is the normal rhythm of the heart
and results from proper activation of the entire heart in proper sequence.
The sinoatrial (or sinus) node located in the right atrium
spontaneously beats periodically to begin each activation,
which proceeds to activate both atria before
reaching the atrioventricular (AV) node.
At the AV node, which under normal circumstances is
the only electrical connection between the atria and ventricles,
activation proceeds more slowly to allow for optimal ventricular filling.
The activation then enters the His-Purkinje system,
which allows a rapid spreading throughout the ventricles
and induces a strong, synchronized contraction.
Any variation from normal sinus rhythm is termed an arrhythmia.
The contracting three-dimensional heart on the left can be rotated, zoomed, and panned by clicking and moving the mouse as indicated.