Body Fluids And Circulation

An electrocardiogram, or ECG, is a graphical record of the electrical activity of the heart during a cardiac cycle. NCERT Class 11, Chapter 18 (Body Fluids and Circulation) describes the P wave as atrial depolarisation, the QRS complex as the depolarisation of the ventricles which initiates their contraction, and the T wave as the return of the ventricles to a normal state, that is ventricular repolarisation. Therefore the QRS complex corresponds to ventricular depolarisation. Repolarisation of the atria is electrically masked within the much larger QRS complex and is not what the QRS itself denotes, making that option incorrect. Atrial depolarisation only is represented by the P wave, which precedes the QRS, so it cannot be the answer. Closure of semilunar valves is a mechanical event of the cardiac cycle, not an electrical deflection recorded by the ECG, and thus lies outside what the QRS measures. Ventricular depolarisation is therefore correct. A sequence plausibility check supports this: in a normal beat ventricular contraction must follow atrial contraction, so the QRS must logically appear after the P wave, exactly as seen in the tracing.