The heart is the hardest working muscle in the human body. Located almost in the center of the chest, the adult human heart is about the size of a fist.
At an average rate of 80 times a minute, the heart beats about 115,000 times in one day or 42 million times in a year. During an average lifetime, the human heart will beat more than 3 billion times – pumping an amount of blood that equals about 1 million barrels. Even at rest, the heart continuously works hard.