The Cardiac Conduction System
The Cardiac Conduction System is what allows the heart to beat in the "lubb-dubb" pattern that it does. Everything begins with the SA Node, the pacemaker of the heart. When the SA Node starts its action potential, the process of atrial systole begins and the left and right atrials contract. From there, the electrical pulse travels to the AV Node through junctional fibers. Because these fibers carry the action potential somewhat slowly, there is a delay between the contraction of the SA Node and the contraction of the AV Node, causing the "lubb-dubb" sound. When the ventricles contract, they experience ventricular systole. (At the same time, the atrials go through atrial diastole, where they relax.) Once this occurs, the electrical pulse is sent to the Bundle of His, which splits into left and right fibers and travels down the interventricular spetum. When it passes the interventricular septum and enters the apex, the left branch travels to the left ventricle and the right branches travels to the right ventricle. At this point, these fibers are called perkinje fibers. The contraction of the ventricles allows the blood to be pushed out of the arteries and to be pumped throughout the body. When veins bring blood back to the heart, the whole process begins again.