Which change occurs in the kidneys when a person becomes dehydrated?

• A. The filtration rate of water remains constant and the reabsorption of water increases.
• B. The filtration rate of water increases and reabsorption decreases.
• C. The filtration rate decreases and reabsorption remains constant.
• D. Both filtration and reabsorption increase.

Answer: (A)

When dehydrated, the body aims to conserve water. Higher plasma osmolarity triggers antidiuretic hormone release, which makes the collecting ducts more permeable to water by inserting aquaporin-2 channels. This increases water reabsorption back into the bloodstream, reducing urine volume and concentrating the urine. The glomerular filtration rate is kept near normal through renal autoregulation, so the amount of filtrate (and water in it) remains constant while reabsorption increases. The other patterns would imply more filtration or less reabsorption, which isn’t how the kidneys respond to dehydration—the priority is reclaiming more water without increasing filtration.