Which ion is primarily responsible for determining the resting membrane potential of a cardiac cell?

The resting membrane potential of a cardiac cell is primarily determined by potassium ions. This is due to the fact that cardiac cells, like all excitable tissues, maintain a voltage difference across their membrane, which is a result of the distribution of ions inside and outside the cell.

Potassium ions are more concentrated inside the cell compared to outside, and the cell membrane is largely permeable to potassium at rest. As potassium ions leak out of the cell through potassium channels, they carry a positive charge with them, which results in a negative charge relative to the outside of the cell. This process establishes a negative resting membrane potential, typically around -90 mV for cardiac myocytes.

Although sodium, calcium, and chloride ions also play roles in the action potential and other cellular activities, they do not primarily influence the resting membrane potential to the extent that potassium does. Sodium ions, although influential during depolarization events due to their influx, do not significantly affect the resting state. Calcium ions also enter during specific phases of depolarization but do not contribute to the resting state. Chloride is involved in other cellular processes but does not play a significant role in setting the resting membrane potential. Thus, potassium is the key player in establishing and maintaining the resting membrane