Thirst is a strong motivational factor. The importance of replacing lost water is underscored by the fact that a person can survive for a month without food but for only several days without water. It appears that both thirst processes help to promote drinking. Researchers have estimated that 64 to 85 percent of the drinking following water loss is caused by cellular dehydration thirst. Hypovolemic thirst accounts for 5 to 27 percent of the drinking, and the remainder is caused by peripheral factors.
The two types of thirst are independent of each other. The receptors for both thirsts are located in the hypothalamic region of the brain, but they are at different locations. Research has shown that lesions in one region will have no effect on thirst regulation in the other region.
Although the motivation to drink in humans is under conscious control by peripheral factors, unconscious control does exert a large influence. A study of cellular dehydration thirst using goats showed that the injection of a saline solution that has a salt concentration of more than 0.9 percent salt (body fluids have a salt concentration of 0.9 percent salt) into the area in which the osmoreceptors are located will produce a drinking response within sixty seconds. Similar results have been found regarding hypovo-lemic thirst; injecting angiotensin II (a converted protein found in the blood) into the hypothalamus causes a drinking response. This occurs even in animals that are fully hydrated. These animals will consume in direct proportion to the amount of angiotensin II injected into the hypothalamus.
Diet can have a profound effect on water balance in humans. Eating salty foods will produce cellular dehydration thirst despite adequate fluid levels, because water will flow out of the cells into the extracellular fluid. In contrast, salt-free diets will produce hypovolemic thirst by causing water to flow into the cells. Other factors also cause thirst. As stated previously, diarrhea, vomiting, and blood loss will cause hypovolemic thirst as a result of the loss of extracellular fluid. Therefore, significant blood loss will cause a person to become thirsty.
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