And that's exactly why the pulmonary semi lunar valve, this valve right here, is closed so that we have a one way movement, a unidirectional movement of blood this way from the right atrium to the right ventricle. Now, at the same exact moment in time, this is what begins to take place at the left side of our heart in the left pump. So just as the right atrium is fully filled, the left atrium is also fully filled, but it's filled with oxygenated blood. And this causes the contraction of this left atrium. And that opens up the bicuspid valve, also known as the mitral valve. And our oxygen blood now flows into the fully relaxed left ventricle of our heart.