Final answer:
Maintenance of normal extracellular fluid volume is largely dependent on the concentration of sodium ions. Sodium is the major cation in the extracellular fluid, vital for maintaining osmotic pressure and electrical neutrality, regulated by kidney excretion.
Step-by-step explanation:
Maintenance of normal extracellular fluid volume depends largely on the extracellular concentration of sodium ions. The correct answer is A. sodium. Sodium is the major cation of the extracellular fluid and is responsible for one-half of the osmotic pressure gradient that exists between the interior of cells and their surrounding environment. Normal sodium concentrations are about 10 times higher outside than inside cells. The excess intake of sodium, which is common in a typical Western diet, is a major factor in hypertension for some individuals. Sodium levels are regulated by the kidneys, which are tasked with excreting excess sodium to maintain a healthy balance.
Regarding the statement "5. The sodium ion is at the highest concentration in:", the answer is B. extracellular fluid, reflecting its importance in maintaining osmotic balance and proper hydration. Chloride, the predominant extracellular anion, works hand in hand with sodium to maintain electrical neutrality in the extracellular fluid and follows similar paths in the renal system.