In the human body, the oxygen is absorbed by the blood stream in the lungs, being then transported to the cells where an elaborated change process takes place.
Oxygen plays a vital role in the breathing processes and in the metabolism of the living organisms.
Probably, the only living cells that do not need oxygen are some anaerobic bacteria that obtain energy from other metabolic processes.
The nutrient compounds, inside of the cell, are oxidized through complex enzymatic processes.
This oxidation is the source of energy of most of the animals, mainly of mammals.
The products are carbon dioxide and water (exhaled air has a relative humidity of 100%), which are eliminated by the human body through the lungs.