So when the SA node generates that electrical signal, the action potential, it increases the voltage, it makes it more positive. And our electrical signal then propagates through these conduction channels shown in purple. And these conduction channels extend through the right atrium and through the left atrium. 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.