What is the role of the kidneys when urine osmolarity is high?

• A. The kidneys are excreting excess water
• B. The kidneys are conserving water
• C. The kidneys are filtering blood less efficiently
• D. The kidneys are unable to concentrate urine

Answer: (B)

The role of the kidneys when urine osmolarity is high is to conserve water. When the body is in a state where urine osmolarity is elevated, it generally indicates dehydration or a need for the body to retain water. The kidneys respond by reabsorbing more water back into the bloodstream, which leads to a more concentrated urine. This process is primarily facilitated by the action of antidiuretic hormone (ADH), which increases the permeability of the collecting ducts in the kidney, allowing for greater water reabsorption.

By conserving water, the kidneys help to maintain homeostasis and ensure that the body does not lose more fluid than necessary, which is particularly important in situations where the individual may be dehydrated or in states of increased plasma osmolarity. Therefore, the correct response accurately reflects the kidney's function in regulating water balance during periods of high urine osmolarity.