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. 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.