From experience one would have to say ‘neither’. Otherwise, just swallow or rinse with water would satisfy the craving. Perhaps, thirst is a signal from the stomach. Yet, being ‘filled up’ does not always quench thirst. So what does make us thirsty? In one elegant experiment, researchers removed the gullet (or throat) of a dog so that no water could reach its stomach. Yet, when this dog was made thirsty, it drank incessantly for hours, never satisfying its thirst.
A study of humans who had lost their salivary glands demonstrated that thirst could be satisfied despite a continually dry mouth. Such observations prove that the sensory mechanisms involved in thirst are not as superficial as mere satiation. The real cause of thirst is a change in body chemistry. The major factor in fluid regulation is found in the bloodstream. As a change occurs in the body’s water content it is registered in blood volume, and subsequently in blood pressure. This decrease in blood pressure stimulates chemical secretions and alerts the brain to signal thirst. Evidence for this mechanism was obtained by tying off the major veins in a rat’s abdomen. These rats showed prolonged drinking.