Treatment for ectopic beats11/22/2023 ![]() The control of the heartbeat starts with a small clump of cells in the right atrium, called the sinoatrial node (the heart's natural pacemaker). A heartbeat happens when this muscle suddenly contracts (tightens) so that the chambers become smaller and the blood inside is squeezed out. The atria and ventricles have walls of muscle. The heart has two upper chambers (the left and right atria) and two lower chambers (the left and right ventricles).
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