Now, this valve, which is known as the aortic valve, is also fully closed, as this valve here to prevent the back flow of blood from the aorter back into our left ventricle. So this takes place at the same exact moment in time. So in stage one, the atria are both fully relaxed. In stage number two, both atria contract while both ventricles fully relaxed. And now let's move on to stage number three. So as the blood fills our two ventricles, they basically fill up with the blood.