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| Description: Electrical activity begins in the SA node and quickly spreads through the atria to the AV node. Branches from the AV node transmit electrical activity throughout the ventricles. The atria and ventricles do not contract until they have become electrically excited. Changes in pressure gradients between the atria and ventricles, and between the ventricles and the aorta and pulmonary arteries, cause the four heart valves to open and close.
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