During a long-distance run on a hot day, an athlete produces large quantities of sweat. As a result, the kidneys change the rate of urine production. Why is this change important?

To regulate water balance in the body

Explanation:

The kidneys are two bean shaped renal organs which perform a host of functions for the body such as regulating fluid balance, filtering minerals from the blood etc. Among their functions is the regulation of water balance in the body through the antidiuretic hormone.

When an athlete completes a physical activity and sweats a lot, the kidney reacts to the loss of water by sweat by adjusting urine output causing the body to produce lower and more concentrated urine.