Each contraction of the heart, termed systole (SIS-to_-le_), is followed by a relaxation phase, diastole (di-AS-to-le), during which the chambers fill. Each time the heart beats, both atria contract and immediately thereafter both ventricles contract. The wave of increased pressure produced in the vessels each time the ventricles contract is the pulse.

Contractions are stimulated by a built-in system that regularly transmits electrical impulses through the heart. The components of this conduction system are shown in Figure 1. They include the sinoatrial (SA) node, called the pacemaker because it sets the rate of the heartbeat, the atrioventricular (AV) node, the AV bundle (bundle of His), the left and right bundle branches, and Purkinje ( pur-KIN-je) fibers.

Although the heart itself generates the heartbeat, factors such as nervous system stimulation, hormones, and drugs can influence the rate and the force of heart contractions.

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