JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Electrical conduction in heart. Illustration of a human heart, showing the specialised tissue that conducts the electrical signals governing heart- beats. The heartbeat starts in the heart's natural pacemaker, the sinoatrial (SA) node (green). This small area of specialised muscle cells emits an electrical impulse about 70 times a minute. The impulse spreads through the muscles of the atria (upper heart chambers), making them contract, and is carried via special fibres to the atrioventric- ular (AV) node (orange). The AV node delays the impulse to allow the ventricles (lower heart cham- bers) to fill, before effecting ventricular con- traction via branches of the bundle of HIS (red).
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