Final answer:
The first heart sound, 'lubb', is heard at the beginning of the ventricular systole marked by the QRS complex on the ECG, which corresponds to the closure of the atrioventricular valves.
Step-by-step explanation:
The first heart sound, or “lubb,” is heard at the point on an ECG known as the QRS complex. The QRS complex represents the depolarization of the ventricles, which precedes the contraction of the ventricles. When the ventricles begin to contract, it causes an increase in pressure within them. This pressure eventually exceeds the pressure in the atria and leads to the closure of the atrioventricular (AV) valves, producing the first heart sound, S₁ or “lubb.” This sound corresponds to the beginning of the ventricular systole, which is marked by the QRS complex on the ECG.