The heart is a fascinating organ! Some people even say that it is the center of the soul, but that debate is best left for the theological web sites to engage in. Your heart can usually perform a terrific job of pumping blood for many years with a minimum of trouble. In fact, if you figure that the normal heart beats approximately 72 times every minute, that adds up to nearly 38 MILLION beats every year! So, it's not surprising that after 30 or 40 years "the engine don't quite run too smooth."
As mentioned above, the heart is a pump. The walls of the heart are actually made of muscle tissue. When the muscular walls of the heart contract, the blood inside the heart gets squirted out into the arteries of the body. Remember from grammar school biology that the arteries bring blood away from the heart and the veins bring the blood back to the heart. The heart has four chambers, two on the left side and two on the right side. Each side (left and right) is further divided into a receiving chamber (atrium) and a pumping chamber (ventricle). The atria and ventricles are separated by special one-way valves which keep the blood flowing in the proper direction. The right side of the heart pumps blood to the lungs (via the pulmonary artery) and the left side of the heart pumps blood to the rest of the organs (via the aorta, which is the main artery of the body). These organs include the brain, the kidneys, the stomach, the intestines, and so on, as well as the skin, bones, and muscles of the entire body. The lungs, of course, serve to transfer oxygen from the air we breathe to the red blood cells, which carry the oxygen on a substance called hemoglobin. The lungs also serve to eliminate excessive carbon dioxide, a by-product of our metabolism. All of our organs require a constant supply of oxygen and nutrients to function properly. Since these substances are carried by the blood, the continuous circulation of blood is vital to maintain all of our bodily functions. The amount of blood the heart pumps every minute is known as the "cardiac output," which is generally measured in liters per minute. If the cardiac output decreases for any reason, the organs may fail to function correctly. In the case of the brain, for example, if the cardiac output drops significantly the brain will be deprived of oxygen, and this can cause lightheadedness, weakness, and even loss of consciousness (fainting).
The heart also needs a constant blood supply. As mentioned above, the left side of the heart ejects blood into the aorta, the main artery of the body. The first branches of the aorta are the small arteries that supply blood to the heart itself. These are the "coronary arteries." One interesting bit of trivia is that of all the organs of the body, the heart is unique in that the arteries that feed this organ run on the outside and branch inward, diving into the muscle to supply the deeper tissues. All other organs' main blood supply is carried to the center of the organ, from where the blood vessels branch outward. The three main branches of the coronary arteries surround the organ; in fact, the coronary arteries got their name from the word "corona." Coronary artery disease refers to the formation of plaques on the inside of the arteries resulting in a narrowing of the opening (called the arterial lumen), similar to a blocked pipe. No one knows what causes these "blockages" to build up. The process of narrowing of the arteries is generally referred to as "arteriosclerosis," or more correctly "atherosclerosis." Certain risk factors increase one's chance of developing blockages. These risk factors include hypertension, diabetes, smoking (you heard me, Joe Camel!), an elevated cholesterol level, and a family history of coronary artery disease. If the blockages are severe enough to restrict the blood flow to the heart, then the heart muscle suffers from a lack of oxygen (a condition known as ischemia, pronounced iss-KEY-mee-ah). This is often perceived as a painful sensation or pressure in the chest, called angina (ann-JIY-nah). If the artery gets occluded completely, blood flow to the muscle stops entirely and the muscle dies, forming an area of scar tissue on the heart. This process is known as myocardial infarction or "MI" for short and is known by it's more common name, a Heart Attack. A heart attack is usually associated with crushing chest pain that can radiate down the arms or up to the jaw, but 10-15% of heart attacks are "silent" since they go unrecognized as such until months or years later when the scar is picked up on a routine test. Once a heart attack occurs, that part of the heart is never able to pump again. This can reduce the cardiac output and in severe cases cause congestive heart failure, a condition where fluid builds up in the lungs because the heart cannot circulate the blood adequately. Another cause of congestive heart failure is known as cardiomyopathy. Cardiomyopathy simply means a disease of the heart muscle and can be due to a number of reasons, including ischemia, valvular heart disease, excessive alcohol use, or what's referred to as "idiopathic cardiomyopathy." The word idiopathic means that we doctors are idiots and we don't know what causes it. Most likely, idiopathic cardiomyopathy is due to a severe viral infection that left the muscle weak. Heart muscle problems like myocardial infarction and cardiomyopathy can permanently reduce the cardiac output. On the other hand, a temporary decrease in the cardiac output can result from disturbances in the heart rhythm.