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. The average adult heart is about the size of a clenched fist and weighs about 11 ounces (310 grams). 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. As a result, the heart is well protected

Got to note this:
  Your heart beats day in and day out, all the time. So in a day an adult heart pumps 7,500 liters of recycled blood by filling and contracting. There are of course different parts of the heart too:

• The heart has four chambers at the  top and bottom on the right and left side. 

•   The top two chambers are called the atria, which are the ones that are filled with blood. 

•   The bottom two chambers are called the ventricles. These guys squeeze the blood out of the heart, which they get from the atria. 

•   The septum separates the left side and the right side of the heart. This is incidentally the center of the circulatory system. 

•   The heart has four valves, which are like little gates. Blood that comes in relies on these four special valves inside the heart. The four valves are as follows:
  1. The mitral valve.
  2. The tricuspid valve (These two work between the atria and ventricles).
  3. The Aortic valve.
  4. The pulmonary valve (These two work to control the flow of the blood that leaves the heart)

Continues...

The Heart | matters of the heart