And as they open, the blood begins to rush into the right ventricle and into our left ventricle. Now, at the same time that our two atrium begins to contract, that signal eventually reaches another specialized section known as the AV node, or the atrial ventricular node. And this note is located in the interatrial septum of the heart in the wall separating our two atrium. Now, when the signal arrives at the AV node, the AV node, what it does is it delays that signal by about zero of a second. And what that does is it gives the atria enough time to fully contract and move all that blood into the fully relaxed ventricles. So we see along our P waves, our atria begins to contract at the tip while the ventricles are actually fully relaxed.