The heart consists of four chambers, two atria (upper chambers) and two ventricles (lower chambers). Blood passes through a valve before leaving each chamber of the heart. The valves prevent the backward flow of blood. Valves are actually flaps (leaflets) that act as one-way inlets for blood coming into a ventricle and one-way outlets for blood leaving a ventricle. Each valve has three flaps (leaflets), except the mitral valve, which only has two flaps.

The four heart valves include the following:

• tricuspid valve - located between the right atrium and the right ventricle.

• pulmonary valve - located between the right ventricle and the pulmonary artery.

• mitral valve - located between the left atrium and the left ventricle.

• aortic valve - located between the left ventricle and the aorta.

As the heart muscle contracts and relaxes, the valves open and shut, letting blood flow into the ventricles and atria at alternate times.

The following is a step-by-step description of how the valves function normally in the left ventricle:

1. When the left ventricle relaxes, the aortic valve closes and the mitral valve opens, to allow blood to flow from the left atrium into the left ventricle.

2. The left atrium contracts, allowing even more blood to flow into the left ventricle.

3. When the left ventricle contracts, the mitral valve closes and the aortic valve opens, so blood flows into the aorta.

Heart valves can malfunction in several ways, including the following:

• regurgitation - the valve does not close completely, causing the blood to flow backward instead of forward through the valve.

• stenosis - the valve opening is narrowed or valves become damaged or scarred (stiff), inhibiting the ability of the heart to pump blood to the body due to the increased force required to pump blood through the narrowed or stiff (stenotic) valve(s).

• atresia - the valve opening does not develop at all, preventing blood from passing from an atria to a ventricle, or from a ventricle to the pulmonary artery or aorta. Blood must find an alternate route, usually through another existing congenital (present at birth) defect such as an atrial septal defect or a ventricular septal defect.