In a Water Loading Test with 20 mL/kg ingestion, what is the normal urine response?

In a Water Loading Test with 20 mL/kg ingestion, what is the normal urine response?

• A. Urine volume is greater than 90% of water ingested
• B. Urine volume is less than 10% of water ingested
• C. Urine osmolality decreases
• D. Serum osmolality increases

Answer: (A)

Normal handling of a water load rests on the kidney’s ability to excrete free water efficiently. After drinking 20 mL/kg, a healthy kidney suppresses ADH, the collecting ducts become less permeable to water, and a large volume of dilute urine is produced to get rid of the excess water. Because most of the ingested water is filtered and excreted, the urine volume should exceed most of what was ingested—typically more than 90% of the water consumed. This shows the kidneys can rapidly dilute urine and eliminate the added fluid without raising plasma osmolality. In this context, what happens to urine osmolality is that it becomes quite dilute, but the key takeaway about a normal response is the high urine volume relative to intake. Serum osmolality would not rise with a water load; it would tend to stay the same or decrease due to dilution.

Normal handling of a water load rests on the kidney’s ability to excrete free water efficiently. After drinking 20 mL/kg, a healthy kidney suppresses ADH, the collecting ducts become less permeable to water, and a large volume of dilute urine is produced to get rid of the excess water.

Because most of the ingested water is filtered and excreted, the urine volume should exceed most of what was ingested—typically more than 90% of the water consumed. This shows the kidneys can rapidly dilute urine and eliminate the added fluid without raising plasma osmolality.

In this context, what happens to urine osmolality is that it becomes quite dilute, but the key takeaway about a normal response is the high urine volume relative to intake. Serum osmolality would not rise with a water load; it would tend to stay the same or decrease due to dilution.