What would go wrong if the valves in atria work?

The valves between atria (the atrioventricular valves) must not be completely closed when the atria contract. Otherwise, blood would stay stuck in the atria, and consequently less oxygenated blood would leave the heart. In case of a mitral stenosis, the mitral valve which connects the left atrium to the left ventricle, does not open fully. This situation impairs the filling of the left ventricle, and it may produce back-pressure in the pulmonary circulation, leading to pulmonary hypertension and eventually right heart failure.