Answer: D. By counting the number of QRS complexes one can determine the pulse rate
Explanation:
Electrocardiograph (ECG)
ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity. For a detailed evaluation of the heart’s function, multiple leads are attached to the chest region.
♣ Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.
♣ The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria.
♣ The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole.
♣ The T-wave represents the return of the ventricles from excited to normal state (repolarisation). The end of the T-wave marks the end of systole.
♣ Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual. Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence, it is of a great clinical significance.

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Reference: NCERT Biology Text, Class: XI, Unit V: Human physiology, Chapter 18: Body Fluids and Circulation, Page no: 286.