Through oxygen cycle.
Humans and animals take in oxygen and give out carbon dioxide.
Plants take in carbon dioxide and give out oxygen.
This ways the oxygen is circulated in the atmosphere.
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The nitrogen and oxygen in Earth's atmosphere together account for a speck more than 99% of it..
78% Nitrogen 21% Oxygen
atmosphere
Much less than the approximate 21% in the air. Perhaps half that. Fish need a large energy expenditure to strain oxygen from the water. Most of the oxygen produced by phytoplankton, for instance, is vented into the atmosphere.
The ratio is approxNitrogen : Oxygen : Argon : Carbon Dioxide :: 39000 : 10500 : 465 : 19.