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| == Systematic definition of normoxia as a reference for hypoxia ==
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| :::: For explaining normoxia and deviations from normoxia, we distinguish environmental, compartmental, and functional definitions of normoxia. These definitions provide a reference for distinguishing corresponding causes for deviations from normoxia or normoxic function.
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| === Categories of normoxia ===
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| ::::: 1. Environmental conditions
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| ::::::* Normoxia may be defined arbitrarily (like altitude relative to sealevel) as air-level ''p''<sub>O<sub>2</sub></sub> at sealevel in air or in the aqueous extra-organismic environment.
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| ::::: 2. Compartmental respiratory cascade
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| ::::::* Normoxia may be defined arbitrarily as the ''p''<sub>O<sub>2</sub></sub> in any given compartment of a living organism (alveolar, arterial, venous, mixed-venous, intracellular) obtained under environmental normoxia in a steady state of physiological routine activity (here ‘routine’ is organismic physiological, whereas ‘ROUTINE’ is cellular physiological).
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| ::::: 3. Biological response
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| ::::::* For any function, normoxic performance is defined as the biological response that does not deviate from the physiological function measured under environmental or compartmental normoxia.
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| === Causes of deviations from normoxia ===
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| :::: Based on definitions of the categories (''1'') environmental normoxia, (''2'') compartmental normoxia, and (''3'') normoxic function, the causes for deviations from normoxia are considered:
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| ::::: 1. Environmental hypoxia and hyperoxia
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| ::::::* Hypobaric conditions: high altitude or low-pressure chamber with air
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| ::::::* Hyperbaric conditions: high-pressure chamber, diving with air
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| ::::::* Normobaric: O<sub>2</sub> deprivation in the environment (environmental normobaric hypoxia), O<sub>2</sub> supplementation (environmental normobaric hyperoxia)
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| ::::: 2. Compartmental hypoxia and hyperoxia
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| ::::::* Environmentally induced hypoxia or hyperoxia on the compartmental level (living organism)
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| ::::::* Physiologically induced hypoxia on the compartmental level: tissue-work related; living organism at high workload of a tissue; ; (mal)adaptive responses of the respiratory cascade to (de)training and lifestyle
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| ::::::* O<sub>2</sub>-transport related hypoxia (pathological: ischemia and stroke, anaemia, chronic heart disease, chronic obstructive pulmonary disease, severe COVID-19, obstructive sleep apnea, CO poisoning)
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| ::::::* Experimental for isolated organs, tissues, cells, and organelles: deviations of incubation O<sub>2</sub> levels of experimental preparations from compartmental or biological normoxia in the intact organism
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| ::::: 3. Biological hypoxia and hyperoxia
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| ::::::* Compartmental: tissue-work related
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| ::::::* Compartmental: pathological
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| ::::::* Genetic: inhibition or acceleration of O<sub>2</sub>-linked pathways (mutations, inherited diseases, knockout, knockin)
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| ::::::* Pharmacological-toxicological: inhibition or acceleration of O<sub>2</sub>-linked pathways (cyanide, rotenone, NO, ..; doping, ..)
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| == Extents of hypoxia ==
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| :::: There is a continuous transition of hypoxia to anoxia, which is best represented on a logarithmic scale of ''p''<sub>O<sub>2</sub></sub>. Only if the transition to anoxia is of interest, then further differentiation of microxia and anoxia is of technical and physiological interest taking into account the limit of detection of a method of determining ''p''<sub>O<sub>2</sub></sub> and different methods to detect functional responses to the presence (microxia) or absence (anoxia) of trace amounts of oxygen.
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