction to support consciousness at the surface. Breath-hold victims typically black out close to the surface, sometimes even as they break surface and have been seen to approach the surface without apparent distress only to sink away. Breath-hold victims are usually established practitioners of deep breath-hold diving, are fit, strong swimmers and have not experienced problems before. Blackout by this mechanism may occur even after surfacing from depth and breathing has commenced if the inhaled oxygen has not yet reached the brain and may be referred to as a surface blackout. Divers ascending using breathing apparatus typically ascend at slower ascent rates to avoid decompression sickness, and the depth at which consciousness is lost tends to follow the oxygen partial pressure of the breathing gas.
   Hypoxic blackout during freediving is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive, when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it. It can be provoked by hyperventilating just before a dive, or as a consequence of the pressure reduction on ascent, or a combination of these. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers and have not experienced problems before.
   Physiological triggering events that may lead to a diving accident, but are not generally the direct cause of death include nitrogen narcosis, dehydration, exhaustion, hypothermia, excessive work of breathing, dehydration, motion sickness and the effects of alcohol and recreational drugs. Occasionally side effects of medical pharmaceuticals may also trigger an accident. Epileptic seizures should not occur, as a history of epilepsy is a bar to diver training, but cases have occurred where the problem was not disclosed and the person subsequently died as a result of drowning after losing their air supply during a seizure. These factors are often overlooked in accident investigations, and this may lead to a misleading conclusion about the cause of death.
   BULLET::::- Freediving: During breath-hold diving there is no additional breathing gas available during the ascent. If the diver stays down long enough to use up the available oxygen to the extent where tissue concentration has dropped below a level sufficient to support consciousness at surface pressure, there is a very high risk of blackout before the surface can be reached.
  
  Section::::Management.
  
  Section::::Management.:Avoidance.
   There are three different mechanisms behind blackouts in freediving:
  
  BULLET::::1. Duration-induced hypoxia occurs when the breath is held long enough for metabolic activity to reduce the oxygen partial pressure sufficiently to cause loss of consciousness. This is accelerated by exertion, which uses oxygen faster or hyperventilation, which reduces the carbon dioxide level in the blood which in turn may:
  
  BULLET::::- increase the oxygen-haemoglobin affinity thus reducing availability of oxygen to brain tissue towards the end of the dive (Bohr effect),
   Central nervous system oxygen toxicity manifests as symptoms such as visual changes (especially tunnel vision), ringing in the ears (tinnitus), nausea, twitching (especially of the face), behavioural changes (irritability, anxiety, confusion), and dizziness. This may be followed by a tonic–clonic seizure consisting of two phases: intense muscle contraction occurs for several seconds (tonic phase); followed by rapid spasms of alternate muscle relaxation and contraction producing convulsive jerking (clonic phase). The seizure ends with a period of unconsciousness (the postictal state). The onset of seizure depends upon the partial pressure of oxygen in the breathing gas and exposure duration. However, exposure time before onset is unpredictable, as tests have shown a wide variation, both amongst individuals, and in the same individual from day to day. In addition, many external factors, such as underwater immersion, exposure to cold, and exercise will decrease the time to onset of central nervous system symptoms. Decrease of tolerance is closely linked to retention of carbon dioxide. Other factors, such as darkness and caffeine, increase tolerance in test animals, but these effects have not been proven in humans.
   Breath-hold diving depth is limited in animals when the volume of rigid walled internal air spaces is occupied by all of the compressed gas of the breath and the soft spaces have collapsed under external pressure. Animals that can dive deeply have internal air spaces that can extensively collapse without harm, and may actively exhale before diving to avoid absorption of inert gas during the dive.
   Not everyone has the physical and mental capacity to dive safely. Professional diving can be physically demanding work, and some diving tasks require considerable strength and stamina, and a sufficient reserve of physical and psychological strength to cope with unexpected situations.
  
presuppositions: Oxygen can only be stored by holding one's breath. No free divers get brain damage.
corrections: Oxygen is also stored in the blood. There's no evidence suggesting that free drivers haven't gotten brain damage.
raw_presuppositions: Oxygen can only be stored by holding one's breath. No free divers get brain damage.
raw_corrections: Oxygen is also stored in the blood. There's no evidence suggesting that free drivers haven't gotten brain damage.