From the eagerly awaited first cry of the newborn to the last gasp of the dying, life is completely dependent on breathing in adequate oxygen and breathing out carbon dioxide. The most important rule of first aid is to assure the airway is open and that breathing or respiration can be maintained.
Brain damage begins to occur within four minutes if breathing stops.
The brain and its nerves cells are the most sensitive in the body to lack of oxygen. A nerve cell will die within four seconds if deprved of oxygen. Fortunately sufficient oxygen is present within the blood and tissues that cells do not immediately lose contact with oxygen when breathing ceases. There's little room in the skull for expansion. This is best done by exercise.