When the body needs to conserve water, the kidneys produce urine that is:

• A. Dilute relative to blood plasma
• B. Similar in concentration to plasma
• C. Concentrated relative to blood plasma
• D. Pure water with no solutes

Answer: (C)

When the body needs to conserve water, the kidneys boost water reabsorption in the collecting ducts under the influence of antidiuretic hormone. Aquaporin channels open, allowing more water to be reabsorbed back into the bloodstream, so the urine becomes more concentrated. This concentrated urine has a higher osmolality than blood plasma, helping to retain water while still excreting waste. Dilute urine would indicate greater water loss, and urine that is similar in concentration to plasma or pure water with no solutes wouldn’t reflect the need to save water. The result is urine concentrated relative to plasma.