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cardiac pacemaker
a small mass of specialized muscle tissue in the heart that sets a rhythm of contraction and relaxation for the other parts
of the heart, resulting in the heartbeat. Usually the pacemaker site is the sinoatrial node, near the junction with the superior vena cava. The normal rhythm, 60 to 100 contractions per minute, increases during physical
or emotional stress and decreases during rest. The pace varies from person to person and is affected by abnormal conditions
such as heart injuries and generalized infections. If the normal pacemaker fails to function, its regulating task may be taken
over by another small mass of special muscular tissue, the atrioventricular node.
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