The pulmonary valve acts like a one-way door from your heart's right ventricle to your lungs. Oxygen-depleted blood flows from the ventricle, through the pulmonary valve and into the lungs, where it picks up oxygen to deliver to your body.
The pulmonary valve consists of three half-moon-shaped flaps of tissue called cusps. These cusps open to allow blood to flow from your right ventricle to your lungs. Then they close so that the blood can't flow backward. If the cusps are malformed, the valve may become narrowed (pulmonary stenosis), reducing the amount of blood reaching the lungs. Or the valve may leak, allowing blood to flow back into the right ventricle (pulmonary regurgitation).
Diseases of the pulmonary valve are fairly uncommon, and most are due to defects present at birth (congenital defects). Sometimes there are no symptoms for decades, so congenital valve defects may not be diagnosed until later in life. In addition, your pulmonary valve can be damaged by infectious diseases, such as endocarditis or a rare condition called carcinoid heart disease. Pulmonary valves can also be damaged by high blood pressure within the lung's blood vessels (pulmonary hypertension).
Read more about endocarditis.