Which function is performed by the kidneys to maintain fluid and electrolyte balance?

• A. Regulate fluid volume, electrolyte concentrations, acid-base balance, and waste elimination.
• B. Produce immunoglobulins.
• C. Regulate respiration rate.
• D. Store vitamins.

Answer: (A)

Kidneys regulate fluid balance, electrolyte levels, acid-base status, and waste removal. They filter blood, then reabsorb the right amount of water and key ions in the tubules, which keeps blood volume and osmolarity stable. They also manage acid-base balance by reabsorbing bicarbonate and secreting hydrogen ions, helping maintain the body's pH. Waste products like urea and creatinine are eliminated in urine, completing the cleanup role. Hormonal signals such as ADH and aldosterone fine-tune these processes in response to hydration and electrolyte needs. That combination of adjusting water, electrolytes, pH, and waste elimination is exactly what the kidneys do.

Immunoglobulin production is an immune system function, respiration rate is controlled by the respiratory system, and storing vitamins isn’t a primary kidney role (though vitamin D activation does occur in the kidneys).