How is urine volume regulated?

• A. By hormones such as ADH, aldosterone, and ANP.
• B. By gravity and hydration only.
• C. By nephron count.
• D. By blood pressure alone.

Answer: (A)

Urine volume is controlled by hormonal signals that adjust how much water and salt the kidneys reabsorb. When the body needs to conserve water, ADH is released and acts on the collecting ducts to insert aquaporin water channels, increasing water reabsorption. The result is smaller, more concentrated urine. When ADH is low, collecting ducts reabsorb less water, producing a larger volume of dilute urine.

Aldosterone also plays a key role by increasing Na+ reabsorption in the distal tubule and collecting duct; since water follows salt, more water is reabsorbed and urine volume decreases. Conversely, ANP rises with high blood volume and works to reduce Na+ reabsorption and promote diuresis, increasing urine volume to help lower volume and pressure.

This hormonal system works with the kidneys’ filtration and osmotic balance to maintain stable plasma osmolality and fluid volume, rather than relying on gravity, nephron count, or blood pressure alone.