Which ion is most abundant in intracellular fluid?

Which ion is most abundant in intracellular fluid?

• A. Potassium.
• B. Sodium.
• C. Chloride.
• D. Calcium.

Answer: (A)

Potassium is the most abundant ion inside cells because the cell maintains a high intracellular potassium level through the Na+/K+-ATPase pump, which moves potassium into the cell while pumping sodium out. Potassium also tends to leak out through channels, but the pump keeps a large reservoir inside. This large intracellular pool helps set the resting membrane potential, making the inside of the cell negatively charged relative to the outside (about -70 to -90 mV in many cells). In numbers, intracellular potassium is roughly 140 mM, while outside the cell it’s about 4–5 mM. Sodium and chloride are much higher outside the cell, and calcium is kept at very low free concentrations inside, reserved for signaling when needed.

Potassium is the most abundant ion inside cells because the cell maintains a high intracellular potassium level through the Na+/K+-ATPase pump, which moves potassium into the cell while pumping sodium out. Potassium also tends to leak out through channels, but the pump keeps a large reservoir inside. This large intracellular pool helps set the resting membrane potential, making the inside of the cell negatively charged relative to the outside (about -70 to -90 mV in many cells). In numbers, intracellular potassium is roughly 140 mM, while outside the cell it’s about 4–5 mM. Sodium and chloride are much higher outside the cell, and calcium is kept at very low free concentrations inside, reserved for signaling when needed.