Final answer:
Potassium is the ion with the highest concentration in the cytosol, maintained by the sodium-potassium pump. Sodium and chloride have higher concentrations in the extracellular fluid.
Step-by-step explanation:
The ion with the highest concentration in the cytosol, or intracellular fluid (ICF), is potassium (K+). Potassium is the major intracellular cation that plays critical roles, such as establishing the resting membrane potential in neurons and muscle fibers. Contrary to potassium, sodium (Na+), chloride (Cl-), and calcium (Ca+2) have higher concentrations outside the cell, in the extracellular fluid (ECF). The sodium-potassium pump is essential for maintaining these concentration gradients, using ATP to actively transport sodium out of the cell and potassium into the cell. This pump ensures high intracellular levels of potassium and low levels of sodium, despite the fact that ions can "leak" through the cell membrane.
Sodium and potassium, along with chloride, are crucial electrolytes that impact the distribution and retention of body water, and their levels are closely regulated in body fluids. Sodium has higher concentrations outside the cell, about 10 times compared to inside, whereas potassium concentrations are about 30 times higher inside than outside of cells.