Dehydration stimulates the posterior pituitary gland to release vasopressin hormone in blood which in turn makes kidneys to reabsorb more water. vasopressin prevents water loss by stimulating the formation of concentrated urine and release of more salts from the body. Since the hormone is released to bring about change in the hydration level of the body (to restore the controlled condition again by reversing the change), it is an example of a negative feedback system.
