Life Science Practice Exam

The timing of a heartbeat is set by the sinoatrial node, the natural pacemaker of the heart. Its pacemaker cells spontaneously generate impulses at a regular rate, establishing the heart’s rhythm. The impulse then travels through atrial tissue to the atrioventricular (AV) node, which adds a brief delay. That delay coordinates atrial contraction with ventricular contraction so the ventricles fill properly before they pump. After the delay, the impulse moves through the His-Purkinje system to rapidly activate the ventricles for synchronized contraction. Atrial tissue mainly serves to conduct and support atrial contraction, not to set the pace. So the primary source that coordinates the heartbeat timing is the SA node, with the AV node refining the sequence by providing the necessary delay.