The heart is a muscular organ composed of four chambers; two upper chambers called atria and two lower chambers called ventricles. These four chambers and their four valves are responsible for pumping blood through the body in a rhythmic, life-giving pattern. 

     Deoxygenated blood enters the heart via the right atrium, flows into the right ventricle via the tricuspid valve, and is pumped through the pulmonary semilunar valve to the lungs.  There, carbon dioxide is released and oxygen is replenished.     The blood then returns to the heart via the left atrium, flows through the mitral (bicuspid) valve into the left ventricle, and is pumped through the aortic semilunar valve into the aorta.   From the aorta, the blood is pumped through 60,000 miles of blood vessels.  What one feels as a "heartbeat," or pulse, is the culmination of one cardiac cycle, and, in the healthy individual, this occurs over 100,000 times a day. 

     Although the heart is full of blood, it cannot receive oxygen and nutrients from the blood inside the chambers. The heart muscle must rely on the arteries on the surface of the heart to nourish it and keep it working properly. These surface arteries are known as the coronary arteries. There are three main coronary arteries: the right coronary artery, the left anterior descending coronary artery and the circumflex coronary artery.  These three arteries branch into thousands of small arteries like a tree trunk brunches into limbs, bringing oxygen and nutrients to the heart muscle cells. 

     Heart disease is actually a very comprehensive term that refers to any disorder that interferes with proper cardiac circulation.  This includes defective heart valves, muscle disease, rhythm disturbances, and congenital defects (abnormalities of the heart that are present at birth).  When most people refer to heart disease, they are probably referring to Coronary Artery Disease (CAD), which is the leading cause of death in the United States.