How the heart works

To understand heart disease, it helps to know how the heart works. Your heart is a pump. It's a muscular organ about the size of your fist and located slightly left of center in your chest. Your heart is divided into the right and the left side. The division protects oxygen-rich blood from mixing with oxygen-poor blood. Oxygen-poor blood returns to the heart after circulating through your body.

The right side of the heart, composed of the right atrium and ventricle, collects and pumps blood to the lungs through the pulmonary arteries. The lungs refresh the blood with a new supply of oxygen, making it turn red. Oxygen-rich blood then enters the left side of the heart, composed of the left atrium and ventricle, and is pumped through the aorta to supply tissues throughout the body with oxygen and nutrients.

Four valves within your heart keep your blood moving the right way. The tricuspid, mitral, pulmonary and aortic valves open only one way and only when pushed on. Each valve opens and closes once per heartbeat — or about once every second while you're at rest.

A beating heart contracts and relaxes. Contraction is called systole, and relaxation is called diastole. During systole, your ventricles contract, forcing blood into the vessels going to your lungs and body — much like ketchup being forced out of a squeeze bottle. The right ventricle contracts a little bit before the left ventricle does. Your ventricles then relax during diastole and are filled with blood coming from the upper chambers, the left and right atria. The cycle then starts over again.

Your heart also has electrical wiring, which keeps it beating. Electrical impulses begin high in the right atrium and travel through specialized pathways to the ventricles, delivering the signal to pump. The conduction system keeps your heart beating in a coordinated and normal rhythm, which in turn keeps blood circulating. The continuous exchange of oxygen-rich blood with oxygen-poor blood is what keeps you alive.

The causes of heart disease vary by type of heart disease.

Causes of cardiovascular disease

While cardiovascular disease can refer to many different types of heart or blood vessel problems, the term is often used to mean damage caused to your heart or blood vessels by atherosclerosis (ath-ur-oh-skluh-ROW-sis), a buildup of fatty plaques in your arteries. This is a disease that affects your arteries. Arteries are blood vessels that carry oxygen and nutrients from your heart to the rest of your body. Healthy arteries are flexible and strong.

Over time, however, too much pressure in your arteries can make the walls thick and stiff — sometimes restricting blood flow to your organs and tissues. This process is called hardening of the arteries (arteriosclerosis). Atherosclerosis is the most common form of this disorder. Atherosclerosis is also the most common cause of cardiovascular disease, and it's often caused by an unhealthy diet, lack of exercise, being overweight and smoking. All of these are major risk factors for developing atherosclerosis and, in turn, cardiovascular disease.

Causes of heart arrhythmia

Common causes of abnormal heart rhythms (arrhythmias), or conditions that can lead to arrhythmias include:

  • Heart defects you're born with (congenital heart defects)
  • Coronary artery disease
  • High blood pressure
  • Diabetes
  • Smoking
  • Excessive use of alcohol or caffeine
  • Drug abuse
  • Stress
  • Some over-the-counter medications, prescription medications, dietary supplements and herbal remedies
  • Valvular heart disease

In a healthy person with a normal, healthy heart, it's unlikely for a fatal arrhythmia to develop without some outside trigger, such as an electrical shock or the use of illegal drugs. That's primarily because a healthy person's heart is free from any abnormal conditions that cause an arrhythmia, such as an area of scarred tissue.

However, in a heart that's diseased or deformed, the heart's electrical impulses may not properly start or travel through the heart, making arrhythmias more likely to develop.

Causes of heart defects

Heart defects usually develop while a baby is still in the womb. About a month after conception, the heart begins to develop. It's at this point that heart defects can begin to form. Some medical conditions, medications and genes may play a role in causing heart defects.

Heart defects can also develop in adults. As you age, your heart's structure can change, causing a heart defect.

Causes of cardiomyopathy

The exact cause of cardiomyopathy, a thickening or enlarging of the heart muscle, is unknown. There are three types of cardiomyopathy:

  • Dilated cardiomyopathy. This is the most common type of cardiomyopathy. In this disorder, your heart's main pumping chamber — the left ventricle — becomes enlarged (dilated), its pumping ability becomes less forceful, and blood doesn't flow as easily through the heart.
  • Hypertrophic cardiomyopathy. This type involves abnormal growth or thickening of your heart muscle, particularly affecting the muscle of your heart's main pumping chamber. As thickening occurs, the heart tends to stiffen and the size of the pumping chamber may shrink, interfering with your heart's ability to deliver blood to your body.
  • Restrictive cardiomyopathy. The heart muscle in people with restrictive cardiomyopathy becomes stiff and less elastic, meaning the heart can't properly expand and fill with blood between heartbeats. It's the least common type of cardiomyopathy and can occur for no known reason.

Causes of heart infection

Heart infections, such as pericarditis, endocarditis and myocarditis, are caused when an irritant, such as a bacterium, virus or chemical, reaches your heart muscle. The most common causes of heart infections include:

  • Bacteria. Endocarditis can be caused by a number of bacteria entering your bloodstream. The bacteria can enter your bloodstream through everyday activities, such as eating or brushing your teeth, especially if you have poor oral health. Myocarditis can also be caused by a tick-borne bacterium that is responsible for Lyme disease.
  • Viruses. Heart infections can be caused by viruses, including some that cause influenza (coxsackievirus B and adenovirus), a rash called fifth disease (human parvovirus B19), gastrointestinal infections (echovirus), mononucleosis (Epstein-Barr virus) and German measles (rubella). Viruses associated with sexually transmitted infections also can travel to the heart muscle and cause an infection.
  • Parasites. Among the parasites that can cause heart infections are Trypanosoma cruzi, toxoplasma, and some that are transmitted by insects and can cause a condition called Chagas' disease.
  • Medications that may cause an allergic or toxic reaction. These include antibiotics, such as penicillin and sulfonamide drugs, as well as some illegal substances, such as cocaine. The needles used to administer medications or illegal drugs also can transmit viruses or bacteria that can cause heart infections.
  • Other diseases. These include lupus, connective tissue disorders, inflammation of blood vessels (vasculitis) and rare inflammatory conditions, such as Wegener's granulomatosis.

Causes of valvular heart disease

There are many causes of diseases of your heart valves. Four valves within your heart keep blood flowing in the right direction. You may be born with valvular disease, or the valves may be damaged by such conditions as rheumatic fever, infections (infectious endocarditis), connective tissue disorders, and certain medications or radiation treatments for cancer.

Jan. 16, 2013