In stage number two, both atria contract while both ventricles fully relaxed. And now let's move on to stage number three. So as the blood fills our two ventricles, they basically fill up with the blood. So let's begin with the right pump. So once the right ventricle is fully filled with our deoxygenated blood, what happens is there is a build up of pressure. And what that means is this ventricle, the right ventricle, begins to contract and that causes the closure of this tricuspid valve and it causes the opening of the pulmonary valve.