CVS Practice Test 2025 - Free CVS Practice Questions and Study Guide

In cardiac action potentials, depolarization is critical for the proper functioning of the heart's electrical system. The correct choice highlights the role of calcium ions in this process. During the depolarization phase of the cardiac action potential, there is a significant influx of calcium ions through voltage-gated calcium channels. This influx occurs more slowly compared to sodium ions, which contributes to the sustainment of a prolonged depolarization phase.

As calcium enters the cardiac muscle cells, it not only contributes to depolarization but also plays a vital role in triggering muscle contraction. This contrasts with sodium, which initially causes rapid depolarization but is not responsible for the sustained phase, and potassium ions, which primarily function to repolarize the cell. Chloride ions, while present, do not play a primary role in the depolarization during the cardiac action potential. Therefore, the modulation of calcium ion inflow is essential to maintaining the depolarization necessary for coordinated heartbeats.