Which organ system maintains internal balance by conserving or excreting excess fluid?

• A. Digestive system
• B. Muscular system
• C. Endocrine system
• D. Kidneys

Answer: (D)

Fluid balance is controlled primarily by the kidneys, part of the urinary system. They continuously filter the blood, reclaim needed water and electrolytes, and form urine. When the body needs to conserve water, hormones signal the kidneys to reabsorb more water in the tubules, reducing urine output and helping maintain blood volume and osmolality. When there is excess fluid, the kidneys increase urine production to excrete the surplus, keeping extracellular fluid volume and electrolyte balance in check. The endocrine system influences this process with hormones like ADH and aldosterone, but the kidneys perform the actual conservation or excretion. The other systems don’t regulate fluid balance in this direct way.