The oxygen is transported to the tissues by the blood, which circulates around every part of the body. This circulation depends on the heart functioning as a pump. If the heart stops, the oxygen does not reach the tissues.
These three factors: airway, breathing and circulation, must be present for survival. If any one of them fails, death will occur.
If the unconscious casualty has a clear airway and has started breathing, then you must check his pulse to see if there is adequate circulation.
