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

A normal mitral valve allows blood to flow from the left atrium, into the left ventricle. The mitral valve closes to prevent the backflow of blood from the left ventricle into the left atrium.

The narrowing interferes with the normal flow of blood from the left atrium into the left ventricle. This places stress on the heart, as it adjusts to keep up with demand.