Cardiac Conduction System
The heart beats at a rate of about 70-80 beats per minute. These beats are controlled by the cardiac conduction system. Near the right atrium is the SA node. It is the pacemaker. The SA node sends impulses to eh atrium and causes them to contract during systole. The impulse also travels to the AV node, through the AV bundle (bundle of His) and into the purkinje fibers. The purkinje fibers help to contract the ventricles during ventricular systole. Also, at this time, the mitral and tricuspid valves close so that blood does not flow backwards from the ventricles to the atriums.