How does dehydration affect kidney function and urine output?

• A. Increases urine production
• B. Improves kidney perfusion
• C. Reduces kidney perfusion, concentrates urine, decreases urine output, and increases risk of stones and renal impairment
• D. Has no effect on urine volume

Answer: (C)

Dehydration reduces circulating blood volume, which lowers kidney perfusion and filtration. In response to higher plasma osmolality, the body releases vasopressin (antidiuretic hormone), signaling the kidneys to conserve water. Water reabsorption increases in the collecting ducts, producing a small, concentrated urine with high osmolality and low volume. If dehydration is severe or prolonged, reduced renal blood flow and glomerular filtration can contribute to renal impairment, and the concentrated urine raises the risk of mineral precipitation and stone formation. This combination—lower kidney perfusion, concentrated urine, decreased urine output, and increased risk of stones and renal impairment—best describes how dehydration affects kidney function. The other options don’t fit because dehydration does not increase urine production, nor does it improve kidney perfusion, and it does alter urine volume.