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god made it that way
Well, for one, oxygen is used during aerobic respiration (think citric acid cycle, glycolysis, etc.) as an oxidizing agent.
recall exothermic reaction and understand that living systems such as human beings need energy in order to survive. Oxygen turns out to be more reactive element than the dominating element in the atmosphere, the Nitrogen; it reacts with Carbohydrates in an exothermic reaction, thus releasing energy. IN details we can talk about the energy currency of most living systems, the ATP, and all and discuss electron transfer phosphorylation and complexes, but the summary of this is that it is the element known to be efficient (without killing us immediately) that we can use to harness the main energy from carbohydrates.
1,500 years ago, god once talked about how we can have low pressure in space, lack of breathing oxygen, and fatal increase in fluids, gases and temperature inside the body.
respiration does not really explain to us why we need oxygen instead of something else. it just gives us the mechanism in harvesting energy
this is why it is very interesting to study cyanobacterias and stromatolites, which may potentially provide us why Oxygen instead of something else. I am not saying, however, that only oxygen is the known element that a living system can use. Recent research in 2010 showed that some metazoans can spend a lifetime under anoxic conditions.
OXYGEN Carries our blood. Hemoglobin. Look up hemoglobin.
The reason why we need oxgyen is because our cells need oxgyen. When cells doesn't have oxgyen, then the cells die. And when the cells die, we can't perform any activites. Therefore, death occur.