The heart, a muscular organ used to transport blood throughout the circulatory system. The human heart is split into four chambers. It has both the left and right atria, as well as a left and right ventricle. The atria is the top part of the heart while the ventricles consist of the lower half. If the heart were to stop beating for four to five minutes, one would be pronounced dead. After that, if the heart were to be mechanically pumped for someone, the chances of them being brain dead are high.