Contractions of the atria arise from the SA (sinoatrial) node and squeeze blood into the ventricles. Signals pass to the atrioventricular (AV) node which lies between the atria and ventricles. Signals from the AV node are conducted through the AV bundle (of His) in the interventricular septum. Purkinje fibers in the walls of the ventricles conduct the signal upward from the apex of the heart. The spread of the electrical signal causes the ventricles to squeeze blood up and out the arteries.
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