And this creates a unidirectional and a continuous flow of blood not only inside the four chambers of the heart but also inside the entire cardiovascular system of our body. So, once again, in stage number one we have the right atrium and our left atrium that are fully relaxed and they are accepting, they are receiving blood inside those atrium. The right atrium is receiving deoxnated blood while our left atrium is receiving oxygenated blood. They are being filled, they are fully relaxed. Now, in stage number two, now, our two atria contract because they are fully filled. That opens up these two atrial ventricular valves and allows the movement of our blood from the atria and into our two ventricles.