The functions of the heart, the brain and the lungs are intimately associated.

Oxygen is indispensable for the life of the tissues, and its supply is dependent upon the integrity of the three organs mentioned, which have been called the tripos of life.
Respiration is brought about by the stimulation of certain nerve cells in the brain, the most effective stimulus to these cells being a diminution of oxygen in the blood supplying them.

These cells send out impulses to the muscles concerned in inspiration, the chest expands, and air is taken into the lungs.

Respiration is then a more complicated process than is the action of the heart, for its contraction, which causes the blood to circulate, is not immediately dependent upon extrinsic influences.

Death is usually more immediately due to failure of respiration than to failure of circulation, for the heart often continues beating for a time after respiration has ceased.
Thus, in cases of drowning and suffocation, by means of artificial respiration in which air is passively taken into and expelled from the lungs, giving oxygen to the blood, the heart may continue to beat and the circulation continue for hours after all evident signs of life and all sensation has ceased.
By this general death is meant the death of the organism as a whole, but all parts of the body do not die at the same time.