Drastically reducing the amount of oxygen traveling to the brain could temporarily cause a person to lose consciousness. Although incredibly dangerous, it is possible to self-induce unconsciousness either by reducing the oxygen intake through the lungs or by causing the blood pressure to drop.
Hyperventilation, characterized by rapid and shallow breathing, is the most common way people who have no pre-existing medical conditions lose consciousness. Air enters and exits the lungs at a much faster rate, upsetting the balance between carbon dioxide and oxygen in the blood, and eventually resulting in a loss of consciousness. Alternatively, dehydration is another possible way to become unconscious. This is because without adequate hydration, the blood pressure plummets and not enough oxygen is able to reach the brain.