The Mitral valve is located between the left atrium and the left ventricle.

Its task is enabling blood flow from the atrium into the ventricle during the contraction of the atria, but it also must prevent back flow into the atrium during the ventricular systole, when the ventricular pressure mounts markedly.

Insufficiency of the mitral valve causes back flow to the atrium, thereby excess load for the atrium and even blood return to the lungs.