question: Why we can’t breathe 100% O2 & stay alive for a few minutes without breathing?
comment: Your blood can only store so much O2. Once it's saturated, which it typically is while you are simply sitting and breathing deeply, it can't fold any more. Breathing air has plenty of O2 to saturate your blood, the air you exhale still has most of the oxygen it had when you inhaled it.
passages: Oxygen (O) must be present in every breathing gas. This is because it is essential to the human body's metabolic process, which sustains life. The human body cannot store oxygen for later use as it does with food. If the body is deprived of oxygen for more than a few minutes, unconsciousness and death result. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes.
   Oxygen toxicity to the lungs and central nervous system can also occur in deep scuba diving and surface supplied diving. Prolonged breathing of an air mixture with an partial pressure more than 60 kPa can eventually lead to permanent pulmonary fibrosis. Exposure to a partial pressures greater than 160 kPa (about 1.6 atm) may lead to convulsions (normally fatal for divers). Acute oxygen toxicity (causing seizures, its most feared effect for divers) can occur by breathing an air mixture with 21% at or more of depth; the same thing can occur by breathing 100% at only .
   The partial pressure of oxygen in the air in the lungs controls the oxygen loading of blood. A critical pO of in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive. This is about 4% oxygen in the lungs and 45% oxygen saturation of the arterial blood. At 30 msw (4 bar), 2% by volume oxygen in the lung gas gives a pO of . At 10 msw (2 bar), for the same 2% oxygen, the pO would be , i.e. marginal. At the surface the same 2% oxygen drops to , ignoring metabolic use.
  
  Section::::Consequences.
   At partial pressures of oxygen of —100% oxygen at 2 to 3 times atmospheric pressure—these symptoms may begin as early as 3 hours after exposure to oxygen. Experiments on rats breathing oxygen at pressures between suggest that pulmonary manifestations of oxygen toxicity may not be the same for normobaric conditions as they are for hyperbaric conditions. Evidence of decline in lung function as measured by pulmonary function testing can occur as quickly as 24 hours of continuous exposure to 100% oxygen, with evidence of diffuse alveolar damage and the onset of acute respiratory distress syndrome usually occurring after 48 hours on 100% oxygen. Breathing 100% oxygen also eventually leads to collapse of the alveoli (atelectasis), while—at the same partial pressure of oxygen—the presence of significant partial pressures of inert gases, typically nitrogen, will prevent this effect.
   An adult human at rest inhales 1.8 to 2.4 grams of oxygen per minute. This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year.
  
  Section::::Biological role of O.:Living organisms.
  
  The free oxygen partial pressure in the body of a living vertebrate organism is highest in the respiratory system, and decreases along any arterial system, peripheral tissues, and venous system, respectively. Partial pressure is the pressure that oxygen would have if it alone occupied the volume.
  
  Section::::Biological role of O.:Build-up in the atmosphere.
   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.
   The lungs and the remainder of the respiratory tract are exposed to the highest concentration of oxygen in the human body and are therefore the first organs to show toxicity. Pulmonary toxicity occurs only with exposure to partial pressures of oxygen greater than , corresponding to an oxygen fraction of 50% at normal atmospheric pressure. The earliest signs of pulmonary toxicity begin with evidence of tracheobronchitis, or inflammation of the upper airways, after an asymptomatic period between 4 and 22 hours at greater than 95% oxygen, with some studies suggesting symptoms usually begin after approximately 14 hours at this level of oxygen.
   For example, myoglobin's "p" for O is 130 pascals while the P for adult hemoglobin is 3.5 kPa. Thus, when O partial pressure is low, hemoglobin-bound O is more readily transferred to myoglobin. Myoglobin, found in high concentrations in muscle tissue, can then transfer the oxygen to muscle tissue muscle fibers, where it will be used in the generation of energy to fuel muscle contraction.
   The oxygen for this in low oxygen conditions comes from gulping air at the water surface.
  
  Section::::Unique brain percentage of body energy consumption.:Large brains.
   Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness.
   At rest, the human brain receives 15% of total cardiac output, and uses 20% of the body's energy consumption. The brain is normally dependent for its high energy expenditure upon aerobic metabolism. The brain as a result is highly sensitive to failure of its oxygen supply with loss of consciousness occurring within six to seven seconds, with its EEG going flat in 23 seconds. If it affected the oxygen and glucose supply to the brain, the metabolic demands of exercise could therefore quickly disrupt its functioning.
   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, hyperbaric medicine, neonatal care and human spaceflight. These protocols have resulted in the increasing rarity of seizures due to oxygen toxicity, with pulmonary and ocular damage being mainly confined to the problems of managing premature infants.
   Section::::Medical breathing gases.
  
  Medical use of breathing gases other than air include oxygen therapy and anesthesia applications. 
  
  Section::::Medical breathing gases.:Oxygen therapy.
  
  Oxygen is required by people for normal cell metabolism. Air is typically 21% oxygen by volume. This is normally sufficient, but in some circumstances the oxygen supply to tissues is compromised. 
   In response, many bacterial pathogens have evolved mechanisms for nitric oxide resistance. Because nitric oxide might have proinflammatory actions in conditions like asthma, there has been increasing interest in the use of exhaled nitric oxide as a breath test in diseases with airway inflammation. Reduced levels of exhaled NO have been associated with exposure to air pollution in cyclists and smokers, but, in general, increased levels of exhaled NO are associated with exposure to air pollution.
   Oxygen toxicity is caused by exposure to oxygen at partial pressures greater than those to which the body is normally exposed. This occurs in three principal settings: underwater diving, hyperbaric oxygen therapy, and the provision of supplemental oxygen, particularly to premature infants. In each case, the risk factors are markedly different.
  
  Section::::Causes.:Central nervous system toxicity.
   The biochemical basis for the toxicity of oxygen is the partial reduction of oxygen by one or two electrons to form reactive oxygen species, which are natural by-products of the normal metabolism of oxygen and have important roles in cell signalling. One species produced by the body, the superoxide anion (), is possibly involved in iron acquisition. Higher than normal concentrations of oxygen lead to increased levels of reactive oxygen species. Oxygen is necessary for cell metabolism, and the blood supplies it to all parts of the body. When oxygen is breathed at high partial pressures, a hyperoxic condition will rapidly spread, with the most vascularised tissues being most vulnerable. During times of environmental stress, levels of reactive oxygen species can increase dramatically, which can damage cell structures and produce oxidative stress.
   In low-pressure environments oxygen toxicity may be avoided since the toxicity is caused by high partial pressure of oxygen, not merely by high oxygen fraction. This is illustrated by modern pure oxygen use in spacesuits, which must operate at low pressure (also historically, very high percentage oxygen and lower than normal atmospheric pressure was used in early spacecraft, for example, the Gemini and Apollo spacecraft). In such applications as extra-vehicular activity, high-fraction oxygen is non-toxic, even at breathing mixture fractions approaching 100%, because the oxygen partial pressure is not allowed to chronically exceed .
  
  Section::::Management.
   An application of as a low-pressure breathing gas is in modern space suits, which surround their occupant's body with the breathing gas. These devices use nearly pure oxygen at about one-third normal pressure, resulting in a normal blood partial pressure of . This trade-off of higher oxygen concentration for lower pressure is needed to maintain suit flexibility.
   Air is the most common source and reservoir of oxygen, containing 20.8% oxygen. This concentration is sufficient for many purposes, such as combustion of many fuels, corrosion of many metals, and breathing of animals. Most humans can function at rest with an oxygen level of 15% at one atmosphere pressure; a fuel such as methane is combustable down to 12% oxygen in nitrogen.
  
  A small room of 10 meter has 2.08 meter (2080 liters) or 2.99 kg of oxygen which would occupy 2.62 liters if it was liquid.
  
  Section::::High pressure.
   The most important function of breathing is the supplying of oxygen to the body and the removal of its waste product of carbon dioxide. Under most conditions, the partial pressure of carbon dioxide (PCO2), or concentration of carbon dioxide, controls the respiratory rate.
   The factors affecting VO are often divided into supply and demand. Supply is the transport of oxygen from the lungs to the mitochondria (including lung diffusion, stroke volume, blood volume, and capillary density of the skeletal muscle) while demand is the rate at which the mitochondria can reduce oxygen in the process of oxidative phosphorylation. Of these, the supply factor is often considered to be the limiting one. However, it has also been argued that while trained subjects probably are supply limited, untrained subjects can indeed have a demand limitation.
   Levels of CO rise in the blood when the metabolic use of O, and the production of CO is increased during, for example, exercise. The CO in the blood is transported largely as bicarbonate (HCO) ions, by conversion first to carbonic acid (HCO), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H and HCO. Build-up of CO therefore causes an equivalent build-up of the disassociated hydrogen ions, which, by definition, decreases the pH of the blood. The pH sensors on the brain stem immediately sense to this fall in pH, causing the respiratory center to increase the rate and depth of breathing. The consequence is that the partial pressure of CO (PCO2) does not change from rest going into exercise. During very short-term bouts of intense exercise the release of lactic acid into the blood by the exercising muscles causes a fall in the blood plasma pH, independently of the rise in the PCO2, and this will stimulate pulmonary ventilation sufficiently to keep the blood pH constant at the expense of a lowered PCO2.
   In some sea snakes, cutaneous respiration can account for up to 30 percent of total oxygen uptake, and is important when diving, during which blood is shunted away the lungs and towards capillaries in the skin, in some cases causing the skin to turn pink. 
  
  Section::::Taxonomic diversity.:Mammals.
   A typical human breathes between 12 and 20 times per minute at a rate primarily influenced by carbon dioxide concentration, and thus pH, in the blood. With each breath, a volume of about 0.6 litres is exchanged from an active lung volume (tidal volume + functional residual capacity) of about 3 litres. Normal Earth atmosphere is about 78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide, and other gases. After just two or three breaths of nitrogen, the oxygen concentration in the lungs would be low enough for some oxygen already in the bloodstream to exchange back to the lungs and be eliminated by exhalation.
   Some animal species are better equipped than humans to detect hypoxia, and these species are more uncomfortable in low-oxygen environments that result from inert gas exposure; however, the experience is still less aversive than CO exposure.
  
  Section::::Physiology.
  
presuppositions: If you breathe 100% O2, you should be able to go without breathing for a few minutes. You can breathe excess O2 to stay alive for a few minutes without breathing.
corrections: Your blood can only store so much O2. Your blood can only store so much O2.
raw_presuppositions: If you breathe 100% O2, you should be able to go without breathing for a few minutes. You can breathe excess O2 to stay alive for a few minutes without breathing.
raw_corrections: Your blood can only store so much O2. Your blood can only store so much O2.