The Physiology of Thirst

It’s a beautiful day to jog and you grab your favorite sneakers and plug in some earphones. The sun is greeting you warmly and after shedding out a good deal of sweat, nothing is more refreshing than drinking a bottle of cool, pure water.

Thirst is the conscious desire for water, and like most of the physiologic processes in the human body, it is also regulated by specific areas in the brain. These areas are collectively called the thirst center. As the neurons located in the thirst center get stimulated, they function to activate the thirst mechanism and thus prompt the need for fluid intake.

An important stimulus is a state of dehydration, such as that which occurs during profuse sweating after some heavy exercise, and the body’s response is obvious: a sensation of thirst is felt until the person is able to drink enough water to restore the normal body fluids. Yet surprisingly, there are other stimuli for thirst that may not be obvious but are equally significant.

Sensation of thirst is promoted by a decrease in the blood volume, perhaps due to hemorrhage. A decreased blood pressure resulting from the blood volume loss also sends signals to stimulate the thirst mechanism. Angiotensin II, an important hormone responsible for the regulation of both body fluid concentration and arterial pressure, is stimulated by factors associated with hypovolemia and low blood pressure. An increase in angiotensin II stimulates the thirst centers and likewise impels the kidneys to decrease fluid excretion.

Dryness of the mouth and mucous membranes of the esophagus also elicit the sensation of thirst. Relief is immediately achieved as soon as the thirsty person drinks water. Gastric distention produces the opposite effect in that it decreases the sensation of thirst.

Humans have the intrinsic ability to gauge fluid intake, an important mechanism to prevent overhydration. About 30 to 60 minutes may be required for the water to be absorbed and distributed in the body after intake. If the thirst sensation were not temporarily relieved after drinking water, the person would need to drink more and more which may eventually lead to overhydration.

The thirst mechanism is a complex means for the body to regulate fluid intake while maintaining the normal concentration of the extracellular fluids. Unknown to us is water lost from the body by evaporation from the lungs and the gastrointestinal tract, and it is also lost through urination and sweating. It is very important to replace lost fluids in order for the body to function optimally.

Next time you feel a little thirsty, make sure to grab some water and definitely obey your thirst.

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