The sodium ion is at the highest concentration in:

a. intracellular fluid

b. extracellular fluid

c. blood plasma

d. none of the above

Electrolytes are solutes that constitute inorganic salts, acids, and bases. The composition of electrolytes varies in different fluid compartments namely, intracellular fluid, extracellular fluid, and blood plasma. Electrolyte balance is essential to maintain fluid volume and regulate membrane potential.

Intracellular fluid refers to the fluid present within the cell membrane namely, the cytoplasm. The intracellular fluid has a high concentration of cation potassium and a low concentration of cation sodium. Hence, option a is incorrect.

Blood plasma constitutes about 55% of the blood and is a yellow-colored liquid. It plays an important role in carrying nutrients to different parts of the body as well as removing waste. The plasma is composed of 90% water and the remaining 10% is composed of proteins, electrolytes, hormones, metabolic nutrients, and metabolic waste. Hence, option c is incorrect.

The extracellular fluid refers to the fluid present outside the cell. Sodium ions are the major cation present in the extracellular fluid and chloride ion is the major cation. Sodium ions play an important role in maintaining the membrane potential of the cell and fluid volume. Hence, option b is correct.