The rate of heartbeat is determined by:

Correct option is D

Correct Answer: (d)
Explanation:
The sinoatrial (SA) node is the natural pacemaker of the heart; it initiates impulses that set the rate and rhythm of the heartbeat. Other conduction components (AV node, Purkinje fibres) conduct or modify impulse timing but do not primarily set the baseline rate.
Information Booster:
SA node located in the right atrium near the SVC opening.
AV node delays impulse to allow ventricular filling.
Purkinje fibres rapidly transmit impulse to ventricular myocardium for coordinated contraction.
Autonomic nervous system modulates SA node firing (sympathetic ↑, parasympathetic ↓).
Pacemaker rhythm can shift if SA node is damaged (AV node or ectopic pacemakers take over).
Additional Information (Incorrect Options):
(a) AV node controls conduction delay and backup pacemaking but not primary rate.
(b) Purkinje fibres are rapid conductors, not rate setters.
(c) Papillary muscles assist valve function, not electrical pacing.