Where does the atria located?

The Heart's own pace maker

The heart beats regularly because it has it's own pacemaker. The pacemaker is a small region of muscle called the sinoatrial, or SA, node. It is in the upper back wall of the right atrium. The node triggers an impulse that causes both atrium to contract. Very quickly, the impulse reaches the atrioventricular, or AV, node at the bottom of the right atrium. Immediately, the atrioventricular node triggers an impulse that causes both ventricles to contract.