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A heartbeat is defined as one complete contraction of the heart.[1] There are several distinct stages which form a full heart beat. Cardiac Systole describes the period at which the heart contracts and Cardiac Diastole describes the period of relaxation, between beats (thus, the terms diastolic and systolic pressure, in relation to blood pressure). They can be further divided into diastole and systole of the atria and ventricles.

The Sino-Atrial (SA) node, situated in the wall of the right atrium, initiates the heartbeat, and the Atrioventricular (AV) node, situated between the ventricles, distributes the wave of impulses.


  1. unk. (n.d.). Respiratory Disorders Glossary. Retrieved on 2008-10-01.