And as the ventricles begin to contract, there is an increase in hydrostatic pressure. And that forces these two valve, the tricuspid and the biconspid valve, to close. And when they shut close, that's exactly what causes that first sound that we hear when we listen to our heart via a stethoscope. So if we listen to our heart, we hear the love dub love dub sound. And the love is caused by the closure of these two valves during the QRS complex. Now, this is when the ventricles actually begin to contract.