Your heart is a specialised muscle, designed to pump blood around your body. In fact, the heart is two pumps. The right side of your heart receives blood from the body and pumps it to the lungs. The left side of the heart does the exact opposite: It receives blood from the lungs and pumps it out to the body. Your hearts has four chambers, two on each side. Blood from all over the body enters the heart through the VENA CAVA, the main vein in the body. The blood flows into the first chamber, the right ATRIUM. A valve then opens and the blood flows into the next chamber, the right VENTRICLE. From there, the blood is pumped out of the heart, through the pulmonary artery, to the lungs (here oxygen replaces carbon dioxide in the blood). The blood then returns to the heart through a vein called the pulmonary vein and enters the left ATRIUM. Then again a valve opens and the blood flows into the left VENTRICLE and is then pumped all over the body through the AORTA, the main artery in the body. The left side of the heart therefore needs to be bigger and stronger then the right side.The heart is unlike all your other muscles in your body. It is made of a special muscle, called cardiac muscle, which never tires or grows too big. You also are not able to control this muscle (in case you told it to stop by accident). Blood is pumped by the heart with great pressure, more than enough to squirt blood over 10 metres. Your hearts beats on average 72 times every minutes. That means roughly 35 million times a year, and never gets tired.
But your hearts needs to be looked after. Eating too much fatty foods or not giving your heart enough exercise can increase the risk of heart attacks. This is when part of your heart gets deprived of oxygen and stops working, sending the heart into an irregular heart beat. For more information about the heart, and the risks of heart disease, visit Habits of the Heart, an excellent website. There are some great (but graphic) videos of heart transplants and coronary bypass surgeries on that website.