Homeostasis refers to maintaining a steady state inside the body. In order to maintain a steady state, exchange of molecules take place.
For example, to maintain homeostasis the nitrogenous wastes should be removed from the body through the excretory and urinary system other wise these will be toxic to the body. The balance of water and other electrolytes is also important. Excess water should be released from the body.
In this way an organism whether a human being, an insect, etc is able to maintain homeostasis by exchanging molecules with its environment.
