Thirst is generally defined as a sensation of dryness in the mouth and throat associated with a desire for liquids. Yet many of us have very little knowledge of the fundamental mechanisms involved in thirst. Because if we do have a thorough understanding of what thirst is, we should know that this condition goes beyond merely having dry mouth and throat.
Compared to hunger, thirst is a far more discomforting sensation. While the sensation of hunger is characterized by intermittence, that of thirst is not. Rather, the sensation created by thirst is one that is continuous, becoming increasingly painful and unbearable until death.
In moderate thirst, the sensation seems confined within the back of the throat; an unpleasant feeling of prickling and mild burning is how this sensation can best be described. It is quite difficult, however, to be completely objective when describing true thirst; it is equally hard to separate the sensation itself from the desire for liquids.
There is a general propensity to associate thirst with the dryness in the mouth and throat. Along this line of thinking, the mechanism of thirst has been theorized, thus: When the water content of the blood and the tissues is diminished, the volume of the fluid secretions of the body - saliva included - is diminished as well. The mouth and throat become dry as a result of the decrease in the secretion of saliva. This sensation of dryness in the mouth and throat is what is commonly referred to as thirst.
This theory on the mechanism of thirst has a loophole though. True thirst is not produced by the mere dryness of the surface of the mouth and throat. This surface dryness may cause a craving for both the mouth and throat to be moistened. A person experiencing true thirst has the condition of water deficiency in the deeper tissues, not just this surface dryness.
As an example, an animal deprived of its salivary glands drinks no more water and is thus, by reasonable assumption, no thirstier than a normal one whose salivary glands continuously supply the fluid necessary to keep its mouth and throat moist.
It appears in all probability that the significant water deficiency in causing the beginning of the thirst sensation is in the deeper tissues of the mouth and pharynx. All the body tissues, therefore, of an animal that is deprived of water become dehydrated. Obviously, the sensory nerves mainly in the throat area are effectively stimulated by this water deficiency, making this specific area act like the sense organ for thirst.