The heart's structure makes it an efficient, never-ceasing pump. From the moment of development through the moment of death, the heart pumps. The heart, therefore, has to be strong. The average heart's muscle, called cardiac muscle, contracts and relaxes about 70 to 80 times per minute without you ever having to think about it. As the cardiac muscle contracts it pushes blood through the chambers and into the vessels.
Nerves connected to the heart regulate the speed with which the muscle contracts.
When you run, your heart pumps more quickly. When you sleep, your heart pumps more slowly. Considering how much work it has to do, the heart is surprisingly small. Located in the middle of the chest behind the breastbone, between the lungs, the heart rests in a moistened chamber called the pericardial cavity which is surrounded by the ribcage. The diaphragm, a tough layer of muscle, lies below.