The heart actually has two natural pacemakers. The sinoatrial node (SA node) is the primary pacemaker and the atrioventricular node (AV node) is the secondary. The AV node is in a bundle of tissues on the border that separates the right atrium and the right ventricle.
When the SA node sends out an electrical impulse, it goes straight to the AV node. The SA node sets the rhythm of your pulse. The AV node sets the rhythm of your heart contractions.
The AV node delays the electrical signal on its way to the ventricle, which gives the atrium time to contract first. The delay lasts for about a tenth of a second. If the ventricle and atrium were to contract at the same time, the ventricle would push out their blood before they were full, which would result in low blood pressure and other problems.
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