Under normal conditions, humans cannot store much oxygen in the body. Prolonged<unk>  leads to severe lack of oxygen in the blood circulation. 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,<unk> c oxygenation (see below), or extracorporeal membrane oxygenation, much longer periods of<unk>  may be tolerated without severe consequences. Untrained humans cannot sustain voluntary<unk>  for more than one or two minutes, since the urge to breathe becomes unbearable. The reason for the time limit of voluntary<unk>  is that the rate of breathing and the volume of each breath are tightly regulated to maintain constant values of CO2 tension and pH of the blood. In<unk> , CO2 is not removed through the lungs and accumulates in the blood. The consequent rise in CO2 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. The lungs start to feel as if they are burning, and the signals the body receives from the brain when CO2 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 and unbearable that continued holding of the breath is impossible. When a person is immersed in water, physiological changes due to the mammalian diving reflex enable somewhat longer tolerance of<unk>  even in untrained persons. 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 and for more than four minutes.<unk> sts, in this context, are people who can hold their breath for a long time.
Apnea
Cornea
Bradypnea
Hypopnea
Hyperpnea
Bronchospasm
Tachypnea
Cyclospasm