Now, when the voltage reaches its maximum, so at the peak of this wave, at the peak of the P wave, that is when these two atriums begin to contract simultaneously. So this ventricle, this atrium and this atrium begin to contract. And as they begin to contract, they cause these two valves, the tricuspid and the Bicuspid valve, to basically open. 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.