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. These valves closed and the right and left atrium are now relaxing. Now, the question is why exactly is the voltage difference here so high? Well, that's because this section of the heart, the ventricles, contain the thickest layer of myocardium.