The heart is a four-chambered pump with the two atria at the top and the larger ventricles at the bottom. The right side receives blood and pumps it to the lungs for oxygenation. From the lungs blood returns to the heart on the left to be pumped out to the rest of the body. This pumping action is coordinated on both sides of the heart to efficiently deliver blood to the organs. The mechanism ensuring proper pumping is the cardiac conduction system.
Sinoatrial Node and Bachman's Bundle
According to CV Physiology, the sinoatrial or SA node sits in the right atrium and is the most rapid pacemaker of the heart. It starts the process of depolarization. Standard electrical wires power devices through the transmission of electrons; the conduction system of the heart also transmits signals using electricity. The inside of the cells is negatively charged relative to the outside. Depolarization occurs when electrically charged ions, usually sodium but sometimes calcium, enter the cell through ion channels and make it more positive. The SA node begins the process of depolarization with calcium ions entering the cell, followed by sodium ions. Bachman's bundle transmits the electrical impulse to the left atria.
Atrioventricular Node and Bundle of His
The electrical signal originating in the SA node travels to the atria and reaches the AV node before transmission into the ventricles. The AV node acts to delay depolarization of the ventricles so the atria can finish pumping blood into them. From the AV node, the depolarization spreads downward into the bundle of His.
Left and Right Bundles
The wave of depolarization passes through the bundle of His to the left and right bundle branches. These are fast-conducting extensions of the His bundle. The electrical impulse spreads rapidly here. It reaches the Purkinje fibers first on the left side of the heart and then on the right.
Purkinje Fibers
The Purkinje fibers, also known as subendocardial branches, conduct the electrical impulse out into the contractile muscles of the ventricles. They are located just below the inner surface of the ventricles. They are specialized to conduct the wave of depolarization synchronously to enable a coordinated contraction of the heart.
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