Which of the following will directly affect the contraction of the ventricles?

The contraction of the ventricles is primarily influenced by the electrical conduction system of the heart. The Purkinje fibers play a critical role in this process. They are specialized conductive fibers that distribute the electrical impulse throughout the ventricles, leading to their contraction. When the impulse reaches the Purkinje fibers, it triggers a rapid depolarization of the ventricular myocardium, resulting in a coordinated and efficient contraction of the ventricles.

While activation of the sinoatrial node initiates the heartbeat and sets the pace, it is the conduction through the Purkinje fibers that directly translates that electrical signal into mechanical contraction of the ventricles. Relaxation of the atria and changes in heart rate do influence overall heart function, but they do not directly cause ventricular contraction; rather, they are parts of the cardiac cycle that facilitate the timing and efficiency of heart function. Thus, the action of the Purkinje fibers is fundamental to the direct control of ventricular contraction.