If blood volume is low and osmolarity is high (dehydration), what happens to urine?

• A. Urine becomes more dilute.
• B. Urine output increases.
• C. Urine becomes highly concentrated due to water conservation.
• D. Urine production stops.

Answer: (C)

In dehydration the body aims to conserve water, so the urine becomes highly concentrated because ADH increases water reabsorption in the kidneys. Osmoreceptors detect high plasma osmolality and signal the posterior pituitary to release antidiuretic hormone. ADH makes the collecting ducts more permeable to water, pulling more water back into the bloodstream. The result is a small volume of urine with a high concentration of solutes (urine osmolality rises), reflecting the emphasis on water conservation. If dehydration is severe, urine output can drop substantially, but it typically does not stop entirely.