Sample 1Easy
Which part of the human heart is called the pacemaker?
- A
Atrio-ventricular node
- B
Bundle of His
- C
Sino-atrial node
- D
Purkinje fibres
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The answer is C.The sino-atrial node can generate the maximum number of action potentials, 70 to 75 per minute, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore it is called the pacemaker.
- A.
The AVN receives the impulse from the SAN; it does not set the rhythm.
- B.
The bundle of His conducts the impulse through the inter-ventricular septum.
- D.
Purkinje fibres spread the impulse through the ventricular musculature.
NCERT: Class 11 Biology, Chapter 15 (Body Fluids and Circulation), Circulatory Pathways, Human Circulatory System
Sample 2Medium
An ECG trace recorded for exactly one minute shows a repeating pattern: a small upward wave P, then a sharp complex QRS, then a broad wave T. The trace has 72 QRS complexes. Which statement about the trace is correct?
- A
P shows depolarisation of the ventricles, and the heart rate is 72 beats per minute
- B
T shows depolarisation of the atria, and the heart rate is 36 beats per minute
- C
QRS shows repolarisation of the ventricles, which ends systole
- D
QRS shows depolarisation of the ventricles, and the heart rate is 72 beats per minute
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The answer is D.In an ECG the P-wave is atrial depolarisation, QRS is ventricular depolarisation and T is ventricular repolarisation. Counting the QRS complexes in a given time gives the heart rate, here 72 per minute.
- A.
P shows depolarisation of the atria. Ventricular depolarisation is shown by the QRS complex.
- B.
T shows the ventricles returning to normal (repolarisation). Each beat has one QRS, so 72 complexes a minute is 72 beats a minute.
- C.
QRS is ventricular depolarisation, which starts ventricular contraction. Systole ends at the end of the T-wave.
NCERT: Class 11 Biology, Chapter 15