Biology · Body Fluids and Circulation · NEET
The P wave represents the electrical excitation (depolarisation) of the atria. This depolarisation leads to the contraction of both atria. In NCERT the exact line is: 'The P-wave represents the electrical excitation (or depolarisation) of the atria.' So P wave = atrial depolarisation, not atrial repolarisation.
The QRS complex is depolarisation of the ventricles. It initiates ventricular contraction. Students often mix it up with the T wave. Remember: QRS = ventricles get excited (depolarise) and squeeze; T = ventricles relax (repolarise). This exact swap has been tested in NEET many times.
The T wave. It represents the return of the ventricles from the excited state back to the normal resting state, which is repolarisation. The end of the T wave marks the end of systole (end of ventricular contraction).
Atrial repolarisation does occur, but it happens at the same time as the large QRS complex (ventricular depolarisation). The QRS is so big that it hides the small atrial repolarisation signal. That is why NCERT only mentions P, QRS and T. For NEET, remember there is NO visible wave for atrial repolarisation.
The Q wave (start of the QRS complex). Contraction starts shortly after Q, so Q marks the beginning of systole (ventricular contraction). NEET 2023 tested this exact idea: Q-wave to 'beginning of systole'.
The interval after the T wave and before the next P wave is when the heart muscle is electrically silent (no active depolarisation or repolarisation). This resting gap between cycles was tested in NEET 2024 as 'heart muscles are electrically silent'.
A reduction in the size of the T wave indicates coronary ischemia, meaning the ventricular muscle is not getting enough oxygen (poor blood supply through the coronary arteries). This clinical use of ECG is why NCERT calls it of 'clinical importance'.
The QRS complex in a standard ECG represents:
Match List I with List II. List I: A. P-wave, B. Q-wave, C. QRS complex, D. T-wave. List II: I. Beginning of systole, II. Repolarisation of ventricles, III. Depolarisation of atria, IV. Depolarisation of ventricles.
Match List I with List II. List I: A. P Wave, B. QRS complex, C. T wave, D. T-P gap. List II: I. Heart muscles are electrically silent, II. Depolarisation of ventricles, III. Depolarisation of atria, IV. Repolarisation of ventricles.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An ECG (electrocardiogram) is a line graph that records the electrical activity of the heart during one cardiac cycle. A machine called an electrocardiograph makes it. For a standard ECG the patient is connected with three electrical leads (one to each wrist and one to the left ankle).
P wave = depolarisation (excitation) of the atria. QRS complex = depolarisation of the ventricles. T wave = repolarisation (return to normal) of the ventricles. These are the three main deflections you must know for NEET.
By counting the QRS complexes in a set time, a doctor can find the heart rate. The shape and size of the waves show if the heart is working normally. For example, a smaller T wave can signal that the heart muscle is short of oxygen (coronary ischemia). This is why NCERT says the ECG is of clinical importance.
ECG is a very high-yield, repeated NEET topic from Body Fluids and Circulation. The exact P/QRS/T matching has appeared almost every year (2019, 2020, 2023, 2024). It is a guaranteed-mark question if you memorise the three waves and the Q-to-systole link correctly.
