What is referred to as the resting phase in the cardiac cycle?

The resting phase in the cardiac cycle is referred to as Phase 4. This phase is crucial as it represents the point when the cardiac muscle cells are in a polarized state, meaning they are not actively contracting and are at rest. During this phase, there is a stable resting membrane potential, and the cardiac cells are prepared for the next action potential that initiates another cycle of contraction.

In Phase 4, ion channels are closed to maintain this resting potential, with potassium ions (K+) being predominant inside the cell, while sodium ions (Na+) and calcium ions (Ca2+) remain outside. This polarization is essential for the heart to function properly, as a stable resting phase ensures that the heart can respond effectively to an electrical stimulus when it arises.

The other phases of the cardiac cycle correspond to different stages of contraction and repolarization, such as the depolarization and contraction processes (Phases 0, 1, 2, and 3). Understanding the significance of Phase 4 helps in comprehending how the heart maintains its rhythm and responds to electrical signals, ultimately ensuring efficient circulation throughout the body.