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1.
Essential life process where oxygen reacts with glucose from the food that you eat to provide your body with energy. The two by-products of this process are carbon dioxide, which is exhaled along with the rest of the harmless gases that entered your lungs but did not diffuse into your blood, and water.

Not to be confused with breathing, which is how the oxygen enters the body.
If you cannot breathe, oxygen cannot enter your lungs and therefore cannot diffuse into your blood. With no oxygen your cells cannot respire and will therefore die.

Respiration:

Oxygen + Glucose -> Energy + Carbon Dioxide + Water
by Jonny3 September 07, 2006