Which of these changes occurs in the kidneys when a person becomes dehydrated?

• A. No changes occur in kidney function.
• B. The filtration rate remains constant and the reabsorption of water increases.
• C. The kidneys filter more water and there is less reabsorption of water from the blood.
• D. The filtration rate of water increases and the reabsorption remains constant.

Answer: (B)

When you’re dehydrated, the body works to conserve water while keeping the kidneys able to filter blood. The glomerular filtration rate is kept roughly constant by the kidney’s autoregulatory mechanisms, so filtration continues, but more of the filtered water is reabsorbed back into the bloodstream. This increased water reabsorption is driven mainly by antidiuretic hormone (ADH), which rises in dehydration and makes the walls of the collecting ducts more permeable to water, pulling more water out of the filtrate. Aldosterone also helps conserve sodium and, with it, water, but the key change that reduces water loss is the rise in ADH and the resulting increase in water reabsorption. So the filtration rate stays about the same while water reabsorption goes up, producing less and more concentrated urine.