What is the osmolality range of serum compared to urine?

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Multiple Choice

What is the osmolality range of serum compared to urine?

Osmolality in blood is kept within a narrow window, about 275–295 mOsm/kg, reflecting a stable concentration of solutes like sodium, glucose, and urea. Urine osmolality, however, varies widely because the kidneys adjust water reabsorption in response to hydration and ADH. It can be very dilute (around 50 mOsm/kg) when the body needs to excrete free water, or very concentrated (up to roughly 1000 mOsm/kg or more) when conserving water. The best choice matches these realities: serum osmolality near 275–295 mOsm/kg and urine osmolality spanning a broad range from about 50 to 1000 mOsm/kg. Other options either shift the serum range too far from the normal tight control or cap the urine range too narrowly, not reflecting the kidneys’ ability to concentrate or dilute urine.

Osmolality in blood is kept within a narrow window, about 275–295 mOsm/kg, reflecting a stable concentration of solutes like sodium, glucose, and urea. Urine osmolality, however, varies widely because the kidneys adjust water reabsorption in response to hydration and ADH. It can be very dilute (around 50 mOsm/kg) when the body needs to excrete free water, or very concentrated (up to roughly 1000 mOsm/kg or more) when conserving water.

The best choice matches these realities: serum osmolality near 275–295 mOsm/kg and urine osmolality spanning a broad range from about 50 to 1000 mOsm/kg. Other options either shift the serum range too far from the normal tight control or cap the urine range too narrowly, not reflecting the kidneys’ ability to concentrate or dilute urine.

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