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The sinoatrial node in the right atrium functions as a pacemaker. In a resting adult 60 to 80 rhythmical (constant) impulses are produced every minute. This impulse stimulates the atria muscles to contract. The impulse is conducted via the atria muscles to the A-V node. The A-V node delays the electrical conduction and by-passes the heart valves, which are prevented from passing on the impulses. This delay is necessary so that the ventricle muscle only contracts after the atrium muscle and not simultaneously. The stimulus is passed on by A-V node through the Bundle of His. This splits into two branches in the interventricular septum, the left bundle branch and the right bundle branch which, because of their high conduction speed, allow simultaneous contraction of all ventricle muscles. The stimulus is relayed to the single heart fibers via the Purkinje fibers.
Safety first!
No one can live without a heartbeat. That's why the body has back-up systems to take over control of the heart if the sinoatrial node fails or the electrical conduction system is disrupted.
At several places in the heart there occurs a build up of tension which, once it has reached a certain level, releases the impulse for the heart muscles to contract.
If the ventricular bundle branches have not been electrically discharged by an order to contract, they build up an impulse approximately 40 times a minute.
The sinoatrial node generates an electrical impulse faster than all and is therefore the first to give the order for the heart to beat. If the sinoatrial node fails or its signals are not propagated/passed-on to the atria, the A-V node takes over this pacemaker function with approx. 50 impulses per minute.
Even the single muscle cells of the heart are in the position to produce 25 to 30 impulses a minute. However, on account of their low frequency and the disjointed sequence of contractions, they are not able to move enough blood.
