A person with pulmonary stenosis has abnormal narrowing of the pulmonic valve in the heart.

A normal pulmonic valve allows blood to flow from the right ventricle, into the lungs where it picks up oxygen, before returning to the left side of the heart. The pulmonic valve closes to prevent the backflow of blood from the pulmonary artery into the right ventricle of the heart.

The narrowing (stenosis) interferes with the normal flow of blood from the right ventricle to the pulmonary artery. This places stress on the heart, as it adjusts to keep up with demand.