What makes us thirsty? Is it dry mouth or dehydrated body tissue that signals thirst?

Thirst and hunger have played a much bigger role in human history than they should have. You would think that a species that domesticated animals and conceptualized agriculture almost half a million years ago would have fulfilled its most basic needs ages ago. Alas, lack of food and water persists even to this day.

For most of you reading this however, the feeling of coming home and downing a glass of cold water is an everyday thing. Yet it still feels so good you can’t help but sigh when you’re done. So what exactly makes us realize we’re thirsty? Of course, lack of water in the body is what makes us thirsty, but what actually signals it? Dry mouth? Parched throat?

From experience one would have to say ‘neither’. Otherwise, just swallowing or rinsing 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 rather morbid 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.

It is obviously a common evolutionary trait we share with almost all other species on this planet. Water is much more essential for life than food. People that have been stranded alone out in deserts for long periods of time have been known to survive without food for almost a month without major complications. This is because your body has a lot of energy stored away just waiting to be utilized; mainly in the form of fats. Therefore, these individuals, when rescued after long periods of isolation without food were found to have lost dozens of pounds in weight.

Water on the other hand is not so easily waived off. The most a person can go without water intake is a week. Severe dehydration follows with death occurring not long after. It is therefore quite understandable that we evolved the need to seek out water as soon as its levels dropped below optimal within our bloodstream. You could even claim that thirst is one of our bodily senses, as the notion of just five senses covering all our body is capable of is quite simplistic and outdated.

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