a cat wanting food
The receptor in this case is also the cat. Here, the cat is realizing that it is very hungry. The receptor is what senses the change.
The control center is the cat's meowing. The control center compares the change with the norm, and in this case, the control center tells the cat to begin making noise in order to get more food.
The effectors are what stay the same as the stimulus until the action is complete. The cat's meowing and hunger stay unsatisfied until the effectors (the humans hearing the meowing and giving the cat food) do their job.
Balance is reached as the is fed and can consume its food.