MONICA SCHROEDER / SCIENCE PHOTO LIBRARY MONICA SCHROEDER / SCIENCE PHOTO LIBRARY
Illustration depicting the heart's electrical system (yellow circuitry). The sinoatrial (SA) node (thick yellow oval upper left) consists of self-excitatory muscle cells that contract rhythmically at around 70 beats per minute. Following each contraction, an impulse spreads through the heart's upper chambers (atria, upper frame) to the atrioventricular (AV) node (thick yellow oval opposite SA node). Contraction of the atria causes blood to move into the lower chambers (ventricles, lower frame).
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