And they both empty out our oxygen blood and begin to fill the left atrium of the heart. So this takes place at the same exact moment in time. So our two atria are fully relaxed and blood begins to fill these two atria, the only distance is here we have deoxynated blood, and here we have the oxygenated blood. Now let's move on to stage number two. In stage number two, the right atrium is now fully filled with our de oxygenated blood, and the left atrium is also filled with our oxygenated blood. And this causes the two atrium to contract.