For thousands of years, a person was pronounced dead when they stopped breathing and their heart stopped beating. In the 1600s anatomical investigations discovered that the heart, lungs and brain were closely linked. It was noted that once blood has stopped pumping around the body death quickly occurs.

We know now that the brain sends signals to the heart to make it pump and the heart sends oxygen carried in the blood to the brain. One cannot work without the other. The two organs are so closely linked it is difficult to tell which one is the cause of death, whether the heart or brain failed first.

Even after death occurs the heart and lungs can be kept alive with a machine (a ventilator). The brain can also sometimes keep those organs working even when in deep coma.