An animal maintains its fluid balance by regulating the gain and loss of water. This maintenance is an example of

Homeostasis

This maintenance is an example of homeostasis.

Homeostasis is the ability of a biological system or an organism to regulate its internal environment regardless of the external changing conditions in order to maintain a stable equilibrium. Homeostasis is usually carried out by a feedback control system, and all integration processes and coordination of functions in the body are examples of homeostatic control.