Explain step by step or in simple terms how does the body regulate water

The kidneys can regulate water levels in the body; they conserve water if you are dehydrated, and they can make urine more dilute to expel excess water if necessary. Water is lost through the skin through evaporation from the skin surface without overt sweating and from air expelled from the lungs.

the correct answer is:

a population changes in response to a change in the environment.