Pulmonary artery stenosis is a narrowing (stenosis) that occurs in the pulmonary artery, a large artery that sends oxygen-poor blood into the lungs to be enriched with oxygen. The narrowing may occur in the main pulmonary artery and/or in the left or right pulmonary artery branches. This narrowing makes it difficult for blood to reach the lungs to pick up oxygen. Without enough oxygen, the heart and body cannot function as they should. In an effort to overcome the narrowing, the pressure in the right ventricle (the chamber that pumps blood into the pulmonary arteries) rises to levels that can be damaging to the heart muscle.