CVS Practice Test

During phase 3 of the cardiac action potential, repolarization occurs primarily due to the efflux of potassium ions (K+) from the cardiac cells. As the membrane potential reaches its peak and starts to decline, the permeability of potassium ions increases significantly because of the opening of voltage-gated potassium channels. This allows potassium to move out of the cell, which helps to restore the negative internal environment of the cell after the positive influx of sodium (Na+) and calcium (Ca2+) ions that occurs during the depolarization phases.

While sodium may play a role in the action potential, it primarily affects the initial depolarization phase and not the repolarization phase. Chloride ions can have a role in certain cell types as well, but they are not the main ions involved in the repolarization process in the heart. Therefore, the critical function of potassium efflux in phase 3 confirms that this answer is accurate, highlighting potassium's key role in returning the membrane potential to its resting state.