what structures prevent blood in the ventricles from backing up into the atria
The valve(s) does not close completely, causing the blood to flow backward through the valve. This results in leakage of blood back back into the atria from the ventricles (in the case of the mitral and tricuspid valves) or leakage of blood back into the ventricles (in the case of the aortic and pulmonary valves). stenosis (or narrowing of the valve).