Final answer:
The highest concentration of sodium ions is in the extracellular fluid. This is maintained by the sodium-potassium pump, which creates a higher sodium concentration outside cells than inside.
Step-by-step explanation:
The sodium ion is at the highest concentration in the extracellular fluid of the body. Sodium (Na+) ions are essential for maintaining osmotic balance and proper cell function. They are found at about 10 times higher concentrations outside cells than inside. The primary mechanism for maintaining this differential is the sodium-potassium pump, which actively transports sodium ions out of cells and potassium ions in. This creates a high concentration of sodium ions in the extracellular fluid which encompasses blood plasma, interstitial fluid, and lymph. For example, the concentration of sodium in the plasma is approximately 0.14 M, whereas in the fluid outside of the plasma, the sodium concentration is about 0.5 M.