The ability to maintain water balance is highly dependent on thirst, a sensation thought to
provide the drive for fluid ingestion. Clearly, it is only through the ingestion of fluid that a
water deficit can be replaced. Fluid ingestion is characterized by two types of drinking
behaviors – primary drinking, which is driven by deficits in body water and acts to restore
those deficits, and secondary drinking, where drinking occurs when no apparent need is
present (thirst associated with excessive talking). Primary thirst is regulated by plasma
tonicity and the volume of blood within the blood vessels (similar to antidiuretic secretion).