Where is most potassium found when the cardiac muscle cell is at rest?

When a cardiac muscle cell is at rest, most potassium is found in the cytosol, which is the liquid component inside the cell. This is primarily due to the function of the sodium-potassium pump (Na+/K+ ATPase), which actively transports potassium ions into the cell while moving sodium ions out. As a result, potassium concentration becomes significantly higher inside the cell compared to the extracellular fluid.

This high intracellular potassium is crucial for maintaining the resting membrane potential, which is essential for normal electrical activity in cardiac tissue. When a cardiac muscle cell is at rest, the interior of the cell is negatively charged relative to the outside, primarily due to this high concentration of potassium and its permeability through the cell membrane.

In contrast, while some potassium is indeed present in extracellular fluid, this concentration is much lower in comparison. The cytosol is also where metabolic processes take place, and the presence of potassium here supports various cellular functions. Other cellular compartments such as the mitochondria and nucleus have different functions and do not primarily store or contain high concentrations of potassium ions compared to the cytosol.