Summary: Mitral stenosis is a disorder in which the mitral valve does not fully open. This restricts the flow of blood.Blood that flows between different chambers of your heart must flow through a valve. The valve between the 2 chambers on the left side of your heart is called the mitral valve. It opens up enough so that blood can flow from the upper chamber of your heart (left atria) to the lower chamber (left ventricle). It then closes, keeping blood from flowing backwards. Mitral stenosis means that the valve cannot open enough. As a result, less blood flows to the body. The upper heart chamber swells as pressure builds up. Blood and fluid may then collect in the lung tissue (pulmonary edema), making it hard to breathe. More >> Less >>

Reference Material:
https://medlineplus.gov/ency/article/000175.htm